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GalvInfoNoteSeries

Table of Contents

Rev 6 Mar-2010 1. Metallic-Coated Products and Specifications1.1. Understanding Coating Weight Designations for Zinc-Based Coatings on Steel Sheet

1.2. Hot-Dip Coated Sheet Products

1.3. Galvanneal - Differences from Galvanize

1.4. 55% Aluminum-Zinc Alloy-Coated Steel Sheet

1.5. ASTM Standards for Coated Sheet Products

1.6. Selecting Coating Thickness (Weight or Mass) for Galvanized Steel Sheet Products

1.7. Advantages of Metallic-Coated Steel Framing in Residential Buildings 1.8. Steels Used in Coated Sheet Products 1.9. Zinc-5% Aluminum Alloy-Coated Sheet 1.10. Useful Galvanized Sheet Metrics

2. Coating Processes and Surface Treatments2.1. The Continuous Hot-Dip Coating Process for Steel Sheet Products

2.2. The Continuous Electroplating Process for Steel Sheet Products

2.3. Continuous Hot-Dip Galvanizing versus General (Batch) Galvanizing

2.4. The Role of Aluminum in Continuous Hot-Dip Galvanizing

2.4.1. Zinc Bath Management on Continuous Hot-Dip Galvanizing Lines2.5. Control of Coating Weight (Mass) for Continuous Hot-Dip Galvanized Sheet Products

2.6. The Spangle on Hot-Dip Galvanized Steel Sheet

2.7. Galvanizing – The Use of Chemical Fluxes2.8. Improving Uniformity of Appearance

2.9. Treatments for Enhancing Formability

2.10. Imparting Resistance to Storage Stain2.11. Preparing Galvanize for Field Painting

2.12. Pretreatments for Metallic-Coated Sheet2.13. Treatments for Resistance to Handling and Fingerprint Marks

3. Corrosion – Mechanisms, Prevention, and Testing3.1. How Zinc Protects Steel

3.2. Protecting Galvanized Steel Sheet Products from Storage Stain

3.3. Dark-Colored Stains on Galvannealed Sheet3.4. The Salt Spray Test

3.5. Fretting Corrosion (Transit Abrasion) on Galvanized Sheet

4. Prepainted Metallic-Coated Steel Sheet4.1. Introduction to Painted, Metallic-Coated, Steel Sheet Products

4.2. Prepainted Metallic-Coated Steel Sheet for Building Panels – Assuring Good Performance4.3. Repair Painting of Prepainted Metallic-Coated Steel Sheet

5. General Coated Sheet Topics5.1. Hardness Measurement of Coated Sheet

5.2. Zinc Grades Used for Continuous Hot-Dip Galvanizing




1

COATING WEIGHT MEASUREMENT

There is a very precise on-linetechnique for measuring coatingthickness. The equipment required is

expensive, uses sophisticated x-rayor radio-isotope fluorescencedevices, and requires considerable

expertise to operate. These gaugesrepeatedly sense the coatingthickness on each surface, average a

large number of readings, and thenconvert the results to the morefamiliar coating weight units.

Laboratory versions of this equipmentare also available. Refer to ASTMTest Method A754 for more

information. These gauges requirecalibration based on weigh-strip- weigh testing.

1. Metallic-Coated Products and Specifications

GalvInfoNote

1.1

Understanding Coating Weight Designationsfor Zinc-Based Coatings on Steel Sheet

Rev 2.0 Mar-2010 Introduction

One of the most confusing aspects of coated steel sheet products are coating weight designations andwhat they mean, particularly with respect to product performance. This article is intended to clarify thisissue.

Coating Weight [Mass] Measurement Systems

Each coated steel sheet product has its own coating weight designation system, which is defined in theappropriate ASTM standard. For example, the most widely used ASTM metallic-coated sheet standard is A 653/A 653M, which covers hot-dip galvanized products. One of the coating weight designation systemsin this standard uses descriptors such as G60, G90, etc. The “G” means the coating is galvanize (zinc),and the numbers refer to the weight of zinc on the surface of the steel sheet in inch-pound(English/Imperial) units. Taking G90 as an example, the coating weight on one square foot of sheet (total,both sides of the sheet) shall have a triple spot test (TST) average minimum of 0.90 ounces. If equally

applied to both sides of the sheet, there would be a minimum of 0.45 ounces on each surface.The other measurement system in widespread use today is the SI (Metric) system. The conversion fromthe inch-pound weight in ounces per square foot (oz/ft

2) to the SI mass in grams per square metre (g/m

2)

is:

1 oz/ft2 = 305.15 g/m2

To convert from oz/ft2 to g/m2, multiply by 305.15

Example: G90 (0.90 oz/ft2) = Z275 (275 g/m2)

If what we are interested in is the coating thickness, why do ASTM

standards not use thickness measurements? The answer is simply

that it is difficult to directly measure the thickness accurately. For

example, a G90 coating contributes about 1.6 mils (0.0016 inches,

or about 42 microns) to the total thickness of the coated sheet. For

a coating equally applied to both sides of the sheet, this means

there is about 0.0008 inches (21 microns) of zinc on each surface.

To accurately determine the thickness of the coating, the coated

thickness must be measured, the coating stripped off, and then the

steel substrate thickness measured using a gauge capable of

accurately reading to the nearest ten-thousandth of an inch. This is

very difficult to do with good accuracy. On-line equipment is

available that can nondestructively do this (see sidebar), but the

most accurate manual method of determining the amount of coating

present is to measure its weight [mass] on a given surface area

using the “weigh-strip-weigh” technique. Weigh-strip-weigh refers to

the procedure of weighing a standard size sample of the productusing a very accurate scale, stripping the coating in an inhibited acid without removing any of the

substrate, then reweighing the coupon to determine the weight [mass] loss. This is the original method of

determining coating weight [mass], and, in fact, is still the referee and standard method used to check

and calibrate nondestructive on-line and laboratory coating thickness gauges. There are weigh-strip-

weigh procedures that can be used for all zinc-based coatings in commercial production today. For the

most common products, these procedures are defined in ASTM Standard A 90/A 90M, and cover

galvanized and galvannealed sheet, 55% aluminum-zinc alloy-coated sheet, and zinc-5% aluminum alloy-



GalvInfoNote 1.1Rev 2.0 Mar-2010


2

coated sheet. There are special procedures required for other types of alloy coatings such as aluminized,

and zinc-nickel alloy electroplated sheet. These are covered by other ASTM standards.

Designation System for Galvanized and Galvannealed Sheet Products

Galvanize – For galvanized sheet, common inch-pound coating weight designations (ordered as A 653)are, in oz/ft

2:

G30 G40 G60 G90 G115

These designations specify the minimum average TST, total both sides, tested per ASTM A 924/A 924M,

e.g., G90 requires a minimum average TST of 0.90 oz/ft2 total both sides. The specification stipulates

that TST samples shall be taken from defined positions at the edge-center-edge of the as-coated sheet.

There are designations for heavier coatings, such as G165 and G210, but these products are used forvery specialized applications and are generally not available on thinner gauge sheet.

In SI units (ordered as A 653M), the comparable coating mass designations for galvanized sheet are, ing/m

2:

Z90 Z120 Z180 Z275 Z350

These designations specify the minimum average TST, total both sides, tested per A 924/A 924M, e.g.,

Z275 requires a minimum average TST of 275 g/m2

total both sides.In 2007 ASTM added the option of ordering single side, single spot test (SST) coating designations to A653/A 653M. These are SI designations only (ordered to A 653M) and specify the minimum andmaximum allowable coating mass per side for any SST. They take the familiar form of automotivecoating designations (numeric characters first – signifying a per side requirement). No inch-pounddesignations are used since single side coatings are traditionally ordered in SI units only. Examples are:

60G 70G 90G

These designations specify the minimum and maximum SST value on each surface, e.g., 60G requires a

minimum of 60 g/m2 and a maximum of 110 g/m

2 of zinc on each surface for any SST.

When specifying single side single spot coatings, the designation for each surface must be shown, e.g.,60G60G.

Coating weight [mass] versus coating life – For galvanized coatings in most applicationsand environments, the corrosion performance is an approximate linear function of coatingweight (thickness). For instance, a G60 coating has twice the thickness of a G30 coating, andthe life of the product (defined, perhaps, as the time to 5% rust) in a given environment isapproximately twice as long. Similarly, a G90 coating is approximately 50% thicker than aG60 coating, and thus would be expected to perform 50% better (in terms of time to 5% rust).For a more thorough discussion on service life, see GalvInfoNote 3.1. Limits on maximumacceptable coating weights for an application are usually determined by other factors such ascost or formability. For a more thorough discussion of this topic see GalvInfoNotes 1.6 and

2.5.

For other metallic-coated sheet products, the life versus coating thickness is typically notlinear; thus determining the coating weight (mass) to use is not as simple as it is forgalvanized coatings. Also, when these products are painted, the behaviour is even morecomplex. The subject of painted hot-dip products is addressed in GalvInfoNotes 4.1 and 4.2.





3

Zinc-Iron (Galvanneal) – The common inch-pound coating weight designations (ordered as A 653) forgalvannealed sheet (zinc-iron alloy-coated) are, in oz/ft

2:

A25 A40 A60

As with galvanized product designators, A40 for example, requires a minimum average TST coatingweight of 0.40 oz/ft

2, total both sides. While the coating contains approximately 8 to 10% iron, resulting in

the density being slightly higher than a zinc coating and the coating thickness being slightly less than aG40 galvanize coating, the difference is too small to be of concern. The effect of density is discussed inthe section on 55% Al-Zn coatings, and in the Appendix. Also, see GalvInfoNote 1.3 for a full explanationof hot-dip galvanneal coatings.

The SI equivalent coating mass designations (ordered as A 653M) for galvannealed sheet are, in g/m2:

ZF75 ZF120 ZF180

ZF120, for example, requires a minimum average TST of 120 g/m2 total both sides.

As with galvanize, the option of ordering zinc-iron coatings to single side, SST coating designations hasbeen added to A 653/A 653M. Again, these are SI designations only (ordered to A 653M), specifying theminimum and maximum allowable coating mass per side for any single spot, and taking the familiar formof automotive coating designations (numeric characters first – signifying a per side requirement). No

inch-pound designations are used since single side coatings are traditionally ordered in SI units only.Examples are:

45A 50A

These designations relate to the minimum and maximum SST value on each surface, e.g., 45A requires aminimum of 45 g/m

2 and a maximum of 75 g/m

2 of zinc-iron alloy on each surface for any SST.

When specifying single side, SST coatings, the designation for each surface must be shown, e.g.,45A45A.

………………………

For galvanized and galvannealed sheet, the relationship between coating weight (mass) and thickness is

as follows (based on zinc density of 446 1b/ft3 or 7140 kg/m

3):

1 oz/ft2

= 0.0017 in = 305.15 g/m2

= 0.0427 mm (1)

Designation System for Electrogalvanized Sheet Products For electroplated coatings (pure zinc and zinc-based alloy coatings), SI system (g/m

2) designators are

most commonly used, although ASTM Standard A 879/A 879M for electrogalvanize was updated in 2004to include the inch-pound [oz/ft

2] designator system. The reason for the initial use of SI designators is that

many electroplated products were and still are used for automotive applications. Auto companies, whoimplemented worldwide coated sheet specifications some time ago, use only SI units.

For electrogalvanized sheet, common inch-pound coating weight designations (ordered as A 879) are, inoz/ft

2:

08Z 13Z 30Z

These designations relate to the minimum and maximum SST value on each surface, as defined in ASTM A 879/A 879M, e.g., 13Z requires a minimum of 0.13 and a maximum of 0.23 oz/ft

2 of zinc on each

surface for any SST. Again, the numeric characters come first, signifying per side requirements.

When specifying, the designation for each surface must be shown, e.g., 13Z13Z.

For electrogalvanized sheet, common SI coating weight designations (ordered as A 879M) are, in g/m2:

24G 40G 90G





4

These designations relate to the minimum and maximum SST value on each surface, as defined in ASTM

A 879/A 879M, e.g., 40G requires a minimum of 40 and a maximum of 90 g/m2 of zinc on each surface for

any SST.

Again, the designation for each surface must be shown, e.g., 40G40G.

See GalvInfoNote 2.2 for an explanation of the electrogalvanizing process.

Keeping Designation Systems Straight

As hot-dip galvanized and galvannealed coatings saw more use by the automotive industry, it became the

practice to manufacture these products to conform to single side, SST g/m2values; that being the

requirement of automotive manufacturers. Products ordered for construction and other general end usescontinue to be ordered to total both sides, TST, inch-pound designations. For hot-dip galvanize, as wehave seen, ASTM uses “G” (prior to the numerals) in the designator for inch-pound coatings and “Z” for SIcoatings – total both sides. On the other hand, for electrogalvanize “G” (after the numerals) means SIunits and “Z” means inch-pound units.

The use of both dimensional units, and the reversal of “G” and “Z” between TST hot-dip, and single side,SST EG in ASTM specifications, certainly can lead to confusion in the marketplace. Table 1 belowsummarizes what the various designations mean in terms of single spot and triple spot requirements.

Table 1 Galvanized Sheet Designations Explained

Product Type and Coating Requirements

Coating Tests Required

Single Side Total Both Sides

CoatingDesignation

Format Specification Coating Units

SST TST SST TST

Gnn A 653 – Table 1 zinc - HD oz/ft2 NONE Min Min Min

Znn A 653M – Table 1 zinc - HD g/m2 NONE Min Min Min

Ann A 653 – Table 1 zinc-iron - HD oz/ft2 NONE Min Min Min

ZFnn A 653M – Table 1 zinc-iron - HD g/m2 NONE Min Min Min

nnZnnZ A 879 zinc - EG oz/ft2 Min & Max NONE NONE NONE

nnGnnG A 879M zinc - EG g/m2 Min & Max NONE NONE NONE

nnGnnG A653 M – Table S2.1 zinc - HD g/m2 Min & Max NONE NONE NONE

nnAnnA A653 M – Table S2.1 zinc-iron - HD g/m2 Min & Max NONE NONE NONE

nnGnnG Auto (typical) 1 zinc - HD & EG g/m2 Min & Max NONE NONE NONE

nnAnnA Auto (typical) 2 zinc-iron - HD & EG g/m2 Min & Max NONE * NONE NONE

Notes: nn = numerals (2 or 3) specific to coating weight [mass]HD = Hot-Dip

EG = ElectrogalvanizeSST = Single Spot TestTST = Triple Spot Test

* some auto manufacturers require a minimum TST





5

For additional clarification, see Table 2 below, which provides the requirements of selected coating weight[mass] examples for galvanized sheet made to ASTM specifications.

It is not easy to keep the terminology straight. Users should be aware that both units are in common usetoday, and are advised to pay close attention when ordering, knowing precisely what is meant by theterminology being used. See the Table 3 at the end of this article, which summarizes the designations

used for most hot-dip products, and may be useful in keeping terminology clear.Table 2 Selected ASTM Galvanized Sheet Designations – Requirements

Product Type Example Designation Requirement

G90(A 653, Table 1, in-lb)

TSTa average 0.90 oz/ft

2 min – total both sides

TST average 0.32 oz/ft2 min – each side

SSTb 0.80 oz/ft

2 min – total both sides

Z275(A 653M, Table 1, SI)

TST average 275 g/m2 min – total both sides

TST average 94 g/m2 min – each side

SST 235 g/m2 min – total both sides

Hot-Dip Galvanize(A 653/A 653M)

60G60G(A 653M, Table S2.1, SI)

c

SST 60 g/m2 min, 110 g/m

2max – each side

A40(A 653, Table 1, in-lb)

TST average 0.40 oz/ft2 min – total both sides

TST average 0.12 oz/ft2 min – each side

SST 0.30 oz/ft2 min – total both sides

ZF120(A 653M, Table 1, SI)

TST average 120 g/m2 min – total both sides

TST average 36 g/m2 min – each side

SST 90 g/m2 min – total both sides

Hot-Dip Galvanneal(A 653/A 653M)

45A45A(A 653M, Table S2.1, SI)

SST 45 g/m2 min, 75 g/m

2 max – each side

13Z13Z(A 879, Table 1, in-lb)

SST 0.13 oz/ft2 min, 0.23 oz/ft

2 max – each side

Electrogalvanize(A 879/A 879M) 40G40G

(A 879M, Table 1, SI)SST 40 g/m

2 min, 70 g/m

2 max – each side

a – Triple Spot Test

b – Single Spot Test

c – For information purposes, Table S2.1 in A 653M shows inch-pound values for the SI coating designations

Total Both Side TST versus Single Side SST Coatings

Because the ASTM total both sides, TST designators allow an uneven split of the coating (one side musthave at least 40% of the specified minimum SST coating weight), it is not possible to precisely convertthem to SST, single side designators since the latter specify exact minimums per surface. It is sometimesuseful, however, to provide an approximate conversion based on the total coating thickness on bothsurfaces.

Figure 1 on the next page is a chart that allows this to be done, both in terms of coating designators and

thickness of the total coating. For instance, it can easily be seen that a G60 coating has a minimum totalthickness of about 1.0 mil, which is the very close to the total minimum thickness (25 microns) of a90G90G coating. Remember, however, that a G60 coating is an average of 3 readings (TST) and canhave an uneven split of the total coating thickness, while a 90G90G coating must have a minimum of 12.5microns on each side for any single spot.

Figure 1 is a guideline only for estimating coating thickness in terms of the two systems and is not meantto suggest equivalency. Also, the values shown are specified minimums. Actual coatings are always afew percent thicker in order to guarantee the minimums.





6

Designation System for 55% Aluminum-Zinc Alloy-Coated Sheet Steel sheet with a 55% aluminum-zinc alloy coating

(55% Al-Zn alloy-coated) is in common use today

throughout the construction and other industries. It, too, has very specific coating designators.Fortunately, there are only a few designators, but that doesn’t mean there is no confusion about themeaning. The designation systems for coating weight and coating mass are given in ASTM Standard A792/A 792M.

The four inch-pound coating weight designations (ordered as A 792) are, in oz/ft2:

AZ50 AZ55 AZ60 AZ70

These designations specify the minimum average of a TST, total both sides, tested per A 924/A 924M,

e.g., AZ50 requires a minimum average TST of 0.50 oz/ft2 total both sides.

These designators are comparable to those used for galvanized sheet in that the dimensions are oz/ft 2 .

But, one has to be aware that the designation AZ60 is not equivalent to a G60 coating with respect to thethickness of the coating. Here is where the issue of density comes into play. The coating on 55% Al-Znalloy coated sheet has about 55% aluminum and 45% zinc. Actually, the coating has a small addition ofsilicon, but for purposes of this discussion the silicon is not important. Since aluminum is less dense thanzinc (a given volume weighs less than the same volume of zinc), an AZ60 coating is thicker than a G60galvanize coating. See the section on theoretical weight [mass] in the Appendix to understand howdifferences in coating density affect the coated sheet.





7

Because 55% Al-Zn alloy coating and a galvanize coating behave quite differently with respect tocorrosion processes, it is not possible to try to draw a performance equivalency curve. There is no answertherefore to the question: What 55% Al-Zn alloy coating is equivalent in performance to a G90 coating?The major use of 55% Al-Zn alloy coated sheet is for construction industry building panels, and for thisapplication the most common coating weights are AZ50 and AZ55. As the differences in performancebetween these two designators are subtle, ask your supplier which coating thickness they recommend for

your application.

For 55% Al-Zn alloy-coated sheet there is also a SI coating mass designator system (ordered as A792M). The SI equivalents to AZ50, AZ55, AZ60 and AZ70 are, in g/m

2:

AZM150 AZM165 AZM180 AZM210

These designations specify the minimum average of a TST, total both sides, per A 924/A 924M, e.g.,

AZM150 requires a minimum TST of 150 g/m2 total both sides.

Since 55% Al-Zn alloy coated sheet is produced only by the hot-dip process, there is no additionalterminology or specification related to the manufacture of an electroplated product. Also, there are noSST, single side designations for this product.

For 55% Al-Zn alloy coated sheet, the relationship between coating weight [mass] and thickness is as

follows (based on a density of 234 lb/ft3 or 3750 kg/m

3):

1 oz/ft2 = 0.0032 in = 305.15 g/m2 = 0.0813 mm (2)

See GalvInfoNote 1.4 for a complete description of 55% Al-Zn alloy-coated sheet.

Designation System for Zinc-5% Aluminum Alloy-Coated Sheet

A third type of zinc-based coating that has not seen much use for sheet products in the United States, butis recognized by ASTM, is zinc-5% aluminum alloy-coated (Zn-5% Al alloy-coated) sheet. Zn-5% Al alloy-coated sheet has a coating that consists of 95% zinc and 5% aluminum, and small amounts of otherelements to improve processing and product characteristics. The designation systems for coating weightand coating mass are given in ASTM Standard A 875/A 875M.

The common inch-pound coating weight designations (ordered as A 875) are, in oz/ft2:

GF30 GF45 GF60 GF75 GF90

For Zn-5% Al alloy-coated sheet, since the coating contains about 95% zinc, and thus has nearly thesame density as zinc, a GF90 coating is approximately equivalent in thickness to a G90 galvanizedcoating.

The equivalent SI coating mass designations (ordered as A 875M) are, in g/m2:

ZGF90 ZGF135 ZGF180 ZGF235 ZGF275

As with 55% Al-Zn al loy-coated sheet, Zn-5% Al alloy-coated sheet is made only by the hot-dip processso there are no designator systems that involve per side terminology.

For Zn-5% Al alloy coated sheet, the relationship between coating weight [mass] and thickness is asfollows: (based on a density of 427 1b/ft

3 or 6840 kg/m

3)

1 oz/ft2 = 0.00175 in = 305.15 g/m2 = 0.0446 mm (3)





8

Summary

This article explains the complexities of coating designation systems and hopefully provides a betterunderstanding of why it is important to be sure that you and your supplier are speaking the samelanguage. Table 2 gives examples of some of the designators discussed above. See GalvInfoNote 1.10for an explanation of how coatings affect the metrics of coated steel sheet products.

Table 3 Designators for Zinc-Based Coatings on Steel Sheet - SUMMARY Example of Common Coating

DesignationsProduct

inch-pound SI

Coating Weightinch-pound

oz/ft2

Coating MassSI

g/m2

Total Both Sides - Minimum Triple Spot Average

ASTM A 653/A 653MGalvanize

G90 Z275 0.90 275

ASTM A 653/A 653M

Galvanneal A40 ZF120 0.40 120

ASTM A 792/A 792M55% Al-Zn alloy-coated

AZ55 AZM165 0.55 165

ASTM A 875/A 875MZn-5% Al alloy-coated

GF75 ZGF225 0.75 225

ASTM A 1046/A 1046MZn-Al-Mg alloy-coated

ZM90 ZMM275 0.90 275

Single Side++ - Minimum Single Spot

ASTM A 653MGalvanize

N/A* 60G 0.20 60

ASTM A 653MGalvanneal

N/A* 45A 0.15 45

ASTM A 879/A 879MElectrogalvanize

13Z 40G 0.13 40

Automotive SpecifiedGalvanize

N/A* 100G N/A* 100

Automotive SpecifiedGalvanneal

N/A* 45A N/A* 45

* Not Applicable++

Single side designators are used to specify the coating mass for each side and are written, for example, 60G60G, or inthe case of differential coating masses, 90G60G.

Copyright! 2010 – IZA

Disclaimer:

Articles, research reports, and technical data are provided for information purposes only. Although the publishers endeavor to

provide accurate, timely information, the International Zinc Association does not warrant the research results or information reportedin this communication and disclaims all liability for damages arising from reliance on the research results or other informationcontained in this communication, including, but not limited to, incidental or consequential damages.




1


GalvInfoNote Hot-Dip Coated Sheet Products

Rev 1.0 Aug-091.2

Introduction

GalvInfoNote 2.1 describes the steel sheet hot-dip coating process, explaining how it is used to make

seven different types of coated products. These products are described in more detail below. For more

information on the ASTM standards covering this material (and how to obtain the standards), refer to

GalvInfoNote 1.5.

Types of Hot-Dip Coatings

Coating Name Coating Composition ASTM Specification

Galvanize Zinc A 653/A 653M

A 1063/A 1063M

Galvanneal Zinc-10% Iron A 653/A 653M

Aluminum-Zinc 55% Aluminum-Zinc A 792/A 792M

Zinc-Aluminum Zinc-5% Aluminum A 875/A 875M

Zinc-Aluminum-Magnesium Zn-5/13% Al-2/4% Mg A 1046/A 1046M

Aluminized Al-5/11% Si, or pure Al A 463/A 463M

Terne Lead-8% Tin A 308/A 308M

General requirements for all hot-dip coatings – ASTM A 924/A 924M

ASTM Hot-Dip Steel Sheet Specifications

A 653/A 653M - Standard Specif ication for Steel Sheet , Zinc-Coated (Galvanized) or Zinc-Iron

Al loy-Coated (Galvannealed) by the Hot-Dip Process

• Covers galvanized and galvannealed steel sheet in coils and cut lengths.• The most commonly used type of coated-steel sheet in manufacturing and construction.

A 1063/A 1063M - Standard Speci ficat ion for Steel Sheet, Twin-Roll Cast, Zinc-Coated (Galvanized)

by the Hot-Dip Process

• Covers steel sheet produced by the Twin-Roll Cast process and galvanized in coils and cut lengths.• Contains only commercial, structural, and high-strength low-alloy grades.

A 792/A 792M - Standard Specif ication for Steel Sheet, 55% Aluminum-Zinc Alloy-Coated by the

Hot-Dip Process

• Covers 55% aluminum-zinc alloy-coated steel sheet in coils and cut lengths.

• Intended for applications requiring high corrosion resistance and/or heat resistance.



GalvInfoNote 1.2Rev 1.0 Aug-09


2

A 875/A 875M - Standard Specif ication for Steel Sheet, Zinc-5% Aluminum Alloy-Coated by the

Hot-Dip Process

• Covers steel sheet, in coils and cut lengths, metallic-coated by the hot-dip process, with a zinc-5 %

aluminum alloy coating.

• Coating is produced as two types: zinc-5% aluminum-mischmetal alloy or zinc-5% aluminum-

magnesium alloy.• Intended for applications requiring corrosion resistance, formability, and paintability.

A 1046/A 1046M - Standard Speci ficat ion for Steel Sheet, Zinc-Aluminum-Magnesium Alloy-Coated


• Covers steel sheet, in coils and cut lengths, metallic-coated by the hot-dip process, with a zinc-5-

13% aluminum, 2-4% magnesium alloy coating.

• Intended for applications requiring superior corrosion resistance and paintability.

A 463/A 463M - Standard Speci ficat ion for Steel Sheet, Aluminum-Coated by the Hot-Dip Process

• Covers aluminum coated steel sheet in coils and cut lengths with two types of aluminum coating.

• Type 1 coating is an aluminum-silicon alloy intended for heat resisting applications and for uses

where corrosion and heat are involved.

• Type 2 coating is commercially pure aluminum intended for applications requiring corrosionresistance.

A 308/A 308M - Standard Specif ication for Steel Sheet, Terne (Lead–Tin Al loy) Coated by the Hot-

Dip Process

• Covers steel sheet, in coils and cut lengths, metallic-coated by the hot-dip process, with a lead-3-

15% tin alloy coating.

• Primary end use is automotive fuel tanks.

A 924/A 924M - Standard Specif ication for General Requirements for Steel Sheet, Metall ic-Coated


• Covers the general requirements that apply to all hot-dip coated steel sheet in coils and cut lengths.

•

Contains the common requirements for all types of hot-dip metallic-coated steel sheet, such asdimensional tolerances for thickness, width, flatness, etc.

Hot-Dip Galvanized Steel Sheet (A 653/A 653M and A 1063/A 1063M)

• A galvanize coating is essentially a zinc coating on steel sheet. The word “galvanize” comes from the

galvanic protection that zinc provides to steel when exposed to a corroding environment.

• It is, by far, the most common hot-dip coated product with a wide range of applications.

• Zinc provides both galvanic and barrier protection. The galvanic protection is greater than for any othertype of hot-dip coating on steel.

• The coating contains aluminum – typically between 0.20 and 0.30% - to control the growth rate of thealloy layer (bond zone between the steel and zinc coating). It is added to dramatically improveadhesion during forming.

•

Coating may contain a small amount of lead and/or antimony for spangle development. Almost allgalvanized product contains “no lead”, and if it does, the lead is less than 0.03%.

• Coating weight (mass) range available: 0.30 – 4.00 oz/ft2 (90 – 1200 g/m

2) for A 653/A 653M, and 0.30

– 1.85 oz/ft2

(90 – 600 g/m2) for A 1063/A 1063M total both sides.

• Coating designations: “G” (Inch-Pound), “Z” (SI).





3

Hot-Dip Galvannealed Steel Sheet (A 653/A 653M)

• A hot-dip galvanize coating that is diffusion-alloyed with the steel by additional heating in the towerabove the coating bath.

• Typical coating contains 8 to 11% iron.

• Intended to be painted for most applications.

• Characterized by its high hardness and brittle behavior during forming.

• Easier to spot weld and paint than galvanized product.

• Performance under paint is synergistically improved because of the excellent bond formed between thepaint and the surface of the coating. Compared with a galvanized product, galvanneal generally exhibitsless undercutting corrosion beneath paint at exposed edges, scratches, or other defects in the paint.

• Used by a number of auto companies for body panels. (Galvanneal used for automotive end uses isordered to auto company specifications).


2) total both sides.

• Coating designations: “ A” (Inch-Pound), “ZF” (SI).

Hot-Dip 55% Aluminum-Zinc Alloy Coated Steel Sheet (A 792/A 792M)

• An aluminum/zinc alloy coating that contains approximately:

• 55% aluminum,

• 43.5% zinc

• 1.5% silicon.

• Offers excellent barrier-coating protection combined with some galvanic protection.

• Retention of galvanic protection is an important feature.

• This particular combination of aluminum and zinc effect the formation of a coating microstructure that isvery important for good performance. Provides a very good balance between galvanic and barrierprotection.

• Silicon is added to control the alloy-layer growth rate. Improves adhesion during forming.

• Much higher resistance to corrosion than galvanize coatings in most environments. Long term durability

has been demonstrated.



• Coating designations: “ AZ” (Inch-Pound), “ AZM” (SI).

Hot-Dip Zinc-5% Aluminum Alloy Coated Steel Sheet (A 875/A 875M)

• A galvanic coating that contains approximately 95% zinc and 5% aluminum.

• Provides approximately the same galvanic protection as galvanized and improved corrosion resistancein most environments.

• Primary attribute is the improved ductility vs. a galvanized coating

• Used mostly for applications that require good coating ductility – deep drawn parts and prepaintedsheets.

• Coating weight (mass) range available: 0.30 – 2.35 oz/ft2

(90 – 715 g/m2

) total both sides.• Coating designations: “GF” (Inch-Pound), “ZGF” (SI).





4

Hot-Dip Zinc-Aluminum-Magnesium Alloy Coated Steel Sheet (A 1046/A1046M)

• A galvanic coating that contains zinc, 5 to 11% aluminum, and 2 to 4% magnesium.

• Provides superior corrosion resistance in many aggressive environments.

•

Used in such applications as transportation infrastructure construction, agricultural, electric power, andautomotive.



• Coating designations: “ZM” (Inch-Pound), “ZMM” (SI).

Hot-Dip Aluminized Steel Sheet (A 463/A 463M)

• Two types of aluminized coatings -

• Type 1 – Aluminum and 5 to 11% silicon

• Type 2 – Pure aluminum coating

• Most common form is Type 1 coating; used for applications that require heat-oxidation resistance suchas furnace parts, small appliances, exhaust systems, etc.

•

Best coating on steel sheet for heat-oxidation resistance• Can be applied over stainless steel to offer even better high temperature performance.

• Pure Al Type 2 coating is used for exterior applications.

• Corrosion performance is based on barrier protection; no galvanic protection in most environments.

• Barrier corrosion protection is very good.

– Forms a stable aluminum oxide film on the surface of the coating.

• Coating weight (mass) range available: Type 1 0.25 – 1.00 oz/ft2 (75 – 300 g/m


Type 2 0.65 – 1.00 oz/ft2 (200 – 300 g/m


• Coating designations: Type 1 “T1” (Inch-Pound), “T1” (SI).

Type 2 “T2” (Inch-Pound), “T2” (SI).

Terne-Coated Steel Sheet (A 308/A 308M)• A lead-alloy coating that contains 3 to 15% tin.

• Tin is added to develop a bond between the coating and steel.

• The coating is very formable. Improves the deep drawing behavior. Also, the product is easily welded.

• Very good resistance to gasoline, although use for fuel tanks is decreasing (related to theenvironmental issue associated with lead; not product performance).



• Coating designations: “LT” (Inch-Pound), “LTZ” (SI).

Copyright 2009 – IZA

Disclaimer:

Articles, research reports, and technical data are provided for information purposes only. Although the publishers endeavor toprovide accurate, timely information, the International Zinc Association does not warrant the research results or information reportedin this communication and disclaims all liability for damages arising from reliance on the research results or other informationcontained in this communication, including, but not limited to, incidental or consequential damages.




1


GalvInfoNote

1.3

Galvanneal – Differences from Galvanize

Rev 1.0 Jan-09

Introduction

This GalvInfoNote explains how hot-dip galvanneal differs from hot-dip galvanize. The hot-dip galvanizing

process is explained in GalvInfoNote 2.1. In manufacturing galvanneal, the main differences in the

process are that a lower level of aluminum (typically 0.11% to 0.14%) is used in the zinc bath, and the

moving, zinc-coated strip is reheated immediately after it passes the air-wiping step above the zinc bath.By heating the strip to between 935 - 1050°F (500 - 565°C), and holding at this temperature for a few

seconds, the zinc coating, by way of diffusion, alloys with the iron in the steel. The end result is that the

coating is converted to layers of zinc-iron intermetallic compounds with a bulk composition of

approximately 90% zinc and 10% iron. These are averages, as the iron percentage varies throughout the

coating thickness, from as low as ~6% at the surface, to as high as ~23% at the steel interface.

Galvanneal coatings have no free-zinc present and have a low-lustre matte appearance, versus the

metallic sheen of galvanized coatings. The final bulk iron concentration depends primarily on the heating

cycle, since the rate of diffusion is a function of time and temperature. The zinc and steel chemistries canalso affect the alloying behavior, but they are secondary to the heating cycle.

The differences in characteristics and performance between galvanneal coatings versus galvanize

coatings are explained below.

Coating Composition

A galvanize coating is essentially pure zinc, with between 0.20 and 0.50% bulk aluminum, although it ishighly concentrated in the thin inhibition layer next to the steel. The aluminum is present, not to affect thecorrosion performance, but to provide adhesion between the coating and the steel substrate when thesheet is eventually formed. See GalvInfoNote 2.4 for an explanation of the role of aluminum in continuoushot-dip galvanizing.

Galvanneal coatings contain 9 -12% bulk iron, along with a small amount of aluminum, which is spreadmore uniformly through the coating thickness than in the case of galvanize. The iron in galvannealcoatings is definitely not uniformly distributed, however, as it is combined with zinc in 3 distinct zinc-ironphases. These phases are shown in Table 1, along with their iron and aluminum contents.

Table 1 Galvanneal Phases and Compositions

Alloy Layer % Fe %Al

Zeta (!) FeZn13 5.2 – 6.1 0.7

Delta (") FeZn10 7.0 – 11.5 3.7

Gamma (#) Fe3Zn10 15.8 – 27.7 1.4

The phases in Table 1 are shown in the order that they occur in the coating, with the high-iron gammalayer next to the steel substrate. It is also important for good appearance and press formability that thetop zeta layer contain no less than about 5% iron or there will be a risk of free zinc on the surface. The



GalvInfoNote 1.3Rev 1.0 Jan-09


2

higher aluminum content of galvanneal coatings is a result of its diffusion outward from the inhibition layernext to the steel.

Figure 1 is a micrograph of a cross section of a galvanneal coating showing the three alloy phase layers.

Figure 1 Cross-section of galvanneal coating showing the gamma (#), delta ( ) and zeta ( ) alloy layers

Galvanneal coatings are hard and brittle. During bending and press forming, coating cracks andpowdering are always present to some degree. Each coating line must develop practices to produce theoptimum coating properties for the particular end use, in order to balance performance in forming versus

coating line throughput.

Production of Galvanneal

The moving strip, with the still liquid zinc coating, enters the galvanneal furnace about 10-15 feet abovethe gas wiping knives. Before the zinc can solidify, reheating of the strip begins. As the strip temperaturerises, the zinc-iron diffusion reaction restarts and breaks down the aluminum-zinc-iron inhibition layer thatformed in the zinc pot at the steel zinc interface. After 5-7 seconds of heating and up to about 10seconds of soaking, enough iron diffuses into the coating to convert it to a dull matte gray appearance.

Coating lines built since the early 1990s use induction galvanneal furnaces. Typically they have 3 or

more zones, which can reheat the strip from about 865°F (463°C) to 1050°F (565°C) in the few seconds

available. Following the heating furnace zones is an electrically heated holding zone that is used tooptimize the iron content of the coating. Older galvanneal coating lines use gas-fired furnaces, with which

it can be more difficult to obtain a well-controlled alloying reaction. Induction galvannealing is inherentlydifferent than the convection/radiation version since, with the former, the heat for diffusion comes fromwithin the strip, not externally as with the latter process.

The reactions that convert a liquid zinc coating to a solid zinc-iron coating begin at the steel interface andare dependent on a number of factors. In approximate order of importance they are: heating time andtemperature, percent aluminum in the coating, coating bath temperature, steel grade, coating weight, andline speed. These variables are not all necessarily independent and each coating line has to determinethe necessary protocol to produce a product for a particular end use. For instance, a higher Al in the

Layer $

Layer $$

# Layer $

1 µm





3

coating requires a higher reheating temperature and/or a longer soaking time. Too high a temperatureand too low an Al will result in high iron and excess powdering. Stabilized IF grades react faster thanplain carbon steels. Steels with higher phosphorous levels react slower in the galvannealing furnace thatlow P steels.

Figure 2 is a schematic representation of the galvannealing process.

Figure 2 The Galvannealing Process

Induction or gas heating furnaceto raise strip temperature to935-1050°F (500-565°C)

Holding furnace tohold strip at or below1050°F (565°C) for upto 10 seconds

Molten zinc bath

temperature865 F (463 C)

Steel sheet

from furnace Air knives to wipe offexcess zinc coating





4

The moving sheet is immersed in the zinc bath where a thin inhibition alloy layer forms at the zinc-steelinterface. As the strip emerges from the bath it drags excess zinc with it, which the air knives remove toobtain the desired coating weight. The still molten zinc coating is converted to zinc-iron alloy layers in theheating and holding furnaces.

Figure 3 depicts the stages the coating goes through during its conversion from zinc to zinc-iron.

Figure 3 The Stages of A lloying Between Steel Sheet and

Molten Zinc Coating to Produce “ Galvanneal”

Unalloyed zinc

Coating is now 100%alloyed with iron

At this stage, the coating

contains about 10% iron(average composition)





5

Coating Weldability, Paintability, Formability and Adherence

Two primary benefits of using galvanneal rather than galvanize are:

• Improved spot-weldability

• Ease of painting and improved coating adhesion

Zinc-iron alloy coatings generally have better spot welding characteristics than pure zinc coatings1. The

coating’s higher electrical resistance, along with its higher hardness and higher melting point, allow goodwelds to be obtained at lower currents with longer electrode life, to the extent that galvannealed sheetspot-welds very much like cold rolled sheet.

Performance of galvanneal under paint is synergistically improved because of the excellent bond formedbetween the paint and the surface of the coating. The reason for the good bond is evident in Figure 4 –the paint can “mechanically lock” with the zinc-iron crystals on the surface. This is why galvannealcoatings can be painted directly without the need for a primer. Compared with a galvanized product,galvanneal generally exhibits less undercutting corrosion beneath paint at exposed edges, scratches, orother defects in the paint. To achieve maximum corrosion protection, galvanneal is usually treated withzinc phosphate before painting.

Figure 4 Surface of Galvanneal – Showing Zinc-Iron A lloy Crystals (magnif ication ~ 2700X)

A galvanize coating is quite soft, and easily scratched. A galvanneal coating is very hard, and thus not aseasily scratched when handling. The harder zinc-iron alloy powders on deformation, unlike pure zinccoatings, which can gall and flake

1.

The good frictional behaviour and ductility of zinc, combined with the excellent adhesion achievedbetween the coating and the steel, allows galvanized sheet to be formed into many intricate shapeswithout any loss in coating adhesion. In fact, because the coating is soft, care needs to be exercised to

prevent flaking resulting from galling.

Even though the galvanneal alloying reaction results in a hard, relatively brittle coating, it can be bent,stretched and drawn when correct sheet manufacturing and part forming procedures are used. Manyparts made from galvannealed sheet require a deep drawing operation. When deep drawn, galvannealedsheet typically exhibits some “powdering” of the coating as a result of high compressive strain that canoccur during the forming operation. By proper control of the steel manufacturers’ processing practices,combined with the proper setup of the drawing dies, and the use of appropriate drawing lubricants, theamount of powdering can be minimized and excellent performance can be achieved.





6

The powdering of a galvannealed coating during forming is a function of many parameters, mostly relatingto the steel manufacturing practices covered previously. The most important characteristic of the coatingthat affects the powdering tendency is the coating thickness. The amount of powdering rises directly asthe thickness increases. For this reason, the maximum coating weight for galvanneal is restricted to A60

(0.60 oz/ft2) [ZF180 (180 g/m

2)]. For many applications, an A60 coating weight is too thick to provide

acceptable powdering performance, and many users specify A40 (0.40 oz/ft2) [ZF120 (120 g/m

2)], or less.

In fact, most automobile applications of galvanneal use the equivalent of about an A30 [ZF90] coating.The tendency for A60 to powder should be considered when selecting a coating weight.

Generally, there are no significant differences in the properties of the steel substrate whether it isgalvanize or galvanneal. Any differences in forming behaviour (splits, etc.) are usually related to thedifferent nature of the two metallic coatings. For example, the substantial difference in coating hardnesscan necessitate changes to the stamping parameters, i.e., die type, die clearances, hold-down forces,lubrication type, etc.

Corrosion Performance

The thickness of a galvanize coating has a direct influence on the corrosion performance and life of theproduct, i.e., the thicker the coating, the longer its life. See GalvInfoNote 3.1 – “How Zinc Protects Steel”for more information.

The corrosion performance of a galvannealed coating is more complicated than its galvanizedcounterpart. Almost all applications of galvannealed sheet involve painting after fabricating. The primaryreason is that, when unpainted, the presence of 10% iron in the coating can lead to a “reddish-colored”corrosion product. The color is related to corrosion of the iron within the coating and does not necessarilysignify that corrosion of the steel substrate is occurring. This discoloration due to the iron in the coating ispurely a cosmetic effect.

Nevertheless many users find this staining unacceptable, thereby requiring most applications forgalvanneal be painted after fabrication. For this reason, most corrosion studies of galvanneal relate to itbeing painted. Since paint systems have a direct influence on product life, the corrosion performance ofgalvanneal is generally not compared with bare (unpainted) galvanize. The importance of the galvannealcoating thickness is often revealed at sheared edges or scratches, etc, i.e., places where the steel andmetallic coating are directly exposed to the corroding environment. At such discontinuities, a thickercoating can improve resistance to “undercutting” the paint film, i.e., a thicker galvanneal coating can slowdown degradation of the paint, as evidenced by edge “creep back” corrosion and eventual total loss ofpaint adhesion. To maximize service life, it is therefore advisable to use as thick a galvanneal coating aspotential powdering problems will allow.

