Corrosion en Metal Liquido

7/29/2019 Corrosion en Metal Liquido

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Material Evaluation of Liquid Metal Corrosion

in Zn-Al Hot-Dip Coating Baths

Matthew L. Burris

Thesis submitted to the

College of Engineering and Mineral Resourcesat West Virginia University

in partial fulfillment of the requirements

for the degree of

Master of Sciencein

Mechanical Engineering

Keh-Minn Chang, Ph.D., ChairBruce Kang, Ph.D.

Kenneth Means, Ph.D.

Department of Mechanical and Aerospace Engineering

Morgantown, West Virginia2000

Keywords: Liquid Metal Corrosion, Iron Aluminide, Bath Hardware Corrosion, Hot-Dip

Coating


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ABSTRACT

Material Evaluation of Liquid Metal CorrosionIn Zn-Al Hot-Dip Coating Baths

Matthew L. Burris

The corrosion of bath hardware used in continuous galvanization lines is a great

challenge within the hot-dip coating industry today. The objective of this research was to

examine the potential of using intermetallic Fe3Al alloys developed at Oak RidgeNational Laboratory in the bath hardware application of the continuous hot-dipping

process.

In order to examine the effectiveness of Fe3Al in this harsh, molten metalenvironment, two sets of liquid metal corrosion tests were carried out. The purpose of

the first set of experiments was to examine the effect of test duration on the corrosion of

the different alloys. The testing was conducted by the immersion of test specimens in a

hot-dip coating bath of a commercial hot-dipping line. These corrosion tests wereperformed on coupons of Fe3Al, and a low carbon stainless steel, 316L, was accompanied

for comparison. The 316L stainless steel alloy is a common alloy used for the

manufacture of bath hardware of continuous hot-dip coating lines. In these tests, thebath material was maintained at a specific temperature used for commercial hot-dip runs.

The sample materials were submerged in four different baths for durations ranging from

2 hours to 240 hours. The baths included: pure zinc, zinc-5wt%aluminum, zinc-55wt%aluminum, aluminum-8wt% silicon.

The purpose of the second group of tests was to examine the temperature effect on

corrosion behavior of the sample alloys. In this group of tests an experimental FeCrSialloy, developed at Oak Ridge National Laboratory, was tested in addition to the Fe3Al

and 316L. Test specimens were statically tested in the same four liquid baths as the on-

line battery of tests, but different temperatures were employed.After the corrosion tests were completed, the specimens were cut, mounted and

ground. The specimens were then etched and the remaining specimen thicknesses were

measured using a Hi-Scope optical microscope from Hirox outfitted with Vision Gauge

PC-based software. Grain size measurements and microstructure examination of the pre-test sample materials were carried out through standard laboratory procedure.

Examination of the interface layers and compositional analyses were then carried out on

the test specimens with the aid of a scanning electron microscope (SEM).Comparison between the corrosion behavior of the 316L, Fe3Al, and FeCrSi

samples in the separate sets of corrosion tests were made. A sound base of corrosion data

for the three test materials in molten zinc, Al-8Si, Zn-5Al, and Zn-55Al is developed andmicrostructure evaluation of the formed alloy layers is presented.

A significant temperature and time effect on the corrosion of the test materials in

the molten baths was found. Through the on-line corrosion testing, a linear time law for

corrosion from 2 hours to 240 hours was found for the 316L, Fe3Al, and FeCrSispecimens. The Fe3Al alloy showed similar results in the Al-8Si bath as the 316L alloy.

The FeCrSi alloy showed relatively low corrosion rates during the static testing, but

demonstrated a pronounced temperature effect in the Zn-5Al bath as the testingtemperature was increased above 560C.


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Acknowledgments

Firstly, I would like to thank my family. Their constant encouragement and

unwavering support mean more to me than they can know. Without them, none of this

would have been possible.

I would next like to thank my research advisor, Dr. Keh-Minn Chang. His advice

and guidance to me throughout the course of this project have helped me immensely.

The knowledge and work ethic that he possesses leave a lasting impression on me. Next,

I would like to thank Dr. Wanhong Yang for helping me with the SEM and EDS work at

NIOSH. I appreciate the effort he put forth to help me with this project and all of my

research. I would also like to thank Jian Mao, Longzhou Ma, and Li Yang for

continually taking time out from their schedules to assist me throughout the course of my

graduate work. These people, and the rest of Dr. Changs research group, I would like to

thank for everything theyve done. It has been a real privilege knowing them and

working with them. I would also like to thank N. Rampura for conducting the on-line

tests and preparing many of the test specimens for me.

Next I would like to thank all the members of the Materials Processing division of

Metals and Ceramics at Oak Ridge National Laboratory, especially Dr. Vinod K. Sikka. I

learned a great deal while working in their facility and thoroughly enjoyed myself. They

have my heartfelt gratitude for the lengths they went to in order to assist me with my

project and testing. They were a tremendous group of people to work with.

Finally, I would like to thank some of my close friends. I would like to thank

Bobby Coulter for keeping graduate school from being too serious, and for helping me

with all of my research work. I also need to thank Butch Kanth, John Clark, and Ben


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Freeman for being life-long friends, making me laugh, and keeping me sane. We had a

lot of good times at WVU, and we made a lot of great memories in the process. Lastly I

would like to thank Carrie Patterson, for always supporting me in everything that I did,

and for being a great friend.


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v

Table of Contents

Title page i

Abstract ii

Acknowledgments iii

Table of Contents v

List of Tables viii

List of Figures ix

Chapter 1 Introduction 1

1.1 Hot-Dip Coating 1

1.2 Performance of Bath Hardware 2

1.3 Intermetallics 4

1.4 Liquid Metal Corrosion 7

1.5 Research Objective 9

Chapter 2 Literature Review 10

2.1 Iron Aluminide 10

2.2 Corrosion in Hot-Dip Coating Baths 11

2.2.1 Galvanization 11

2.2.2 Aluminization 14

Chapter 3 Experimental Procedure 18

3.1 Corrosion Tests 18

3.1.1 Alloy Preparation 18

3.1.2 On-line Corrosion Tests 19

3.1.3 Static Immersion Tests 21

3.2 Sample Sectioning 23


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3.2.1 On-line Corrosion Specimens 23

3.2.2 Static Immersion Specimens 24

3.3 Sample Preparation 24

3.4 Thickness Measurements 25

3.5 Calculation of Corrosion Rates 26

3.6 Scanning Electron Microscopy (SEM) 28

3.7 Microstructure Evaluation of Base Materials 29

Chapter 4 Results and Discussion 31

4.1 Microstructure of Base Materials 31

4.2 Corrosion of On-Line Test Specimens 33

4.2.1 Al-8Si Bath 33

4.2.2 Zinc Bath 35

4.2.3 Zn-5Al Bath 36

4.2.4 Zn-55Al Bath 37

4.2.5 Discussion 37

4.3 Corrosion of Static Immersion Specimens 39

4.4 SEM/EDS Analysis 44

4.4.1 Al-8Si Bath 44

4.4.2 Zinc Bath 50

4.4.3 Zn-55Al Bath 55

4.4.4 Discussion 62

Chapter 5 Conclusions and Recommendations 64

5.1 Conclusions 64

5.1.1 On-line Corrosion Testing 64


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5.1.2 Static Corrosion Testing 65

