A Study of Trivalent Chrome Process Coatings on Aluminum Alloy 2024-T3

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A Study of Trivalent Chrome Process Coatings on

Aluminum Alloy 2024-T3

Dissertation

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of

Philosophy in the Graduate School of The Ohio State University

Yang Guo, M.S.

Graduate Program in Materials Science and Engineering

The Ohio State University

2011

Dissertation Committee:

Prof. Gerald Frankel, Advisor

Prof. Rudy Buchheit

Prof. Sheikh Akbar

Yang Guo

2011

iii

ABSTRACT

Chromate conversion coatings (CCCs) have been employed in the surface finishing process for AA2024-T3 for their excellent ability to resist localized corrosion and to promote paint adhesion. However, due to the toxic effects of chromium compounds, a significant amount of effort has been extended to develop alternative corrosion inhibitor systems. Trivalent Chrome Process (TCP) coatings recently have gained wide acceptance and are considered an environmentally friendly replacement for chromate conversion coating, because the TCP bath and the resulting film contain no Cr (VI) species.

In this study, the application of TCP coatings as an alternative to CCCs has been investigated. During TCP coating formation, activation of the Al surface leads to the reactions of oxygen reduction and hydrogen evolution, which result in the local pH increase and the deposition of the TCP coating. TCP coating is characterized as a dense layer consisting of rounded particles hundreds of nm in size, similar to the CCCs. The thickness of the TCP was in the range of 40-120 nm depending on the conversion time, considerably thicker than the zirconium based coating without chromium species. No

Cr (VI) was found on the TCP surface assures its application as an environmentally friendly replacement for CCC. A two layered structure is suggested, with zirconium-chromium mixed oxide in the outer layer and aluminum oxide or oxyfluoride at the metal/coating interface. The high vacuum condition in the traditional

ii SEM dehydrates the coating quickly; consequently, mud-crack artifacts were always

observed. The TCP coating provides corrosion protection to the AA2024-T3 through

suppressing the oxygen reduction reaction on aluminum alloy surfaces by acting as a

protective barrier layer.

The effects of two pretreatments on the TCP formed on AA2024-T3 surface

were investigated. The growth of TCP following Process I (Henkel Chemicals) started

faster compared to Process II (silicate treatement). The size of the particles was around

tens of nm which was smaller compared to Process II. The abnormal round clusters

were not easily observed after Process I. Nucleation uniformly occurred on the sample

surface after Process I, while non-uniform nucleation occurred after Process II. Longer

immersion in the TCP bath resulted in increased thickness of TCP film for both

pretreatments. Considering all that was found, there was not much difference of TCP

formation on the aluminum surface after these two pretreatments.

The self healing properties of TCP on AA2024-T3 have been assessed and quantified using the artificial scratch technique. TCP treatment can greatly improve the corrosion resistance of AA2024-T3 to sustain the long time exposure to simulated corrosive environments. The chromium is able to be released from the TCP coating according to ICP-OES and the released chromium can be transported to uncoated area nearby. EIS data showed the polarization resistance of uncoated surface was twice as much as uncoated controls, which suggested that TCP can provide mild active corrosion inhibition for Al alloys.

iii

DEDICATION

Dedicated to my parents and husband

ACKNOWLEDGMENTS

I would like to thank my advisor, Dr. Jerry Frankel, for all the guidance,

encouragement, support and patience. I treasure the invaluable experience here

working with him, both professionally and personally. I greatly appreciate his

understanding and help especially during the hard times. I would also like to

acknowledge the advice and encouragement from Dr. Rudy Buchheit. I would like to thank Dr. Sheikh Akbar for the guidance and advice to be a part of my oral examination committee. Special thanks to SERDP for providing us the funding for the project.

My sincere thanks to all FCC members for being so supportive and helpful, especially, Dr. Yumei Zhai, Dr. Belinda Hurley, Dr. Kinga Unocic, Dr. Bastian Maier,

Dr. Fariaty Wong, Dr. Zhijun Zhao, Dr. Xiaodong Liu, Dr. Santi Chrisanti, Dr. Yiyun

Li, Liu Cao, Anusha Chilukuri, Xiaoji Li, Brendy C. Rincon. I would like to express my gratitude to Mr. Cameron Begg, Mr. Daniel Huber, Mr. Henk Colijin and Mr.

Ross Baldwin for all the help with instruments and facilities usage. Thanks to Dr. Lisa

Hommel for helping me with XPS. Administrative help from Ms. Christine Putnam is

also acknowledged.

I am also thankful to my friends who helped and supported me over the course of my study at The Ohio State University, in particular, Dr. Lin Li, Dr.

Hongqin Sun, Dr. Li Sun.

Most of all, I would like to thank my parents, my grandparents for all the love, understanding and encouragement these past years. They have always inspired me and motivated me to do my best. Lastly, I would like to thank my loving husband,

Jianbo, for his emotional support and encouragement through all these hard times.

VITA

June 2005…………………………………B.E. Material Science and Engineering

Zhejiang University, Hangzhou, China

June 2008………………………………….M.S. Material Science and Engineering

The Ohio State University, Columbus, OH

September 2005 – present…………………Graduate Research Associate

The Ohio State University, Columbus, OH

PUBLICATIONS

1. Y. Zhai, Y. Guo, G.S. Frankel, J. Zimmerman and W. Fristad, “Chromate-Free

and Phosphate-Free Surface Treatments for Al Alloy and Steel Substrates,”

Proceedings CD, Proceedings of the 17th ICC, Las Vegas, (2008).

2. Saikat Adhikari, Y. Guo, Brendy Rincon-Troconis, Gerald Frankel.

“Chromate-Free and Phosphate-Free Surface Pretreatments”, Proceedings CD,

Materials Science & Technology Proceedings, Houston, TX, (2010).

FIELDS OF STUDY

Major Fields: Materials Science and Engineering

vii

TABLE OF CONTENTS

ABSTRACT…...... ii DEDICATION...... iv ACKNOWLEDGMENTS ...... v VITA………… ...... vii LIST OF TABLES ...... xi LIST OF FIGURES ...... xii

CHAPTER 1 INTRODUCTION...... 1

CHAPTER 2 LITERATURE REVIEW...... 7 2.1 INTRODUCTION...... 7 2.2 METALLURGY OF ALUMINUM ALLOY 2024-T3...... 9 2.3 SURFACE PRETREATMENT OF ALUMINUM ALLOYS ...... 11 2.3.1 Alkaline cleaning ...... 12 2.3.2 Desmutting...... 13 2.4 CHROMATE IN SOLUTION...... 14 2.4.1 Background...... 14 2.4.2 Speciation...... 15 2.4.3 Protection by Soluble Chromate ...... 16 2.5 CHROMATE CONVERSON COATING ON ALUMINUM ALLOYS ……………………………………………………………………….17 2.5.1 CCC Processes...... 17 2.5.2 Coating Formation ...... 18 2.5.3 Structure and Composition ...... 20 2.5.4 Activators and Accelerators ...... 22 2.5.5 Corrosion Protection ...... 23 2.6 ZIRCONIUM BASED CONVERSION COATINGS...... 29 2.6.1 Coating Morphology and Composition ...... 29 2.6.2 Electrochemical Behavior...... 31 2.7 TRIVALENT CHROME PROCESS (TCP) COATINGS...... 32 2.7.1 Background...... 32 2.7.2 Formation...... 32 2.7.3 Corrosion Inhibition and Adhesion...... 34 2.8 KEY UNSOLVED ISSUES ...... 35

CHAPTER 3 Characterization of Trivalent Chrome Process Coating on AA2024-T3 ...... 52 3.1 INTRODUCTION...... 52 3.2 EXPERIMENTAL PROCEDURES...... 54

viii 3.3 RESULTS ...... 58 3.3.1. Surface Morphology of TCP Coating ...... 58 3.3.2. Dehydration of TCP coating ...... 59 3.3.3. Surface Composition of TCP on AA2024...... 60 3.3.4. Structure of TCP Coating on AA2024-T3 ...... 60 3.3.5. Polarization of TCP in dilute Harrison’s Solution...... 62 3.4 DISCUSSION ...... 62 3.4.1. Formation and Morphology of TCP ...... 62 3.4.2. Structure and Composition of TCP coating on AA2024-T3...... 65 3.4.3. Electrochemical behavior of TCP coating in dilute Harrison’s solution...... 66 3.5 CONCLUSION ...... 67

CHAPTER 4 Effect of Pretreatment on Trivalent Chrome Process Coatings ...... 83 4.1 INTRODUCTION...... 83 4.2 EXPERIMENTAL PROCEDURES...... 86 4.3 RESULTS ...... 89 4.3.1. AA2024-T3 Surface after Alkaline Cleaning ...... 90 4.3.2. AA2024-T3 Surface after Desmutting...... 90 4.3.3. Formation of TCP Coating after Different Pretreatments...... 92 4.3.4. Chemistry and Thickness of TCP Coating after Different Pretreatments...... 93 4.3.5. Electrochemical Behavior of TCP Coating after Different Pretreatments………………………………………………………94 4.4 DISCUSSION ...... 95 4.4.1. Effect of Process I on the TCP coating ...... 95 4.4.2. Effect of Process II on the TCP coating...... 97 4.4.3. Effect of Pretreatments on the TCP coating Chemistry and Thickness ...... 98 4.5 CONCLUSION ...... 99

CHAPTER 5 Assessment of Active Corrosion Inhibition of Trivalent Chrome Process Coatings on AA2024-T3 ...... 121 5.1 INTRODUCTION...... 121 5.2 EXPERIMENTAL PROCEDURES...... 123 5.3 RESULTS ...... 127 5.3.1 Visual Observation of Corrosion Protection in Artificial Scratch Cells ...... 127 5.3.2 Inhibitor Release and Surface Morphology ...... 127 5.3.3 Chromium Transport in the Artificial Scratch Cell Experiments...128 5.3.4 EIS Characterization of TCP Coatings in the Artificial Scratch Cells ...... 129 5.3.5 Artificial Scratch Cell Tests with Electrical Connection and Different Area Ratio ...... 133 5.4 DISCUSSION ...... 134 5.4.1 Corrosion Protection of TCP Coated Surface ...... 134 5.4.2 Active Corrosion Protection by Self-Healing...... 135

ix 5.4.3 Possible Mechanism of Active Corrosion Protection ...... 137 5.5 CONCLUSION ...... 138

CHAPTER 6 CONCLUSIONS AND FUTURE WORK...... 162 6.1 SUMMARY OF KEY RESULTS...... 162 6.2 FUTURE WORK ...... 165

BIBLIOGRAPHY...... 166

LIST OF TABLES

Tables Pages

4.1: Samples treated by Process I with different times of degreasing, desmutting and coating………………………………………………………………………….101

4.2: Samples treated by Process II with different times of degreasing, desmutting and coating………………………………………………………………………….101

LIST OF FIGURES

Figures Pages

2.1: Chromate aqueous chemistry as a function of pH and concentration [36]……...37

2.2: Mechanism for Cr(OH)3 “backbone” formation. (b) Condensation of Cr(VI) on the

Cr(III) backbone [8]……………………………………………………………..38

2.3: SEM of shrinkage cracking in a CCC on 2024-T3 [105]…………………….….39

2.4: AFM image of a CCC formed on the surface of a freshly polished AA2024-T3 after

exposure for 3s to a commercial CCC solution at 5oC [51]………………………40

2.5: Transmission electron micrograph of cross-sectional area of 5 min CCC on

2024-T3 [61]……………………………...………………………………………41

2.6: Schematic illustration of the formation of chromate conversion coatings

[106]…………………………………...………………………………………...42

2.7: Schematic illustration of the structure of Zr/Ti based coatings [37]……….……43

2.8: Schematic illustration of the structure of Trivalent Chrome Process Coatings

[13]………………………………………………………...…………………….44

3.1: SEM micrograph of the TCP foil prepared by FIB………………………………68

3.2: SEM micrographs of 10 min TCP on AA2024-T3 at different magnifications….69

xii 3.3: Crack formation during dehydration as observed by ESEM (by sequence of

cumulative exposure time). Imaging conditions are (a) 3oC, 5 Torr; (b) and (c)

10oC, 5 Torr; (d), (e) and (f) 22oC, 2 Torr………………………………………70

3.4: Crack formation after atmospheric aging for (a) 6 h, (b) 12 h, as observed by ESEM

under the condition 24oC, 7.7 Torr, 35% RH……………………………………71

3.5: Crack formation at different locations on TCP surface after atmospheric aging for

24 h as observed by ESEM. Imaging condition is 24oC, 7.7 Torr, 35% RH……72

3.6: Crack formation at different locations on TCP surface after atmospheric aging for

48 h as observed by ESEM. Imaging condition is 24oC, 7.7 Torr, 35% RH……73

3.7: XPS survey on 5 min TCP coated AA2024-T3 surface………………………….74

3.8: Cr 2p spectra of 5 min TCP-coated AA2024-T3…………………...…………….75

3.9: Transmission electron micrograph of 5 min TCP on AA2024-T3………………..76

3.10: Nano-EDS line profiles of 10 min TCP on matrix of AA2024-T3…………..…77

3.11: Layered structure model for TCP coating deposited on AA2024-T3…………...78

3.12: Potentiodynamic polarization curves in aerated dilute Harrison’s solution. a.

anodic polarization curves, b. cathodic polarization curves…………………….79

4.1: SEM micrograph of as-received AA2024-T3 surface……………………..……102

4.2: SEM micrograph of AA2024-T3 surface after degreasing by Process I for 15 min.

Two regions of the surface are shown…………………………………………..103

4.3: SEM micrograph of AA2024-T3 surface by Process II after degreasing for 2

min………..…………………………………………………………………….104

4.4: Si 2p spectra of surfaces after different pretreatments………………………….105

4.5: SEM micrograph of AA2024-T3 surface after desmutting by Process I for (a) 1 min,

xiii (b) 5 min, (c) 10 min……………………………………………………………106

4.6: SEM micrograph of AA2024-T3 surface by Process II after desmutting for 3

min……………………………………………………………………………...107

4.7: SEM micrograph of AA2024-T3 surface after given treatment (a) Process I 10 s, (b)

Process I 30 s, (c) Process II 10 s, (d) Process II 30 s………………………….108

4.8: (a) SEM micrograph of TCP on AA2024-T3 after Pretreatment II 30s. (b) EDX

profile…………………………………………………………………………..109

4.9: SEM micrograph of AA2024-T3 surface after given treatment (a) Process I 60 s, (b)

Process I 2 min, (c) Process II 60 s, (d) Process II 2 min………………………..110

4.10: SEM micrograph of AA2024-T3 surface after given treatment (a) Process I 3 min,

(b) Process I 5 min, (c) Process II 3 min, (d) Process II 5 min…………………111

4.11: SEM micrograph of AA2024-T3 surface after given treatment (a) Process I 7 min,

(b) Process I 10 min, (c) Process II 7 min, (d) Process II 10 min………………112

4.12: 2 min TCP on matrix of AA2024-T3 after Process I (a) Transmission electron

micrograph, (b) Nano-EDS line profiles………………………………………113

4.13: 2 min TCP on matrix of AA2024-T3 after Process II, (a) Transmission electron

micrograph, (b) Nano-EDS line profiles………………………………………114

4.14: 5 min TCP on matrix of AA2024-T3 after Process I, (a) Transmission electron

micrograph, (b) Nano-EDS line profiles………………………………………115

4.15: 10 min TCP on matrix of AA2024-T3 after Process I, (a) Transmission electron

micrograph, (b) Nano-EDS line profiles………………………………………116

4.16: 10 min TCP on matrix of AA2024-T3 after Process II, (a) Transmission electron

micrograph, (b) Nano-EDS line profiles………………………………………117

4.17: Bode plots obtained by EIS on the TCP coated AA2024-T3 after Process I and

xiv Process II………………………………………………………………………118

5.1: (a). Schematic drawing of artificial scratch cell, (b). Photographs of shorted

artificial scratch cell………………………………………..……….…………140

5.2: Photographs of leaching cells………………………………………..…………141

5.3: Photographs of artificial scratch cell sheets exposed to 0.5 M NaCl; (a) & (b)

TCP-treated and corresponding bare surface exposed for 21 days; (c) & (d) bare

surface controls exposed for 14 days………………………………………….142

5.4: Inhibitor released after static exposure in the dilute Harrison’s solution by