Considering the relative corrosion rates:

• A pure zinc coating (galvanize) provides a high degree of galvanic protection to exposed steel suchas at sheared edges and scratches.

• A galvannealed coating is about 10% less galvanically active in most environments because itcontains 10% iron. Its bare corrosion rate may, in fact, be less than pure zinc but is masked by the“reddish-colored” corrosion products that form on the surface from the iron in the coating.

• The more galvanically active galvanize coatings could be quickly consumed when acting as agalvanic protector to any exposed steel. The less galvanically active galvanneal coatings do notoffer as much galvanic protection, and therefore are not as rapidly consumed during the corrosionprocess. It is interesting to note that over the last 2 decades, automotive galvanneal coatingsequivalent to about A30, have performed just as well as approximate G50 galvanize coatings withrespect to the corrosion resistance of painted outer auto body panels.





7

• The specific needs of the application and the corrosion performance requirements dictate which

coating will perform best. Other requirements for the application, such as weldability and the

specific capabilities of each product manufacturers’ paint shop need to be considered when

deciding which product is best for a given situation.

Which Product to UseWhen considering whether to use galvanize or galvanneal for a specific application, find the answer tosuch questions as:

• What are the corrosion demands of the end use and the environment? Coating thickness is theprimary consideration.

• Is spot welding involved? Galvanneal might perform better.

• Is the product going to be used unpainted? In most cases, galvanize is preferred.

• Is deep drawing involved? Before applying galvanneal, stamping trials may need to be conducted inorder to assure that the amount of powdering is acceptable.

In almost all applications, there is more than one issue involved. The correct product recommendation bythe producer requires in-depth consideration of all processing steps included in manufacturing, plusknowledge of end-use requirements.

Reference:

1) J. P. Landriault, F.W. Harrison: CIM Bulletin, August 1987, pp. 71-78

Copyright% 2009 – IZA

Disclaimer:






1


GalvInfoNote

1.4

55% Aluminum-Zinc Alloy-Coated Steel Sheet

Rev 1.0 Feb 2010 Introduction

The most widely used metallic coating for the corrosion protection of steel is zinc (galvanize). It offers avery good combination of galvanic and barrier protection. Its excellent performance for many applicationsis well documented. However, in the desire to improve, there is always a quest to find even betterproducts. Researchers continually attempt to develop superior steel coatings that can be commerciallyapplied. Often, the target is to find better products for specific end uses or environments, e.g. ones havingsuperior corrosion resistance, or better coating formability. These attempts meet with little success mostof the time, due either to an undesirable product attribute, or because manufacturing is too expensive ordifficult, but every so often a breakthrough coating is discovered.

One hot-dip product that was successfully developed is 55% aluminum-zinc alloy-coated steel sheet. This

product is known by many different trade names throughout the world. Galvalume is the most widely

used name. It is a registered trademark of BIEC International Inc., and is used by many of its licensedproducers, including ArcelorMittal Dofasco Inc., and Severstal Sparrows Point. Steelcscape Inc. uses the

trademark ZINCALUME® for this product, while Ternium brands it as Cincalum-Galval . Following its

introduction in 1972, Galvalume! was well received in the marketplace, particularly for metal building

roofing. The cumulative worldwide production now exceeds 70 million tons1, with an annual production

level currently approaching 7 million tons. The coating is comprised of 55% aluminum, 43.5% zinc, and1.5% silicon. Steel sheet with this coating has proven to be an excellent product for long-life buildingcladding, especially low slope roofing on industrial buildings. It has been widely applied as bare(unpainted) sheet with the coating being directly exposed to the atmosphere. The product is also used asa substrate for prepainted sheet and this use has also grown significantly. The ASTM productspecification for 55% Al-Zn coated sheet is A 792/A 792M, and the prepainted sheet version is specifiedin A 755/A 755M.

Galvalume! alloy-coated sheet has performed extremely well for over 35 years, particularly in the case of

unpainted, low-slope roofing.

In this GalvInfoNote, the basis for the excellent corrosion performance of 55% aluminum-zinc alloy-coatedsteel sheet is explained. More information can be obtained at www.galvalume.com/

Manufacture

Keep in mind that the exact composition of the 55% Al-Zn alloy is more precisely 55% aluminum, 43.5%zinc and 1.5% silicon. Although the corrosion performance is primarily related to the composition andmicrostructure of the aluminum-zinc alloy coating, the addition of approximately 1.5% silicon is vital. Thepurpose of the silicon is to control the growth of a brittle intermetallic layer that would otherwise formduring manufacture of the product. (Refer to GalvInfoNote 2.4 for a discussion of a similar, but thinner,intermetallic layer that forms when steel is zinc-coated using the hot-dip process).

As with hot-dip galvanized sheet, controlling the interaction between the steel sheet and the molten

coating during the manufacturing process is vital to achieve good adhesion of the coating during eventualforming operations by the customer. Even so, the intermetallic alloy layer is hard and brittle and it istherefore important for this layer to be kept as thin as possible. This is the role of silicon in a 55% Al-Znbath. It dramatically restricts growth of the alloy layer, allowing the product to be readily formed aftermanufacture. The silicon is not added to enhance the corrosion performance.

In some applications, especially those that involve deep drawing, the coating adhesion of the as-produced product is not as good as that of a galvanized coating. The inhibition of the alloy layer growth is



GalvInfoNote 1.4Rev 1.0 Feb 2010


2

not as effective with the addition of silicon to a 55% Al-Zn bath as it is when aluminum is used in agalvanizing bath. For this reason, and also for reduced galling behavior, galvanized sheet is often thepreferred product when deep drawing; bi-axial deformation is involved. However, recent advancementsin coating technology, in particular the use of acrylic coatings, permit 55% Al-Zn to be used for somedeep drawing applications.

Galvalume! sheet products are coated on processing lines that are almost identical to those used toproduce galvanized sheet, and which are described in GalvInfoNote 2.1. In many cases, production linesthat produce 55% Al-Zn coatings are dedicated to this product, although there are lines that can producetwo coatings by using dual interchangeable coating pots.

Coating Microstructure

The microstructure of the 55% Al-Zn coating is shown in Figure 1. The coating has two principal phasesin its microstructure. One phase is the primary aluminum-rich dendritic phase that begins to grow initiallyduring solidification. The other is an interdendritic zinc-rich region that forms when the zinc concentrationin the solidifying liquid reaches a high level. The origin of these phases is explained by reference to thealuminum-zinc phase diagram, and is beyond the scope of this GalvInfoNote. This microstructure;aluminum-rich dendrites plus a network of zinc-rich interdendritic areas, is essential to obtain the desired

corrosion resistance. Other phases in the microstructure of the coating include small discrete needles ofelemental silicon, and the intermetallic layer at the steel-coating interface.

The coating relies on an extensive labyrinth of zinc-rich regions throughout the microstructure to optimizethe anti-corrosion performance. This labyrinth forms during the post dip cooling section and is monitoredduring the manufacturing process.

Zinc-rich

interdendritic

area

Figure 1: Microstructure of 55% Al-Zn Coating (note the interdendritic zinc-rich areas)

Silicon

Needle

Al-rich

region





3

Corrosion Resistance

Table 1 contains corrosion performance data comparing the performance of 55% Al-Zn with galvanizedcoatings. The data

2 indicate that the performance is superior versus galvanize coatings in all three types

of environments – marine, industrial and rural.

Table 1: Ratios of Average Corrosion Rates of 55% Al-Zn and Galvanize Coatings3

LocationRatio of Average Corrosion Rates

55% Al-Zn: Galvanized*

Kure Beach, NC 25-meters (Severe Marine) 3.8

Kure Beach, NC 250-meters (Moderate Marine) 8.2

Bethlehem, PA (Industrial) 6.4

Saylorsburg, PA (Rural) 18.7

*Ratio gives the relative improvement of the 55% Al-Zn coating versus galvanize for coatings of approximately the

same thickness (G90 [Z275] galvanize and AZ50 [AZM150] Galvalume!). Note: since the density of the 55% Al-Zn

coating is much lower than for the galvanize coating, coatings with the same thickness (20 µm) are much lighter for

55% Al-Zn (150 g/m2

) than for galvanize (275 g/m2

).

The unique dendritic structure of the alloy coating is now widely recognized as the primary reason for theimproved corrosion resistance of the 55% Al-Zn coating. When it is exposed to the environment, the zinc-rich areas corrode first. Since these areas are located in a labyrinth of interdendritic regions in thecoating, the products of corrosion tend to fill the interdendritic interstices and the corrosion ratedecreases

4. This leads to a parabolic corrosion rate in most environments. This contrasts with the linear

behavior typical of galvanize.

Corrosion of a 55% Al-Zn coating is therefore not the uniform thinning process of a galvanized coating.During the early stages of the product life, the aluminum-rich dendrites are largely unaffected by mostenvironments. In a sense, the aluminum-rich dendrites perform like a barrier coating, while the zinc-richareas provide the galvanic protection that is needed to minimize the tendency for rust staining at shearededges and other areas of exposed steel.

The most common coating designations for 55% Al-Zn product to be used in outdoor environments are AZ50 [AZM150], AZ55 [AZM165], and AZ60 [AZM180], as described in ASTM Specification A 792/A792M. The AZ50 [AZM150] coating is approximately as thick as a G90 [Z275] galvanize coating. Theproduct is also used for corrugated steel pipe with a coating of AZ70 [AZM210] as described in A 929/A929M. Furthermore, it is specified for use as cold formed steel framing in A 1003/A 1003M with a coatingdesignation of AZ50 [AZM150].

Refer to GalvInfoNote 1.1, pages 6 & 7, for a full explanation of these coating designations and how theyare related to coating thickness.





4

There are two notable exceptions to the improved corrosion performance of 55% Al-Zn coated sheetversus galvanized sheet.

1. Perhaps, the most important exception is the performance in animal confinement buildings for swine,cattle, and poultry. Buildings that house these intensive farming activities present problems for 55% Al-Zn alloy-coated sheet. They can result in the creation of waste decomposition by-products, which

can be extremely aggressive towards this coating, creating significant corrosion problems5. Someproducers

5, 6 neither recommend nor warrant the use of

55% Al-Zn alloy-coated sheet for animal

confinement buildings.

2. Unpainted 55% Al-Zn alloy-coated sheet should not come in direct contact with wet concrete.Concrete’s high alkalinity attacks the aluminum, causing the coating to become porous and prone tocorrosion

6, 7. The product can be used over hardened and cured concrete.

High Temperature Resistance

Due to the high aluminum content of the 55% Al-Zn coating the sheet can withstand surface temperaturesof up to 750°F [400°C] without discoloration, and up to 1200°F [650°C] without heavy oxidation andscaling.

Summary

The composition of the aluminum-zinc alloy used in 55% Al-Zn coatings has been proven to outlastgalvanized steel by two to four times, depending on the environment. Building panels fabricated from55% Al-Zn alloy-coated steel sheet will provide many years of trouble-free service when properlydesigned, installed and maintained.

References:1. ZAC Insider, May 2005, Issue Twenty-Four

2. A. Humayun, The Basics of 55% Al-Zn Coated Sheet’s Legendary Performance, National Conference on CoilCoating and Continuous Sheet Galvanizing, New Delhi, India, Sept. 10-11, 1997.

3. H. E. Townsend and A. R. Borzillo, 55% Al-Zn Alloy Coated Sheet Steel: The Versatile, Long Lasting BuildingPanel Steel, 5

th International Conference on Zinc Coated Sheet Steel, Birmingham, England 1997.

4. H.J. Cleary, The Microstructure and Corrosion Resistance of 55% Al-Zn Coatings on Steel Sheet, 16th Annual

Technical Meeting of the International Metallographic Society, July, 1983. 5. Technical Bulletin #8, Steelscape, Inc., 1999 www.steelscape.com/products/

6. Technical Bulletin TP 2005.2, U.S. Steel, www.uss.com/corp/construction/bulletins/

7. Spec Data Sheet, 1995, NamZAC

Copyright" 2010 – IZA

Disclaimer: Articles, research reports, and technical data are provided for information purposes only. Although the publishers endeavor to





1


GalvInfoNote ASTM Standards for Coated Sheet Products

Rev 1.0 Aug-091.5

Introduction

The most common way to order coated steel sheet products from the steel manufacturers is to refer to

ASTM Standards. These standards, which are written by voluntary standards writing committees within

the ASTM International organization, cover all of the details related to ordering coated steel sheet

products. These details include:

Coating type

Coating weight [mass]

Surface finish

Steel strength

Steel ductility/formability

Steel dimensions

o Thickness

o Width

o Flatness

Tolerances – steel dimensions, coating thickness, etc.

Coating Steel Product Standards

For coated-steel sheet products, the standards are written by ASTM Committee A05, a body comprised ofknowledgeable volunteers who oversee the development and revision of the specific standards related to

coated steel products.

The common terms that are included in many customer requirements such as G60, A40, CommercialSteel (CS), Deep Drawing Steel (DDS), etc. all have their origin within the appropriate ASTM Standard. Ifyou need to learn the meaning of many of the terms used for coated steel sheet products, the best thingto do is obtain a copy of the appropriate ASTM Standard. These can be purchased from ASTMInternational.

Ways to contact ASTM International:

Phone: 610-832-9500

E-Mail: [email protected]

Website: www.astm.org



GalvInfoNote 1.5

Rev 1.0 Aug-09


2

ASTM Volume 01.06 contains all the standards related to coated steel products. It can be purchased as

an entire volume, or individual standards can be purchased separately. ASTM offers both printed and

electronic versions of all standards.

The table on page 3 of this GalvInfoNote contains a list of coated steel sheet standards. This list coversboth hot-dip coated products and electroplated products. Also, see GalvInfoNote 1.2 for an in-depth

description of the ASTM hot-dip coated sheet standards.

In addition to these “product” standards, there are two “general requirement” standards that cover issues

such as:

Ordering information

Tests for coating properties

Tests for mechanical properties

Dimensions and permissible variations

The two documents that cover the general requirement issues are:

1. ASTM Standard A 924/A 924M for hot-dip coated steel sheet products.

2. ASTM Standard A 917 for the electroplated coated steel sheet products.

When using ASTM standards for ordering purposes, be aware of the requirements in both the specific“product“standard and the appropriate “general requirements” standard.

Inch-Pound versus SI

When an ASTM standard number is written as, for example, A 653/A 653M, it means it is a “Dual”standard and allows the product to be ordered in either “Inch-Pound” (English) units, or “SI” (Metric) units.The two sets of units are to be regarded separately as standard units. In the text the SI units are shownin brackets. Keep in mind that the values stated in each system are not exact equivalents and eachsystem must be used independently of the other. In this example, when ordering galvanize to inch-poundunits, specify A 653. When ordering to SI units, specify A 653M.

Almost all ASTM standards for coated sheet products are dual specifications. The exceptions are someof the electrogalvanize products, viz., A 917 and A 918. These standards, while not having an “M” afterthe number, specify only SI units.



GalvInfoNote 1.5

Rev 1.0 Aug-09


3

ASTM Standards for Coated Sheet Products

Copyright 2009 – IZA

Disclaimer:


Coated Sheet

Common Name ASTM Standard Comments

Hot-dip galvanize A 653/A 653M

A 1063/A 1063MZinc-coated sheet

Hot-dip galvanneal A 653/A 653M Zinc-iron alloy-coated sheet

Electrogalvanize A 879/A 879M Electroplated zinc-coated sheet

55% Al-Zn Alloy A 792/A 792M 55% aluminum/45% zinc alloy-coated sheet

Zn-5% Al Alloy A 875/A 792M 95% zinc/5% aluminum alloy-coated sheet

Zn-Al-Mg Alloy A 1046/A 1046M Zinc-5-11% aluminum-2-4% magnesium alloy-coated sheet

Aluminized A 463/A 463M 2 types of aluminum coatings• aluminum/5% -11% silicon alloy-coated sheet

• pure aluminum-coated sheet

Terne A 308/A 308M Lead/tin alloy-coated sheet

Electroplated Zinc/Nickel A 918 Zinc/9-16% nickel alloy electroplated coatedsheet




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GalvInfoNote

1.6

Selecting Coating Thickness (Weight or Mass)for Galvanized Steel Sheet Products

Rev 1.1 Jan-09 Introduction

The proper selection of coating thickness to meet a galvanized steel sheet user’s needs requiresknowledge of the corrosiveness of the environment in which the product will be used. The thickness of thezinc coating largely determines its ultimate life, but it is not used directly to specify the amount of coating.GalvInfoNote 1.1 explains why galvanized sheet coatings are specified, not as thickness, but as coatingweight (inch-pound system) or coating mass (SI system).

Effect of Atmospheric Conditions

The corrosion rate of a zinc coating varies widely depending upon many environmental factors. Forexample, “time of wetness” is an important issue that affects corrosion rate, i.e., outdoor applications inthe dry Southwest United States are very different from locations that experience high annual rainfall orextended foggy periods. Also, the presence of impurities such as sulfates, chlorides, and nitrates can

dramatically affect the rate of corrosion. Other variables, including the amount of oxygen present in theelectrolyte, and the temperature of the environment are important determinants for predicting product life.

In 2003, the American Galvanizers Association (AGA) published an updated service life chart for batch-

galvanized articles that have zinc coatings up to 6+ mils thick. The chart also applies to continuous

galvanized sheet, although the life expectancy of their thinner coatings is hard to read on the AGA graph

(www.galvanizeit.org). The original AGA data was obtained by the GalvInfo Center, which allowed Figure

1 below to be generated. Essentially it is a magnified version of the lower left hand corner of the AGA

chart.

The life expectancy lines shown by this chart reflect outdoor corrosion rates produced using the Zinc

Coating Life Predicto r (ZCLP). This software was developed by Gregory Zhang of Teck Cominco, and

can be found at www.galvinfo.com in the GalvInfo Library – Additional Information section. It is

applicable to all zinc-coated steel; that is, coatings composed of zinc exclusively. It does not apply tozinc/aluminum or aluminum/zinc coatings. It performs calculations based on statistical models, neural

network technology and an extensive worldwide corrosion database. The environmental input data

obtained by the AGA was from the World Wide Web. The calculated corrosion rates used to generate the

service life chart in Figure 1 are an average for six different North American cities in each of the five

climate categories.

Six common ASTM A 653 coating weight “bars” have been overlaid on the chart. For each bar, the leftedge is an assumed one-half ! of the minimum allowed triple spot test coating thickness, and the right

edge is one-half the maximum TST thickness that would typically be produced. The middle of a bar width

is therefore a good estimate of the service life of the coating designation in a given environment, e.g.,

G90 will last 20 years before 5% red rust in an average suburban locale. To determine the corrosion rate

for a specific locale, the documented actual environmental data for the ZCLP can be looked up and input

onto the software.

! ASTM TST coating weight specifications allow one side to be as low as 40% of the total specified minimum. Figure 1 uses a

50/50 split as the minimum, since this is by far the most typical coating distribution produced on modern coating lines.





2

Product Life Considerations

The performance requirements, i.e., the desired product life, will be a factor in determining the requiredcoating weight [mass] needed for a given application. For example, consider an application such as ametal building roof where the desire is for no red rust being visible for many years. In this case, the timeto failure might be defined as the time for the onset of red rust (the time for the zinc coating to beconsumed in a large enough area for rusting of the steel to be observed). This application requires a thickzinc coating. Another example is an application in which the time to failure is defined as the time whenperforation of the steel sheet is observed. In this case, failure is affected by the thickness of the steelsheet (and the corrosion rate of the steel) as well as the thickness of the zinc coating.

Once the desired product life is determined, it is important to match the desired life with corrosion rateinformation for any specific application. By combining the rate of corrosion (zinc coating thicknessloss/year) for a specific application with the desired life in years, one can then readily determine the zinccoating weight [mass] to specify.

Designation System for Ordering a Specific Coating Weight [Mass]

For galvanized steel sheet products, the coating weight [mass], and hence the thickness, is defined by

the designator system in ASTM Specification A 653/A 653M. The inch-pound coating weight designators(as A 653) range from designations G30 (0.30 oz/ft

2 of sheet) to G235 (2.35 oz/ft

2 of sheet), with many

intermediate coating weights between these two. The equivalent SI coating mass designators (as A653M) are Z90 (90 g/m

2 of sheet) to Z700 (700 g/m

2 of sheet). This is almost an eight-fold difference in

weight [mass] of zinc. These coating designations are total-both-sides, meaning that the coating weight[mass] on one side of the sheet is nominally one-half of the indicated value. Refer to GalvInfoNote 1.1 formore details on coating designations.





3

For many outdoor applications of bare (unpainted) galvanized sheet, the most common coating weight

[mass] in use today is G90 [Z275]. This product is also specified for indoor applications where there is the

potential for considerable amount of dampness due to condensation, etc. For other indoor applications

where the environment is relatively dry, a G40 [Z120] or G60 [Z180] coating weight [mass] are usually

sufficient. Outdoor applications such as corrugated steel pipe (CSP) for drainage applications require veryheavy coatings. The most common coating weight [mass] for CSP is G200 [Z610] .

Effect of Coating Weight [Mass] on Product Life

Although the corrosion rate can vary considerably depending on the environmental factors, as Figure 1shows, the life of a zinc coating is a linear function of coating weight [mass] for any specificenvironment. This means that to achieve twice the life for any specific application, twice the coatingweight [mass] is required.

Examples –

• A G60 coating weight will exhibit approximately twice the life of a G30 coating weight

• A G90 coating weight will exhibit about 50% longer life than a G60 coating weight

Additional information on this topic is contained in Appendix X4 of ASTM Specification A 653/A 653M.

Corrosion Rate Data

In addition to The Coating Life Predictor that is available at www.galvinfo.com, the following two referencebooks are excellent sources for additional and more detailed information on the corrosion behaviour ofzinc-coated steel sheet products. These publications go beyond the information available using TheCoating Life Predictor in that they contain information on corrosion rates in various aqueous solutions, aswell as in organic and inorganic solutions, and in soils.

1. Corrosion and Electrochemistry of Zinc, X. Gregory Zhang, Published by Plenum Press, 1996.

2. Corrosion Resistance of Zinc and Zinc Alloys, Frank C. Porter, Published by Marcel Dekker, Inc.,

1994

These publications document that corrosion can range from very low rates – in the order of less than 0.01

mil*/year (0.254 µm/yr) – to much higher rates. If the rate of corrosion were, for example, 0.05 mil/year

(1.25 µm/year), the life of a G90 coating would be approximately 16 to 17 years, since a G90 coating is

approximately 0.83 mil (21 µm) thick on each side of the coated steel sheet. In some environments, the

rate of corrosion is so high that galvanized steel is not the preferred product. Generally, such applicationsare those that have either very acidic or very basic environments.

Another source of zinc corrosion rate data can be found in ASM Metals Handbook Vol. 13B Corrosion:Materials, 2005, pp. 402-417,available at: http://asmcommunity.asminternational.org/portal/site/www/

It should be emphasized that much of the zinc corrosion data given in the above references was

generated in the 1950–1970 era, while the data used to generate Figure 1 on page 2 is more recent, afteraggressive pollutants such as sulfur dioxide declined from their higher levels of the mid 20

th century. The

service life of galvanize in, say, urban industrial areas is now longer than it was 30 to 50 years ago. On

the other hand, corrosion rates in marine environments are not so much changed, since the rate of zinc

loss is governed more by the amount of deposited sea salt than airborne pollutants.

*(1 mil=0.001 in)





4

Contact the GalvInfo Center

The correct selection of coating thickness is but one of the many factors that need to be considered when using

galvanized sheet products. Others include the steel thickness, the steel strength, the steel formability, the

surface treatment applied to the galvanized coating, etc. To assist you with these many considerations, please

contact the GalvInfo Center by either phone or e-mail.

Toll-free phone: 1.888.880.8802

E-mail: [email protected]

Note: Additional information on corrosion rates in various environments, ASTM coating designations, andgalvanized steel specifications can be found in GalvInfoNotes 3.1, 2.5, and 1.5, respectively.


Disclaimer:

Articles, research reports, and technical data are provided for information purposes only. Although the publishers endeavor toprovide accurate, timely information, the International Zinc Association does not warrant the research results or information reported

in this communication and disclaims all liability for damages arising from reliance on the research results or other informationcontained in this communication, including, but not limited to, incidental or consequential damages.




1


GalvInfoNote

1.7

Advantages of Metallic-CoatedSteel Framing in Residential Buildings

Rev 1 Jan-09 Introduction

Metallic-coated steel framing has many advantages over competitive building materials in load-bearing

residential buildings. While these advantages include a higher strength to weight ratio, non combustibility, and

improved dimensional stability, the purpose of this GalvInfoNote is to provide an introduction to the durability

and service life benefits of metallic-coated steel framing. Along with short descriptions of the products used to

manufacture steel framing, summary information on durability is provided along with links to more in-depth

reports and data, including expected service life.

Source: CSSBI A principle reference document on the durability of steel framing is published byThe Steel Framing A lliance, and can be obtained at:

http://store.steelframingalliancestore.com/duofcostfrme.html; “Design Guide 4 – Durability of

Cold-Formed Steel Framing Members”; available for download - $15.

Benefits of Residential Steel Framing

1. Variety of steel thicknesses and strengths available allowing design flexibility, longer floor spans, andhigher, straighter walls

2. More resistant to fires, hurricanes, earthquakes, insects and mold

3. Expected service life of hundreds of years under normal conditions

4. At the end of a steel-framed home’s useful life, the steel components are recyclable

These are a few of the benefits of steel framing. Below are references and/or links to more detailed informationon these and other advantages:



GalvInfoNote 1.7Rev 1 Jan-09


2

American Iron and Steel Institute (AISI): www.steel.org

Click on Construction, Framing – general information and numerous links related to the topic, includingSteel Framing Alliance; www.steelframing.org Click on Media Center, Fact Sheets, “The New Steel, Building in New Markets”

International Zinc Organization (IZA):

http://www.iza.com/zwo_org/Applications/zwo00-index-App.htm - “At home with zinc and steel”

Types of Metallic-Coated Steel Framing

ASTM International; www.astm.org; Specification A 1003/A 1003M, Steel Sheet, Carbon, Metallic- andNonmetallic-Coated for Cold-Formed Framing Members, covers the steel sheet used in the manufacture of thisproduct. The metallic coatings allowed are; zinc (galvanize), zinc-iron alloy, 55% aluminum-zinc alloy, zinc-5%aluminum alloy, aluminum coated Type 1, aluminum-coated Type 2, and electrolytic zinc-coated. A descriptionof these coatings can be found in GalvInfoNotes 1.2 and 1.5 of this GalvInfo Center series.

Specification A 1003/A 1003M can be purchased and downloaded from the ASTM website, as can theindividual product specifications for each of the coated sheet types listed above. A link to all ASTM metallic-coated specifications can also be found at the GalvInfo website, http://www.galvinfo.com/index-3a.html .

Note that most cold-formed framing members are produced using galvanized sheet. The process for producinggalvanize is described in GalvInfoNote 2.1. The minimum coating weight designation in A 1003/A1003M forgalvanized cold-formed steel framing is G40.

How Metallic Coatings Protect Steel

There are two primary mechanisms by which metallic coatings protect steel, viz., barrier and galvanic (cathodic)protection. The details of these mechanisms are described in GalvInfoNote 3.1.

For in-depth study, an excellent reference text on the corrosion of zinc is:

“ Corrosion and Electrochemistry of Zinc” , Xaioge Gregory Zhang, Plenum Press, New York, 1996

Corrosion of Cold-Formed Framing Members in Residential Construction

As galvanized steel sheet is the predominant material used to manufacture steel framing members, the longterm corrosion performance studies of this product are based largely on zinc coatings, although there is dataavailable on aluminum-zinc and zinc-aluminum coatings.

All worldwide test results to date indicate that there is little corrosion of galvanized steel framing in residentialconstruction under normal conditions. Minor corrosion, if present, will not adversely affect the anticipated life of astructure. The environment of building wall interiors is discussed in more detail in the following document,available at:

International Zinc Organization (IZA):

http://www.iza.com/zwo_org/Applications/zwo00-index-App.htm; “Housing for Generations” byRoger Wildt

As stated in the Introduction on page 1, an excellent document that provides guidance for designers inselecting coated steels and enhancing durability in residential (and commercial) buildings that utilize cold-formed steel framing members is again referenced below. It is a comprehensive guide that provides designinformation, not only on such issues as contact with non-metallic materials and other metals, and the corrosion





3

properties of zinc, but cites data that shows the expected life of metallic coated framing is in the hundredsof years. The reference is:

The Steel Framing Alliance (SFA):

http://store.steelframingalliancestore.com/duofcostfrme.html; “Design Guide 4 – Durability of

Cold-Formed Steel Framing Members”; available for download - $15. This reference providesdata on the expected service life of framing members manufactured using, zinc, aluminum-zinc,

and zinc-aluminum coatings.

A second document that provides much of the data supporting the above guide is:

http://www.steelframing.org/PDF/research/RP06-1.pdf :Galvanized Steel Framing for ResidentialBuildings” – Research Report RP06-1, 2006, available for download - $25. This reference provides

detailed data on the expected service life of framing members manufactured using zinc,aluminum-zinc, and zinc-aluminum coatings. A summary version of this report can be

downloaded from the GalvInfo website www.galvinfo.com in the GalvInfo Library, Additional

Resources section.

A useful related document is:http://store.steelframingalliancestore.com/coofgafausin.html; A report providing information on

the corrosion of fasteners is: Report 12, “Corrosion of Galvanized Fasteners used in Cold-Formed

Steel Framing”; available for download - $25

Of concern to many users is the durability of cold-formed steel members in aggressive environments,such as coastal areas. Issues such as chloride ion concentration and time of wetness impact the servicelife of metallic-coated steel framing members. These and other issues are covered in a number of

documents that are available on-line, and links are given below:

The Steel Framing Alliance;

http://www.steelframingalliance.com/mc/page.do?sitePageId=1081 “Corrosion Protection for Life”

http://store.steelframingalliancestore.com/tepu1.html; “Corrosion Protection for Cold-Formed Steel

Framing in Coastal Areas -140”, available for download - $5

http://data.memberclicks.com/site/sfa/305framework.pdf ; “Framework”, Mar-Apr, 2005, pp 26-29.

This issue provides information on the corrosion of fasteners in coastal environments, and design

recommendations for cold-formed steel framing and fasteners in coastal environments.

Users often ask about the coating weight required for cold-formed steel framing. For drywall steel,

the minimum for galvanize is G40, while for structural members it is G60. For situations where extra

protection is needed, G90 is recommended. Many of the references listed above expand on thistopic, and additional information can be found at:

http://www.steelframingalliance.com/mc/page.do?sitePageId=4057; Frequently Asked Question pageof the SFA website.





4

One important consideration in prolonging the life of steel framing is the control of condensation within

the structure. A fact sheet on how condensation forms in metal buildings and how to control it has beenissued by the Metal Building Manufacturers Association, and can be found at:

http://www.mbma.com/pdf/condensationfactsheet.pdf ;

Another technical resource on controlling corrosion of metal in buildings in coastal areas can be found at:

http://www.fema.gov/pdf/fima/corr.pdf ; “Corrosion Protection for Metal Connectors in Coastal

Areas”

Summary

Steel framing has many advantages, including extraordinary service life, and is a growing market for

metallic coated sheet steel (see: http://www.steelframingalliance.com/mc/page.do?sitePageId=1083).This GalvInfoNote provides the reader with summary information on the benefits of using steel framing

members and cites references for the reader to find more detailed data on how best to utilize it.

Source: Steel Framing Alliance


Disclaimer:






1


GalvInfoNote

1.8

Steels Used in Coated Sheet Products

Rev 0 Jan-2010

Introduction

Zinc and zinc alloy-coated sheet is produced using many different types and grades of steel. This

GalvInfoNote briefly describes these steels and the basic metallurgy involved in their production andapplication.

Steel Substrates for Galvanizing

Steels for continuous galvanizing can be categorized into the following major groups (most of theterminology is as defined in the ASTM product specifications described in GalvInfoNote 1.2):

Commercial Steel (CS) – carbon levels range between 0.04% and 0.10%, and manganese from 0.2% to0.6%, depending on the product being made. The substrate is cold rolled anywhere from 50 to 80%

reduction prior to it being processing through a galvanize line.

Forming Steel (FS) – carbon levels between 0.04-0.08%, and manganese at about 0.25%. This steel iscold reduced between 60 to 80% and is used to produce a slightly softer product than CS in order to giveimproved formability.

Structural Steel (SS) – carbon levels range between 0.04% and 0.20%, and manganese from 0.4% to1.6%, depending on the product being made. The sheet is cold rolled anywhere from 50 to 70%reduction. SS grades must meet minimum mechanical property requirements and have yield strengthsbetween 33 and 80 ksi [230 and 550 MPa].

Deep Drawing Steel (DDS) & Extra Deep Drawing Steel (EDDS) – generally made from ultra-lowcarbon (10-15 ppm) stabilized steels, although some DDS is made using extra low carbon (0.015-0.020%) steel. EDDS, and some DDS, is designed to be fully stabilized (non-ageing) after in-line

annealing and coating. To maximize annealing response, cold reduction is generally 75% minimum.

Solut ion Hardened Steel (SHS) & Bake Hardenable Steel (BHS) – ultra-low to low carbon (0.12% max)steel that has yield strengths from 26 to 44 ksi [180 to 300 MPa]. SHS is strengthened usingsubstitutional elements such as M, P, or Si, while BHS relies on strain ageing after forming forstrengthening.

High Strength-Low Alloy Steel (HSLAS) – typically made from micro-alloyed low carbon steel. Theprimary micro-alloying element is niobium (Nb). Cold reduction rarely exceeds 60% due to the high coldrolling loads necessary to reduce the thickness of these steels. They have a yield strengths of 40 to 80ksi [275 to 550 MPa].

Advanced High Strength-Low Al loy Steel (AHSS) – Produced using higher levels of alloying elementsand carefully controlled annealing and cooling cycles. Cold reduction rarely exceeds 60% due to high cold

rolling loads. The yield strength of these steels is typically between 50 and 80 ksi [340 and 545 MPa].





2

Effects of Alloying Addition on Steel Properties

• Carbon – Is the most important steel-alloying element, having the greatest effect on steel properties.Carbon is part of integrated steelmaking operations starting with the blast furnace. Figure 1 showsthe influence carbon has on steel properties.

• Manganese – Was initially used to control “hot shortness”, a problem associated with sulfur in steel.Now it is used for strengthening and is intentionally added.

• Sulfur – Undesirable in almost all steels. Comes from the sulfur in coal or the ore. In most casesprocessing practices are in place to minimize the sulfur content.

• Phosphorus – Often present at very low levels, residual amounts – less than 0.01%. Can be addedto increase strength.

• Silicon – Usually present only as a residual element, typically less than 0.01%. Can be added as astrengthener to produce high strength steels. May cause problems during hot-dip coating, as it isdifficult to “reduce” silicon oxides in continuous annealing furnaces.

• Alum inum – Added to “kill” the steel during casting, i.e., prevent oxygen out-gassing problems duringsolidification. Also, can tie up the nitrogen to minimize “aging”. Used to make deep-drawing steels.

• Nitrogen – Present as an impurity; coming from handling molten steel in air.

• Niobium, Titanium & Vanadium – Intentionally added to strengthen steels. Nb and Ti are also usedas stabilizers in IF steels.

• Copper, Nickel & Chromium – Typically present only as impurities. When added, are used forhardening and/or strengthening.

(a) (b)

Figure 1 Effect of C on mechanical properties, (a); effect of alloying elements on yield strength, (b)





3

Ultra Low Carbon Stabilized Steel

Carbon (C) and nitrogen (N) in sheet steel results in higher mechanical properties, age hardening, anddeterioration of the r-value (measure of resistance to thinning and drawability).

Liquid steel is processed through a degasser to reduce C and N to levels low enough that the remainder

can be “stabilized” by small additions of titanium (Ti) and niobium (Nb).Ti and Nb are strong carbide/nitride formers, taking the remaining C and N out of solution in liquid iron,after which these latter two elements are no longer available to reside in the interstices between solidifiediron atoms.

The term “Interstitial-Free” or “IF” steel refers to the fact that there are no interstitial solute atoms to strainthe solid iron lattice, resulting in very soft steel.

Non-ageing IF steel has no yield point elongation, which means fluting and stretcher strains are never aproblem.

IF steel made using only Ti is very common and is used to produce the best mechanical properties fordeep drawing. It is very reactive in a zinc bath and is usually coated only as galvanize (GI).

Another popular type of IF steel is stabilized with both Ti and Nb. The synergy of these two elements

allows complete stabilization to be achieved at lower levels of each element. Depending on the relativeamounts of Ti and Nb, the steel needs to be annealed at a higher temperature during galvanizing and hasslightly inferior mechanical properties to the Ti type. Ti-Nb type IF is also less reactive in a zinc bath andis usually employed when producing galvanneal (GA).

IF steels are ideal for directly producing DDS and EDDS hot-dip products by the continuous annealprocess. During the zinc coating and galvannealing steps the strip is reheated above the overageingtemperature. If low carbon steel were being used, carbon would redissolve, and could cause strainageing. With IF steels, cooling and reheating is irrelevant, since carbon (and nitrogen if present) are notavailable to be redissolved and cause aging.

One type of EDDS made using stabilized steel is actually a higher strength steel with a minimum yieldstrength of 30 ksi [205 MPa]. Made using phosphorous additions of up to 0.06%, it combines goodformability with high strength, producing good dent resistance on exterior panels

Stabilized, ultra low carbon (ULC), interstitial-free steel has the ability, during continuous annealing, toform crystal orientations favorable to deep drawing. This is not possible with low carbon, unstabilizedsteel on continuous annealing lines. The high r-values needed for good steel drawability require plentiful“cube-on-corner” crystal orientations to form during annealing. This becomes increasingly possible whenthe carbon level is below 0.01%, and is optimized at 0.001% (10 ppm).

Most formable steels (whether IF or carbon) have manganese levels below 0.20%, and the formabilityimproves as the carbon level is lowered. Manganese becomes more damaging to r-values as the carbonlevel increases. R-value is maximized when ULC steel with 10-ppm carbon and about 0.15% Mn iscompletely stabilized using Ti.

Some advantages of ULC stabilized steel are: superior stamping, forming, and drawing performance; theability to make more complex parts, perhaps using a fewer numbers of dies; age hardening resistance(long shelf life for stored steel); and improved coating adhesion for galvanized products.

The main disadvantage of ULC stabilized steel is that it can be very soft, resulting in shearing andpunching difficulties, and its use may result in parts that are not as ‘strong’, i.e., dent resistant, comparedto parts made from carbon steel.





4

Bake Hardenable Steel

Using ultra-low-carbon vacuum-degassed steel, and precise alloying additions, partially stabilized steelcan be produced that has a low amount of solute carbon available after precipitation reactions arecompleted on the galvanizing line.

Bake hardenable steel (BHS) takes advantage of the low solute carbon to produce controlled carbonstrain aging to augment the yield strength of formed automotive panels, thus improving dent resistance orpermitting some thickness reduction. The strain comes from press forming and the aging is acceleratedby the paint baking treatment. BH steels contain enough supersaturated solute carbon that the agingreaction typically adds 4 to 8 ksi [27 to 55 MPa] to stamped panel yield strength.

This approach to providing higher strength panels has the advantage of presenting formable low yieldstrength material to stamping operations so as to avoid panel shape problems due to elastic deflectionassociated with initial yield strengths exceeding 35 ksi [240 MPa]. BHS is the practical consequence ofmodern manufacturing technologies, which permit control of supersaturated solute carbon at a levelwhich is just high enough to provide a useful amount of accelerated strain aging, without aging duringtransport/storage. The BHS process produces a coated product that will be free from stretcher strains forat least 2 to 3 months after its production, allowing stampers time to consume it before its mechanicalproperties begin to deteriorate due to aging.

Figure 2 illustrates the concept of bake hardening, with BH representing the flow stress increase onbaking. This chart also represents the typical strain and baking conditions for the least formed areas ofautomotive panels.

Figure 2 Bake hardening phenomenon

It can be seen that when producing BHS on a hot-dip CGL the most critical parts of the process involvetrapping solute carbon by fast cooling through the carbide precipitation range, and avoiding cementiteprecipitation by quickly passing through the overaging zone to the zinc bath entry temperature.

High Strength Steel

There are various approaches to making high strength steels. For many years galvanize with 80 ksi yieldstrength has been produced using a “full hard” (unannealed or recovery annealed) method (ASTM A653,Grade 80 [550 MPa]). This product is strong but has very limited ductility. It is typically used for suchproducts as roll-formed building siding.

High strength steel sheet can be produced using solid solution strengthening or precipitation hardening.

Solid solution hardening is used mostly for high strength structural steels and is accomplished by usingalloy additions (solute) that are interstitial and/or substitutional in the solvent metal as illustrated in Figure





5

3. It achieves high strength with moderate formability. The interstitial approach uses elements such ascarbon and nitrogen that stretch the ferrite lattice. This mechanism is usually combined withsubstitutional elements such as manganese, silicon, and phosphorous, which replace iron, also stretchingthe ferrite lattice.

Figure 3 Interstitial (a) and substitutional (b) solution strengthening

The above 2 mechanisms are used to produce galvanized steels with yield strengths up to about 65 ksi[450 MPa] with reasonable formability. Steels of this type have a characteristic low (<0.75) yield/tensile

(Y/T) ratio and are mostly used for structural applications.Precipitation hardening and grain refinement is used in the production of high strength-low alloys steel,using alloying elements, such as V, Nb, and Ti, to combine with C and/or N to form very smallcarbide/nitride precipitates. These steels are more formable than structural high strength steel and havea high (>0.80) Y/T ratio.

Advanced high strength steels (AHSS) are a relatively new class of high strength steels produced usinghigher alloy levels combined with special in-line thermal treatment. They combine very high strength withgood ductility and have lower Y/T ratios than HSLAS.

High Strength-Low Alloy Steel (HSLAS)

High strength-low alloy galvanize is produced using precipitation hardening reactions during annealingand uses alloying elements, such as Nb, and Ti, to combine with C and/or N to form very smallcarbide/nitride precipitates. Hardening results from the precipitates preventing or altering dislocation(lattice defect) movement in the steel. Precipitates also act as grain refiners by pinning therecrystalization interfaces. Also, recrystalization is delayed until the carbides grow in size, resulting inmuch smaller grain size. Yield strength increases since it is inversely proportional to ferrite grain size.Niobium at a level as low as 0.005% is effective because of its high atomic weight, and NbC precipitatesdo not dissolve at continuous annealing temperatures, making them available for both precipitationhardening and grain refinement. These techniques are used to produce HSLAS with yield strengths from40 to 60 ksi [275 to 410 MPa].

Vanadium not used as microalloying element for galvanize because VN precipitates dissolve at thecontinuous annealing temperatures used, the N combines with Al, and the precipitates are lost.

Figure 4 shows a drawing of the nature of HSLAS microstructure.





6

Figure 4 Microstructure of HSLA steel

The typical HSLA stress-strain curve (lower curve in Figure 5) has very little difference between the yieldpoint and the ultimate tensile strength (a high Y/T ratio, ~0.80-0.84). It is moderately formable but will

fracture at stresses close to its yield point. This is the primary reason behind HSLAS losing favour forautomotive applications, i.e., it cannot match the performance of the higher strength, more formablegrades in the AHSS family.

