HOT DIP GALVANIZING Fundamentals & Guidelines Galvanizing...Dip Galvanizing – a process comprising...

HOT DIP GALVANIZINGFundamentals & Guidelines

Hunter Galvanizing

Hunter Galvanizing Hunter Galvanizing

Hot Dip Galvanizing Hunter Galvanizing

Hunter Galvanizing is proudly Australian owned and operates two galvanizing plants in Tomago, NSW. Commissioned in 2002 to meet the needs of fabrication, structural and mining industries in the Newcastle and the surrounding region, our company has grown to incorporate handling facilities in Sydney and a transport fleet, servicing clients throughout the eastern states.

Hunter Galvanizing plant facilities are operational 24 hours, 7 days a week catering for a diverse range of fabricated steel items in processing baths of the following sizes:•Plant 1: 10m long x 2.4m deep x 1.5m wide

•Plant 2: 7m long x 3m deep x 1.8m wide

We provide durable, industrial strength zinc coatings for a wide range of industries including:

Upon request Hunter Galvanizing offers*:

•Quality Assurance Certificates

•Coating Thickness Reports

•Project Warranties

• heavy engineering and material handling

•mining equipment and infrastructure

• water treatment and sewerage

•structural buildings and projects

•food refrigeration and production

•highway and road hardware

•agricultural and irrigation equipment

•domestic construction

•rural and domestic fencing

•transportation, sport and recreation

The prevention of corrosion is a costly issue faced by councils, government departments, manufacturers and industrial facilities. It attacks steel items around our homes, sporting venues, shopping centres, roadways, marinas and farms.

The hot dip galvanized process has been instrumental in providing protection for steel items since 1837. Time proven and well documented, this process offers many benefits unable to be provided by other coating systems:

Hunter Galvanizing is a member of the following industry groups:

Governed by Standard AS/NZS 4680:2006 “Hot Dip Galvanized (zinc) Coatings on Fabricated Ferrous Articles” minimum and average coating thickness requirements are detailed and constantly achieved. As zinc coating thicknesses correlate to the time to first maintenance; specifiers can designate hot dip galvanized coatings with confidence assured of reliability and minimum service life expectations.

•layered barrier protection system

•industrial strength durability

•ease of inspection

•low initial cost & upkeep

•zinc to steel metallurgical bond

•total surface coverage

•cathodic coating properties

•high resistance to abrasion

•predictable service life

• high performance in most environments

• coating thickness relative to steel thickness

• coating life 3–5 times longer than other zinc coatings

•Australian Steel Institute

•Australasian Corrosion Association

•International Zinc Association

•American Galvanizing Association

•UK Galvanizers Association We pride ourselves on our commitment to quality, customer service, our employees and the environment.

Hunter Galvanizing Hot Dip Galvanizing Guidelines for Design & Fabrication 4th Edition, August 2014.

Information contained in this publication has been gathered from a range of industry sources, including the UK and American Galvanizers Association, and has been compiled by Judy Russell of

Hunter Galvanizing. Whilst every effort has been made to ensure information contained in this booklet is correct, the guidelines are intended to offer general advice to designers and fabricators.

For specific information relating to individual items or design, contact should be made with Hunter Galvanizing staff. Hunter Galvanizing acknowledges graphics & layout by THINK Graphic

Communication. Photographs supplied by Kyle Hesketh, Judy Russell and other industry bodies. Hunter Galvanizing stipulates that this publication may not be reproduced or any part there of

duplicated in any manner.*charges & conditions apply

Hunter Galvanizing

Hunter Galvanizing

Contents

Differentiating Zinc Coatings Relevant Standards for Zinc Coatings on Steel Different Zinc Coatings Zinc Metallizing, Zinc Rich Paint, Continuous In-Line, Electroplating, Sheridizing, Mechanical Plating

Corrosion Understanding Corrosion How Zinc Protects Cathodic Protection, Galvanic Series of Metals, Galvanic Corrosion, Other Materials and Hot Dip Galvanized Coatings

Hot Dip Galvanized Coatings How the Coating Forms Coating Thickness

Suitable Steel for Hot Dip Galvanizing Steel Type & Embrittlement Strain Age Embrittlement, Hydrogen Embrittlement, Liquid Embrittlement Effects of Steel Chemistry Silicon and Phosphorus, Delamination, Differing Cooling Rates

Suitable Surface Conditions Large Items & Thick Steel Rusted Items Steel Coatings & Contaminants Adhesive Residue, Cutting Oil, Marking Pens, Masking, Penetrant Dye, Painted Sections, Pre-Existing Zinc Coatings, Thermal Cut Edges

Identification of Items

Welding Welding Items for Hot Dip Galvanizing Weld Quality, Welding Media, Welding Slag & Spatter, Welding Painted Sections Welding Galvanized Steel

1112-3

4466-8

131313-14

14-1517-18

1919192020-23

252525-27

27

24

9911

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Distortion Double (End) Dipping Plate, Sheet and Coil sections Channel Sections Hollow Sections

Requirement for Holes Holes for Hanging Wire, Chain or Other Touch Marks Holes to Prevent Entrapment Air Locks, Zinc Pooling, Ash Formation Holes for Venting Overlapping Surfaces Holes for Venting and Draining Hollow Sections External Holes, Internal Holes Holes for Venting Tanks & Hollow Vessels

Inspection and Dressing Items Bare Spots, Acid Leeching, Smoothness, Oxide Lines, Touch Marks & Wire Marks, Colour & Lustre, Dross Pimples, Spangle, Chromate Colouring

Passivation and Storage Passivation Packaging & Storage Light White Oxidisation, Wet Storage Stain

Environmental Performance Coating Life Warm Dry Atmosphere, Rural Areas, Industrial Areas, Coastal Areas Time to First Maintenance

Duplex Coatings Preparation & Pre-Treatment of Galvanized Steel Abrasive Blasting, Powder Coating, Wet Brush/Spray Coating

Moving & Threaded Parts

2829303132

52525353-54

4849-51

3334363737414244-4647

555555-56

56

575757-58

59

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Hunter Galvanizing | 2Hunter Galvanizing | 1

Differentiating Zinc Coatings

Relevant Standards for Zinc Coatings on Steel

Some steel products are zinc coated by means of various processes carried out in-house by the manufacturer. Pre 2000, AS1650-1989 “Hot dipped galvanized coatings on ferrous articles” covered all zinc coatings regardless of product type or application method.

In 1999, Australia Standards and New Zealand Committee MT/9 recognized the unique characteristics of hot dip galvanizing on fabricated items and how these varied from those of other coating systems. A number of separate standards were created to distinguish different coating processes and the attributes of each:

AS/NZS 4680 Hot Dip Galvanized (zinc) coatings on fabricated ferrous articles The scope of this standard covers structural steel, steel reinforcements, steel sheet fabrications, assembled steel products, tubular fabrications, fabricated wire work, steel forgings, steel stampings, ferrous castings, nails and other small components. The standard applies to both centrifuged and non-centrifuged articles. The standard does not apply to products such as wire and welded wire fabric, sheet, or open sections and tube hot dip galvanized in continuous, semi-continuous or specialized plants. Definition of Hot Dip Galvanizing – a process comprising of pre-treatment, and molten zinc baths in which steel products are dipped so as to form adherent zinc and zinc-iron alloy coatings.

Zinc-iron alloy coatings are formed when molten zinc reacts with elements of the steel’s surface in the galvanizing bath at a nominal operating temperature of 450°C. Hunter Galvanizing provides hot dip galvanized coatings with zinc-iron alloy layers in most cases harder than the base metal upon which they are applied. An average coating mass of 600g/m² is achieved on fabricated items of steel thickness 6mm (and greater) in accordance with AS/NZS 4680. Details of this process form the basis of this manual.

Standards for other Zinc Coating Processes:AS1397 Steel sheet and strip – Hot dipped zinc coated or

aluminium/zinc coatedAS/NZS 4534 Zinc and zinc/aluminium – alloy coatings on steel wireAS/NZS 4791 Hot dip galvanized (zinc) coatings on ferrous open sections,

applied by an in-line processAS/NZS 4792 Hot dip galvanized (zinc) coatings on ferrous hollow sections,

applied by continuous or specialized process

Different Zinc Coatings

As recognized by the Standards committee, it is advisable that specifiers and fabricators have an understanding of the advantages and limitations of the variety of zinc based coatings available. Coatings defined as a barrier coating will protect the base metal only as long as the coating remains intact. Coatings with both barrier and cathodic properties will continue to protect the metal even if damaged. As coating and service life are determined by the zinc thickness, each coating type provides a different level and period of corrosion protection. Furthermore, some processes do not use heat to form a metallurgical bond and subsequent zinc-iron alloy layers and as such, these do not meet the defined criteria of a galvanized coating. A summary of the different zinc coatings follows:

• Zinc Metallizing / Zinc Metal Spraying / Thermal Spraying A barrier type coating where melted zinc powder is manually or mechanically sprayed from a heated gun onto an abrasive blasted surface. This process allows large items to be coated which are unable to fit into galvanizing baths or cannot be removed from site. Coatings of 250μm (microns) can be achieved, however the coating is not alloyed to the base steel therefore not a ‘galvanized’ coating. Zinc spraying has difficulty in reaching recesses, cavities, and hollow spaces. Coatings may be uneven and costly.

• Zinc Rich Paint / Cold Galvanizing Zinc dust in organic or inorganic binders is applied to an abrasive blasted surface by brush or spray. Zinc rich coatings are barrier coatings and do not form alloyed layers with the base metal thus reference to galvanizing is incorrect. Suitable zinc rich paint coatings provide a useful repair media for hot dip galvanized coatings. As with zinc spraying it is difficult to reach recesses, cavities, and hollow spaces however the application provides an alternative if items are unable to be removed from site or placed in a galvanizing bath due to size limitations.

• Continuous Galvanizing / In-Line Galvanizing / Galvanized Sheet / Galvanneal / Zincanneal Sheet steel is cleaned, pickled and galvanized on a processing line run at very high speeds. The automated process allows accurate control of the coating process. Aluminium is included in a shallow zinc bath to suppress the formation of the zinc-iron alloys, resulting in a thin coating (25μm microns)that is mostly pure zinc. In-line products have thin coatings and may require additional protection for outdoor exposure. Galvanized strip is further processed into pipe and tube hollow sections. If sheet or strip is to be painted, an additional process is carried out called galvannealing / zincannealing. An in-line heat treatment process is used to produce a fully alloyed coating which will provide alloy layers on the steel surface to help paint adhesion. Coating mass quoted on continuous/in-line product refers to to the total of all surfaces. Galvanized sheet with a coating mass of 300 grams/m² has in effect 150 grams/m² each side. In comparison coating mass quoted on a hot dip galvanized item details the zinc thickness on each side: eg 600 grams/m2.