ICP-OES………………………………………………………………………143

5.5: Aluminum content after static exposure in the dilute Harrison’s solution by

ICP-OES………………………………………………………………………144

5.6: XPS spectra measured from untreated AA2024-T3 surface in the artificial scratch

cell: (a) after 14 days in 0.5M NaCl; (b) after 0, 7, 14 and 28 days in dilute

Harrison’s solution…………………………………………………………….145

5.7: Cr 2p spectra of uncoated surface of the artificial scratch cell after 14 days

exposure……………………………………………………………………….146

5.8: Al 2p spectra of nonTCP surface of the artificial scratch cell after different

exposure time………………………………………………………………….147

5.9: Bode plots by EIS for the TCP treated surface in the artificial scratch cell

containing 0.5 M NaCl………………………………………………………...148

5.10: Bode plots by EIS for the nonTCP treated surface in the artificial scratch cell

containing 0.5 M NaCl………………………………………………………...149

5.11: Bode plots by EIS for the control nonTCP treated surface in the artificial scratch

xv cell containing 0.5 M NaCl……………………………………………………150

5.12: Bode plots by EIS for the TCP treated surface in the artificial scratch cell

containing dilute Harrison’s solution………………………………………….151

5.13: Bode plots by EIS for the nonTCP surface in the artificial scratch cell containing

dilute Harrison’s solution……………………………………………………...152

5.14: Bode plots by EIS test for the nonTCP surface in the control cell containing dilute

Harrison’s

solution………………………………………………………………………..153

5.15: Low frequency impedance at 0.01Hz for TCP and nonTCP surfaces in the

artificial scratch cell and control cell containing dilute Harrison’s solution; (a)

exposure within 24h, (b) exposure after 24h………………………………….154

5.16: Equivalent circuit models for resistance and capacitance calculations of coated

surface…………………………………………………………………………155

5.17: Polarization resistance for TCP and nonTCP samples in the artificial scratch cell

and control cell containing dilute Harrison’s solution…………………………156

5.18: Surface capacitance for TCP and nonTCP samples in the artificial scratch cell and

control cell containing dilute Harrison’s solution……………………………...157

5.19: Open Circuit Potential collected for TCP coated and bare surfaces of artificial

scratch cells containing dilute Harrison’s solution. “AR” is area ratio of

TCP/nonTCP surface. “Open” is used for non-shorted cell. The underlines in the

legend signify the surfaces on which the OCP values were measured, whereas the

other surface in the cell is given after the slash………………………………...158

5.20: Low frequency impedance at 0.01Hz for TCP and uncoated surfaces in the

artificial scratch cell containing dilute Harrison’s solution……………………159

xvi

CHAPTER 1

INTRODUCTION

Aluminum alloy 2024-T3 has been widely used in aerospace applications

because of its superior mechanical properties including strength-to-weight ratio, which

result from the alloying elements such as copper, magnesium, and manganese [1].

However, the corrosion performance of the material can be a problem because the

addition of copper and magnesium leads to the formation of various intermetallic particles that make the alloy highly susceptible to localized corrosion, especially pitting corrosion and intergranular corrosion.

Chromate conversion coatings (CCCs) have been employed in the surface finishing process for AA2024-T3 and other metal alloys for their excellent ability to resist localized corrosion and to promote paint adhesion for a long time [2]. However, due to the toxic and carcinogenic effects of chromium compounds on the environment and human health, there have been increasingly stringent legislations regarding their application and waste disposal [3-4]. Consequently, a significant amount of effort has been extended to develop alternative corrosion inhibitor systems that can provide

1 comparable performance with minimal health and environmental concerns [5]. Among

these alternative coatings, zirconium based conversion coatings currently have wide

applications as a substitute for chromate conversion coatings because of their simple

application and good adhesion to subsequent paint coatings [2, 6-9]. One promising

candidate from the fluorozirconate coatings family, Trivalent Chrome Process (TCP),

recently has gained wide acceptance because it provides comparable performance to

the CCCs in both paint adhesion and corrosion resistance only by adding a small amount of trivalent chromium species [10-11]. TCP is considered to be an

environmentally friendly replacement for CCC the TCP bath and the resulting film

contain no chromate species, only Cr(III).

In the present study, TCP coatings on aluminum alloys have been studied.

Several aspects of TCP coatings have been investigated including coating nucleation and growth, composition and structure, effects of pretreatment on the coating performance, and mechanism of inhibition of TCP on aluminum.

This dissertation consists of six chapters. Chapter 1 (current chapter) gives a brief background of this project. Chapter 2 is a brief review of relevant studies published in the literature. The microstructure of aluminum alloy 2024-T3 is reviewed

with a focus on corrosion behavior related to intermetallic particles. This review is also

concentrated on the formation of chromate conversion coating, composition and

structure and corrosion protection mechanisms. Fluorozirconate based coating is

another focus here because these coatings are similar to the current TCP coatings. And

finally a review is given on trivalent chrome process including the unique properties

that might be exploited to develop a chromate-free corrosion inhibitor of AA2024-T3.

Chapters 3 through 5 give the detailed experimental results obtained from this

2 study on TCP coating. Chapter 3 describes the characterization of TCP coating formed

on AA2024-T3 by various analysis techniques (Scanning Electron Microscopy and

EDX, Transmission Electron Microscopy and STEM, X-ray Photoelectron

Spectroscopy, potentiodynamic polarization, Electrochemical Impedance Spectroscopy)

to understand the coating morphology, composition and structure. The distribution and

chemistry of chromium in the coating were investigated. Additionally, environmental

scanning microscopy (ESEM) has been used to characterize morphological changes

due to dehydration.

Chapter 4 focuses on investigation of the effects of two pretreatments on the

TCP formed on AA2024-T3 surface. The general surface features after each step of treatment were examined by SEM. The nucleation and growth of TCP film after various stages were characterized by SEM and TEM analysis. The evolution of surface morphology, chemistry and structure were studied.

The self healing properties of TCP on AA2024-T3 are assessed and quantified

using the artificial scratch technique in Chapter 5. In this technique, a bare, untreated

surface representing bared metal at the base of a scratch is exposed in close proximity to

a coated surface, separated by a thin layer of electrolyte. This separation allows for

independent assessment of the performance of the bare and coated surface by EIS.

Analysis of the untreated surface by XPS and ICP-OES also provides evidence of

transport of chromium because inhibitor species found on the untreated surface must

have migrated from the coating.

Chapter 6 summarizes the key findings of this project and draws conclusions

as to the coating nucleation and growth, composition and structure, effects of

pretreatment on the coating performance, and mechanism of inhibition on aluminum.

3 Some suggestions are also made for future study and development of the TCP coating.

REFERENCE

1 . I.J. Polmear, Light Alloys Metallurgy of the Light Metals, Third ed., London:

Arnold, 1995.

2 . R.W. Hinton, Metal Finishing, 89 (1991) 55.

3 . P. O'Brien, A. Kortenkamp, Transition Metal Chemistry, 20 (1995) 636-642.

4 . Fedral Register, 69 (October, 2004).

5 . L. Xia, E. Akiyama, G.S. Frankel, R.L. McCreery, J. Electrochem. Soc., 147 (2000)

2556-2562.

6 . A.E. Hughes, J.D. Gorman, P.J.K. Patterson, Corrosion Sci., 38 (1996) 1957.

7 . R.G. Buchheit, A.E. Hughes, Chromate and Chromate-Free Conversion Coatings,

ASM Handbook: Fundamentals, Testing, and Protection, 2003.

8 . D. Galjard, Finishing, 7 (1983) 34.

9 . J.H. Nordlien, J.C. Walmsley, H. Qsterberg, K. Nisancioglu, Surface and Coating

Technology, 153 (2002) 72-78.

5 10 . W.C. Nickerson, E. Lipnickas, TriService Corrosion Conference Proceedings,

(2003).

11 . C. Matzdorf, M. Kane, J. Green, U.S. Patent No. 6,375726, (2002).

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

Aluminum alloy 2024-T3 has been widely used in aerospace applications

because of its superior mechanical properties including strength-to-weight ratio, which

result from the alloying elements such as copper, magnesium, and manganese [1].

However, the corrosion performance of the material can be a problem because the

Chromate conversion coatings (CCCs) have long been employed in the surface finishing process for AA2024-T3 and other metal alloys for their excellent ability to resist localized corrosion and to promote paint adhesion [2]. Chromate coatings are known for their “self-healing” or “active corrosion inhibition” property, which is considered to involve three steps: 1) release of Cr(VI) into a corrosive

7 environment, 2) transport of Cr(VI) through the aqueous phase to a non-coated or

damaged area, and 3) reduction of soluble chromate to insoluble and protective chromium hydroxide [3-11]. The self healing property of CCC along with the excellent protectiveness of chromium hydroxide coatings on light alloys makes CCC a highly

anticorrosion coating [10-14].

One approach to study self healing of protective coatings is the artificial

scratch technique [10]. In this technique, a bare, untreated surface representing bared

metal at the base of a scratch is exposed in close proximity to a coated surface,

separated by a thin layer of electrolyte. This separation allows for independent

assessment of the performance of the bare and coated surface. Analysis of the

untreated surface also provides evidence of transport because inhibitor species found

on the untreated surface must have migrated from the coating.

Due to the toxic and carcinogenic effects of hexavalent chromium compounds

on the environment and human health, there has been increasingly stringent legislation

regarding their use and waste proposal. Consequently, significant efforts have focused

on chromate-free corrosion inhibitor systems that can provide comparable performance

[12]. Among these alternative coatings, zirconium based conversion coatings currently

have wide applications in the container, aluminum appliance, automotive, and

extrusion industries as an alternative to chromate conversion coatings because of their

simple application and good adhesion properties for the paint applied on top [16-20].

One promising candidate from the fluorozirconate coatings family, Trivalent

Chrome Process (TCP), recently has gained wide acceptance. The trivalent chrome

process (TCP) was first developed at NAVAIR and is currently one of the leading

non-chromate conversion coatings on the market as it has been shown to provide

8 excellent corrosion protection and paint adhesion in standardized tests [13]. The TCP

process is a drop-in replacement for hexavalent chromate treatments and it contains no

Cr(VI) in the coating bath and resulting film [14]. The TCP coating systems are similar

to commercial fluozirconate/fluorotianate-based conversion coatings, but are enriched

in Cr(OH)3 and Cr2O3, which improve the coating to provide better protection [13].

In this chapter, general aspects of aluminum alloy 2024-T3 are reviewed with

a focus on metallurgy and microstructure related to corrosion susceptibility. Secondly,

this review summarizes the work related to the two major ways chromate is used for

corrosion protection: chromate ion in solution and chromate conversion coating (CCC).

Chromate pigments are also added to organic primers, but they act by dissolving into

solution. The morphology, structure, composition and protection mechanism of CCC

are discussed. Zirconium based coating is another focus here because these coatings are

similar to TCP coatings. And finally a review is given on TCP including the unique

properties that might be exploited to develop a chromate-free corrosion inhibitor of

AA2024-T3.

2.2 METALLURGY OF ALUMINUM ALLOY 2024-T3

Aluminum alloy 2024-T3 has been widely used since the 1930’s in aerospace

applications because of its superior mechanical properties and distinguished

strength-to-weight ratio resulting from the alloying elements such as copper, magnesium, and manganese [2]. However, the performance of the material can be

greatly compromised because the addition of copper and magnesium leads to the

formation of various intermetallic particles, making the alloy highly susceptible to

9 localized corrosion, especially pitting corrosion and intergranular corrosion.

Aluminum alloy 2024-T3 has a nominal composition of Al, 4.4% Cu, 1.5%

Mg, 0.6% Mn, 0.5% Fe, 0.5% Si, 0.25% Zn, 0.10% Cr, 0.15% Ti and 0.15% other

elements [1]. The addition of copper and magnesium as major alloying elements as

well as a specific heat treatment make the AA2024-T3 a highly strengthened alloy for

aerospace applications. Copper provides both solid solution strengthening and

precipitation hardening [15-16]. The solubility of Cu in aluminum decreases from

5.65 wt% at 548oC to lower than 0.1 wt% at room temperature [2]. The addition of

magnesium accelerates and intensifies the precipitation process. Manganese acts as a strengthener and controls grain size. Chromium and titanium are added primarily as grain refiners while iron and silicon are unwanted impurities in the alloy [15].

The designation “T3” indicates sheet product that was solution heat treated, quenched, cold worked and naturally aged at room temperature to a fully stable condition [2, 16]. Solution-annealing produces a homogeneous single-phase solid solution by uniformly distributing the alloying elements through diffusion. Quenching leads to a thermodynamically unstable condition in which both solute atoms and vacancies are supersaturated. As a consequence, precipitation occurs at dislocations and grain boundaries, which impedes the movement of dislocations and sliding of grain boundaries, providing the aged AA2024-T3 superior mechanical properties [1].

While they greatly improve the mechanical properties of AA2024-T3, the same alloying elements also make the alloy inherently susceptible to localized corrosion because of the formation of micrometer sized second-phase intermetallic particles [1]. These constituent particles form during the original solidification and are not dissolved during the following heat treatment process. The micro-constituents

10 lead to non-uniform attack on the alloy surface resulting in pitting and intergranular

corrosion.

Al-Cu-Fe-Mn particles with irregular shape comprise a major fraction of

IMCs in AA 2024-T3 with chemistry ranging from Al7Cu2Fe, Al12Si(Mn, Fe) to

Al20Cu2Mn3 [2]. The corrosion potential of these second-phase particles is usually

more positive than the aluminum matrix; thus they are potential sites of cathodic reactions [17].

Constituent S phase, round-shaped Al2CuMg, which tends to dominate at

lower Cu/Mg ratios, and θ phase, Al2Cu, which tends to be more predominant at high

Cu/Mg ratio, can also be present in the alloy [18-19]. S phase particles comprise up to

~60% of the particles that are larger than 0.5~0.7 μm on the alloy surface[18].

Investigations have revealed that S phase particles are initially active to the surrounding

matrix and these second phase particles will undergo dealloying, leading to quick

dissolution of Mg, the enrichment of Cu and finally reversal of their galvanic

relationship to the matrix which results in subsequent pitting at their periphery

[20-21].

2.3 SURFACE PRETREATMENT OF ALUMINUM ALLOYS

When aluminum is exposed to air, a thin layer of native oxide is formed on

the surface with nanometer thickness. However, for commercial aluminum, alloying

elements and impurities are always present in the surface, which result in a thinner

oxide film [2]. Thus this naturally formed passive layer is not uniform and is

11 susceptible to localized corrosion [1]. Therefore, it is necessary to treat the surface in certain ways in order to obtain a more protective barrier layer [22]. Prior to the application of various kinds of conversion coatings, it is essential to clean the metal surface physically or chemically to remove the oils, contaminants and surface metal oxides developed during manufacture and transportation. Proper substrate pretreatment is critical for obtaining well-adhering and continuous conversion coatings since pretreatments not only clean the surface but also alter the surface chemistry and morphology [22-23]. High strength aluminum alloys used for aerospace application undergo two pretreatment steps: alkaline cleaning and acidic desmutting [24].

2.3.1 Alkaline cleaning

The importance of surface cleaning procedures cannot be overstated for finishing cycles of almost all types of metal substrates. Alkaline cleaning is widely used in industry for the purpose of removal of oils, organic contaminants, and the oxide layer from the aluminum surface [22, 25]. The pH of alkaline cleaners used for aluminum needs to be carefully adjusted to relatively low alkalinity since aluminum is amphoteric and easily dissolves in alkaline solution. There are two major types of alkaline cleaners: etching and non-etching. Sodium hydroxide is most principally used as an etching cleaner for aluminum due to its efficiency and low cost. On the other hand, the non-etching alkaline cleaners typically contain sodium carbonate, sodium phosphate and sodium silicate [22]. The cleaning periods generally last for several seconds to several minutes. A water break-free surface indicates cleanliness as water rinses off a clean surface without breaking.

12 For the application of TCP coatings, non-etching cleaners were typically used

[26]. The combination of sodium carbonate and sodium metasilicate is a common

non-etching cleaner for aluminum substrates because they provide alkalinity and

buffering at a low cost [27]. A balanced mixture is used for minimum etching attack

of the aluminum surface at the pH 11.1~11.7. The particular reason for addition of

sodium metasilicate is that a protection layer (a mono-molecular layer of hydrated

silica) forms when metasilicate salts react with the aluminum or alumina surface,

which minimizes the etching attack on the base metal [22].