Figure 5 High strength steel tensile behaviors

Advanced High Strength Steel (AHHS)

As automobile companies are committed to lowering the CO2 emissions of their products, and weight reductionis an integral part of achieving this, AHSS technology offers an excellent means of contributing to this goal. Another important requirement for vehicles is to perform well in collisions. This requires steels with tensile

strengths as high as are compatible with the demanding formability requirements required by the fabricationprocesses. Currently, AHSS grades with about 50 ksi [340 MPA] YS, and about 85 ksi [600 MPa] tensilestrength (TS) are the most widely used. Development has proceeded actively in Europe, Japan and Americaover the last decade and includes Dual Phase (DP), Multi Phase (MP) or Complex Phase (CP), andTransformation Induced Plasticity (TRIP) steels.

Current production is mostly Dual Phase (DP). Active development of Transformation Induced Plasticity (TRIP)coated sheet is underway. Typical alloying strategies involve the use of elements such as C, Si, Mn, P, Cr, Mo,and Al. Rapid cooling and isothermal holding are required during continuous annealing to achieve the required

HSLA





7

mechanical properties. The main goal is better formability at a given strength level and, in some cases,post forming strengthening. Figure 5 illustrates the tensile properties of these steels compared the HSLAS.

The microstructure of DP and TRIP steels are shown in Figure 6. It is the very hard martensite and bainiteconstituents in the soft ferrite matrix that give these steels their combination of high strength and good ductility.TRIP steels also have untransformed austenite when they are made, which then transforms to harder

constituents as a result of the energy input from forming operations. This “delayed reaction” produces strongerfinished parts, allowing further reduction in steel thickness.

(a) (b)

Figure 6 Microstructure of Dual Phase (a) and TRIP (b) steels

DP and TRIP steels have much lower Y/T ratios (~0.60) compared to HSLAS as shown in the upper twostress-strain curves in Figure 5. After yielding, they have the capacity to absorb considerably moredeformation before fracturing. Consequently, the finished part ends up with a much higher strength thanif made with HSLA. This allows a thinner steel to be used to produce a part of equivalent strength.

TRIP steels (top curve in Figure 5) have a similar Y/T ratio to DP steels, but are stronger, and can workharden more, with equivalent or better formability. The low Y/T ratio that is characteristic of DP and TRIPsteels is being used to advantage in more than one way by automotive designers. Not only is there thebenefit of weight savings, but these steels also provide gains in crash energy management, resulting insafer vehicles. The larger capacity for work hardening absorbs more energy during a crash, energy thatis not transferred to the vehicle occupants.

Figure 7 Benefit of DP versus HSLAS in final material strength

TRIP

DP





8

Figure 7 illustrates the benefit of using Dual Phase steel over HSLA in the manufacture of structural parts.The lower Y/T ratio of the DP allows a much higher work hardening component to add to the strength ofthe part. An added benefit is that DP steel also has a bake-hardening component that adds strength afterthe part is heated to paint baking temperatures.

Summary

The relationship of tensile strength to ductility of the family of steels used for coated sheet (exceptmartensitic – MART – steel) is illustrated in Figure 8. It is evident that the advantages of AHSS are onlybeginning to be tapped, since there are few developed grades with TS greater than 700 MPa. While thecontinuous galvanizing industry has advanced significantly over the last two decades in developing a hostof steel grades for many different markets, there is still much work to be done in the quest for stronger,more formable grades of coated sheet products.

Figure 8 Types of steels used in hot-dip galvanizing


Disclaimer:

Articles, research reports, and technical data are provided for information purposes only. Although the publishers endeavor toprovide accurate, timely information, the International Zinc Association does not warrant the research results or information reportedin this communication and disclaims all liability for damages arising from reliance on the research results or other information

contained in this communication, including, but not limited to, incidental or consequential damages.




1


GalvInfoNote

1.9

Zinc-5% Aluminum Alloy-Coated Steel Sheet

Rev 1.0 Mar-2010

IntroductionThe most widely used metallic coating for the corrosion protection of steel is zinc (galvanize). It offers avery good combination of galvanic and barrier protection. Its excellent performance for many applicationsis well documented. However, in the desire to improve, there is always a quest to find even betterproducts. Researchers continually attempt to develop superior steel coatings that can be commerciallyapplied. Often, the target is to find better products for specific end uses or environments, e.g. ones havingsuperior corrosion resistance, or better coating formability. These attempts meet with little success mostof the time, due either to an undesirable product attribute, or because manufacturing is too expensive ordifficult, but every so often a breakthrough coating is discovered.

One hot-dip coating that was successfully developed is zinc-5% aluminum (Zn -5% Al) alloy-coated steel

sheet, the most popular version of which is known throughout the world as GALFAN. It is a registered

trademark of the Galfan Technology Center, Inc. (GTC) http://galfan.com/home.html. This website lists all

the active producing Galfan

®

licensees.The GTC website summarizes the history of Galfan

® as follows:

“Galfan® as a trademark has been around since the International Lead Zinc Research Organization

(ILZRO) obtained worldwide patents on this new alloy for anti-corrosion coating in 1981. This grew from an

ILZRO-organized project co-sponsored by Arbed, Cockerill Sambre, Usinor and Sacilor (now all part of

ArcelorMittal), British Steel, Fabrique de Fer de Maubeuge (now all part of Corus), New Zealand Steel (now

part of Bluescope), and Stelco (now U.S. Steel Canada) at the Centre de Recherches Metallurgiques

(CRM) in Belgium. This project showed that an alloy combining 95% zinc, nearly 5% aluminum, plus

specific quantities of rare earth mischmetal, could be reliably used in the hot-dip coating process, and

conferred substantially improved performance to the end-product. Licenses to use the revolutionary Galfan®

technology have been granted to manufacturers worldwide.

“The name Galfan® was given to the new alloy product during a business meeting one evening after the first

large-scale production campaign at Sacilor’s, Ziegler S.A. works in Mouzon, France on July 8-10, 1981.Upon reviewing the success of this campaign, in which 150 tonnes of coils were coated, with high quality

product obtained after running the first 250 meters of strip through the continuous coating line, J-L. Pagniez,

head of the French Coated Steel Information Center (CITAG), christened the product "galvanisationfantastique". This was shortened to Galfan

®, and this name was then trademarked by ILZRO.”

The ASTM steel sheet product specification for Zn-5% Al alloy-coated sheet is A 875/A 875M. The

coating is available in two types: Type I alloy coating contains small additions of rare earth mischmetal

(Zn-5Al-MM) and is what is used to produce sheet under the trade name Galfan®. Type II contains 0.1%

magnesium (Zn-5Al-Mg). Zinc-5% Al alloy-coated sheet is also manufactured and sold under other trade

names.

Both Type I and Type II can be used for prepainted sheet as specified in A 755/A 755M.

Specifications EN 10214 and ISO 14788 are other documents that can be used to specify Zn-5% Alcoated sheet.

Manufacture

In the 1970s, research in America, Europe, and Japan experimented with zinc coatings containing up to15% aluminum. This research found that zinc with 5% aluminum provided the best corrosion resistancebut the problem of small, unwetted bare spots stymied commercialization of the alloy as a coating

1.



GalvInfoNote 1.9

Rev 1.0 Mar-2010


2

Research that ILZRO later commissioned CRM to conduct on the Zn-5% Al system determined that theaddition of small amounts of rare earth mischmetal

1 containing cerium and lanthanum improved fluidity,

wettability, coating ductility, and inhibited intergranular corrosion. This mischmetal formulation is whatlater became known as Galfan

®.

Composition of the mischmetal-bearing alloy used to produce GALFAN is specified in ASTM B750. The

technology used to manufacture this alloy is licensed technology 1

. Also, see GalvInfoNote 5.2 for thespecified chemistry limits.

Galfan® sheet products are coated on processing lines that are almost identical to those used to produce

galvanized sheet, and which are described in GalvInfoNote 2.1. In some cases, production lines thatproduce Zn-5% Al coatings are dedicated to this product, although most lines that produce GALFAN alsoproduce galvanize through the use of dual, interchangeable coating pots.

Coating Metallurgy and Microstructure

Zinc with 5% aluminum is a eutectic alloy . A eutectic composition is that ratio of elements having thelowest melting temperature, and is located at the intersection of the elements’ liquidus curves in a phasediagram (a representation of what occurs when elements are mixed together). The eutectic is a uniquetemperature-composition point for two or more elements. Lowering the temperature to just below the

eutectic temperature results in a reaction where the all of the liquid mixture freezes to a complete solid atthat temperature. In non-eutectic mixtures, freezing occurs over a range of temperatures, and elementscan segregate into phases. For more information on this phenomena refer to literature about the zinc-aluminum phase diagram, as a more detailed explanation here is beyond the scope of this GalvInfoNote.

Figure 1 Microstructure of Zn-5% Al Coating (note the lamellar plate-like structure) 1

Regular zinc galvanizing alloys freeze into a single-phase microstructure. Al-Zn alloys such as

Galvalume® form two-phase microstructures in which aluminum phases are surrounded by the lower

freezing temperature zinc phase (see GalvInfoNote 1.4). Galfan®, however, forms a hypoeutectic

microstructure in which the high zinc phase and high aluminum phase freeze into very thin, alternating

parallel plates called lamellae, as shown in Figure 1. Control of the cooling rate is necessary to ensure

that the entire microstructure is lamellar.

The intermetallic alloy layer that forms between the Galfan® coating and the steel substrate is a very thin

(1 !m) ternary Al-Fe-Zn compound. This layer is thinner than a similar layer that forms on galvanized

sheet, and is the reason for the extremely good formability of Galfan®.

1 The mischmetal used in GALFAN is a mixture of cerium (Ce), lanthanum (La), two of the 15 elements between atomic

numbers 57 and 71, sometimes called rare earths. Their percentage in the alloy is extremely small but their effect in

improving wettability is very significant.



GalvInfoNote 1.9

Rev 1.0 Mar-2010


3

Corrosion Resistance

Aluminum corrodes more slowly than zinc in most atmospheres because of its surface barrier layer ofvery passive aluminum oxide. However, this passive layer prevents aluminum from adequatelycontributing towards cathodic (sacrificial) protection. Cathodic protection is the strong point of zinccoatings, in that if the coating is cut or scratched, the zinc near the exposed steel will corrode first.

Galfan® combines the strengths of both zinc and aluminum, giving better passive barrier protection thanregular galvanize, and better sacrificial protection than alloy coatings with lower zinc compositions.

Regular galvanized coatings corrode in the atmosphere by the zinc being continually converted to zincoxide and zinc carbonate. The lamellar eutectic microstructure of Galfan

®, shown in Figure 1, interferes

with this mechanism, as corrosion must follow the direction of the thin lamellae. Also, the dense, morepassive, aluminum-rich corrosion products that are left behind lower the reactivity of the surface.Naturally, corrosion will be slowest when the lamellae are parallel to the sheet, but even the randomorientation that the lamellae usually take greatly reduces the corrosion rate.

Galvanize exhibits a linear corrosion rate. ILZRO research has shown that the corrosion rate of Galfan®

is parabolic rather than linear. Galfan’s weight loss during the first two or three years is slightly less thangalvanize, but as its surface passivates, it’s rate of weight loss decreases parabolically. This is illustratedin Figure 2.

Figure 2 Coating Thickness Loss in Marine Environment from New Zealand Steel Study1

The results of field studies have shown that Galfan® has a minimum of two times better outdoor corrosion

resistance than conventional galvanize of the same coating weight. This is the case in rural, industrial,

and marine environments.

The issue of whether to use galvanize or Galfan® for a particular end use therefore becomes one of

overall value versus initial cost. If the objective is to have products last over 2 times longer than those

made with galvanize, then the same coating weight as used with galvanize can be specified. If lower

initial cost is paramount, then the same service life can be obtained by specifying one-half the normalgalvanize coating weight.



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4

Summary

Years of testing and evaluation have allowed the comparison chart shown in Figure 3 to be compiled. It

shows that Galfan® is equal to or better than galvanize in all the important coated sheet attributes.

Figure 3 Galfan® Compared to Other Metallic Coatings

Zinc-5% aluminum hot-dip coated sheet has corrosion resistance at least twice as good as galvanize. It

has been in use for 25 years and has application in many markets, including construction, agriculture,

appliance, automotive, highway, and utility.

More information on this product can be found at: http://galfan.com/home.html

References:1 GALFAN Product Manual, ILZRO, June 1993


Disclaimer: Articles, research reports, and technical data are provided for information purposes only. Although the publishers endeavor toprovide accurate, timely information, the International Zinc Association does not warrant the research results or information reported





1


GalvInfoNote

1.10

Useful Galvanized Sheet Metrics

Rev 0 Mar-2010 Introduction

Galvanized sheet is produced in large quantities worldwide. It is ordered to a many different

combinations of thicknesses, widths, and coating weights [masses]. In most cases, galvanized sheet is

sold by weight [mass] and put into use by area, and purchasers continually strive to obtain the maximum

number of parts/pieces per coil(s) received. This is usually done by tracking the length of sheet received

per coil, and/or the number of pieces obtained. From this information the actual thickness of the sheet

can be calculated and compared to what was ordered, whether that be a minimum or nominal thickness

value. In doing this it is important to know the correct weight [mass] per unit area of galvanized sheet.

Another aspect of galvanized sheet is the effect of coating weight [mass] on the cost of the product. Zinc

is a world commodity that can fluctuate in price to an extent that can make a significant difference to the

price of galvanized sheet. This difference becomes larger the thinner the sheet, as a unit weight [mass]

of thinner product has a higher fraction of zinc on it. As an example, the price of thin galvanized sheet

rises more that that of thick sheet when zinc prices increase. It is therefore helpful to know the effect ofsheet thickness and coating weight on the amount of zinc in a coil.

This GalvInfoNote provides tools for both producers and users to more fully understand the details

involved in the above two concepts, so as to help in more effective utilization of galvanized sheet.

Theoretical Weight [Mass] per Unit Area

Because the densities of zinc-based hot-dip coatings are lower than the density of steel, the weight per

unit area of coated sheet is less than uncoated sheet of the same thickness. This small difference can be

important when large volumes of coated sheet are being consumed. The adjustment in weight [mass]

varies as a function of the thickness of the steel substrate, the coating, and the coating type in the case of

zinc alloys. For example, 0.013” G90 sheet is about 1.2% lighter than cold rolled sheet of the same

thickness. This difference lessens for thicker sheet and/or thinner coatings. Using the densities of steel

and the various coatings, plus the actual typical applied coating thickness, a “Coating Factor” can becalculated for each coating type and designation. To arrive at the theoretical weight [mass] per unit area

of sheet, the Coating Factor is subtracted from the weight [mass] of uncoated steel sheet of the same

thickness. The Coating Factor is actually the difference in weight [mass] between the coating metal and

steel of the same thickness as the coating metal. Knowing these factors is important in being able to

closely track the sheet area obtained per unit weight [mass] of coated sheet product.

Theoretical Weight

The formula for calculating the Theoretical Weight of galvanized sheet in lb/ft2 is:

TW = t x 40.833 – CF (1)

Where: TW = Theoretical Weight in lb/ft2

t = actual sheet thickness in inches

40.833 = weight in lbs of 1 ft2 of 1” thick steel

CF = Coating Factor in lb/ft2

For example; galvanized sheet 0.020 inches thick with a G90 coating has a coating factor of 0.006 lb/ft2,

based on the relationship in (1) and a typical actual G90 coating weight of 0.96 oz/ft2. Therefore:

TW = 0.020 x 40.833 – 0.006 = 0.8107 lb/ft2



GalvInfoNote 1.10Rev 0 Mar-2010


2

Theoretical Mass

The formula for calculating the Theoretical Mass of galvanized sheet in kg/m2

is:

TM = t x 7.85 – CF (2)

Where: TM = Theoretical Mass in kg/m2

t = actual sheet thickness in millimetres

7.85 = mass in kg of 1 m2 of 1mm thick steel

CF = Coating Factor in kg/m2

For example; galvanized sheet 0.50 mm thick with a Z275 coating has a coating factor of 0.029 kg/m2,

based on the relationship in (1) and a typical actual Z275 coating mass of 293 g/m2. Therefore:

TM = 0.50 x 7.85 – 0.029 = 3.896 kg/m2

Using the above relationships, Theoretical Weight [Mass] can be calculated for all combinations of sheet

thicknesses and coating types/thicknesses. For quick reference, producers of coated sheet usually have

tables available for their customers showing this information for the products and thicknesses they sell.

The above formulas can be used to interpolate between the thicknesses shown in these tables. If you do

not have access to such information, Table 1 shows the coating factors for typically produced coatingweights [masses] of most of the commonly ordered coating designations for galvanize, 55% Al-Zn alloy

coated sheet, and Zn-5% Al alloy coated sheet.

The following example illustrates how a consumer of galvanized sheet steel might utilize the above

information:

A roll forming operation orders 1000 tons of 0.020” min thickness x 48” wide in 10 ton coils that will

be used for manufacturing wall panels for a large project. The roll former wishes to track how close

the supplier came to producing steel to the minimum thickness, realizing that the sheet must be

somewhat above the minimum to guarantee there is no material that is under thickness.

From Equation (1) above and from Table 1, the weight per square foot of 0.020” thick G90 sheet is

0.8107 lb/ft2 (if this same thickness of sheet were uncoated it would weigh 0.8167 lb/ft

2 – a

difference of 0.74%, which is 7.4 tons on a thousand ton order). If the entire order were exactly

0.020” thick, at 0.8107 lb/ft2, it would yield 616,670 feet of 4-foot wide sheet. Since the thickness

must be slightly above the minimum, the actual footage obtained is therefore a measure of how

close the steel is to minimum gauge. Keep in mind that the actual width must be measured and

used in the calculations, since it is always slightly above ordered width. Yield losses at the coil

ends must also be accounted for. From the actual usable footage measured on each coil, the

actual average thickness received can be calculated. i.e.,

Actual thickness (in) = [Coil wt – yield loss] (lb) ÷ [40.833-CF] lb/inft2 x width (ft) x length (ft)

If the actual dimensions of coated sheet are to be closely monitored, it is apparent that the weight per unit area

must be exactly known to be able to obtain accurate results using coil footage measurements. Table 1 shows

the Coating Factors for various coating designations, which are for the coating weights [masses] shown, and

which are typical of normal production product. Adjusting the weight per unit area of sheet by the coating factor

is necessary to avoid incorrect calculations of actual average steel thickness received. If the coating factor wasignored, the calculated thickness would be higher than it actually is.





3

Table 1 Coating Factors for Zinc-Based Coated Sheet

Inch-pound SI (Metric)

ASTM CoatingType

Designation

TypicalProduced

CoatingWeight(oz/ft

2)

CoatingFactor(lb/ft

2)

Designation

TypicalProducedCoating

Mass(g/m

2)

CoatingFactor(kg/m

2)

G30 0.40 0.0025 Z90 120 0.012

G40 0.48 0.0030 Z120 144 0.014

G60 0.66 0.0041 Z180 198 0.020

G90 0.96 0.0060 Z275 293 0.029

G115 1.23 0.0076 Z350 375 0.037

G140 1.50 0.0093 Z450 482 0.048

G165 1.76 0.0109 Z500 533 0.053

G185 1.98 0.0123 Z550 588 0.058

G210 2.25 0.0140 Z600 643 0.064

G235 2.54 0.0158 Z700 756 0.075

G300 3.25 0.0202 Z900 975 0.097

G360 3.90 0.0242 Z1100 1190 0.118

A25 0.35 0.0022 ZF75 105 0.010

A40 0.46 0.0029 ZF120 138 0.014

A 653 – Galvanizeand Galvanneal

A60 0.66 0.0041 ZF180 198 0.020

AZ50 0.55 0.0375 AZM150 165 0.180

AZ55 0.61 0.0416 AZM165 180 0.196A 792 - 55%

Aluminum-Zinc

AZ60 0.66 0.0450 AZM180 198 0.216

GF30 0.40 0.0047 ZGF90 120 0.023

GF45 0.51 0.0060 ZGF135 153 0.029

GF60 0.66 0.0078 ZGF180 198 0.038

GF75 0.82 0.0097 ZGF225 245 0.046

GF90 0.96 0.0114 ZGF275 293 0.055

GF115 1.23 0.0146 ZGF350 375 0.071

GF140 1.50 0.0177 ZGF450 482 0.091

GF210 2.25 0.0266 ZGF600 643 0.122

A 875 - Zinc-5%Aluminum

GF235 2.54 0.0301 ZGF700 756 0.143





4

What Portion of a Galvanized Coil is Zinc?

A coil of 0.012” thick G90 sheet consists of about 12.5% zinc, whereas a coil of 0.050” thick G90 containsabout 3% zinc. It is therefore apparent why the price changes more for thin gauge galvanized sheet thanfor thick, when the price of zinc changes. It is useful to know the relationship between the amount of zincon galvanized sheet as a function of sheet thickness and coating weight. This relationship is shown inFigure 1. It can be helpful when evaluating the cost of a zinc coating versus the corrosion demands that itmight be put to, as a function of sheet thickness.

As the fraction of zinc that makes up a coil of galvanize is highest for thinner gauges, only thicknessesfrom 0.012” through 0.057”, plus 0.10”, are shown. Note that four common coating weights are shown,plus G140 for the thicker gauges. It is not generally possible to apply G140 to thin gauge. The points for0.01” thick sheet are given to show that above this thickness, percent zinc as a function of sheet weightbecomes small.

Copyright

! 2010 – IZA

Disclaimer:





2. Coating Processes and Surface Treatments

GalvInfoNote

2.1

The Continuous Hot-Dip Coating Process forSteel Sheet Products

Rev 0 Jan-07

IntroductionThe continuous hot-dip coating process for steel-sheet products is widely used and employed in all corners ofthe globe today. It was originally developed over fifty years ago for galvanizing (zinc coating). Now it is also usedto apply other metals to steel sheet and the early practitioners would hardly recognize the coating lines of today.It has become a very sophisticated, technically advanced operation, enabled by the availability of advancedelectrical/mechanical computerized control systems. Originally, the product was used for applications that didnot demand a high quality finish or a high degree of formability. Today, the consuming industries are using hot-dip coated product for the most demanding applications; items such as automotive hoods, fenders and doors.Not only do these applications require excellent surface quality, but they also demand a high degree offormability. Hot-dip coated sheet is produced in thicknesses from 0.010 in. to 1.70 in. (0.25 mm to 4.30mm) in widths up to 72 in. (1830 mm).

The Process – Basic Principle

As the name implies, continuous hot-dipcoating involves the application of amolten coating onto the surface of steelsheet in a non-stop process. The steelsheet is passed as a continuous ribbonthrough a bath of molten metal at speedsup to 600 feet per minute. In the molten-metal bath, the steel strip reacts (alloys)with the molten metal to bond the coatingonto the strip surface. As the stripemerges from the molten bath, it dragsout excess liquid metal, much like when

an object is pulled rapidly from acontainer of water. Using a gas-wipingprocess, a controlled thickness ofcoating, usually expressed as weight(mass) of coating per unit area, isallowed to remain on the strip surface.

How Much Coating is Needed?

Hot-dip galvanized coatings can be applied at levels ranging from as lowas 0.30 oz/ft

2 (90 g/m

2) to greater than 2.00 oz/ft

2 (600 g/m

2). The amount

of coating required depends on the application. Thin galvanized coatingsprovide sufficient corrosion protection for applications where the corrosionrate is low (interior electrical equipment, interior wall panels, computerequipment, etc.). Thick coatings are intended for applications where thecorrosion rate is high and long service life is needed (marineenvironments and underground applications). For a detailed discussionon coating weight terminology, refer to GalvInfoNote 1.1

Similar considerations related to the corrosiveness of the applicationneed to be made for the other types of hot-dip coatings. In all cases, aword of caution is in order. The coating thickness may affect otherproperties of the coated-steel sheet product; thus, all applicationrequirements need to be considered when selecting the specific coatingweight. For example, does the part being made involve a large amount offorming? This may limit the thickness of the coating to avoid loss ofcoating adhesion. Is spot-welding involved? This may limit the maximumcoating thickness for a given application. See GalvInfoNotes 1.6 and 3.1for more information on selecting coating weight.

The Process Details

Continuous hot-dip coating processing lines consist of a series of steps, which may include the followingsequential operations:

• An entry-end welder to join the trailing edge of one coil to the leading edge of the succeeding coil to allowthe process to be continuous

• An alkaline cleaning section to remove the rolling oils, dirt, and iron fines (surface contaminants from thecold reduction process) that are on the sheet surface


1



GalvInfoNote 2.1

Rev 0 Jan-07

• An annealing furnace that is used to heat the steel to high temperatures to impart the desired mechanicalproperties (strength and formability) to the steel sheet

• A bath of the molten coating metal being applied to the steel surface

• Gas knives to wipe of the excess coating metal and obtain the required coating weight

• Galvanneal furnace to produce a zinc-iron coating

• A cooling section to cool the strip and solidify the coating as it emerges from the bath

• A temper mill to impart the desired surface finish to the coated steel

• A tension leveler to flatten the strip to meet the end use requirements

• A treatment section to apply a clear, water-based treatment to the coating to prevent storage stains thatcan form on the coating surface when moisture is present (condensation and/or water infiltrationoriginating from improper shipping or storage)

• An oiling section used most often to apply a rust-inhibitive oil; at times, used to apply a forming oil

• A recoiler to rewind the finished coil of steel that now has:

• the desired steel strength and formability

• an adherent, corrosion-resistant coating• the desired surface finish

• a high level of flatness

• a clear chemical treatment and/or oil to assist with preventing degradation of the coated-sheetappearance

Below is a schematic of a hot-dip coating line.

Source: Voest-Alpine


2



GalvInfoNote 2.1

Rev 0 Jan-07


3

In most cases, the incoming steel is “full-hard” sheet coming directly from a cold reduction mill. The coldreduction mill is used to decrease the thickness of the hot-rolled pickled strip to the desired thickness. Coldrolling makes the steel very hard with limited formability. For heavier sheet thicknesses, the product may beentered into the coating line directly after hot rolling and pickling. In either case, the sheet is uncoiled andwelded to the tail end of the coil ahead of it in the processing line. It is then cleaned in a process unit thattypically uses an alkaline liquid combined with brushing, rinsing and drying. From the cleaning section, the strippasses into the heating (annealing) furnace to soften the full-hard strip and impart the desired strength andformability to the steel.

In the annealing furnace, the strip is maintained under a reducing gas atmosphere to remove any vestiges ofoxide on the steel surface. The gas atmosphere is composed of hydrogen and nitrogen. This oxide reductionstep (iron oxide converted to iron by reacting with hydrogen), is very important to obtain complete wetting

1 of the

steel surface during the short time that it is immersed into the coating bath.

The exit end of the furnace is connected directly to the molten coating bath by a “snout” to prevent any air fromre-oxidizing the heated steel strip prior to it reacting and alloying with the molten coating metal. In the coatingbath, the strip passes around a submerged roll and then exits the bath in a vertical direction. At the exit point, aset of gas knives (usually high pressure air), wipe off excess molten metal, leaving behind a closely controlledthickness of molten metal.

The coating is then cooled to allow the metal to freeze on the steel surface. Freezing of the coating, orsolidification, has to be accomplished before the strip contacts another roll to avoid transferring the coating ontothe roll. To accomplish this, coating-process lines usually have a high tower above the coating bath, perhaps ashigh as 200 feet on some of the newer lines.

Galvanneal is produced from galvanize by reheating the coated sheet above the wiping knives to alloy the zincwith the iron in the steel. See GalvInfoNote 1.3 for a detailed description of the galvannealing process.

After cooling to close to room temperature, the strip feeds into the exit end equipment - temper mill, tensionleveler, chemical treatment section, oiler, and then is recoiled on an exit-end mandrel.

The continuous strip is sheared at the weld that was made at the entry end of the line to remove the weld, aswell as to preserve coil-to-coil identity.

Not all hot-dip coating lines have all of the above processing steps. For example, some do not include the

aqueous cleaning stage, relying instead on “flame” cleaning in the entry end of the annealing furnace. Othersmight not have a temper mill; temper rolling is not necessary, and perhaps not desired, for some applications ofhot-dip coated products.

Alternate Process (Flux Coating)

There is another hot-dip process that is used in some parts of the world, including the United States, whichinvolves a significantly different approach. In this process, called the flux-coating process, the steel is annealedahead of the coating line in a separate operation, either by batch annealing or continuous annealing. Thus, thecoating line does not have the large continuous annealing furnace that was mentioned above. In the flux-coatingprocess, the steel is cleaned to remove oils, dirt, etc., and then pickled to remove the thin oxide coating that ispresent on the steel surface. After rinsing, the steel is then passed into an aqueous flux solution to apply a fluxcoating onto all areas of the steel surface.

When the flux-covered strip enters the coating bath, the chemical action of the flux behaves similar to theperformance of fluxes used for soldering; that is, the flux reactions help to obtain rapid “wetting” of the molten

1Wetting is the term used to define the reaction between the steel surface and the molten coating metal. This reaction is very

important to obtain complete coverage by the coating and good adhesion between the coating and the steel.



GalvInfoNote 2.1

Rev 0 Jan-07

coating to the steel surface. Recall that this wetting reaction is required to obtain complete coverage of themolten coating, and a good bond between the coating and the steel.

The remainder of the flux-coating process is essentially the same as that used in the process described abovefor the lines that have in-line annealing furnaces.

Some references to the flux-coating process refer to it as the “cold” process in contrast with the term “hot”

process used to define the lines that have in-line annealing furnaces.

By far, the number of “hot” lines exceeds the number of “cold” lines.

See GalvInfoNote 2.7 for a more complete description of the flux coating process.

Summary

The hot-dip process for steel sheet is used today to make a variety of hot-dip coated products, i.e., sevendifferent coating metals. These coatings include:

• Galvanize (zinc)

• Galvanneal (zinc/8-10% iron alloy)

• Two alloys of zinc and aluminum

• 55% aluminum/45% zinc alloy

• 95% zinc/5% aluminum alloy

• Alloy of zinc, aluminum and magnesium

• Two aluminum based alloys

• aluminum/5-11% silicon alloy

• pure aluminum

• Terne coating (lead/3-15% tin alloy)

As of 2006, there are approximately 90 hot-dip lines in North America. With the current exception of zinc-aluminum-magnesium, each of these lines has the capability of applying one or more of the above coatings.

See GalvInfoNote 1.2, which describes the above coatings in more detail.

Copyright© 2007 – ILZRO

Disclaimer:

Articles, research reports, and technical data are provided for information purposes only. Although the publishers endeavor toprovide accurate, timely information, the International Lead Zinc Research Organization does not warrant the research results orinformation reported in this communication and disclaims all liability for damages arising from reliance on the research results orother information contained in this communication, including, but not limited to, incidental or consequential damages.


4




GalvInfoNote

2.2

The Continuous Electroplating Process forSteel Sheet Products

Rev 0 Jan-07

Introduction

The steel sheet electroplating process utilizes the same basic principle as that for conventional decorative finish

electroplating. However, the steel sheet process differs in that the electroplated coating is applied by passing the

strip at high speeds through a series of plating cells, building the coating thickness by a small amount each time

the strip passes through an individual cell. This continuous process for electroplating steel strip requires the

necessary equipment to transport the strip at high speeds (500 to 700 feet per minute and higher) through a

series of individual plating cells, and is not as simple as it sounds. In this GalvInfoNote, some of the complexities

of the process will be covered.

An Electroplating Cell

The simplest electroplating cell is shown in this sketch where the plating solution bath is zinc sulphate.

Anode Cathode

+ - Zn

++

SO4--

Power Source

Plating Solution (Battery)

Oxygen is generated at this rod Zinc is plated onto this rod

Zinc sulphate solution

This simple plating cell illustrates the actions during the plating process. At the cathode (steel, for example), zincions dissolved in the zinc sulphate solution combine with 2 electrons and form elemental zinc, which depositsonto the cathode surface. At the anode, water is converted to oxygen and hydrogen ions to maintain electricalbalance. The oxygen forms a gas and nothing is deposited on the anode surface. The plating solution carriesthe current between the cathode and anode.


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GalvInfo Center email: [email protected]

Plating of Steel Sheet in a Continuous Process

How is this plating operation extended to the plating of steel sheet as wide as 1800 mm (70+ inches) on acontinuous basis at high speeds? Imagine a series of cells like the one above, except much larger, aligned in arow. Connect each anode/cathode set to an electrical power source. Add the necessary rolls and motors totransport the sheet between an anode/cathode set in each cell. Use an uncoiler at the entry end of the line to

feed the coiled sheet into the processing section, and a recoiler at the exit end of the line to rewind the sheetinto a coil.

Series of horizontal anodes oneach side of the steel strip

Toll-free phone: 1-888-880-8802

2

Steel sheet is the cathode

Anodes connected to powersupplies to provide current

Of course, many additional pieces of equipment and electrical controls are needed to complete the line. Tomake the process continuous, an accumulator is needed at the entry end to allow the tail end of one coil to bewelded to the head end of the succeeding coil. Alkaline cleaning to remove dirt and oils and a pickling operationto remove the fine film of iron oxide on the steel surface are important operations ahead of the plating cells.Remember, the coating is bonded to the steel by inter-atomic bonding; there is no diffusion reaction like thatwhich occurs in the hot-dip process. Thus, the surface of the incoming steel has to be very clean to achievegood adhesion.

There are many types of anode arrangements. Some are horizontal, others are vertical, and one processutilizes a radial cell wherein the strip passes around large diameter rolls inside each plating cell, and the anodeshave a radial design to match the diameter of the large rolls submerged into the plating solution. Each type ofanode arrangement and design has advantages and disadvantages; thus, it is easy to see why differentmanufacturers use different methods. Each requires very close control of the anode-to-strip spacing to achieveefficient plating, avoiding arc spots and other defects in the coating.

Maintenance of the large volume of plating solution that is contained in all the cells is a science unto itself.Whether the plating solution for electrogalvanizing is based on zinc sulphate or zinc chloride chemistry,maintenance of the proper ranges of zinc ion concentration and solution pH are important control features.Besides plating zinc, some manufacturers have the ability to deposit alloy coatings. This requires, at a minimum,at least one more level of control of the plating solution. For example, producing a zinc/nickel alloy coatingrequires close control of the concentrations of both the dissolved zinc and nickel in the solution. Solution control

has to be accomplished on a dynamic basis since these lines operate continuously.





Power Requirements

The electroplating process requires a large amount of electric power to deposit a metallic coating. The totalpower requirement is a direct function of the coating thickness that is needed to meet the customer’sspecification. For example, the power required to deposit a zinc coating mass of 80 g/m

2 is approximately twice

that required to deposit a coating of 40 g/m2. A typical line that has the capability to process 70 to 120 tons/hour

with a coating mass of 50 g/m2

will consume hundreds of thousands of amperes during this one hour ofprocessing time. It is easy to see why power costs are major cost component for a facility that processes largequantities of electroplated sheet product.

Product Types

The most common electroplated coating for steel sheet products is zinc. Electrogalvanized zinc coatings areused by a number of automotive companies for exposed car-body panels, where the typical coating massranges from about 50 to 80 g/m

2per side. These coatings are considerably thicker than the electrogalvanized

coatings typically used for non-automotive applications, so the lines built to make products for automotiveapplications usually have a large number of plating cells. Also, they have the ancillary equipment needed toproduce a high quality surface and require a large capital outlay to build. The products are included in ASTMSpecification A 879/A 879M. Also, each automotive customer has their own specific coated-productspecification.

Another attribute associated with the use of electrogalvanized coatings for automotive applications is theexcellent surface finish that is attainable with the electroplating process. Twenty-five years ago, whenautomotive companies began using large amounts of galvanized sheet for exposed body panels to improvecorrosion protection, one of the few coated sheet products that could meet the demanding surface qualityrequirements was electrogalvanized. Hot-dip galvanized was, and still is, used for unexposed body parts. As thesurface of hot-dip products improves, they continue to replace electrogalvanized sheet for exposed automotivebody panels.

Other zinc electroplating lines have been built through the years to make thinner coatings. Typically, the sheetthat is made on these lines has a coating mass of less than 25 g/m

2. The applications for this product are often

indoors; applications where the environment is not very corrosive. Many applications involve a painted product.These coating lines often have the ability to apply paint pre-treatment so that the customer can paint directly

without additional in-house treating. These lighter coating weight electrogalvanized sheet products are alsocovered by ASTM Specification A 879/A 879M.

A second type of electroplated coated-steel sheet being manufactured today has a coating composed of azinc/nickel alloy. Typically, the nickel content is 10 to 16 percent with the balance being zinc. The uniquefeature of this process is that the zinc and nickel ions are co-deposited to make a true alloy coating. It is notcomposed of alternating layers.

The application for this product has been limited primarily to a few automotive companies. These companieshave developed in-house product design and manufacturing processes to take advantage of the uniquecharacteristics of the zinc/nickel coating. For these automotive applications, the metallic coating is often coatedwith a special corrosion-resistant thin organic coating on top of the zinc/nickel. The zinc/nickel alloy coating iscovered by ASTM Specification A 918.

A third type of electroplated coating is a zinc/iron alloy coating. The attributes of this specialized coating aresomewhat like those of hot-dip galvannealed product. Like the zinc/nickel alloy, the zinc/iron coating is co-deposited as an alloy coating. The iron is uniformly deposited throughout the coating thickness. Also, like thezinc/nickel coating, the zinc/iron coating is used predominantly by the automotive industry.

The attributes of electroplated zinc/iron is that it is relatively easy to weld and paint if the proper electro-primingequipment is available to the automotive manufacturer. Also, the coating is very hard; making it is less


3





susceptible to scratching during stamping and handling. This is an important feature since the zinc/iron alloycoated-sheet product is being used almost exclusively for exposed car-body panels.

Corrosion Resistance of Electroplated Coatings

Concerning the corrosion behaviour of an electrogalvanized versus a hot-dip galvanized coating, it is importantto note that it is essentially equivalent for identical coating masses. A coating mass of 100 g/m2 will provideessentially the same amount of corrosion protection whether it is a hot-dip galvanized or electrogalvanizedcoating. See GalvInfoNote 3.1 for more information on how zinc protects steel.

The reason that the automotive companies can successfully use a coating mass in the 50 to 80 g/m2 range is

because they apply additional treatments on top of the metallic coating, including a zinc phosphate coating, anelectro-deposited organic-based coating, a primer, and multiple-layer finishing paint coatings. Clearly, thecorrosion resistance needed to protect a car body panel for over 10 years is more than that afforded by themetallic coating alone. Application of the above coatings over the electroplated metallic layer results in asynergistic system, whose corrosion resistance is more than the sum of its individual components.

Summary

Electroplated zinc- and zinc-alloy coated sheet products are a special type of metallic-coated steel. Theapplications involve either a coating mass of 50 to 80 g/m2 (per side), or a coating mass of less than 25 g/m2.The heavier coatings are used for automotive applications predominantly while the lighter coating masses areapplied for applications (often indoor) that do not require a high degree of corrosion protection.

Remember that the conversion from g/m2 to oz/ft

2 is approximately: 305 g/m

2 = 1 oz/ft

2. Compared with a G90

hot-dip coating, even the heaviest coating masses of electrogalvanized product are considerably less thanmuch of the hot-dip galvanized product in use today for exterior applications. The capital expenditure required tomanufacture electroplated zinc coating equal to a G90 coating would be prohibitively expensive as would thepower costs during production.

In summary, electroplated coatings on steel sheet have found unique applications in the industries using steelsheet products. The high quality surface of the electroplated product, combined with the fact that a coating massof 50 to 80 g/m

2 is sufficient to meet the corrosion requirements, make electroplated sheet products ideal for

exposed panels on a car. Also, the other category of electrogalvanized coating, sheet with a coating mass ofless than 25 g/m

2, is ideal for relatively non-corrosive applications. Production of an equally thin hot-dip coating

is not practical.


Disclaimer:



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1


GalvInfoNote Continuous Hot-Dip Galvanizing versusGeneral (Batch) Galvanizing

Rev 0 Jan-072.3

Introduction

There are two different processes for applying a zinc coating to steel by the hot-dip method. Both involve

immersing the steel in a bath of molten zinc. Since zinc melts at 419°C (787°F), and must be further heated to a

temperature of approximately 455°C (850°F) or higher for the galvanizing process to be effective, both

operations are referred to in general as the “hot-dip” process.

One process involves the application of zinc onto a continuous ribbon of steel sheet as it passes through a bath

of molten zinc at high speeds – hence, the term “continuous” hot-dip galvanizing. As a coil is processed through

the coating line, another is welded to its trailing end. The process is truly “continuous”, as the line may operate

for days without interruption. The other process involves the application of a zinc coating to the surface of steel

parts after they have been fabricated. It is not continuous in that the parts are immersed as a discrete “batch”

into the zinc bath; hence, the names “batch”, “after fabrication” or “general” galvanizing - terms that are used

interchangeably. Parts as small as fasteners, to as large as bridge structural girders, are galvanized by thebatch method.

Continuous Galvanizing

As briefly described above, the continuous galvanizing process applies a zinc coating to the surface of a

continuous ribbon of steel sheet as it passes through a zinc bath. The coated sheet coils are either directly roll-

formed or fed into stamping presses, or blanked/sheared and then formed into parts. The sheet thickness might

be as thin as 0.25 mm (0.010 inch) or less, to as thick as 6.3 mm (0.25 inch). The facilities in operation world-

wide are typically “light-gauge”, “intermediate-gauge” or “heavy-gauge” coating lines. Product from light-gauge

lines is used mostly for applications in the construction industry (roofing sheets, building sidewall panels,

flashing, etc.) The largest application for product made on intermediate-gauge lines is automotive body panels.

Product from heavy-gauge lines is used for culvert, automotive structural parts, grain bins, etc.

In this process, the steel sheet is passed through the molten zinc bath at speeds as high as 200 meters per

minute (>600 feet per minute). As the moving sheet exits the coating bath, it drags out molten zinc. The desired

coating thickness is attained by the use of “gas knives”. These knives typically use air as the gas, and are

directed at both sides of the sheet to remove excess zinc. The coated steel is then cooled and the zinc solidifies

on the surface of the sheet.

The continuous galvanizing process for producing coated steel sheet involves a series of complex

steps, one of which is to anneal the steel to soften it and make it more formable. More details about the

continuous galvanizing process are given in GalvInfoNote 2.1.

One of the most important features of the continuous galvanizing process is the formation of a strong bond

between the steel and its zinc coating. At the processing speeds used on continuous galvanizing lines the strip

is only in the zinc bath for 2 to 4 seconds. During this brief time the molten zinc and steel must react to form a

strong metallurgical bond by way of diffusion. The bonding region is an intermetallic compound, termed the

“alloy layer”.

This thin alloy bonding zone, which is usually only 1 to 2 micrometers thick, is critical because after the coating

is applied and the sheet cooled to room temperature, it is recoiled and shipped to customers for forming into the

desired shape. For example, the sheet might be deep drawn to form a canister, it might be stamped into a car





2

fender, or it might be roll-formed into a building roof panel. For the forming operation to be done successfully,

the steel and zinc have to be well bonded to each other. If the bond zone is not formed, or not formed correctly,

the steel and zinc would not “stick” together during the many critical forming steps that the coated sheet might

undergo.

An adherent and formable bond zone requires that the alloy layer be thin and of the correct composition. This is

because the intermetallic compound that the bond layer consists of is very hard and brittle, an inherent

characteristic of such alloy layers. There is no metallurgical process that will make the bond zone soft and

ductile. By producing a thin alloy layer of the correct composition, the coated steel sheet can be formed into

many intricate shapes without loss of adhesion between the steel and zinc coating. If the alloy layer becomes

too thick, or is of the wrong composition, cracks develop in it during forming and the steel and zinc coating may

disbond when formed. A thin alloy layer of the correct composition can be bent and stretched without cracking

and disbanding.