• Electroplating / Zinc Plating / Electrogalvanizing Surfaces are cleaned then submerged in a zinc salt solution. Rods or balls of pure zinc are charged with an electrical current and a very thin coating of between 5µm–15µm (microns) is applied to the steel surfaces producing a very even and metallic sheen. Electroplated components should be finished with a layer of paint or other organic coating prior to outdoor exposure in order to increase their service life.

Metallizing

Hot Dip Galvanizing

Electroplating Continuous In-Line Hunter Galvanizing


Zinc MetalSpraying600-1500

g/m²

Hot DipGalvanizing

300-900g/m²

ContinuousGalvanizing

40-240g/m²

ZincPlating60-80g/m²

150microns

10 microns

BASE METAL

BASE METAL

Hot DipGalvanizing

Zinc MetalSpraying

Electroplating

ContinuousGalvanizing

MechanicalPlating Sheridizing

ZincRichPaint

microns50

microns50

microns50

microns25

100microns

Corrosion

Understanding Corrosion

Corrosion is a natural electrochemical reaction involving the movement of electrolytic cells. To fully appreciate the benefits of hot dip galvanizing, an understanding of the cause and effect of corrosion is required.

Electrolytic cells are made up of an anodic and a cathodic element and are found on the surface of all steels. For corrosion to occur, the following four specific components are required to be in contact with each other: • an anode: an electronegative active metal on which corrosion occurs (the electrode where galvanic reactions generate electrons)

• a cathode: an electropositive noble metal protected from corrosion (the electrode that receives electrons)

• conductive material: the metallic connection for the anode and cathode (under lying metal which transfers the electrical current)

• an electrolyte: a conducting solution which carries the current (aqueous solutions, water, moisture, dampness or other liquids)

If unprotected, the electrolytic cells of iron particles and other impurities in the surface of steel react with moisture and allow the formation of rust. Steel is a combination of impurities, oxygen and metal elements both anodic (active) and cathodic (less active). The metal elements on the surface of a piece of steel form interlocking areas of anodes and cathodes connected by the underlying steel which is the conductive material.

When the steel is exposed to moisture, the electrochemical reaction occurs. As negative electrons flow from anode to cathode, they are charged and converted to positive ions which in turn react negatively with hydrogen in the moisture. The anodic area depletes and forms a pit and new anodes and cathodes are exposed from underneath. The cycle continues and corrosion occurs.

The greatest benefit of hot dip galvanizing is realised with an understanding of how a zinc coating provides anodic (sacrificial) protection to the entire steel item. This is discussed in How Zinc Protects.

Comparison of Zinc Coating Thickness

Electrolytic Cell

Comparison of Zinc Coating ThicknessTypical thickness of zinc coatings range from 10 microns to 150 microns. Protective life is proportional to the zinc thickness and as such hot dip galvanizing provides coating protection second only to thermal spraying.

Comparison of Zinc Coating MassZinc coating mass is measured in grams per square metre. Hot dip galvanized coatings have an average 600 grams of zinc per m2. This is more than twice the zinc coating mass found on in-line galvanized products such as sheet, wire and hollow sections.

• Sherardizing Steel components are cleaned with acid and packed in a drum with zinc powder and sand. The drum is rotated and heated. With continued rotation, iron and zinc galvanize and iron/zinc alloys are formed on the steel surface. Sherardizing produces relatively thin coatings between 15μm–50μm (microns) with dark grey surfaces.

• Mechanical Plating / Peen Plating Items mechanically plated receive a flash coating of copper followed by the zinc coating. Coating thickness range available is between 5µm–110µm (microns). The mechanical bond between zinc and steel in this process is weaker than the metallurgical bond found in hot dip galvanizing. Edge, corner and thread coating thicknesses are usually lower due to minimal peening action at these locations.

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How Zinc Protects

When an item is hot dip galvanized, it forms a barrier between the steel surface and moisture; without moisture contacting the steel, corrosion cannot occur. This barrier provides a metallurgical protective system. Bonded to the base metal with impenetrable adhesion, it has high abrasion qualities and shields the steel from the effects of it’s environment.

Unique to a hot dip galvanized coating is the combination of barrier protection and cathodic protection properties.

• Cathodic Protection As discussed in Understanding Corrosion, the variance in the electrical potential between zinc and steel in the alloy layers of a galvanized coating will create an electrolytic cell. When zinc is used to protect an item, it provides anodic (sacrificial) properties for the base steel. In the event a zinc coating is damaged and the base steel is exposed to moisture, an electrochemical reaction will occur causing the zinc anode to oxidize in preference to the cathodic bare steel. For this reason, galvanized coatings are referred to as sacrificial coatings or coatings with sacrificial properties. Metals which provide sacrificial properties to others are detailed in The Galvanic Series of Metals on the following page.

Cathodic protection is evident in day to day fabrication and building where:• continuous in-line galvanized purlins, roofing sheets and fasteners have bare metal exposed at cut ends and where holes are punched

•in-line galvanized wire or mesh is cut to length

• zinc plated nuts have uncoated internal threads.

In the above instances, the existing coating protects the uncoated areas and will continue to do so whilst sufficient zinc is present.

The cathodic reaction can also be experienced when a hot dip galvanized item is in contact with an uncoated piece of steel, steel filings or drill shavings. The zinc on the galvanized item will commence to oxidize or sacrifice itself where it is in contact with the uncoated steel.

Left in contact for an extended period, the benefits of the zinc’s cathodic properties are unnecessarily depleted; wasted on an object not associated with the galvanized piece of steel.

The cathodic properties of a hot dip galvanized coating will protect small areas of bare metal.

C

C

C

C

CC

CC

C C C

CCC

C

H O2H O2

A

A

A

A

AA

AC

C C C

CCC

C

H O2H O2

A A A A A

C

C

C

C

CC

CC

C C C

CCC

C

H O2

A A A A A A

Electrolyte(Water or Humidity)


Galvanic Protection

Steel


Zinc Coating

Zinc Coating

Anodic and Cathodic CellsAnodic and cathodic cells are present on the surface of the steel. When the steel is exposed to water (electrolyte), electrons flow through the water from the anode cells to the cathode cells. The electrons are then transferred via the underlying steel (conductive metal) back to the anode. The anodes subsequently dissolve and rust forms.

Hot Dip Galvanizing is a BARRIER coatingHot dip galvanized coatings provide a barrier, preventing moisture from attacking the steel surface.ADDED BONUS — Most steel is cathodic when compared to zinc.As zinc is one of the most ACTIVE (electronegative) metals, all cells on the surface of the steel under the galvanized coating become cathodic.

Hot Dip Galvanizing also offers CATHODIC PROTECTIONShould the galvanized coating be damaged and an area of bare metal is exposed, the anodes will detract the moisture from attacking the cathodes. Cathodic protection occurs as the anodes “sacrifice” themselves. As long as anodic material exists (zinc) rust will be prevented from forming.

Anodic and Cathodic Properties

Base

Hunter Galvanizing


•strong acid solutions

•insecticides in solution

•organic lubricants

•freshly treated timber

•unseasoned timber

Sacrificial properties of zinc is demonstrated where bare metal is exposed at cut edges.

Any element listed above mild steel in the Galvanic Series of Metals is anodic and will sacrifice itself to protect the cathodic steel.

• Galvanic Series of Metals As discussed in Cathodic Protection, the galvanized zinc coating provides sacrificial properties to the surface of the steel. Metals which provide protection to cathodic elements are detailed in The Galvanic Series of Metals.

This table details metals in decreasing order of electrical activity as found when submerged in seawater. It is used as a guide to determine which metal will have a greater tendency to lose electrons or experience galvanic corrosion when electrically connected to another.

At the top of the list are metals such as magnesium, zinc and aluminium. Metals located here are the most susceptible to corrosion and can be used as a sacrificial element to protect other metals lower on the table. They are referred to as being anodic or less noble. In turn they will sacrifice their properties to protect lower listed metals. Whilst magnesium, aluminium and cadmium are also listed above steel, zinc is cost efficient and is widely used for this purpose.

Metals listed at the lower end of the table are cathodic metals. These are least susceptible to corrosive attack and are referred to as being more noble and have less electron activity.

The ability to provide cathodic benefits is one of the key advantages of a hot dip galvanized coating. In contrast, paint and other non-galvanized coating systems depend on their ability to provide a seal over the steel surface. Paint systems also require anti-corrosive inhibitors to be added. Should the coating fail or become damaged, barrier coatings offer little if any protection to the exposed steel and corrosion will quickly commence and spread.

Further examples of the anodic and cathodic effect are found where other metallic objects are in contact with a zinc coated item. This is discussed in Galvanic Corrosion.

MagnesiumZincAluminium

Cadmium

Mild steel

Cast Iron

Stainless steel, type 410 (active)

Lead-tin solder, 50/50

Stainless steel, type 304 (active) Stainless steel, type 316 (active)

Lead

Tin

Brasses

Gunmetals

Aluminium bronzes

Copper

Copper-nickel alloys

Monel

Titanium

Stainless steel, type 304 (passive) Stainless steel, type 316 (passive)

Silver

Gold

Platinum

An

od

ic

C

atho

dic

Galvanic Series of Metals

• Galvanic Corrosion Galvanic corrosion occurs when an anodic element is in contact with a cathodic metal whilst subject to moisture. Zinc, (anodic) a more active metal than most, will rapidly lose electrons and sacrifice its properties to corrode in preference to the lesser (cathodic or more noble) metal. Contact of zinc coated items with aluminium, cadmium and stainless steel is generally fine in moderate environments. However, in contact with metals such as copper, the loss of properties of the zinc will be extremely high. An example of galvanic or electrolytic corrosion is where dissolved particles of copper or brass in run off water from pipes is in contact with a galvanized item. The cathodic properties of the zinc will be activated; it will corrode in preference to the copper; in turn wasting its energy and depleting its ability to protect the actual steel item it is coating. Other cathodic metals that have a detrimental effect when in contact with zinc in a moist environment or in liquid, include cast irons, chromium, bronze, nickel and hard solders. As discussed in Differentiating Zinc Coatings, other coatings offer various levels of corrosion protection and service life. When two or more different zinc coatings are in contact, they will act independently. When moisture is present on both, the thinnest zinc coating type will be the first to oxidize and corrode. This is often evident where zinc plated fasteners are attached to hot dip galvanized structural steel and exposed to rain and moisture; the electroplated items will corrode rapidly in comparison.