However, this silicate modified layer is not easily removed through rinsing

and is carried over to the next step of surface cleaning on the metal surface [22]. A

layer of insoluble silicic acid is formed when the silicate residues are transferred to an

acid solution. The silicate salt in the alkaline cleaner was reported to inhibit the

formation of TCP coating on aluminum surface [28]. Therefore, sodium

tripolyphosphate is used instead to eliminate the silicate residue after the degreasing

process [26]. The sodium tripolyphosphate provides alkalinity, strong buffering, and

the ability to soften hard water salts. The alkaline cleaning gives rise to a surface

covered by a thin layer of amorphous aluminum oxide containing zinc, magnesium,

silicate and other intermetallic particles that are insoluble in the alkaline solution [22].

2.3.2 Desmutting

In order to remove the smut layer formed during the alkaline cleaning and

insoluble intermetallic particles, a subsequent step called desmutting is usually performed on the aluminum surface. Mixtures of mineral acids, organic acids, and acid

13 salts are commonly used as desmutter. An acid solution containing 25~50 vol% of nitric

acid is frequently employed and can effectively remove normal smut layers after

alkaline cleaning. Nitric acid is also effective when magnesium oxide scale is present.

However, when a silicon compound is present on the metal surface, a small amount of

fluoride is always needed [22]. Sulfuric acid combined with ferric sulphate is also

frequently employed for various alloys. The time of desmutting generally ranges from

several seconds to several minutes.

Previous publications have shown that, prior to the application of chromate

conversion coating, it was particularly important to remove the smut layer and

undesired intermetallic particles from the surface of the Al alloy [29-30]. In a more recent work by Harvey and coworkers, the effect of bromate-nitric acid desmutter on

AA2024-T3 was investigated and proved to be efficient in removal of the oxide and large intermetallic particles from the aluminum surface [39]. Other combinations of mineral acids have also been proven to be effective. For example, Hughes et al. studied the desmutting of AA2024-T3 with various mixture of acids and found hydrofluoric-nitric acid solution produced a surface free of copper-rich smut [31]. After desmutting, surface roughening takes place, leaving behind voids and pits on the metal surface. An over-desmutted surface typically exhibits a dull or matte appearance [22].

2.4 CHROMATE IN SOLUTION

2.4.1 Background

Hexavalent chromium compounds have been used to protect high strength

14 aluminum alloys and other light alloys in the aircraft industry for decades. Chromate

salts are added in desmutters, conversion coatings and paints. Unfortunately, as a result of the wide range of toxic effects on environment and health, there has been increasingly stringent legislation to eliminate it from manufacturing environments

[32-33]. It is believed that the reduction of Cr6+ to Cr3+ in biological bodies and the resulting molecular debris cause critical changes in DNA that can lead to cancer

[34-35]. As a consequence, an environmentally benign replacement for chromate system is strongly desirable [12, 35]. A better understanding of inhibition mechanisms afforded by chromate compounds may assist the development of non-chromate coating alternatives for aluminum alloys.

2.4.2 Speciation

Hexavalent chromate compounds in aqueous solution mainly appear in the

2- 2- following forms: chromate ion (CrO4 ), dichromate ion (Cr2O7 ), and chromic acid

(H2CrO4) [36-37]. The major species present depends on concentration of Cr(VI) and

2- pH values, as shown in Figure 2.1. The dichromate ion (Cr2O7 ) dominates at

2≤pH≤6 for higher concentrations of total hexavalent chromium, whereas for lower

6+ - concentration of Cr at acidic environment (pH 2~4), the bichromate ion (HCrO4 ) is

2- the most abundant species. Chromate ion (CrO4 ) dominates in neutral and alkaline

environments [36]. The tetrahedrally coordinated hexavalent chromium oxoanions are

readily dissolved in aqueous solution, while the trivalent chromium species are

octahedrally coordinated and form stable compounds in water [38].

15 2.4.3 Protection by Soluble Chromate

The corrosion protection by Cr(VI) ions on aluminum has been investigated in detail and several theories have been developed. An in situ Raman study on

AA2024-T3 showed that dilute Cr(VI) with a concentration of 10-3 M was transported

into a corroding pit, resulting in the adsorption of Cr(VI) on Al(OH)x. Electrostatic

bonding between Cr(VI) and cationic Al(OH)x was suggested at low pH, while

covalent bonding was also possible at higher pH. Thus an Al(III) - Cr(VI) mixed

oxide was formed in the pit, which can provide corrosion resistance via mechanisms

such as surface-charge neutralization, and displacement of chloride ion [39]. This is in

agreement with another study in which the dissolution kinetics of aluminum in an artificial crevice cell was studied and the addition of dichromate ions was proved to be incapable of suppressing the active dissolution. Thus protection mechanisms other than the anodic inhibition were suggested [40]. By using a split-cell, Clark et al. found that dilute chromate solution strongly suppressed the oxygen reduction rate on both

Cu and AA2024-T3 by reduction of Cr(VI) followed by formation of a Cr(III) oxyhydroxide monolayer. This monolayer inhibited electron transfer and occupied the active chemisorption sites and thus made Cr(VI) an excellent irreversible cathodic inhibitor for AA2024-T3 in near neutral and alkaline solutions [41-42].

Interaction of chromate with intermetallic particles on AA2024-T3 was also studied. In situ examination by AFM was conducted by Schmutz and coworkers [43].

Addition of 10-4 M dichromate in 0.5 M NaCl was found sufficient to inhibit the dissolution of Al-Cu-Mg particles during AFM scratching, while localized breakdown on the aluminum matrix was observed. With increasing dichromate concentration, the degree of Al-Cu-Mg dissolution was limited although not completely suppressed [43].

16 The localized breakdown depended on the AFM scratching forces applied In a more recent study, the addition of 10 mM chromate to the NaCl solution retarded the dealloying of S-phase particles significantly and restricted the copper redeposition on the alloy surface due to the formation of mixed Cr(III) and Cr(VI) oxide [44].

2.5 CHROMATE CONVERSON COATING ON ALUMINUM

ALLOYS

2.5.1 CCC Processes

Owing to their strong corrosion resistance, chromate conversion coatings have been used for high strength aluminum alloys in the aircraft industry where anodizing is not feasible for some parts [22]. The chromate coating baths in the market differ from each other in chemistry but the major components are generally chromic acid, fluoride and a mixture of ferricyanide and ferrocyanide. The chromate processes can be applied by immersion, spraying and brushing. Typical processes usually include (depending on the manufacturer) an alkaline degreasing and acid desmutting pretreatment, rinse, coat, rinse and dry. The colors of CCCs appear to vary from colorless, iridescent yellow or brown depending on the types of substrates, immersion time and coating thickness [22]. If the coated surfaces require further painting, it is common to do so applied within 24 hours after CCC formation [45].

17 2.5.2 Coating Formation

Chromate conversion coatings when properly applied are spontaneously

formed 0.1 to 2 μm thick films that can provide corrosion resistance and improved paint adhesion of subsequently applied coatings [37]. Coatings are typically formed in an acidic bath with pH 1.2 - 3.0 containing 50 mM chromium trioxide (CrO3), 30 mM

to 40 mM mixture of fluoride salts, and 2 mM to 5 mM potassium ferricyanide

(K3Fe[CN]6) [27, 37].

It is well accepted that electrochemical nature of the chromate coating

formation involves oxidation of aluminum and reduction of chromium (Cr6+) species

[3-4, 46-49]. The formation reactions for CCCs on aluminum are typically described as Equation 1 through 3, producing the overall formation reaction shown in Equation

4 [49-51]:

2Al → 2Al3+ + 6e- (in presence of HF) (1)

3+ + 2Al + 3H2O → Al2O3 + 6H (2)

- + 2- 6e + 8H + Cr2O7 → 2Cr(OH)3 + H2O (3)

+ 2- 2Al + 2H2O + 2H + Cr2O7 → Al2O3 + 2Cr(OH)3 (4)

Katzman and coworkers proposed the first model about CCC formation on aluminum alloys in 1979 [3]. They suggested that the aluminum oxide must be removed to make sure the aluminum metal can contact directly with the chromate coating bath in order to start the deposition of hydrated aluminum and chromium oxide. AlOOH in the mixed oxide is dissolved by fluoride ion in the solution leaving

18 behind the less soluble CrOOH on the coating surface. Thus as film continues to grow,

aluminum at the aluminum/coating interface is oxidized and Al3+ diffuses outward

through the coating, while the chromate ions adsorbed on the coating surface are

reduced to CrOOH and hydrated chromium oxide layer is formed. However, based on

this model it is difficult to understand why there is little aluminum in the CCC.

Brown et al. performed a detailed investigation into CCC nucleation and

growth on high purity aluminum [29, 52]. The authors highlighted the great effects of

spatial separation of the cathodic and anodic reactions on aluminum surface. Their

basic principle was that the natural oxide present on the aluminum surface is not

completely removed but is thinned until appreciable tunneling of electrons becomes

possible. Segregated flaw sites such as grain boundaries and impurities provide easy

paths for electron tunneling and become the preferential sites for cathodic reduction of

chromate.

Xia and coworkers developed newer interpretations of CCC formation on

aluminum alloys based on a sol-gel mechanism [7-9], as illustrated in Figure 2.2.

According to their explanation, CCCs form by sol-gel processes involving reduction,

hydrolysis, polymerization, and condensation of the Cr(III) species. A hydrated Cr(III) oxy-hydroxide barrier coating is formed to about 200-300 nm in thickness [53]. Cr(III) polymerization reaction occurring within the first several minutes of CCC formation is promoted by the relatively high concentration of Cr(III), and the Cr(VI) is bound to the Cr(III) oxy-hydroxy polymer during this process [54-55]. These condensation and polymerization reactions in company with dehydration would continue during air during drying, as shown in Figure 2.3. This Cr(III) backbone also serves as a host for

Cr(VI). Chromate ions adsorbed onto the matrix of Cr(III) oxy-hydroxide with

19 covalent Cr3+-O-Cr6+ linkage can later be released. It is believed that the equilibrium

Cr(VI) concentration is related to the formation of reversible bonds of Cr(VI) to Cr(III) according to an expression similar to a Langmuir isotherm [10]. While high solution concentration of Cr(VI) in the coating bath help the Cr(VI) binding, forming a mixed oxide that is approximately 25% Cr(VI) and 75% Cr(III) [7, 56], release of Cr(VI) from a CCC is favored with low concentration of Cr(VI) in field application.

The formation of CCC was found to exhibit two-stage growth in experiments with electrode arrays [48]. In the first stage (~30 s), CCC formation is characterized by intense measurable electrochemical activity. In the remaining period of CCC formation up to 300 s, a quiescent condition occurs during which little measurable electrochemical activity is detected. However, coating evolution continues during the second stage based on Raman spectroscopy results, which indicates an increase of

Cr(VI) concentration in the coating. Previous work has shown that spherical particles with a diameter of 2~4 μm are precipitated on aluminum surface and the initial high rate of growth of the coating is decreased when the surface is covered by a successive layer [57]. The electrochemical quiescent second stage of CCC growth suggests chemical reactions such as sol-gel processes predominate in this period [27].

2.5.3 Structure and Composition

Many authors have studied the structure of chromate conversion coatings formed on aluminum alloys. The coating layer is typically composed of layers of small spherical particles within the range of tens of nanometers and is non-uniform over the entire surface [51, 57-58], as illustrated in Figure 2.4. The larger particles

20 tend to nucleate in voids on the surface left by the cleaning pretreatment or close to

second phase particles [59]. The particulate nature of the coating increases the surface

area which leads to promotion of adhesion [60]. An XPS study showed the chromate

film consists of a layer of hydrated chromium oxide, Cr2O3⋅xH2O (~30 nm), covered

by a chromium ferricyanide monolayer (~2 nm) [30]. TEM micrographs of ultramicrotomed sections of 5 min CCC on AA2024-T3 showed the coating thickness was about 80 nm, as shown in Figure 2.5 [61]. Fluoride is present at the interface between the conversion coating and the metal matrix [30, 62]. X-ray absorption spectroscopy (XAS) showed that the coating was composed of amorphous chromium hydroxide (Cr(OH)3), and partial dehydration occurred during drying leading to the

formation of chromium oxide (Cr2O3). A small portion of Al2O3 was also present [63].

In a more recent study, a duplex layered structure composed of an external thick

porous layer and inner thin dense barrier layer was suggested [64]. X-ray absorption

near-edge spectroscopy (XANES) minimizes the risk of Cr(VI) photodecomposition

reduction, which can occur during XPS analysis in ultrahigh vacuum, and thus provides precise evaluation of Cr(VI) content in CCCs [6, 65-66]. CCCs contain a significant amount of Cr(VI), ~20%, based on the XANES spectra and the concentration of hexavalent chromium decreases exponentially after exposure to the

NaCl environment [46]. Based on characterization of CCC by Raman spectroscopy,

Xia and coworkers developed a model consisting of Cr(OH)3 polymer with linkage to

Cr(VI) in the outer portions of the coating [8, 11]. It is also suggested that the Cr/Al

and Cr(VI)/Cr(III) ratios increase as the conversion coating thickens with the Cr(VI)

to total chromium ratio up to 40% in thicker coatings. Dehydration and resultant

cracks as a result of high levels of thermal stress during the drying step of the layer

21 are observed by scanning electron microscopy (SEM) and atomic force microscopy

(AFM) [58, 60, 64, 66].

Intermetallic particles on the aluminum surface result in nonuniformity of

CCCs. It was reported that the formation of chromate conversion coatings over

second phase particles is slower than on the surrounding aluminum matrix and results in thinner films with lower Cr content in thickness order: matrix > copper-rich particles > iron-rich particles [50]. It has been observed that essentially no Cr(VI) is involved in the coating formed over Cu-rich intermetallic particles, while some Cr was detected on Fe-rich phases [67]. However, on a high purity aluminum substrate,

CCCs grow faster along grain boundaries than the matrix after short time immersion leading to a thicker deposit with higher Cr content [52]. This enhancement is due to ready electron transfer along the grain boundaries, which facilitates the Cr(VI) reduction. AFM showed that the nucleation and growth of CCCs on the two major second phase particles are remarkably different. The coating deposition is faster on

Cu-Mn-Fe rich phases than Al matrix while Cu-Mg intermetallic particles tend to inhibit the coating formation [51].

2.5.4 Activators and Accelerators

Additives acting as alternate activators and accelerators are often involved in commercial CCC formulations to optimize the formation of conversion coatings [30,

35, 75]. CCC formation is greatly assisted by additions of fluoride, nitrate, and sulfate ions. Fluoride species such as sodium fluoride (NaF) and mixed-metal fluoride salts are particular effective and commonly added to CCC baths to a concentration of 30

22 mM to 40 mM [27]. The fluoride species are effective because they can help dissolve the native aluminum oxide film, complex aluminum ions in solution, promote the oxidation of the aluminum surface by Cr6+ and result in higher chromium content in

the coating [3].

The presence of potassium ferricyanide (K3Fe[CN]6) as an “accelerator” in

commercial formulations with a concentration of 2 to 5 mM is crucial for the coating

growth [27]. It has been reported that ferricyanide can increase the thickness of

coating thus resulting in enhanced corrosion resistance [62]. It was suggested that

ferricyanide functions as a redox mediator for CCC to increase the reaction kinetics. It

3- was observed that aluminum was rapidly oxidized by ferricyanide (Fe[CN]6 ) while

4- ferricyanide was readily reduced to ferrocyanide (Fe[CN]6 ). At the same time, Cr(VI)

is reduced to Cr(III), while ferrocyanide is quickly oxidized back to ferricyanide [9],

as illustrated in Figure 2.6. However, studies have also shown that the formation of

CCCs over second phase particles might be inhibited because cyanide might easily

complex with copper on copper-rich inclusions in Al-Cu-Mg alloys, which suppresses

the reduction of Cr(VI) and thus the coating growth [4, 76].

2.5.5 Corrosion Protection

2.5.5.1 Cathodic Inhibition

In order to consume electrons generated by the oxidative half reaction and

maintain charge neutrality, a cathodic reduction must occur simultaneously for

corrosion to take place. Therefore, inhibition of the cathodic reduction reaction can

23 be extremely effective at reducing the rate of corrosion. It has been shown that the

rate of the primary cathodic reaction, oxygen reduction, is greatly reduced by the

reduction of Cr(VI) to Cr(III) and irreversible formation of a Cr(III) layer on Al alloy

surfaces [42].