In summary, it is very important for the steel and zinc to form a proper bonding zone, and that this zone is thin.

This is readily accomplished by the producers of hot-dip galvanized sheet by focusing on two primary control

points:

1. the addition of a controlled amount of aluminum (approximately 0.15 to 0.20%) to the molten zinccoating bath, and

2. control of the steel sheet temperature at the point where it enters into the molten zinc and control of the

temperature of the zinc coating bath.

The impact of the addition of aluminum to the zinc coating bath used for continuous hot-dip sheet galvanizing is

covered in detail in GalvInfoNote 2.4. It is a complex issue that needs to be discussed as a specific topic.

Nevertheless, when the process is properly controlled, the coated steel sheet made by the continuous hot-dip

galvanizing process is a well engineered product; one that is being used for the manufacture of many

sophisticated end products.

General (Batch or After Fabrication) Galvanizing

The second hot-dip process involves the application of zinc onto a “fabricated” shape. This means the steel isshaped into the final product; a structural beam, a large diameter pipe, or a small fastener, and then dipped intomolten zinc to apply the zinc coating. These items are coated either one at a time or, in the case of small parts,as a number of parts contained in a “basket”. Hence, the terms “batch” or “after fabrication” are used to describethis process.

In a sense, the general or batch process is the same as the continuous process in that the objective is to apply

an unbroken coating of corrosion resistant zinc onto the surface of steel. However, these two methods have

many differences.

The typical batch process involves three steps prior to the immersion of the part(s) into the molten zinc bath:

• Caustic cleaning

• Pickling

• Fluxing

Caustic cleaning involves the use of a hot alkali solution to remove organic contaminants such as oils and

greases. These surface contaminants need to be removed prior to pickling so that the surface can be “wetted”

by the pickling solution.





3

Pickling involves the immersion of parts into an acid solution (typically heated sulphuric acid or ambient

temperature hydrochloric acid) to remove surface scale or rust (both oxides of iron). The term “scale” is typically

used to describe the oxides of iron that form at high temperatures such as during hot rolling, annealing in air, or

welding. Rust is the product of corrosion of the steel surface when it gets wet. Both types of iron oxide need to

be removed prior to the application of the zinc coating.

Fluxing involves the application of a special chemical coating onto the surface of the steel part. This “flux”

serves the same purpose as fluxes used during soldering operations. The fluxing chemical (zinc ammonium

chloride) is designed to chemically remove the last vestiges of oxides just as the steel is being immersed into

the molten zinc, and allow the steel to be wetted by the molten zinc. Fluxing can be either “dry” or “wet”. Dry

fluxing involves immersion of the steel part into an aqueous solution of the flux. Upon removal, the flux solution

is dried prior to immersion into the zinc bath. (Note that there is a continuous galvanizing process that uses dry

fluxing. It is described in GalvInfoNote 2.7). In wet fluxing, a blanket of liquid (molten) zinc ammonium chloride

is floated on top of the molten zinc bath. The part to be coated is then immersed through the molten flux as it is

being introduced into the coating bath. (Wet fluxing works because zinc ammonium chloride has a melting point

below that of molten zinc and is less dense than molten zinc, thereby floating on the bath surface).

As with continuous galvanizing, the application of the zinc coating in batch galvanizing involves immersion of the

steel into a bath of molten zinc. However, in contrast with the continuous process wherein the steel is immersedfor a very brief time, the batch process requires that the part be immersed for much longer times, typically

measured in minutes, not seconds. There are two reasons for needing longer immersion times. One is to allow

the part to reach the bath temperature. Immersion of a relatively cold thick-walled large pipe, for example,

results in a skin of zinc freezing onto its surface when it is first immersed. For the coating to bond metallurgically

to the steel, the pipe has to reach the bath temperature to “remelt” the zinc. After this, additional time is required

to develop the iron/zinc alloy bond zone.

Unlike the continuous process, where the alloy layer has to be kept very thin to accommodate subsequent

forming into the final shape, for batch-galvanized parts the alloy layer can be allowed to grow much thicker. In

fact, a thicker alloy bond layer is often desired to provide a longer life to the final product, i.e., a longer time

before the onset of rust. Like zinc itself, the alloy layer is galvanically protective to the steel part and a thicker

alloy layer means longer life. Yes, the alloy layer is hard and brittle, but since the part is already fabricated, there

will be no additional forming that can crack the alloy. The brittle alloy layer is not deleterious. It will not result incoating damage during shipment and subsequent handling at the jobsite. A representative photomicrograph of

the alloy layer that forms while the steel is immersed in the bath is shown in Figure 1. As can be seen in this

photo, the alloy layer is as much as 50% of the total coating thickness and it actually consists of two or more

distinct zinc/iron layers. Each of these distinct layers combines to form the “total” alloy layer zone. Each layer

actually has a specific amount of iron and zinc. The layer closest to the steel has the highest iron content while

the layer immediately adjacent to the pure zinc outer layer has the lowest iron content. The composition and

properties of these alloy layers are shown on Table 1.





4

Figure 1: Cross-section of a batch hot-dip galvanized coating.

Remember, the alloy layer grows by an intermixing diffusion reaction between the atoms of the steel and zinc.

This is a time dependent process, and for most steels, a longer immersion time provides a thicker alloy layer. In

fact, for batch galvanized parts, additional immersion time is often needed to achieve the final required thickness

of the protective coating (the thickness is a combination of the alloy layer and the pure zinc outer coating metal).

Table 1: Composition and Properties of Alloys Layers in Batch Hot-Dip Galvanize

Layer Alloy Iron, % Melting Point

Crystal Structure Alloy

Characteristics °C °F

Eta (η) Zinc 0.03 419 787 Hexagonal Soft, ductile

Zeta (ζ) FeZn13 5.7-6.3 530 986 Monoclinic Hard, brittle

Delta (δ) FeZn7 7.0-11.0 530-670 986-1238 Hexagonal Ductile

Gamma (Γ) Fe3Zn10 20.0-27.0 670-780 1238-1436 Cubic Thin, hard, brittle

Steel Base

Metal

Iron 99+ 1510 2750 Cubic ------

As a result of the ability to accommodate long immersion times, the final thickness of the coating (pure zinc +

alloy layer) on batch galvanized parts is often considerably thicker than the coating on continuous galvanized

sheet product – at least the thickness can be much thicker if desired/required. This is one major difference

between the batch galvanizing process and the continuous galvanizing process.

Eta

Zeta

Delta

GammaSteel





5

There are production issues that often need to be considered with respect to the maximum alloy layer thickness

that can be achieved during batch galvanizing. As the alloy layer thickens, its rate of growth slows down

because diffusion through the thickening alloy layer takes longer, resulting in a practical limit to the final

thickness. Also, for some steel compositions, the uniformly thickening alloy bond does not form on the surface.

Instead, the alloy grows to a certain thickness and then begins to spall off the steel surface. When this type of

behaviour is experienced, the practical maximum coating thickness is less than when the alloy continues to

grow as a compact layer.

Zinc Bath Composition for General (Batch) Galvanizing

Historically, the zinc bath used for general galvanizing contained between 0.5 and 1.0% lead. The leadhad two effects. First, it caused the formation of the typical, attractive large spangled surface, whichthrough the years was “the way to identify galvanized coatings”. Second, the lead was beneficial toaccommodate “free drainage” of excess zinc as the part was removed from the zinc bath. In someinstances today, bismuth is being substituted for lead to achieve free drainage of the excess zinc. Alloysthat contain bismuth for use by the general galvanizing industry are available today from a number of zincsuppliers.

Another alloying addition to zinc that is receiving some attention today as a way to further improve the

coating performance is the addition of nickel to the galvanizing bath. The influence of nickel is importantwith respect to the development of the zinc/iron alloy layer, especially when galvanizing high silicon-containing steels. This development is still quite new and the metallurgical aspects related to the additionof small amounts of nickel are still being discovered.

The addition of 0.15 to 0.20% aluminum to the coating bath – a required addition to the bath whencontinuous galvanizing – is not a typical practice for general galvanizing. In general galvanizing, thedevelopment of a thick alloy layer is important to the achievement of the required coating thickness.

Aluminum acts as an inhibitor and interferes with this action.

Part Thickness

Another difference in the two processes, batch vs. continuous, relates to the thickness of the steel thatcan be galvanized without experiencing “heat distortion” of the steel. In the continuous process, very thinsteel can be coated. The reason that this can be accomplished is that during continuous galvanizing, thesteel sheet is held under some amount of tension while being processed. Tension needs to be applied to“pull” the ribbon of steel through the coating line, and to maintain the flatness of the sheet. Distortion ofthe sheet can occur during exposure to the high annealing temperatures. Tension prevents distortion, andallows a controlled, even application of zinc onto very thin sheet, which otherwise would not be possible ifit were off-flat.

In the batch process, the products immersed into the coating bath are not constrained by the applicationof outside forces. The part has to be designed to be dimensionally stable during the exposure to the bathtemperature. This is accomplished by using both thicker steels and part design principles that preventheat-generated distortions. Also, temporary bracing can be used for thin-walled parts to minimizedistortions caused by the heating. Stated simply, it is not easy to batch galvanize fabricated parts madefrom thin steel sheet, nor is it easy to continuous galvanize heavy steel plate.

For more extensive information on the after fabrication general galvanizing process, visit The AmericanGalvanizers Association at www.galvanizeit.org/ .





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Summary

Both continuous and batch galvanizing processes have been in use for many years. Both processes provide acorrosion resistant zinc coating that has been a proven value-added method for protecting the steel substrate ina multitude of applications. Through the years, both processes have undergone advances in technology thatcontinue to expand the markets for galvanized steel.

The two processes described in this GalvInfoNote are generally applicable to different spectrums of steelthickness. Yes, there is some overlap in that thick steel can be continuous galvanized and thin steel can bebatch galvanized, but to a large extent, the two processes are complementary and allow the protective nature ofzinc to be used for a wide range of steel products.


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GalvInfoNote The Role of Aluminum inContinuous Hot-Dip Galvanizing

Rev 1.1 Mar-092.4

Introduction

Continuous hot-dip galvanized sheet is manufactured almost exclusively on coating lines where the

molten zinc contains a small amount of aluminum. This has been the case for many years and is, in fact,

the primary reason that galvanized sheet is used today for a wide variety of very demanding products.

The addition of aluminum is not made to enhance corrosion performance, but to ensure good coating

adhesion during forming of the sheet. This GalvInfoNote explains the influence of aluminum and why it is

so important to the successful manufacture and use of continuous hot-dip galvanized sheet.

Metallurgy of the Coating

Continuous hot-dip galvanized sheet is made by immersing steel sheet, as a continuous ribbon, into abath (pot) of molten zinc. The process is explained in detail in GalvInfoNote 2.1 and again in GalvInfoNote

2.3. Both sides of the steel sheet are very clean and free of surface oxides when introduced into the

coating bath. Typically, the sheet is cold rolled steel and receives an in-line anneal at temperatures above

1200°F [650°C] ahead of the pot, and is then cooled to approximately 875-925°F [470-490°C] before it

enters the bath. The zinc, which melts at 787°F [419°C], is usually at a temperature of 870°F [465°C].

Steel sheet has sufficient high temperature strength so that it can be pulled through both the annealing

furnace and the zinc bath without tearing or deforming.

During the time that the sheet is immersed in the bath (on some coating lines, as brief as ~2 seconds),

the steel and molten zinc undergo a metallurgical reaction. What happens? The surface atoms of the

steel sheet, which are in the solid state, interact with the zinc atoms in the bath, which are in the molten

state. This interaction is called diffusion. Zinc atoms move in the direction of the steel and iron atoms in

the steel migrate towards the molten zinc. The result is the formation of a solid “mixed” layer between thesteel and the molten zinc. This layer contains zinc and iron atoms in specific proportions, and is called an

intermetallic compound. The mixing of atoms of different metals is known as alloying and the diffusion

zone that is formed during galvanizing is an intermetallic alloy. It is this alloy zone, when properly

formed, which provides the excellent bond between the steel and the zinc coating.

Surface tension forces cause an outer layer of molten zinc to adhere to the sheet when it leaves the

molten bath. After excess zinc is wiped off, the remaining liquid solidifies when it cools below 787°F

[419°C]. The final product (galvanized sheet) consists of the steel core, with an intermetallic alloy layerand outer zinc layer on both surfaces.

If the zinc bath was aluminum free, a cross-section of the coating might look similar to that in Figure 1.





2

The intermetallic alloy layers shown in Figure 1 are a mixture of zinc and iron atoms. They provide a high

degree of bonding between the steel and the zinc outer coating. Unfortunately, these alloys have very

poor ductility, i.e., they are very hard and brittle. When the sheet is formed into a shape, there is a high

probability of shear cracks developing in the alloy, and the zinc coating flaking off. This behaviour

seriously limits the ability to form the sheet into shapes such as drawn cups, roofing panels, tight

lockseams, or highly stretched automotive fenders.

Overcoming the Brittle Alloy Layer

An alloy layer is vital to achieving a good bond between steel and zinc. This layer must also be

continuous (over the entire surface area of the sheet) for the coating to be free from pores. Without

interfering with the formation of an alloy bond zone, how can the nature of the alloy be changed so thatforming into intricate shapes is possible?

Over 75 years ago, it was discovered that the addition of a small amount of aluminum to the coating bath

is a perfect answer to this problem. Initially, the reason it worked so effectively was not understood, but it

was observed that having aluminum in the zinc bath made the alloy layer very thin compared to that from

an aluminum-free bath. Aluminum is an inhibitor that greatly slows down the zinc-iron reaction rate. This

thinner, and hence more ductile, alloy layer allows the coated sheet to be formed into many complex

shapes without loss of coating adhesion, since it is not prone to the development of large internal shear

cracks.

Using aluminum, at a level of approximately 0.15%, became the standard for galvanizing baths in

continuous lines. To this day an aluminum addition practice is used; however there is now a much better

understanding of the metallurgy of aluminum in zinc, with the result that the aluminum concentration ismore closely controlled. Some manufacturers use as much as 0.20 to 0.25% aluminum, but the standard

practice involves the use of about 0.15 to 0.19%. When making zinc-iron coatings (galvanneal), the

aluminum level is lowered to the range of 0.12 to 0.14%.

Zinc outerlayer

Intermetallic

alloy zone

Steel

Figure 1: Cross section of coating produced in an aluminum-free galvanizing bath





3

Although the addition of such a small amount of aluminum has a pronounced effect on the ability to form

galvanized sheet, it does not have much effect on other attributes of the product. For example, its

influence on bulk corrosion behaviour is insignificant. Because the aluminum tends to concentrate in the

alloy layer, and to some extent at the surface of the zinc, it can adversely affect issues such as spot

welding, soldering, and white rust occurrence. However, these drawbacks are insignificant in comparison

with the beneficial effect that aluminum exerts on the ability to form the sheet without loss of coating

adhesion.

Why Aluminum Changes the Alloy Layer

How can this small amount of aluminum have such a pronounced influence on the alloy layer growth

rate? It is because when aluminum at this level is in the coating bath, the normal zinc-iron alloy

compound, FeZn7, that forms on the steel surface (and grows at a fast rate), is no longer the most stable

compound. Aluminum has a greater affinity for iron than zinc, so immediately (within 0.15 seconds)i after

the steel enters the coating bath, the stable intermetallic compound that forms is not a zinc-iron

compound, but an aluminum-iron compound, i.e., Fe2 Al5. This alloy layer (also known as a barrier or

inhibition layer ) is extremely thin and retards the zinc-iron reaction. By the time the strip leaves the bath

(~2-4 seconds later) some zinc is taken into this alloy layer, but its nature is completely different from that

which occurs in the absence of aluminum. It is a very thin, ternary alloy layer composed of approximately

45% Al, 35% Fe and 20-35% Zn (Fe2 Al5-XZnX). Instead of the high diffusion rate that occurs when liquid

zinc and solid iron form a binary FeZn7 alloy in aluminum-free baths, the diffusion rate is now dependent

on the transport characteristics of zinc through the barrier created by the aluminum-iron compound. The

reaction between zinc and iron is delayed, and the net result is that the final thickness of the alloy layer is

much less that when it is dependent on the diffusion rate across a growing zinc-iron alloy zone.

The nature of the alloy layer, when aluminum is added to the galvanizing bath, is shown in Figure 2. The

alloy layer is the thin layer seen near the bottom of the figure. When the alloy layer is this thin, the coated

sheet can be bent or formed into many useful shapes without the alloy layer cracking and resulting in loss

of coating adhesion.

Figure 2: Cross section of galvanize coating from an aluminum containing bath (magnification 7000X)

Zinc coating layer

Thin, ternary alloy

Steel





4

What a discovery! This development single-handedly allowed the growth of the large continuousgalvanizing industry, which now produces over 25 million tons per year of galvanize and galvanneal inNorth America alone. The product of these lines is used in many applications, including those that requireit to accommodate very severe forming.

Aluminum Content in the Coating

Recall that for the production of galvanize, the coating bath contains 0.15-0.17% aluminum. When acoating made from such a bath is later analyzed, it is found to have bulk aluminum content of from 0.25 to0.40%. How does this apparent increase occur? The answer lies in the strong affinity aluminum has foriron. The initial alloy that forms is Fe2 Al5 – by weight over 55% aluminum. Aluminum actuallyconcentrates at the steel zinc interface and is taken out of the bath with the strip. The thickness of theinhibition layer is independent of coating weight (mass). This is why a lighter coating weight (mass)contains a higher overall percentage of aluminum. The rate and method of aluminum addition to the bathmust take into account situations that cause its removal rate to vary, e.g., coating light gauge sheet (highsurface area) with a thin zinc coating removes aluminum at a much higher rate than running heavy gaugesheet with a thicker coating. There are other factors that control the amount of aluminum in the coating,such as: immersion time, aluminum addition rate, zinc bath temperature, and steel type. Fossen et aldiscuss these topics in a paper given at Galvatech ‘95

ii. All these factors must be taken into account

when planning how to replenish the aluminum in the bath. Most galvanize producers, with the assistanceof their zinc suppliers; have developed aluminum addition algorithms for their zinc pots. These predictionmodels forecast aluminum levels depending on product mix and stipulate how to add aluminum bearingzinc to keep the aluminum in the bath at the desired concentration.

In addition to the high percentage of aluminum in the inhibition layer, the outermost atomic layers at thezinc surface contain a significant amount of aluminum, mostly in the form of aluminum oxide. This occursdue to solute rejection during freezing of the zinc coating, which forces aluminum to the surface. In orderthat they can bond to the zinc, chemical passivation and prepainting treatments are designed to removethe few high aluminum-containing atomic layers at the surface.

Aluminum in Galvannealed Coatings

Making galvanneal involves growing zinc-iron alloy compounds throughout the coating, so that it has

approximately 9 to 10% bulk iron content. Refer to GalvInfoNote 1.3 for a complete description of howgalvanneal is made. Since the presence of aluminum dramatically restricts growth of the alloy layer, how

does this affect the production of galvanneal?

The reheating of the strip that is necessary to produce galvanneal restarts the zinc-iron diffusion reaction.

Within seconds the heat breaks down the aluminum-iron inhibition layer. For this to happen consistently

requires aluminum control that was not possible in the early days of continuous galvanizing. Producing

galvanneal depends on the zinc and steel alloying at a rate high enough so that complete diffusion of iron

throughout the coating can be accomplished in a reasonable timeframe (allowing production to be done at

economical rates). Inhibition layers formed in galvanneal zinc baths are generally thinner than those

formed in galvanizing baths1. In fact, the inhibition layer formed in higher aluminum galvanizing baths may

not allow the galvanneal reaction to proceed properly in the time allotted. When the aluminum

concentration in the bath is reduced to less than 0.15% (in the range of 0.12 to 0.14%), the ternary alloy

layer is thin enough, and of the right composition, that reheating allows the required amount of diffusion to

occur in a matter of seconds.

After the coating has been converted to galvanneal the aluminum-iron alloy interface layer no longer

exists. It, and the small amount of extra aluminum at the surface, becomes diffused into the zinc-iron

coating.





5

A manufacturer making both galvanize and galvanneal products on the same coating line typically uses0.15 to 0.19% aluminum for galvanized production, and then allows the aluminum level to drop to lessthan 0.15% to make galvanneal. In practice, this is not as easy as it appears, since the transition needs tobe accomplished in a rapid timeframe. For this, precise aluminum control is needed, requiring accuratemeasuring capabilities.

Summary

The inhibiting action of aluminum in a galvanizing bath was an important discovery. Galvanizing lines

throughout the world now use aluminum in their zinc baths. Through the years, much has been learned

about the metallurgy of aluminum in galvanizing baths, which in turn has greatly influenced processing

practices. The use of aluminum to reduce the thickness of the alloy layer is the one development that

made continuous hot-dip galvanizing the large industry that it is today. For more detailed information on

aluminum in continuous galvanizing line zinc baths, refer to the associated GalvInfoNote 2.4.1 Zinc Bath

Management on Continuous Hot-Dip Galvanizing Lines.

Copyright© 2009 – IZA

Disclaimer:


i Price, S.E., et al, Formation and development of aluminum inhibition layers during galvanizing/galvannealing, La Revue de

Metallurgie-CIT, Mars 1999, pp. 381-393ii

Fossen, E., et al, Aluminum Control on Stelco’s Z-Line, Proceedings Galvatech ’95, pp. 795-800




1


GalvInfoNote

2.4.1

Zinc Bath Management onContinuous Hot-Dip Galvanizing Lines

Rev 1.0 Aug-09

IntroductionThis GalvInfoNote expands on Note 2.4 to provide more detailed information on the management of the

zinc bath chemistry and other parameters needed to produce high quality zinc-coated sheet products on

continuous coating lines.

General

• Control of the steel-zinc reaction has a major effect on coating quality, i.e., coating adherence, formability,weldability, uniformity, and appearance. In successfully managing these important characteristics, it is

also vital that the strip surface be free of oxides and soils as it enters the zinc bath.

• A clean strip surface, free from iron debris, is also critical to minimizing dross formation.

• In a zinc bath without aluminum, there is a high diffusion rate between the molten zinc and any immersedsteel, with the alloy layer growing very fast. This is the case in batch galvanizing. Various brittle, binary

FeZn intermetallic alloys immediately form, resulting in poor coating adherence if the material is laterformed. It is for this reason that batch galvanizing is done after all fabrication/forming is complete.

• Small amounts of aluminum added to the zinc act as an inhibitor, greatly restricting the rate at which the

zinc-iron alloying reaction proceeds in the early stages of immersion.

• When aluminum is at the correct level, the zinc reaction with the steel instantly forms a very thininterfacial, ternary alloy layer (see Fig 2 in Note 2.4) with a composition of 45% Al, 35% Fe, and ~20% Zn(Fe2 Al5-XZnX). The extent of this ternary alloy reaction (and the amount of zinc incorporated into it) is very

sensitive to the amount of Al in the bath, to the immersion time, and to the bath and strip temperatures.

• During the typical immersion times in a CGL zinc bath (2 to 8 seconds depending on line speed) theternary alloy remains stable if there is sufficient Al present. However, a high zinc temperature and/or longimmersion time will begin to overwhelm and consume any ternary layer. Obtaining a ternary composition

with a low percentage of zinc is extremely important for good coating adherence.

• Dross is a by-product of hot-dip galvanizing and consists of iron-containing particles that form in the zinc

bath, and which can detract from the coating quality. The particles can be aluminum-iron-zinc (top dross,or !-Fe2 Al5-XZnX) or zinc-iron (bottom dross, or "-FeZn7), with the iron coming from strip dissolution and

any iron fines on the surface.

Understanding Effective Aluminum (AlEFF)

• Total aluminum (AlTOT) is all of the Al in the bath; Al in solution in zinc and Al that is part of all the

intermetallic dross phases. AlTOT is determined by chemical analyses of bulk bath samples.

• Effective aluminum (AlEFF) is the aluminum dissolved in the molten zinc, and is the determining factor informing the desired coating microstructure

1. It does not include the aluminum that is tied up in

intermetallic dross particles, since that is not available to react with the iron in the steel strip. Usingcomputer programs to make necessary adjustments for bath temperature and Fe level, AlEFF is measuredindirectly using techniques such as AA, OES, or ICP.

• Aluminum changes the solubility of iron in molten zinc:

• Increasing Al decreases the amount of Fe that the zinc can hold without precipitating top drossparticles in the bath.

• Zinc with no Al at 860°F [460°C] can hold in excess of 0.035% Fe in solution, whereas with Alat 0.20% it can only hold about 0.010% Fe in solution (refer to Figure 1).



GalvInfoNote 2.4.1Rev 1.0 Aug-09


2

Figure 1 – Phase diagram of the Zn-rich corner of the Zn-Fe-Al system at 860°F [460°C] as per reference 1

• When producing galvanize (! + L region of Fig 1), determining AlEFF consists of plotting the bath analysis

results for Al and Fe on a phase diagram that is correct for the bath temperature at the time of sampling. A tie line is then drawn parallel to the intermetallic composition line, as shown in Fig 2. In this example,

AlEFF is the intersection of the tie line drawn from the 0.20% Al/0.035% Fe point, parallel to the !

“composition” line (see Fig 1), to where it intersects the Fe/Al solubility line at 0.175% wt% Al. Again, thisis only correct for a given temperature (in this case 860°F [460°C]) because the solubility curves shift upor down with the bath temperature.

Figure 2 – Determining AlEFF using the Zn-Fe-Al phase diagram (Courtesy of Xstrata)

• It is important to emphasize that the solubility diagrams vary as a function of temperature, which affects

the determination of AlEFF. To obtain correct AlEFF results the sampling temperature must be known.

• Because of the effect of temperature on AlEFF, it is impractical to use multiple solubility curves for itsdetermination. Fortunately, zinc suppliers have developed computer tools that can determine AlEFF for thecomplete range of bath chemistries and operating temperatures used by galvanizing companies.





3

Zinc Bath Operations

• Coatings with good surface quality, and having a proper composition and appearance, require a stablebath composition/temperature and effective control of dross build-up.

• Most CGL lines today use lead-free zinc, resulting in galvanize with a spangle free, smooth appearance.

Bath Compositions

• Total aluminum (AlTOT) – typically 0.16 to 0.20% for galvanize, and 0.11 to 0.14% for galvanneal.

• Iron (Fe) – typically 0.015 to 0.03%.

• Lead (Pb) – typically zero (0.007% max); may be up to 0.10% if a spangle is desired.

• Antimony (Sb) – typically zero; may be up to 0.10% if a spangle is desired.

• Zinc (Zn) – Balance.

NOTE: Most galvanized sheet produced in the western world is spangle-free, i.e., zero Pb or Sb.

Bath and strip temperatures

• Bath temperatures range from 855 to 880°F [455 to 470°C] (typically 865-870°F [463 to 465°C]).

• Incoming strip temperature runs from 800 to 900°F [425 to 480°C], although the aim is to be at, but nomore than, 5°F [2°C] above the bath temperature in the case of ceramic pots. This brings just enoughexcess heat into the bath so as to minimize the need to operate the inductors to keep the zinc at the

correct temperature. The bath is more quiescent when the inductors are not operating.

• Whatever temperature practice is employed, it is important to maintain it consistently in order to avoid

zinc temperature fluctuations. The reason for this is explained in the section on dross control.

• A high entry temperature can result in an uncontrollable rise in bath temperature, plus zinc dust build-

up in the snout – leading to surface defects.

• An entry temperature that is too high can also result in more enrichment of Al in the ternary alloy layer,causing a higher depletion rate of Al from the bath and overall poor Al level control. It also results inmore dissolution of the steel strip – leading to more dross generation.

Producing Galvanize (GI)

• Effective Al levels above 0.14% produce adherent coatings. When AlEFF is below 0.14%, binary FeZnintermetallics can form, which are brittle and can lead to poor adherence.

• At 0.14% AlEFF, the entire zinc coating (including the ternary alloy layer) contains about 0.20% Al. The

higher the Al level is above 0.14%, the higher the bulk Al level in the coating. If the bulk Al content ofthe coating reaches levels above 0.30%, spot weldability problems could result.

• When Al in the bath is at the appropriate level (0.14% and higher), the galvanizing reaction forms a thin,interfacial, ternary alloy layer on the steel having a composition of 45% Al, 35% Fe, and 20% Zn(Fe2 Al5-XZnX).

• Spangles are the zinc crystals or grains formed during solidification of the coating. The difference incrystal orientation from spangle to spangle manifests itself as variations in reflectivity. Spangles are

thickest at the center and thinnest at the grain boundary, more so in the case of large spangles.

• Pure zinc freezes on most steel substrates with a very small (< 0.5mm diameter) spangle that is barelydiscernible to the naked eye, resulting in a very smooth and evenly reflective surface. To achieve alarger spangle, additions of lead or antimony must be made to the zinc bath. These elements have theeffect of reducing the number of nucleation sites, and/or increasing the dendrite growth velocity,allowing spangles to grow larger before touching their neighbors. Refer to GalvInfoNote 2.6 for more

information on spangles.• Lead reduces surface tension and enhances fluidity and wettability. It increases the propensity for

crazing at bends and can cause intergranular corrosion and delayed adhesion failure. Lead increasesthe tendency for sagging of the coating at low line speeds and produces a high relief spangle. At levelsof 0.10% - 0.15% the spangles can be very large (~ 25 mm diameter). The small spangle that results

from lead free zinc is very flat and easy to convert to extra smooth by temper passing.

• Antimony also reduces surface tension and enhances wettability. Its presence can result in a brittlecoating if the aluminum level is not proper. Antimony produces a smaller spangle than lead, all other

factors being equal.





4

• If the strip is too hot at bath entry it causes Al content in the coating to increase, while if it is too cold the Al in the coating will decrease. It is very important to control the strip temperature as it enters the zincbath. Ideally the strip should be at the liquid zinc temperature to no more than 5°F [2°C] above it.Operating in this fashion on lines with a ceramic pot brings in a small amount of extra heat to minimize

the need for the inductors to be on, helping to keep the bath quiescent.

• The reason for the above behavior is the strong affinity between Al and Fe, the speed of the reaction,

and the effect that temperature has on this reaction. At a given bath Al level and temperature, thesame amount of ternary alloy instantly forms, regardless of strip speed or total coating weight.

• Since the amount of Al extracted from the bath by the ternary alloy layer is independent of line speedand coating weight, it is important that operators know the rate of Al extraction at all times so that it canbe replenished. As stated in GalvInfoNote 2.4, most galvanize producers, with the assistance oftheir zinc suppliers, have developed aluminum addition algorithms for their zinc pots. Theseprediction models forecast aluminum levels depending on product mix and stipulate how to add

aluminum bearing zinc to keep the aluminum in the bath at the desired concentration.

Producing Galvanneal (GA)

• Galvanneal is a zinc-iron alloy coating that is made by reheating the strip from as low as 820°F

[438°C] to 935 to 1050°F [500 to 565°C], and holding in this range for about ten seconds. The

zinc is still liquid when the strip enters the galvanneal furnace. Reheating restarts the zinc-iron diffusionreaction, breaks down the inhibition layer that formed while the strip was in the zinc bath, and after 5 to10 seconds a dull gray matte coating is created. This coating consists of intermetallic zinc-iron alloylayers having an overall bulk iron content of between 9 and 12%. See Table 1 in GalvInfoNote 1.3for the composition of the alloy phases in a galvanneal coating.

• The AlEFF level in the zinc bath when making galvanneal is typically 0.11% to 0.13%. This level islower than that when making galvanize in order to produce an inhibition layer that breaks downeasier during conversion of the coating to galvanneal. Higher Al levels would require more heatduring conversion, producing coatings with a higher iron level that would be susceptible topowdering. Keep in mind that the AlEFF level used to make galvanneal is unsuitable for producinggalvanize, as the ternary alloy layer formed would have a high zinc-iron content that could lead to

flaking of the coating during severe forming operations.

• Most lines producing galvanneal use induction furnaces having three or more zones, which reheat the

strip to 935 to 1050°F [500 to 565°C] in a few seconds. Induction furnaces, combined with soaking

and cooling zones, provide the means to do this in a controlled, fast, and efficient fashion, resulting in a

coating with good appearance and adherence.

• The galvanneal reaction starts at the steel interface and is dependent on: time, strip temperaturereached, AlEFF, bath temperature, steel grade, coating weight, and line speed. A higher AlEFF contentrequires higher temperature and/or longer time to produce an optimum coating, with more danger of“overcooking”, that could result in powdering. If the temperature is too low, the coating will not be fullyalloyed. If the AlEFF is too low (<0.11%), the coating adhesion will be poor, again due to powdering.Running too fast does not allow proper soak times. Keep in mind that each coating line is different and

operators must determine the optimum windows for all the above variables.

• Stabilized IF substrate converts to galvanneal faster than ultra-low carbon or plain carbon substrate,and amongst IF substrates, Ti stabilized reacts faster than Ti-Nb stabilized. Rephosphorized IF steels

are slower to respond to the galvanneal reaction and must be heated to a higher temperature.

Control of Dross

• Dross is an iron-containing particle that forms in the coating bath.

• Sources of iron are: strip dissolution, iron fines on the surface of the strip, and sink roll & other

submerged hardware.

• Dross is a very hard, sand-like particle. Top dross is an aluminum-iron compound and bottom dross is

a zinc-iron compound. Both types of dross interfere with good coating quality.

• Clean strip surface, free of iron debris, is also important in minimizing dross formation.





5

• At the steel-zinc interface a ternary Fe2 Al5-XZnX intermetallic forms, which has a density less than zinc

and floats, contributing to top dross. It floats because it is less dense than zinc due to its high Al

content.

• Since they float, any addition of 10% Al alloy bars to the bath results in most of the Al reporting directlyto the top dross and leaving the bath on the surface of the strip. For this reason, it is a much betterpractice to use pre-alloyed jumbo zinc ingots, or 5% Al alloy bars (which sink), as the means of adding Al, since it will be more uniformly distributed throughout the entire bath volume.

• On many lines, robots are used to skim off top dross.

• Binary intermetallic ("-FeZn7) can also form. It has a density greater than zinc and sinks to form bottom

dross. Again, by the proper use of pre-alloyed jumbo zinc ingots to control aluminum, the formation of"-FeZn7, and thus bottom dross, can be reduced.

• The aluminum level in the bath therefore affects the type of dross that is formed. At greater than about0.14% AlEFF, the stable dross particle is a primarily aluminum-iron particle – less dense than the moltenzinc (top dross). The more the AlEFF is below 0.14%, the more the stable dross particle that forms is azinc-iron compound – denser than the molten zinc (bottom dross). For this reason, bottom dross can

build-up during galvanneal campaigns.

• A key element in minimizing bottom dross generation is stability of the bath with respect to bothtemperature and aluminum content. A sudden drop in temperature can cause precipitation of zinc-ironfrom the melt. Also, sudden increases in aluminum percentage can generate excess top dross.Minimizing Al gradients by using modified feeding practices can control dross1. In short, don’t shock

the system.

• Again, to avoid rapid Al fluctuations, use pre-alloyed zinc jumbos, or 5% Al alloy bars, as much aspossible as the means of getting Al into the bath, rather than 10% Al alloy bars. Following this practicehas been found effective in minimizing bottom dross generation during long galvanneal runs. Refer toGalvInfoNote 5.2 for the range of Al levels available in jumbo zinc ingots.

Bath Control During Product Transitions (GA GI and GI GA)

• This article emphasizes that having stable bath operations is important to producing galvanized sheet.However, on coating lines that produce both GI and GA sheet, bath stability must be disturbed whentransitioning from one product to another. Transitions are a complex topic that is too large to cover indetail here. Reference 1 contains information on the best practices to use when making these

transitions. In short:• The GA GI transition involves adding large amounts of Al to the bath such that the Al level

is increased quickly.

• The GI GA transition consists of adding only SHG ingots (Al-free) to the bath so that Al

decreases via removal by the strip.

Measuring and Controlling Bath Variables

• An important element of bath management involves measuring and controlling the chemistry of the

zinc, the temperature of the strip and zinc, and the bath level.

• For each line a sampling plan must be developed for the bath metal. This involves:

o The location(s) where the samples are taken

o The sampling technique and sample type

o

The method(s) of determining total and effective aluminum o The measurement of other elements (Fe, Pb, Sb, Cd, etc.) in the bath metal

• AlEFF in the zinc can be measured indirectly by a number of analytical techniques, including atomicabsorption (AA), inductively coupled plasma (ICP), and optical emission spectroscopy (OES). Thedirect reading must be corrected, however, by knowing the values of Al and Fe measured by thesetechniques, and the bath temperature at the time of the sampling. Computer tools and software areavailable to perform this task.

• AlTOT determination typically requires wet chemical analysis, as the aluminum tied up in the dross mustbe dissolved before it can be analyzed.





6

• The appropriate strip and zinc temperature and zinc level sensors must be part of the line equipment.

• Shown below are the important factors involved in good management of zinc bath chemistry:

Factors for Bath Management and Control of Effective Aluminum

1. Material

• Zinc ingot chemistry

• Aluminum/Antimony bar chemistry

• Steel strip – chemistry, thickness, width, roughness

2. Method

• Coating weight

• Steel strip – speed, temperature

• Bath temperature, pot heating method

• Sampling method

3. Machine

• Pot –condition, inductors or heating elements, hardware condition

• Sampling tool

• Sampling mold

4. Man

• Additions – zinc ingot, aluminum bar, antimony bar

• Dross removal method

• Sampling practice and location

5. Measurement System (accuracy, precision, resolution, calibration)

• OES, ICP (bath chemistry)

• Pot thermocouple (bath temperature)

• Sensors – strip thickness, coating weight, strip temperature, line speed

Copyright$ 2009 – IZA

Disclaimer: Articles, research reports, and technical data are provided for information purposes only. Although the publishers endeavor to


1 McDermid, J.R., Baril, E., Goodwin, F.E., Galvanizing Bath Management During Galvanizing to Galvanneal and Galvanneal

to Galvanize Product Transitions, Proceedings Galvatech ’04, pp. 855-869




1


GalvInfoNote Control of Coating Weight [Mass] forContinuous Hot-Dip Galvanized Sheet Products

Rev 1 Jan-092.5

Introduction

Hot-dip galvanized sheet products are manufactured to exacting coating thickness standards. It is the

coating thickness that determines coating life in any given application, even though the normal practice is

to specify and manufacture to coating weight (inch-pound system) or coating mass (SI system) per unit

area. The reason for this is explained In GalvInfoNote 1.1. Since the density of zinc is well known, it is

easy to calculate the thickness of a galvanized coating once the coating weight [mass] is determined.

This GalvInfoNote explains how coating weight [mass] is controlled.

Corrosion Performance

It is well established that, in most environments, the corrosion rate of a galvanized coating is

approximately linear. Twice the coating thickness gives approximately twice the product life before the

onset of steel corrosion. For example, a 1-mil thick (0.001 inch or 25.4 microns) coating provides a life of

30 years in a rural environment, and a 2-mil thick coating (0.002 inch or 50.8 microns) lasts about 70

years before the onset of rusting of the steel sheet. This relationship – the life of the product being a

linear function of the coating thickness – is relevant for almost all applications of galvanized sheet. It is

important, therefore, that:

1. the customer ascertain and order the coating weight [mass] needed for the intended application,and

2. the manufacturer of the galvanized sheet produces the correct coating thickness uniformly acrossthe entire width of the sheet and on both surfaces.

For the customer, it is important to have the following two questions answered:

1. What is the corrosion rate in the environment that the product will be used?

2. What is the desired life of the product?

The answers will then dictate what coating weight [mass] is required. Refer to GalvInfoNote 1.6 foradditional reference information on selecting the appropriate coating weight [mass].

Controlling the Coating Weight [Mass] During Coating

Modern coating lines are very capable of controlling the thickness of the coating to meet end-user needs. As noted in GalvInfoNote 2.1, these lines operate at high speeds – as fast as 600 feet/minute and higher. At these speeds, very specialized equipment is required to ensure the correct coating thickness is appliedto the sheet. Figure 1 shows a general layout of the bath equipment, and strip pass-line, in a modern hot-dip coating line.





2

Figure 1 General Arrangement of Coating Bath Equipment in a Continuous Hot-Dip Coating Operation

In this arrangement, the sheet is exiting the bath at high speeds, and as it exits, it drags out more zincthan is needed for the coating. The higher the line speed, the more zinc is dragged out of the bath. Thethickness of zinc on the sheet is then controlled by using “gas knives” to wipe off excess zinc while

allowing the desired amount to pass through the knives.





3

Typical gas knives employ low-pressure/high-volume gas streams (in most cases air, but sometimesnitrogen) that impinge against the sheet surfaces. The pressurized air is generated by blowers. The airflows from the blower through piping up to a position parallel and adjacent to the strip. It is then allowed toescape through a precisely designed and machined slot opening or orifice that is placed about ½ inch orless from the traveling strip. The resulting air jet acts as a knife, stripping the excess molten zinc andforcing it back in the direction of the coating bath surface. Pressure/volume is the principal control

parameter, although height above the bath, distance to the strip, angle of the knives, and knife orifice gapare also controlled.

Automatic coating weight [mass] control systems using artificial intelligence technology have been

installed on many lines to produce consistent coating thickness with low standard deviation. The degree

of control required depends on the thickness that is being applied. The thinner the coating the more

control required. As shown in Figure 1, the manufacturer can use a set of small rolls located immediately

beneath the zinc bath surface to keep the sheet uniformly distant from each knife orifice. This, or a

similar, roll arrangement is very important to obtain the desired uniform thickness of coating on both sides

of the sheet. Figure 2 is a cross-section schematic of an air knife operation. In addition to flat sheet or

strip, this technology is also used when coating wire or tubing.

Figure 2: Schematic of Air Knife Operation

Range of Coating Weights [Masses] on Galvanized Sheet

There are limitations to the minimum/maximum coating thickness that can be applied to continuous

galvanized sheet products.

Minimum Coating Thickness

The minimum thickness is limited primarily by the amount of air (pressure and volume) that is

practical to use during manufacture. As the air pressure and volume are increased, or the strip to

knife distance is decreased, the coating thickness will decrease. The rate of decrease in coating

thickness as the knife parameters are adjusted becomes limiting when the coating thickness gets

Source: www.coatin control.com





4

down to about 0.00025 inch (about 6 microns). This is not an absolute number as the design of the air

knives and the processing speed govern the lowest coating thickness that is achievable. If the air

pressure and volume are increased further, or the knives are moved too close to the strip, the zinc

exhibits a tendency to freeze at the air knife location. If this occurs, there is obviously no further

“wiping” action.

The speed of the sheet when it exits the coating bath has a large influence on the volume of zinc that

needs to be wiped off. The higher the sheet speed, the higher the air pressure and volume needed to

obtain a specific coating thickness, therefore the thinnest coating achievable is influenced by line

speed. Since the processing speeds used on coating lines are usually dictated by the annealing

furnace design, it is common for thin gauge sheet to be processed at high speeds and for thick gauge

sheet to be processed at lower speeds. It is reasonable to expect that the thinnest coatings might

only be possible on heavier gauge strip. This is true except for one offsetting factor. As explained in

GalvInfoNote 2.4, the sheet and coating metal react to form an alloy layer during the time that the

sheet is immersed in the coating bath. This alloy layer is solid and cannot be wiped off by the air

knives. The longer the sheet is immersed in the bath, the thicker the alloy layer. Therefore, heavy

gauge sheet, being processed at lower speeds, is immersed in the coating bath longer than light

gauge sheet, and typically has a thicker alloy layer. Since the alloy layer is a part of the final total

coating thickness, it is not necessarily true that heavy gauge sheet can coated with the thinnestcoatings.