5.1.3 Bath / Base Material Interactions 67

5.1.4 Summary 69

5.2 Future Work 69

References 71

Appendix A Thickness Loss Graphs 74

Appendix B Raw Data 89

Vita 106


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viii

List of Tables

Title Page Number

Table 2.1: Phases Identified in the Galvanization of Steels Substrates 14

Table 2.2: Fe-Al System Phases 16

Table 2.3: Confirmed Phase Identities in Fe-Al-Si System 17

Table 3.1: Chemical Compositions of Corrosion Test Specimens 18

Table 3.2: Bath Temperatures for On-line Corrosion Tests 20

Table 3.3: Static Immersion Tests Conducted 21

Table 3.4: Static Immersion Specimen Dimensions 22

Table 3.5: Density of Test Materials 27

Table 3.6: Etchants Used for Microstructure Evaluation of Base Materials 29

Table 4.1: Grain Sizes of Test Materials 31

Table 4.2: Corrosion Rates for On-line Corrosion Tests 37

Table 4.3: Average Thickness Losses for Static Immersion Tests 40

Table 4.4: Corrosion Rates for Static Immersion Tests 40

Table 4.5: Phases formed Upon Corrosion in Zn-Al Hot-Dip Baths 62


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List of Figures

Title Page Number

Figure 1.1: Schematic of Hardware in Hot-Dipping Process 3

Figure 1.2: Iron-Aluminum Phase Diagram 6

Figure 1.3: Unit Cell of FeAl (B2) and Fe3Al (D03) superlattices 6

Figure 3.1: On-line Corrosion Test Specimen 19

Figure 3.2: 316L Specimens upon Removal from the Aluminizing Bath 20

Figure 3.3: Static Immersion Testing Set-up 22

Figure 3.4: 316L Specimens Sectioned at Position 2 23

Figure 3.5: Hi-Scope Optical Microscope Set-up 25

Figure 3.6: Location and Designation of Specimen Thickness Measurements 26

Figure 4.1: Microstructure of Fe3Al, 316L, and FeCrSi specimens 31

Figure 4.2: Grain structures in Fe-Cr-Si alloy 32

Figure 4.3: Plot of Thickness Loss of 316L in Al-8Si bath at 660C 33

Figure 4.4: Thickness Loss of 316L and Fe3Al in Al-8Si Bath 34

Figure 4.5: Thickness Loss of 316L and Fe3Al in Zinc Bath 35

Figure 4.6: Thickness Loss of 316L and Fe3Al in Galfan Bath 36

Figure 4.7: Thickness Loss of 316L and Fe3Al in Galvalume Bath 37

Figure 4.8: Corrosion Rates for On-line Corrosion Tests 38

Figure 4.9: Corrosion Rates for Static Immersion Tests in zinc 42

Figure 4.10: Corrosion Rates for Static Immersion Tests in Zn-5Al 43

Figure 4.11: Transverse Section of Hot-Dip Aluminized 316L Sample 46

Figure 4.12: EDS Spectrums of 316L in Al-8Si 46


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Title Page Number

Figure 4.13: SEM Micrograph of Fe3Al in Al-8Si 47

Figure 4.14: EDS Spectra of Fe3Al in Al-8Si Bath 48

Figure 4.15: SEM Micrograph of FeCrSi in Al-8Si 49

Figure 4.16: EDS Spectra of FeCrSi in Al-8Si 50

Figure 4.17: SEM Micrograph of 316L in Zinc 51

Figure 4.18: EDS Spectrum of Fe-Al-Zn Ternary Phase Formed on 316L in Zinc 51

Figure 4.19: SEM Micrograph of Fe3Al in Zinc 52

Figure 4.20: EDS Spectrum of FeAl3 Phase Formed on Fe3Al in Zinc 53

Figure 4.21: EDS Spectrum of Zr Phase in Fe3Al Alloy 53

Figure 4.22: SEM Micrograph of FeCrSi in Zinc Bath 54

Figure 4.23: SEM Micrograph of 316L in Zn-55Al Bath 56

Figure 4.24: EDS Spectra of 316L in Zn-55Al Bath 57

Figure 4.25: SEM Micrograph of Fe3Al in Zn-55Al Bath 58

Figure 4.26: EDS Spectra of Fe3Al in Zn-55Al Bath 59

Figure 4.27: SEM Micrograph of FeCrSi in Zn-55Al Bath 60

Figure 4.28: EDS Spectra of FeCrSi in Zn-55Al Bath 61

Figure A.1: Thickness Loss of 316L in Al-8Si for On-line Tests 76

Figure A.2: Thickness Loss of Fe3Al in Al-8Si for On-line Tests 77

Figure A.3: Thickness Loss of 316L in Zinc for On-line Tests 78

Figure A.4: Thickness Loss of Fe3Al in Zinc for On-line Tests 79

Figure A.5: Thickness Loss of 316L in Zn-5Al for On-line Tests 80

Figure A.6: Thickness Loss of Fe3Al in Zn-5Al for On-line Tests 81


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Title Page Number

Figure A.7: Thickness Loss of 316L in Zn-55Al for On-line Tests 82

Figure A.8: Thickness Loss of Fe3Al in Zn-55Al for On-line Tests 83

Figure A.9: Thickness Loss in Al-8Si for Static Tests 85

Figure A.10: Thickness Loss in Zinc for Static Tests 86

Figure A.11: Thickness Loss in Zn-5Al for Static Tests 87

Figure A.12: Thickness Loss in Zn-55Al for Static Tests 88

Figure B.1: Level 1 Thickness Measurements in Al-8Si for On-line Tests 90



Figure B.4: Level 1 Thickness Measurements in Zinc for On-line Tests 93



Figure B.7: Level 1 Thickness Measurements in Zn-5Al for On-line Tests 96






Figure B.13: Thickness Measurements in Al-8Si for Static Tests 102

Figure B.14: Thickness Measurements in Zinc for Static Tests 103

Figure B.15: Thickness Measurements in Zn-5Al for Static Tests 104

Figure B.16: Thickness Measurements in Zn-55Al for Static Tests 105


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Chapter 1 Introduction

1.1Hot-Dip Coating

In the world today, the need for corrosion resistant, structural materials is continually

increasing. For almost two centuries, hot-dip coating has proven to be a high quality and

highly economical corrosion protection method with a vast number of applications.1

Hot-

dip coated products have a multitude of uses ranging from the automobile industry, to the

heating industry, to the construction industry. The hot-dip coating process remains

relatively simple and highly effective even after many years of development. New

variations and uses for hot-dip coated products are continually being developed by

companies world-wide.

One reason for the overall effectiveness of hot-dip coating for corrosion control stems

from the metallurgical bond that is formed between the base metal and the coating metal.

During the coating process, the molten coating material reacts with the steels surface to

form a series of alloy layers. This bond provides very strong protection, with excellent

impact resistance provided by the ductile outer layer and excellent abrasion resistance

provided by the hard, inner alloy layers.

Two major sub-divisions within the field of hot-dip coating include the processes of

aluminization and galvanization. Each of these processes involves the coating of a steel-

based substrate with the respective coating alloy. Though the separate processes are very

similar in their execution, the coated products that they yield harbor differing properties,

and are often used in different applications.

There are two main methods of protecting a steel sheet from corrosion: barrier

protection and cathodic protection.2

Galvanization utilizes protection from both of these


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important mechanisms. In the cathodic protection process, the zinc coating acts as the

anodic area on the steel. Because it is the anode, the zinc will preferentially corrode,

leaving the base metal intact. Also, the zinc coating applied to the steel base is extremely

dense and considerably less permeable than other coating methods such as painting. The

intermetallic layers formed by the alloying of the zinc and the steel form an excellent

type of barrier protection. Because of this alloying, the bond between the zinc coating

and the base material is extremely strong.

The coatings formed from the process of aluminizing have many of the same

properties as the coatings formed in the galvanization process. With aluminum, primary

corrosion protection of the steel base is supplied by the formation of an impervious oxide

barrier. The barrier property comes from the ability of aluminum to generate rapidly, a

very thin surface film of alumina, which is practically impermeable and insoluble to most

oxidizing media.3

Though coatings formed through the aluminizing of steel are not as easily welded or

as inexpensive as their zinc counterparts, they have excellent heat and light reflectivity

properties. Aluminized steel is widely used in vehicle exhaust systems and domestic

appliances such as dryers, heating boilers, and cookers.3

Aluminized and galvanized

steels have both encountered high demand in the construction industry.

1.2Performance of Bath Hardware

In continuous hot-dip coating lines, the immersed bath hardware (e.g. bearings, sink,

stabilizer, and corrector rolls, and also support roll arms and snout tip) is subject to

corrosive attack by the molten bath material.4

Figure 1.1 shows a schematic diagram of a


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continuous hot-dipping bath. In addition to the corrosive effects of the molten bath

material, the bath hardware is subject to erosive and abrasive attack from the hard

intermetallic particles formed within the bath.

The attack on the bath hardware hinders line operation, degrades the surface quality

of the coated product, and requires frequent and expensive bath hardware changes. A

typical campaign for the submerged bath hardware used in the hot-dip coating industry

currently ranges from approximately one to six weeks.5

Figure 1.1: Schematic Diagram Showing Positions of Hardware Rolls in

Continuous Hot-Dipping Bath

A shutdown of these continuous hot-dipping operations is costly because of both a

loss of production time and additional energy to restart the process. A major factor

contributing to the shutdown of these lines involves the limited life of the roll bearings

GAS

KNIVESSNOUT

BATH

Sink

Roll

STRIP

DIRECTION

StabilizingRolls


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submerged in the liquid metal bath. Upon being subjected to the molten metal corrosion

and extensive erosion within the bath, the bearings will lose their original dimensions.6

Because of these dimensional, losses vibrations within the line develop. These vibrations

can be very hazardous, even catastrophic. Line vibrations can also create uneven

material coatings and cause intermetallic particles from the bath hardware and melt to be

stirred up and mixed in with the clean coating material. These effects severely detract

from the quality of the applied coating and may even render the coated material unusable.

Numerous materials are currently being developed and tested to assess their

effectiveness in the bearing application of a continuous hot-dip coating line. A

satisfactory material must possess the required mechanical strength at high temperatures

while also resisting corrosion and erosion from the molten bath material. Current

materials used in the bearing application of continuous hot-dip coating lines include

specialized ceramics, Stellite alloys, and multiple other materials. Though many

materials have been examined in the search for corrosion resistant bath hardware, none of

these have emerged as an obvious leader in this field. An improvement in the material

used for bearings in the hot-dip coating process would mean a decrease in the frequency

of line shutdowns and substantial cost and energy savings. The continuous hot-dip

coating lines could run for longer periods and produce higher quality products.

1.3 Intermetallics

In recent years, increasing research interest has been seen in the development of

intermetallic compounds. Intermetallics constitute a unique class of materials that have

many exciting and advantageous properties for use in a wide variety of applications.


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These materials are currently used in such diverse applications as resistors, magnets,

superconductors, heating elements, and corrosion-resistant coatings.7

A number of

intermetallic compounds are also being developed for use in structural applications.