Hot dip galvanized items in contact with each other rarely form issues. However, accelerated anodic reaction may occur between two or more hot dip galvanized items in areas which experience continual extreme high levels of humidity.

• Other Materials and Hot Dip Galvanized Coatings Galvanized coatings perform well when exposed to sewerage, soaps, detergents, diesel, fuel, glycerine, mineral lubricants, refrigerants and dried timber products. When moist, concrete and mortar may initially etch the surface of a hot dip galvanized coating, however this should not pose an issue to the corrosion protection properties of the zinc. The following materials are detrimental to zinc and hot dip galvanized items should not be placed in contact with:Hunter G

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CAUSTICDEGREASING BATH

HYDROCHLORICACID BATH

MOLTEN ZINCBATH

QUENCHBATH

PAINT & OILREMOVED

SURFACE RUSTREMOVED

ZINC & STEELGALVANIZE

COATINGPASSIFIED

The Hot Dip Galvanizing Process.

Eta Layer: Pure outer zinc coating, 70 DPN hardness

Patina

Zeta Layer: Zinc-iron alloy containing 94%zinc & 6%iron, 179 DPN hardness

Delta Layer: Zinc-iron alloy containing 90%zinc & 10%iron, 244 DPN hardness

Gamma Layer: Zinc-iron alloy containing 75%zinc & 25%iron, 250 DPN hardness

Base Steel: Typically 159 DPN hardness

A hot dip galvanizing coating consists of 3 zinc-iron layers, plus 1 layer of pure zinc. A protective patina forms over time as the coating is exposed to the natural weather conditions.

Hot dip galvanized coatings are often thicker around the edges of steel items.

Hot Dip Galvanized Coatings

How the Coating Forms

The process of hot dip galvanizing starts in the cleaning and preparation of the steel. Items are suspended on hanging frames and placed in a series of pre-treatment chemical baths to remove rust, contaminants and light pre-existing coatings. Once clean, the items are moved into a bath of molten zinc with a nominal operating temperature of 450°C.

The coating is achieved when zinc reacts with iron contained in the steel’s surface. A unique, layered protective system is formed as the zinc ‘galvanizes’ with the base metal; covering corners, sealing edges and penetrating all internal and external recesses. Unlike paint based systems, the coating metallurgically bonds with the entire surface area of the item and does not shrink from the edges of the steel sections.

Three alloying layers Gamma, Delta and Zeta form on the surface of the steel. Harder than the base metal which is typically 150 DPN (Diamond Pyramid Number), these layers provide the durability and high resistance to abrasion for which hot dip galvanizing is recognized. Iron content through the layers ranges from 6%–25% producing hardness levels between 179 DPN–244 DPN.

Eta, a fourth relatively soft, pure zinc layer forms on top. Should this outer layer sustain impact or damage, the harder inner layers continue to offer abrasion resistance together with cathodic protection if bare metal is exposed. In addition to the physical barrier provided by these four layers, a fifth layer called the patina forms over a period of time after the item is despatched.

The patina is a series of films on the zinc surface initially consisting of zinc oxide and zinc hydroxide. A final transition occurs as the oxides react to carbon dioxide in the air forming a dull grey zinc carbonate coating.

It is the formation of the zinc carbonate film which changes the lustre of previous bright coatings to a matt weathered appearance. Subject to the conditions of the immediate environment the item is located, this transition may occur quickly or over a period of months. The formation of the patina (weathering) completes the protective armour of the hot dip galvanized coating and is a critical key to long term corrosion protection.

Micrograph of a Hot Dip Galvanized Coating Alloying Layers of a Hot Dip Galvanized Coating

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Coating thickness will vary subject to steel thickness.

Bright lustre on newly galvanized items will ‘weather’ and gradually become dull over time as the protective zinc oxides form the patina. A different coating standard and coating thickness applies to mass produced in-line continuous galvanized products.

AS4680:2006 Coating Thickness Requirements Items Not Centrifuged

AS4680:2006 Coating Thickness Requirements Centrifuged Items

Article Thickness

Up to & including 1.5mm

Above 1.5mm-3mm

Above 3mm-6mm

Greater than 6mm

Average Coating Thickness (Min)

45μm (microns) or 320 grams per m²




Local Minimum

35μm (microns)

45μm (microns)

55μm (microns)

70μm (microns)

Article Thickness

Under 8mm

8mm and above

Average Coating Thickness (Min)



Local Minimum

25μm (microns)

40μm (microns)

Hunter Galvanizing achieves the minimum coating thicknesses as specified in the governing standards AS/NZS 4680:2006 “Hot Dip Galvanized (zinc) Coatings on Fabricated Ferrous Articles” as detailed in the following tables.

Small items may be spun at high speeds in a centrifuge, which rotates the items very quickly to remove excess zinc.

Coating Thickness

The thickness of the coating is determined by the reactivity of the steel’s metallurgy with the zinc and the thickness of the steel it is covering.

It is for this reason that minimum coating thicknesses are stipulated in the Galvanizing Standard AS/NZS 4680 and are generally easy to achieve. Maximum thickness however cannot be dictated. High reactivity between zinc and the steel’s composition will generate thicker coatings as will the effect of abrasive blasting prior to galvanizing. This is advantageous as the coating thickness will determine the longevity of the coating and subsequent service life of the steel item. Further information relating to the composition of steel can be found in Effects of Steel Chemistry.

AS/NZS 4680:2006 is specific to the “hot dip galvanizing” process. It states ”The galvanized coating shall be continuous, adherent, as smooth and evenly distributed as possible, and free from any defect that is detrimental to the stated end use of the coated article. On silicon killed steels, the coating may be dull grey, which is acceptable provided the coating is sound and continuous. The integrity of the coating shall be determined by visual inspection and coating thickness measurements. With reference to adhesion the galvanized coating shall be sufficiently adherent to withstand normal handling during transport

and erection”.

The appearance of hot dip galvanized coatings should, and can not, be compared with that of the continuous in-line galvanizing system which allows a high degree of control over the zinc coating and provides comparatively low zinc content (with subsequent reduced corrosion protection properties).

Similarly, the smoothness of hot dip galvanized coatings cannot be judged by the same standards as those with a high degree of automation and control. Thicker coatings offer longer protection to the base metal. Uneven coatings are outside of our control and are acceptable under the standard.

Some confusion exists regarding the industry term double dipping. This term has no bearing on coating thickness. Double dipping refers to the dipping of one end of an item, then the other to permit items with dimensions larger than those of the galvanizing baths to be coated. Further information regarding double dipping can be found in the chapter, Distortion.Hunter G

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Suitable Steels for Hot Dip Galvanizing

Steel Type & Embrittlement

Hot dip galvanized coatings are able to be achieved on most ferrous materials and general steel grades without difficulty. However, as hot dip galvanizing is a form of heat treatment and items are soaked in acid, some susceptible grades of steel maybe prone to embrittlement which is outside of our control.

• Strain Age Embrittlement Strain age embrittlement is caused in certain low quality steels when areas stressed by cold working are exposed to elevated temperatures (including hole punching and tight radius bending in thicker steel sections). Steels generally have many impurities which gather in high stress areas and in certain steels cracking may occur prior to galvanizing. It is recommended where possible that items are worked after galvanizing; any flaking or cracking will be limited to the zinc coating which can be repaired using zinc rich paint.

• Hydrogen Embrittlement Generally occurring in steels with a tensile strength equal to or higher than 1000 MPa and harder than 340 DPN, hydrogen embrittlement rarely affects structural steels. This form of embrittlement is likely to be observed when an item is in service and under load. Hydrogen is absorbed during the acid pre-treatment process and then discharged quickly during galvanizing. Specialised steels such as Bisalloy and other susceptible steels should be abrasive blasted immediately prior to galvanizing to eliminate the requirement for soaking in pre-treatment chemicals.

• Liquid Embrittlement Embrittlement in this form may occur on high carbon and stainless steel where zinc atoms are absorbed by the susceptible metal. In critical applications, stainless steel items should not be hot dip galvanized. When galvanizing non-critical stainless steel items, additional pre-treatment may be required to enable the zinc coating to form.

• Other Issues Other issues related to steel type are generally limited to old iron work items or castings which are often porous. Castings may have sand embedded which cannot be removed by pre-treatment processing. Items should be abrasive blasted prior to delivery. Of additional note, soft solder and aluminium rivets must not be used in any fabrication as they will not withstand galvanizing temperatures. Brazed itemsshould be discussed with Hunter Galvanizing staff to confirm suitability.

Effects of Steel Chemistry

Hunter Galvanizing provides industrial zinc coatings formed by metallurgical reaction between steel and molten zinc. The smoothness, thickness and colour of hot dip galvanized coatings are not factors which can be controlled as the steel thickness combined with the steel chemistry will determine the aesthetics of the coating. Thicker steel will attract thicker zinc coatings which by nature will be darker in colour. Items also may display a bright sheen through to a dull or matt grey finish. It is impossible for galvanizers to conform to a specific shade of silver or grey or to control the lustre of a coating.

The metallurgical structure of the steel may encourage a variety of effects to appear in the coating. Localized areas can display a lacework or snakeskin pattern, dull grey patches or large bright spangles. These effects may appear in one area or across the entire surface of a piece of steel. Extreme levels of silicon and phosphorous have dramatic effects relating to colour, lustre and smoothness of a hot dip galvanized coating.

Stainless steel components may require additional processing to achieve successful coatings. Sand trapped in castings will cause coating issues. Variances in steel chemistry in different sections within one fabrication are clearly visible after galvanizing.

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Some manufacturing processes of steel processing can also alter the formation of the free zinc layer creating a number of effects on hollow sections. Fish bone effect can occur on large diameter pipes where the difference in the surface chemistry will cause varying reaction rates between the steel and zinc.

Patches of dull grey can present in a striped or spiralled sequence along lengths of pipe, RHS and SHS, where zinc has reacted to stresses in the surface chemistry produced during manufacture of these sections.

As discussed in Welding, zinc is attracted to welding media high in silicon. Weld material used in the production of pipe and tube sections is highly reactive with zinc and welding seams will be highlighted by heavier coatings.