Maximum Coating Thickness

The maximum coating thickness is limited by a number of factors, but clearly one is the amount of

zinc that can be “dragged” from the bath. Since it is governed by the surface tension of the liquid zinc,

the amount of zinc being dragged at low speeds is less than at high speeds, so it is difficult to achieve

a thick coating on heavy gauge sheet. Remember, heavy gauge sheet is usually processed at lower

line speeds because of the limitations of the annealing furnace. Since heavy gauge sheet is often a

product that is expected to be in corrosion-free service for many years (corrugated steel pipe, for

example), the product needs to have a thick coating. To accomplish this, galvanized sheet producers

apply special practices to heavy-gauge sheet to achieve thicker coatings. One such practice is to

increase the surface roughness of the steel substrate. The rougher surface results in more zinc being

dragged out at any given speed, and provides more “holding power” to prevent the liquid zinc from

running back down the sheet before it freezes.

Besides being limited by the amount of zinc that is dragged out of the coating bath, there is another

practical limitation. If the coating is too thick after it passes through the air knives towards the top roll

above the coating pot, there is a tendency for the molten coating metal to “sag” simply because of

gravity. The coating immediately adjacent to the steel surface is “held” in place by surface tension

between the molten coating and the solid alloy layer on the smooth or rough steel sheet. Also, the

outer surface of the molten coating has a “solid” but very thin layer of oxide. This oxide layer attempts

to hold the molten coating in position until it has totally solidified. However, as the thickness of the

molten layer increases, there is a tendency for the coating to “break through” the oxide layer, and as

a result local sagging can occur. This results in a non-uniform coating thickness on the sheet surface,

one that can be unsightly as well as affecting the time before rusting of the steel sheet begins. The

coating needs to be uniformly thick to avoid localized uneven onset of red rust.

The practical maximum thickness depends on many specifics of the particular coating line, but

realistically, coatings thicker than about 2 mils (0.002 inch or 50.8 microns) often have coating sags.





5

ASTM Coating Designations

Specifications such as A 653/A 653M, the ASTM Standard that covers continuous hot-dip galvanized

sheet products, take into account the limitations that were discussed in the previous sections on

minimum/maximum coating thickness. Table 1 at the end of this GalvInfoNote contains the coating

designations that are recognized in A 653/A 653M.

In Table 1, the maximum coating thicknesses, G360 and G300, can only be applied to thicker sheet (for

most producers, approximately 0.060” [1.5 mm] and heavier), and the tendency for sags to develop in

coatings this thick is high. In fact, coating designations of G115 (Z350) and heavier typically have

minimum sheet thickness to which they can be applied. This limitation varies by producer as explained in

the next section.

On the other end of the range, the thinnest coating, G01, has no specified minimum thickness. This

designation clearly recognizes that there is a physical limit to the thinnest achievable coating thickness.

Even a G30 coating at approximately 0.0003-inch (7 microns) thick is beyond the thinnest designation

that is achievable on some coating lines when processing the sheet at high speeds.

Producer Capability

The preceding discussion highlights the importance of determining the specific coating thickness needed

for a given application. It also shows that there are some very definite limitations to the thickest and

thinnest coatings achievable by the continuous hot-dip process.

Each continuous hot-dip production line has specific capabilities with regard to the thickest and thinnest

coatings that can be applied uniformly. These limitations depend on some very specific features of the

line including:

• the processing speed for any specific sheet thickness/width combination,

• the design of the air knife equipment, and

• the ability of the steel company to control sheet surface finish (surface roughness) on the

incoming steel. A rough steel surface is needed for thicker coatings, but such a surface is notpossible on thin sheet which requires a relatively smooth surface.

These and other reasons are why producers have very specific limits with respect to their minimum and

maximum coating weight [mass] capability for each of their coating lines. Typically, these capability limits

have been developed on the basis of experience, and they take into account the needs of the end-user

community with respect to coating thickness uniformity and coating appearance. Issues such as forming,

welding, and corrosion performance all are very dependent on the application of a uniform coating

thickness. To find out what coating weights [masses] are produced as a function of sheet thickness, it is

necessary to contact the intended galvanize producer.

Other Types of Hot-Dip Coatings

The other types of continuous hot-dip-coated sheet products have limitations much like galvanized

coatings with respect to coating thickness. The capability range is somewhat different because ofdifferences in the density and viscosity of the specific liquid coating alloy, but each type of product - pure

aluminum coatings, aluminum-5 to 11% silicon coatings, 55%aluminum-zinc coatings, and 95%zinc-

aluminum coatings – has coating metal attributes that make the commercially available coating weight

[mass] range a very specific defined window. If your application involves one of these other products,

recognize that it is still important to have a uniformly thick coating, and the commercially available range

of coating weight [mass] has been established taking into account many of the same parameters as those

discussed here.





6

Batch-galvanized parts are made in a very different manner than continuous galvanized sheet product,

and as a result, the range of commercially available coating thickness is very different than for sheet

product. Much thicker coatings can be applied by the batch process. For batch-galvanized parts, the

immersion time - the time the part is submerged in the molten coating bath – is much longer than that for

continuous-galvanized sheet. The manufacturer takes advantage of this to allow the alloy layer to grow

quite thick, if desired. Since the alloy layer provides good galvanic protection to the steel part, it is a vital

component of the coating life. Thus, items such as transmission towers can be batch galvanized to

provide sufficiently thick coatings to last for more than 50 years without maintenance.

Table 1 Coating Designations for Hot-Dip Galvanized Sheet Products*

Units Coating DesignationMinimum Coating**

Weight (oz/ft2)

Inch-Pound G01 No minimum

G30 0.30

G40 0.40

G60 0.60

G90 0.90G115 1.15

G140 1.40

G165 1.65

G185 1.85

G210 2.10

G235 2.35

G300 3.00

G360 3.60

Mass (g/m2)

SI (Metric) Z001 No minimum

Z90 90

Z120 120

Z180 180

Z275 275

Z350 350

Z450 450

Z500 500

Z550 550

Z600 600

Z700 700

Z900 900Z1100 1100

* Source: ASTM Annual Book of Standards Volume 01.06

**Minimum coating weight [mass], total both sides of the sheet, triple-spot average. Refer to Specification ASTM

A 653/A 653M for additional requirements pertaining to single spot and per side requirements.





7

Summary

The life of galvanized sheet is a direct function of the coating thickness. To determine what coating

weight [mass] to order, the customer needs to know both the desired life of the product and the corrosion

rate of the environment it will be exposed to. Modern coating lines have excellent capability in controlling

the thickness of all hot-dip zinc and zinc-alloy coatings. For continuous hot-dip galvanized products,coating weights available range from G01 to G360 (masses from Z001 to Z1100). Batch hot-dip

galvanizing can apply even thicker coatings.


Disclaimer:





1


GalvInfoNote The Spangle on Hot-Dip Galvanized Steel Sheet

Rev 1.3 Aug 2011 2.6

Introduction

For many years, galvanized articles made by hot-dip coating techniques were identified by acharacteristic spangle appearance. In some cases, this is still true today. However, because of changesin zinc refining processes, in the galvanizing process, and in the demands of the marketplace, not all hot-dip galvanized steel sheet made today has a visible spangle. The explanation for this is given later in thisGalvInfoNote.

What is a Spangle?

The dictionary defines “spangle” as a glittering object. When the word spangle is used to describe thesurface appearance of galvanized steel sheet, it means the typical snowflake-like or six-fold star patternthat is visible to the unaided eye. Figure 1 shows the details of a typical spangle pattern on a galvanizecoating at a magnification of about 10X.

Fig. 1 The spangle structure of a hot-dip galvanize coating.

The features shown here encompass a number of quite complex metallurgical phenomena. ThisGalvInfoNote explains why these features are present.

Note the 6-foldsymmetry of thisspangle

Blue arrow definesthe direction ofgrowth of the“primary” dendritearm within thesolidifying grain ofzinc

“Secondary” dendritearms growinglaterally away fromthe primary arm



GalvInfoNote 2.6Rev 1.3 Aug 2011


2

The Solidification Process

Spangles develop when the molten zinc adhering to the steelsheet is cooled below the melting point of zinc, which is

approximately 787°F [419°C]. At this temperature, the randomlyarranged atoms in the liquid zinc begin to position themselves into

a very ordered arrangement. This occurs at many randomlocations within the molten zinc coating. The transformation from adisordered arrangement of atoms (liquid state) into an orderedarrangement is the process of solidification or crystallization. Thesmall solidifying regions within the molten zinc are defined as“grains” (see sidebar). As individual atoms in the molten zincattach themselves to a solidifying grain (causing grain growth),they do so in an ordered fashion and form into a distinct array, orcrystal. In the case of zinc, the crystals form with hexagonal (six-fold) symmetry. As the solid zinc grains grow larger, individualatoms of zinc arrange themselves into the often-visible hexagonalsymmetry of the final spangle. When the coating is completelysolidified, individual spangles define individual grains of zinc.

“Nucleation” is the term used to define the process oftransformation of randomly arranged atoms of molten metal into asmall, organized array of atoms in the “seed” crystals at the initial stage of solidification. A high rate ofnucleation during the freezing process tends to cause the formation of numerous small grains in the finalsolidified structure, while a low rate tends to favour the growth of large grains.

Dendritic Growth

There is another aspect of the solidification process that leads to the snowflake pattern in galvanizecoatings, viz., “dendritic” (meaning tree-shaped) growth. Dendritic growth causes the individual growing(solidifying) grains to grow into the melt (the molten zinc coating) with a distinct leading rounded edge. A“primary” dendrite arm is identified in Figure 1. There are secondary dendrite arms that grow laterallyaway from the “primary” dendrite arms.

Dendritic growth of grains during the solidifying of metals is very common. The reason that the dendritesare readily visible in a galvanize coating is that we are basically seeing a two-dimensional version of an

as-cast, dendritic, solidified grain structure. Remember, the coating is less than 0.001 in (25 µm) thick,considerably less than the diameter of a spangle. In other metals (for instance the steel substrate), theoriginal as-cast, three-dimensional, dendritic structure of the grains is subsequently broken up into manysmaller, more equiaxed grains. This is due to the effects of hot rolling (for example, rolling a 9-inch [230mm] thick slab of steel reduced in thickness to as low as 0.050-inch [1.3 mm] thick steel sheet), coldrolling and recrystallization during the sheet annealing process.

The rate of growth of the dendrite arms during the solidification of a galvanize coating competes with therate of nucleation of new grains within the molten zinc. This process determines the final size of thecompletely solidified structure. In the case of Figure 1, which is a galvanized coating with a well-definedlarge spangle pattern, the rate of dendrite growth dominated the solidification process leading to a smallnumber of large spangles. One characteristic of such spangles is that they are thickest at their centers

and thinnest at their edges, or grain boundaries. The grain boundaries can be said to be “depressed” andare difficult to smooth by subsequent temper (skin) passing. Galvanize coatings with small spanglesgenerally have less depressed grain boundaries, and can be smoothed more easily by skin passing.

The Effect of Zinc Bath Chemistry

The nature and rate of dendritic growth during the solidification process is affected by the presence ofother metallic elements in the molten metal. These can be either intentionally added alloying elements orimpurities. In the case of galvanize coatings on steel sheet; the most common reason for the well-defined

Grains

Metals, like many solids in nature, have

a crystal or “grain” structure. For

example, the steel sheet beneath the

galvanized coating consists of many

small grains of iron-carbon alloy (steel).

The individual grains of steel are very

small compared with the grains of zinc

in the zinc coating, and are “glued” to

one another by atomic bonding forces.

Think of this as “grains of sand” in a

sandstone rock. The size of the

individual grains of sand may be larger

than the grains in the steel sheet, but

this analogy allows the concept of grain

structure to be visualized.





3

dendritic growth pattern is the presence of lead in the coating. It had long been thought that the reasonlead results in large spangles is that it reduces the number of nucleation sites. In research performed inthe late 1990s

i, it was proposed that the presence of lead decreases the solid/liquid interfacial energy in

the solidifying coating. This leads to an increase in dendrite growth velocity, resulting in large spangles.Lead precipitates at the coating surface, and the varying distribution of lead particles across the surface,define the optical appearance (dull vs. shiny spangles).

Lead is a common impurity in zinc. In years gone by, the most common method of zinc metal productioninvolved smelting, distillation and condensation. Lead is a common metal found in zinc-containing ores,and this refining process carried it through as an impurity in the zinc. In the early days of galvanizing, leadwas almost always present in the zinc, and it was common to see a spangle pattern. Galvanize coatingson steel became identified by the characteristic spangle. Essentially, all hot-dip galvanized coatings had aspangle appearance. If the spangle wasn’t visible, the users “knew” that the steel had not been

galvanized.

The first galvanize coatings contained as much as 1% lead. During the past 40 years, the presence ofsuch high lead levels has not been common in galvanize coatings on steel sheet, at least not in North America, Europe, and Japan. Typical concentrations of lead (where it was intentionally used) in mostgalvanized sheet made during this time has been less than 0.15%, often as low as 0.03 to 0.05%. Eventhis lower amount of lead is still sufficient to develop dendritic growth behaviour during the solidification

process. Today, a typical level of lead in the coating bath on lines where the product requires a well-developed spangle pattern is in the range of 0.05 to 0.10% lead.

As there are now environmental concerns about the use of lead, some galvanized sheet manufacturershave established practices on their older or low speed lines that use lead-free zinc, whereby a smallamount of antimony is added to the zinc coating bath. Antimony influences spangle formation in a similarfashion to lead. The final result is a smooth, visibly spangled coating. Typically, the amount of antimony inthe coating bath is about 0.03 to 0.10%.

To obtain smoother coatings with lead-bearing zinc, it is possible to suppress spangle growth on thesheet by rapidly cooling the coating. This is done by the use of a spangle “minimizing” device above thezinc bath. These devices direct steam or zinc dust at the surface to rapidly freeze the zinc and keep thespangle small. Such technology is not required in the case of lead-free zinc for the reasons explained inthe next section.

Non-Spangle Coatings

In recent times, the production of zinc from zinc-containing ores has been changed to an electrolyticrecovery method. In this method of zinc production, the refined zinc is very pure, with the lead beingexcluded. This change occurred at a time when many users of galvanized sheet, especially those desiringa high quality finish after painting, such as the automotive and appliance industries needed a non-spanglecoating. Removing the lead gave them the product they desired. The amount of lead in the coating forlead-free coatings is less than 0.01%, and often less than 0.005%.

Lead-free coatings still have a grain pattern that is, at best, barely visible to the unaided eye. Typically,the spangles are about 0.5 mm in diameter and are clearly visible when seen at 5 to 10X magnification.However, the grains no longer grow by a dendritic mode but by a cellular mode of growth. Essentially,zinc grains nucleate on the steel surface, and grow outward toward the free surface. The absence of leadtakes away the strong driving force for growth in the plane of the sheet, preventing the formation of large

spangles. Rapid spangle growth cannot occur and the absence of lead results in the coating appearinguniformly shiny. Grain boundary depressions, for all intents and purposes, do not exist in lead freecoatings.

This non-spangle coating, when combined with temper rolling by the galvanized sheet producer, can bemade very smooth. The large grain boundary depressions and surface relief of a spangle coating are notpresent. The coating can then be painted to give a very smooth finish.





4

An added advantage of producing a lead-free galvanize coating is that it is not susceptible to a problemknown as delayed adhesion failure. This is a coating failure mechanism, occurring primarily in dampenvironments, related to the fact that lead concentrates at the spangle boundaries and allows smallcorrosion cells to form.

Another feature of the very small spangles of lead-free coatings is that the shiny metallic appearance of

the coating is very uniform, unlike the appearance of large spangle lead-bearing zinc coatings, where theluster of each spangle differs, giving the sheet a non-uniform appearance.

Why is Lead Still Used on Some Galvanizing Lines?

The manufacture of non-spangle coatings, free of lead (or antimony), is not so easily done. The reasonrelates to the influence of even a small amount of these additions on the viscosity of the molten zinc. It isdifficult to avoid small sags and ripples in the zinc coating when lead/antimony is not present in themolten zinc, due to its higher viscosity. The thicker the coating, the greater the tendency to form sags andripples during freezing. Fortunately, the automotive and appliance industries need only relatively thincoatings (typically 60 to 80 g/m

2/side) of zinc to obtain the level of corrosion resistance their customers

demand. Also, the products used by these industries are made on relatively new high-speed lines, orolder lines that have been refurbished to allow production at high speeds. The combination of highprocessing speeds and low coating weights allows producers to use lead-free coating baths, avoid the

development of spangles, and still attain a ripple-free coating. Improved gas-wiping technology andpractices has also helped in producing smoother coatings.

If the end user requires a heavier coating mass (100 g/m2/side and higher), there is a tendency for the

coating, when applied from a lead-free bath, to develop very visible sags and ripples. The result is thatthe surface is not smooth and the coating is composed of locally thick and thin regions. This tendency forsags is exacerbated at low line speeds (<75 meters/minute). Older, low speed coating lines designed toprocess heavy-gauge sheet, and those that are used to make heavy coating weight products (heavierthan 275 g/m

2or G90), typically still have some amount of lead in the coating bath to improve the final

coating uniformity. The concentration of lead in the zinc bath is typically between 0.05 and 0.10%. Antimony additions of between 0.03 and 0.10% provide the same effect.

The net result is that the final product from many lines still has a visible spangle pattern. This meets withthe marketplace needs in that a number of industries, especially those that use bare (unpainted)galvanized sheet, still want the large, bright, reflective spangle pattern.

Specifying Spangle Size

Users often ask if there are specifications that govern the size (diameter) of galvanize spangles.Unfortunately there are no quantitative specifications that regulate this feature of galvanized sheet.Spangle size can be affected not only by the zinc chemistry and cooling rate, but also by other factorssuch as the smoothness of the substrate. Consistently controlling spangle formation to a specified size,and then verifying compliance, would be an extremely difficult task. For this reason, spangle sizeterminology is qualitative. It is defined in ASTM A653/A653M, Specification for Steel Sheet, Zinc-Coated(Galvanized) as follows:

• Regular spangle – zinc-coated steel sheet with a visible multifaceted zinc crystal structure. Thecooling rate is uncontrolled, which produces a variable grain size.

• Minimized spangle – zinc-coated steel sheet in which the grain pattern is visible to the unaided eye,and is typically smaller and less distinct than the pattern visible on regular spangle. The zinc crystalgrowth is arrested by special production techniques, or is inhibited by a combination of coating bathchemistry plus cooling.

• Spangle-free – zinc-coated steel sheet with a uniform finish in which the surface irregularities createdby spangle formation are not visible to the naked eye. The finish is produced by a combination ofcoating bath chemistry, or cooling, or both.





5

In the absence of specifications for galvanized sheet spangle size, Figures 2, 3, 4, & 5 are suggested sizeratings provided by the GalvInfo Center. While spangle-free products are the result of non-lead bearingrequirements, and are preferred for many end uses, some users still desire galvanized sheet having avisible spangle. Keeping in mind that it is generally not possible to order by spangle size, and thatspangle products are not available in all regions of the world, the photos in Figures 2 – 5 illustrate what

can still be obtained from producers in some parts of the world.

Fig. 2 Large – Spangles ! 15 mm across Fig. 3 Medium – Spangles up to 10 mm across

Fig. 4 Small – Spangles up to 5 mm across Fig. 5 Spangle-free – Spangles " 0.5 mm across





6

Summary

The spangle on hot-dip galvanized steel sheet has been its primary identifying feature for many years.The demand for both lead-free coatings and very smooth products has resulted in spangle size beingreduced by many producers until it is no longer visible to the unaided eye. This was, and to some extent

still is, of concern to certain segments of the marketplace, but gradually users of galvanized sheet havebecome accustomed to a product that does not have a large, easily seen spangle. While in the futurethere may be no demand for a visible spangle, some consumers today still desire to use galvanize with aspangle for their products.


Disclaimer:



i J. Strutzenberger, J. Federl: Metall. Trans. A, 1998, vol. 29A’ pp. 631-646




GalvInfoNote

2.7

Galvanizing – The Use of Chemical Fluxes

Rev 0 Jan-07

Introduction

As discussed in GalvInfoNote 2.1, “The Continuous Hot-Dip Galvanizing Process for Steel Sheet Products”, it is

absolutely necessary to have the steel sheet free from any surface oxide as it enters the molten zinc coating

bath. On high-speed processing lines, where running at speeds of up to 600 feet/minute is common, the sheet

is in the coating bath for times as short as 2 to 4 seconds. In order to develop the alloy layer essential for good

adhesion between the steel and zinc coating, the incoming sheet has to be very clean and oxide-free.

Hydrogen Gas Cleaning for Continuous Galvanizing

The most common method used today for obtaining a super-cleansurface at coating bath entry is to have an aqueous alkalinecleaning section (sometimes with an electrolytic assist) ahead of theannealing furnace. The annealing furnace gas atmosphere contains

a small amount of hydrogen (typically 5-6%) and is kept as free ofoxygen as possible to assist with hydrogen-reduction of the oxidelayer. Nitrogen is used as the inert carrier gas to maintain adequateinternal pressure within the furnace. As the sheet is heated to hightemperatures in the furnace to anneal the steel and obtain thedesired mechanical properties, the hydrogen gas reacts with anyremaining iron oxides to produce a very clean surface; one that canbe readily wet by the liquid zinc bath metal. In this manner, the zincand steel are able to develop a complete alloy bond in a very shorttime.

Other Gas Cleaning Methods

Some lines also have a non-oxidizingcleaning furnace section between the

wet cleaning and annealing sections.Other lines omit the wet cleaningsection, but have a non-oxidizingcleaning furnace before the annealingfurnace. Non-oxidizing furnaces burn offsurface hydrocarbons and provide somereduction of surface oxides, whileheating the sheet to temperatures justbelow the annealing point.

The chemical reaction in the furnace is shown by the following:

Iron oxide + Hydrogen (H2) Iron (Oxide-free surface) + Water (H2O)

To prevent re-oxidation after being annealed and cooled to the approximate temperature of the zinc bath, themoving sheet is kept under the protection of the hydrogen-containing atmosphere between the end of thefurnace and the coating bath. This is accomplished via a fixed enclosure (usually referred to as a “snout”)between the furnace and the molten zinc bath. The upper end of the snout is bolted tightly to the furnace, and itslower end submerged into the coating bath, ensuring that the steel never encounters air prior to immersion intothe molten zinc. It is at the snout where the pressurized atmosphere gas is introduced and forced to flow backtowards the entry end of the line, against the movement of the steel sheet. This ensures that gas with the mostreducing potential is in contact with the sheet just before it enters the molten zinc.

Flux Cleaning Method for Continuous Galvanizing

There is one other method for providing a clean oxide-free steel surface to a galvanizing bath, viz., the use of

chemical fluxes. It is far less common than the process described above, but it is a proven means of obtaining

very good coating adhesion. The flux galvanizing process is also used in the after-fabrication, batch galvanizing

industry for articles such as structural shapes, pipe, etc. Refer to GalvInfoNote 2.3 for information on batch

galvanizing.


1





The normal procedure for continuous flux galvanizing of steel sheet involves a cleaning/degreasing step, oftenusing a similar aqueous alkaline cleaning solution to that used for the hydrogen cleaning process. Following thisis an acid-pickling step (usually hydrochloric acid) to remove surface oxides. After pickling and during the timethat the sheet is rinsed and dried, a very thin oxide layer reforms on the steel surface. The reason this happensis that oxide-free low carbon steel reacts very quickly in air to form a thin surface oxide layer. It is essentiallyimpossible to prevent this reaction in air. This oxide layer does not change the appearance of the steel surface

perceptibly; although the surface may be slightly darker than an absolutely oxide-free surface. The color is notthe usual black or red iron oxide normally associated with rusting; nevertheless a thin oxide is present. This thinfilm must be removed in order to get rapid, complete wetting of the steel by the molten zinc. Therefore one morestep is needed ahead of the coating bath.

Since flux coating lines do not have an annealing furnace as part of the process (the sheet’s mechanicalproperties are obtained in an annealing operation ahead of the galvanizing line), hydrogen cleaning is notpossible. Instead, chemicals are used to dissolve the last vestiges of oxide. These chemicals are called fluxes,much like the fluxes used for processes such as soldering. They are simply compounds capable of dissolvingthe oxides of iron.

For galvanizing, the most common flux in commercial use, and one that has been around for many years, is

based on the inorganic chemical “zinc ammonium chloride”. The weight ratios of zinc chloride to ammonium

chloride can be adjusted to meet individual customer needs. Typically, these solutions also contain specialproprietary wetting agents, anti-foaming agents, and possibly other viscosity-adjusting additions. Zinc

ammonium chloride fluxes are used for all types of galvanizing - after-fabrication galvanizing as well as

continuous sheet, wire, and tube galvanizing operations.

As flux is a relatively low melting temperature inorganic chemical, the steel sheet cannot be heated to hightemperatures ahead of the galvanizing bath. If the steel temperature became too hot, the chemical flux would beburned, detracting from its performance. This means the sheet must enter the galvanizing bath at a temperature

considerably below the zinc metal temperature (about 460-470°C). The zinc pot therefore has to have a muchhigher heating capability than a typical coating pot used on lines that have in-line annealing. This high heatingcapability, combined with the need to remove the “spent” flux from the surface, usually leads to a less efficientuse of the zinc metal than for coating lines that utilize in-line annealing and hydrogen cleaning. Flux fumes arealso generated and must be collected by hoods located above the zinc pot. Another feature of flux coating lines

is that the coated product has natural small, flat spangle – even with lead-bearing zinc. This is a result of thefast post-pot cooling resulting from the sheet’s low pot entry temperature.

In the continuous sheet galvanizing process, the flux can be applied as a “preflux”, that is, applied from a water-based solution that contains the dissolved flux chemicals, or it can be applied as a “top flux”, that is, a moltenflux layer floating on top of the galvanizing bath. In some cases, both types of flux application are used. Again,the flux is a chemical that reacts with iron oxide to remove it and leave behind a very clean oxide-free surface forrapid wetting as the sheet enters into the molten galvanizing metal.

Continuous galvanizing with the use of fluxes has been a commercial process for many years. In fact, beforethe development of continuous galvanizing, the galvanizing of steel sheet was done by operators immersingsheets, one at a time, into a bath of molten zinc. Zinc ammonium chloride fluxing was part of this operation. Themovement to continuous galvanizing, using fluxes as a part of the process, was a natural outgrowth of the one-sheet-at-a-time process.

Flux galvanizing lines are also known as “Cold Galvanizing Lines”, as “Wheeling Lines” (the first steel companyto use them), and as “Cook-Norteman Lines” (the process developers).


2





Summary

The flux-galvanizing process serves as a viable process for the continuous galvanizing of steel sheet. Although

it has its own unique processing problems, such as the formation of flux fumes and flux ashes that need to be

handled as waste products, it is a method of galvanizing that allows the construction of a galvanizing line without

the expense of the typical large annealing furnace associated with the hydrogen cleaning method. For many

producers of galvanized sheet, the annealing of steel sheet is an integral part of the complete steel processing

procedure, and they incorporate the annealing furnace into the galvanizing line so that the hydrogen cleaning

concept can be used. But, for those smaller manufacturers who want to purchase steel sheet and apply a zinc

coating for corrosion protection, a flux galvanizing line is an alternative process – one that requires less capital

expenditure.


Disclaimer:



3




1


GalvInfoNote

2.8 Improving Uniformity of Appearance


The surface of zinc and zinc alloy-coated steel sheet can be treated using one or more of many methods.

This GalvInfoNote deals with a mechanical surface treatment used to improve uniformity of

appearance. Other treatments are used for different reasons, namely:

• Treatments for enhancing formability (see GalvInfoNote 2.9)

• Imparting resistance to storage stain (see GalvInfoNote 2.10)

• Preparing galvanize for field painting (see GalvInfoNote 2.11)

• Pretreatments for metallic-coated sheet (see GalvInfoNote 2.12)

• Treatments for resistance to handling and fingerprint marks (see GalvInfoNote 2.13)

While most of the above treatments are performed directly on the hot-dip line after the metallic coating

has been applied, some can also be carried out on separate process lines/facilities, or in the field.

Improved Surface Uniformity

Whether it is galvanize, galvanneal, aluminum-zinc, or aluminum coatings, many end uses require a

surface that is more topographically uniform than obtainable directly off hot-dip coating lines. This is so

that the underlying surface does not show through paint coatings used for such appearance critical

applications as exterior automotive body panels, some appliance parts, and prepainted building panels.

The method most often used to make the surface more uniform is known as “temper passing”, “temper

rolling” or “skin passing” and is done with a temper mill or skin mill. Figure 1 shows strip passing through

the work rolls of a stand-alone 4-high temper mill.

Fig 1 Temper Mill

(Photo source: Blair Steel Strip Company)





2

Figure 2 includes a schematic of a 4-high temper mill located in a continuous coating line. Both in-line

and stand-alone temper mills typically consist of two back-up rolls and two work rolls. The work rolls

contact the two sheet surfaces with up to several hundred tons of force. This load combined with the

sheet being under high tension between the entry and exit bridle rolls imprints, in part, the work roll finish

to the sheet surface. Depending on the load employed, the sheet is extended in length (and reduced inthickness) by as much as 2%, although in most cases it is in the range of 0.5 to 1.0%.

Fig 2 Schematic of in-line 4-Hi temper mill

With coated steel sheet, temper rolling is generally performed with work rolls having a blasted surface

finish. The roll finish is partially imparted to the sheet surface, and has the effect of reducing the metallic

sheen of bright hot-dip coatings to a uniform dull appearance. With zinc-iron alloy (galvanneal) coatings,

the change to surface appearance is not so obvious. Whether the product is galvanneal or zinc coated,

for automotive end uses the primary purpose of temper rolling is ensure the surface conforms to surface

requirement similar to those of Figure 3.

Fig 3 Automotive specified surface finish parameters

This figure is a chart of average surface roughness (Ra) versus peaks per inch (ppi). Temper passing

using work rolls with the appropriate degree of surface roughness and mill load ensures that the finish

imparted to the sheet has a Ra and ppi that falls into the upper left region. This guarantees the correct





3

“distinctness of image” (DOI) on the final painted automotive panel. Sheet with too low a ppi and/or too

high a Ra will result in an unsuitable painted appearance.

In the case of galvanize coatings made with lead-free zinc, the surface is quite flat after coating and

temper passing using a moderate mill load is able to produce a matte surface with controlled roughness

and little, if any, evidence of the zinc spangle or metallic lustre. It is difficult to flatten galvanize that is

produced with large spangles because of the spangle boundary depressions. For a more in-depthdiscussion of spangle, see GalvInfoNote 2.6. Most galvanize products made today use lead-free zinc.

As a matter of interest, temper rolling of coated sheet is performed “wet” with a very small amount of

water sprayed onto the sheet as it enters the bite of the work rolls. This is to wash away the small flakes

of zinc or zinc-iron that the work rolls abrade from the sheet surface to prevent them from being rolled

back into the surface as “zinc pick-up”. The moving sheet is immediately dried as it leaves the tempermill.

As most temper rolling of coated sheet is done on a mill stand that is located in the coating line (although

it can be done on a stand-alone mill), it is important that the sheet flatness not be adversely affected by

the operation. Obtaining good sheet flatness is part of the operational control exercised during temper

rolling. Given that the sheet is usually extended 0.5 to 1.0%, its thickness is reduced accordingly. To

achieve the optimum final sheet thickness, allowance for temper rolling should therefore be made when

calculating the required cold-rolled sheet thickness set-point value.

Eliminating Stretcher Strains

The small amount of cold work imparted to the steel by temper rolling to smooth the surface finish has the

added benefit of eliminating the yield point phenomenon exhibited by low carbon steel (if not temper

rolled, leveling of the sheet also accomplishes this). The upper yield point is the peak in the stress-strain

curve shown in Fig 4, at the left near YS. For many end uses it is important to suppress this peak

because if not done, the sheet will show discontinuous yielding behavior due to yield-point-elongation

(YPE), and will exhibit stretcher strains when stretch-formed, or flute when bent. The upper yield point

and YPE is associated with interstitial elements such as carbon and nitrogen in the steel. Yield-point-

elongation is deformation that occurs at constant load.

Fig 4 Stress-strain curve with yield point phenomenon - (Source: Auto/Steel Partnership)

The small amount of cold work imparted to the sheet during temper rolling (or leveling) suppresses the

yield point phenomenon as evidenced in the shape of the stress strain curve in Fig 5 (a). With the

discontinuous yielding behavior having been masked, the steel can be formed with no concern about

stretcher strains or fluting. The resulting small changes in mechanical properties are generally not harmful

to subsequent forming operations or the properties of the final part. Heavy temper passing could impart

too much cold work to the sheet and detract from formability. Lead-free zinc coatings help in minimizing





4

unwanted cold work, as only moderate temper mill loads are necessary to achieve the necessary

smoothness. A rougher, spangled coating might require such high rolling loads that the mechanical

properties, formability, and flatness characteristics would be adversely affected.

(a) (b)Fig 5 Stress-strain curve with no yield point phenomenon (a), and 0.2% offset YS (b) - (Source: Auto/Steel Partnership)

For the stress-strain curves of Fig 5, the yield strength can no longer be measured as a distinct yield

point, so the most popular convention is to measure it at 0.2% offset strain. With this method, shown if

Fig 5 (b), the YP is defined as the point where a line, offset at 0.2% strain, and drawn parallel to the

slanted, elastic modulus line, crosses the plastic rounded part of the stress-stain curve. The value

obtained by this method may, in fact, be slightly less than the upper yield point of Fig 4. It must always be

remembered, however, that with ageing, the yield point will return over time. The steel must be

consumed within a reasonable period to avoid this happening.

Discontinuous yielding does not occur on steels made from stabilized ultra-low carbon steel. The

complete absence of solute carbon and nitrogen in these steels removes the yield point. Stress strain

curves for such steels are the same as those of Fig 5 without temper rolling or leveling and they are not

subject to strain lines.

Non-Matte Smooth Surfaces on Coated Sheet

While the above discussion focuses on the use of blasted temper mill work rolls, some non-automotive

coated sheet is temper passed using smooth-ground work rolls. These rolls impart to the sheet a smooth

and non-matte shiny appearance that has a very low average roughness (Ra) value. Such an

appearance is preferred for some end uses.

Summary

When required, the surface finish of metallic-coated steel sheet can controlled very closely to provide asmooth and uniform appearance for critical end uses. The use of in-line equipment to achieve this allowsit to be done not only at will, but with consistent results. Improvements to mechanical properties can alsobe achieved.


Disclaimer: Articles, research reports, and technical data are provided for information purposes only. Although the publishers endeavor toprovide accurate, timely information, the International Lead Zinc Association does not warrant the research results or informationreported in this communication and disclaims all liability for damages arising from reliance on the research results or other

information contained in this communication, including, but not limited to, incidental or consequential damages.




1


GalvInfoNote

2.9

Treatments for Enhancing Formability



This GalvInfoNote deals with surface treatments to enhance the formability of coated sheet. Other

treatments are used for different reasons, namely:

• Improving uniformity of appearance (see GalvInfoNote 2.8)





While most of the above treatments are performed directly on the hot-dip line after the metallic coating

has been applied, some can also be carried out on separate process lines/facilities, or in the field.

There are a number of surface treatments to improve formability of coated sheet.

Oils

Oils are applied to coated sheet for two reasons – primarily to aid subsequent forming, but also to

improve storage stain resistance. In most cases, oils designed to be applied to coated sheet are

formulated to achieve both of these goals, even though storage stain resistance benefits of oil are

generally limited to excluding condensation water from penetrating between coiled or stacked sheet

surfaces. It is important to note that oils are not effective in preventing damage from bulk water (rain,

splashing, flood, etc.).

In the case of coated sheet used by the automobile industry, it is always produced without surfacepassivation (chemical treatment) applied to the surface. This is because many passivation treatments

interfere with spot welding and painting operations. Unpassivated sheet is at great risk of water damage

if it gets wet. Oiling at the coating line usually provides enough protection so that condensation moisture

cannot penetrate between sheets or laps and cause storage stain.

Oils applied to the surface of metallic-coated steel sheet provide lubricity to aid subsequent roll forming or

stamping operations. Some end uses require heavy oiling, while most need only a very small amount.

Oil exuding from the sidewalls and dripping onto warehouse floors is a problem with heavy oiling. Oil

reduces galling, scratching, and fracturing during fabrication. The steel supplier applies the oil on the

coating line just before the sheet is recoiled. Typically, it is applied using a device that first atomizes the

oil, and then deposits it on both sheet surfaces in a controlled manner using electrostatic forces. This

allows close control of the amount of oil deposited on the surface.

Most oils used to aid forming of coated sheets are referred to as mineral or “slushing” oils. There are

many different brands with varying viscosity and levels of volatile components and rust inhibitors. Other

oils with a high volatile content are designed to evaporate when the sheet is exposed to the air and hence

are called “vanishing” oils. Some oils are thixotropic and partially solidify after they are applied. They do

not exude from the coil walls after application. Details regarding the best oil to use for an application can

be obtained from oil suppliers.





2

Soap Lubricants

Soap lubricants are also known as dry film lubes and their purpose is to provide superior lubricationduring difficult forming operations. They are based on alkaline chemistry and are usually roll-applied atthe exit end of a coating line using an aqueous solution followed by drying. The end use determines theaim coating weight. One problem with these coatings is absorption of moisture in humid environments.

This could lead to surface corrosion and problems during forming. Their typical use is on prepaintedsheet although they can be applied to zinc-coated sheet for difficult forming applications.

Dried-in-Place Phosphate Coatings

These are phosphate coatings designed specifically to aid the formability of coated sheet. They consistof tri-metal (Zn-Mn-Ni), microcrystalline phosphate crystals that are applied from aqueous solution usingrubber rolls. Coating weights are typically between 0.5 and 1.5 g/m

2. After the roll application these films

are dried using IR or convection ovens. Usually the phosphate coating is oiled with a mill oil in order toprevent moisture pick-up and subsequent corrosion. After parts are formed they can be cleaned andrephosphated prior to painting.

Acrylic Coatings

Acrylic polymer coatings are water-borne solid films applied at coating weights of between 150-350mg/in

2. They are of two types; permanent and alkaline-removable. During application on the coating line

they require a peak metal temperature of at least 125ºC to drive off the water so as to inhibit moisturemigration through the coating. Characteristically they have low friction coefficients and therefore do notrequire oiling to achieve their excellent formability enhancement. These coatings offer excellent storageand transit corrosion protection. The permanent versions of these treatments are paintable unless theywere formulated with silicon pigments or wax. Standard alkaline cleaners easily remove non-permanentvarieties, which can then be phosphate treated and/or painted.

A source of additional information on formability enhancing mill-applied surface treatments can be foundin Appendix X2 of ASTM A 924/A 924M Standard Specification for General Requirements for Steel Sheet,Metallic-Coated by the Hot-Dip Process, available at www.astm.org .


Disclaimer:






1


GalvInfoNote

2.10

Imparting Resistance to Storage Stain



This GalvInfoNote deals with surface treatments used to impart resistance to storage staining .

Other treatments are used for different reasons, namely:






While anti storage stain treatments are always applied directly on the hot-dip line after the metallic coatinghas been applied, some of the others listed above can also be carried out on separate process

lines/facilities, or in the field.

There are a number of surface treatments available to improve resistance to storage stain. First, the

reason for the formation is storage stain is explained.

Storage Stain

Storage stain is a corrosion product that is typically white, but which can also take the form of a grey or

black deposit on the surface. Since the most common form of discoloration is white, storage stain is often

called white rust . It can occur when sheets of galvanized steel that are in close contact (in a coil or

stacked in lifts/bundles) get wet, either by water intrusion, or by condensation from moist air trapped

between the sheets. The discoloration is due to the corrosion products that form after zinc reacts withmoisture in the absence of free air circulation. Refer to GalvInfoNote 3.2 for more information on storage

stain.

White rust on storage stain-damaged galvanize is initially zinc hydroxide, and turns to zinc oxide when left

to dry in the open air. The white rust inside a wet stack or coil will eventually turn black if left unchecked.

When storage stain on galvanize turns black it usually means that iron has become part of the corrosion

product. When iron becomes involved, enough zinc has been consumed to expose the steel substrateand the remaining zinc is of little, if any, protective value.

The stain that forms on water-damaged galvanneal is grey to black in color due to the iron in the coating.

See GalvInfoNote 3.3 for more information on stained galvanneal. The corrosion products that form on

water damaged zinc-aluminum coatings often have a black to grey appearance – the result of hydrated

aluminum hydroxide formation.

Chemical Treatments – Chromate Based

To reduce the susceptibility of metallic coated steel sheet to storage stain, the practice for many years

has been to treat it with an aqueous solution of chromic acid, chromium salts and mineral acids to

produce a thin-film coating on the surface. This inorganic chemical or “passivation” treatment is applied

near the end of the coating line. The solution dissolves a very small amount of the coating metal and

forms a protective film containing complex chromium and metal compounds1. Traditional chromium





2

based treatments contain chrome in two valence states, trivalent (Cr +3

), and hexavalent (Cr +6

). The

presence of Cr +6

designates these as “chromate” passivates. The formation mechanism of a chromate

coating is a dissolution and precipitation process similar to what occurs during phosphating. The

thickness and color of chromate coatings depends mainly on the chromate concentration, pH, and dipping

time. They are usually applied so thin that they are essentially invisible. Thicker chromate coatings mayhave a yellowish or greenish appearance and could be anywhere from 0.1 to 0.6 µm thick. The total

chromium content of a normal, invisible coating is usually 1-2 mg/ft2, with less than half as hexavalent

chromium in a complex mixture of metal salts and oxides.

Protection of the zinc is afforded through barrier and passivation effects. The complex chromium oxide

acts a barrier while the hexavalent chromium contained in the film serves to re-passivate exposed metal.

Water that comes in contact with the film dissolves the hexavalent chromium, forming a chromate

solution, which then forms a fresh passivation film on the surface. This is the reason for the “self healing”

ability of chromate passivation films. This self-healing attribute is limited under wet conditions. The result

is that chromate passivate films do not prevent the eventual formation of storage stain if the water is

allowed to remain between contacting surfaces. In any case however, even when the sheet is kept dry,Cr

+6 eventually oxidizes to Cr

+3.

Chromate passivation films are generally not considered paintable without the use of extreme removal

procedures. Also, they cannot be effectively phosphate treated.

Any type of chromium passivation interferes negatively with the spot weldability of galvanized sheet.

Chrome “poisons” the copper electrodes, softening them, and reducing tip life. For this reason, almost all

galvanized sheet intended to be fabricated using automated spot welding equipment, is ordered and

produced as non-chemically treated (unpassivated).

Due to health, safety, and environmental concerns, the use of hexavalent chrome is being discontinued2.

This move began with the European Union RoHS Directive to eliminate hexavalent chromium and other

substances from essentially all new electrical and electronic equipment by July 1, 2006. This regulation is

being used as a model in many other countries. It is significant that these regulations do not prohibit the

use of chromium; rather they prohibit the use of chrome in the hexavalent state2. The use of chrome in

other valence states, e.g., Cr +3

, is acceptable. As a result, the development of passivates to replace

chromate is evolving in two directions: products containing trivalent chromium (Cr +3), or products entirely

free of chromium.