Some examples of these materials include Ni3Al, NiAl, Ni3Si, Fe3Al, FeAl, Ti3Al, TiAl,

and MoSi2.8

The field of intermetallic research is a relatively new area with many

exciting possibilities for further research and future applications.

An intermetallic compound can loosely be defined as an ordered alloy phase

formed as a combination of two or more metal elements, generally falling at or near a

fixed stoichiometric ratio and ordered on at least two or more sublattices.

7

The ordered

structure of an intermetallic compound gives the material some extremely attractive

properties at higher temperatures. These exceptional high temperature properties are due

to the long-range-ordered superlattice which reduces dislocation mobility and diffusion

processes at high temperatures.8

These characteristics contribute to the increased high-

temperature strength and creep resistance of intermetallic materials. In addition,

hundreds of binary intermetallics have been identified that have melting points in excess

of 1500C. These factors contribute to some of the appeal for the use of intermetallics in

a variety of widespread applications.

Iron aluminides have been of interest since the 1930s when the excellent

corrosion resistance of compositions with more than about 18at% Al was first noted.9

At these compositions, the resistance of iron aluminides to corrosion in oxidizing and

sulfidizing environments is well noted, particularly at elevated temperatures. Despite the

fact that iron and aluminum can possibly form multiple intermetallic phases, two phases

in particular carry the most possibility for use in structural applications.


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Figure 1.2: Iron-aluminum phase diagram9

These two ordered phases are FeAl and Fe3Al, as can be seen in the Fe-Al binary phase

diagram in Figure 1.2. The ordered DO3 structure of Fe3Al and the B2 crystal structure

Figure1.3: Unit cell of FeAl (B2) and Fe3Al (D03) superlattices9

of FeAl both have derivatives from the body-centered cubic structure.9

The crystalline

unit cells for each phase are illustrated in the Figure 1.3.


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1.4Liquid Metal Corrosion

Since the earliest days of metal processing there has been great concern about the

corrosion of a solid metal exposed to a liquid-metal environment. As the metal

processing industry grew, so did the need to contain and transport molten metals. In

addition to the melts used during material processing, molten metals were being utilized

as high-temperature reducing agents in the production of metals, and because of their

excellent heat transfer properties they were being used as coolants in a variety of power-

producing systems.

Liquid metals have been observed to attack solid metals in various ways. The

phenomena of liquid-metal corrosion can be placed in the following categories:

dissolution, impurity and interstitial reactions, alloying, and compound reduction.18

These classifications however have an arbitrary nature, due to the fact that the four

individual categories of corrosion phenomena do not act independently of one another.

Dissolution is the simplest type of corrosion that can occur in a liquid metal system.

In this simple-solution type of attack, the amount of damage the solid metal undergoes is

dependant upon the surface area of the solid exposed and the volume of the liquid metal.

The rate of the dissolution reaction is governed by the kinetic properties of the rate-

controlling step in the dissolution reaction. The net dissolution rate at which the solid

metal enters solution can be seen in the equation:

J = k (C c) (1.1)

where J is the net dissolution rate of the solid, k is the solution rate constant for the rate-

controlling step, C is the solubility of the particular element in the liquid metal, and c is

the instantaneous concentration of this element in the melt.18


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Impurity or interstitial reactions refer to the interaction of light elements in the

solid or the liquid metal. In situations where the solid metal has a low solubility in a

particular liquid metal, reactions involving light elements such as oxygen, nitrogen, and

carbon may dominate the reaction process. Examples of this type of reaction include the

decarburization of steel in lithium and the oxidation of steel in sodium.18

Another type of corrosion that occurs between liquid metals and solid metals is

alloying or alloy layer formation. In this type of reaction stable products are formed from

the reaction of atoms within the solid metal with those from the liquid metal. These

reactions occur without the participation of interstitial or impurity elements. The basic

alloying reaction can be expressed by the equation:

xM + yL = MxLy (1.2)

where L is the chemical symbol for a liquid-metal atom and M is one species from the

solid metal.18

The product formed by this reaction may either be soluble or insoluble in

the liquid metal. Assuming that chemical contact is maintained between compact product

layers, then at equilibrium, the growth of the phase layers is controlled by volume

diffusion.

Compound reduction represents another mechanism of corrosion observed in

liquid metal systems. In compound reduction, structural integrity of the solid metal is

lost through reduction-induced removal of the nonmetallic element in the solid. An

example of this aggressive situation would be the exposure of most oxides to molten

lithium.18


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1.5Research Objective

The main objective of this research is to examine, by means of corrosion testing and

microscopic examination, the effectiveness of Fe3Al-type intermetallic alloys as materials

of construction for bath hardware in continuous hot-dip coating lines. The iron-

aluminide alloy, which was developed at Oak Ridge National Laboratory, was compared

to specimens of 316L, low-carbon stainless steel. Each of the selected materials was

tested in similar conditions so that the results could be compared. Two different batteries

of tests were run on the materials, the first being on-line corrosion tests to examine the

testing duration effect on corrosion, and the second being laboratory, static immersion

tests to examine the temperature effect on corrosion of the materials. During the static

immersion tests an experimental FeCrSi alloy that was developed at Oak Ridge National

Laboratory was tested alongside the Fe3Al and 316L specimens.

The microstructure and composition of the interface layers formed from the reaction

between the test specimens and the molten bath were successfully examined with the aid

of the scanning electron and optical microscopes. This paper discusses the differences in

performance of the selected materials in the four separate coating baths: pure zinc, Zn-

5Al, Zn-55Al, and Al-8Si (weight percents).


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Chapter 2 Literature Review

2.1 Iron Aluminide

Iron aluminides compete with the 300 and 400 Series stainless steels and some

nickel-based alloys in a variety of different applications.10

They offer relatively low

material cost, conservation of strategic elements, and a lower density than stainless

steels.8

These properties contribute to the reason iron aluminides have potential uses in

numerous structural applications.

One factor contributing to the excellent performance of iron aluminides in

oxidizing and sulfidizing environments stems from the systems ability to form a

protective Al2O3 coating. This coating protects the underlying base material from attack

by sulfidizing and oxidizing environments in excess of 1400C.9 An example of iron

aluminides being used in a highly sulfidizing environment include the use of the material

as filters in coal-gasification processes. Here iron aluminide powders are sintered to form

porous gas-metal filters that are used to remove particulate from the gas produced during

a coal gasification process.10 This application exploits the excellent sulfidation resistance

of the material. Iron aluminide is also used to produce heating elements, furnace fixtures,

and catalytic converter substrate to take advantage of the superb oxidation resistance of

the material. The application of iron aluminide as heating elements also utilizes the high

resistivity of the material, which remains constant up to 1000C.


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2.2Corrosion in Hot-Dip Coating Baths

2.2.1 Galvanizing Lines

Though the hot-dip coating processes greatly enhance the corrosion resistance of

the steel substrate that is passed through the molten coating material, the intrinsic

conditions of the procedure have severe effects on the submerged bath hardware. This

hardware is subjected to high temperatures, severe corrosion, and continuous erosion

from intermetallic particles within the molten coating bath. The same process that

produces the beneficial alloy layers that protect a steel sheet from corrosion is constantly

at work shortening the life of the bath hardware. A considerable amount of research has

been conducted to find a bath hardware material that can withstand these conditions for

an extended period of time. Typical materials used for galvanizing operations are Type

316L stainless steel for the bath hardware rolls and Stellite for the roll bearings.11

One corrosion study compared the mass loss of various materials due to liquid

zinc attack in a 480C bath for 120 hours. In that study, low carbon steel specimens were

found to corrode at a rate almost three times that of type 304 and 316 stainless steel,

which experienced approximately the same mass loss.11 Slight compositional changes to

the base material were found to have very significant effects on the corrosion of the

tested material. For example, increasing the silicon content of the Type 316 stainless

steel from .4 wt% to .5 wt% was found to have a beneficial effect, while increasing the

chromium content in binary iron-chromium alloys was detrimental to their performance

in molten zinc.11

Also, martensitic grades of stainless steel, such as Type 410, were found

to be corroded more readily in molten zinc baths than austenitic grades such as Types 304

and 316.11


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Factors such as bath chemistry, sample microstructure, and surface condition have

also been found to have significant effects on the corrosion of steels in molten zinc baths.

Lampe et al.12

found that although small additions of aluminum to a pure zinc melt have

no effect on the attack rate, an addition of 4% aluminum to the zinc melt greatly reduces

the corrosion rate of the base steel. In hot-dip galvanizing, the formation of Fe2Al5 (-

phase) has been implicated in the inhibition of Fe-Zn reaction diffusion even when

aluminum is present in the zinc melt in relatively low concentrations.3

In the same series

of tests, the researchers found that the pre-oxidation of an AISI H13 alloy decreased the

corrosion of the specimen in the bath. Brunnocket al.

11

found that the grain size of the

base steel had an effect on the mass loss independent of compositional variations. In

every test run, the sample with the smaller grain size showed the higher amount of mass

loss. This work confirmed previous studies, which stated that in steels, molten zinc

attacks grain boundaries preferentially.