In high strength Al alloys, S phase intermetallic particles are attacked

resulting in non-Faradic Cu release, which is followed by replating and preferential

dissolution of aluminum from the matrix, leading to a Cu-rich surface [18]. Intact

Cu-rich intermetallic particles may also serve as catalytic sites for the O2 reduction

reaction at rates depending on catalyst surface area, chemisorption energy and kinetics,

and accompanying solution reactions. Although there may be extremely different

mechanisms on varied metals, it is generally regarded that chemisorption is a critical

step to catalyze O2 reduction, and Cu enrichment over large areas of the surface will

increase the cathodic current and drive corrosion [68-73]. A detailed investigation

showed that Cr(VI) adsorption to catalytic sites and rapid irreversible reduction to

Cr(III) hydroxide surface layer, which is strongly bonded to the surface, would reduce

the activity of cathodic sites (possibly Cu-rich IMC particles), permanently interfere with O2 reduction and thus provide corrosion protection [41]. Because of the

inhibition of further chromate reduction as well as oxygen reduction, only

near-monolayer of Cr(III) species forms under certain conditions [41-42]. There is

general agreement that mobility, adsorption and irreversible reduction of Cr(VI) on

potential cathodic sites are major aspects of corrosion protection provided by

chromate conversion coatings. The split cell technique was employed to study the

cathodic inhibition of CCC [41]. The oxygen reduction rate was lowered by two

orders of magnitude on a copper cathode due to the presence of Cr(VI), while no

24 obvious effect was observed on the anodic current at the aluminum electrode. This

was in agreement with the conclusion of Ilevbare et al. based on their studies on

oxygen reduction reaction kinetics on intermetallic particles [74].

Other measurements supported cathodic inhibition by showing that addition

of 10-5 M dichromate ions to 1 M NaCl solution significantly suppressed the cathodic

current on AA2024-T3 [75]. Pit growth rate measurements using the foil penetration

technique showed that dichromate additions had little effect on pit growth rate at

applied anodic potentials, but small concentrations of dichromate effectively inhibited

open circuit pit growth, apparently by inhibiting the cathodic reaction [75].

Electrochemical measurements indicated cathodic protection of CCCs was greatly lost

during aging, which possibly resulted from dehydration of the coating and the

subsequent immobilization of Cr(VI) in the coating [49].

2.5.5.2 Anodic Inhibition

The Cr (III) layers have also been observed to act as a modest anodic inhibitor in several studies, though the extent depends on various environmental factors such as Cl/Cr ratio, pH and alloy substrate types. The anodic inhibition of localized corrosion by CCCs results from the reduction of the initiation but not propagation stage of the localized attack. It is shown that propagation rate of localized

corrosion in aluminum alloys is not reduced with the addition of chromate [76]. The

results of artificial crevice electrode experiments revealed that inhibition of anodic

dissolution in an active pit or crevice was not the key component of the inhibition

mechanism of localized corrosion of Al alloys by CCCs [85-86]. Under some

25 conditions, chromate can actually serve as an oxidizing agent and result in an increase in the rate of localized corrosion [77].

Pride and coworkers have shown that soluble chromate is capable of

decreasing the rate of metastable pitting, the magnitude of metastable pitting events,

the growth rate of individual pits, and the apparent pit current density; hence the possibility that metastable pits grow into stable pits before repassivation is largely reduced [78].

There is also evidence that the breakdown potential required to form pits is increased by chromate [43, 79]. The pitting potential, which is the potential at which breakdowns of the protective film become stable sites of attack, can act as a comparative gauge of localized corrosion resistance, where an increased or more noble pitting potential is related to better resistance to pitting attack [80]. The pitting potential of AA2024-T3 in 0.1 M Na2SO4 + 0.005 M NaCl was significantly raised by

30 s CCCs from -0.4 V vs SCE to -0.2 V vs SCE, which was likely due to film

stabilization by the coating rather than suppression of the oxygen reduction reaction

kinetics [79].

The release of Cr(VI) from CCCs and migration into active pit sites are

believed to increase the pitting potential [27, 78]. Adsorption of Cr(VI) to Al(OH)X within the pit and the subsequent formation of Al/Cr mixed oxide are responsible for the decrease in the nucleation rate of metastable pitting and the pit growth rate, although the detailed mechanisms are not completely clear. One possible reason is that local pH within the pit is increased by the reduction of Cr(VI) and consumption of H+. It was also suggested that cation charge on the oxide surface is neutralized by

Cr(VI) adsorption preventing the further adsorption of chloride ions [39].

26 Special care has to be taken of when studying the anodic inhibition of CCC

because breakdown potentials tested using electrode arrays after 2 min CCC coating

were raised about 300 mV higher than bare Al while a similar experiment on 1 cm2 bulk sample suggested no change of breakdown potential [48]. Therefore, the electrode area, alloy purity, and environmental aggressiveness such as the chloride concentration should be taken into consideration when determining pitting potential in anodic polarization measurements [48].

2.5.5.3 Self-healing

It is well accepted that chromate coatings not only contain hexavalent chromium but also have a “self-healing” or “active corrosion inhibition” property that is considered to involve three steps: 1) release of Cr(VI) into a corrosive environment,

2) transport of Cr(VI) through the aqueous phase to a non-coated or damaged area, and 3) reduction of soluble chromate to insoluble chromium hydroxide to resist further attack [4-7, 9-11]. The additional property of storage and release of Cr(VI) by

CCC in addition to the excellent performance of chromium hydroxide coatings on light alloys makes CCC a highly anticorrosion coating [12-14]. The important

“self-healing” behavior of CCCs is attributed to the storage, release, and migration of

Cr6+. The Cr(VI) reservoir in CCC has been thought to be on the order of 10-7 mol/cm2 depending on the coating weight and thickness [11]. A spectroscopic study

indicated that exposure to water or salt solution causes the covalent Cr(III)-O-Cr(IV)

bond in the matrix of Cr (III)/Cr(VI) mixed oxide to break, as described by the

following equation [11]:

- 2- + Cr(III)-O-Cr(VI)O3 (s) + H2O → Cr(III)-OH(s)+Cr(VI)O4 (aq) + H (aq) (5)

Then the Cr(VI) is released from a CCC into aqueous surface films or bulk solution,

to a level of approximately 10-4 M [10-11]. The low solution concentration of Cr(VI)

anticipated in the environment promotes the release of Cr(VI) from a CCC [10]. It is

proposed that the Langmuir-like adsorption behavior is followed during the release of

chromate from a CCC depending on solution ionic strength and the ratio of CCC

surface area to solution volume [11]. In addition, it is also shown in Equation 5 that

the Cr (III)/Cr(VI) equilibrium is pH dependent with chromate uptake at low pH and

2- - release at high pH. This Cr(VI) in solution, usually in the form of CrO4 or HCrO4 can migrate to damaged locations in the coating and interact with bare metal.

Accumulated Cr(VI) at susceptible sites can act as another reservoir for further protection since the chromate and its reduction products can greatly depress the cathodic reaction [10].

The ability to release Cr(VI) from the CCCs is gradually lost during exposure at ambient temperature, which suggests that the self-healing property of CCCs is temporary [81]. In the study based on the use of extended X-ray absorption fine structure (EXAFS) measurement and EIS on aged CCCs, significant loss of corrosion resistance was observed after 20 days. In addition, a shortening in Cr(III)-Cr(III) nearest neighbor distances was observed by EXAFS, which indicated consolidation of the coating backbone. Immobilization of Cr(VI) and subsequent loss of corrosion resistance resulted from the shrinkage associated with dehydration [81].

28 2.6 ZIRCONIUM BASED CONVERSION COATINGS

Zirconium forms a stable oxide in its highest oxidation state. Patents exist

based on the peroxo complexes and acid fluorides, which stabilize at low pH [82-83].

Nowadays, zirconium based conversion coatings have wide applications in the

container, coil coating, aluminum appliance, automotive, extrusion industries as an

alternative to chromate conversion coatings because of their simple application and

good adhesion properties for the subsequent paint layer [37, 84-86]. In addition, these

coatings have been used in aircraft industry for space shuttles and fighter jet

applications [37].

2.6.1 Coating Morphology and Composition

The conversion coating bath is typically composed of an acidic hexafluoro

metal complex (H2ZrF6) and sequestering agents to prevent precipitation of aluminum dissolved during the pretreatment process [87-88]. The coating bath is usually

operated at room temperature under acidic (pH < 4) conditions. In current versions,

polymer compounds, such as polyacrylic acid, ammonium polyacrylate, phenolic

resins, are the common additives to the coating solution to obtain a base for

subsequent painting and to improve corrosion resistance [37, 82]. A small amount of

HF is also added to activate the alloy surface by removing the naturally formed oxide

film [85, 89-91]. Recently, a product with a small amount of component containing Si

and Cu was proved to provide comparable corrosion resistance to phosphate

conversion coatings when coated with an organic paint overcoat. [88].

29 Zirconium based conversion coatings were found to have a multilayer

structure by the use of a variety of complementary surface analysis techniques such as

AES, XPS and SIMS [86, 92]. Without the presence of polymer additives, the coating

was believed to contain an aluminum oxide layer close to the metal interface and an

outer layer composed of zirconium oxide with a mixed Zr/O/F layer on top of it, as

shown in Figure 2.7 [37, 92-93]. When the polymeric component was added, instead

of ZrO2 and Zr/O/F layer, the structure of a complex hydrated oxyfluoride containing

Zr, Ti and Al was detected. The polymer compound usually added into the conversion bath was found to be concentrated toward the surface [94]. When Cu is present in the formula, it is enriched at some spots with a concentration up to 60wt% [88].

A colorless layer with a thickness of 10-25 nm is formed on Al alloy surfaces after 10 seconds of immersion in the coating bath and only a minor increase of thickness is observed after extending the immersion time [94, 99, 102]. A similar coating thickness, ~ 30 nm, is found on the surface of cold rolled steel [88]. It has been concluded that the conversion coating is formed in the following manner [86].

The native aluminum oxide on the metal surface is removed or thinned by fluoride ion in solution and becomes hydrated. The more stable aluminum fluoride bonds substitute the aluminum-OH bonds quickly, presumably with the help of the net positive charge on the alumina surface at low operating pH. A mixed oxyfluoride containing Zr thus begins to develop. The deposition reaction is described as follows

[16, 103] :

4+ + Zr +3H2O → ZrO2 ·H2O+ 4H (6)

30 The presence of intermetallic particles greatly influences the formation of Zr

conversion coatings. It was found that nucleation of the Zr based conversion coatings

on AA6061 occurred preferentially on cathodic intermetallic particles due to their

electrochemical nature [87, 95]. The cathodic activity of these particles was then

decreased by the lateral growth of the film and further deposition of the coating was

impeded resulting in a surface that was not completely covered by the conversion

layer.

The Zr based coatings can be applied by a non-rinse technique or through conventional processes (e.g. rinse, immersion or spray) [90, 96]. Although the Zr based conversion process is being used as a replacement for phosphate conversion coatings in the automotive industry, the use of these Zr based coatings to replace

CCCs in the aerospace application could be limited due to their inability to provide active corrosion protection [27].

2.6.2 Electrochemical Behavior

In a study based on fluorotitanate/zirconate coating on AA6061, only slight

cathodic inhibition and no anodic protection was observed in 0.1 M NaCl solution,

which was likely resulted from poor deposition on cathodic Al(Fe,Mn)Si particles

[97]. Zirconium oxide is formed on magnesium substrates after treatment in

zirconium oxychloride bath. The solution with 20 g/L zirconate produces coatings

with the best corrosion resistance according to the EIS and polarization measurements,

which is likely due to the blocking of active sites from chloride ions [98].

31 2.7 TRIVALENT CHROME PROCESS (TCP) COATINGS

2.7.1 Background

Trivalent chrome process (TCP) coatings are currently among the leading

non-chromate conversion coatings on the market and have shown to provide excellent

corrosion protection and paint adhesion in standardized tests [13]. The TCP process is

a drop-in replacement for hexavalent chromate treatments as it is a simple immersion

process, but it contains no Cr(VI) in the coating bath and resulting film [14].

Agarwala and colleagues first reported corrosion resistant coatings formed on

aluminum alloys by immersion in aqueous solutions containing Cr2(SO4)3 and

Na2SiF6 or NaF [99]. The current coating systems are similar to commercial

fluozirconate based conversion coatings, but are enriched in Cr(OH)3 and Cr2O3, which improve the coating to provide better protection and good paint adhesion [13].

TCP is currently the only chromate-free product that meets the requirements according to Class 1A of MIL SPEC MIL-ETL-5541F, Chemical Conversion

Materials for Coating Aluminum and Aluminum Alloys [28].

2.7.2 Formation

Pearlstein and Agarwala developed corrosion resistant coatings containing exclusively Cr(III) by immersion of aluminum samples in aqueous bath containing chromic sulfate (Cr2(SO4)3), fluoride ion from compounds such as fluosilicate

(Na2SiF6) or fluoride (NaF), and having a pH range of 3.3 to 5.5 [99-100]. An alkali

was added near or slightly beyond the precipitation of the insoluble basic compounds.

32 Tan-colored films were produced on aluminum substrate after 10 min immersion in the trivalent chromium bath, violet films after 20 minutes and blue after 40 minutes

[99]. Although the detailed mechanism of the film formation is complex and not well understood, film formation may initiate with the attack at the aluminum oxide film by fluoride ions resulting in an increased pH. The local alkalization leads to the precipitation of insoluble hydrous chromic oxides and finally hydrated chromium hydroxide on the aluminum surface. A sol-gel process similar to the formation of chromate conversion coatings was also proposed during which hydrolyzed Cr(III) ions were polymerized [27]. A post-treatment with peroxide or permanganate to further improve the corrosion resistance for aluminum substrate was suggested [100].

However, the formation of Cr(VI) species in the conversion coating may occur after treatment in these oxidizing baths, which should be avoided.

The current version of TCP coatings are different from the original one.

Alodine T5900 developed by Henkel Corporation is one of the typical TCP products that are commercially available nowadays. The coating bath contains zirconium fluoride salt and chromium (III) compound [101]. The coatings are based on formation of hydrated zirconium oxide and trivalent chromium oxide in solution, which is driven by local alkalization due to dissolution of aluminum oxide layer and cathodic reactions at Cu-rich IMCs [102]. The structure of these TCP coatings was proposed, as shown in Figure 2.8 [13]. Metalast TCP-HF is another TCP product patented by NAVAIR. The formation of this TCP coating on thin AA2024-T3 film was studied by Neutron reflectivity and X-ray reflectivity [103]. Layered structure was proposed and the composition of the film was suggested as Cr2O3 2.10H2O

0.85 (ZrO2 1.60H2O).

2.7.3 Corrosion Inhibition and Adhesion

For the original version of TCP developed by Agarwala, coatings formed on

AA7075-T6 and AA2024-T3 after 10 min immersion provided corrosion protection of more than 96 hours when exposed in 5% NaCl salt spray [100]. When post-treated by oxidizing agents, the corrosion resistance of the coatings for aluminum substrates extended over 168 hours. For the coating which is post-treated with peroxide and leached for 30 min, release of hexavalent chromium is detected which is responsible for the self-healing of such surface [99].

The current trivalent chromium conversion coatings based on fluozirconate are different from the original one. Characterization of these TCP coatings focusing on the level of hexavalent chromium revealed no detectable Cr (VI) was present in the

TCP solution or on the coatings formed on aluminum substrate [14]. The current TCP coatings are the only non-chromate products that have been qualified according to

Class 1A of MIL SPEC MIL-DTL-5541F [13]. Investigations of corrosion protection on aluminum alloys by EIS measurements revealed that AA7075-T6 was well protected by TCP with corrosion resistance similar to CCCs while only poor protection of AA2024-T3 was observed, perhaps due to the Cu-rich intermetallic particles on the surface [13]. The anodic and cathodic current density was increased a full order of magnitude in polarization curves on AA2024-T3 compared to CCCs.

Coating performance has shown to be enhanced after the removal of the Cu-rich layer by desmutting. Modified TCP formulations have been reported to provide better protection [104]. Excellent paint adhesion performance of TCP coatings on anodized

34 aluminum which is comparable to CCCs is rated according to ASTM D3359 Test

Method A and ASTM D 714 [105].

2.8 KEY UNSOLVED ISSUES

For the past 30 years, a considerable amount of effort has been dedicated in the development of environmentally friendly alternatives to chromate conversion coatings. In this literature review, the background of aluminum metallurgy, chromate conversion coatings, zirconium based coatings and TCP coatings is provided, which is required to understand several aspects related to TCP formation, structure, composition and corrosion protection. The TCP coatings are currently used in military and industry applications as a drop-in replacement to CCCs, however, lack of fundamental understanding of the coatings raises critical issues.

First, a detailed understanding of TCP coating composition and structure is needed. It is essential to understand TCP formation process on aluminum compared to the growth of CCCs. The distribution of elements in the coating is of great interest and characterization of the coating surface and cross-section would help understand the composition and structure of the TCP coating. Additionally, this would give a better idea about the corrosion protection mechanism. In order to obtain the most adhering and corrosion resistant coating, the use of proper surface pretreatment is critical. It is imperative to choose the alkaline cleaner and acidic desmutter that promote uniform coating formation.