Trivalent Chromium Passivation Treatments

The use of trivalent chromium products allows the retention of some of the advantages of chromium-

based systems yet avoids the use of hexavalent chromium treatments2. These products have been in

use for several decades and are less costly than non-Cr based passivates. Obtaining all the benefits of

chromate is difficult to achieve with trivalent Cr so the latter must be applied at heavier coating weights to

obtain the same corrosion protection as the former. Trivalent treatments can be applied successfully on

both flood/squeegee and chemcoater application systems. Many of these treatments are paintable.

Henkel, a GalvInfo Sponsor, now produces a Cr +3

passivate, called Bonderite® 6020, that can be utilized

at coating weights as low as 2.0 mg/ft

2

, has improved paintability, and has been approved by majorelectronic manufacturers.

Chromium-Free Passivation Treatments

Chrome-free coatings are manufactured from both organic and inorganic materials that can contain many

different ionic species, including molybdates, tungstates, vanadates, titanates, and fluorides. While they

can be applied using flood/squeegee and chemcoater units, the application parameters have been found

to be more critical compared to those for Cr +6

systems and coating weights need to be as high as 30





3

mg/ft2. Corrosion performance tests compare favorably with hexavalent systems with excellent

formability. Paintability, however, is similar to chromate passivation.

Henkel, produces a chromium-free passivate, called Passerite® 5004, that is being used in several

galvanize mills to achieve RoHS compliance.

Another non-chromium product has been developed by Henkel2 to replace Cr

+6 containing acrylic

coatings. Thick acrylic coatings are used on zinc and aluminum-zinc coated sheet, not only as

passivates, but as clear protector films to avoid handling marks and to keep the sheet brighter for a longer

period. The Henkel product is known as Granocoat® 342 and is an inorganic system containing vanadium,

cerium, zinc, molybdenum, and titanium reacted with fluoride and phosphate. The thickness is as high as

250 mg/ft2 and is applied using a roll coater at ambient temperatures. The paintability is improved over

hexavalent-based treatments.

Is it Passivated?

It is sometimes necessary in the field to find out if coated sheet has been passivated. In most cases it is

not possible to visually determine this. There are a variety of methods for detecting passivation.

o Surface passivation can be quickly evaluated with 5% hydrochloric acid. A drop will “fizz” on

unpassivated zinc surfaces but show little reaction on passivated zinc.

o The amount of chromium on the surface can always be tested by using chemical stripping and

laboratory analysis but the standard of the industry is to use x-ray fluorescence; for example the

Cianflone PortSpec.

o Analysis for the presence of chromate can also be performed by placing a drop of a

diphenylcarbohydrazide solution on the surface of the sheet and observing if there is a color

change or not. If the drop remains clear, no hexavalent chromium is present. This test is

described in ASTM D 6492 – Detection of Hexavalent Chromium on Zinc and Zinc/Aluminum

Alloy Coated Steel, available at www.astm.org .

o Another quick method of finding out if galvanized sheet has been passivated, with chrome or non-

chrome treatments, is to use a simple condensing humidity test. Place a 4” square of thegalvanize sheet as a lid on a beaker containing 140°F water and leave for 15 minutes. If theunderside remains shiny it is passivated. If it is stained to any degree it is not passivated.

Oils

An alternative to using passivation treatments is to apply oil to the sheet surface. Specially formulated

oils are used that contain rust inhibitors, which are usually polar products designed to strongly adsorb

onto metal surfaces. They are effective in providing protection from humidity rust due to their ability to

prevent moisture from condensing between the laps of a coil or sheets in a bundle. They are not

effective, however, in preventing the penetration of bulk water, e.g., rain, between laps.

Oil also has the benefit of being easily cleaned off at a paint line, so some temporary protection can be

given to metallic-coated sheet without the risk of contaminating the cleaning and pretreatment chemicals

with chrome. For added protection, and/or to assist lubrication during forming, passivated sheets canalso be oiled.





4

A source of additional information on mill-applied surface treatments for storage stain protection can befound in Appendix X2 of ASTM A 924/A 924M Standard Specification for General Requirements for SteelSheet, Metallic-Coated by the Hot-Dip Process, available at www.astm.org .

1 Zhang, Xiaoge Gregory: Corrosion and Electrochemistry of Zinc, Plenum Press, New York, 1996, pp. 16-17.2 Cape, Tom, et al: Non-Hexavalent Chromium Coating Technologies for Galvanized Steel, Galvanizers Association Conference, Montreal,

QC, 2007


Disclaimer:

Articles, research reports, and technical data are provided for information purposes only. Although the publishers endeavor to provideaccurate, timely information, the International Zinc Association does not warrant the research results or information reported in this

communication and disclaims all liability for damages arising from reliance on the research results or other information contained in this

communication, including, but not limited to, incidental or consequential damages.




1


GalvInfoNote

2.11

Preparing Galvanize for Field Painting



This GalvInfoNote deals with surface treatments to enhance field paintability of galvanized sheet .







Preparing for Field Painting of Galvanize

It is very difficult to obtain good paint adherence on new zinc-coated surfaces passivated with chromate

solutions. Where possible, the sheet should be allowed to “weather” for at least 12 months to allow the

surface to oxidize. In some environments it may take up to 18 months for sufficient oxidation to occur to

provide good paint adhesion. Passivated galvanize remains brighter longer than unpassivated, but even

the latter must weather for a period of time before it is ready for painting.

Weathered galvanize has oxidized to the point where chromate and very thin alumina films have been

washed away and the zinc at the surface has been converted to zinc oxy-carbonate. Good paint

adhesion is more readily achieved on such a surface. To determine if the galvanize is ready to be

painted, check if it is water break-free (no beading of water when the surface is wet). If it is not then itmust weather for a longer time or be treated with phosphoric acid or zinc phosphate before painting.

Sources of more detailed information on painting galvanize are: ASTM D 7396 Standard Guide for

Preparation of New, Continuous Zinc-Coated (Galvanized) Steel Surfaces for Painting; and ASTM

D 6386 Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product

and Hardware Surfaces for Painting, available at www.astm.org.

On any galvanized surface that is to be painted, it is extremely important that the surface be clean and

dry. Any surface dirt or rust must be removed with a stiff wire brush. Grease and oil must be removed

with mineral spirits or detergent and water. All traces of soap should be removed by thorough rinsing.

Paint only when the surface is completely dry. Again, having a water break-free surface is preferred to

obtain good adhesion.

Numerous proprietary pretreatment solutions, including zinc phosphate, are available from many

suppliers. The use of these solutions should be seriously considered to maximize the adherence, and

thus protective life, of the paint.

Many different primers are available that are designed for galvanize. The paint supplier should be

consulted as to whether a primer is necessary and compatible with the topcoat being used. Similarly, the

topcoat should be designed for metal surfaces, whether or not a primer is used. Many paints designed

for wood do not perform well on galvanized steel. Products of decomposition in the oils of these paints

react with the zinc surface and cause the paint to peel. Many paints designed for galvanize are based on

acrylic resins.





2

To summarize, when field painting galvanized steel:

• Allow adequate time for weathering or pretreat the surface

• Paint on a clean and dry surface

• Use a paint designed for galvanized steel, including any primer that is used

• Follow the paint suppliers recommendations


Disclaimer:






1


GalvInfoNote

2.12

Pretreatments for Metallic-Coated Sheet



This GalvInfoNote deals with pretreatments used to prepare coated sheet for factory painting .






• Treatments for resistance to handling and fingerprint marks (See GalvInfoNote 2.13)

All coil painting lines have the ability to apply pretreatments prior to application of the paint. Also, some

hot-dip lines can apply zinc phosphate to the coated sheet.

Phosphate Pre-treatments

A common class of pretreatment used to obtain good bonding between paint and galvanized or

galvannealed coatings is phosphate pretreatment. Two of the most widely used versions are zinc

phosphate and iron phosphate. Zinc phosphate is used as a pretreatment on coil prepainting lines and in

post fabrication factory paint processes, including automotive body plants. It can also be applied directly

on galvanizing lines to provide a product designed for field painting. Iron phosphate is used primarily in

post-fabrication factory painting operations to ensure good paint adhesion. In addition to the excellent

effect phosphate coatings have on paint adhesion, they decrease (more so in the case of zinc phosphate)the tendency for paint disbondment during atmospheric exposure in a corrosive environment.

Zinc Phosphate – It can be applied to galvanized sheet by the steel manufacturer (for field painting) or

the coil coater (manufacturer of prepainted sheet), or can be factory applied to cut sheets or fabricatedarticles by the end-use manufacturer. It is very difficult, if not impossible, to successfully phosphate

galvanize that has been chemically treated with chromate, unless the chromate has been removed – a

very difficult task.

The usual zinc phosphating process involves several steps, whether application is on a coil line or to

formed parts. If there are oils present on the surface of the galvanized or galvannealed steel, the first step

is to remove the oil by degreasing. This might involve cleaning with the use of a hot aqueous, alkaline

cleaning solution or by other forms of degreasing using solvents. Hot alkali cleaning is preferred because

is very difficult to get a clean enough surface (water break-free) using solvent cleaning. The next step is

a conditioning stage; the application of a titanium phosphate conditioner to prepare the

galvanize/galvanneal surface for the deposition of a superior zinc phosphate coating. Titanium phosphate

aids in the development of a uniform coating having small zinc phosphate crystals. While several

mechanisms have been suggested, the conditioner can be thought to create seed crystals, which

promote the growth of zinc phosphate crystals on the surface of the sheet. Again, the surface must be

water break-free (no beading when the surface is wet) for this conditioning stage to be effective.

After conditioning, the zinc phosphate coating is applied by immersion in a zinc phosphate solution or by

spraying it onto the surface of the galvanized or galvannealed sheet or part. During the time that the

surface is in contact with the acidic phosphate solution it actually dissolves a small amount of the coating.

At the surface of the zinc, the acid attack of the zinc phosphate produces a localized increase in the pH,





2

resulting in the precipitation and deposition of insoluble zinc phosphate crystals on the surface of the zinc

or zinc-iron coating. After allowing the reaction to take place for some time, this crystallizing action leaves

behind a continuous, relatively thick solid film of zinc phosphate. After this film is deposited, the steel isremoved from contact with the solution and then thoroughly rinsed and dried.

Zinc phosphate coatings often receive a final sealing rinse treatment. Typically, the sealer contains

chromates for enhanced corrosion protection, although chrome-free sealers are available.

The steps in a 6-stage zinc phosphating operation are:

• Alkaline cleaning

• Water rinse

• Titanium activator rinse

• Application of the zinc phosphate solution (spray or immersion)

• Hot water rinse

• Sealing rinse

The total cycle might take several minutes. For example, a spray phosphating time might be of the order

of 3 minutes to develop a film weight of 150 to 300 mg/ft

2

of surface area. However, for coil-linephosphating, typical treatment times are in the range of 5 to 10 seconds, requiring solution parameters to

be adjusted accordingly.

To accomplish the development of the preferred fine zinc phosphate crystalline surface, it is important to

closely follow the specified temperatures, times, and chemical concentrations in each of the abovestages.

For both zinc and iron phosphating, the first way the product is improved is that the somewhat rough and

porous phosphate film allows for mechanical keying between the phosphate and the paint. The

substantial quantity of oxygen in the phosphate film also promotes chemical bonding (hydrogen bonding)

between the paint and phosphate coating. In the case of zinc phosphating, the zinc in the coating

significantly reduces the rate of paint undercutting at areas where the integrity of the paint is destroyed.

Since prepainted steel is fabricated after painting, it can have uncoated shear cut edges exposed to theenvironment and can be subject to damage during installation. Prepainted steel that is pretreated with

zinc phosphate therefore has excellent bond line durability, giving more resistance to paint undercutting

that can start at sheared edges or damaged areas. This is also the case on factory painted articles that

may be subject to paint damage during use.

To further describe this benefit (reduced rate of paint undercutting corrosion), consider a prepainted

galvanized surface. If the paint is damaged, the natural tendency is for the corrosion reaction to undercut

the paint and move laterally along the sheet surface by corroding and dissolving the galvanized coating

near the bond line. This breaks the bond between the paint and the sheet, and the paint can then peel off.

A well-developed zinc phosphate pretreatment slows down this lateral rate of undercutting corrosion, and

a considerably longer product life results.

Some investigators claim that zinc phosphating is particularly effective on zinc coatings containing highlevels of iron1, e.g., galvanneal, as the amount of adhesion-enhancing phosphate formed increases with

increasing iron content in the coating. It may be that the nature of the galvanneal coating surface (see

GalvInfoNote 1.3, Figure 3) simply results in less under-paint corrosion because of superior mechanical

bonding that occurs. In any case, the galvanneal (zinc-iron) coated sheet used by many automotive

companies for auto body panels has time-tested excellent corrosion resistance when coated with zincphosphate and the various multilayer paints systems used by auto body manufacturing plants.





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As stated earlier, some steel sheet manufacturers produce phosphate-treated galvanize directly off

galvanizing lines. This product has a matte grey appearance and provides a corrosion inhibiting,

crystalline zinc phosphate, micro-porous surface that promotes exceptional adherence and corrosion

resistance of field-applied paints. This product is commonly known as “ Bonderized Steel” , and is

illustrated in Figure 1 in the unpainted state.

Figure 1 – Unpainted Bonderized steel roof (courtesy of Steelscape Inc.)

Figure 2 is close-up of Bonderized sheet that has been coated with a clear acrylic lacquer. The

popularity of this material for unpainted architectural uses is growing rapidly in some areas of the United

States. The lacquer coating preserves the Bonderized look for a longer time.

Figure 2 – Bonderized and lacquered galvanized sheet panel (courtesy of Steelscape Inc.)

Iron Phosphate – Many post-fabrication factory phosphating operations use iron phosphate. As it is

easier to apply than zinc phosphate, iron phosphating is generally performed using a 3-stage process

(clean, iron phosphate, rinse/sealer), although there are some 5 stage processes. Iron phosphating is





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less costly than zinc phosphating and does not offer the same corrosion resistance benefits as the zinc-

bearing version. However, if an iron phosphate bath is run with the proper conditions, a zinc phosphate

coating can be applied on zinc coated sheet as it incorporates some zinc from the galvanized layer. Iron

phosphating does result in excellent paint adhesion by the same method described earlier for zinc

phosphating. Since iron phosphating is used primarily for treating fabricated assemblies, the entire

surface gets treated then painted, leaving few if any uncoated edges where corrosion can easily begin.

Many powdercoating operations use iron phosphate pretreatments. The heavier thickness of the paintapplied by this method is a good barrier against the onset of corrosion.

Additional information on phosphate coatings can be obtained from ASM Handbook Volume 5 Surface

Engineering, 1994, pp. 378-404, available at http://asmcommunity.asminternational.org/portal/site/www/

Chromate Conversion Pre-treatments

Chromate conversion treatments change the zinc surface to a complex oxide layer about 0.5 -3 mm thick,

and contain chromium hydroxide, zinc hydroxy-chromate, and zinc chromate2. When used as paint pre-

treatments on prepainting lines these coatings are usually heavier than when used as passivation

treatments, and thus have a greenish/yellow-iridescent, brown or drab appearance. The color varies with

bath formulation, process parameters, film thickness, and substrate. These treatments are used on both

zinc and aluminum-zinc coated steel sheet to enhance the corrosion resistance of the final prepaintedproduct.

On prepainting lines, these treatments can be applied with the traditional tank-spray process, or by a dry-

in-place (DIP) method using roll coaters. Galvanize intended for prepainting is usually produced asunpassivated. On the other hand, passivated Al-Zn (GALVALUME!) is routinely pre-treated on prepaint

lines to remove some of the passivation chrome, then fresh chrome pre-treatment is deposited on top of

the remaining chrome passivate to give excellent corrosion resistance and paint adhesion. Chemical

treatment suppliers should be consulted for specific products to be used for this application.

Chromium based pre-treatments may contain both trivalent and hexavalent chromium. The environmentaldrive to cease using hexavalent chromium, e.g., EU RoHS initiative (see GalvInfoNote 2.10), has resultedin these treatments beginning to be phased out and replaced with the less environmentally sensitive,chrome-free, zinc phosphate pre-treatments. While a well-applied chromate conversion coating does

afford substantial added corrosion resistance to many prepainted systems used on zinc-coated steelsheet, zinc phosphate treatments have gained favor because of their superior resistance to under-filmcorrosion as described in the previous section.

Additional information on chromate conversion coatings can be obtained from ASM Handbook Volume 5Surface Engineering, 1994, pp. 405-411, available at http://asmcommunity.asminternational.org/portal/site/www/


Disclaimer:



1 Zhang, Xiaoge Gregory: Corrosion and Electrochemistry of Zinc, Plenum Press, New York, 1996, p. 262.

2 Fudge, Duane W; Favilla, John R; Coil Passivation, Galvatech ‘04 Conference, Chicago, IL, April 4-7, 2004.




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GalvInfoNote

2.13

Treatments for Resistance toHandling and Fingerprint Marks



This GalvInfoNote deals with pretreatments used to prevent handling and fingerprint marks during

fabrication and installation of products made with coated sheet . Other treatments are used for

different reasons, namely:

• Improving uniformity of surface appearance (see GalvInfoNote 2.8)


• Treatments for improving resistance to storage stain (see GalvInfoNote 2.10)


• Pretreatments for metallic-coated sheet (See GalvInfoNote 2.12)

Some metallic-coated sheet products are susceptible to surface marking during processing and handling.

For instance, galvanized sheet can be permanently marked by the perspiration of workers who come in

contact with it during the manufacture of heating/ventilating ductwork. While not harming performance,

the marking affects the esthetics of the product when intended for an exposed end use, such as shown

Figure 1. The white stains are most likely the result of the salt from the worker’s perspiration permanently

marking the surface. Once stained in this manner, there is no known method of restoring the original

metallic lustre.

Figure 1 – Fingerprinting and handling marks on exposed galvanized ducting





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Aluminum-zinc coated sheet is subject to roll forming and handling marks that appear as permanent black

smudging. Contact with the forming rolls in roofing sheet lines can leave permanent black abrasion lines

on the sheet surface. Workers constructing roofs can leave handprints and boot marks that turn dark and

remain visible for years.

To provide a product that is resistant to marking, the industry has developed special coatings (generally

acrylic-based) for metallic-coated sheet. They are usually applied at the coating line using a roll-coating

technique and infrared and other curing ovens. The coating is clear and typically consists of a water-

soluble acrylic resin and inorganic corrosion inhibitor. Benefits may include being able to be roll formed

dry without need of vanishing oil, resistance to hand and/or foot marking during handling/installation, good

resistance to storage stain/transit corrosion, and retention of brightness over a longer time.

Keep in mind that these products are not all alike. Some are more paintable than others, and if not

painted, tend to dissipate after 12 to 18 months. Non-permanent types also tend to be less roll-formable.

The coatings that are more roll-formable (and have a tendency to be less paintable) are good at staying

on the surface for many years and thus enhance the long-term corrosion resistance, and brightness of the

sheet product. One such product produced by Henkel, a GalvInfo Sponsor, is sold under the trade name

of Granocoat® 342.

Most producers of acrylic-coated metallic-coated sheet market the product with the term “Plus” added totheir normal coated sheet trade names, e.g., “GALVALUME! Plus” or “ZINCALUME! Plus”.

A source of additional information on acrylic-based surface treatments can be found in Appendix X2 of

ASTM A 924/A 924M Standard Specification for General Requirements for Steel Sheet, Metallic-Coated

by the Hot-Dip Process, available at www.astm.org .


Disclaimer:






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3. Corrosion – Mechanisms, Prevention, and Testing

GalvInfoNote How Zinc Protects Steel

Rev 1 Jan-093.1

Introduction

Steel sheet is a very versatile product. It comes in many sizes and types, and is applied to many end

uses including steel buildings, automotive panels, signs, and appliances. The low cost, strength and

formability of steel sheet are some reasons for its widespread use. Unfortunately it is prone to rusting, a

phenomenon that causes the surface to become unsightly and, over time, may contribute to product

failure. For this reason, steel is protected by a variety of methods ranging from internal alloying, e.g.,

stainless steel, to coating with metallic coatings and/or paints.

Corrosion is an electrochemical process that, in the case of steel sheet, oxidizes the iron in the steel and

causes the sheet to become thinner over time. Oxidation, or rusting, occurs as a result of the chemical

reaction between steel and oxygen. Oxygen is always present in the air, or can be dissolved in moisture

on the surface of the steel sheet. During the rusting process, steel is actually consumed during the

corrosion reaction, converting iron to corrosion products. In the case of most low-carbon steel sheet

products, iron oxide (rust) develops on the surface and is not protective because it does not form as a

continuous, adherent film. Instead, it spalls, exposing fresh iron to the atmosphere which, in turn, allows

more corrosion to occur. This aspect of steel sheet behaviour is very undesirable, both aesthetically and

from the aspect of service life. Eventually, often sooner than desired, steel sheet corrodes sufficiently to

unduly shorten its service life, i.e., loss of structural strength, or perforation and intrusion of water.

Fortunately there are many coatings that can be applied to steel in a very cost-effective manner to confer

sufficient corrosion protection to steel so that it can be used for a multitude of demanding applications.

Refer to GalvInfoNote 1.6 for more information on choosing coating types and designations.

Paint Coatings

One well developed coating for steel is paint. Numerous types of paint have been developed through the

years that offer excellent performance on steel sheet. Paints are barrier coatings that, when applied andused properly, give sufficient corrosion protection to steel for many common applications. However,

paints are not impervious to moisture and rust can occur underneath even a perfectly applied paint if the

exposure time to moisture is sufficiently long or at shorter times if the moisture contains corrosive

chemicals. Also, exposed areas on the steel, such as sheared edges and scratches through the paint, are

susceptible to the same rusting mechanisms as unpainted steel. Further rusting in these exposed areas

can cause degradation of the paint adhesion. For example, the iron oxide that forms at a scratch lifts the

paint film immediately adjacent to the scratch. This allows further under-film corrosion of the steel and the

eventual loss of paint adhesion. Some paints are better at resisting this than others, but eventually, if

there is sufficient moisture present, and especially if the moisture contains highly corrosive chemicals,

unacceptable levels of rusting will occur. Even if the amount of rust does not adversely impact the

strength of the steel, it does lead to an unsightly appearance.

Metallic Coatings

Metallic coatings are a well-developed method of protecting steel and provide it in two ways:

1. like paint, they provide barrier protection, and

2. galvanic protection in most instances.



GalvInfoNote 3.1Rev 1 Jan -09


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These protection mechanisms are described below.

Barrier Protection

The main mechanism by which galvanized coatings protect steel is by providing an impervious barrierthat does not allow moisture to contact the steel, since without moisture (electrolyte) there is no

corrosion. The nature of the galvanizing process ensures that the metallic zinc coating has excellentadhesion, abrasion, and corrosion resistance.

Galvanized coatings will not degrade (crack, blister, and peel) as with other barrier coatings such aspaint. However, zinc is a reactive material and will corrode and erode slowly. For this reason, theprotection offered by a galvanized coating is proportional to its thickness and to the corrosion rate. It istherefore important to understand zinc's corrosion mechanism and what factors affect the rate.

Freshly exposed galvanized steel reacts with the surrounding atmosphere to form a series of zinccorrosion products. In air, newly exposed zinc reacts with oxygen to form a very thin, tenacious zincoxide layer. When moisture is present, zinc reacts with water, resulting in the formation of zinchydroxide. The final corrosion product is zinc carbonate, which forms from zinc hydroxide reacting withcarbon dioxide in the air. Zinc carbonate is a thin, tenacious, and stable (insoluble in water) layer thatprovides protection to the underlying zinc, and is the primary reason for its low corrosion rate in mostenvironments. Refer to GalvInfoNote 3.2 for more information on the films that form on zinc.

Other metallic coatings, such as aluminum, also provide good barrier protection for steel sheet. Why isthis the case with aluminum? Similar to steel and zinc, aluminum reacts in air to form an oxide film onits surface. However, contrary to the behaviour of iron oxide, and similar to what happens with zinc, thealuminum oxide film that forms does not spall off, and remains as an intact, very tightly adhering film onthe surface of the aluminum. By preventing exposure of fresh aluminum to air and moisture this intactfilm stops further corrosion of the underlying aluminum. The oxide remains as a stable non-corrodingfilm.

Galvanic (Cathodic) Protection

The second shielding mechanism is zinc's ability to galvanically protect steel. When base steel isexposed, such as at a cut edge or scratch, the steel is cathodically protected by the sacrificial corrosionof the zinc coating. This occurs because zinc is more electronegative (more reactive) than steel in thegalvanic series, as shown below.

Galvanic Series of Metals and AlloysCorroded End - Anodic

(Electronegative) Magnesium

Zinc AluminumCadmium

Iron or SteelStainless Steels (active)

LeadTin

CopperGold

(Electropositive) Protected End - Cathodic or most noble

Note: Any one of these metals and alloys will theoretically corrode while protectingany other that is lower in the series as long as both form part of an electricalcircuit.





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In practice, this means that a zinc coating will not be undercut by rusting steel because the steeladjacent to the zinc coating cannot corrode. Any exposure of the underlying steel, due to severecoating damage or a cut edge, will not result in corrosion of the steel until the adjacent zinc has beenconsumed. Unless relatively large areas of steel are exposed there is minimal effect on the overallservice life of the coating.

The distance over which the galvanic protection of zinc is effective depends on the environment. Whencompletely and continuously wetted, especially by a strong electrolyte, e.g., seawater, relatively largeareas of exposed steel will be protected as long as any zinc remains. In air, where the electrolyte isonly superficial or discontinuously present (such as from dew or rain), smaller areas of bare steel areprotected. The “throwing power” is nominally about 0.125 in (3.2 mm), although this can varysignificantly with the type of atmosphere.

If the coating is consumed, why use it? In the case of a zinc coating, the rate of corrosion that itundergoes while protecting steel is considerably lower than that of the steel (by at least a factor of 10). Inthis way, a thin coating of zinc can protect steel for a long time. For example, in a rural atmosphere,where the number and concentration of pollutants in the air is generally quite low, zinc might corrode at a

rate of 1.0 μm/year (0.04 mil/year), while low-carbon steel in this same environment might corrode at a

rate 10 times as high (10 μm/year or 0.4 mil/year), or even higher. The primary reason for the reduced

rate of zinc corrosion versus the rate for steel is that, as it corrodes, zinc forms an adherent, protectiveoxide/carbonate film on its surface similar to the oxide film on the surface of aluminum. This film helps toprevent contact between the environment and fresh zinc, and the rate of corrosion is kept low. Recall thatsteel typically does not form a protective film in that the oxide layer spalls, constantly exposing fresh ironto the environment.

The film that forms on the surface of zinc is not as protective as the aluminum oxide film on the surface ofmetallic aluminum. One reason is that zinc oxide/carbonate is susceptible to dissolution if the moisture issufficiently acidic. This is good and bad. It is good in that, if the oxide film were totally protective, the zincwould no longer offer galvanic protection to the steel at exposed areas. Rusting of steel would thereforeoccur at scratches and other exposed areas. The downside of the somewhat incompleteness ofprotection of the oxide film on a galvanized sheet is that the coating does corrode and is eventuallyconsumed.

Other Galvanically Protective Coatings

Among the commercially available metallic-coated steel sheet products, zinc (galvanize) offers the mostgalvanic protection. Zinc-5% aluminum alloy coated behaves similarly with respect to the level of galvanicprotection that it provides. Steel sheet with a 55% aluminum-zinc alloy coating offers somewhat reducedgalvanic protection versus a galvanized or Zn-5% Al alloy coating. What does this mean about the relativeperformance of these products?

As with most things in life, everything comes with a price. Galvanically protective coatings are consumedby corrosion eventually. That is why a galvanized sheet has a definite life span; time before corrosion ofthe steel begins. Thus, the amount of zinc applied to the steel during manufacture, described as thecoating weight (mass), is important to the life of the product. Coating weight (mass) is expressed usingterminology such as G60 [Z180], G90 [Z275], G200 [Z600] etc. per ASTM Specification A 653/A 653M.

G60 [Z180] means that the coating weight (mass) is 0.60 oz/ft2

[180 g/m2

], minimum (total coating on bothsides of the sheet). This coating weight (mass) can be translated into thickness; a G60 [Z180] coating ofzinc is about 0.00055 in. (0.014 mm) per side of the sheet.

For a galvanized coating, the rate of corrosion is typically linear in most environments, i.e., twice thecoating thickness translates to twice the “life”. Of course, different environments are more or lesscorrosive than others, so that the “life” of the coating varies considerably for different “environment types”.This behaviour is shown in the following figure.





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Note: for a quantitative version of this chart refer to GalvInfoNote 1.6

These three types of atmospheres all exhibit different rates of corrosion, but the behaviour is linear. Forexample, a G90 [Z275] coating will exhibit a longer life in a rural atmosphere than in a polluted industrialatmosphere, but still the rate is linear in both environments. Twice the coating thickness or coating masswill give twice the coating life.

Now, consider the life of a 55% Al-Zn alloy coating, a product that clearly exhibits less galvanic protectionthan a galvanized coating, at least after some time of exposure. A 55% Al-Zn alloy coating, being lessgalvanic in nature, is less reactive, and as a result, the life of this coated product is usually considerablylonger than for a comparably thick galvanized coating. This behaviour of a 55% Al-Zn alloy coating is thereason why it is being used so successfully for bare roofing applications. Of course, there are applications

where the highly galvanic nature of zinc is desired. Also, there are other considerations that need to betaken into account when selecting a product for a specific application.

Life of a Galvanized Coating in Different Environments

We have established that a galvanized coating protects steel:

1. by two mechanisms – barrier protection and galvanic protection, and

2. by a linear rate of corrosion for any specific environment type, and

3. for considerably different time periods depending on the specific environment

What is the life of the product in specific applications? The answer to this question is very complex. Thereare textbooks written on this subject. Two excellent references are:

Corrosion Resistance of Zinc and Zinc Alloys , by Frank C. Porter, published Marcel Decker, Inc., 1994

Corrosion and Electrochemistry of Zinc , by Xiaoge Gregory Zhang, published by Plenum Press, 1996

Increasing time of exposure

Thickness ormass loss Rural atmosphere

Marine

atmosphere

Industrial atmosphere





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The following table contains a small amount of the data in the book compiled by Frank Porter.

Location/Environment Type Corrosion Rate, m/year for 5 years

State College, PA/rural 1.0

South Bend, In/semi-rural 1.9

Middletown, OH/semi-industrial 1.3

Kearny, NJ/Industrial-urban 4.5

Kure Beach, NC/marine 1.1

Daytona Beach, FL/marine 1.8

Montreal, Quebec/industrial-marine 3.5

Esquimalt, BC/rural-marine 0.5

Sheffield, Great Britain/industrial 5.1

These data are for exposure in the atmosphere. There are many other applications for galvanized sheetin which the corrosion rate might be different. These include: contact with water, buried in soil, contactwith concrete, sheltered areas on buildings such as under eaves, ductwork inside buildings, etc. For eachof these and other applications, the corrosion performance depends on many of specific aspects of theapplication. For example, when used in contact with concrete, how often does the concrete/galvanizedsheet interface get wet? When buried in soil, what is the pH of the soil and the soil permeability, theoxygen content, etc? When used for ductwork, does the product experience condensation periodically oron a regular basis? Are there pollutants in the condensate? Many of these types of questions are

answered in the referenced textbooks.

As explained in GalvInfoNote 1.6, it should be emphasized that much of the zinc corrosion data given inthe above references was generated in the 1950–1970 era. Aggressive pollutants such as sulfur dioxideare much declined from their higher levels of the mid 20

th century. The service life of galvanize in, say,

urban industrial areas is now longer than it was 30 to 50 years ago. On the other hand, corrosion rates inmarine environments are not so much changed, since the rate of zinc loss is governed more by theamount of deposited sea salt than airborne pollutants.

The life expectancy of galvanize sheet is now more reliably determined using the Zinc Coating LifePredictor (ZCLP). This software was developed by Gregory Zhang of Teck Cominco, and can be foundat www.galvinfo.com in the GalvInfo Library – Addi tional Information section. It is applicable to allzinc-coated steel; that is, coatings composed of zinc exclusively. It does not apply to zinc/aluminum oraluminum/zinc coatings. It performs calculations based on statistical models, neural network technology

and an extensive worldwide corrosion database. The environmental input data obtained by the AGA wasfrom the World Wide Web. The calculated corrosion rates used to generate the service life chart in Figure1 of GalvInfoNote 1.6 are based on the ZCLP model, and are an average for six different North Americancities in each of the five climate categories.





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Summary

Hopefully, this discussion helps explain some of the issues associated with the effect of coatings on thecorrosion behaviour of coated-steel sheet products. This field of technology has many aspects thatcannot be explained in a brief article such as this. If you have additional questions on this very extensivesubject, please contact the GalvInfo Center. Refer to GalvInfoNote 1.6 for more information on choosing

coating types and designations.

Another source of information on how zinc protects steel can be found in ASM Metals Handbook Vol. 13B

Corrosion: Materials, 2005, pp. 402-417,available at: http://asmcommunity.asminternational.org/portal/site/www/


Disclaimer:





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GalvInfoNote Protecting Galvanized Steel Sheet Productsfrom Storage Stain

Rev 1.1 Mar-093.2

Introduction

Storage stain, when related to galvanized sheet products, is a corrosion stain that is typically white, but which

can also take the form of a grey or black deposit on the surface. Since the most common form of discoloration is

white in appearance, storage stain is often called white rust. It can occur when sheets of galvanized steel that

are in close contact (in a coil or stacked in lifts/bundles) get wet, either by direct water intrusion, or condensation

between the surfaces. The discoloration is due to the corrosion products that form after zinc reacts with moisture

in the absence of free air circulation.

Building erected using roll-formed galvanizedpanels from a bundle that had extensive amountsof white rust.

Note: This voluminous amount of white rust did not occur aftererection of the building siding. When freely exposed to air, theproducts of zinc corrosion form a thin, tenacious film. Thesurface accumulation seen here occurred while the sheetswere stored in a wet bundle.

Before discussing the issue of storage stain in more detail, let’s first review what happens when a galvanized

(zinc) coating corrodes in the environment.

Why Does Zinc Protect Steel?

Zinc, by its very nature, is a “reactive” metal and tends to corrode quite readily when exposed to moisture. Why

then does it protect steel when a zinc-coated sheet is exposed to the atmosphere?

When zinc corrodes in the presence of air and moisture it undergoes a series of chemical reactions, changing

from metallic zinc on the surface to other chemical compounds. In air, newly exposed zinc reacts with oxygen to

form a very thin oxide layer. In the presence of moisture the zinc oxide reacts with the water, resulting in the

formation of zinc hydroxide. Over time, and under the influence of cyclic weathering, the final corrosion product

is zinc carbonate (formed by the reaction between zinc hydroxide and carbon dioxide in the air). Zinc carbonate

is a thin, tenacious, compact, and stable (insoluble in water) film. When the surface is further exposed to rain or





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condensation, the protective film serves as a barrier between the moisture and the zinc underneath. This type of

chemical layer is called a passive film. It is the presence of this passive film that slows down the reactivity of

the zinc, thereby dramatically reducing the corrosion rate of the zinc coating.

The series of chemical reactions described above are:

Zn + ½O2 ZnO

ZnO + H2O Zn(OH)2

Zn(OH)2 + CO2 ZnCO3 + H2O

A through-thickness schematic that shows the cont inuous, protective oxide/carbonate

passive film that forms on galvanized sheets exposed to the atmosphere.

Typically, when low carbon steel corrodes, the corrosion products (iron oxide and/or iron hydroxide) do not form

a continuous, protective, passive film. Instead, they tend to spall or develop cracks, which allows moisture and

air continued access to the iron, continuing the corrosion reaction. The difference in oxide film-forming

behaviour between iron and zinc is the underlying reason for galvanizing extending the life of steel. For

more information on how zinc protects steel, see GalvInfoNote 3.1.

In most applications, the passive surface film that forms on zinc, while tenacious, is not totally protective, andcontinued corrosion does occur over time. However, because of the nature of the passive film, the corrosionrate of a zinc coating is diminished substantially – typically by a factor of 10 compared to bare steel.

Why is Zinc Susceptible to Storage Stain (White Rust)?

As stated above, zinc is very reactive metal. It exhibits a low corrosion rate only because a continuous passivefilm forms on the surface. A key part of the corrosion mechanism is that the surface needs to dry in air in orderto develop and maintain this passive layer. It is during the drying part of a rain cycle that the zinc carbonatepassive film develops. Atmospheric wet-dry cycles are therefore necessary for zinc to develop passivity.

When galvanized sheet gets wet while still in coil form, or stacked into bundles at a roll-forming plant or jobsite,storage stain can result. Storage stain (white rust) is simply the chemical compound, zinc hydroxide (ZnOH),which forms when zinc is in contact with moisture. It does not convert to a zinc carbonate passive film because

Steel sheet

Zinc coating

Continuous zinccarbonate passivefilm





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the tightly packed sheets are not freely exposed to oxygen/carbon dioxide-containing air. A protective zinccarbonate film never gets a chance to form. Since the corrosion reaction continues to proceed as long as thesurfaces are wet and starved for oxygen and carbon dioxide, a large accumulation of zinc hydroxide can form.Zinc is a very reactive metal in the presence of moisture when conditions do not allow the passive film to form.

When white rust does occur, there is an actual loss of zinc coating, and some of the zinc that is intended to

protect the coated steel product while in service is consumed by oxidation. The extent of the damage is primarilydependent on:

1. the exposure time to moisture,

2. the temperature that is experienced during storage, and

3. the presence of accelerating corrosive agents, such as chloride-containing salts.

Often, the amount of white rust appears to be quite heavy when, in fact, the amount of zinc corroded is small.This occurs because ZnOH is somewhat flocculent, and builds up in the area of the wetness. When ZnOH driesin the open air it converts to zinc oxide (ZnO). If the application is not aesthetically critical, the galvanizedcoating should perform very well and meet the requirements and expectations of the end user. In mostinstances involving outdoor exposure, the white rust will disappear over time, as it is either washed off by rainfall

or is converted to zinc oxide and then zinc carbonate.

The surface of the zinc coating in the area that experienced white rust is “etched” and no longer has the bright,reflective appearance of as-produced galvanized sheet. Removing the white rust (see section at the end of thisarticle) will not eliminate the etched appearance. This is why, for applications where appearance is critical,galvanized sheet with white rust may not be acceptable.

There are times, albeit seldom, when the sheets have been wet for a long time; long enough that the corrosionof the zinc coating is severe. In these cases, the product may no longer provide the corrosion resistance desiredfor the application. In addition, the storage stain may take on a dark grey or black appearance. When the stainson galvanize turn black it usually means that iron has become part of the corrosion product, i.e., enough zinchas been consumed to expose the steel substrate. Nevertheless, it takes a trained observer to determinewhether or not the amount of corrosion that has occurred is severe or not.

Preventing Storage Stain

Clearly, it is very desirable to take every precaution to avoid storage stain on galvanized sheet. Often, thecustomer’s application requires the aesthetic appearance of a bright galvanized surface, and no amount ofstorage stain is acceptable. Fortunately, practices have been developed that allow the shipment and storage ofgalvanized sheets without the formation of storage stain.

Chemical Treatments (Passivation)

The best way of minimizing the chance of white rust forming during shipment and storage of coils, shipmentand storage of lifts of sheared blanks, or shipment and storage of stacked bundles of roll-formed panels at a

jobsite, is the application of a chemical treatment by the steel sheet manufacturer. This passivation coatingis applied on the galvanizing line. It is very thin, and usually invisible. The most common type of passivationtreatment has been a water-based chromate coating. Chromate treatments contain hexavalent chromium.

They are typically applied by spraying the solution onto the surface, after which the excess is “squeeged” offusing rubber-coated rolls. Following this, the passivating film is dried thoroughly before recoiling the coatedsheet at the exit end of the galvanizing line.

Due to health, safety, and environmental concerns, the use of hexavalent treatments is being discontinued.They are being replaced by treatments that are free of hexavalent chromium; either those where thechrome is only in the trivalent state, or products entirely free of chrome. Refer to GalvInfoNote 2.10 formore information on these new treatments.





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Passivation coatings have been in use for many years, and their performance has been exceptional withrespect to minimizing the tendency for staining when the sheets get wet in coil or bundle form. Steel sheetmanufacturers use the term “passivation treatment” or “chemical treatment” for this surface treatment. Bothterms are used interchangeably. When an order is placed, it is necessary to specify whether chemicaltreatment “ is” or “ is not” required.

It is important to remember that mill-applied passivation treatments minimize the tendency for storagestain; they do not eliminate its occurrence if the product is subjected to very adverse conditions. Anexample would be having a coil get wet during transit to a customer, and then allowing the coil to sit in awarehouse for a long period without any attempt to dry it. Even if the product is ordered with a chemicaltreatment, it is still important to keep moisture from between the wraps while in coil form, blanked sheets, orin bundles.

Surface passivation treatments also assist the product in another way. When coated sheets are put intoservice and exposed to the atmosphere, the passivation treatment helps to maintain the bright, shinyappearance. Eventually, the brightness diminishes, but the passivation treatment aids in keeping the shiny,metallic appearance for a considerable time. The longevity of this effect depends on the type of treatment,the environment, and the relative corrosivity of each location. Also, as the surface dulls, it tends to do so in amore uniform fashion than if the sheets were unpassivated.

Visually, it is generally not possible to tell if galvanized sheet has been passivated. ASTM StandardPractice D 6492 – Detection of Hexavalent Chromium on Zinc and Zinc/Aluminum-Alloy Coated Sheet, canbe used to determine if the sheet has been treated with a chromium based passivating solution. As anyhexavalent chrome in the treatment eventually oxidizes to trivalent chrome, this test only works on freshly(at most a few months) passivated sheet.

Another quick method of finding out if galvanized sheet has been passivated, with chrome or non

chrome treatments, is to use a simple condensing humidity test. Place a 4” square of the galvanize

sheet as a lid on a beaker containing 140°F water and leave for 15 minutes. If the underside remains

shiny it is passivated. If it is stained to any degree it is not passivated.

For more information on passivation see GalvInfoNote 2.10.

Passivating Oils

Besides the use of chemical passivation treatments, other surface treatments can be used. The mostcommon are rust-inhibitive oils. These are oils containing corrosion inhibitors that provide some protectionfrom storage stain. The inhibitors are usually polar products designed to strongly adsorb onto metalsurfaces. The oil serves as the carrier solution for the inhibitor. As with chromate treatments, the oil isapplied by the steel sheet manufacturer on the galvanizing line. A common method of applying the oil is byan electrostatic applicator. These oils are not intended to provide sufficient lubrication for applications suchas deep drawing, but they do provide some lubrication and can assist with some forming operations.

Another type of oil is “vanishing oil”. It is a volatile compound that evaporates when exposed to air, andleaves behind a corrosion inhibitor on the sheet surface.

Oils are effective in providing protection from humidity rust due to their ability to prevent moisture fromcondensing between the laps of a coil or sheets of a bundle. They are not so effective, however, in

preventing damage from the penetration of bulk water, e.g., rain, between laps.

Which Treatment to Apply

Often, the end use defines whether a passivation treatment or rust-inhibitive oil should be applied. Typically,when the end use is one that does not involve painting, the passivation method is best, although somepassivates are compatible with painting. If the application requires painting in the manufacturer’s plant, rust-inhibitive oil is usually best, but check with the producer to see if they use a passivate that is paintable.