All of the above phenomenon have no effect on the corrosion protection properties of the hot dip galvanized coating or on the integrity of the steel section. Items displaying any of these effects are not cause for rejection.

Hunter Galvanizing accepts steel items for hot dip galvanizing based on their design and fabrication. Galvanizers can not be aware of the potential for high reactivity of the steel with molten zinc, unless specific material specifications have been supplied and discussed prior to hot dip galvanizing.

Thicker coating along the seam is a result of high reactivity between the zinc and silicon in the weld material. Note the differing colours and coating thickness of other items.

Fish bone effect on large diameter pipe is a result of manufacturing stresses and not an acceptable cause for rejection.

Rough coating appearance reflects the metallurgical substrate of the pipe section. This differs from the attached piece of RHS.

Variances in steel chemistry within the same fabrication or piece of steel is common.

Extreme reactivity between steel chemistry and zinc will result in coatings which may not be smooth.

Reaction between zinc and surface stresses have formed dull grey areas on the above SHS sections.

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Delaminated coatings are caused by continued alloy growth in highly reactive steel and is outside the control of the galvanizer.

As silicon and phosphorus are not always distributed evenly throughout the steel, some areas within the same piece of steel can have higher/lower silicon and/or phosphorus levels; creating coatings that grow differently than the surrounding areas.

Differing cooling rates may produce dull grey areas around edges of small plates and holes. This is in stark contrast to the remaining bright shiny coating.

For general fabricators it is not possible to determine steel chemistry accurately prior to processing. Steel analysis certificates can detail only batch testing levels. As silicon and phosphorus are not always distributed evenly throughout the steelmaking process, samples of the total heat may not be representative of each individual piece of steel. Where aesthetics is critical, trial samples of product can be galvanized; however, again they may not provide a true indication of chemistry across a product batch. Items displaying chemistry related issues are acceptable under the galvanizing standard and are not means for rejection.

Two chemical components in steel have the ability to affect coating thickness and appearance.

• Silicon Very high or very low levels of silicon will generate high reactivity with the zinc and in turn stimulate rapid growth of the zinc–iron layers. Silicon in the ranges between 0.04%–0.14% (low extreme) and silicon above 0.22% (high extreme) will have varying degree of effects. Zinc-iron layers will grow less in steels containing between 0.15%–0.22% silicon and in general will display lighter coloured coatings (subject to steel thickness).

• Phosphorous When phosphorous is present in steel in levels above 0.05%, reactivity is also increased and will result in thicker, matt coatings. Recommended level of phosphorus should be below 0.05%. If steel has a combination of extreme levels of silicon (either very high or very low) and high levels of phosphorous, the coating produced will be excessively thick and the outer layers may be brittle and easily chipped during handling and transportation. The following formula can be used as a guide when determining the steel’s chemistry as to its suitability for hot dip galvanizing: Suitable Steel = %Silicon + (2.5 x %Phosphorous) < 0.09%

• Delamination Extremely reactive steel can cause a void to form between the top two layers of the galvanized coating causing the outer layer to peel. This is referred to as delamination. Sufficient zinc generally remains in the underlying (hard alloy) layers which continue to be metallurgically bonded to the base steel offering the protective qualities of a sound hot dip galvanized coating. This effect is often outside of our control and not a suitable means of rejection. Delamination occurring after galvanized items have been abrasive blasted in readiness for painting, is not the responsibility of the galvanizer. Procedures for the correct blasting of hot dip galvanized coatings are detailed in Duplex Coatings

• Differing Cooling Rates Some cut edges of sections within a fabricated item may cool much quicker than others and result in shiny effects surrounding dull grey areas. This different rate of cooling allows free zinc to form on top of the existing alloy layers causing the zinc patterns, whilst other areas not affected by the free zinc produce a dull grey colouring. This effect is not deemed to be an issue, as the shiny areas will also change to the dull grey colour as the coating weathers and the coating patina forms.

Items with coating effects described are no less suitable in service than those with a bright finish or a spangle effect typically displayed on very thin steels. A dull grey coating is likely to indicate a thicker coating of zinc; and as service life is proportional to coating thickness, these coatings will perform longer, extending the life of the underlying steel. Further information relating to dull grey colouring and weathering is detailed in Hot Dip Galvanized Coatings. Control of colour, lustre and texture remains outside of the control of Hunter Galvanizing.

Example of high reactivity steels: analysis displayed 0.19% silicon (acceptable range), 0.021% phosphorus (extreme). In combination 0.19+(2.5*0.021) = 0.24% well above the recommended level for hot dip galvanizing.

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Surface rust is easily removed in our process.

Pitted surfaces will be evident after galvanizing.

Suitable Surface Conditions

Large Items or Thick Steels

Very thick coatings may form on items which are large and consist of heavy steel sections. Longer immersion and handling times required to process these sections may result in the metallurgical properties of the steel having high reactivity with the zinc whilst in the galvanizing bath. Further information relating to zinc reactivity is available in Effects of Steel Chemistry.

Rusted Items

Light mill scale and surface rust on items can be removed within our pre-treatment process. Heavily rusted steel will require abrasive blasting prior to delivery to remove the rust layers. If the steel surface is pitted after blasting, this effect will be evident after the hot dip galvanized coating is applied.

Steel Coatings

The bond between the steel and zinc is unable to be achieved if any form of substance remains on the steel surface after chemical treatment. Resistant substances which can prevent the coating from forming include (however are not limited to) pre-existing zinc coatings, paint, lacquer and adhesive residue from identification labels and stickers.

• Adhesive Residue Manufacturer’s steel identification stickers will deteriorate during pre-treatment processing; however non visible adhesive residue may remain and prevent successful formation of the zinc coating. Stickers should be removed and the immediate area prepared by suitable means to remove the adhesive substance prior to despatch.

Adhesive residue contaminates the steel surface.

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Oil based paint markings should be ground from the steel surface prior to hot dip galvanizing.

Application of a high temperature tape or sealants will prevent zinc from forming in small areas.

Drilling lubricant can contaminate the steel surface preventing successful coating.

• Cutting Oil Some cutting lubricants can become baked onto the steel surface during fabrication. Oil based fluids are not visible during pre-treatment processing and may contaminate the surface preventing the coating from forming. Cutting oils should be cleaned from the surface prior to delivery.

• Marking Pens Paint pigmentation from marking pens may be resistant to chemical cleaning. During pre-treatment processing paint layers are removed, however, non-visible pigments may remain. The zinc coating will form around the residue pigmentation and remnants of workshop markings may remain evident after galvanizing. Oil based paint markings should be removed by suitable means from steel surfaces prior to delivery for galvanizing.

• Masking In some applications, small mating or threaded areas may be required to be uncoated. This can be achieved by applying a small amount of a suitable adhesive or sealing product on the area creating a barrier to pre-treatment acids and zinc. The following products will have varying success in preventing zinc coatings from forming and some clean up of the surrounding surfaces will be required by the fabricator after galvanizing. –Tapes high temperature tape / duct tape –Sealants silicone adhesive sealant / petroleum gel / household chalk –Paints Maskote / Stop Galv

Dyes utilised to check for weld penetration can contaminate the steel surface and should be removed prior to delivery.

• Penetrant Dye for Welds Dyes utilised for weld checks can create issues if unable to be removed by pre-treatment chemicals. Dyes should be removed from steel surfaces with a suitable paint removing solution or by light sanding prior to despatch for galvanizing.

• Painted Sections Most pipe and tube sections manufactured in Australia are painted with water based coatings which generally can be removed within our process. Some local manufacturers of pipe and tube products and most offshore producers coat their product range with clear varnish or black bituminous paint. These coatings are resistant to chemical removal within our galvanizing plant and are required to be abrasive blasted prior to delivery. To avoid additional costs and extended processing times, hollow sections should be stipulated that they be suitable for hot dip galvanizing when ordering from your steel supplier. All steel sections with powder coated, brush or spray paint coatings must be abrasive blasted prior to despatch to Hunter Galvanizing.

Primer paint coatings can be removed from most domestically produced pipe and tube sections during our process.Hunter Galvanizing


Pre existing zinc coatings must be removed prior to hot dip galvanizing.

Flame cut edges should be ground prior to delivery for galvanizing.

• Pre Existing Zinc Coatings Pre existing zinc coatings must be stripped from all items including in-line produced lengths of hollow section, purlins, wire and mesh prior to hot dip galvanizing. Additional charges are necessary for this procedure and lead times may be increased. Some pipe and tube products have an external light coating of zinc and a painted internal surface. These items will require internal abrasive blasting prior to delivery to our facility in addition to acid stripping.

Thermal Cut Edges

Flame, laser and plasma cutting will change the structure of the steel composition in the immediate area of the heat source. These areas may present thinner coating thicknesses and a lack of adhesion, reducing the ability of the zinc alloy layers to bond with the base metal.

The high temperatures utilised to cut material depletes the alloying elements in surface of the steel. As discussed in Hot Dip Galvanized Coatings the formation of the coating structure relies on the alloying of iron and zinc in the galvanizing bath. Should insufficient alloying elements be present, the minimum coating thickness may not be able to be achieved; and cohesion of the galvanized coating will also be limited. To eliminate such issues, grind the heat affected surface and bevel the cut edge.

For permanent identification which will be visible after the hot dip galvanizing process, item details can be stamped or welded onto the steel surface. Steel identification tags can also be stamped or welded and wired to individual items.

For non-permanent markings, chalk or non-permanent pens are suitable. Water soluble paint markers can be used, however, if applied heavily paint pigmentation can prove difficult to remove. Often not visible after acid cleaning, residues can contaminate the surface and prevent zinc bonding to the steel surface. All workshop markings should be kept to a minimum. Refer Marking Pens.

If using oil based paint markers, all paint must be removed from each item prior to delivery, as the paint pigmentation may remain evident after galvanizing. The immediate area should be ground to remove the paint residue which is not visible after chemical treatment.

Hunter Galvanizing utilizes proprietary printed tags to assist with traceability. Tags are produced and wired to individual items upon receipt. Withstanding pre-treatment acids and galvanizing conditions, they remain on the item for identification at the customer’s location.

Stamped tags can be wired to items. Welded identification. Water based paint markers for non-permanent identification.

Hunter Galvanizing produced tags

Identification of Items

Hunter Galvanizing


Welding

Welding Items for Hot Dip Galvanizing

Poor welding techniques affect the quality of a hot dip galvanized coating. By understanding the hot dip galvanizing process, most issues related to welding can be avoided.