As does the coated steel sheet, stainless steel or iron based bath hardware forms a

series of alloy layers upon submersion in a zinc or zinc alloy coating bath. In one study,

these coatings were found to be fully alloyed on submerged steel panels in an immersion

time as brief as three minutes.12

The series of zinc/iron alloy layers formed from the

metallurgical reaction of the bath material and the submerged material will be

intermetallic phases with specific stoichiometric ratios. When a submerged material is

removed from the zinc bath, it carries with it a covering of pure zinc. This outer layer is

known as the -layer. Progressing in towards the base steel, three other alloys layers

have been observed to form: the -FeZn13 layer, the -FeZn10 layer, and the -Fe3Zn10

layer.1

From the outer layer to the inner -Fe3Zn10 layer, the alloy layers have


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increasing iron contents. The exact compositions and presence of the alloy layers formed

on an experimental steel specimen submerged in liquid zinc is under debate however.

In a study conducted by Verma et al.14

at the University of Cincinatti, it was

found that at a testing temperature of 455C the well-known three-phase structure, i.e., -

FeZn10, -FeZn13, and, was formed. The thin, innermost -Fe3Zn10 layer did not form.

The researchers here formed comparisons among the coatings formed on a standard steel

substrate in the temperature range 520 - 555C. It was found that coatings at the lower

end of the temperature range had well defined layers on top of a layer.14 The coatings

changed to primarily the alloy layer at the higher end of the temperature range.14 In

the first part of this study, which was a description of coatings formed at 560C, the

experimenters found that the coating consisted of monoclinic crystals of the phase

embedded in a layer which was a solid solution of the and phase. This layer was

formed on top of a very thin layer of the alloy, which was present at the zinc-steel

interface.12

It was found that by lowering the immersion temperature from 555 to 530C

that the solid solution of + grew at the expense of the pure phase.12 After the

immersion of an AISI 1012 steel specimen in a molten zinc bath for eight hours, Lampe

et al.12

identified the formation of the noted , , , and layers.12 The researchers

further divided the alloy layer into two separate sub-layers, designating these layers the

1 and 2 sub-layers because of their different morphologies. Table 2.1 illustrates the

phases observed upon the corrosion of steel substrates in molten zinc baths.

Despite the fact that much research is being conducted in order to find improved

materials for the bath hardware application in continuous galvanization lines, only a

handful of materials are being put into actual use in industry today. In 1996, the


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International Lead Zinc Research Organization, Inc. (ILZRO) surveyed continuous

galvanizers within Africa, Asia, North America and Europe in regard to their operating

practices and experiences in the area.5

Of the bearing materials the galvanizers listed in

the survey given, three were ceramic, eighteen were a cobalt-based Stellite alloy, and one

was an alloy known as Tribaloy T-800.5

Even though these currently utilized alloys yield

unsatisfactory operating lives, they continue to be used because a superior material for

this application has not been identified.

Table 2.1: Phases Identified in the Galvanization of Steel Substrates

2.2.2 Aluminizing Lines

Similar to reactions in molten zinc, when a solid iron or steel substrate is

submerged in a molten aluminum bath metallurgical reactions between the bath and

immersed material will occur. Jones and Denner stated that the coating formed on the

submerged material will have the following features: the number and order of formation

of constituents it contains will accord with the predictions of the equilibrium phase

Source Base Material

Testing

Time

(min)

Testing

Temperature

(C)

Phases Identified

Verma et al.14Rolled structural

steel panel3 455 -FeZn10, -FeZn13


steel panel3 520 -FeZn10, -FeZn13


steel panel3 555 -FeZn10

Verma et al.12 Rolled structural

steel panel 3 560 -Fe3Zn10, -FeZn10, -FeZn13

Lampe et al.13 AISI 1012 Steel 480 470- Fe3Zn10, 1-FeZn10, 2-FeZn7, -FeZn13


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diagram, and the overall thickness of the coating layer will increase parabolically at

reaction temperatures.15

At currently accepted hot-dip aluminizing temperatures

(approximately 660 - 730C), three intermediate phases should form between the limiting

solid solution of aluminum in iron and the liquid solution of iron in aluminum.14

Richards stated that the addition of silicon in the Type 1 aluminizing melt results in

several substantial differences between Type 1 and Type 2 hot dipped aluminized steels.

A Type 1 aluminizing melt generally contains 8-11 wt.% silicon and the Type 2 melt is

an unalloyed aluminum bath. The addition of the silicon inhibits alloy layer growth in

the Type 1 aluminized steels and results in the formation of a planar, steel/alloy layer

interface. A serrated, or saw-tooth, interface is said to be typical of Type 2 aluminized

steels.3

In an X-ray diffraction study of Type 1 specimens produced in baths containing

9 11% Si, Coburn concluded that the single detected intermetallic layer was composed

entirely of silicon bearing -Fe2Al5, this overlying a region of Fe-Al solid solution.3

Refer to Table 2.2 for phases formed in the Fe-Al binary system.

In an analysis of industrially produced Type 1 samples, Liu and Wu found that the

composition of the single layer formed during immersion testing consisted of FeSiAl2.

In this study, no Fe-Al solid solution zone was found.3

Fotouhi found that the alloy layers formed in hot dip aluminized steels at

temperatures below 750C were comprised of two distinct intermetallic strata.

3

For

immersion times of 300 seconds, the outer, thicker strata was identified as -Fe2SiAl8,

while the inner, much thinner layer consisted of-FeAl3. At temperatures higher than


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approximately 750C, the thinner FeAl3 layer was not identified, and the alloy layer

consisted entirely of-Fe2SiAl8.14

For immersion times greater than 900 seconds and temperatures up to

approximately 730C, Denner and Kim found a similar alloy layer structure to that

observed by Fotouhi.3 The researchers found a distinct, but discontinuous layer of -

FeAl3 which was observed to form between a layer of Fe2SiAl7 and an -Fe2Al5 layer

which formed next to the steel base material.

Table 2.2: Fe-Al system phases3

Richards et al.3

gives a comprehensive view of the phases formed upon the hot-

dip aluminizing of a steel substrate in a Type 1 aluminizing bath. Table 2.3 represents a

compilation of the results from multiple research projects dealing with Type 1

aluminization.

-Fe2Al3 (complex cubic, bcc) not relevant to hot dip aluminizing (high-temp phase) FeAl6(monoclinic) metastable phase Fe2Al7 no recent evidence supporting existence

Phase Stoichiometry Crystal Structure

-FeAl bcc

1 Fe3Al Cubic (BiF3-type)

FeAl Disordered, bcc

2 FeAl Ordered, bcc

FeAl2 Monoclinic,

(47-50 wt%) (rhombohedral?)

Fe2Al5 Orthorhombic(52-54 wt%)

FeAl3 Monoclinic(57-62 wt%)

Al-Fe fccSolubility of Fe in Al

700C ~ 2.5 wt%

600C ~ < 0.1 wt%


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Table 2.3: Confirmed phase identities in Fe-Al-Si system3

Phase

Reported

stoichiometry

Approximate

composition, wt-%

_______________Fe Al Si

Reported

crystal

systems

2() FeSiAl3 35.5 49.5 15 One report cubic;

one monoclinic

6() Fe2Si2Al9 27.0 49.5 13.5 Several reports

monoclinic;one tetragonal

4() FeSi2Al4 27.0 48.0 25.0 All reports

FeSi2Al3 tetragonal

Fe15Si28Al57Fe

15Si

38Al

47

5() Fe2SiAl7 32.2 59.8 8.0 One report cubic;

Fe2SiAl8 all othersFe3Si2Al12 hexagonal

Fe5Si2Al20


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Chapter 3 Experimental Procedure

3.1 Corrosion Testing

3.1.1 Alloy Preparation

The Fe3Al intermetallic alloy used throughout the course of this research was

prepared by Oak Ridge National Laboratory, Oak Ridge, Tennessee. The induction

melting of high-purity, raw materials was performed. The composition of the iron

aluminide alloy is listed in Table 3.1. Ingots were cast from the alloy melt in cylindrical

graphite molds that were 75 mm in diameter and 180 mm in length. The cylindrical

ingots were heated to and held at 1100C for two hours and then extruded by a hydraulic

press into billets with a 19 mm by 38 mm cross-section. The head and tail of the billets

were cut off and the remaining material was then hot rolled to a thickness of 3.2 mm.

Test specimens were then cut from the as-rolled plates in the longitudinal direction.

The stainless steel used in the corrosion tests was commercial grade 316L

stainless steel plate with a thickness of 3.2 mm. The chemical compositions of the 316L

stainless steel and other test materials are listed in Table 3.1.

Table 3.1: Chemical Composition of Corrosion Test Specimens (wt%)

The FeCrSi alloy was an experimental alloy produced at Oak Ridge National

Laboratory. The FeCrSi alloy was produced by the vacuum induction melting (VIM) of

high purity laboratory materials. This mixture was poured at 1600C and the resulting

Fe Al Cr Zr C Ni Mo Mn Si P S

Fe3Al Bal 15.83 5.45 .96 .01 -- -- -- -- -- --

316L Bal -- 17.00 -- .03 12.00 2.5 2.00 1.00 .045 .030

eCrSi Bal -- 35.00 -- -- -- -- -- 2.50 -- --


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ingot had dimensions of 25.4 mm x 101.6 mm x 152.4 mm. The ingot was then hot-

rolled to yield the final thickness of 6.35 mm.