Secondly, although several alternatives to CCCs have been reported, none of these are comparable to the current chromate technology because most of them are

35 merely adhesion promoters and provide poor corrosion inhibition for bare applications.

TCP has been proven effective on aluminum substrate but, with only Cr(III) in the film, how comparable protection to CCCs is achieved needs to be investigated. More importantly, there are conflicting reports on the self-healing property of TCP of exposed Al substrate. With the absence of mobile Cr(VI) in the coating bath and the coating, the mechanism for such protection needs to be assessed. It is of great importance to determine whether the chromium is released from the coating and the oxidation states in which it exists.

Figure 2.1. Chromate aqueous chemistry as a function of pH and concentration [36].

Figure 2.2. (a) Mechanism for Cr(OH)3 “backbone” formation. (b) Condensation of Cr(VI) on the Cr(III) backbone [8].

Figure 2.3. SEM of shrinkage cracking in a CCC on 2024-T3 [105].

Figure 2.4. AFM image of a CCC formed on the surface of a freshly polished AA2024-T3 after exposure for 3s to a commercial CCC solution at 5oC [51].

Figure 2.5. Transmission electron micrograph of cross-sectional area of 5 min CCC on 2024-T3 [61].

Figure 2.6. Schematic illustration of the formation of chromate conversion coatings [106].

Figure 2.7. Schematic illustration of the structure of Zr/Ti based coatings [37].

Figure 2. 8. Schematic illustration of the structure of Trivalent Chrome Process Coatings [13].

REFERENCES

1 . I.J. Polmear, Light Alloys Metallurgy of the Light Metals, Third ed., , London: Arnold, 1995.

2 . J.E. Hatch, Aluminum: properties and physical metallurgy, American Society for Metals, Metals Park, Ohio, 1984.

3 . H.A. Katzman, G.M. Malouf, R. Bauer, G.W. Stupian, Applications of Surface Science, 2 (1979) 416-432.

4 . P.L. Hagans, C.M. Haas, Surf. Interface Anal., 21 (1994) 65-78.

5 . T. Drozda, E. Maleczki, Journal of Radioanalytical and Nuclear Chemistry, 95 (1985) 339-349.

6 . K. Asami, M. Oki, G.E. Thompson, G.C. Wood, V. Ashworth, Electrochim. Acta, 32 (1987) 337-343.

7 . G.S. Frankel, R.L. McCreery, The Electrochemical Society Interface, 10 (2001) 34.

8 . L. Xia, R.L. McCreery, J. Electrochem. Soc., 145 (1998) 3083-3089.

9 . L. Xia, R.L. McCreery, J. Electrochem. Soc., 146 (1999) 3696-3701.

10 . J. Zhao, G. Frankel, R.L. McCreery, J. Electrochem. Soc., 145 (1998) 2258-2264.

11 . L. Xia, E. Akiyama, G.S. Frankel, R.L. McCreery, J. Electrochem. Soc., 147 (2000) 2556-2562.

12 . A. Nylund, Aluminum Transactions, 2 (2000) 121-137.

13 . W.C. Nickerson, E. Lipnickas, TriService Corrosion Conference Proceedings, (2003).

45 14 . A. Iyer, W. Willis, S. Frueh, W. Nickerson, A. Fowler, J. Barnes, L. Hagos, J. Escarsega, J. La Scala, S. Suib, Plating and Surface Finishing, (May 2010) 32-42.

15 . L.F. Mondolfo, Aluminum Alloys: Structure and Properties, London, U.K.: Butterworths, 1976.

16 . W.F. Smith, Structure and Properties of Engineering Alloys, McGraw Book Company, 1981.

17 . R.G. Buchheit, J. Electrochem. Soc., 142 (1995) 3994-3996.

18 . R.G. Buchheit, R.P. Grant, P.F. Hlava, B. McKenzie, G.L. Zender, J. Electrochem. Soc., 144 (1997) 2621.

19 . P. Leblanc, G.S. Frankel, J. Electrochem. Soc., 149 (2002) B239-B247.

20 . R.G. Buchheit, L.P. Montes, M.A. Martinez, J. Michael, P.F. Hlava, J. Electrochem. Soc., 146 (1999) 4424-4428.

21 . R.G. Buchheit, M.A. Martinez, L.P. Montes, J. Electrochem. Soc., 147 (2000) 119-124.

22 . S. Wernick, R. Pinner, P.G. Sheasby, The surface treatment and finishing of aluminum and its alloys, 5th ed., Ohio : ASM International ; Teddington : Finishing, 1987.

23 . A.E. Hughes, R.J. Taylor, K.J.H. Nelson, B.R.W. Hinton, L. Wilson, Mater. Sci. Technol., 12 (1996) 928-936.

24 . R.G. King, Treatment and Finishing of Aluminium Pergamon Press, Headington Hill Hall, Oxford OX3 0BW, England, 1988.

25 . G.J. Cormier, ASM Handbook, Surface Engineering, Alkaline Cleaning, Volume 5.

26 . Henkel, Turco 6849, Aqueous Alkaline Degreaser, Material Safety Data Sheet, ID: 238832.

27 . M.W. Kendig, R.G. Buchheit, Corrosion, 59 (2003) 379-400.

28 . W. Nickerson, Personal communication, (2011).

46 29 . G.M. Brown, K. Shimizu, K. Kobayashi, G.E. Thompson, G.C. Wood, Corrosion Sci., 34 (1993) 1045-1054.

30 . J.A. Treverton, N.C. Davies, Metals Technology, 4 (1977) 480-489.

31 . A.E. Hughes, G. Theodossiou, S. Elliott, T.G. Harvey, P.R. Miller, J.D. Gorman, P.J.K. Paterson, Mater. Sci. Technol., 17 (2001) 1642-1652.

32 . Fedral Register, 69 (October, 2004).

33 . Final NCMS Report 0273RE95, National Center for Manufacturing Sciences, (1995).

34 . P. O'Brien, A. Kortenkamp, Transition Metal Chemistry, 20 (1995) 636-642.

35 . Toxicological Profile for Chromium, ATDSR/TP-88/10, , in: U.S.P.H.S. Agency for Toxic Substance, Washington, DC (Ed.), 1989.

36 . C.F. Baes, R.E. Mesmer, The Hydrolysis of Cations, Robert E. Krieger Publishing Company, Inc, Malabar, Fl, 1986.

37 . R.G. Buchheit, A.E. Hughes, Chromate and Chromate-Free Conversion Coatings, ASM Handbook: Fundamentals, Testing, and Protection, 2003.

38 . F.A. Cotton, G. Wilkinson, Advanced inorganic chemistry 5th ed., New York : Wiley, 1988.

39 . J.D. Ramsey, R.L. McCreery, J. Electrochem. Soc., 146 (1999) 4076-4081.

40 . E. Akiyama, G.S. Frankel, J. Electrochem. Soc., 146 (1999) 4095-4100.

41 . W.J. Clark, J.D. Ramsey, R.L. McCreery, G.S. Frankel, J. Electrochem. Soc., 149 (2002) B179-B185.

42 . W. Clark, R.L. McCreery, J. Electrochem. Soc., 149 (2002) B379-386.

43 . P. Schmutz, G.S. Frankel, J. Electrochem. Soc., 146 (1999) 4461-4472.

44 . A. Kolics, A.S. Besing, A. Wieckowski, J. Electrochem. Soc., 148 (2001) B322-B331.

45 . U.S. MILITARY SPECIFICATION, MIL-C-5541E, (1990).

47 46 . M.W. Kendig, A.J. Davenport, H.S. Isaacs, Corrosion Sci., 34 (1993) 41-49.

47 . B.L. Hurley, R.L. McCreery, J. Electrochem. Soc., 150 (2003) B367-B373.

48 . W. Zhang, B. Hurley, R.G. Buchheit, J. Electrochem. Soc., 149 (2002) B357-B365.

49 . A.E. Hughes, R.J. Taylor, B.R.W. Hinton, Surf. Interface Anal., 25 (1997) 223-234.

50 . P.L. Hagans, C.M. Haas, "Chromate Conversion Coatings," in Surface Engineering, in: Materials Park, ASM International, OH, 1987, pp. 405.

51 . J.R. Waldrop, M.W. Kendig, J. Electrochem. Soc., 145 (1998) L11-L13.

52 . G.M. Brown, K. Shimizu, K. Kobayashi, G.E. Thompson, G.C. Wood, Corrosion Sci., 33 (1992) 1371-1385.

53 . J.H. Osborne, Prog. Org. Coat., 41 (2001) 280-286.

54 . J. A. J. Aldykiewicz, A.J. Davenport, H.S. Isaacs, J. Electrochem. Soc., 143 (1996) 147-154.

55 . J. A. J. Aldykiewicz, H.S. Isaacs, A.J. Davenport, J. Electrochem. Soc., 142 (1995) 3342-3350.

56 . J. Sinko, Prog. Org. Coat., 42 (2001) 267-282.

57 . D.J. Arrowsmith, J.K. Dennis, P.R. Sliwinski, Trans. Inst. Met. Finish., 62 (1984) 117.

58 . W.J. Wittke, Metal Finishing, 87 (1989) 24-26.

59 . J.A. Treverton, M.P. Amor, J. Mater. Sci., 23 (1988) 3706-3710.

60 . J.A. Treverton, M.P. Amor, Transactions of the Institute of Metal Finishing, 60 (1982) 92.

61 . X. Sun, R. Li, K.C. Wong, K.A.R. Mitchell, T. Foster, J. Mater. Sci., 36 (2001) 3215-3220.

62 . J.A. Treverton, N.C. Davies, Surf. Interface Anal., 3 (1981) 194.

48 63 . F.W. Lytle, R.B. Greegor, G.L. Bibbins, K.Y. Blohowiak, R.E. Smith, G.D. Tuss, Corrosion Sci., 37 (1995) 349-369.

64 . P. Campestrini, G. Goeminne, H. Terryn, J. Vereecken, J.H.W.d. Wit, J. Electrochem. Soc., 151 (2004) B59-B70.

65 . G.P. Halada, C.R. Clayton, J. Electrochem. Soc., 138 (1991) 2921-2927.

66 . D.C. Koningsberger, R. Prins, X-ray absorption : principles, applications, techniques of EXAFS, SEXAFS, and XANES, New York : Wiley, 1988.

67 . M.Jaime Vasquez, G. P. Halada, C.R. Clayton, J.P. Longtin, Surf. Interface Anal., 33 (2002) 607-616.

68 . A. Kolics, A.S. Besing, P. Baradlai, A. Wieckowski, J. Electrochem. Soc., 150 (2003) B512-B516.

69 . M. Kendig, C. Thomas, J. Electrochem. Soc., 139 (1992) L103-L104.

70 . A.E. Hughes, J.D. Gorman, P.J.K. Patterson, Corrosion Sci., 38 (1996) 1957.

71 . A. Aballe, M. Bethencourt, F.J. Botana, M.J. Cano, M. Marcos, Mater. Corros., 53 (2002) 176-184.

72 . A.E. Hughes, J.D. Gorman, P.R. Miller, B.A. Sexton, P.J.K. Paterson, R.J. Taylor, Surf. Interface Anal., 36 (2004) 290-303.

73 . B.R.W. Hinton, D.R. Arnott, N.E. Ryan, Metals Forum, 7 (1984) 211-217.

74 . G.O. Ilevbare, J.R. Scully, J. Electrochem. Soc., 148 (2001) B196-B207.

75 . A. Sehgal, G.S. Frankel, B. Zoofan, S. Rokhlin, J. Electrochem. Soc., 147 (2000) 140-148.

76 . D.R. Arnott, N.E. Ryan, B.R.W. Hinton, B.A. Sexton, A.E. Hughes, Appl. Surf. Sci., 22/23 (1985) 236-251.

77 . C. Wang, F. Jiang, F. Wang, Corrosion, 60 (2004) 237-243.

78 . S.T. Pride, J.R. Scully, J.L. Huston, J. Electrochem. Soc., 141 (1994) 3028-3039.

79 . G.O. Ilevbare, J.R. Scully, J. Yuan, R.G. Kelly, Corrosion, 56 (2000) 227-242.

49 80 . D.A. Jones, Principles and Prevention of Corrosion, 2 ed., NJ: Prentice-Hall, Inc, Upper Saddle River, 1996.

81 . V. Laget, C.S. Jeffcoate, H.S. Isaacs, R.G. Buchheit, J. Electrochem. Soc., 150 (2003) B425-B432.

82 . S.E. Dolan, U.S. Patent No. 5449415, (1995 ).

83 . L.S. Sander, E.M. Musingo, U.S. Patent No. 4,921,552, (1 May 1990).

84 . D. Galjard, Finishing, 7 (1983) 34.

85 . R.W. Hinton, Metal Finishing, 89 (1991) 55.

86 . Nelson J. Newhard Jr., in: J. Henry Leidheiser (Ed.) Corrosion Control by Coatings, Proceedings of the Meeting on corrosion control by coatings, SCIENCE PRESS, Princeton, 1979, pp. 225.

87 . J.H. Nordlien, J.C. Walmsley, H. Qsterberg, K. Nisancioglu, Surface and Coating Technology, 153 (2002) 72-78.

88 . S. Adhikari, K.A. Unocic, Y. Zhai, G.S. Frankel, J. Zimmerman, W. Fristad, Electrochim. Acta, (2010).

89 . D.Narayan, US Patent No 5139586, (1992).

90 . D.Y. Dollman, T.J. O'Grady, US Patent No 4191596, (1980).

91 . L.Hallman, WO No 9012902 A1, (1990).

92 . T. Schram, G. Goeminne, H. Terryn, W. Vanhoolst, P. Van Espen, Trans. Inst. Met. Finish., 73 (1995) 91-95.

93 . G.G. T. Schram, H. Terryn, W. Vanhoolst, P. Van Espen, Trans. Inst. Met. Finish., 73 (1995) 91-95.

94 . P.D. Deck, D.W. Reichgott, Metal Finishing, 90 (1992) 29-35.

95 . F. Andreatta, A. Turco, I. de Graeve, H. Terryn, J.H.W. de Wit, L. Fedrizzi, Surface & Coatings Technology 201 (2007) 7668-7685.

96 . L. Hallman, US Patent No 4992116, (1991).

50 97 . O. Lunder, C. Simensen, Y. Yu, K. Nisancioglu, Surface & Coatings Technology, 184 (2004) 278-290.

98 . A.S. Hamdy, M. Farahat, Surface & Coatings Technology, 204 (2010) 2834-2840.

99 . F. Pearlstein, V.S. Agarwala, Plating and Surface Finishing, 81 (1994) 50-55.

100 . F. Pearlstein, V.S. Agarwala, U.S. Patent No. 5304257, (1994).

101 . Henkel Alodine T5900 Material Safety Data Sheet, ID:239583DLP470.

102 . L. Li, G.P. Swain, A. Howell, D. Woodbury, G.M. Swain, Michigan State University, unpublished research.

103 . X. Dong, P. Wang, S. Argekar, D.W. Schaefer, Langmuir, 26 (2010) 10833-10841.

104 . C.A. Matsdorf, W.C. Nickerson, E. Lipnickas, NACE paper 2005-54. in NACE International, (2005).

105 . E.N. Beck, TriService Corrosion Conference Proceedings, (2003).

106 . W. Zhang, R.G. Buchheit, Corrosion, 59 (2003) 356-362.

107 . D. Chidambaram, C.R. Clayton, G.P. Halada, J. Electrochem. Soc., 151 (2004) B151-159.

CHAPTER 3

Characterization of Trivalent Chrome Process Coating on

AA2024-T3

3.1 INTRODUCTION

Aluminum alloy 2024-T3 has been widely used in aerospace applications

because of its superior mechanical properties including strength-to-weight ratio, which

result from the alloy elements such as copper, magnesium, and manganese [1].

However, the corrosion performance of the material can be a problem because the

52 due to the toxic and carcinogenic effects of chromium compounds on the environment

and human health, there have been increasingly stringent legislations regarding its

application and waste disposal [3-4]. Consequently, a significant amount of effort has

been extended to develop alternative corrosion inhibitor systems that can provide

comparable performance with minimal health and environmental concerns [5]. Among

these alternative coatings, zirconium based conversion coatings currently have wide

applications in the container, aluminum appliance, automotive, and extrusion industries

as an alternative to chromate conversion coatings because of their simple application

and good adhesion properties for the top painting [2, 6-9].