Internal stresses developed during welding can result in distortion during the galvanizing process. Items should be designed so they can withstand weld stress despite reduction and elasticity. Further information is provided in Distortion.

General guidelines: • Face joints of lap welds downwards to avoid collection of moisture and sediment

•Where possible, use butt welds in preference to lap welds •Stagger welds to minimise heat related stresses.• Eliminate trapped processing liquids and air between welds by allowing a 2.5mm gap between surfaces.

• Where possible restrict the size of the welding seam and apply in a symmetrical or even manner in a fabricated item.

• Weld Quality During pre-treatment, chemicals can become trapped in very small gaps or pin holes in the welds. The trapped liquid may boil out of these areas and cause a ‘miss’ in the immediate area. Alternatively the chemicals can boil and ‘blow out’ onto further areas of the same item or onto other items being processed. Any pre-treatment liquid at this stage will contaminate the surface and prevent the zinc from galvanizing in the affected areas.

Narrow gaps less than 2.5mm should be avoided between plates, back to back angles or channels. Gaps of this dimension will allow pre-treatment acids to escape and molten zinc to penetrate the area.

Chemicals that do not escape may crystallize in the holes or gaps. Later when exposed to moisture they can leech or weep, creating a yellow brown stain on the surface of the coating. Care should be taken to ensure pin holing does not occur.

• Welding Media Zinc reacts differently when in contact with a number of elements; including silicon deposits in welds. When items are galvanized with welding media high in silicon, thick coatings form on the weld as the zinc growth is rapidly accelerated. This is a metallurgical reaction between the silicon and zinc and outside the control of the fabricator and the galvanizer. This has no detrimental effect on the integrity of the weld.

Acid can leech from gaps and pin holes in welds.

Acid trapped in pin holes in welds will boil out and cause a ‘miss’ in the coating.

Pin holes can trap dried chemicals which will leech when in contact with moisture at a later date.

‘Blow out’ of acid trapped in welds.

Silicon in weld material will attract thick coatings of zinc.

• Welding Slag and SpatterThe residue from flux or slag is inert, unable to be removed by pre-treatment chemicals. If present when galvanized, it will leave uncoated areas on the welded joints. The use of uncoated electrodes or electrodes which create self detaching slag is recommended when fabricating items for hot dip galvanizing.

To ensure maximum zinc coverage; flux, slag, residues from welding ferrels or stud insulators should be ground, brushed, scraped or abrasive blasted from the welded areas as they prevent the zinc from bonding to the weld surface.

Weld spatter should also be removed prior to delivery as it remains visible after processing and may later become dislodged leaving uncoated areas.

Weld spatter should be removed prior to delivery.

Weld spatter can dislodge after galvanizing and result in localized oxidisation

Residue from welding electrodes will prevent zinc bonding with the weld surface if not removed prior to hot dip galvanizing. Affected joints in service can be cleaned and coated with a suitable zinc enriched paint.

Hunter Galvanizing


• Welding Painted Sections When items are welded to painted or lacquered hollow sections, pigmentation can become baked onto the steels surface. Residues will remain ingrained and prevent zinc from bonding to the base steel. We recommend that the area around the heat affected zone of the weld be ground to remove all residues prior to delivery.

Welding Galvanized Steel

Satisfactory high quality welds can be made on hot dip galvanized steel with tensile, bend and fatigue properties identical to those of welds made on uncoated steel. However, welding speeds will be slower and there will be increased spatter.

General Guidelines:• All welding and oxy cutting processes can readily be used on galvanized steel with minimal variations required.

• Zinc coatings should be ground 25mm–100mm either side of the intended weld zone and on the surface and underside.

• All welds made in galvanized articles should be protected against rust as soon as welding is finished.

• Care should be taken to ensure adequate ventilation is provided to minimise the possibility of adverse fume reaction.

• Extraction equipment should be utilised when welding galvanized steel in confined areas.

Paint pigmentation remains in the heat affected zone.

Distortion

The percentage of items which are affected by distortion issues is relatively low given the volume of items which are hot dip galvanized. An item’s dimensional stability can be compromised by a number of factors. By understanding the causes of distortion and adopting simple design principles, the effects can be minimised.

Inherent internal stress is present in every steel section. Stresses may be a result of the steel mill rolling, handling and transport methods or incurred during subsequent manufacturing processes; cutting, welding, hole punching or other cold working.

At galvanizing temperatures (approx 450°C) steel sections can experience a reduction of up to 50% in their yield strength. Whilst normal strength is returned upon cooling; this effect combined with release of internal stresses can result in distortion in some items.

Distortion can also be the result of different thermal expansion and contraction rates occurring when thin items such as sheet, plate and mesh are used in conjunction with items of thicker sections. Thin sections or weaker areas within a fabrication may lose their shape as heat is transferred through the item. Thermal and contraction rates also differ during the process of double dipping, increasing the propensity of some sections to lose their shape or distort. Refer to Double (End or Side) Dipping.

Non symmetrical sections or fabrications with cleats or plates welded to one side may bow. This potential is prevalent in channels and large welded beams.

General Guidelines:• Where possible items should be designed so they are able to be immersed into our galvanizing baths in one single dip. Hunter Galvanizing bath sizes are: Plant 1 10m long x 2.4m deep x 1.5m wide Plant 2 7m long x 3m deep x 1.8m wide

• To allow even heat transfer, avoid using combinations of thick and thin materials in the same fabrication.

•Bend curved members to the largest possible radii.• Venting and draining holes should be as large as possible to allow timely immersion and withdrawal from the galvanizing bath.

• Heat induced stress can be minimised by staggering welds. Welding should be as symmetrical as possible and use opposing weld shrinkage forces to balance each other.

• Symmetrical sections such as SHS, RHS and pipe should be used where possible and avoid designs where plates and cleats are welded to one side.

• Fabricated items which are stronger in one area may suffer distortion in the weaker plane. Brace items that are weaker over large areas. Bracing material should be the same thickness as the item.

• Where angles or channels are used to rim or frame a tank, apertures must be provided in the corners.

• Items with large areas of unsupported plate or sheet may suffer from liquid drag when being withdrawn from below the surface of the molten zinc. Coupled with reduced yield strength some loss of shape can occur.

Please note as distortion is generally a result of poor design and outside of our control, Hunter Galvanizing cannot be held responsible for items not meeting the recommended criteria.

Hunter Galvanizing


Design should allow for handrails to be bolted to large fabrications after hot dip galvanizing.

Items should be designed in sections suitable for Hunter Galvanizing bath sizes.

Large items can be double dipped.

Double dipping refers to dipping a large or long item one end or side at a time.

Items fabricated using thin sheet are at high risk of distortion.

• Double (End or Side) Dipping Subject to the overall size and degree of difficulty; items with dimensions greater than our baths may be double dipped. The term double dipping has no bearing on coating thickness; it relates to dipping a large item into the galvanizing bath one end or one side at a time. To achieve a hot dip galvanized coating over the entire surface one end is pre-treated and dipped in the molten zinc; the item is withdrawn, and turned end to end. The second end or side is then cleaned in pre-treatment chemicals and dipped.

Large open bins or tanks should be braced. Bracing material should be the same thickness of the walls of the tank.

A

A

B A

B

B

B

A

Plate, Sheet and Coil Sections

The potential for distortion is increased when utilising large areas of unsupported plate. Items should be designed to incorporate the smallest area of unsupported plate possible, as small sections of plate will hold their shape better than large sheets.

General GuidelinesSubject to overall dimensions and weld stresses, thicker plate sections hold less risk of distortion. Large areas of plate 12mm thick and under have a high probability of distorting if unsupported. • Floors for platforms and panels of perforated sheet should be galvanized separately then bolted to supporting structures.

• Floors in box trailers will not remain flat and will display some degree of buckling.

• Plate 6mm and under will ripple when hot dip galvanized.• Plate sections should be limited to our bath sizes to eliminate the requirement for double dipping.

• Plate able to be suspended diagonally and submerged in one dip will allow heat to transfer evenly throughout the section.

• Folded, ribbed or corrugated sheet sections are less prone to distortion and generally hold their shape.

• Long lengths of thin plate will require a number of holes to allow the plate to be supported along one edge.

•Equalize stresses by cutting all edges in the same manner. Cold cutting methods such as guillotine will induce less stress than heated methods.• Welding will increase the stress in plate items. A stitch or staggered weld will induce less stress than a full continuous weld; however this may lead to ‘blow outs’ as discussed in Welding.

All items incorporating large areas of plate should be discussed with Hunter Galvanizing staff prior to fabrication.

Staggered welding will minimise stresses.

Flat panels should be braced, ribbed or dished with openings provided in the corners.

Small areas of thin plate are less likely to distort.

Large areas of unsupported thin sheet distort easily.

The potential for distortion is increased as sections of the fabrication remain cool whilst others are heated. As one end is heated to galvanizing temperatures (450°c) it will expand, whilst the opposite end remains at ambient temperature. The section may bow when the second end is immersed in the molten zinc. Additional lifting points may be required to facilitate double dip handling. Distortion potential of items in this category should be discussed with Hunter Galvanizing staff prior to delivery.

Hunter Galvanizing


Channels

As channels (Parallel Flange Channels and Tapered Flange Channels) have a non symmetrical profile, heat is transferred unevenly and the risk of distortion is increased.

General Guidelines:• Subject to their length, channels have the tendency to camber or sweep when galvanized. Where possible these sections should not be double dipped without discussion with Hunter Galvanizing staff.

• Welding cleats on one side of a channel will encourage the channel to camber along its length at galvanizing temperatures.

• Lintels fabricated from channel and flat bar present dual issues. A combination of the cross section, coupled with welding stresses may increase the opportunity of camber or sweep. Staggered welding to minimise the additional stress is recommended, however pre-treatment chemicals may become trapped and result in blow outs or staining during galvanizing as discussed in Welding.

Welded Beams

Large fabricated beams or columns, including certain sizes of the pre-welded product range pose some issues when hot dip galvanized.

General Guidelines:• Sections where the web thickness is less than half that of the flange thickness are prone to distortion as the thinner web expands at a faster rate than the thicker flanges.

• Subject to the depth of the of the web, the top flange may cool more rapidly as it is withdrawn from the bath increasing the stress over the rest of the section.

• Twisting may result in large welded beams and columns due to the combination of longitudinal, transverse and sheer stresses within the section coupled with the loss of yield strength (up to 50%) at galvanizing temperatures.