3.1.2: On-line Corrosion Tests

A schematic of the test specimens used in the on-line corrosion tests is illustrated

in Figure 3.1. During the corrosion tests, one iron-aluminide specimen and one 316L

specimen were separated by a 25 mm wide spacer and bolted to a rig which hung over the

coating bath. The rig was positioned over the hot-dip coating bath so that approximately

one-third of the specimen was immersed in the melt.

Figure 3.1: On-line corrosion test specimen

The on-line corrosion tests in the galvanizing, galvalume (Zn-55Al), and Type 1

aluminizing bath (Al-8Si) were conducted on a commercial production line of the

Wheeling Nisshin plant in Follansbee, WV. The on-line corrosion tests in the galfan (Zn-

Level S

Level 1

Level 3

Level 2

Immersion Line

50.8mm

304.8mm

292.1mm

247.6

5mm

203.2mm


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5Al) bath were performed at the Weirton Steel facility in Weirton, WV. The operating

temperatures of the various hot-dip coating processes are listed in Table 3.2.

Table 3.2: Bath Temperatures for On-line Corrosion Tests

Five specimen pairs of Fe3Al and 316L were immersed in the baths for times of 2,

8, 24, 72, and 240 hours. One by one, the specimens were removed from the hanging rig

assembly after their specified immersion times had expired. Figure 3.2 shows a typical

example of several test specimens after being removed from one of the molten coating

baths.

Figure 3.2: 316L Specimens upon Removal from the Aluminizing Bath

Bath Material Bath Temperature (C)

Al-8Si (Type 1 aluminizing bath) 660Zinc (galvanizing bath) 460

Zn-5Al (Galfan) 490Zn-55Al (Galvalume) 600


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3.1.3 Static Immersion Tests

The static immersion testing for this project was conducted at Oak Ridge National

Laboratory in the Materials Processing Group of the Metals and Ceramics Division. The

bath materials used for the static immersion tests were of the same chemistry as the

molten baths used during the first series of tests. In addition to the Fe3Al and 316L, the

FeCrSi alloy was also tested for comparison.

The specimens were tested at temperatures of 460, 560, and 660C for 24-hour

periods in order to investigate the temperature effect of the molten metal corrosion in the

specified bath materials. Because of the melting points of the specified bath materials, all

three temperatures could not be tested in every bath. For example, the melting point of

the Type 1 aluminizing bath, Al-8Si, has a melting point slightly over 600C and

therefore testing could not be run in this bath material at the 460 and 560C testing

temperatures. The static corrosion tests that were run in the four bath materials are listed

in Table 3.3 below.

Table 3.3: Static Immersion Tests Conducted

The specimens were cut to varying dimensions due to a limited amount of usable

material. Table 3.4 lists the nominal dimensions of the test specimens that were used

during the static immersion tests. The machined specimens of the three test materials

were cleaned in methanol and weighed before the immersion tests were run.

316L Fe3Al FeCrSi

Bath (melting point) 460C 560C 660C 460C 550C 660C 460C 560C 660C

Al-8Si (605C) X X XZn (420C) X X X X X X X X X

Zn-5Al (390C) X X X X X X X X XZn-55Al (570C) X X X

Test Materials and Test Temperature


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Table 3.4: Static Immersion Specimen Dimensions

Crucibles composed of 90% alumina were used for the static immersion. The

inner surface of the crucibles was coated with zirconium oxide to prevent the molten bath

metal from wetting to the crucible surface upon removal from the furnace. The furnaces

used were Thermolyne single-phase resistance furnaces controlled by a Barber Coleman

560 temperature controller. Figure 3.3 shows a schematic representation of the static

corrosion testing set-up.

Figure 3.3: Static Immersion Testing Set-up

The bath material was melted, brought to the desired testing temperature, and

stabilized. The specimens were cleaned, measured, and weighed before being introduced

to the molten bath. They were immersed for a duration of 24 hours. At the end of the

Material thickness (mm) width (mm) length (mm)

316L 3.175 25.4 50.8Fe3Al 2.54 25.4 50.8

FeCrSi 6.35 25.4 50.8

External Thermocouple

Test Specimen

Resistance Furnace

Alumina Crucible

Molten Bath Material


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testing time the crucibles were removed from the furnace. The test specimens were then

removed from the bath material and weighed.

3.2 Sample Sectioning

3.2.1 On-line Corrosion specimens

Samples were cut from the tested 316L and Fe3Al in order to quantitatively

compare the corrosion rate of the materials in the different testing scenarios. The tested

specimens were cut at four different positions, which lied at different heights on the

tested plate. These height designations are illustrated in Figure 3.1. Position 1 was 12.7

mm from the immersed end of the specimen and Position 2 was 57.2 mm from the

immersed end. Positions 1 and 2 were both fully submerged in the molten bath material.

Position 3 lied just above the immersion line of the specimen (refer to Figure 3.1).

Position S lied 50.8 mm from the top of the specimen and served as the dimensional

reference. Figure 3.4 shows specimens of 316L that have been cut at position 2 upon

removal from the Al-8Si bath.

Figure 3.4: 316L specimens embedded in resin. Samples were cut at position 2 upon

removal from the Al-8Si bath. The specimens were tested for periods of 0, 2, 8, 24,

and 72 hours (left to right).


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3.2.2 Static Immersion Specimens

The specimens used for the second series of tests differed from the on-line

specimens in their dimensions (refer to Table 3.4). The static specimens were smaller,

coupon-type specimens which were totally immersed in the testing bath. After testing,

the specimens were cut in half horizontally with an abrasive cutting wheel. Care had to

be taken to assure that the cuts lied perpendicular to the specimen edge. The reference to

be used for the quantitative corrosion analysis was the original dimensions of the test

specimens, which were individually measured before each test.

3.3 Sample Preparation

Cut specimen sections were cold-mounted. After the sections were mounted,

grinding was done using a Buehler Ecomet 2 two-speed grinder-polisher outfitted with a

Buehler Automet 2 power head. The specimens were consecutively ground using 240,

320, 400, 600, and 800-grit sandpaper. They were then polished using the Buehler

Ecomet polisher and one-micron deagglomerated alumina polishing paste.

Before thickness measurements of the specimens were taken, the polished

specimens were first etched in a solution of 3 parts hydrochloric acid and 5 parts water.

This etch was used to introduce a contrast between the matrix material and the alloy

layers formed as a result of the corrosion. Without this etching, the interface between the

alloy layers and the base material could not be clearly distinguished.

Upon viewing the etched specimens under the optical microscope, very bright

edges were produced as a result of light being reflected from the polished surface. These

bright edges that appeared on the polished specimens inhibited the taking of accurate


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thickness measurements. Because of this the specimens were very lightly ground with

600-grit sandpaper. This procedure eliminated this bright edge effect and allowed more

accurate thickness measurements to be taken with the optical microscope.

3.4 Thickness Measurements

Thickness measurements of the various test specimens were taken with the aid of

a Hi-Scope KH-2400R optical microscope by Hirox. The optical microscope was used in

conjunction with Vision Gauge version 4.98 imaging software by VisionX. The Hirox

microscope and Vision Gauge software allowed accurate, reproducible measurements to

be made and stored with the aid of a personal computer. The resolution of the

measurements taken was 0.006 mm. Figure 3.5 shows the Hi-Scope used for the

thickness measurements of the corrosion specimens.

Figure 3.5: Optical Microscope Set-up Used for Thickness Measurements


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The thickness of every tested specimen was measured at multiple locations across

the cross-section of the specimen. Figure 3.6 shows the locations and designations of the

thickness measurements that were made on each cross-section.

Center

B1 B10T10 T1

1 mm.

Figure 3.6: Location and Designation of Specimen Thickness Measurements

Thickness measurements were made at 1mm intervals across the cross-sections. The

measurements were not taken fully to the end of the specimen, but only taken 10 mm to

either side of the specimen center. This was done in order to eliminate erroneous data

that might result from the tapering of the specimen on the ends due to accelerated

corrosion in these areas.

3.5 Calculation of Corrosion Rates

From the thickness measurements taken along the cross-section of each corrosion

specimen, average thickness losses were found. These average thickness losses were

used to calculate corrosion rates. The formula used to calculate the corrosion rates of the

tested alloys is given below:

Specimen

cross-section


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dw/dt = * (dy/dt) * (1/2) (3.1)

where dw/dt = corrosion rate (g/cm2*hr)

= density of base material (g/cm3)

(dy/dt) = average thickness loss per unit time (cm/hr).

The factor of (1/2) in the equation is used to reflect the fact that the average

thickness change of the specimen is caused by the corrosion of both faces of the

specimen. The densities of the tested materials are listed in Table 3.5.

Table 3.5 Density of Test Materials

Test Material Density (g/cm3)

316L 7.9Fe3Al 6.72

FeCrSi 7.45

Corrosion rates for the on-line corrosion tests were calculated from the thickness

losses of the Level 1 sample sections. This was done because of the fact that the on-line

test specimens showed nearly identical corrosion losses at the Level 1 and Level 2 height

designations. At the Level 3 height designation, very little corrosion was seen. Level 1

measurements were found to give the most accurate depiction of corrosion losses due to

immersion in the molten baths. The remainder of the graphs and data presented for the

on-line corrosion tests are derived from thickness losses measured at the Level 1

designation.