One promising candidate from the fluorozirconate coatings family, Trivalent

Chrome Process (TCP), recently has gained wide acceptance since it provides

comparable performance to the CCCs in both paint adhesion and corrosion resistance

only by adding a small amount of trivalent chromium species [10-11]. TCP is

considered to be an environmentally friendly replacement for chromate conversion

coating, because the TCP bath and the resulting film contain no chromate species, only

Cr(III). Agarwala and colleagues first reported corrosion resistant coatings formed on

aluminum alloys by immersion in aqueous solutions containing Cr2(SO4)3 and Na2SiF6 or NaF [12]. The zirconia-based Trivalent Chrome Process (TCP) coatings, however, are quite different in nature and have improved properties. Currently, TCP bath is the only chromate-free product that meets the stringent corrosion resistance requirements of MIL-DTL-5541F [11].

Zirconia-based conversion coatings have a multilayer layer structure consisting of an aluminum oxide layer close to the metal interface and an outer layer composed of zirconium oxide with a mixture of Zr/O/F layer on top of it [8, 13-14].

53 After 10 seconds of immersion in the treatment bath, a colorless layer with a thickness

of 10-25 nm is formed on Al alloy substrates and only a minor increase of thickness is

observed after extending the immersion time [13, 15].

Because of its increasing usage as a metal pretreatment, the structure and

composition of TCP and its corrosion protection mechanism need to be fully studied

and understood. Since the process is thought to be an environmentally friendly

replacement for CCC, the presence of chromium species and their oxidation states in the TCP film need to be investigated to assure its potential usage. In this chapter, the

TCP coating formed on AA2024-T3 is characterized by various analysis techniques, namely Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray

Spectrocopy (EDS), Transmission Electron Microscopy (TEM) and scanning TEM

(STEM), X-ray Photoelectron Spectroscopy (XPS), potentiodynamic polarization to understand the coating morphology, composition and structure. The distribution and

chemistry of chromium in the coating was investigated. Additionally, environmental

SEM (ESEM) has been used to characterize morphological changes due to dehydration.

3.2 EXPERIMENTAL PROCEDURES

Materials and chemicals. Aluminum alloy 2024-T3 sheets with a thickness of

2 mm (ALCOA) were cut from a commercial sheet stock with chemical composition of

Al, 3.8-4.9%Cu, 1.2-1.8%Mg, 0.30-0.9%Mn, 0.5%Fe, 0.5%Si, 0.25% Zn, 0.10%Cr,

0.15%Ti and 0.15% other elements [16]. All chemicals used in the study were of

reagent grade. Deionized water with a resistivity of 18.2 MΩ⋅cm was used to prepare

all solutions. The solutions studied include 0.5 M NaCl and dilute Harrison’s solution,

54 which is much less aggressive with composition of 0.05 wt% NaCl + 0.35 wt%

(NH4)2SO4 [17].

Coating Preparation. Samples were prepared in the following manner: (i) degreasing for 15 min at 55oC in 15% (v/v) Turco 6849TM, (ii) deoxidation for 5 min at room temperature in 20% (v/v) Liquid Turco Smut-go NCTM, (iii) then coating with

TCP in AlodineTM 5900S bath for 5 min at room temperature (treatment solutions are products of Henkel Corp.). Deionized water rinsing was performed between each step.

Samples were given a final rinse and blown dry with compressed air at room temperature at least 1h prior to use if not mentioned otherwise. The pH of Turco 6849 is

11.7~12.2 and the major ingredients are nonylphenoxypoly ethanol, sodium xylene sulfonate, ethanolamine and sodium tripolyphosphate [18]. The Smut-go NC solution has a pH ~ 0 and contains mainly ferric sulfate, nitric acid and sodium bifluoride [19].

Scanning Electron Microscopy. TCP coated AA2024-T3 samples with dimensions of 10 x 10 x 2 mm were examined using a Phillips Electronics model

XL-30F ESEM with a field emission gun. A gold coating was sputtered onto the TCP coating surfaces to make them conductive during the imaging process. A secondary electron detector was used to image the coating with a 15 kV accelerating voltage.

The formation and evolution of shrinkage crack morphology due to dehydration was examined by ESEM. One major advantage of ESEM is that hydrated specimens can be examined in-situ. Samples with a diameter of 6 mm and thickness of

2 mm were conversion coated for 10 min and rinsed. These samples were not dried at all or coated with Au, but rather were kept wet during transfer into the ESEM chamber.

55 The temperature in the chamber was cooled to 0.5oC and the water partial pressure was

maintained at 5 Torr to keep water in the liquid phase. By raising the temperature

slowly with a Pelletier stage, the samples were then gradually dehydrated, during which

time the formation of shrinkage cracks was monitored by SEM imaging.

The formation of shrinkage crack morphology after atmospheric aging was

also examined by ESEM. Samples were conversion coated for 5 min, rinsed, and then

kept in a sealed container with relative humidity controlled at about 34% by saturated

magnesium chloride solution. A separate sample was removed from the container after

6 h, 12 h, 24 h and 48 h aging, and transferred into the ESEM chamber, without an Au

overlayer. The temperature in the chamber was maintained at 24oC and the water partial

pressure was at 7.8 Torr to keep the relative humidity at 35%, which was similar to the

atmospheric condition.

Transmission Electron Microscopy. An FEI Nova 600 Dual Beam

SEM/FIB instrument was used to prepare cross-sectional samples from TCP treated

specimens. FIB milling was conducted with a 30 keV Ga ion beam and a 5 keV electron

beam. A layer of Pt with a thickness of ~1.5 μm was sputter deposited on the area of

interest to protect the TCP coating from milling, as shown in Figure 3.1. For some samples, a layer of Au was first vapor deposited by sputtering for added protection.

Electron transparent TCP foils with dimensions of 15 μm x 5 μm were produced by FIB and finally thinned to a thickness of ~100 nm. The extraction of TCP foils was performed by a micromanipulator with a pyrex needle (~1 μm in diameter) under an optical microscope. Then the thin foils were placed on a 200-mesh Ni grid with a carbon support film for TEM examination.

56 An FEI Tecnai F20 microscope was used to characterize the cross-sectional

TCP foils at high magnification. EDX line profiles were acquired at 200 kV and

FEI/Emispec TIA software was used to analyze the data. To investigate the relationship

of conversion time and coating thickness, cross-sectional foils of TCP-coated

substrates treated for various immersion times up to 10 min were prepared by FIB and

coating thickness was then examined directly in the TEM.

X-ray Photoelectron Spectroscopy (XPS). XPS measurements were

conducted on TCP-coated AA2024-T3 coupons (10 x 10 x 2 mm) to examine the

presence of chromium species on the surfaces and their oxidation states. A Kratos

AXIS Ultra instrument controlled by a VISION data system was used. A

mono-chromatic Al Kα X-ray source was operated at 1486.6 eV and 150 W. Typical

operating pressures were around 2x10-8 Torr. All spectra collected were calibrated by

the carbon 1s peak at 284.6 eV.

Potentiodynamic Polarization. All polarization scans were carried out using a Gamry Reference 600 potentiostat and GamryTM software. The measurements were

conducted in quiescent dilute Harrison’s solution (0.05 wt% NaCl + 0.35 wt%

(NH4)2SO4) in a corrosion flat cell supplied by EG&G Princeton Applied Research

(Model K0235). A three-electrode configuration was used with a saturated calomel reference electrode (SCE), a Pt mesh counter-electrode and AA2024-T3 as working electrode with 1 cm2 exposed area. The control samples in polarization experiments

were prepared by ultrasonic cleaning in ethyl alcohol to eliminate organic

contamination. This condition is termed “as received” hereafter.

57 In all potentiodynamic polarization tests, the open circuit potential (OCP) was

monitored before polarization until it reached a steady-state value (typically in 30 min).

Anodic polarization curves were collected starting at -0.005 V versus OCP and ramped

in the positive direction until the current density reached 2 mA/cm2. Cathodic

polarization scans started at 0.005 V positive to the OCP and ramped in the negative

direction until the current density reached -2 mA/cm2. A scan rate of 0.5 mV/sec was

used for all measurements. At a minimum, all experiments were replicated three times.

3.3 RESULTS

3.3.1. Surface Morphology of TCP Coating

Figure 3.2 shows SEM images of TCP coating formed on the substrate of

AA2024-T3 after 10 min exposure to the TCP bath. The sample surface was entirely

covered with dispersed rounded particles of 500 nm diameter, which merged together to form a compact film. The TCP surface morphology was similar to CCCs showing

macroscopic uniformity [20]. The TCP coating exhibited typical mud-crack

morphology, which also is similar to CCC when observed by SEM [20-21]. The TCP

coating peeled off the substrate at certain locations due to the cracks. It has been shown

that exposure to the vacuum during SEM promotes dehydration of the CCCs rather

rapidly [22].

58 3.3.2. Dehydration of TCP coating

The drying of TCP coating formed on AA2024-T3 after 10 min immersion

was studied. A series of micrographs taken during TCP dehydration in the

environmental SEM (ESEM) are shown in Figure 3.3. ESEM imaging was started on a freshly coated and still wet sample surface so that the initiation and evolution of mud-crack morphology could be monitored in-situ. Two major parameters in the

ESEM chamber, pressure and temperature, were well controlled to maintain water in the liquid phase before imaging. Then they were adjusted in a manner that water evaporated gradually and the TCP dried slowly for imaging of film crack initiation.

Under the initial condition of 3oC and 5 Torr (Fig 3.3a), the TCP coating

appeared to be un-cracked and consisted of layers of merged spherical particles, which

was similar to the CCC morphology reported by Arrowsmith and Treverton [20-21].

When the temperature was raised to 10oC (Figure 3.3 b and c), the particle margins

became sharper and were more easily identified in the ESEM. Upon heating to 22oC

and maintaining the relative humidity at 10%, formation of cracks started to be

observed and new cracking occurred as the aging continued. The coating appeared to

flake off the substrate at different regions on the surface, as shown in Figure 3.3 d, e and

The crack formation during natural aging was also investigated. Freshly

prepared TCP coated samples were kept in a sealed container with relative humidity of about 34%. After 6 h and 12 h aging, the coating surfaces were examined by ESEM under the condition of 24oC, 7.8 Torr and 35%RH. The surfaces were characterized by

the presence of a compact film consisting of spherical particles and no cracking was

observed, as shown in Figure 3.4. After 24 h atmospheric aging, the coating still

59 showed macroscopic uniformity. However, very small cracking was observed at certain

locations, as pointed by arrows in Figure 3.5. The density the cracking was

considerably increased after 48 h aging and the size of the cracking was extended over 5

μm, as shown in Figure 3.6.

3.3.3. Surface Composition of TCP on AA2024

X-ray photoelectron spectroscopy was conducted on TCP coated AA2024-T3

and a full range survey is given in Figure 3.7. The presence of the characteristic

elements Zr, O, Cr was observed, which supported the idea that the TCP coating is

mainly composed of zirconium oxide and chromium oxide. Carbon is a significant

impurity at the surface, presumably coming from atmospheric contamination.

A high resolution XPS spectrum for Cr 2p was collected from the 5 min TCP

coated surface, as shown in Figure 3.8. In the energy range of 568 to 593 eV, two Cr

peaks corresponding to the Cr 2p1/2 and Cr 2p3/2 electron configurations were observed.

Fitting of the peak binding energies and widths indicated that chromium was present in

the trivalent state as Cr2O3 and Cr(OH)3 [23]. The content of Cr2O3 was ~3 times higher

than Cr(OH)3 according to the area under the deconvoluted peaks. No hexavalent chromium was detected in the TCP coating, supporting its potential usage as a replacement of chromate conversion coating.

3.3.4. Structure of TCP Coating on AA2024-T3

Figure 3.9 is a TEM image of the cross-section of a TCP coating on an

AA2024-T3 substrate. TEM studies of cross-sectional specimens equivalent to those

60 examined in SEM confirmed the large variation in the thickness of the conversion layer

across the sample surface. Coating thickness in the range of 60-100 nm was observed.

The coating appears to be delaminated from the surface in many regions that were

observed in the TEM. It is likely that this separation is an artifact that occurred during

sample preparation, such as in the high vacuum of the FIB tool by a process related to

the cracking described above. Delamination of TCP in service is generally not a

problem so it is unlikely that cracks were present in the as-prepared sample. However, such separations were not observed with CCCs, which might indicate that the adhesion

of TCP coatings is not as good as CCCs [24].

EDS compositional line profiles of the TCP coating along the line shown on

the inset STEM micrograph from the Pt to the matrix are shown in Figure 3.10. Based

on the position where Al signal intensifies, the films are on the order of 70 nm in

thickness. The coating is mainly composed of Zr, Cr, O, Al and F, with Zr comprising

40 wt% in the TCP region. The Cr content is only one fourth the Zr content. It is

therefore reasonable to consider TCP to be a zirconate film that contains Cr. The Zr and

Cr signals exhibit similar intensity patterns indicating that the mixed Zr-Cr oxide is

uniform across the thickness. The O signal extends beyond where the Zr and Cr signal

decreases and overlaps with decay of the Al signal. The F level appears to be enriched

at the metal-film interface. This indicates that the coating is bilayered with a layer

aluminum oxide between the metal matrix and the Zr-Cr oxide, as shown in Figure

3.11.

61 3.3.5. Polarization of TCP in dilute Harrison’s Solution

Anodic and cathodic potentiodynamic polarization curves measured in aerated

dilute Harrison’s solution are shown in Figure 3.12. The corrosion potential of TCP

coated surface was about -450 mV vs. SCE, whereas that for an as received AA2024-T3

sample was about -600 mV vs. SCE. However, the corrosion potential for a

Smutgo-treated surface (the treatment step prior to TCP) was similar to that of the TCP

coated sample. In fact, the whole anodic polarization curve for the TCP-treated sample

was similar to that of the Smutgo-treated sample. A large passive region was observed

for the as recieved surface with a breakdown potential of ~-450 mV vs. SCE. A passive region was found for the TCP coated surface, the breakdown potential was raised to

-250 mV vs. SCE. The cathodic polarization curves in quiescent Harrison’s solution

(Figure 3.12 b, measured separately) indicate that the oxygen reduction limiting current

density, found in the potential range of -600 mV to -1000 mV vs. SCE, was lowest for the TCP-coated sample. The limiting current density for the as received surface was the highest, almost one order of magnitude higher than that of the TCP sample and similar to the Smutgo surface.

3.4 DISCUSSION

3.4.1. Formation and Morphology of TCP

The TCP Coating Morphology. Figure 3.2 shows TCP coating morphology formed on the AA2024-T3. A relatively uniform coating consisting of rounded nodules

about 0.2 μm in diameter was observed. The XPS spectra revealed the presence of

62 extensive amounts of zirconium oxide along with trivalent chromium compound,

which indicated a mixed oxide layer formed on the aluminum substrate after immersion

in the AlodineTM5900 bath. Previous work has shown that, on commercial purity

aluminum substrate, CCC grew by precipitating layers of spherical particles of 30 nm

to 4 μm diameter depending on the details of the coating procedures [20-21, 25].

Verdier et al. found that a zirconia based coating was a compact layer consisting of

micro-sized spherical particles [26]. Thus, it appears that the TCP coating has very

similar surface morphology to both CCC and Zr-based coating. Note that the coating

baths of TCP and Zr-based coating are different in that a small amount of chromium is

added into TCP coating bath maintained the dense barrier layer [11].

TCP coating formation. On exposure to the TCP bath, a mixed oxide layer

precipitates on the surface. Based on the similarity to Zr-based coatings, it is likely that

the formation of TCP takes place in a similar manner. The TCP formation bath contains

2- 3+ 2- - ZrF6 , Cr , SO4 and F , with a pH of 3.8-4.0. During coating formation, the fluoride

ions break down the passivating Al oxide layer, which leads to the dissolution of aluminum metal accompanied by a shift of the electrode potential to the negative direction [27]:

Al → Al3+ + 3e- (1)

At this potential, both the reduction of dissolved oxygen and hydrogen evolution occur,

as represented by the following reactions:

- - O2 + 2H2O + 4e → 4OH (2)

+ - 2H +2e → H2 (3)

A local increase in pH results from these cathodic reactions, but it is balanced to a

63 certain extent by partial cation hydrolysis. The alkalization favors the precipitation of a

hydrated zirconia film on the surface along with hydrated chromium oxide, as shown in

the reactions:

2- - - ZrF6 + 4OH → ZrO2-2H2O + 6F (4)

3+ - Cr +3OH → Cr(OH)3 (5)

Thus, the deposition of the TCP coating induced by the local pH increase is fundamentally different from the formation of chromate conversion coating, which involves a redox reaction between chromate ions and the metal substrate [28-30].