• Designers incorporating these sections should select sizes with the thickest possible web to assist in minimising the twisting action.

• The provision of symmetrical stiffening ribs may assist in the control of the expansion and contraction of the section.

• Double dipping of welded beams and columns should be avoided at all times.

Channels have a non-symmetrical cross section.

Fabricated lintels pose dual distortion issues.

Staggered welding is recommended for fabricated lintels however this may allow acid to weep from gaps in the weld.

Holes near each end are required to help support channel sections.

WebThickness

FlangeDepth

Fillet

Flange Width

Flange Thickness

Large welded beams or columns with a flange to web ratio greater than 2:1 may not be suitable for hot dip galvanizing.

Hollow Sections

Pipe and tube hollow sections (Circular Hollow Sections, Square Hollow Sections, Rectangular Hollow Sections) hold stresses induced in their rolling and manufacture. At galvanizing temperatures the relieving of these stresses coupled with the loss of yield strength may result in camber or sweep forming in longer lengths.

General Guidelines• Lengths of tubular sections should remain proportional to their diameter or cross section and lifting lugs or holes should be provided at quarter points.

• Venting and draining holes should be as large as possible to allow timely immersion in the bath and adequate drainage of zinc to prevent the loss of shape.

• Bracing at the end of long pipes may be required to prevent zinc drag or loss of shape when items are lifted from the galvanizing bath.

Diameter and cross section of hollow sections should remain proportional to length.

• All channel sections including those fabricated as lintels or with cleats, should have holes placed near each end to enable them to be suspended and dipped with their toes up. Longer lengths may require additional lifting points; please confirm with Hunter Galvanizing staff.

Hunter Galvanizing


In order for items to progress through the series of pre-treatment and galvanizing baths at our facility, they must be suspended in a suitable manner to ensure all liquids are able to clean and galvanize all surfaces. If items are small enough, they can be out sourced and centrifuged in perforated baskets (spinning). This process is ideal for large numbers of nuts, bolts etc. Touch marks are often an issue with centrifuged items where they make contact whilst being processed.

In house, some products are processed in specialized dipping frames or racks and allow large quantities of straight lengths to be galvanized at the same time. Minimal areas of the surface of each section will be in contact with parts of the dipping racks resulting in small touch marks which, subject to quantity and end use of the items, may or may not be touched up with repair paint.

Large assemblies are supported by chain slings or lifting fixtures. To enable safe handling, lifting points should be incorporated into the fabrication’s design distributing the weight equally over 4 points. Lifting lugs and heavy duty washers can be welded at the required points and then removed after galvanizing. These are preferable to chain or wire marks remaining after galvanizing. The coating can be repaired with appropriate zinc enriched paint if aesthetics is an issue after removing unwanted lifting points.

Most general fabrications are suspended by wire on apparatus referred to as headframes and hung vertically or on an angle to maximise drainage of pre-treatment chemicals and molten zinc. Additional holes or lifting lugs may be required after fabrication to enable successful galvanizing of items.

In order to understand where to place a hole for galvanizing, the function of each hole must be understood.

The various functions of holes can be categorized into four requirements: hanging, prevention of pooling and entrapment, venting and draining and to relieve pressure from overlapping surfaces.

Reference hereon is made to the high end (air exit point) and low end (zinc entry point).

Requirement for Holes

Items are suspended to allow total coverage of processing liquids and zincs.

Holes for Hanging

The shape and dimensions of an item will determine how it is suspended during the hot dip galvanizing process. Where possible holes located in cleats, flanges or base plates will be utilised to suspend general fabricated items. Should holes not be available, they will be required to be added to enable the item to be hung in the correct plane, allow processing liquids to drain and to minimize distortion.

General Guidelines• Subject to weight; items less than 2m require a hole or lifting lug placed at one end. Longer and heavy items will require holes or lifting lugs at both ends. Holes should be a minimum of 10mm diameter, large enough for jigging wire to be passed through.

• Larger fabrications will require numerous wire strands to be used and hole sizes should be adjusted accordingly. Further information should be sought from Hunter Galvanizing staff regarding size and location of holes specific to each item.

Holes in endplates, cleats and gussets are utilised to suspend items.

High End – Air Exit

Low End – Zinc Entry

Hunter Galvanizing prefers external lifting lugs to be fitted to long or heavy items.Hunter G

alvanizing


• Wire, Chain or Other Touch Marks All items are processed either in dipping frames, held by chains or suspended by wire.

Items supported in dipping frames may have small touch marks evident where they have been in contact with the frame structure.

Chains are utilised for heavy items and will leave touch marks in their immediate area as will wire if required to be wrapped around an item or through hanging holes. The wire sticks to the surface of the galvanized coating as the item is withdrawn from the galvanizing bath.

Touch marks or chain marks are usually completely galvanized affecting only the outer free zinc layer of the coating and therefore not a reason for rejection.

Holes are required where end plates are not available to secure wire. The large pipe above (top left) has holes in flanges coupled with additional lifting lugs.

Hunter Galvanizing


• Air Locks Pre-treatment acids are critical in preparing the steel surface for galvanizing. The acids remove contaminants including surface rust, soluble oils and water based paint coatings. When an item is suspended on a headframe, air can become trapped in corners at the high points. An air pocket prevents cleaning solutions from preparing the steel surface and the zinc coating will not form in these areas.

• Zinc Pooling Molten zinc is very dense and solidifies immediately upon withdrawal from the galvanizing bath. Excess zinc will collect in corners of fabrications subject to the item’s hanging position. Zinc pooling will increase the overall weight of an item and may affect the cost and end use application.

Holes to Prevent Entrapment

When an item is suspended on a headframe, it remains in the same hanging position throughout the hot dip galvanizing process. Holes are required to be in the appropriate location to ensure pre-treatment acids, molten zinc and zinc ash can flow freely from all item surfaces as it is submerged and withdrawn from each processing bath.

General Guidelines:• A hole, gap or mitre in the corners of gussets or stiffeners will assist processing products to drain.

• Holes through end plates or web plates will also provide suitable access for zinc and zinc ash to drain and means of air to escape.

• By adopting a “hole in every corner” principle, the majority of issues relating to draining can be eliminated and the best possible hot dip galvanized finish can be achieved.

• We recommend holes should not be less than 12mm in diameter, (larger if the design permits) to enable zinc and zinc ash to escape freely as items are withdrawn from the galvanizing bath.

The following aesthetic issues are generally deemed acceptable under the governing standards for hot dip galvanizing. Simple allowances whilst fabricating can minimise these effects.

An airlock will form if pre-treatment chemicals are unable to escape from corner areas.

Zinc collects in corners of items if holes are not correctly positioned.

Holes or cropped corners are recommended.

• Ash Formation Zinc ash is a bi-product of the zinc iron alloying process. Ash forms on the surface of the molten zinc and is skimmed away from items as they are withdrawn through the zinc’s surface. Subject to the number of items processed in the galvanizing bath at one time, some surfaces may not be reached by operating staff and ash may adhere to the steel as it is withdrawn. A light skin or film may form in isolated areas on the surface of an item. Heavier deposits of ash may remain trapped within a fabricated item or hollow section. Upon drying the ash appears as a yellow-brown powder or in clumps. As ash is relatively pure zinc it does not represent any concern to the coating properties other than aesthetics and can be easily brushed from the surface.

Light film of ash. Ash can become trapped on surfaces within structural steel sections.

Ash will be caught in long lengths of hollow sections.

Hunter Galvanizing


Recommended holes or mitres for general fabrications, universal beams and columns.

By placing a hole in every corner of general fabrications, most issues relating to entrapment of air, zinc and zinc ash will be minimised.

“A HOLE IN EVERY CORNER”

Recommended location of holes, mitres or cropped corners

Hunter Galvanizing


Holes for Venting & Draining Hollow Sections

Molten zinc is extremely dense. Consideration of venting and draining requirements is mandatory when fabricating items for hot dip galvanizing to eliminate potential hazards:

– items may float on top of the molten zinc– air may become trapped or is slow to escape

Sealed hollow sections will float on the surface until such time they are either pushed or dragged below the zinc’s surface. As discussed in Holes for Venting Overlapping Surfaces; at galvanizing temperatures trapped air or moisture will very quickly convert to super heated steam. The resultant pressure can expand, distort or tear weaker areas within a fabrication (either in steel thickness or weld) with explosive force. Galvanizing staff are at risk as this force may cause steel fragments or molten zinc to be blown from the galvanizing bath causing injury and rendering processing equipment inoperable.

In order for items to be processed safely, vent holes are required to enable the item to be submerged and allow air and moisture to escape at the same rate as the zinc enters. The following rules must apply regarding size and location of holes for hollow sections to permit hot dip galvanizing in a controlled and safe manner.

General Guidelines:

• All capped hollow sections must have a minimum of one hole or cropped corner diagonally placed at each end to permit zinc entry at the low end and air escape at the high end when suspended on a headframe.

• Hollow sections open one end (low end) require a minimum of one hole or cropped corner at the capped end (high end).

• Hollow sections welded within a fabrication must have holes placed at both ends of each hollow section.

• Holes or cropped areas should be as large as the design or end use will allow for expanding air, processing liquids, zinc and zinc ash to escape from within.

• The total area of holes must be equal to or no less than 25% of the diameter of cross section of the hollow piece. Numerous holes can be placed in order to meet this venting requirement. Holes less than 10mm are not functional as they can easily become blocked; Hunter Galvanizing recommends 12mm diameter holes minimum.

Holes for Venting Overlapping Surfaces

When steel sections are welded together, air is trapped between the overlapping surfaces. At galvanizing temperatures, the entrapped air converts to super heated steam with pressure sufficient to force weak areas within a fabrication (either in steel thickness or weld) to expand, distort or tear.

Galvanizing staff are at risk as this force may cause steel fragments or molten zinc to be blown from the galvanizing bath causing injury and rendering processing equipment inoperable. In order for items to be processed safely, holes are required to enable air and moisture to escape and pressure to be relieved. The following rules must apply regarding size and location of relief holes.

General Guidelines:•Overlapping areas 10cm² or greater must have one hole every 100mm.•Thin, long overlapping areas require one hole every 300mm in length.• Where possible avoid designing items with back to back channels and angles unless a gap of 2.5mm or greater is allowed.

•Overlapping areas greater than 40cm² should be avoided at all times.• Holes may be placed through one or both steel surfaces and must be greater than 10mm in diameter.