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3.6 Scanning Electron Microscopy (SEM)

After the thickness measurements, the corrosion specimens were prepared for

examination under the scanning electron microscope (SEM). Test specimens from the

static immersion testing in the Al-8Si bath were chosen for examination with the SEM.

Also, specimens from the 24-hour on-line tests in the zinc and Zn-55Al baths were also

selected for SEM analysis. In every case, the level 1 specimens were examined. Also,

since the FeCrSi specimens were not tested in the first, on-line series of tests, specimens

were selected from the static series of tests for examination. The FeCrSi specimens from

the Al-8Si and galvalume (Zn-55Al) baths that were chosen for examination were

immersed for 24 hours at 660C. A FeCrSi specimen statically tested in the zinc bath for

24 hours at 460C was chosen for observation under the scanning electron microscope.

Selected specimens were re-ground. This grinding was done in order to remove

the surface of the specimen that had been attacked by the previous etching treatment.

Much of the SEM analysis was conducted at the National Institute for Occupational

Safety and Health (NIOSH) located in Morgantown, WV. The scanning electron

microscope used at this facility was a JEOL JSM-6400. This microscope allowed high

magnification observation and analysis of the corrosion specimens. The SEM

accelerating voltage was set to 20 kV and the condenser lens current was set to 0.6 mA.

The energy dispersive x-ray spectroscopy (EDS) capability of the SEM was

extensively used during the course of this project. When the electron beam of the SEM

bombards a sample, electrons are ejected from the atoms comprising the materials

surface. The resulting electron vacancy is filled with an electron from a higher shell, and

an x-ray is emitted to balance the energy difference between the two electrons.17

The


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energy of these emitted x-rays are characteristic of the elements from which the x-rays

were emitted. The EDS detector collects, counts, and sorts the x-rays and displays the

results as an EDS spectrum, which is a plot of energy versus the relative counts of the

detected x-rays.

EDS spectra of the alloy layers and the base materials were taken and compared.

EDS spectra were collected for a time of 100 seconds. Elemental mapping of several

specimens was also performed. The mapping indicated the distribution and relative

concentrations of elements across the sample surface using image brightness intensity.17

3.7 Microstructure Evaluation of Base Materials

The three base materials were etched to reveal the microstructure so that optical

microscopy could be performed. The etching solutions used for the three test materials

are listed in Table 3.6.

Table 3.6: Etchants Used for Microstructure Evaluation of Base Materials

Optical microscopy was performed on a Leitz Laborlux 12 ME optical microscope

outfitted with a COHU high-performance CCD camera. Optimas 4.02 imaging software

was used in conjunction with the optical microscope and CCD camera to capture images

of the samples.

Base Material Etching Solution Used

316L 1 part HNO3, 1 part HCl, 1 part waterFe3Al 50 ml CH3COOH, 30 ml HNO3, 20 ml HCl, 10 ml water

FeCrSi 3 parts HCl, 1 part HNO3, 1 part glycerol (Glyceregia)


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The images taken with the Leitz optical microscope were used for grain size

measurement. Grain sizing calculations were performed with the aid of Scion Image

software. Grain sizes that were calculated by the use of the Scion Image software were

compared with grain size measurements taken with the Hirox optical microscope to

confirm the values.


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Chapter 4 Results and Discussion

4.1 Microstructure of Base Materials

The three different base materials used throughout the course of this corrosion

study contained distinctly different microstructures. Optical micrographs of the 316L,

Fe3Al, and FeCrSi alloys can be seen in Figure 4.1.

Figure 4.1: Microstructure of Specimens used for Corrosion Testing. Fe3Al (left),

316L (center), FeCrSi (right)

Compared to the Fe3Al, the grain size of the low-carbon stainless steel 316L was

substantially smaller. The grain size of the FeCrSi alloy was extremely large when

compared to the other two alloys. Listed in Table 4.1 are the grain sizes of the three test

materials.

Table 4.1: Test Material Grain Sizes

Material Grain Size (ASTM #)Average Grain Diameter

(microns)

316L 8.5 17.7

Fe3Al 5.5 55.6

FeCrSi -0.7 458.4


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The grains of the 316L and FeCrSi specimens were found to have straight grain

boundaries in contrast to the grain boundaries of the iron aluminide specimen. The grain

boundaries of the Fe3Al specimen appeared globular in nature and did not have the flat

edges that appeared in the grain structure of the two other specimens. Also, within the

structure of the iron aluminide specimen, small dark particles were observed. The

particles appeared to be evenly distributed across the surface, and may be precipitates

within the grain structure of the Fe3Al.

It can be seen from Figure 4.2 that the FeCrSi alloy contained a very large grain

structure. Upon examining the FeCrSi alloy samples, the effect of mechanical rolling on

the grain structure of the alloy was very apparent. In various areas on the specimen

surface elongated grain structures, a result of the rolling process, were observed.

However, within the same specimen, other areas of undeformed grain structures were

present. It was from these non-elongated grain areas that the grain size calculations were

done for this material. In Figure 4.2 one can clearly see the distinct differences between

the areas in the FeCrSi alloy affected by the rolling process, and those that were left

undeformed.

(a.) (b.)

Figure 4.2: Grain structures in FeCrSi alloy. (a.) unaffected grain

structure (b.) elongated grain structure


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From these photographs, it can be seen that the rolling of the FeCrSi alloy did not

have an equal effect on all areas of the material. The center of the FeCrSi specimens that

were examined under the optical microscope showed this unaffected grain structure,

while the outer edges of the specimens more prevalently showed the elongated grains.

4.2 Corrosion in On-line Tests

4.2.1 Al-8Si Bath

Figure 4.3 illustrates a plot of specimen thickness loss measured at specified

locations on the cross-section of a test specimen.

Figure 4.3: Thickness Loss of 316L specimens in Al-8Si bath at 660C

From Figure 4.3 the thickness loss of the 316L specimens can be observed to increase

with an increase in testing time. The data in Figure 4.3 was measured from the Level 1

height designation of the respective test specimens. An edge effect on corrosion can be

noticed in the 72-hour test. The data for this test shows greater thickness losses occurring

0.0

0.51.0

1.5

2.0

2.5

3.0

3.5

T10

T8

T6

T4

T2 C

B2

B4

B6

B8

B10

Specimen Location

ThicknessLoss(mm)

72 hr

24 hr8 hr

2 hr


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at the end of the specimen, compared to the specimen center. Similar graphs for the other

material/bath combinations can be found in Appendix A.

From the thickness loss data, such as in Figure 4.3, average thickness losses were

calculated and these values were plotted versus time. Figure 4.4 shows the results of the

corrosion testing of the two materials in the Al-8Si bath.

Figure 4.4: Thickness Loss of 316L and Fe3Al Specimens in Al-8Si bath at 660C

Through this graph it can be seen that the two materials perform similarly in the

Type 1 aluminizing bath. The iron aluminide specimen, however, seems to have a slightly

lower resistance to corrosion than the stainless steel alloy. The linear nature of the

thickness reduction rate (dy/dt), the slope of the fitted line, can also be observed from the

presented data. This linear trend seems to hold true for all on-line tests.

Fe3Al

316L

Time (hours)

ThicknessLoss(mm)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 20 40 60 80


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4.2.2 Zinc Bath

Figure 4.5: Thickness Loss of 316L and Fe3Al Specimens in Zinc Bath at 460C

From Figure 4.5 the performance of the two alloys in the pure zinc bath can be

seen. While the 316L shows very little corrosion, even after a time of 240 hours, the

Fe3Al alloy shows a substantial thickness loss. Considering that the initial iron aluminide

specimens had a thickness of approximately 3.2 mm, a thickness loss of close to 2.5 mm

demonstrates the severe amount of corrosion that occurred with this material in the zinc

bath.

Time (hours)

ThicknessLoss(mm

)

Fe3Al

316L

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 100 200 300


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4.2.3 Zn-5Al Bath

Figure 4.6: Thickness Loss of 316L and Fe3Al Specimens in Galfan Bath at 490C

Though both materials seemed to perform similarly in the galfan (Zn-5Al) bath,

the 316L specimens appeared to have a better resistance to corrosion than the iron

aluminide. Even after 240 hours in the bath, the 316L specimen showed little thickness

loss. For the same time of 240 hours, the Fe3Al specimen showed approximately .75 mm

of thickness reduction across the specimen cross-section. The linear nature of the

thickness reduction rate is evident from this graph and the previous graphs. This linearity

however, contradicts the study by Lampe et al.13

in which it was stated that in a zinc melt

containing 4% aluminum, the time law for the corrosion of steel was found to be

parabolic up to 500C. Our study indicates a linear time law at 490C in a zinc bath

containing 5% aluminum.

316L

Fe3Al

ThicknessLoss(mm)

Time (hours)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 100 200 300


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4.2.4 Zn-55Al Bath

Figure 4.7 illustrates the performance of the stainless steel and iron aluminide

alloys in the on-line testing in the galvalume (Zn-55Al) bath. As in the galfan bath, the

316L stainless steel alloy shows a higher resistance to corrosion than the Fe3Al alloy.