5

Because of the changing situation with passivation treatments, it is best for a user to discuss their needswith the galvanized sheet producer. For instance, it is possible to order a product with both chemicaltreatment and oil. Typically, this combination provides better white-rust protection than either chemicaltreatment or oil used separately, and should be considered when harsh storage conditions are expected.

When the end use involves spot welding or coil prepainting it may be necessary to order the product as

unpassivated. When this is the case, there must be absolute certainty that it w ill not get wet before itis used. Precautions that can be taken are explained below.

Other Ways to Protect Galvanize from White Rust

Besides the use of oils or chemical treatments, there are other ways to minimize the tendency for storage stain.

A common method involves “wrapping the coil” by the sheet manufacturer. Both plastic and paper wrapping

materials are available. The packaging material may have a corrosion inhibitor impregnated into it to provide

even better protection.

In addition, the prevention of storage stain is strongly influenced by the methods and practices used forshipment from the steel manufacturer to the customer. It is vital to prevent water intrusion and to use practicesthat minimize the tendency for condensation during transit and storage. It is especially important to maintaincontrolled temperature storage (sometimes even during transit), to prevent condensation that can occur if thetemperature of drops below the dew point, or cold steel coils/bundles are moved into a warm, moistenvironment.

How do Coils Get Wet?

Coils or bundled sheets get wet in two ways:

1. Water from rainfall gets between the sheets while the product is in transit or while it is sitting at a jobsite

2. Condensation.

Condensation occurs when the coil or stacked sheets are below the dew point of the local atmosphere. Oneway for this to occur is when coils are shipped during the winter, and then placed into a warehouse that is

warmer than the galvanized steel, and where the humidity is not at a controlled low level. Under theseconditions, moisture simply condenses onto the steel’s surface as the cold coil causes the nearby airtemperature to drop. This is similar to the condensation of moisture onto a cold windowpane.

Condensation can occur in other ways that are not as obvious as that above. For example, even if the coiltemperature and the temperature inside the local warehouse are about the same when the coil arrives at acustomer’s plant, but the warehouse is not temperature controlled, cooling overnight (or other rapid temperaturedrop) might allow condensation to occur on the coil and even between adjacent wraps. Once moisturecondenses, it takes a long time for it to dry because there is so little air movement between wraps in a coil orbundle.

Because there is no absolute way to totally prevent storage stain once the material gets wet, it is important forthe best practices to be applied at all steps in the process.

Best Practices

1. The steel manufacturer needs to apply the chemical treatment and/or oil in a controlled manner tocover the entire surface area of the sheet.

2. If possible, the coils should be wrapped with either paper or plastic that is specially made for thisapplication.





6

3. The shipper needs to protect the steel during shipment to the customer’s plant. Even if hecoils/bundles are wrapped, ship only in covered watertight conveyances. If it is necessary touse an uncovered conveyance, wrap the load completely with a tarp to assure no waterintrusion if it rains during shipment. Avoid tearing the paper.

4. The customer should store the coils in a climate-controlled warehouse. Use the material promptly.

Whenever possible, do not allow the product to remain in storage for extended periods of time (inexcess of two months).

5. For shipping from the customer’s plant to the final location, the product again needs to be protected,especially if the sheets are in intimate contact with each other. When this is the case, the product isvery susceptible to storage stain as the sheets will not dry properly if they get wet.

a. Paper wrapping is one way to protect the sheets while in transit or during storage at a jobsite.Be careful to not wrap the bundle while the sheets are wet. This traps moisture in the bundleand prevents drying.

b. Do not wrap the sheets tightly in plastic. Allow the product to “breathe” by providing aircirculation.

c. Store the lifts of sheets indoors if possible.

d. Store the panels above ground by at least 12 inches to allow air circulation beneath thebundle. If bundles are stacked, ensure free circulation of air between bundles using curedlumber spacers.

e. Inspect frequently to assure that the panels have not become wet.

f. Elevate one end of a bundle of sheets to allow water to drain if moisture gets into the lift ofsheets. Make sure there are no low spots along the length so as to allow water to flow freely ifnecessary.

Treatment of Galvanized Steel Damaged by Storage Stain

Galvanized sheet damaged by storage stain generally cannot be restored to its original high lustre appearance.The stain, depending on severity, irreversibly alters the surface characteristics of the zinc to varying degrees.Nevertheless, there are treatments that are helpful in improving the appearance, depending on the severity ofthe stain.

1. For less severe initial white rust, rub/brush the surface with a mixture if mineral oil and sawdust. Themild abrasive action may remove the stain, although this treatment is not of much help for advancedwet storage stain. Many users have found that if lightly stained panels are used as is, the stain“weathers off” after being exposed to the outdoor environment for a year or so, depending on thelocation.

2. If the stain is not too severe, it may be removed by washing with white vinegar, followedimmediately by a thorough rinsing with water to neutralize the surface. The removal can be

assisted by the use of a stiff nylon brush. The sheets must be dry before restacking. Thistreatment will remove some of the metallic lustre from non-stained areas, but not to an excessivedegree.

3. Other commercial products that will clean white rust in a manner similar to white vinegar are:CLR

®, limejuice, Naval Jelly

® Rust Dissolver, and Picklex™ 10G

1.

1 Duran, B., Langill, T., Cleaning Wet Storage Stain from Galvanized Surfaces, 2007, American Galvanizers Association





7

4. For more severe staining, a solution of 5% (by volume) of phosphoric acid in water, with a wettingagent added, can be brushed onto the sheets. The sheets must be immediately well-rinsed toneutralize the surface and then thoroughly dried. This treatment will remove most of the metalliclustre, even in non-stained areas.

5. If the stain has progressed to dark grey or black in color, removal is probably not possible.

6. One method of restoring the protective value of the zinc coating, and improving the appearanceof storage stain-damaged sheets, is to apply a good, color matched, zinc-rich paint. The surfacemust be thoroughly cleaned using any of the products described above, brushed, rinsed anddried beforehand. After two years or so, weathering will largely remove any difference inappearance between the zinc-rich paint and the galvanized surface.

Two excellent references that were consulted when preparing this article and that discuss storage stain ingreater detail are:

[1] Zhang, Xiaoge Gregory: Corrosion and Electrochemistry of Zinc, Plenum Press, New York, 1996, pp. 236-239.[2] Porter, Frank C.: Corrosion Resistance of Zinc and Zinc Alloys, Corrosion Technology Series, Vol. 6, P.A.

Schweitzer (ed), Marcel Dekker, New York, 1994, pp. 64-66 and 372-373. Summary

Storage stain, or white rust, is the surface corrosion that occurs on galvanize when sheets get wet while tightly

bundled (in coils, or in lifts of blanked sheets/roll-formed panels), and then not immediately separated and

allowed to dry. The continual wetness prevents the formation of a protective passive film on the zinc surface.

The result is a stained, discolored sheet for which it is virtually impossible to return to its original shiny metallic

appearance.

To prevent white rust, galvanized sheet must be protected from contact with moisture whenever the sheets are

in close contact (coiled or bundled) and free airflow is not available to dry the surface.

Copyright©

2009 – IZA

Disclaimer:





GalvInfoNote

3.3

Dark-Colored Stains on Galvannealed Sheet

Rev 0 Jan-07

Introduction

Normally, galvannealed steel sheet has a very uniform matte-grey appearance, as is shown in Figure 1.

Figure 1: Normal appearance of galvannealed steel sheet

The color may be slightly lighter or darker, reflecting a respectively lower or higher percentage of iron in thecoating.

At times, the users of galvannealed sheet complain about localized dark stains on the surface of a galvannealedcoating. Often, these stains have a dark grey to black appearance. The dark stained areas usually transition to alighter grey around their periphery, but at times, there is anything from a straw colored to a dark purple huearound the edges.

Dark areas can range in size from small to large (less than 1 inch to many inches in diameter), be rounded orvery irregular in shape, be uniformly dark or exhibit variable darkening, or show a gradation in darkness as theytransition to the unaffected areas.

Figure 2 shows two photographs of galvannealed coatings with areas of dark stain and the transitioning to anormal grey appearance.


1






2

Normal grayappearance

Transition region showingdifferent shades of color

Dark stained region

Figure 2: Dark stains on galvannealed steel sheet





Source of Dark Staining on Galvanneal

Regardless of the color or shape, there is one primary cause for darkening. This is the presence of moisture,either from intrusion during storage and shipping or from condensation within the coil during transit and storage.

With galvanized sheet product, if moisture gets trapped between the adjacent wraps in a coil, or between sheets

in a lift, the usual result is “white rust”. Most users of galvanized sheet have encountered this phenomenon. Thewhite color is that of the corrosion products (primarily zinc hydroxide) that form when moisture corrodes the zincin the absence of free air circulation. Typically, white rust is present as a flocculent, loose powder . For a detaileddiscussion of white rust, please refer to GalvInfoNote 3.2.

For galvannealed sheet product, corrosion of the coating also occurs if the coil or bundle of sheets gets wet andhas no way of drying. Instead of forming white rust, the first appearance change is the formation of an adherentoxide that is most often dark grey to black in color. The iron in the coating is the main reason for nature of thiscorrosion product – the result of localized galvanic coupling on the Zn-Fe surface alloy layer. The stain does notform as a loose powder, but is a thin, often adherent continuous film. Its dark grey to black color is associatedwith the light absorbing characteristics of the thin film of corrosion product. If the sheet stays wet for long periodsand large amounts of corrosion product form, the stain may begin to exhibit the same flocculent white corrosionproduct that is observed on storage-stained galvanized sheet.

Why is Galvannealed Sheet Susceptible to Dark Staining?

The primary reason that the dark corrosion film is seen more than perhaps expected on galvannealed sheet isbecause this product is often shipped “dry – no oil and no surface passivation (chemical treatment)” to thecustomer. There is no rust-inhibitive oil applied at the exit end of the galvanizing line, nor is the surfacepassivated with a clear chromate treatment to minimize white rust.

The reasons for shipping without any type of corrosion inhibiting surface film are:

1. Chemical treatments may interfere with obtaining good paint adhesion. Since galvanneal is intended tobe painted, most customers do not want to be concerned about the possible adverse effect onpaintability due to a surface passivation film.

2. Many customers do not want to be concerned about removing the rust-inhibiting oils present on thesurface, which is necessary, because as with a passivation film, oil can also interfere with paintadherence. Excellent cleaning methods are required to get rid of all the oils, especially since thegalvannealed surface is somewhat rough and porous. If the oil is not totally removed, paint blisteringcan occur during baking of thermally cured paints. If the paint is air-drying, the presence of any oilsbetween it and the galvannealed surface can lead to premature separation of the paint during service.

3. During any subsequent zinc or iron phosphating treatments that the product may undergo, chromatepassivation treatments will prevent the deposition of the phosphate compounds onto galvanneal (andgalvanize) surfaces.

4. Passivation treatments interfere with the spot welding of galvannealed (and galvanized) steel parts.The presence of the chromate poisons the copper based electrodes, causing a severe drop-off in the

life of the electrode tips and degradation of spot weld quality.


3





Recommendations

1. As the susceptibility of galvannealed sheet to exhibit dark grey to black surface discoloration is causedby the intrusion of water during shipping or storage, or condensation of moisture onto the sheet

surfaces, it is extremely important to keep the product dry. For example, when a coil stored at 70°F in awarehouse is shipped to a customer during the wintertime, there is a high potential for condensation tooccur within the coil. If, at the customer’s site, the coil is then allowed to sit for a period of time beforeuse and no attempt is made to remove the condensation, black stains may form. Extreme care istherefore needed to prevent the corrosion reaction that produces dark surface stains. Precautions mustbe taken when shipping, especially during wintertime. If transit times will be long, heated trailers/railcars should be used. Packaging must be good. Good storage conditions at the customer’s plant areimportant. Also, inventory control is important to assure that any specific coil is not allowed to remainunused for a long time. Refer to GalvInfoNote 3.2 for more information on protecting against storagestain.

2. Consider ordering the product “oiled”, and then utilize good cleaning practices that employ stagedalkaline cleaning. The presence of rust-inhibitive oil will dramatically extend the shelf life ofgalvannealed sheet, and help prevent storage staining.

3. In some instances, cooperative work amongst the steel producer, surface treatment supplier, and paintcompany has allowed the customer to paint over the surface passivation film applied at the exit end ofthe hot-dip coating line. If passivated galvanneal can be successfully painted, then the probability ofdark storage stains developing is dramatically reduced.

4. If dark stains are present on a galvannealed sheet, it does not necessarily mean that paint adhesion willbe “poor” for all applications. Typically, the dark stain corrosion product that is formed is a thin adherentfilm on the surface of the galvannealed coating. Paint trials are recommended to determine if the darkstain does indeed adversely affect paint adhesion. If not, it may then be possible to use the stainedgalvannealed product.

If this salvage method is tried, it is important to be not only be sure that the dark stained oxide on thesurface of the galvannealed sheet is adherent to the galvannealed coating, but that the paint hassufficient hiding power to prevent “show through”. If the amount of storage stain is severe and the dark

corrosion product is not adherent to the galvannealed coating, it is often better to not use the productfor a painted application unless the surface is subjected to aggressive cleaning.

Summary

The dark stains that are at times visible on galvannealed sheet are caused by the same wet storage conditionsthat cause white rust on galvanized surfaces. Since galvannealed sheet is often shipped “dry, no oil, no surfacepassivation”, it is very important to control the shipping and storage of this product. Often, if storage stains arepresent and the stain is a thin, adherent film, the product can still be used for the intended application. Paintadhesion tests should be conducted to assure that the mechanical adhesion and appearance of the paint is notadversely affected over the stained areas.


Disclaimer:



4




1


GalvInfoNote The Salt Spray Test

Rev 1.0 Jan-093.4

Introduction

This GalvInfoNote concerns the performance of coated-steel sheet products in accelerated corrosion

testing. Specifically, the discussion will concentrate on the salt spray or salt fog test. Both terms, spray

and fog, refer to the same test procedure, and are used interchangeably when describing and discussing

this test.

Accelerated Corrosion Testing

The purpose of an accelerated corrosion test is to duplicate in the laboratory the field corrosionperformance of a product. This provides scientists and engineers with a means of quickly developing newproducts. For many years the salt spray test has been used extensively for this purpose by researchers inthe evaluation of new metallic coatings, new paint coatings, as well as testing miscellaneous types ofchemical treatments and paint pretreatments for use with metallic-coated steel sheet products.

For an accelerated corrosion test to be truly useful, a prime requirement is that the results correlate withperformance in the real world, something that has never been demonstrated with the salt spray test. Thishas lead many researchers to conclude that the test has no relevance, and should be discontinued.However, the results of salt spray testing are extensively used in product literature, customerspecifications, product data sheets, as well as the technical literature. Typical data gives the “life” of agiven type of coating, the benefits of new paint systems, the salt spray requirements for the acceptanceby an end customer of an alternative product, etc., so it seems virtually impossible to stop using the saltspray test at this time. In fact, there are so many specifications in use today that require a product toexhibit a specified number of “hours to failure” in the salt spray test, that any change to the test or itselimination is improbable. Clearly, any push to eliminate it would require that alternate acceleratedcorrosion tests be accepted by architects, specification writers, etc. Simply put, the corrosion performanceof different products has been compared using this test for so long that it would be difficult for today’s

researchers to not have salt spray test results when they are presenting performance data on a newproduct to a potential end user. That’s how commonly accepted the test and its data is by the end-usercommunity. Also, salt spray testing, while severe, is a good screening test because results can begenerated in a timely manner and “poor contenders” can perhaps be eliminated early on in the evaluationprocess.

The Salt Spray Test Procedure

Basically, the salt spray test procedure involves the spraying of a salt solution onto the samples being

tested. This is done inside a temperature-controlled chamber. The solution is a 5% salt (sodium chloride –

NaCl) solution. The samples under test are inserted into the chamber, following which the salt-containing

solution is sprayed as a very fine fog mist over the samples. The temperature within the chamber is

maintained at a constant level. Since the spray is continuous, the samples are constantly wet, and

therefore, constantly subject to corrosion. Through the years, there have been some new twists added tobetter simulate special environmental conditions, but the most common procedure by far in North America

is the test as described in ASTM B 117 Standard Practice for Operating Salt Spray (Fog) Apparatus.

In summary, the procedure is:

• Wooden racks are contained in the salt fog chamber (3’ high, 3’ deep, 5’ wide)

• Place samples on a the wooden rack at a small tilt angle





2

• 5% NaCl in tap water pumped from a reservoir to spray nozzles

• Solution mixed with humidified compressed air at nozzles

• Compressed air atomizes NaCl solution into a fog at the nozzles

• Heaters maintain a 95°F cabinet temperature

• Test duration can be from 24 hours to 1000 hours and more for some materials

Within the chamber, the samples are rotated frequently so that all samples are exposed as uniformly aspossible to the salt spray mist.

When the salt spray test is used for testing metallic-coated steel sheet, the corrosion performance is ratedin the following ways:

• Number of hours until rusting of the steel is first evident

• Number of hours until 5% of the surface area is rusted

• Number of hours until 10% of the surface area is rusted, etc.

The onset of red rust on a sample of galvanized sheet, for example, means that the coating has beenconsumed by the corrosion reaction, and the corrosion of the base steel is beginning. There is no onebest performance criterion. It simply depends on what the user defines as failure. The following table isone guideline that that can be used as a measure of expected performance of three zinc-containing hot-dip coatings.

Guideline to Salt Spray Resistance of Zinc-Containing Hot-Dip Coatings

Product Approximate Time to 5% Red Rust

(per micron [μm] of coating thickness)

Galvanize (zinc-coated) 10 hours A

Galfan® (zinc-5% aluminum alloy-coated) 25 hoursB

Galvalume® (55% aluminum-zinc alloy-coated) 50 hoursC

®Galfan is a trademark of the Galfan Technology Center, Inc. ®Galvalume is a registered trademark of BIEC International, Inc.

AGalvanize Z275 – typical coating thickness/side is 20.5 μm, so approximate time to 5% red rust is 205 hours in salt spray.

BGalfan ZGF275 – typical coating thickness/side is 21.5 μm, so approximate time to 5% red rust is 540 hours in salt spray.

CGalvalume AZM150 – typical coating thickness/side is 21.5 μm, so approximate time to 5% red rust is 1075 hours in salt spray.

When the salt spray test is used to rate the performance of paint pretreatments, paint primers and/ortopcoats, the normal rating schemes involve:

• Measuring the width of paint undercutting either along a scribed line through the paint or at asheared edge after 250, 500, 750 etc hours of exposure in the test chamber,

• Measuring the amount of paint blistering that has occurred on the surfaces of the painted steelpanel in 250, 500, 750 etc. hours.





3

There are other ways to define failure, but the above two are very common.

Since the salt spray test does not involve any exposure to ultraviolet light, paint fading and chalking arenot measured in this test.

An ASTM standard has been developed for modified salt spray testing. It is G 85 Standard Practice for

Modified Salt Spray (Fog) Testing, and has several modifications involving cyclic additions of acid andSO2. This standard is not as widely used as B 117. Method A5 of G85 has been found by someproducers to be able to rank materials and coatings similar to that witnessed in actual service

i.

Historic Problems

Through the years, various challenges to the applicability of salt spray test data have been made. Clearly,

many field applications do not involve exposure to salt chemicals, and rarely at a concentration level of

5%. How meaningful, therefore, can salt fog data be? For example, galvanized steel experiences a higher

rate of corrosion in sulphide atmospheres compared to sulphide-free atmospheres, and corrosion

reactions will not be the same in a chloride atmosphere as in a sulphide atmosphere, so salt spray test

results would not be expected to correlate with outdoor performance in sulphide environments. Also,

manufacturers do not recommend the use of coated steel sheets for applications that involve continuous

exposure to moisture (as occurs in the salt spray test). In fact, the good performance of zinc basedcoatings on steel requires drying between periods of wetness, and the need for these wet/dry cycles is

generally well known. It is the development of a passive and relatively stable oxide and/or carbonate film

during the drying cycle that contributes to the good performance of galvanized coatings. The continual

wetness during the salt spray test does not allow this passive oxide/carbonate layer to develop.

When painted material is evaluated using the salt spray test, there is no exposure to ultraviolet light, a

common cause of deterioration for paints and primers. This is a serious omission, since the failure

mechanisms that eventually cause painted steel sheet to deteriorate are typically not included as

conditions in the salt spray test.

There are other vagaries that often show up in the salt spray test. For example, sample-to-samplevariability for supposedly identical samples has been large. Also, test data gathered in two different

cabinets, even though they are identical in design and operated as recommended, have shown a highamount of variance.

There are many reasons for the salt spray test not correlating with most real world exposure conditions.Three of the most significant are:

• The surface of the test coupons are constantly wet, with no cyclic drying, which does not happenin the field.

• The test chamber temperature is at a constant elevated 95°F, which increases water, oxygen andion transport compared to the field.

• The chloride content is very high at 5%, preventing zinc from forming a passive film as it does inthe field.

These are unusual and severe conditions that rarely, if ever, occur during natural weathering.





4

Does the Test Have Any Value?

There are those in the scientific community that claim the test has limited or no value. One can make asolid argument for this conclusion. Certainly, the practice of using the salt spray test to rank the relativeperformance of different coatings and/or paints is meaningless with respect to service performance.However, partly because there are so many historical data in the literature, there are some general waysin which the test has value.

For example, consider the performance of galvanized coatings on steel. The salt spray test shows alinear relation between the thickness of the coating and its life (such as time to first rust). This is similar tothe performance correlation in real world exposures. In most types of environmental exposure, thecoating life is linear, i.e., twice the zinc coating thickness provides twice the product life. Although the saltspray test does not correlate with outdoor exposure sufficiently to claim that a specific number of hours inthe salt spray test will provide “x” number of years life in a real world application, it can be used to confirmthat a specific lot of material has approximately the coating thickness claimed by the seller. For example,if the life to 10% rust is only 40 hours, it is essentially certain that the coating does not meet the thicknessrequirements of the most commonly used G60 and G90 coatings.

To show the test’s value in another way, consider the performance of painted galvanized panels. The

benefit of having a thick galvanized coating beneath the paint coating can be shown in the salt fog test. Incomparing a thin electroplated zinc coating to a thicker G90 hot-dip galvanized coating, after salt spraytesting the thin zinc coated sample will exhibit considerably more paint undercutting corrosion along asheared edge of the panel compared to a painted G90 panel. This result means that a thicker zinc coatingis preferable beneath a paint coating for applications where high rates of corrosion are expected (outdoorroofing, for example). Indeed, the value of a thicker zinc coating has been clearly demonstrated forapplications where the corrosivity of an application is severe. Through the years, there have been anumber of misapplications of painted galvanized steel sheet where the zinc-coating thickness was notsufficient to provide the service life expected by the user.

The salt spray test can be used to demonstrate the benefit of using a thicker galvanized coating toimprove the product life in the field. Be aware, however, that these are qualitative evaluations. Thelimitation is this: Using a thicker zinc coating to reduce the rate of paint undercutting corrosion along asheared edge by one-half in the salt spray test in no way means that the same reduction in undercuttingcorrosion will be observed in real-world applications.

Another example where the salt spray test has been demonstrated to have some value is as a qualitycontrol test for painted steels. If a well applied paint system (pretreatment, primer and topcoat) has beenshown to perform well in service, the periodic sampling of production materials has merit. For example, ifthe normal performance in the salt spray test is 500 hours before the onset of a specific amount ofundercutting corrosion, the periodic testing of production lot samples is a quick way to determine if thereare any major production problems affecting the product quality. The salt spray test may not showconclusively that the product quality is acceptable, but if the performance in this test is substandard, theoutdoor performance may also be diminished. In this instance, a lack of proper quality control might beindicated.

There may be some natural environments where the salt spray test may more closely correlate

qualitatively with field results. One such environment is the splash zone of a sea coast.

As mentioned earlier, another value of the test is that it is severe – candidate materials may be able to beeliminated earlier in the selection process and perhaps save the expense of field testing.





5

The Future

Today, the salt spray test is so deeply embedded in the mindset of many users of coated steel sheet

products that its elimination as a test procedure seems impossible. There are two primary reasons for

this:

• Compliance with the salt spray test is contained in many industry and customer specifications inalmost all consuming industries. In addition, many of the companies who use these specificationsmake claims in their own product literature about the salt spray test “corrosion life” of the coatedsteels that they use.

• There is no one universal accelerated corrosion test to replace the salt spray test. If the steelindustry, the paint industry and the treatment suppliers really desire to replace it, they need aneasy to use alternative. As of today, no such alternative test exists. Several cyclic tests have beendeveloped specifically for the automotive and prepainted building panel industries, but they havenot been widely accepted as a replacement for salt fog testing. It may be too simplistic to expectthat any one accelerated corrosion test will correlate with all types of applications.

If the user community is knowledgeable about what the salt spray test really means, they can then

understand its limitations and use the results in a judicious manner.


Disclaimer:



i K.M. DeSouza, ASTM Prohesion Test Predicts Service Performance of Prepainted Steel Sheet, Galvatech’04 Conference, Chicago, IL, April 4-8, 2004




1


GalvInfoNote Fretting Corrosion (Transit Abrasion)on Galvanized Sheet

Rev 1 Feb-09

3.5

IntroductionGalvanized sheet surfaces sometimes exhibit a surface imperfection that appears as permanent black

spots, marks, lines, or patches. This defect has many names, including transit abrasion, friction oxidation,

wear oxidation, and chafing; all being terms for a form of erosion-corrosion known as fretting. It is a

phenomenon that is more commonly seen on metal surfaces in mechanical assemblies (e.g., bolted,

riveted, keyed, or pinned joints) and electrical contacts,i but can occur on galvanized sheet surfaces

under certain conditions. While superficial, black fretting marks on galvanized sheet are almost

impossible to remove, and are not the direct result of bulk water damage – which can also cause black

(along with white) stains in its most severe form. When fretting occurs on galvanized sheet surfaces,

liquid water is not necessary for its creation, although fretting can occur in the same areas of sheets that

are additionally damaged by storage stain from entrapped moisture.

Mechanism of Fretting Corrosion

Fretting corrosion refers to corrosion damage at the asperities of contact surfaces. This damage occursunder load and in the presence of repeated relative surface motion, as often induced by vibration.

ii

The requirements for fretting corrosion are: the interface must be under load, vibration or repeatedrelative motion must occur, and the load and relative motion must be sufficient to produce deformation onthe surface

iii. Two mechanisms are proposed for fretting corrosion, wear-oxidation and oxidation-wear.

The first theorizes that cold welding (fusion) occurs at contacting asperities under load and duringsubsequent relative motion these contact points rupture, with small fragments of metal being removed.These small fragments immediately oxidize. The process is repeated, resulting in the build-up of oxideresidue. The oxidation-wear model proposes that the oxide layer is already present and when thecontacting surfaces are subjected to relative motion under load, this layer is ruptured at the high pointsproducing the oxide debris. The exposed metal is oxidized and the process repeats. Investigations haveshown that both of these mechanisms operate to produce fretting corrosion.

Displacements as little as 4x10-9

in [10-4

μm] can cause fretting.iv It is seldom seen above amplitudes of

0.001 in [25 μm] and reaches a maximum at 0.0003 in [7.5 μm].v

The reason fretting damage can be a severe problem is that it so often happens at the interface of twohighly loaded surfaces that are not designed to move against each other. In the case of machinery, it canunknowingly and prematurely wear out parts, and also induce cracks that can become fatigue failures. Inelectrical equipment it can increase resistance and cause intermittent connections or unexpected circuitfailures.

Fretting Corrosion on Galvanized Sheet

For many years fretting problems have been observed on galvanized steel in both coil form and bundlesof cut length sheets. The defect is never seen at the production line and when found is almost always atcustomer facilities. It tends to be more prevalent on coil form and on material thicker than 0.030 in [0.8mm].

vi Without fail, it is also characterized by a lower intensity mirror image on the reverse side of the

sheet.



GalvInfoNote 3.5

Rev 1 Feb-09


2

Figures 1 and 2 illustrate the appearance of fretting on galvanized surfaces. In this case the marks arenear the edge of the sheet and were reported to have mirror images on the opposite surface.

vii Figure 2

is a close-up of the mark on the right hand side of Figure 1.

Figure 1 Fretting near coil edge Figure 2 Close-up of right side of Figure 1

(Courtesy of Shantanu Chakraborty/Rajib Chatterjee – TATA STEEL - India)

Another observation made about the nature of these marks, in the case of coil-form material shown inFigures 3 and 4, is that they are located exclusively in the area of contact of the saddle support thatcradled the coil(s). The coils had been shipped with their eyes horizontal and aligned perpendicular tothe direction of movement.

Figure 3 Fretting in area of contact Figure 4 Fretting in area of contact

(Courtesy of Shantanu Chakraborty/Rajib Chatterjee – TATA STEEL - India)



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Figure 5 is fretting on corrugated galvanized sheet that was observed after transport to a jobsite from aroll-forming mill.

Figure 5 Transit Abrasion (fretting) on corrugated galvanized sheet

Given that relative motion between surfaces is a requirement for fretting to occur, where do these markson galvanize come from, and why are they black and impossible to remove short of abrading them off?

The relative motion comes from vibrations that occur during shipment of the product. While this type ofdamage can occur on truck shipments, it is rare, probably because truck transport tends to involve shorterdistances (fewer vibration cycles), and perhaps vibration amplitudes above the maximum known to cause

fretting (25 μm). How material is supported on trucks (bearing points), and road conditions can have aneffect, however, on the propensity for transit abrasion. Transport by train and ship/barge is typically of alonger duration, therefore any vibrations causing low amplitude relative surface motion have much moretime to do damage to the surface, and the nature of the movement over steel rails and through water mayplay more of a part in generating low amplitude vibration. Also, the high power diesel engines used topropel trains and ships may be a factor in contributing to the generation of these vibrations.

The second factor that contributes to transit abrasion is load on the surfaces. The black marks are rarelyover the entire surface area of a sheet, but are concentrated in specific regions, that have been noted insome cases, e.g., Figs 3 & 4, to be the point(s) that bear the weight of the entire coil or bundle, andperhaps the additional load of product stacked on top. These bearing points are where the most pressurewould result on any surface asperities, and is where fretting would begin if relative motion betweensurfaces does occur.

As for the observation that fretting is less common on sheets thinner than 0.8 mm, it is quite possiblyrelated to the number of laps per unit weight of coil, or number of sheets per unit weight of cut-to-lengthbundles. The thinner the sheet, the more contacting surface area per unit volume to resist relativemovement induced by vibration. Also, in the case of coil-form, thin sheet is recoiled using higher tensionthan for thick sheet, thus allowing less chance for surfaces to slide against each other. In other words,everything else being equal, thicker sheets will slide easier against each other than the same weight ofthinner sheets because there is less total frictional force (per unit volume) to resist sliding.



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The reason the marks on galvanize are black is believed to come from the nature of the extremely smallzinc oxide particles (wear debris) that are the likely result of small amplitude vibration. Zinc oxide that isformed from corrosion of zinc in the atmosphere, or manufactured intentionally for industrial use, is awhite powder. Black transit abrasion marks on galvanized surfaces have been shown to be zinccombined with a higher percentage of oxygen than is the case with the metallic zinc on the rest of thesurface (see Figure 6)

viii, indicating a different form of zinc oxide. It is theorized that this oxide is black,

either because of different optical properties than zinc oxide that appears white, or the manner in whichthe very fine oxide particles are bonded to the surface of the underlying zinc.

O Al Si Fe Zn

Black 14.74 1.17 0.63 1.55 81.92

White 6.97 2.25 0.68 0.88 89.22

Figure 6 Analysis indicating black area has at least twice the oxygen content of the normal white zinc(Courtesy of Shantanu Chakraborty/Rajib Chatterjee – TATA STEEL - India)

Minimizing Fretting Corrosion on Galvanized Sheet

There are a host of preventive measuresix,x

that can be taken to minimize fretting corrosion in mechanicalassemblies. These include: lubricating with low viscosity oils or greases, optimizing the surfaceroughness to alter friction coefficients, isolating from vibration, increasing the recoiling tension or load toreduce slip, and decreasing the load at bearing surfaces.

All of the above measures are not practical in the case of galvanized sheet, but investigations at one steelsupplier

xi have indicated that some are effective to varying degrees. An action that is very effective is

redesigning support saddles to reduce concentrated point loading on the bottom of coils. By distributingthe weight of the coil over the entire area of the saddle(s), there is less pressure at any one point,

resulting in less transit damage given that vibration will always be present. A slightly less effective way ofaccomplishing the same result is to reduce the coil size, but this is perhaps not a desirable option for allsituations. With either of these actions, care should be taken to avoid stacking coils during transit, asmaterial on the bottom could become overloaded, even with well-designed saddles under them.

Another option to reduce fretting is to oil the sheet, thereby reducing friction. Oiling has been found not tobe effective in all circumstances and has other drawbacks, such as telescoping of coil walls; oil oozingfrom the walls; and being unacceptable to the customer.

Mag x 12



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An obvious cure would be to eliminate the small amplitude vibration of coiled or stacked galvanize sheet. Accomplishing this is very unlikely given the nature of long distance shipping methods.

Sheet with Fretting Marks – Suitability for Use

Fretting marks on galvanized sheet are surface oxide phenomena that can be a major aesthetic issue, but

there is no evidence they have a negative affect on corrosion resistance. Bright galvanize has a coveringlayer of zinc oxide that is not visible, whereas any fretted spots have an oxide layer that is black. Thisbeing the case, the product can generally be used in situations where appearance is not a factor, e.g.,hidden structural members. In fact, specification EN 10326 Continuously hot-dip coated strip and sheetof structural steels, technical delivery conditions; states in clause 11.2 that darks spots resulting fromfriction during shipping generally only impair the appearance.

Summary

The unsightly black marks sometimes seen on galvanized sheets have been shown to be the result offretting corrosion that occurs during transit of the product, either from the producer to thefabricator/service center, or from the fabricator/service center to a jobsite. The most effective methods ofminimizing fretting damage (transit abrasion) are to reduce bearing point loads on coils or bundles, and tooil the surface. Product with fretting marks is generally suitable for use if appearance is not a factor, such

as in the case of hidden structural members.

Copyright

©

2009 – IZA

Disclaimer:


i Bradford, S.A., Corrosion Control, 1993, Van Nostrand Reinhold, (Intl), p.110ii www.corrosion-doctors.org

iii Fontana, M.G., Corrosion Engineering, 1986, McGraw Hill Companies, p.106

iv Ibid. p.106

v

Bradford, Op. Cit., p.111vi Chatterjee, Rajib, TATA STEEL, private communication with the GalvInfo Center

vii Ibid.

viii Ibid

ix Bradford: Op. Cit., p.111

x Fontana, Op. Cit., p.108

xi Chatterjee, Op. Cit.




1

4. Prepainted Metallic-Coated Steel Sheet

GalvInfoNote Introduction to Painted, Metallic-Coated,

Steel Sheet Products

Rev 1 Mar-094.1

Introduction

Paint is usually considered to be a means of making a surface more appealing. Metals, including coated

steel sheet products, are often painted for this reason. However, in addition to the use of paint to provide

color, there is another reason to paint coated steel sheet; namely, additional corrosion protection!

Unpainted galvanized and other metallic coatings on steel sheet provide good, long-term protection fromcorrosion. Many years of protection can be obtained through the astute use of the metallic coatingsavailable today. Nevertheless, the application of high quality paint can add substantially to the overall lifeof coated steel sheet products. A classic example of the improvement in product life provided by a goodpaint system is the enhancement achieved with exposed automotive body steels. The metallic coating onautomotive body panels is relatively thin compared to that used on steel sheet for many other types ofapplications, yet the synergy produced by the paint and an automotive metallic coating creates a systemthat enables auto body panels to resist corrosion for a long time.

Of course, automotive paint systems are quite complex, as they are meant to withstand severeconditions. Typically, they include a phosphate pre-treatment, a high quality thick electrophoteric primer, acolor coat, and a clear coat. These types of complex, thick paint systems are not needed to gain a verysubstantial improvement in the life of many non-automotive coated sheet products. There are manyexamples of increased service life using paints (primer plus topcoat) as thin as 1-mil (0.001 inch).

Why Do Paints Improve Product Life?

Paints add additional protection to metallic coated steel by two primary means:

1. Acting as barrier between the coating and moisture, oxygen, and other corrosion-inducing agents.

2. By containing specific corrosion-inhibiting agents.

Although paint acts as a barrier film, it is not impervious to moisture. Water can penetrate the paint, andreach the metallic coating if the panels are wet for long periods. For this reason, the barrier aspect of thepaint alone is not sufficient. There must be corrosion-inhibiting agents at the interface between the paintand metal coating to mitigate corrosion of the metal. This is important to prevent loss of adhesion(blistering) between the paint and metal coating. Also, at locations where the paint integrity is lost, suchas at a scratch or cut edge, the presence of the treatment and other corrosion-inhibiting agents helpprevent undercutting corrosion of the paint.

The improvement in product life after painting is dependent on many factors. These include:

• thickness of the paint,

• stability of the paint (resistance to degradation by sunlight, moisture, etc.),

• use of a paint pre-treatment containing corrosion inhibiting agents,

• use of a primer coating beneath the paint, and

• additions to the paint that reduce the water permeability of the paint.

Each aspect of the total paint system plays a role in providing long life. Paint and treatment technologyhave both evolved to the point that each application needs to be considered individually to optimize thepaint, treatment, and metallic coating type.





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Product Life

Product life can be defined in many ways. For some applications, the life of a paint coating is the timeuntil some degree of fading has occurred. In these instances, it is important to select a paint that has highstability when exposed to ultraviolet light. In another instance, failure might be defined as when the paintloses adhesion to the coated-steel sheet. In this case, it is therefore important to properly clean the metalsurface and select a good pre-treatment and primer coating to maximize the adhesion of the paint to themetal coating. For another type of application, the product life might be defined as the time until a specificamount of steel corrosion (red rust) is observed. In yet another end use, the product life might be definedas the time until the steel sheet is corroded to perforation.

For applications where chalking and/or fading of the paint constitutes “failure”, it needs to have excellentresistance to ultraviolet light and chemicals that might be in the environment. This type of servicedepends primarily to the properties of the paint, and is best discussed with the paint manufacturerinvolved.

For applications where failure is defined by excessive corrosion of the metallic coating and steel sheet,it is important to address the entire coating system; the metallic coating type and its thickness, the type ofpre-treatment and its compatibility with the metallic coating, the type of primer and its thickness, and thetype of paint and its thickness. All of these factors need to be addressed to maximize productperformance. The specific environmental conditions must be taken into account. For example, is the

environment close to the sea? Is it in an industrial zone? Is it an environment that has constant highhumidity, or high times-of-wetness?

Corrosion failures of painted, metallic-coated steel sheet are the result of a corrosion reaction thattypically consumes bulk amounts of the metal coating and the steel sheet itself. It is especially importantin these types of applications to consider the performance of the metallic coating, and how it impacts thetotal system performance.

In the applications where bulk corrosion of the metal coating and steel are important considerations, it isimportant to maintain the integrity of the paint in order to maintain the life of the coated steel indefinitely.This means, when evidence of paint failure is noticed (blisters, edge creep along scratches, etc.), repairpainting and/or complete repainting should be done to regain the full original corrosion-resisting integrityof the metallic-coated sheet. For more details on the repair painting of prepainted sheet, seeGalvInfoNote 4.3.

A classic example of the improvement in product life possible by painting is a metal roof exposed to theatmosphere. In a moderately corrosive environment, a G90 galvanized coating might last about 12 to 15years before red rust becomes evident. This red rust occurs in areas where the galvanized coating hasbeen completely consumed by corrosion. If a high quality paint system is applied prior to exposure, thelife before initial signs of red rust might extend to 20 to 25 years, or longer. Furthermore, if the roof isrepainted or repair painted when the initial signs of steel corrosion are visible, the life of the roof might beextended another 10 to 15 years before corrosion is again evident.

What is a High Quality Paint System?

Paints can be applied to coated sheet steel either by “prepainting” the sheet while still in coil form

(prepainting or coil coating) , or by “post- painting” the sheet after it has been shaped into the final part

design. Either way, the “system” most often consists of:

• a thin pre-treatment coating to improve the adhesion between the paint and the metal and in turnthe corrosion resistance, and

• a primer coating that provides added adhesion, and added corrosion resistance, and

• a topcoat paint that consists of an organic binder and various pigments to provide the desiredcolor, gloss, and resistance to ultraviolet light degradation.

Pre-treatments are designed to optimize the performance on specific types of metal coatings. Not all pre-treatments are compatible with all metallic-coating types. For example, zinc phosphate is an excellentpre-treatment for galvanized sheet, but is not acceptable for 55% aluminum-zinc alloy-coated sheet.





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One of the sponsors of the GalvInfo Center is Henkel Surface Technologies, who are veryknowledgeable about pre-treatment technology. For assistance with pre-treatment questions, pleasecontact this company. Their link can be found by clicking the Our Sponsors button on the Home Page ofthe GalvInfo Center.

Similarly, primers are often made for very specific types of metal coating, although there are “universal”primers available in the marketplace that work very well on all the common types of metallic coatings.

Types of Paint Systems

There are many types of paints used for topcoats. Typically, most topcoats are compatible with all types

of metallic coatings. The important step is to make sure that the topcoat is compatible with the primer type

and that the topcoat’s properties are consistent with the end users’ needs with respect to chalk

resistance, color stability, flexibility, hardness, gloss, etc.

Some paints (topcoats) are specialty paints applicable for very specific applications. Others are quite

universal, both in their applicability for most environments and their ability to provide a cost-effective

desired color. Some offer very excellent colorfast qualities; that is, excellent resistance to fading when

exposed to sunlight. Others are very hard, and offer tremendous resistance to marring. Others are very

glossy and reflective. Still others offer a high quality, uniform matte finish. Some paint coatings are very

thick and relatively soft, offering good corrosion protection in harsh environments.

It is not our intention to elaborate on all the special types of paint available in the marketplace today. This

issue is best discussed with the individual paint-company technical experts.

A document that provides a general review of the available paints for metallic coated steel sheet products

is ASTM Specification A 755/A 755M, Steel Sheet, Metallic Coated by the Hot-Dip Process and

Prepainted by the Coil-Coating Process for Exterior Exposed Building Products. This specification can be

obtained at ASTM’s website, www.astm.org . Another source of information is Tool Kit #8 from the

National Coil Coaters Association at www.coilcoating.org .

Paint System Durability

Although paint systems offer a significant enhancement to the life of metallic-coated steel, the system

does eventually “fail” in some fashion. This can take the form of chalking or fading to a color that is no

longer acceptable to the user. It can also take the form of blistering or flaking, which can occur byseparation along the paint/primer bond line, the primer/pre-treatment bond line or the pre-

treatment/metallic coating bond line. The specific nature of blistering and/or flaking, if either one occurs, is

dependent on many factors associated with the specific combination of paint, primer, pre-treatment,

metallic coating, and the environmental conditions. Corrosion of the steel sheet substrate can also cause

the failure of the system.

Loss of paint adhesion can take several forms. The most common are:

1. Lateral undercutting corrosion at a scratch in the paint or at a sheared edge (where thepaint/primer/metallic/coating/steel is all exposed to potential corrosion). The net effect of thislateral undercutting corrosion is the loss of adhesion between the paint and the metal substrate.The corrosion can occur by a chemical reaction along the paint/metallic coating interface whichcan cause the chemical adhesion bond to be degraded, or by bulk corrosion of the metalliccoating leaving the paint totally “unconnected” to the steel sheet.