•Alternatively, staggered welding can provide sufficient means for air to escape.•In some instances, it may be suitable to leave one edge free of weld.• As detailed in Welding pre-treatment chemicals become trapped between overlapping surfaces and may result in aesthetic issues.

The overhanging edge remains free of weld to allow air to escape.

Overlapping areas will require suitable venting holes in place.

Entrapped acid causes issues in weld pin holes and overlapping surfaces.

Gaps should be provided between back to back channels and plates.

Holes can be through one or both surfaces.Holes must be located in diagonally opposite corners of SHS and RHS.

Holes or V notches must be present in all hollow sections within a fabrication.

Recommended location of holes, mitres or cropped corners

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• External Holes SHS, RHS, CHS & Pipe Sections In open-ended SHS, RHS and pipe sections, a hanging hole each end is required to allow hanging wire to pass through. If the hollow section is capped, additional holes for venting are required. For SHS and RHS sections, holes should be placed in the corners. In pipe sections holes are required to be as close to the outside diameter as possible. Holes placed in the centre of end caps will allow air pockets to form or zinc to pool. The minimum acceptable number of venting holes is one each end placed diagonally opposite. If there are no means of holding the item two holes each end are required, with holes located diagonally opposite each other. The size of the venting holes is critical for processing hollow sections. Sufficient air must escape at the same rate zinc is entering the section. Holes should be no less than 25% of the cross section of the hollow section. *The minimum hole size acceptable on any item is 10mm, however as holes this size can prove ineffective we recommend holes sizes of 12mm or greater.

The overall dimensions of an item and hanging method must be considered when fabricating items which include hollow sections.

Small holes may result in floating or drainage issues. Holes should be as large as possible to facilitate ease of galvanizing and to allow the best possible coating to be achieved.

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Hole Placement Options for RHS & SHS Sections

Preferred Option

No endplate

Holes forhangingCorrect Holes

Hole Placement Options for RHS & SHS Sections

Preferred Option

No endplate

Holes forhangingCorrect Holes

Hole Placement Options for CHS & Pipe Sections

Preferred Option

No endplate

Holes forhanging

Hole Placement Options for CHS & Pipe Sections

Preferred Option

No endplate

Holes forhanging

Examples

• A section of 100 x 100 SHS or 100 x 50 RHS must have holes at each end equivalent to approximately 25mm either as: 1 x 25mm hole each end OR 2 x 12mm holes each end OR 4 x 6mm holes each end

• A section of 50mm diameter pipe must have holes at each end equivalent to approximately 12mm either as: 1 x 12mm hole each end OR 2 x 10mm* holes each end

Fabricated items, gates, handrails and fencing require thought regarding placement of holes. Ideally holes should be placed externally to allow quick visual inspection by the galvanizer. Unwanted holes can later be filled with epoxy filler, lead or threaded plugs.

• Internal Holes SHS, RHS, CHS & Pipe Sections Internal holes are not recommended due to inherent safety concerns. Where internal holes are utilised, sections should be interconnected using mitred joints or interconnecting holes. Internal holes should be as close as possible in size to the cross section of the hollow section to eliminate air pockets and zinc pooling inside the fabrication. The ends of the hollow sections must remain open.

Hunter Galvanizing requires signed documentation to guarantee holes have been correctly placed.

1. External Holes 2. Internal Holes 3. Mandatory - Holes in Bends 4. Mandatory - Ends Remain Open

Air and zinc must flow freely within the hollow sections

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Inspection and Dressing of Items

The primary function of a hot dip galvanized coating is to provide corrosion protection. It is an industrial coating specified for it’s ability to protect steel with a system of protective layers formed by the metallurgical reaction between the base metal and molten zinc.

Coupled with cathodic properties, it offers extended longevity to steel items in most environments. Hot dip galvanizing is often incorrectly marketed as an aesthetic or architectural finish with a shiny and smooth coating. These features are subject to many factors outside the control of the galvanizer and are not attributes upon which a coating can be gauged.

Visual inspection and coating thickness testing is simple and the most critical means of assessing the quality of a hot dip galvanized coating.

General Guidelines:• Patterns, colour and finish will be determined by the metallurgy and the rate of reactivity between each piece of steel and the molten zinc which bonds to the surface. The process forms an initial coating of four layers. Later, whilst the item is in service, the fifth and most important layer forms; the dull grey patina of zinc carbonate.

• Hot dip galvanizing is not a colour; the coating can be dull grey to bright silver. Colour can vary within the one fabrication and on the same piece of steel.

• Coatings on all items will turn dull grey as they are exposed to natural weathering and the patina forms on the outer surface.

• Coating may have a spangled effect, others may display lacework or reptilian like patterns as elements within the steel chemistry react differently to zinc.

• Coatings cannot be ordered as an architectural finish. Smoothness and roughness are not qualities a galvanizer can control as chemistry and stress within the surface of the steel will dictate this.

Refer to How the Coating Forms for more information.

Holes for Venting

• Tanks and Hollow Vessels Specific attention to venting and draining is required when preparing large hollow items such as tanks, pods or vessels for galvanizing. The design of tanks and closed vessels must allow for pre-treatment chemicals, air and zinc to enter, fill and flow out of the enclosed space. A large filling hole (minimum of 50mm diameter for each 0.5 cubic metres) is required at the low end when suspended for galvanizing. A vent hole of equal dimensions will be needed diagonally opposite the filling hole to allow air to escape. Internal baffles in tanks must have their corners cropped prior to installation or with large drainage holes to permit free flow of the air and zinc within. Access ports, bosses and openings should be finished flush inside and should be positioned so that all processing fluids can be drained out during the galvanizing process. Whilst processing tanks, a large volume of zinc will pass through the vessel. As a safety precaution we require heavy duty lifting lugs attached to each item. We recommend discussing this requirement with Hunter Galvanizing staff prior to fabrication.

General Guidelines•Venting holes are required to be diametrically opposite.• Minimum acceptable hole size is 50mm in diameter. Subject to tank size additional holes may be required.

•Internal baffles should be cropped top and bottom.•Lifting lugs will be required to facilitate handling. • Design should incorporate an inspection hole to enable internal surfaces to be viewed.

Internal baffles of tanks must be cropped

Drain

Vent

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• Bare Spots Bare spots are generally a result of issues outside of our control, such as rolling defects, or contamination from paints and adhesives as outlined in Suitable Surface Conditions. Subject to size they may be repaired with zinc enriched paint. Cathodic protection will prevent corrosion on small uncoated areas. The governing standard, AS/NZS 4680:2006 states: “The size of the area able to be repaired shall be relevant to the size of the object and the conditions of service but shall normally be in accordance with the provisions of AS/NZS 4680 - Repair after Galvanizing. For objects galvanized after fabrication, the sum total of the damaged or uncoated areas shall not exceed 0.5% of the total surface area or 250cm². No individual damaged or uncoated area shall exceed 40cm²”.

Welding spatter can be easily dislodged in handling or transport after hot dip galvanizing. Light oxidisation will occur very quickly in these areas.

• Acid Leeching Brown or red stains may appear as pre-treatment chemicals leak from unsealed joints after an item has been hot dip galvanized. During galvanizing, chemicals crystallize leaving anhydrous residues in the small holes in welds or overlapping surfaces. Later the crystals absorb water from the atmosphere and weep out onto the immediate surface. Acid leeching is not the responsibility of the galvanizer and is not cause for rejection. Information on preventing entrapment of pre-treatment chemicals can be found in Welding and Requirement for Holes.

• Smoothness Drainage spikes form as the molten zinc solidifies as the item is withdrawn from the galvanizing bath. They are removed to facilitate safe handling. General zinc runs are unavoidable in the hot dip galvanizing process and will not be removed under normal dressing procedures. If additional processes post galvanizing dictate a high degree of smoothness, we recommend the fabricator allows for additional dressing after despatch. Smoothness of the galvanized coating may be affected by abrasive blasting prior to galvanizing. Similarly, surface steel chemistry and manufacturing processes of some steel products may also result in roughness. Refer to Suitable Steels for Hot Dip Galvanizing for more information.

• Oxide Lines Oxide lines form as items are withdrawn from the galvanizing bath. This effect will fade over time as the entire zinc surface oxidizes and the dull grey patina forms. An aesthetic issue only; they have no effect on the corrosion performance of the coating and are not a cause for rejection.

• Touch Marks & Wire Marks In order to hold items adequately during the hot dip galvanizing process items are either held in processing jigs or suspended by wire which in turn become coated in zinc. A touch mark or wire mark will occur where items have rested on a jig or where wire has been removed. If required, areas may be coated with a zinc enriched paint. These occurrences are unavoidable and are not suitable means for rejection. Further information is detailed in Requirement for Holes.

• Colour and Lustre When galvanizing occurs, the thickness of the steel together with its composition, will determine certain aspects of the coating appearance. Thicker steel will attract thicker zinc coatings which by nature will be darker in colour. Coatings may display a bright sheen through to a dull or matt grey finish. As such, it is impossible for galvanizers to conform to a specific shade of silver or grey. The metallurgical structure of the steel may encourage a variety of effects to appear in the coating. Localized areas may display a lacework or snakeskin pattern, dull grey patches or large bright spangles. These effects may appear in one area or across the entire surface of a piece of steel. Extreme levels of silicon and phosphorous have dramatic effects relating to colour, lustre and texture of a hot dip galvanized coating. This issue is discussed in detail; refer to Suitable Steels for Hot Dip Galvanizing.

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• Dross Pimples Dross is created when free iron particles in the galvanizing bath react with the molten zinc. Generally not an issue, dross inclusions do not affect the corrosion resistance of the coating.

• Chromate Colouring As discussed in Passivation & Storage all items are quenched in a passivation solution to assist in the prevention of light white oxidisation. Quench colouring will vary subject to thickness of the steel and will fade as the hot dip galvanized coating forms its natural patina whilst the item is in service. Very thick sections may display deep yellow or green shades which are unavoidable and not an acceptable cause of rejection.

Passivation and Storage

Passivation

For a short period after galvanizing the outer layer of the hot dip galvanized coating is susceptible to the formation of zinc oxides. To minimise effects of light white oxidisation, items are passed through a bath of passivation solution. This process can impart a yellowish film to the galvanized coating. On most items the film will be barely visible; however, on items of heavier thickness it may appear much darker. This does not detract from the quality or performance of the coating and will generally remain visible for approximately 6 weeks upon which time with natural weathering the patina will form. Further information relating to the patina layer can be found in How the Coating Forms.