The stainless steel alloy does show slightly more corrosion in the galvalume bath

compared to the galfan bath. Iron aluminide specimens have a similar corrosion trend in

the galvalume bath to those obtained from the on-line testing in the same bath.

Figure 4.7: Thickness Loss of 316L and Fe3Al Specimens in Galvalume Bath at

600C

4.2.5 Discussion

Table 4.2 presents the corrosion rates from the on-line tests.

Table 4.2: Corrosion Rates for On-line Corrosion Tests (gm/cm2*hr)

316L

Fe3Al

ThicknessLoss(mm)

Time (hours)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 100 200 300

Bath (Temperature(C)) 316L Fe3Al

Al-8Si (660) 1.41 x 10-2 1.51 x 10-2

Zinc (460) 3.56 x 10-5 3.33 x 10-3

Zn-5Al (490) 1.58 x 10-5 1.04 x 10-3

Zn-55Al (600) 2.37 x 10-4 1.11 x 10-3


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The data from Table 4.2 is also presented in Figure 4.8 for comparison. Figure 4.8

compares the corrosion rates of the two test materials in the four different hot-dip coating

baths. The corrosion rate (dw/dt) was calculated from the thickness loss rate (dy/dt) by

using equation 3.1. From Figure 4.8 one can see the drastic difference between corrosion

rates in the Type1 aluminizing bath compared to the zinc containing baths.

Figure 4.8: Corrosion Rates for On-line Corrosion Tests

The corrosion rates are much higher for both the 316L and the Fe3Al alloys in the Al-8Si

bath than their corrosion rates were in any of the zinc containing baths. The corrosion

rate in the Al-8Si bath may have been affected by the high operating temperature

(660C).

C

orrosionRate(gm/cm^

2*hr)

Bath Material

0.0E+00

4.0E-03

8.0E-03

1.2E-02

1.6E-02

Al-8Si Zinc Zn-5Al Zn-55Al

316L Fe3Al

0.0E+00

5.0E-04

1.0E-03

1.5E-03

2.0E-03

2.5E-03

3.0E-03

3.5E-03

Zinc Zn-5Al Zn-55Al


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The corrosion rates in the Type 1 aluminizing bath were very similar for both the

316L and iron aluminide specimens. For the on-line series of tests, this was the only bath

in which the corrosion of the stainless steel and the intermetallic were comparable. In

each of the zinc baths, the 316L clearly exhibited higher corrosion resistance than the

Fe3Al specimens. For example, in the pure zinc bath the 316L yielded a corrosion rate of

3.56 x 10-5

gm/cm2*hr while the iron aluminide specimen showed a corrosion rate of 3.3

x 10-3

gm/cm2*hr. From the inserted figure in Figure 4.8 it can be clearly seen that the

316L alloy performs far better in each of the zinc-containing baths than the iron

aluminide alloy did. Although the corrosion rates for the two materials were similar for

the on-line tests in the aluminizing bath, in the zinc baths the 316L showed dramatically

less corrosion. Of the zinc baths, the pure zinc produced the highest corrosion rate for the

Fe3Al, while the galvalume bath yielded the highest corrosion rate for the 316L alloy.

4.3 Corrosion of Static Immersion Specimens

Table 4.3 lists measured thickness loss data from the static immersion tests.

From the average thickness losses the corrosion rates of the specimens in the static

immersion tests were calculated, and this data is presented in Table 4.4.


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Table 4.3: Average Thickness Losses for Static Immersion Tests (mm)

Table 4.4: Corrosion Rates for Static Immersion Tests (gm/cm2*hr)

316L Fe3Al

Bath 460C 560C 660C 460C 560C 660C 460C

Al-8Si --- --- 5.00E-03 --- --- 3.40E-03 ---

Zinc 3.80E-03 4.90E-03 1.71E-02 5.10E-03 7.10E-03 3.56E-02 1.10E-03

Zn-5Al 0.00E+00 0.00E+00 3.00E-04 1.00E-04 2.29E-02 3.58E-02 1.80E-03

Zn-55Al --- --- 6.50E-03 --- --- 3.56E-02 ---

Specimen Material and Test Temperature

Specimen Material and Test Temperature

316L Fe3AlBath 460C 560C 660C 460C 560C 660C 460C

Al-8Si --- --- 3.02E-01 --- --- 2.41E-01 ---

Zinc 2.30E-01 2.99E-01 1.04E+00 3.61E-01 5.06E-01 2.541E+00* 6.80E-0

Zn-5Al 0.00E+00 3.00E-03 1.90E-02 4.00E-03 1.64E+00 2.554E+00* 1.13E-0

Zn-55Al --- --- 3.94E-01 --- --- 2.541E+00* ---


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Because the melting point of the Type 1 aluminizing mixture is approximately

605C, the tests could not be run at the 460 and 560C temperatures. From the data in

Table 4.4 it can be seen that the corrosion rate of the 316L specimen was found to be

approximately .005 gm/cm2*hr. However, when this value is compared to the value

calculated from the first series of tests, there is a large discrepancy between the two.

From the on-line battery of tests the corrosion rate of the 316L in the Al-8Si was

calculated to be approximately .014 gm/cm2*hr. This value is nearly three times as large

as the value received from the static testing. The exact reason for this discrepancy is not

clear. This could result from the fact that the corrosion rates for the static immersion

tests were calculated only at one testing time, 24 hours. The corrosion rates for the on-

line series of tests however were calculated from multiple testing times, and therefore

most likely give a better overall interpretation of the corrosion rate of the material.

The static corrosion rate of the Fe3Al also shows a considerable reduction when

compared to the corrosion rate received from the dynamic tests. Another reason for this

reduction in corrosion rate may be due to the small, static bath that was used for the

second series of corrosion tests. The small volume of this bath becomes saturated with

dissolved elements from the specimen and this may have slowed the specimens

corrosion. In addition, brittle intermetallic layers that formed on the outside of a

corroded specimen would be more likely to spall off into the melt in a moving bath. The

breaking off of these alloy layers would facilitate new growth and faster corrosion in the

on-line testing. The experimental FeCrSi alloy seems to outperform both the 316L and

the Fe3Al in the Al-8Si bath. This fact can be observed from the corrosion rates listed in

Table 4.4.


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Within the Al-8Si bath, the 316L alloy showed the highest corrosion rate during the static

immersion tests. Though the corrosion rate of the FeCrSi alloy was lower than the 316L

and Fe3Al alloys, the performance of all three test materials was very similar in the

aluminizing bath.

Figure 4.9: Corrosion Rates for Static Immersion Tests in Zinc

Figure 4.9 illustrates the corrosion rates of the 316L, Fe3Al, and FeCrSi alloys

after static testing in the zinc bath for a period of 24 hours. The temperature effect on

corrosion rate of the test materials can clearly be seen in this figure. Though an increase

in testing temperature from 460 to 560C slightly increases the corrosion rate in all three

materials, an increase from 560 to 660C has a drastic effect on the corrosion rates. In

this range the iron aluminide shows the most prominent increase in corrosion rate. At the

final testing temperature the corrosion rate of the Fe3Al, .0356 gm/cm2*hr was more than

twice that of the other two materials.

Temperature

CorrosionRa

te(gm/cm2*hr)

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

460C 560C 660C

316L Fe3Al FeCrSi


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The Figure 4.10 illustrates the temperature effect on static corrosion in the galfan

(Zn-5Al) bath.

Figure 4.10: Corrosion Rates for Static Immersion Tests in Zn-5Al

Increasing the testing temperature from 460C to 560C, and eventually to 660C

had virtually no effect on the corrosion rate of the 316L. The corrosion rate of the iron

aluminide steadily increased across the three testing temperatures. The corrosion rate of

the FeCrSi alloy remained relatively constant after the first two series of tests, but

showed a drastic increase at 660C, which can be observed in Figure 4.10.

Because of the melting point of the galvalume bath material, the static immersion

tests could only be run in this bath at 660C, similar to the testing in the Type 1

aluminizing bath. At this temperature the iron aluminide showed the highest corrosion

rate, while the FeCrSi alloy demonstrated the lowest rate, as can be seen in Figure 4.4.

CorrosionRate(gm/cm2*hr)

Temperature

0.00

0.02

0.04

0.06

0.08

0.10

460C 560C 660C

316L Fe3Al FeCrSi


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All three test materials showed higher corrosion in the zinc bath at 660C than in

the Al-8Si bath at the same testing temperature. The iron aluminide alloy totally

dissolved into the melt upon testing in the zinc, Zn-5Al, and Zn-55Al baths at 660C, as

can be seen in Table 4.3. Upon testing in the Zn-5Al melt at 660C, the FeCrSi specimen

also underwent 100% dissolution into the bath. The 316L alloy conversely, showed

lower corrosion rates in the galfan bath than in the pure zinc bath at all three testing

temperatures. The FeCrSi alloy demonstrated lower corrosion rates than the 316L alloy

and the Fe3Al alloy in the Al-8Si, zinc, and Zn-55Al baths.