The Dehydration of TCP Coating. Figure 3.3 records the coating morphology

of TCP coated surface during drying in the ESEM. Formation and evolution of

characteristic mud cracks were monitored in situ. Earlier studies pointed out the

presence of cracks in zirconium-based coatings. For example, Andreatta et al. observed

cracks on the AA6061 surface by SEM after pre-treatment in Zr/Ti based solution for

300 s [31]. On the basis of the results in the present study, however, it is evident that the

freshly prepared TCP coating formed on AA2024-T3 surface is not defective. Aging

under low humidity resulted in the loss of water from the coating, resulting in shrinkage

and crack formation. The high vacuum condition in the traditional scanning electron

microscope dehydrates the water quickly; consequently, the mud-crack morphology is

an artifact that was always observed in the SEM micrographs. When the samples were

aged and examined both under atmospheric environment, as in the present study, cracks

were first observed after 24 h aging. After 2 days aging, the extent of cracking increased

considerably. This result suggests that the TCP coating dries slowly under atmospheric

conditions but that the overcoat should be applied within about 24 h before severe

64 cracking forms.

In the case of CCC, formation of shrinkage cracks due to dehydration resulted

in the major loss of corrosion resistance [5, 22]. However, if the CCCs are sealed by a layer of impermeable organic coating or aged in 100oC steam, the dehydration stops [22,

32]. Therefore, the crack morphology of TCP coating is considered to be an artifact but

not a demonstration of inability to provide corrosion resistance.

3.4.2. Structure and Composition of TCP coating on AA2024-T3

Figure 3.10 shows the nano-EDS line profiling of 10 min TCP formed on

AA2024-T3 substrate, which has a thickness of about 70 nm. The TCP coating

appeared to be mainly composed of zirconium oxide that contained a small amount of

chromium. A layer of aluminum oxide and/or oxyfluoride was formed at the interface

of the TCP coating and the metal matrix. Previous work by Schram et al. has shown a

similar two-layer structure of a zirconium-based conversion coating, except that

fluorine was enriched in the top layer [14]. In a more recent work, a three layered

structure was proposed based on XPS and AES depth profiling, which has Al/O/F at the

metal/coating interface, a Cr/Zr/O/F layer on top and a zirconium-chromium oxide in

between [11]. TEM analysis of several samples indicated that fluorine was only

detected at the interface of TCP layer and the metal substrate if it was detected at all.

The structure of TCP layer on AA2024-T3 was proposed based on the EDX line

profiling, as shown in Figure 3.11.

Chromium in the TCP coating was identified by XPS to be mainly in the

trivalent state as Cr2O3 and Cr(OH)3. Since the TCP formation bath contains only

65 trivalent chromium salt, it is likely that a local pH increase resulted in precipitation of

the otherwise soluble chromium complex to form the chromium oxide or oxyhydroxide

film. There was no apparent redox Cr chemistry in the formation bath.

It has been suggested that zirconium-based coatings have a thickness less than

10 nm that is reached within the first 10 s of immersion, after which little thickening occurs [14]. The TCP coatings evaluated in this study were at least 4 times thicker than

the Zr-based coating possibly due to the addition of chromium and other differences in the bath chemistry.

3.4.3. Electrochemical behavior of TCP coating in dilute Harrison’s solution

Zirconium based coatings alone have been shown to have excellent

performance in the standard industry adhesion tests, but poor behavior concerning the

corrosion protection property if not covered with paint [33-34].

Anodic polarization curves measured in the aerated dilute Harrison’s solution

showed that the breakdown potentials of TCP coated surface and desmutted surface

were 200 mV higher than the as received control, which indicated the desmutting in

Smut-go NC solution altered the surface and made the surface more corrosion resistant.

The corrosion rate for the TCP coated surface was 6x10-8 A/cm2, which was lower than

the as received surface. These observations combined with the fact that the cathodic

current density of TCP surface was lower than the as received control, suggests that the

oxygen reduction reaction on aluminum substrates is suppressed by the TCP coating.

Below the potential of about -1.3 V vs. SCE, the shape of the cathodic curve

for the as received surface suggested that the dominant reaction shifted from the

diffusion limited oxygen reduction to the water reduction reaction. .

3.5 CONCLUSION

In this study, TCP coating formed on the AA2024-T3 substrate was

characterized by means of SEM, TEM/STEM, XPS and electrochemical measurements.

The coating morphology, structure and composition have been studied. The

electrochemical behavior of the TCP coating was evaluated by potentiodynamic

polarization. The following conclusions are drawn:

(1) TCP coating is a compact film consisting of layers of rounded particles hundreds of

nm in size, similar to that of CCC.

(2) The thickness of the TCP is in the range of 40-120 nm depending on the conversion

time, considerably thicker than the zirconium based coating without chromium

species.

(3) The TCP coating consists of a two layered structure, with zirconium-chromium

oxide in the outer layer and aluminum oxide or oxyfluoride at the metal/coating

interface.

(4) No Cr (VI) was found on the TCP surface, which supports its designation as an

environmentally friendly replacement for CCC.

(5) Dehydration of the TCP coating occurred during exposure to high vacuum.

Atmospheric aging also resulted in shrinkage cracking but at a largely reduced rate.

(6) The TCP coating provides corrosion protection to the AA2024-T3 through

suppressing the oxygen reduction reaction on aluminum alloy surfaces.

Figure 3.1. SEM micrograph of the TCP foil prepared by FIB.

Figure 3.2. SEM micrographs of 10 min TCP on AA2024-T3 at different magnifications.

(a) (b) (c)

2μm

(d) (e) (f)

Figure 3.3.Crack formation during dehydration as observed by ESEM (by sequence of cumulative exposure time). Imaging conditions are (a) 3oC, 5 Torr; (b) and (c) 10oC, 5 Torr; (d), (e) and (f) 22oC, 2 Torr.

(a) (b)

2 μm 2 μm

Figure 3.4.Crack formation after atmospheric aging for (a) 6 h, (b) 12 h, as observed by ESEM under the condition 24oC, 7.7 Torr, 35% RH.

(a)

(b)

5 μm

Figure 3.5.Crack formation at different locations on TCP surface after atmospheric aging for 24 h as observed by ESEM. Imaging condition is 24oC, 7.7 Torr, 35% RH.

(a)

(b)

5 μm

Figure 3.6.Crack formation at different locations on TCP surface after atmospheric aging for 48 h as observed by ESEM. Imaging condition is 24oC, 7.7 Torr, 35% RH.

8x104

7x104 O 1s 6x104

4 5x10

4x104 p Cr 2 Cr Zr 3d Zr Cr Cr

4 3 Zr Intensity (CPS) Intensity

3x10 Zr Zr C 1s 2x104

Zr 3s Zr

4 1x10 Zr Zr 4s Zr

0 600 400 200 0 Binding Energy (eV)

Figure 3.7. XPS survey on 5 min TCP coated AA2024-T3 surface.

1x104 Cr on Fresh TCP A: Cr2O3 Cr 2p3/2 1x104 B: Cr(OH)3 1x104 A 1x104 Cr 2p1/2

1x104

Intensity (CPS) Intensity A 9x103

8x103

7x103 B B 595 590 585 580 575 570 Binding Energy (eV)

Figure 3.8. Cr 2p spectra of 5 min TCP-coated AA2024-T3.

Pt layer

Au layer TCP

Al matrix

Figure 3.9. Transmission electron micrograph of 5 min TCP on AA2024-T3.

60 Al Cr F 40

wt% O Pt Zr 20

0 20 40 60 80 100 120 140 160 180 Position (nm)

Figure 3.10. Nano-EDS line profiles of 10 min TCP on matrix of AA2024-T3.

Zr, Cr oxide

Aluminum Oxide with F enrichment

Aluminum Alloy

Figure 3.11. Layered structure model for TCP coating deposited on AA2024-T3.

0.2

-0.2 TCP

smutgo -0.4 E (V vs SCE) as received

-0.6

-0.8 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 2 i (A/cm )

-0.4

smutgo -0.6

as received -0.8 TCP

-1 E (V vs SCE)

-1.2

-1.4 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 2 i (A/cm )

Figure 3.12. Potentiodynamic polarization curves in aerated dilute Harrison’s solution. a. anodic polarization curves, b. cathodic polarization curves.

REFERENCES

1 . I.J. Polmear, Light Alloys Metallurgy of the Light Metals, Third ed., , London: Arnold, 1995.

2 . R.W. Hinton, Metal Finishing, 89 (1991) 55.

3 . Fedral Register, 69 (October, 2004).

4 . P. O'Brien, A. Kortenkamp, Transition Metal Chemistry, 20 (1995) 636-642.

5 . L. Xia, E. Akiyama, G.S. Frankel, R.L. McCreery, J. Electrochem. Soc., 147 (2000) 2556-2562.

6 . J.H. Nordlien, J.C. Walmsley, H. Qsterberg, K. Nisancioglu, Surface and Coating Technology, 153 (2002) 72-78.

7 . D. Galjard, Finishing, 7 (1983) 34.

8 . R.G. Buchheit, A.E. Hughes, Chromate and Chromate-Free Conversion Coatings, ASM Handbook: Fundamentals, Testing, and Protection, 2003.

9 . A.E. Hughes, J.D. Gorman, P.J.K. Patterson, Corrosion Sci., 38 (1996) 1957.

10 . C. Matzdorf, M. Kane, J. Green, U.S. Patent No. 6,375726, (2002).

11 . W.C. Nickerson, E. Lipnickas, TriService Corrosion Conference Proceedings, (2003).

12 . F. Pearlstein, V.S. Agarwala, Plating and Surface Finishing, 81 (1994) 50-55.

13 . Nelson J. Newhard Jr., in: J. Henry Leidheiser (Ed.) Corrosion Control by Coatings, Proceedings of the Meeting on corrosion control by coatings, SCIENCE PRESS, Princeton, 1979, pp. 225.

14 . T. Schram, G. Goeminne, H. Terryn, W. Vanhoolst, P. Van Espen, Trans. Inst. Met. Finish., 73 (1995) 91-95.

80 15 . P. Laha, T. Schram, H. Terryn, Surf. Interface Anal., 34 (2002) 677-680.

16 . J.E. Hatch, Aluminum: properties and physical metallurgy, American Society for Metals, Metals Park, Ohio, 1984.

17 . ASTM, ASTM G85-09 Standard Practice for Modified Salt Spray (Fog) Testing, in, ASTM.

18 . Henkel, Turco 6849, Aqueous Alkaline Degreaser, Material Safety Data Sheet, ID: 238832.

19 . Henkel Turco liquid Smut-go NC MSDS ID:239357DLP55G.

20 . D.J. Arrowsmith, J.K. Dennis, P.R. Sliwinski, Trans. Inst. Met. Finish., 62 (1984) 117.

21 . J.A. Treverton, M.P. Amor, J. Mater. Sci., 23 (1988) 3706-3710.

22 . V. Laget, C.S. Jeffcoate, H.S. Isaacs, R.G. Buchheit, J. Electrochem. Soc., 150 (2003) B425-B432.

23 . D. Chidambaram, G.P. Halada, C.R. Clayton, Appl. Surf. Sci., 181 (2001) 283-295.

24 . Q.J. Meng, G.S. Frankel, Surf. Interface Anal., 36 (2004) 30-42.

25 . J.A. Treverton, M.P. Amor, Transactions of the Institute of Metal Finishing, 60 (1982) 92.

26 . S. Verdier, N. van der Laak, F. Dalard, J. Metson, S. Delalande, Surface & Coatings Technology, 200 (2006) 2955-2964.

27 . L. Li, G.P. Swain, A. Howell, D. Woodbury, G.M. Swain, Michigan State University, unpublished research.

28 . L. Xia, R.L. McCreery, J. Electrochem. Soc., 145 (1998) 3083-3089.

29 . L. Xia, R.L. McCreery, J. Electrochem. Soc., 146 (1999) 3696-3701.

30 . G.S. Frankel, R.L. McCreery, The Electrochemical Society Interface, 10 (2001) 34.

31 . F. Andreatta, A. Turco, I. de Graeve, H. Terryn, J.H.W. de Wit, L. Fedrizzi,

81 Surface & Coatings Technology 201 (2007) 7668-7685.

32 . A.L. Glass, Materials Protection, 7 (1968) 26-30.

33 . L.S. Sander, E.M. Musingo, U.S. Patent No. 4,921,552, (1 May 1990).

34 . Final NCMS Report 0273RE95, National Center for Manufacturing Sciences, (1995).

CHAPTER 4

Effect of Pretreatment on Trivalent Chrome Process Coatings

4.1 INTRODUCTION

When aluminum is exposed to air, a thin layer of native oxide nanometers in

thickness is formed on the surface. This naturally-formed passive film is not sufficient

to prevent localized corrosion of high strength Al alloys because of the effects of

intermetallic particles.[1]. Thus it is necessary to treat the surface in certain ways to

obtain a more protective barrier layer.

Conversion coatings are surface treatments that convert the native metal oxide

into a more protective film with good paint adhesion and improved corrosion resistance

[2]. Chromate conversion coatings (CCCs) have long been employed in the surface

finishing process for AA2024-T3 and other metal alloys for these purposes [3]. Due to the toxic and carcinogenic effects of hexavalent chromium compounds on the environment and human health, there has been increasingly stringent legislation

regarding their use and waste proposal. Among the chromate-free alternative coatings,

83 Trivalent Chrome Process (TCP) recently has gained wide acceptance. TCP was first developed at NAVAIR and is currently one of the leading non-chromate conversion coatings on the market as it has been shown to provide excellent corrosion protection and paint adhesion in standardized tests [4]. The TCP process is a drop-in replacement for hexavalent chromate treatments and it contains no Cr(VI) in the coating bath and resulting film [5].

Prior to the application of various kinds of conversion coatings, it is essential to clean the metal surface physically or chemically to remove the oils, contaminants and surface metal oxides developed during manufacture and transport. Proper substrate pretreatment is critical for obtaining well adhering and continuous conversion coatings.

Pretreatments not only clean the surface but also alter the surface chemistry and morphology [6]. For high strength aluminum alloys used in aerospace applications, the common pretreatment consists of alkaline cleaning and acidic desmutting [7].

Alkaline Cleaning. Alkaline cleaning is widely used in industry to remove oils, organic contaminants, and the oxide layer from the aluminum surface [2, 8]. There are two major types of alkaline cleaners: etching and non-etching. The etching alkaline cleaners are based on sodium hydroxide, the etching effect of which can only be reduced by the addition of sodium silicate. The non-etching type cleaners usually contain sodium carbonate and sodium phosphate, and the etching action can be fully stopped by the addition of sodium silicate. For the application of Trivalent Chrome

Process (TCP) coatings, the non-etching cleaners are typically used [9]. Sodium carbonate and sodium metasilicate are a common combination for a non-etching cleaner for aluminum substrates because they provide alkalinity and buffering at a low

84 cost [2]. A balanced mixture minimizes etching attack of the aluminum surface at pH

11.1~11.7. Sodium metasilicate is added to form a protective layer (a mono-molecular

layer of hydrated silica) by the reaction of metasilicate salts with the aluminum or alumina surface, which minimizes the etching attack on the base metal. However, this silicate modified layer is not easily removed through rinsing and is carried over to the next step of surface cleaning. The silicate salt in the alkaline cleaner was reported to inhibit the formation of TCP coating on the Al alloy surface in some cases [10].

Therefore, sodium tripolyphosphate is often used instead to provide alkalinity, strong buffering as well as the ability to soften the hard water salts [2]. Alkaline cleaning in phosphate gives rise to a surface covered by a thin smut layer of amorphous aluminum oxide containing zinc, magnesium, silicate and other intermetallic particles, which are insoluble in the alkaline solution.

Acidic Desmutting. To remove the smut layer and insoluble intermetallic particles formed during the alkaline cleaning, the Al alloy surface is usually subjected to a subsequent desmutting step [2]. Mixtures of mineral acids, organic acid, and acid salts are commonly used as desmutting agents. Previous publications have shown that, prior to the application of chromate conversion coating, it was particularly important to remove the smut layer and undesired intermetallic particles from the surface of alloy

[11-12]. In a more recent work by Harvey and coworkers, bromate-nitric acid desmutter on AA2024-T3 was investigated and proven to be efficient in removal of oxide and large intermetallic particles from the Al alloy surface [11]. Other combinations of mineral acids are also effective. For example, Hughes et al. studied the deoxidation of

AA2024-T3 with various mixture of acids and found hydrofluoric-nitric acid solution

85 produced a surface free of copper-rich smut [12]. After desmutting, surface roughening

is found as it leaves behind voids and pits on the metal surface.