• Spangle In reference to hot dip galvanizing, spangle is characterized by a snowflake shape pattern visible in the coating. Spangles may be small and of a uniform bright silver finish, or large and an array of matt grey and bright silver shades. The presence or absence of spangle has no affect on the performance or quality of a hot dip galvanized coating. Its occurrence may be subject to many variables including (but not limited to) coating thickness, zinc bath chemistry, steel chemistry or the rate and/or method of cooling. It occurs during the crystallization or freezing of the outer layers of the zinc.

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• Wet Storage Stain Wet storage stain is a more severe form of light white oxidation caused by prolonged storage of items packed closely together in a humid, damp or poorly ventilated environment.

Product such as lintels, guardrail, and other items which are tightly nested are at risk of incurring wet storage stain if stored externally in their configured bundles. Air must be permitted to circulate freely, around all zinc surfaces when stored. Wet storage stain will not occur on items where moisture is unhindered and can evaporate naturally.

If items are bundled in close configuration for an extended period; fresh water or moisture such as rain, dew and condensation can react with the pure zinc outer layer of the hot dip galvanized coating. Unable to escape, this moisture will form zinc oxide and zinc hydroxide.

In the majority of cases, this does not reduce the expected life of the zinc coating. However, if moisture remains trapped between zinc surfaces, medium to heavy buildup can cause extreme damage to the coating and may result in the items being required to be stripped and re-galvanized.

Light wet storage stain will disappear once bundles are opened, items separated, and allowed to dry. The wet storage stain will convert to the dull grey zinc patina as it reacts with carbon dioxide in the air. Items which have formed their protective patina layer generally will not be susceptible to wet storage stain.

Medium or heavy wet storage stain is very dark or black in colour. At this stage, a significant amount of the zinc coating has been consumed and the patina will be prevented from forming.

The storage stain deposits should be brushed from the area and the coating reinstated by applying an approved zinc enriched epoxy paint to a thickness of approximately 100 microns.

Whilst passivation of items after galvanizing will minimise the occurrence of wet storage stain, the best precaution is to avoid stacking products in poorly ventilated, damp conditions. Wet storage stain is found most often on tightly stacked and bundled items, such as galvanized sheets, plates, angles, bars, hollow sections and pipe.

To ensure wet storage stain does not occur, we strongly recommend packaging utilised to facilitate safe handling and transportation of items be removed as soon as possible after despatch to allow adequate air flow across all surfaces. Break all bundles of items apart to allow moisture to evaporate and prevent the occurrence of bulky white deposits or wet storage stain.

Staining and wet storage deposits are outside of our control and we are unable to accept any responsibility for such occurrences.

Packaging & Storage

Unseasoned timber, mud and clay may discolour hot dip galvanized coatings if in direct contact with their surface.

Uncoated steel, iron filings, scrap metal or uncoated fasteners should not be permitted to be in contact with galvanized coatings. Cathodic reaction will cause the zinc to sacrifice itself to protect the base metal and corrosion may be accelerated by these uncoated items. Further information relating to this issue is in How Zinc Protects.

Items should be packed in a suitable manner to facilitate safe handling and transportation, whilst allowing drainage of any condensation which may occur prior to despatch.

• Light White Oxidisation Light white oxidisation may occur as a thin film of chalky powder. It may occur in times of high humidity or after a period of rain on freshly galvanized zinc coatings. Provided the items are well ventilated and permit moisture to drain or evaporate quickly. Light white oxidisation rarely progresses past this superficial stage. If required, it can be easily removed by applying a low acidic product such as vinegar or CLR Clear with a nylon brush. Otherwise it will generally wash off in service and with normal weathering the patina layer will develop. In this form, it is not detrimental to the hot dip galvanized coating and remedial treatment is not required.

Packed items should be broken apart as soon as possible after despatch.

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Coating Life

The corrosion rate of any metal is determined by the immediate environment in which it is located. Corrosion rates of hot dip galvanized coatings in different environments have been extensively researched and documented worldwide and have been found to corrode between one seventeenth (1/17) and one eightieth (1/80) slower than uncoated steel.

Coating life is directly proportional to the coating thickness of the zinc, and as corrosion rates in specific environments are known, the life expectancy of the hot dip galvanized coating can be estimated.

The following information regarding general atmospheric conditions is supplied as a guide only. Specific microclimates within a general area will influence the rate of corrosion further and must also be taken into consideration. These issues can be discussed further with Hunter Galvanizing staff.

• Warm, Dry Atmosphere The anticipated life of galvanized coatings in an arid dry environment is long term. Protection may continue indefinitely as zinc stability is very high resulting in minimal coating loss. Zinc coatings on steels of thicknesses of above 6mm may remain stable for 100 years or longer.

Environmental Performance • Rural Areas Coating life may be affected by the effects of aerial spraying of fertilizers or insecticides. In dry form these elements will pose little threat, however in solution, fertilizer and insecticides will attack galvanized coatings. Long term protection can be expected if not subjected to hostile chemicals with a coating loss of approx 1–3 microns per year. Based on steel thickness above 6mm it is plausible to expect coatings to remain effective 25 to 85 years.

• Industrial Areas In light industrial areas, hot dip galvanized coatings will generally perform well with coating loss average of 3–5 microns per year. Based on steel thickness above 6mm and subject to exposure to adverse contaminants 15–30 years may be achieved by coatings. High levels of sulphuric gases and chemicals located in some heavy industrial areas will increase coating loss to approximately 5–8 microns per year. In this environment, a duplex coating system of hot dip galvanizing and paint may prove beneficial.

• Coastal Areas Corrosion rates are higher in the presence of salt air. Hot dip galvanized coatings perform well in comparison to other protective systems, however, duplex coating systems of hot dip galvanizing and paint provide the optimal protection. Coating loss will average between 5–15 microns per year subject to the proximity to the ocean and levels of rainwater to wash marine salts from the coating surface. Life span of coatings may range between 5–10 years based on steel thickness over 6mm.

Time to First Maintenance

It is important to know the specific corrosion rate of a given environment to effectively plan for long term sustainable coating life. The most commonly used method for estimating corrosion life of hot dip galvanized items is the use of general values for the different types of atmospheres. These values are referenced as Time to First Maintenance and utilised across the world.

Information regarding Time to First Maintenance can be found on the American Galvanizing Website www.galvanizeit.org/inspection-course/galvanizing-process/time-to-first-maintenance or by contacting Hunter Galvanizing.

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Duplex Coatings

Hot dip galvanizing is a long lasting and cost effective means of protecting steel from corrosion. When organic coatings such as paint or powder coatings are applied over hot dip galvanized steel, the resulting combination is known as a duplex coating.

Preparation & Pre-Treatment of Galvanized Steel

All galvanized items are dressed in accordance with the Australian Standard for Hot Dip Galvanizing; sharp edges and dags are removed however runs and general roughness of the zinc surface will remain. The result of high gloss and smoothness when paint has been applied over extruded metal, planed timber or pre galvanized sheet, wire and tubing will not necessarily be achieved when applied to an item which has been hot dip galvanized. Additional dressing may be required by the fabricator prior to powder coating or painting to achieve the smoothness required for a paint finish standard.

Please Note: if undertaking further dressing of items, care must be taken not to damage the zinc coating by heavy or excessive grinding.

In order to provide a sound substrate for a duplex coating, the galvanized surface will require abrasive blasting or application of a suitable primer.

• Abrasive Blasting In order to create a suitable surface for paint coatings to adhere to newly hot dip galvanized coatings an abrasive sweep or brush-blast may be used.

- Blast material should have a particle size no larger than 0.5mm or between 200 to 500 microns. Aluminium/magnesium silicate, limonite and other suitable mediums can be used .

-Blast pressure should not exceed 40 psi to ensure the minimum amount of zinc oxide is removed.

Paint coatings should be applied as soon as possible after abrasive blasting.

It is important that this operation is performed carefully as the removal of excessive zinc will compromise the quality of the hot dip galvanized coating

Should hot dip galvanized coatings be abrasive blasted in an incorrect manner the coating will delaminate or peel. As this procedure is a form of mechanical damage we do not accept responsibility should peeling or delamination occur.

• Wet Brush or Spray The painting of hot dip galvanized steel requires different paint systems and preparation than if painting uncoated steel. Not all paint types will adhere to a hot dip galvanized coating and as paint formulas vary from one manufacturer to another, a technical expert in the paint field should be consulted.

• Powder Coating Powder coatings are applied by the electrostatic spraying of dry powders which are then heat fused at moderate temperatures to form a continuous, homogeneous coating. To ensure surface treatment applications utilised at Hunter Galvanizing do not interfere with the powder coating process purchase orders supplied with items should clearly state that a powder coating will be applied after galvanizing.

Please Note: Hunter Galvanizing cannot be held responsible for the integrity of the galvanized coating on any item once a subsequent process outside of our control has commenced.

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Hunter Galvanizing Hunter Galvanizing | 59

NotesMoving & Threaded Parts

Depending how an item is suspended for processing, a build up of zinc may occur as it solidifies when withdrawn from the galvanizing bath. Whilst the hot dip galvanized coating is relatively thin, 1mm–2mm minimal radial clearance on moving parts such as drop handles, shackles, hinges and shafts should be allowed. Where possible items should be designed to permit moving parts to be assembled after galvanizing and hinges should be of the loose-pin type.

If the galvanizing process freezes the moving parts, they can be gently reheated and worked free. Reheating may cause discolouration of the galvanized coating; however this should not be detrimental to the coatings ability to prevent corrosion.

It is recommended to cut threads and tap nuts oversize to provide additional clearance of between 0.5mm–1mm. As with moving parts, threads may be gently reheated and cleared of excess zinc with a wire brush. Should you require the thread not be coated, a suitable masking product can be applied prior to delivery for galvanizing to minimise zinc adhesion. Further information relating to masking is in Steel Coatings.

Hunter Galvanizing

Sydney

P 1300 617 778

F (02) 9625 8356

E [email protected]

A Warehouse A, 2 Glendenning Rd, Glendenning, NSW (Cnr Glendenning Rd & Woodstock Ave, entry via Woodstock Ave)

We look forward to being of assistance to you.Please contact us regarding design, lead times, and pricing of our hot dip galvanizing & freight service.

Newcastle

P (02) 4964 9555

F (02) 4964 9333

E [email protected]

A 13 Old Punt Rd, Tomago, NSW

www.huntergalvanizing.com.au

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