4.4 SEM/EDS Analysis

Analysis of the alloy layers formed during the corrosion testing was conducted

with the aid of a scanning electron microscope and the EDS capability of the microscope.

4.4.1 Type 1 Aluminizing Bath (Al-8Si)

Figure 4.11 shows the alloy layers formed by the immersion of the low-carbon

stainless steel specimen in the aluminizing bath. The specimen shown in Figure 4.11 was

taken from the laboratory tests after immersion in the aluminizing bath at 660C for a

period of 24 hours. A back-scattered electron image was used to show the compositional

contrast.

Upon immersion in the Al-8Si bath, the 316L specimen formed several distinct

alloy layers. Figure 4.12 shows the corresponding EDS spectra taken of the 316L

specimen and the formed alloy layers. The bright area on the left of the micrograph is the

316L base material (Figure 4.12a). To the right of the matrix, a relatively thin alloy layer


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was observed. This alloy layer was approximately 20 microns in thickness and had a

very uniform, continuous structure. From the EDS characterization of this thin alloy

layer, Figure 4.12b, the chemical composition was determined to be the -Fe2Al5 phase.

This alloy layer is noted to form on the surface of steels that have been hot-dip coated in

Type 1 aluminizing baths and was mentioned in various literature sources.3

A small

amount of chromium that has diffused from the matrix material can also be seen in the

EDS spectrum for the first alloy layer.

The next alloy layer contained less iron than the -Fe2Al5 phase, and was

identified as -FeAl3. Within this second alloy layer, a crack can be seen that propagates

within the brittle alloy layer, parallel to the edge of the original specimen. This second

layer is much thicker than the -Fe2Al5 layer, which formed adjacent to the 316L matrix.

The third stratum that formed was an inhomogeneous layer that had a tree-like

structure upon observation. This last layer had a high aluminum content, and showed a

marked decrease in iron content compared to the previous alloy layer. Instead of being

one single alloy layer, this layer was actually formed from tree-like projections that

protruded from the second layer. The remaining space between these projections was

filled with material from the surrounding bath. This unique structure gave the area its

tree-like appearance. An EDS analysis of these projections revealed that they had the

same chemical composition as the second alloy layer, and therefore were composed of

the -FeAl3phase.


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Figure 4.11: SEM Micrograph of 316L Specimen after Static Testing in Al-8Si Bath

at 660C for 24 Hours

Figure 4.12: EDS Spectra of Alloy Layers Formed From the Static Corrosion

Testing of 316L in an Al-8Si Bath at 660C for 24 Hours

Cr

Fe

Ni

Al

Cr

Fe

Al

Cr FeFeCr

Al

Si Si

316L Matrix 1st Alloy Layer

2nd Alloy Layer Outer layer

(-Fe2Al5)

(-FeAl3)

a.)b.)

c.) d.)


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Figure 4.13 reveals the alloy layers formed by the corrosion of an Fe3Al

specimen in the Al-8Si bath. As with the stainless steel specimen, several alloy layers are

also observed on the surface of the iron aluminide upon corrosion in the Al-8Si bath.

The first alloy layer formed on the Fe3Al specimen had the same chemical

composition as the first alloy layer formed on the 316L specimen in this bath, and was

therefore identified as the -Fe2Al5 phase. The EDS spectra of the formed alloy layers

are displayed in Figure 4.14. Instead of forming a thin layer of the -Fe2Al5 phase

however, the iron aluminide specimen formed a thick layer of this phase. In Figure 4.13

a crack can be seen in this alloy layer that propagates parallel to the specimen surface.

The second alloy layer to form was identified as the -FeAl3 phase. This second

layer was composed of the same phase as the second alloy layer formed on the 316L

sample. However, instead of forming a thick alloy layer as with the stainless steel

sample, the FeAl3 formed a relatively thin alloy layer of FeAl3, as can be seen in Figure

4.13.

Figure 4.13: SEM micrograph of Alloy Layers Formed from the Static Testing of

Fe3Al (left) in an Al-8Si Bath at 660C for 24 Hours


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Figure 4.14: EDS Spectra of Alloy Layers Formed During the Static

Corrosion Testing of Fe3Al in Al-8Si Bath

Figure 4.15 shows the alloy layers that formed upon the corrosion of the FeCrSi

alloy in the static Al-8Si bath after 24 hours. The FeCrSi matrix is the lighter section on

the left of Figure 4.15. The relatively heavy elements in the base material appear as a

lighter area when using the back-scattered imaging capability of the SEM. The EDS

spectra corresponding to Figure 4.15 are displayed in Figure 4.16.

a.) Fe3Al Matrix b.) First alloy layer (Fe2Al5)

c.) Second alloy

layer (FeAl3)d.) Al-8Si bath

Al

Fe

Cr

Fe

Al

Al

Fe

Al

Si


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One distinct alloy layer was formed on the FeCrSi specimen. The alloy layer that

formed consisted of a ternary phase of aluminum, iron, and chromium. Refer to Figure

4.16 for the EDS spectra of this ternary phase. The exact phase that composed this alloy

layer requires further identification. This layer contained a relatively high amount of

aluminum, compared to the amounts of chromium and iron.

Though the relative thicknesses of the alloy layers formed on the 316L and Fe 3Al

in the Al-8Si bath differed, the actual phase compositions of the alloy layers were the

same. This similar alloy layer structure may have contributed to the similar performance

of the two materials in the aluminizing bath. The 316L alloy showed a lower corrosion

rate than the Fe3Al in the on-line corrosion tests, and a slightly higher one in the static

immersion tests (refer to fig. 4.8 and Table 4.4). The FeCrSi alloy showed a slightly

lower corrosion rate than the stainless steel and intermetallic in the second series of tests

Figure 4.15: SEM micrograph of Alloy Layers Formed from the Static Testing of

FeCrSi (left) in an Al-8Si Bath at 660C for 24 Hours


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Figure 4.16: EDS Spectra of FeCrSi after Static Testing in Al-8Si Bath at

660C for 24 Hours

4.4.2 Zinc Bath

SEM analysis was also conducted on the 316L, Fe3Al, and FeCrSi specimens that

had been immersed in the pure zinc bath. Figure 4.17 shows a back-scattered electron

micrograph of a 316L specimen that has been immersed in a pure zinc melt for 24 hours.

In the micrograph, only one distinct alloy layer is present. This alloy layer can be seen as

the dark, non-uniform layer lying adjacent to the 316L matrix.

The EDS spectrum for this alloy layer is illustrated in Figure 4.18. The actual

phase structure requires further identification.

0.0 2.0 4.0 6.0 8.0 10.0

keV

FeCr

Si

Mn

0.0 2.0 4.0 6.0 8.0 10.0

keV

FeCr

Al

0.0 2.0 4.0 6.0 8.0 10.0

keV

Al

Cr FeSi

a.) FeCrSi Matrix b.) Alloy Layer (Fe-Al-Cr phase)

c.) Al-8Si


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Figure 4.17: SEM micrograph of Alloy Layers Formed from the On-line

Testing of 316L (left) in a Zinc Bath at 460C for 24 Hours

Figure 4.18: EDS Spectrum of Fe-Al-Zn Ternary Phase Formed from the

On-line Testing of 316L in a Zinc Bath at 460C for 24 Hours

Al

Zn

Fe

Zn

keV

0.0 2.0 4.0 6.0 8.0 10.0


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The alloy layer contains a high amount of aluminum. This fact was of particular interest

considering that the 316L base material contains no aluminum and the nominal bath

composition was pure zinc. This high aluminum peak was present in all EDS spectra that

were taken of 316L specimens tested in the commercial zinc bath. The enrichment of Al

in the layer was attributed to residual aluminum contained in the commercial hot-dipping

line. Instead of forming binary phases with the zinc in the bath, the iron from the 316L

specimen reacted with this aluminum to form a ternary Fe-Al-Zn phase.

As did the 316L specimen in the zinc bath, the iron aluminide specimen formed a

single alloy layer, as can be seen in Figure 4.19.

Figure 4.19: SEM micrograph of Alloy Layers Formed from the Immersion of Fe3Al

(left) in a Zinc Bath at 460C for 24 Hours


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Figure 4.20: EDS Spectrum of-FeAl3 Phase Formed from the On-line Testing of

Fe3Al in Zinc Bath at 460C for 24 Hours

Figure 4.21: EDS of Zirconium Rich Phase Observed in Fe3AL Matrix after On-line

Corrosion Testing in Zinc Bath at 460C for 24 Hours


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This alloy layer was identified as the -FeAl3 phase. An EDS spectrum for this

alloy layer can be seen in Figure 4.20. Upon analysis, the white areas within the matrix

were found to be rich in zirconium. An EDS spectrum of these zirconium rich phases is

illustrated in Figure 4.21. This zirconium was possibly introduced into the material

during processing, where zirconium oxide is often used as a melt release during the

casting procedure.

In contrast to the 316L and Fe3Al specimens, the FeCrSi specimen was found to

form no alloy layers upon immersion in the zinc bath. A micrograph of the FeCrSi after

testing in the zinc bath can be seen in Figure 4.22.

Figure 4.22: SEM micrograph of FeCrSi (left) after Static Testing in Zinc at 460C

for 24 Hours

In the left of Figure 4.22 the FeCrSi base material can be se

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