2. Blistering beneath the paint caused by corrosion reactions under the paint film. Remember, paintsare not impervious; water can penetrate through the paint to the substrate surface during times ofwetness. If the initial bond strength is not good, if the pollutants in the environment are particularlyinsidious for the type of paint system used, or if the “time of wetness” is unusually long, blisterscan develop even though there are no discontinuities in the paint. As the blisters grow larger andbegin to combine, the net effect can be gross flaking of the paint in large areas.





4

To minimize the tendency for loss of paint adhesion through undercutting corrosion or blistering, take intoaccount any specific recommendations from the steel supplier and paint manufacturers. The “best”coated product design requires that the user pay attention to the type and thickness of the metalliccoating, the type of pre-treatment, the type and thickness of the primer, and the type and thickness of thetopcoat. The recommendations from the suppliers will take into account issues such as:

• Types and concentrations of corrosive contaminantso acid rain,

o coastal salts,

o manufacturing plant effluents in the area, if any, etc.

• Wetness of the environment, particularly the duration of the wet periods (time of wetness)

• Amount of ultraviolet light exposure

• Customer expectations with respect to performance and aestheticso paint fading

o chalking of the paint

o rust stain at sheared edges

• Desired product life

For information on achieving good performance of pre-painted, metallic-coated steel sheet for building

panels, refer to GalvInfoNote 4.2.

Summary

When properly designed and applied, paints add considerably to the life of metallic-coated steel sheetproducts. The long life that is desired requires careful selection of the:

• type and thickness of the metallic coating,

• type of pre-treatment,

• type and thickness of the primer,

• type and thickness of the paint topcoat, and

• the application

Also, it requires that the metallic coating be properly prepared (cleaned) to remove any oils, dirt, etc. priorto painting regardless of whether the paint is applied via prepainting (painting prior to manufacture of theend product) or a post-painting (painting after fabrication of the end product).

Furthermore, to optimize the life of the painted metallic-coated steel, periodic repair painting and/orcomplete repainting may be needed. The need for repainting depends on many factors. These include theaesthetic requirements, the desired product life, and the severity of the environment, among others. Byproper attention to the paint integrity and the degree of degradation that occurs over time, very long life ofmetallic-coated steel sheet products can be attained. For example, many exterior applications are visibletoday where the proper selection of metallic coating and paint system has led to high performance for 20years or longer without the need for any repainting.

For more information on the use of these products in building applications, refer to GalvInfoNote 4.2 –Prepainted Metallic-Coated Steel Sheet for Building Panels – Assuring Good Performance. Forinformation on repair painting, see GalvInfoNote 4.3.


Disclaimer:





1


GalvInfoNote

4.2

Prepainted Metallic-Coated Steel Sheet forBuilding Panels – Assuring Good Performance

Rev 1 Jan-09 Introduction

Prepainted metallic-coated steel sheet for buildings has been used very successfully for many years. One

indicator of the popularity of this product is the large number of roofs made with prepainted steel in place

around North America and other parts of the world. An example of one such application is shown in thephotograph below and illustrates how this material can be used in a striking and effective manner.

Source: www.mbma.com

A metal roof lasts 2 to 3 times longer than a non-metal roof (see www.metalroofing.com). Metal buildings

comprise almost half of low-rise, non-residential construction, and a high fraction of these buildings use

prepainted metallic-coated steel sheet for both the roof and wall panels. It is a cost-effective and longlasting product. Its appearance and longevity has been made even better because steel producers, paint

manufactures, coil coaters, panel manufacturers and building producers, when designing and

manufacturing these products, have taken into consideration many of the guidelines in this GalvInfoNote.

Prepainted Metallic-Coated Steel SheetPrepainted metallic-coated steel sheet products are made using the coil-coating process to

apply a heat-cured paint system to the sheet substrate. The two most common types of

metallic-coated steel sheet substrates used worldwide today are hot-dip galvanized sheet and

hot-dip coated 55% aluminum-zinc alloy coated steel sheet. A third substrate is hot-dip coated

zinc-5% aluminum alloy coated steel sheet. Refer to GalvInfoNote 4.1 for an introduction to

prepainted products. One important feature of the prepainting process is that it allows theapplication of thermally cured paint coatings, a process that provides superior paint properties

(fade and chalking resistance, for example) compared with most field applied or shop applied

air-dry paints. Also, the superior bonding of the paint to the corrosion resistant, zinc-

containing layer creates a synergistic total coating system that is the reason for the long

product life.



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There are many applications today where the proper selection of the paint system (pretreatment, primer

and topcoat) is providing painted metallic-coated steel sheet roofs and walls with a life of 20 years and

longer. To achieve this long service life, the manufacturers of the prepainted steel sheet and the buildingconstructors, take into consideration issues related to:

• nature of the service environment,

• the metallic-coated steel sheet,

• type of paint system,

• the prepainted coil-coating process,

• panel design and the roll-forming process,

• storage and handling of the roll-formed sheets,

• building design,

• installation practices, and

• field maintenance

Service Environment ConsiderationsOne of the first considerations in the selection of a prepainted metallic-coated steel sheet product is theservice environment to which it will be exposed

1. The environment encompasses both the general

climate of the region, and localized effects.

General climatic factors to be taken into account include:

• the amount and intensity of UV radiation

• time of wetness

• acidity of the rain

• presence of chlorides near sea coast locations

The amount and intensity of UV radiation that the product is exposed to is governed by the latitude of thelocation, the hours per year of sunlight, and the angle of exposure of the prepainted sheet. Obviously, alow angle (flat) roof on a building located in a desert area in the low latitudes requires a primer and topcoat system that is very resistant to UV radiation to avoid premature fading, chalking and cracking. On theother hand, UV radiation damage would be of much less concern for vertical wall cladding on a buildinglocated in a high-latitude, cloudy climate.

Time of wetness refers to the length of time the cladding on roofs and walls are wet due to rain, highhumidity, fog, and condensation. Paint systems are not impervious to moisture. If wet long enough, themoisture will eventually reach the substrate under any paint system, and corrosion will begin. Theamount of chemical contaminants, e.g., sulphur dioxide, chlorides, etc., present in the atmosphere willthen govern the corrosion rate. Some paint systems are more impervious to moisture than others.

Local or micro-climate effects that need to be taken into account include:

• wind direction

• pollution fallout from industrial plants

• marine environments

The prevailing wind direction should be considered when selecting a coating system. If the buildinglocation is downwind from a source of chemical contamination then caution is advised. Gaseous and



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particulate exhaust emissions can have a serious effect on paint systems. Within 5 km of heavy industry,corrosivity could range from moderate to heavy depending on wind direction and local weather conditions.Beyond 5 km, the effects associated with pollution fallout from industrial plants are usually reduced.

If prepainted buildings are located close to the sea coast, the effects of salt water can be severe. Within300 m of a coastline can be critical, while significant effects can be felt up to 5 km inland, and further,

depending on offshore winds.

If the corrosiveness of a proposed building site is not obvious, an investigation of the local area can bevery helpful. Data from environmental monitoring stations are useful as these data provide information onprecipitation, humidity and temperature. Examine unwashed surfaces in protected exposures to learnabout particulate fallout from industry, roads, marine salts, etc. Examine the performance of structures inthe immediate vicinity. If building materials such as galvanized fences and galvanized or prepaintedcladding, roofs, eaves troughs, flashing, etc., are in good shape after 10 to 15 years, the environment isprobably not aggressive. If structures show distress after only a few years, a cautious approach is justified.

Paint suppliers have the knowledge and experience to recommend paint systems for specificenvironments.

Metallic-Coated Sheet Considerations

The thickness of the metallic coating beneath the paint has a significant effect on the life of a prepaintedsheet in the field, particularly in the case of galvanize. The thicker the metallic coating, the lower the rateof undercutting corrosion at a sheared edge, at a scratch or any other place where the paint film integrityis lost.

At locations where the paint is cut or damaged and the zinc or zinc-based alloy is exposed, there is lateralundercutting corrosion of the metallic coating. As the coating is consumed by the corrosion reaction, thepaint loses adhesion, and peels back or flakes off the surface. The thicker the metallic coating, the slowerthe rate of undercutting corrosion, and the lower the rate of lateral paint delamination.

In the case of galvanize, the importance of zinc coating thickness, especially for roofing, is one reasonthat many of the manufacturers of galvanized sheet products recommend a G90 [Z275] coating for mostprepainted galvanized sheet applications. For prepainted 55% aluminum-zinc alloy coated the issue ofcoating thickness is more complex for several reasons. AZ50 [AZM150] is often the recommendedcoating as it has been shown to be very adequate for long-term performance.

One aspect to keep in mind is that coil coating operations can generally not use metallic-coated sheetthat has been passivated with chrome-based chemicals. These chemicals can contaminate the cleanerand pretreatment solutions on a paint line, so the most common practice is to use unpassivated sheet.GalvInfoNote 2.10 on surface treatments describes passivation and its effect on the prepainting processin more detail.

Paint System Considerations

Clearly, one of the most important aspects governing good performance is the paint system used for the

job. For example, in areas that receive a lot of sunlight (high UV exposure), i t’s important to use a topcoat

that is very resistant to fading, while in regions where there are high times of wetness it is vital that the

pretreatment and topcoat are resistant to moisture penetration. Issues relating to the paint system to be

used for a specific application are many and complex, and will not be discussed in this GalvInfoNote. This

subject is best covered by the paint manufacturers and coil coated sheet producers, who possess the

knowledge needed to make specific recommendations. A document that provides a general review of the



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available paints for metallic coated steel sheet products is ASTM Specification A 755/A 755M, Steel

Sheet, Metallic Coated by the Hot-Dip Process and Prepainted by the Coil-Coating Process for Exterior

Exposed Building Products, available from ASTM at www.astm.org

Prepainted Coil-Coating Process Considerations

An important variable that impacts the life of prepainted products in the field is the manufacture of the

prepainted sheet. The prepainted coil-coating process can dramatically affect the field performance. For

example, good paint adhesion is important to prevent paint delamination or blistering in the field. Good

adhesion requires well controlled coil-coating operating practices. This topic will not be discussed indetail, but remember that the coil-line painting process can influence the field life. Issues involved are:

• Good cleaning practices prior to application of the pretreatment,

• The proper application of a good chemical pretreatment, appropriate for the end use ,

• The application of an adequately thick primer and topcoat, and

• Proper thermal curing of both the primer and paint.

Coil-coating producers that manufacture prepainted sheet for buildings have well-developed qualitysystems that ensure the above issues are under excellent control. Refer to http://www.coilcoating.org/ formore information on this topic.

Roll Forming and Panel Design Considerations

The importance of panel design, specifically the bend radii along the formed ribs, is another importantissue. As noted previously, zinc corrosion occurs at areas where the paint film is damaged. If the paneldesign is such that the bend radii are small, there is always the tendency for cracks to develop in thepaint coating. Typically, these cracks are small, and are often referred to as “micro-cracking”.Nevertheless, the metallic coating is exposed and the potential is present for an increased rate ofcorrosion along the bend radii on a roll-formed panel.

The potential for micro-cracking at bends does not mean that deep profiles are not possible. But, toaccommodate these deep profiles, the design should include as large a bend radius as possible. Thisaspect of performance involves roll-former design and is well understood by the roll-forming industry.

In addition to the importance of panel and roll-former design, the operation of the roll-former influences

the field performance. For example, alignment of the roll sets influences the actual bend radii. If the

alignment is not proper, the bends may develop sharp kinks in the profile bends instead of smooth-

flowing, gradual bend radii. These “tight” bends may lead to more severe micro-cracking. Also, it is

important that the mating rolls do not abrade the paint coating as this degrades the ability of the paint to

accommodate the bending operation. Spring-back is another relevant issue that needs to be recognized

when roll-forming. The usual way to allow for spring-back is to “over-bend” the panel. This is needed, but

over-bending during the roll-forming operation does tend to cause more micro-cracking. Again, thequality control procedures of building panel producers are set up to deal with these issues.

A condition known as “oil canning” or “pocket waves” can sometimes occur on roll formed prepainted

steel sheet panels. Panel profiles with wide web or flat areas (architectural profiles) are particularly

susceptible. This condition creates an unacceptable wavy appearance when the panels are installed on

roofs and walls. While oil canning can be caused by a number of reasons2, including poor incoming sheet

flatness, poor roll former operation and poor installation practices, it can also result from elastic buckling

of the sheet during forming because compressive stresses occur in the longitudinal direction of the panel.

This elastic buckling results because the steel has a low or zero yield-point-elongation (YPE). YPE is the



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strain associated with discontinuous yielding in the stress-strain curve. During roll forming, sheet at the

bend tries to thin in the thickness and contract in the longitudinal direction. In steel with low YPE, the

non-deformed regions adjacent to the bend prevent contraction longitudinally and are placed in

compression. Pocket waves occur in the web areas when the compressive stresses exceed the elastic

buckling limit stress. Steel with a high YPE improves roll formability because it thins locally at a bend with

little transfer of strain in the longitudinal direction. In general, prepainted steel with YPE greater than 4

percent will roll form satisfactorily. Lower YPE material may roll form without oil canning depending on

roll former set-up, steel thickness and panel profile. Oil canning severity decreases as: more stands are

used to form a profile, as steel thickness increases, as bend corner radius increases and as web width

decreases. If the YPE is higher than 6 percent, fluting may occur during roll forming. Temper rolling the

steel sheet at the time of manufacture will control this. Steel manufacturers need to be aware of when

they are supplying prepainted sheet for architectural panels so that manufacturing processes can be usedthat will produce YPE in the acceptable range.

Storage and Handling Considerations

Perhaps, the most important issue related to storage at the jobsite prior to putting the panels onto abuilding is “keeping the panels dry”. If moisture is allowed to permeate between adjacent panels, eitherfrom rainfall or condensation, and the panel surfaces are then not allowed to dry very soon thereafter,several undesirable things can happen. One is that the paint adhesion can be adversely affected. Thiscan lead to the development of small blisters between the paint and the zinc coating even before thepanels are placed into service. Needless to say, this behaviour potentially accelerates the loss of paintadhesion in service.

At times, the presence of moisture between the panels at a jobsite can actually cause the formation of

white rust (corrosion of the zinc coating) on the panels. This is undesirable aesthetically and may even

render the panels unusable.

If the bundle of sheets at a jobsite cannot be stored inside, be sure to wrap the bundles with paper. The

paper needs to be applied in a manner that does not allow water to accumulate in the bundle. At a

minimum, cover the bundles with a tarp. Keep the tarp covering open at the bottom so that water can flow

away freely and so that the bundles can have free airflow to allow the bundles to dry if for some reason

condensation does occur.

For further information on this topic refer to National Coil Coaters Association (http://www.coilcoating.org/)

publication: Toolkit #1: Preventing Job Site Storage Corrosion of Prepainted Building Panels.

Building Design Considerations

As mentioned previously, corrosion is greatly influenced by the time of wetness. One of the mostimportant design rules, therefore, is to ensure that all rain and melting snow can run off a building

1).

Water should not be allowed to collect and sit in contact with a building. Following are some othersuggestions related to roof and wall design.

Roofs

• Roofs with low slopes are subject to the most severe corrosion conditions. They encounterhigh levels of ultraviolet (UV) radiation, acid rain, particulate fallout, and wind-bornechemicals. Every effort should be made to avoid water ponding at overlaps, ventilators, airconditioning equipment and other objects. For maintenance traffic, walkways should beprovided to prevent damage to the coating.



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• Drip edge puddling is a function of roof slope. The greater the slope, the better the corrosionperformance at drip edges.

• Dissimilar metals, e.g., steel, aluminum, copper, lead, should be separated electrically toprevent galvanic corrosion. Also, to avoid another source of galvanic corrosion, the runoffpath should be directed to prevent water running from one type of material to another.

• Consider using lighter colours on roofs to lessen the damage from UV radiation.

• In areas where the roof of a building experiences heavy accumulation of snow, and the snowis on the roof for long periods, there is the possibility of shortened panel life. If the buildingdesign is one where the space immediately beneath the roof panels is warm, the snow nextto the sheet may be kept melted all winter long. This continual slow melting leads to asituation where the painted panel is in constant contact with water, i.e., a long time ofwetness. As explained earlier, the water eventually permeates the paint film and corrosioncan be severe, leading to an abnormally short roof life. If the interior roof is insulated so thatthe roof panel stays cold on the underside, then the snow in contact with the exterior surfaceis not constantly melted, and the paint blistering and zinc corrosion associated with longtimes of wetness are avoided. Also, keep in mind that the thicker the paint system, the longerit is before moisture permeates to the substrate.

Walls

• Vertical side walls receive less exposure to the weather than other parts of a building andsuffer less deterioration, with the exception of protected exposures.

• Cladding located in protected exposures, e.g., wall reliefs, overhangs, etc., receives lessexposure to sunlight and rainfall. Corrosion is increased in such locations becausecontaminants are not washed away by rainfall. In addition, wetness due to condensation isnot dried due to the absence of direct sunlight. Protected exposures in industrial or marineenvironments, or close to major thoroughfares, should receive special attention.

• Horizontal portions of wall cladding should be adequately sloped to prevent the accumulation

of water and contaminants. This is particularly important for base flashings, as an inadequateslope can allow both it and the cladding resting on it to corrode.

• Dissimilar metals, e.g., steel, aluminum, copper, lead, should be separated electrically toprevent galvanic corrosion.

• As with roofs, corrosion can be an issue for sidewall panels in areas that receive high

amounts of snow. If possible, remove the snow from areas adjoining the building, or use good

insulation practices so that snow build-up against a building is not continually melting on thepanel surface.

• Good insulation practices are helpful for many reasons. Most importantly, do not allowinsulation to become wet, and if it does, never let it come in contact with prepainted panels.Once insulation gets wet, it doesn’t dry very quickly, if at all. Again, this leads to a situationwhere the panel is subject to long times of wetness; a condition that will lead to acceleratedfailure. A common situation in the field is when the insulation at the bottom of sidewall panelsgets wet because water sits on the footer. A design that involves the panels overlapping thefooter, rather than one where the panel bottom is set directly onto the footer, seemspreferable to minimize this potential problem.

• Prepainted Galvalume sheet should not come in direct contact with wet concrete. Concrete’shigh alkalinity attacks the aluminum, causing the coating to peel

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If the application involves the use of fasteners that penetrate through the panels, the fasteners need to beselected so that their life matches the life of the prepainted panel. Today, there are screws/fasteners withan organic coating on their heads that provide corrosion protection and are available in colors to matchthe roof/wall cladding.

Installation Considerations

Perhaps the two most significant issues relating to field installation, especially when the application is aroof, are the way the panels are moved around on the roof and the impact of the worker’s footwear andtools. If the panel edges have any type of burr from the shearing operation, the paint film can bescratched through to the zinc containing coating when panels are slid across one another. As notedpreviously, at any location where the paint integrity is compromised, the metallic coating begins tocorrode more rapidly and the life of the prepainted panel is adversely affected. Similarly, worker’sfootwear can cause similar scratching damage. It is important that shoes or boots do not allow smallstones, steel drillings, etc., to be embedded into the soles.

During the installation, there are often small drillings and/or cuttings (called “swarf”) from the fasteningand trimming operations. Remember, these contain steel. After the job is done, or even before, the steelwill corrode and leave behind a rust discoloration that is objectionable, especially if the paint color is a

light tone. Too often, this discoloration is thought to be actual premature degradation of the prepaintedpanels, and in addition to the aesthetic issue, the building owner needs to be convinced that the buildingis not failing prematurely. All swarf should be immediately removed from the roof.

If the application involves a low-slope roof, the possibility of water ponding is real. Even though the slopedesign might be adequate to provide for free drainage, there might be local issues that lead to ponding ofwater. Small dents caused by workers (walking or placement of tools etc.), can leave behind areas thatdo not experience free drainage. If free drainage is not allowed, the ponding may lead to paint blistering,then to paint disbondment in larger areas, and then to more aggressive corrosion of the metallic coatingbeneath the paint. After erection, settling of the building may lead to improper roof drainage.

Maintenance Considerations

Simple maintenance of the prepainted panels on the building involves washing with water from time totime. This is not usually necessary for installations where the panels experience rainfall such as a roof.But, in protected exposure areas, such as the soffit and wall sections beneath eaves, washing every sixmonths is beneficial to remove corrosive salts and debris from the surface of the panels.

Care with washing is needed.

• Do not use strong cleansers as these may damage the paint.

• Do not use scouring powders as these assuredly will damage the paint surface.

• One cup of a mild non-abrasive detergent (one that contains less than 0.5% phosphate) dissolvedinto five gallons of water is a common cleaning agent.

• If mildew or other fungal growth is present, a recommended cleanser is one gallon of householdbleach in five gallons of water along with one cup of mild soap to aid wetting.

• Never use a hard bristle brush; use only a very soft bristle brush or a soft cloth

• After cleaning, wash the surface thoroughly with clean water.

It is recommended that any cleaning be done by first “test cleaning” a small surface area in a location that

is not boldly exposed to be certain that satisfactory results are achieved.



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Also, for roofing applications, the removal of loose debris such as leaves, dirt or building effluent (dust or

other residues around roof vents), etc., is important. Even if these residues do not contain corrosive

chemicals, they prevent the quick drying that is vital for a long-life roof.

Another thing to be careful about – don’t use a metal bladed shovel to remove snow from roofs. This cancause severe scratching of the paint.

Prepainted metallic-coated steel sheet for buildings is designed to provide many years of problem freeservice. Eventually, however, all paint coatings will change in appearance, perhaps to a degree thatrequires repainting. Recommendations for repainting weathered coil-coated building panels are given inGalvInfoNote 4.3.

Summary

Prepainted galvanized steel sheet has been successfully used for decades in various climates for buildingcladding (roofs and walls). With proper selection of the paint system, careful design of the building, andregular maintenance, long and trouble-free service is achieved.

Source: www.mbma.com

References:

1) “Stelcolour Prefinished Sheet Steel for Building Construction”, Technical Bulletin 23/December 1983, published by Stelco Inc.2) Metal Construction Association, Technical Bulletin #95-1060, Revised 1/033) Bethlehem Steel Corporation, Descriptive Data Sheet: SPEC-101, April 2000


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GalvInfoNote

4.3

Repair Painting ofPrepainted Metallic-Coated Steel Sheet

Rev 0 Jan-07

Introduction

Coil-coated paint finishes on metallic-coated steel sheet building

panels are designed to provide many years of problem-free

service, with a minimum of maintenance. These paint systems

are resistant to change and in most instances will not require

touch-up or repair painting for a long period of time. While these

prepainted finishes are much more durable and long-lasting than

ordinary field applied paints, they will eventually change in

appearance, perhaps by losing gloss or becoming discolored. If

the service environment is more harsh and aggressive than the

coating was designed to withstand, deterioration of the paint

coating may occur sooner than might normally be expected.

Prepainted Steel SheetPrepainted galvanized steel sheet products

are made using the coil-coating process to

apply a heat-cured paint to a metallic-

coated steel sheet substrate. The two

most common types of metallic coated

steel sheet substrates used world-wide

today are hot-dip galvanized sheet and

hot-dip coated Galvalume® steel sheet.

Refer to GalvInfoNote 4.1 for an

introduction to prepainted sheet, and to

GalvInfoNote 4.2 for recommendations on

maximizing performance.

The degree to which the paint has changed in appearance,and the requirements of the owner of the building, will

determine when repainting is required. This GalvInfoNote

provides suggested guidelines and practices to be followed

when it has been decided that prepainted building panels are to be repainted. It is also recommended

that the services of a qualified painting contractor be engaged.

The information in this GalvInfoNote does not apply to new or i nsuffic iently weathered coil-coated

paint finishes. These surfaces do not easily accept field painting. Paint manufacturers should be

contacted for advice on repainting new or insufficiently weathered prepainted steel sheets.

Touch-up repair painting may sometimes be required to restore small areas of paint damage. This

GalvInfoNote does not apply to touch-up repair. What looks like a good color match when freshly

painted, may turn into a very poor match after weathering. Consult a paint manufacturer for advice ontouch-up repair painting of prepainted steel sheet.

Surface Preparation

Cleaning

In order to assist with good paint adherence, it is necessary to thoroughly clean prepainted metallic-coated sheet building panels prior to repainting. In fact, it is recommended that coil-coated finishes onbuildings be cleaned routinely using the procedures outlined on page 7 in GalvInfoNote 4.2. By doing so,surface dirt is not allowed to build up, which helps to prolong the life of the finish. Sometimes athorough cleaning of building panels thought to require repainting will restore panel finish to thepoint that repainting is not required.

Any mildew present on the panels can be cleaned using the procedures in GalvInfoNote 4.2. Aftercleaning it is important to thoroughly rinse the panels with clear water to remove any cleaner residue thatmay be present. These residues will interfere with proper adhesion of field applied paints.

Surface Imperfections

Minor scratches which have not exposed the metal substrate should be lightly sanded to provide asmoother surface for repainting. It is important to not to expose any of the substrate. Exposed substrate


1





will require application of a primer as described in the next section. Deep scratches and other majorimperfections that have exposed large areas of bare metal, or are badly corroded, should be replaced.

Bare Metal and Rusting

Bare metal must be treated prior to repainting to improve corrosion resistance. If the mill hot-dip metallic

coating is not present or is badly corroded, serious consideration should be given to replacing the panelswith new material. If it is decided to paint over rusted panels, remove all traces of corrosion products(red, white or black rust) by vigorous wire brushing, taking care to not to remove any of the hot-dipmetallic coating. Clean and remove all loose debris. Lightly sand all edges of the areas to be repainted.

All exposed metal should be painted with a high quality bare metal primer 1. Be certain to follow all

instructions offered by the manufacturer of any bare metal primer that is used.

Intercoat Adhesion - Testing

It is important to achieve good intercoat adhesion between the coil coated finish and the new finish coator peeling may occur. Before proceeding with the repainting work it is strongly advised to perform anintercoat adhesion test. Below are two test procedures that can be followed, depending on the topcoat.

1. Enamel finish coat – Clean a small area representative of the surface to be repainted. Apply acoat of the field repaint enamel according to the manufacturer’s instructions. Allow the test area tothoroughly dry – at least overnight. When dry, firmly apply about 8 inches of gray “duct” tape ontothe repainted area while firmly holding the free end of the tape. Rapidly pull and remove the tapefrom the test area. Examine the underside of the tape. If any paint adheres to the tape thenadditional surface preparation is necessary.

2. Latex finish coat – Clean a small area representative of the surface to be repainted. Apply a coatof the field repaint latex according to the manufacturer’s instructions. Allow the test area tothoroughly dry – at least overnight. Use a utility knife to cut a 2 inch “X” into the repainted testarea. Place a 3 inch strip of “Scotch” tape over the “X” and rub 10 times with heavy pressure.Leave one-half inch of tape free for easy removal. Pull the tape back over itself at a 180° angle.Examine the tape and the panel for any signs of latex paint removal. If the tape removes morethan 1/16” of the repaint latex from the “X” cut, or if any paint is removed from the test area, then

additional preparation is necessary. (This test procedure is based on ASTM A 3359 – Method A)

Additional Surface Preparation

If recleaning the surface does not result in a satisfactory intercoat adhesion test, then it may be necessary

to roughen the surface with a 400 mesh abrasive or a green 3M Scotchbrite

abrasive pad. Professional

power washing can also be used. It is extremely important that the suitability of any power washingprocess be verified on a small area before washing the entire surface. Be certain that eitherprocess (sanding or power washing) does not damage or strip the prepainted finish to exposebare metal. These procedures are not recommended, or necessary, for plastisol coatings.

Repainting Procedures

Once the surfaces have been prepared and tested for adhesion, they must be coated within 24 hours withthe field applied topcoat.

The surface must be completely dry prior to repainting, which should not be done in the early morningwhen dew is still present on the panels. Do not paint when the ambient temperature is below 50°F.


2

1 PPG Galvanized Steel Primer

6-209 or equivalent primers designed for adhesion to galvanized surfaces.





Follow the instructions of the paint manufacturer for applying the topcoat. Usually the aim is to achieve adry paint film thickness of 1 mil.

Summary

Factory painted building panels have a proven performance record in providing many years of satisfactoryperformance. When their appearance eventually does begin to suffer, they can usually be refurbished andgiven a fresh appearance by repainting, using the recommendations in this GalvInfoNote.


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1

5. General Coated Sheet Topics

GalvInfoNote

5.1

Hardness Measurement ofCoated Sheet Products

Rev 0 Jan-07

Introduction

“The hardness of a material is a poorly defined term which has manymeanings depending on the experience of the person involved.”

1

In general, hardness implies a resistance to deformation, and for metals it is a measure of their resistance topermanent or plastic deformation. Metallurgists, including those working in the coated steel sheet industry, areconcerned with the mechanics of testing, and define hardness as a measure of the resistance of the base steelto indentation by an indenter of fixed geometry under static load. The quotation preceding this paragraphalludes to the fact (contrary to what many people may believe) that “indentation hardness is not a fundamentalproperty of a material”

2. This is the case even though relationships have been developed between hardness

tests and other material properties, such as tensile strength. To a design engineer such a relationship could be

important; because hardness would mean an easy and specified quantity that indicates the strength of thematerial. Certainly, much information can be derived from a hardness test, although it requires intelligentappraisal of hardness test results using knowledge of the steel’s composition and condition when tested.Factors influencing the accuracy of the test must also be understood. This GalvInfoNote offers guidance in theuse of hardness values as they relate to coated steel specifications, and its suitability for the intended end use.

Usefulness of Hardness Testing

Process control and inspection – Many processes that steel undergoes, e.g., heat treatment and cold

working, result in changes to its hardness. Hardness testing is therefore an excellent method of rapidly and

nondestructively monitoring the product in order to assist in process control. Specification limits for hardness

are established at key processing locations in the product stream, and periodic tests are conducted to ensure

the product of the process is within defined limits. These tests can be done rapidly, allowing quick confirmation

that the process is under control, e.g., that a galvanizing line is achieving, to a high degree of probability, the

proper degree of annealing of the base steel. The hardness control limits enforced in such a process are

developed for the specific coating line and product, and are not necessarily taken from an industry product

specification. They may, in fact, not be representative of the product hardness when it is eventually consumed

because, with time, some steels “age harden”. Age hardening is a natural phenomenon with many low carbon

steels, and while a hardness test on aged steel may show a higher value, the steel may still be suitable for its

intended end use

Other properties of coated sheet estimated from hardness – While it is possible to use hardness toestimate the approximate tensile strength of heat treated low carbon steel, a hardness test is no substitute forthe tensile test. In the coated sheet production industry, hardness testing is used mostly as verification thatprocesses that alter steel properties are within broad control limits. For many of these products, it is necessaryto further verify suitability for end use using more time consuming and complex tensile and formability tests.Hardness testing alone cannot supply reliable information about how steel behaves during forming. A hardness

test can easily distinguish amongst full hard, ½ hard, ¼ hard, and fully annealed sheet steel. It can also discernthe difference between commercial and drawing grades of annealed sheet, but provides no dependableinformation about the stretchability and drawability properties of these grades.

For heat treated high carbon and/or alloy steel parts, hardness testing is a very reliable quality control test thatcan discern small differences in hardness

3. For soft low carbon annealed steel sheet, it is an “in the ballpark”

test, that in cases where formability is critical, must be backed up by more extensive mechanical testing, as asmall difference in hardness could, or could not, be significant. The hardness of many heat treated high carbon





2

and/or alloys steels has be correlated with their tensile strength, but it is significant that published hardnessconversion charts (available from equipment suppliers – see reference 2) do not show approximate tensilestrength for hardness values less than Rockwell B 72. Below that hardness value the relationship is consideredto be inexact.

Types of Hardness Tests

While there are three general methods of hardness testing, viz., scratch hardness, indentation hardness, andrebound or dynamic hardness; for all intents and purposes, indentation hardness testing is the only methodused on steel products. There are numerous indentation hardness methods. The most common are: the Brinellhardness test, the Rockwell hardness test, the Vickers hardness test, and the micro hardness test. For moststeel products the Rockwell hardness test is used. It has a series of scales capable of covering the range ofhardness encountered in steel products. This article deals only with Rockwell hardness testing, focusing on thescales used for annealed steel sheet.

Rockwell Hardness Testing of Steel Sheet

This Rockwell test is the most widely accepted hardness test, not only for steels, but many other metals. It is avery rapid test, taking only about 5 to 10 seconds and can be used on sheet as thin as 0.006 in. [0.15 mm]. ARockwell test is based on measuring the penetration depth of an indenter, with the result displayed directly on a

dial gauge or digital display. Specifically, it measures the additional depth to which a carbide ball, or diamondpenetrator is forced into the material by a heavy (major) load, beyond the depth of a previously applied light(minor) load

4. High Rockwell hardness numbers represent hard steels and low numbers soft steels.

There are two main types of Rockwell tests – Regular and Superficial. While there are over 30 differentRockwell hardness scales between these two categories, for steel sheet only three or four scales are suitable.These are the Regular Rockwell B scale and the Superficial Rockwell 45T, 30T and 15T scales.

• The Regular Rockwell B test uses a 1/16” ball, a minor load of 10 kgf (kilograms force), and a major loadof 100 kgf

• The Superficial Rockwell 45T test uses a 1/16” ball, a minor load of 3 kgf, and a major load of 45 kgf



From this it is obvious that the latter two scales would penetrate the steel less, so are used on thinner sheet.The 45T scale is sometimes used on thicker sheet steel as an alternative to the B scale.

To fully understand why the use of 4 scales is required, it is necessary to understand what happens during apenetration hardness test. The material around a Rockwell indentation is “cold worked”, which means it ispermanently deformed. While the degree of cold work depends on the material and previous cold workhardening of the steel, investigations have found that the thickness of steel affected is about 10 times the depthof the indentation. Therefore, if sheet thickness is less than 10 times the depth of the indentation, an accuratehardness result cannot be expected because the underlying support anvil can affect the result. On the otherhand, it is always desirable to use the heaviest load possible so as to produce a larger indentation that testsmore of the material, making the result more accurate and representative. Choosing the correct scale for thematerial involved is a careful trade-off. It is advisable to refer to ASTM specification E 18 Standard TestMethods for Rockwell Hardness and Rockwell Superficial Hardness of Metallic Materials (available atwww.astm.org) for detailed procedures on how to select the proper scale.

From the above it can be seen that the Rockwell B test is used on thicker steel sheet. In most cases, at 0.040”[1 mm] and above, the Rockwell B test is used, although the 45T scale is an option. Below this thickness, the Bscale can be used down to about 0.026” [0.66 mm] if the steel is hard enough. Below 0.026” the Rockwell 30Tscale is used, down to about 0.014” [0.36 mm] depending on hardness. Below this thickness, the Rockwell 15Tscale is used, and is required for soft steels 0.016” [0.41 mm] and thinner. Refer to specification ASTM E 18 toobtain exact thickness break-points between scales as a function of sheet hardness. To visually check if a sheetis too thin for a given scale, examine the test piece directly beneath the indentation to determine if the metal is

http://www.astm.org/

http://www.astm.org/





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disturbed, or a bulge exists. If this is the case then the sheet is not thick enough for the load used. The “anvileffect” is present and the next lowest scale should be used. It is not acceptable to test multiple thicknesses ofsheet in an attempt to avoid the anvil effect.

For annealed steel sheet it is the usual practice to convert readings obtained using the Superficial Scales backto the Rockwell B scale. While it is best if Rockwell results are reported in the same scale that was used fortesting, converting results to the B scale helps avoid confusion, albeit at the risk of losing some accuracy in theconversion process. For sheet steel the conversion chart used should be based on Table 2 in ASTM E 140.

In addition to choosing the proper scale to avoid the anvil effect, it is important that a number of other simpleprecautions be observed in order to ensure useful and reproducible test results

5, 6. These are:

• The indenter and anvil should be clean and well seated

• The surface to be tested should be clean, dry, smooth and free from oxide

• The surface should be flat and perpendicular to the indenter

• It is important for accuracy that the test sample is held securely and the test piece be centered over theanvil. It is easier to accomplish this if the sample size is not too large, e.g., 4” x 4” maximum

• If a diamond anvil is used, ensure it is not cracked

• If a steel anvil is used, ensure it is flat with no indentations

The definition of what consists of one hardness result varies throughout the industry. Some laboratoryprocedures require 5 individual readings for each test, with the high and low discarded, and the remaining 3averaged to obtain a final result. In any case, most test procedures avoid relying on just a single reading.

Rockwell Testing and ASTM Coated Sheet Specifications

As much as hardness testing is used to monitor the production of steel sheet, it is not possible to order coatedsheet to ASTM specifications using hardness ranges. Many grades of coated sheet are sold on the basis ofmeeting specified mechanical properties, or a guarantee of being able to be fabricated into a specified part.When being sold to mechanical property specifications it is necessary to use fundamental measures such asyield/tensile strength, or elongation. To guarantee to be within a certain range for a non fundamental propertysuch as hardness would mean that such a range would be too wide to be meaningful. There could also be therisk of making a product that would not fabricate into the intended end use.

On the other hand, attempting to produce a product to an overly restrictive Rockwell hardness range runs therisk of unnecessarily rejecting material that might well form into the intended end use without difficulty.

Coating on versus coating off – given that ASTM specifications for coated sheet are silent on the topic ofhardness values, it follows that no information is provided on whether tests should be performed with thecoating on or off. It is the usual practice in coated sheet producing mills to strip the metallic coating offbefore hardness testing. Since it is the steel substrate which governs the mechanical properties of the sheet,it is the steel substrate that should be directly measured. Some laboratories test hardness with the coating on inthe interest of time, or because they may not have the means to properly strip the metallic coating off.Performing the test in this manner is known to affect the result, although the degree has not been quantified.Hardness testing done with the coating on is less reliable – the more so the heavier the coating.




Summary

The Rockwell hardness test is a quick and very useful tool in the metals industry. On hardened steels it caneasily and nondestructively discern small differences in properties. In the annealed low carbon steel sheet

industry the test is useful for production control, and sorting amongst distinct hardness classes of sheet.However, ASTM coated sheet specifications do not contain hardness range limits, for the basic reason that it isnot a fundamental property, and cannot be reliably used to predict the behavior of annealed product in formingoperations.

References:

1) Dieter, Jr, George E., Mechanical Metallurgy, McGraw-Hill, New York, 1961, p. 282.2) Wilson Instruments, Fundamentals of Rockwell Hardness Testing, WB1226, www.wilsoninstruments.com 3) Dieter, Op. Cit. p. 290.4) Wilson Instruments, Op. Cit. p. 75) American Society for Metals, Metals Handbook, 1948, pp. 93-105.6) McGhee, Douglas B., Common Problems in Rockwell Hardness Testing, Heat Treating Progress, May/June, 2004.


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http://www.wilsoninstruments.com/

http://www.wilsoninstruments.com/




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5. General Coated Sheet Topics

GalvInfoNote

5.2

Zinc Grades Used forContinuous Hot-Dip Galvanizing

Rev 1.3 Feb-2010 IntroductionZinc plays a crucial role in continuous hot-dip galvanizing. Using the correctly specified grade of zinc,

continuous galvanizing grade (CGG) alloy, or master alloy is key to producing a galvanized product that

meets the requirements of the marketplace1. For example, and as described in GalvInfoNote 2.4, close

control of the amount of aluminum in the zinc is critical to achieving good adhesion to the steel substrate.

To accomplish this, the galvanizer must be able to depend on a supply of raw zinc ingots that meet

specific composition limits. This is accomplished through a series of ASTM standards that cover zinc

products. This GalvInfoNote reviews the zinc grades available for continuous hot-dip galvanizing and the ASTM documents that govern them.

ASTM Zinc Standards

There are a number of ASTM standards that specify, not only the chemistry of zinc and various zinc

alloys used in hot-dip galvanizing, but the configuration of zinc jumbo and block ingots, and the colorcodes used for visual identification of zinc and zinc alloy ingots. These standards are available atwww.astm.org.

B 6 Standard Specification for Zinc

• Specifies the chemical requirements and other delivery conditions for 5 zinc grades, includingSpecial High Grade (SHG), High Grade (HG), and Prime Western Grade (PW). These grades,and scores of nonstandard variations of them, were once all that were available for use by thecontinuous galvanizing industry. Some are still employed in certain instances, e.g., SHG(99.990% Zn) is used to reduce the aluminum content in coating line zinc baths. The grades inthis standard are also used in the general galvanizing and zinc die-casting industries. Thecompositions of SHG and HG are shown in Table 1 below.

B 852 Standard Specification for Continuous Galvanizing Grade (CGG) Zinc for Hot-DipGalvanizing of Sheet Steel

• This standard specifies eight CGG grades of zinc having aluminum levels from 0.25% to 1.0%.Recognizing that lead is, for the most part, an unwanted impurity in galvanize coatings, it restrictslead content to a maximum of 0.007% in all but one of these grades. The chemistries of eachgrade are shown in Table 1. While this specification does allow for other compositions, it hasachieved a significant reduction in the number of custom grades of zinc that were once in use bygalvanizing lines.

B 860 Standard Specification for Zinc Master Alloys for Use in Hot-Dip Galvanizing

• This specification covers zinc-aluminum and zinc-antimony alloys, one of the uses of which is tomodify the composition of zinc baths on continuous coating lines. The two main alloy types are90% zinc-10% aluminum and 95% zinc-5% aluminum (see Table 1).

B 897 Standard Specification for the Configuration of Zinc and Zinc Alloy Jumbo and Block Ingot

• Specifies the dimensions of 2400 lb (1089 kg) jumbo and block ingots designed for use withautomatic handling systems that add zinc to the baths on continuous galvanizing lines. Theintroduction of this specification standardized the dimensions of these products, allowing areduction in the multiple ingot designs that were specific to individual coating lines.



GalvInfoNote 5.2Rev 1.3 Feb-2010

B 914 Standard Practice for Color Codes on Zinc and Zinc Alloy Ingot for Use in Hot-DipGalvanizing of Steel

• This practice specifies the color code system used to identify zinc and zinc alloy ingots. There isa unique color code for each zinc and zinc alloy grade to avoid confusion in the user’s plant.

Zinc Grades for Continuous Galvanizing

Table 1 lists the chemical composition requirements for the grades of zinc and zinc alloys that are used inthe continuous galvanizing industry.

Table 1 Chemical Composition Limits for Zinc & Zinc Alloys Used in Continuous Galvanizing(Wt%, Range or Max)

ASTMGrade(UNS)

Al Pb Cd Fe Cu Others

B6 - SHG Z13001 0.002 0.003 0.003 0.003 0.002 0.010 (all)

B6 - HG Z15001 0.01 0.03 0.01 0.02 - 0.10 (all)

Z80310 0.22 - 0.28 0.007 0.01 0.0075 0.01 0.01

Z80411 0.31 - 0.39 0.007 0.01 0.0075 0.01 0.01

Z80511 0.40 - 0.50 0.007 0.01 0.0075 0.01 0.01

Z80531 0.40 - 0.50 0.01-0.03 0.01 0.0075 0.01 0.01

Z80610 0.49 - 0.61 0.007 0.01 0.0075 0.01 0.01

Z80710 0.58 - 0.72 0.007 0.01 0.0075 0.01 0.01

Z8810 0.67 - 0.83 0.007 0.01 0.0075 0.01 0.01

B852CGG

Z80910 0.90 - 1.10 0.007 0.01 0.0075 0.01 0.01

B750** Z38510 4.2 - 6.2 0.005 0.005 0.075 - 0.04

Galvin Fo Notes

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Transcript of Galvin Fo Notes