In this chapter, the effect of two pretreatments on the TCP formed on

AA2024-T3 surface is investigated. These two pretreatment processes are commonly

used for aluminum substrates. Silicated alkaline cleaner, which has been used for

aluminum substrate degreasing prior to chromate conversion coating treatment [13],

was reported to negatively influence the formation of TCP film [10]. The purpose of

this work is to compare the effects of the pretreatment containing silicate (Process II)

with the one without silicate (Process I) on the formation of TCP films. The general

surface features were examined by SEM after each treatment step. The nucleation and

growth of TCP film after various stages was characterized by SEM and TEM analysis.

The evolution of surface morphology, chemistry and structure were studied.

4.2 EXPERIMENTAL PROCEDURES

Materials and Chemicals. Aluminum alloy 2024-T3 sheets with 2 mm

thickness and nominal composition of Al, 3.8-4.9%Cu, 1.2-1.8%Mg, 0.30-0.9%Mn,

0.5%Fe, 0.5%Si, 0.25% Zn, 0.10%Cr, 0.15%Ti and 0.15% other elements [14] was

used. All chemicals used in the study were of reagent grade. Deionized water with a

resistivity of 18.2 MΩ•cm was used to prepare all solutions.

Coating Preparation. Two different pretreatments for AA2024-T3 prior to

TCP treatment on the properties were investigated. Process I was carried out in the

86 following manner:

(i) degreasing for 15 min at 50°C in 15% (v/v) Turco 6849

(ii) desmutting for 5 min at room temperature (RT) in 20% (v/v) Liquid

Turco Smut-go NC

(iii) conversion coating for 5 min at RT in AlodineTM 5900S bath.

The pH of Turco 6849 is 11.7~12.2 and the major ingredients are nonylphenoxypoly ethanol, sodium xylene sulfonate, ethanolamine and sodium tripolyphosphate [15]. The

Smut-go NC solution has a pH ~ 0 and contains mainly ferric sulfate, nitric acid and sodium bifluoride [16]. The chemicals are all commercially available from Henkel

Corp, Madison Heights, MI.

The second treatment procedure, Process II, consisted of:

(i) degreasing at 65°C for 2 min in a solution of 32.4 g/l Na2SiO3 + 48 g/l

Na2CO3

(ii) desmutting at 55°C for 3 min in a solution of 72 ml/l HNO3 + 30 g/l

Sanchem 1000 (sodium bromate based, Sanchem Inc., Chicago, IL).

(iii) conversion coating at RT for 5 min in Alodine TM 5900S bath.

Samples were rinsed with deionized (DI) water after each step and blown dry with compressed air at RT at the end. Experiments were started after the coated samples were kept at least 1 h in air if not mentioned otherwise. Besides the standard coating procedures described above, various times for degreasing, desmutting and coating were investigated, as shown in Tables 4.1 and 4.2.

Scanning Electron Microscopy and Energy Dispersive Spectroscopy.

TCP-coated AA2024-T3 samples with dimensions of 10 × 10 × 2 mm were examined

87 using Scanning Electron Microscopy (SEM, Phillips Electronics model XL-30F ESEM

with field emission gun) and Energy Dispersive Spectroscopy (EDS). A gold coating

was sputtered onto the sample surfaces to make them conductive during the imaging

process. A secondary electron detector was used to image the coating with a 15 kV

accelerating voltage.

Transmission Electron Microscopy. Cross sections of TCP films were prepared using focused ion beam (FIB) sectioning (FEI Nova 600 Dual Beam

SEM/FIB). FIB milling was conducted with a 30 keV Ga ion beam and a 5 keV electron beam. A layer of Pt with a thickness of ~1.5 μm was deposited on the area of interest to protect the TCP coating during milling, as shown in Figure 3.1. For some samples, a layer of Au was first deposited by evaporation for added protection. Electron transparent TCP foils with a thickness of ~100 nm with other dimensions of 15 μm x 5

μm were finally produced. The TCP foils were extracted by a micromanipulator with a pyrex needle (~1 μm in diameter) under an optical microscope. The thin foils were placed on a 200-mesh Ni TEM grid with a carbon support film for TEM examination.

Transmission Electron Microscopy (TEM, FEI Tecnai F20 microscope) was used to characterize the cross-sectional TCP foils at high magnification. EDS was used in the TEM to collect compositional line profiles at 200 kV and FEI/Emispec TIA software was used to analyze the data.

X-ray Photoelectron Spectroscopy (XPS). AA2024-T3 coupons (10 x 10 x 2 mm) were treated after degreasing and desmutting following Process I and Process II respectively. XPS measurements were performed to examine the presence of silicon

88 species on the surfaces. A Kratos AXIS Ultra instrument controlled by a VISION data

system was used. A mono-chromatic Al Kα X-ray source was operated at 1486.6 eV

and 150 W. Typical operating pressures were around 2x10-8 Torr. All spectra collected

were calibrated by the carbon 1s peak at 284.6 eV.

Electrochemical Impedance Spectroscopy (EIS). EIS measurements were

performed with a Gamry Reference 600 potentiostat to evaluate the corrosion

resistance of TCP coating after two pretreatments. A cell supplied by EG&G Princeton

Applied Research (Flat Cell, Model K0235) was used for all experiments. Samples

were exposed in the dilute Harrison’s solution (0.05 wt% NaCl + 0.35 wt% (NH4)2SO4).

The sinusoidal voltage amplitude was 10 mV around the OCP, which was applied at frequencies ranging from 105 Hz to 10-2 Hz. For all EIS measurements, the reference

electrode was saturated calomel electrode and a Pt mesh served as the counter

electrode.

4.3 RESULTS

A scanning electron micrograph of the as-received AA2024-T3 surface is

presented in Figure 4.1. Pits with the size of several micrometers and rolling lines

generated during manufacturing process are visible. The as-received alloy surface also

exhibited an extensive amount of submicron-sized contaminant particles. The focus of

this study was to investigate the effects of the two pretreatments applied to the surfaces

of the AA2024-T3 samples prior to immersion in the Alodine T5900 TCP bath. It is

critical to examine the surfaces following each pretreatment to understand their effects

89 on the subsequent treatment step.

4.3.1. AA2024-T3 Surface after Alkaline Cleaning

Alkaline cleaning is used to degrease and partially etch the surface of the

component. It was carried out with Turco 6849 in Process I and with Na2SiO3 +

Na2CO3 in Process II. The morphology of AA2024-T3 surface after Turco 6849 treatment is presented in Figure 4.2. This alkaline cleaning, which was performed at

50oC for 15 min, did not completely dissolve the characteristic rolling lines and pits, yet

resulted in a relatively smooth surface with a largely reduced amount of contaminant particles on the surface.

After degreasing in the silicated alkaline cleaner in Process II, the aluminum

surface was observed to be similar to the surface after Process I. Typical rolling lines

and voids were present on the surface with greatly decreased remnant contamination, as

shown in Figure 4.3.

X-ray photoelectron spectroscopy was performed on AA2024-T3 samples after

the two degreasing methods. The spectrum for Si 2p is plotted in Figure 4.4. A

significant amount of silicon was detected after degreasing in the silicated alkaline

cleaner in Process II, while on the surface cleaned after Process I no trace of silicon was

detected.

4.3.2. AA2024-T3 Surface after Desmutting

The second step in the coating preparation, acidic desmutting, has been proven

to be critical because it not only alters the morphology and chemistry of the

90 AA2024-T3 alloy surface but also has a significant effect on the formation of the

conversion coating [2, 13].

After degreasing in Turco 6849 for 15 min, desmutting in Smut-go NC bath

(Process I) was carried out for 1, 5, and 10 min at RT. SEM micrographs of the surface

after various desmutting times are shown in Figure 4.5. Immersion in the Smut-go NC

bath for 1 min resulted in minor attack of the surface with the appearance of a large

number of nm-sized pits. The larger pit in Figure 4.5 (a) was likely already present after

the alkaline cleaning step. After 5 min desmutting, the surface was characterized by the

presence of pits hundreds of nanometers in diameter or larger over the entire surface,

Figure 4.5 (b). More severe attack of the surface was observed after 10 min desmutting,

as shown in Figure 4.5 (c). Many pits had grown laterally, compared to shorter immersion times. There was considerable scalloping and many intermetallics were totally or partially removed leaving round and irregular shaped voids on the surface.

The base of the scalloped area was etched as well.

After degreasing in the silicate/carbonate bath and desmutting in the nitric acid +

Sanchem1000 bath for 3 min (Process II), significant surface roughening also occurred, as shown in Figure 4.6. Etching of the metal surface was observed with the evidence of formation of μm-sized pits, which was comparable to the surface after 5 min immersion in the Smut-go NC bath according to Process I.

A high resolution XPS spectrum for Si 2p was also collected on AA2024-T3 samples after the two desmutting methods, as shown in Figure 4.4. After desmutting in

Sanchem 1000 and nitric acid following Process II, the silicon residue on the metal surface after degreasing in silicated cleaner was largely reduced since the intensity of Si

2p peak was greatly decreased leaving only a bump. For the surface treated in Smut-go

91 bath in Process I, no trace of silicon species was found.

4.3.3. Formation of TCP Coating after Different Pretreatments

Conversion coating in the TCP bath at RT was carried out for 10 s, 30 s, 1 min,

2 min, 3 min, 5 min, 7 min and 10 min. SEM micrographs of the surface developed at

various stages during conversion coating were recorded. The formation and growth of

TCP films after these two pretreatments are different.

After 10 s TCP conversion treatment following Process I, no apparent

nucleation of TCP coating was observed, as shown in Figure 4.7 (a). However, after 30 s conversion, small TCP nuclei were observed at isolated sites on the surface, as pointed out by arrows in Figure 4.7 (b). These particles tended to agglomerate together to form clusters.

Following Process II, round shaped nuclei of TCP conversion layer ranging from tens to hundreds of nm in size were observed by SEM after 10 s immersion, as pointed out by arrows in Figure 4.7 (c). These particles become evenly distributed over the entire surface after 30 s immersion, as presented in Figure 4.7 (d). At some spots on the surface after 30 s TCP treatment, as indicated in Figure 4.8, large micron-sized clusters of round particles were present and EDS analysis indicated the presence of Zr,

O, Cr and Al.

Immersion in the TCP bath for 60 s or 120 s following Process I resulted in a surface covered by a continuous conversion layer consisting of small particles, approximately tens of nm in diameter, as shown in Figure 4.9 (a) and (b). After minutes

92 of immersion in the TCP bath following Process II, the surface was characterized by the presence of a higher density of particles compared to shorter immersion time as presented in Figure 4.9 (c) and (d). The trenches and voids on the alloy surface were also covered by the particles. Two min immersion resulted in a somewhat better surface coverage because the particles started to be connected and grow into a film. However, the film formation was still relatively non-uniform.

Figure 4.10 (a) and (b) indicate that the TCP film continued to thicken after longer immersion time for Process I. Immersion in the TCP bath for 5 min resulted in a compact uniform coating on the surface. Immersion in the TCP bath for 3 min following Process II greatly increased the surface coverage as the TCP film almost covered the entire surface, as shown in Figure 4.10 (c) and (d). Cracks were observed in the SEM after 5 or 7 min for Process I or Process II pretreatment, respectively, as shown in Figure 4.10 (b) and Figure 4.11 (a, c). SEM micrographs of the surface after 10 min immersion are presented in Figures 4.11 (b) and (d). TCP films formed after both pretreatments started to peel off the Al substrates after this prolonged immersion in the coating bath. The cracks and delaminations are artifacts associated with dehydration of

TCP film in the SEM as discussed in Chapter 3.

4.3.4. Chemistry and Thickness of TCP Coating after Different Pretreatments

Analytical TEM analysis was performed on a series of sections of TCP films deposited on AA2024-T3 following both pretreatments. TCP film deposited on the

AA2024-T3 surface after 2 min immersion following Process I is shown in Figure 4.12.

Coating thickness in the range of 20 to 50 nm thick was observed. Figure 4.12 (b)

93 shows the compositional line profiles of the TCP coating along the red line in Figure

4.12 (a), as measured by EDS. The coating was mainly composed of Zr, Cr, O, Al and F.

A noticeable peak of the F signal, ~20 wt% or about four times higher than the average,

was observed at the interface of the film and metal substrate. The Cr and Zr signals

showed similarity in their intensity patterns.

In the case of the 2 min TCP film following Process II, the coating thickness

appeared to be comparable to sample after Process I, as shown in Figure 4.13 (a). The

Cr and Zr signals were largest in the outer layer of TCP film, which was consistent with

the results from Process I. However, only a trace amount of F was observed at the

interface of TCP and metal matrix indicating F was not necessarily involved in the TCP

film.

As can be seen in Figures 4.14 and 4.15, longer immersion time in the TCP bath

following Process I resulted in a thicker coating. The oxygen content in the inner layer

was reduced to ~10 wt% compared to ~40 wt% for 2 min TCP samples. However,

accurate quantification of the O signal in EDS measurements is not possible [17]. Thus

the oxygen content from the EDS line profiling may indicate a certain trend of the real

composition in the series of specimens but it is not conclusive. In the case of 10 min

TCP following Process II, as shown in Figure 4.16, the Zr signal was high ~45 wt% in the TCP region and was relatively lower at the film/metal interface.

4.3.5. Electrochemical Behavior of TCP Coating after Different Pretreatments

EIS measurements were carried out to clarify the effects of two different

pretreatments on AA2024-T3. However, little difference was found. The low

94 frequency impedances were 22 kΩ⋅cm2 after Process I and 17 kΩ⋅cm2 after Process II, as shown in Figure 4.17.

4.4 DISCUSSION

Prior to TCP application, alkaline cleaning and acidic desmutting are conducted. Hughes et al. characterized the AA2024 surface after degreasing by XPS and detected the presence of Cu, Fe, Mg , Si and Zn on the surface probably due to the mild etching of the aluminum surface. [6] After desmutting, the Mg content was found largely lowered and enrichment of Fe and Cu was observed. Moreover, the surface roughness was also increased. [14, 20] These two pretreatment steps not only clean the surface but also alter the surface chemistry and morphology, which make the proper substrate pretreatment critical for obtaining well adhering and continuous TCP coatings.

4.4.1. Effect of Process I on the TCP coating

The first step in Process I was degreasing with Turco 6849 for 15 min at 50oC, which resulted in the removal of contamination, leaving a smooth surface. This aqueous alkaline degreaser contains sodium tripolyphosphate with pH 11.4~12.2. It was evident from the SEM micrographs that contamination at the edge of surface voids and along the rolling lines was removed after the alkaline degreasing step. The alkalinity of the degreaser might also result in mild dissolution of the aluminum matrix, which can be represented by

- - Al + H2O + OH Æ AlO2 +3/2 H2 (1)

Subsequent immersion in the Smut-go NC for various times resulted in

increasing degrees of acid etching of the aluminum surface, from minor pits over the

surface to eventually severe roughening and scalloping. The intermetallic particles

were totally or partially removed due to the dissolution of aluminum matrix adjacent to

them, leaving round and irregular shaped holes. Smut-go NC, which contains ferric

sulfate, bifluoride, nitric acid and small amount of sulfuric acid is a gentle desmutter

with low etch rate [16]. The smut layer remaining on the surface after the alkaline

degreasing was dissolved and brightening of the alloy surface was observed. This result

is generally consistent with the work reported by Toh and co-workers that a mixture of

ferric salt and hydrofluoric-nitric acid was an effective desmutter in removal of surface

oxide layer on AA7475-T7651 [18].

The growth of TCP following Process I started after 30 s immersion as evidenced by the presence of a partial film formed on the surface already at that time.

The particles were around tens of nm in size, which is smaller than those formed by 30 s in the TCP bath following Process II. On the other hand, complete coverage over the alloy surface was observed after 1 min of conversion coating. This is faster than after

Process II, which required more than 2 min for complete coverage. After complete coverage was achieved, the coating thickened. Cracks in the TCP coating were observed for samples exposed to the TCP bath 5 min and longer. However, such cracks are artifacts associated with the SEM vacuum level.

96 4.4.2. Effect of Process II on the TCP coating

The silicate based alkaline cleaner has been used prior to application of

chromate conversion coating for aluminum substrate[13], however, it was reported that the silicate residue after the step negatively impacted the formation of the TCP film

[10].

The combination of sodium bromate and nitric acid, the active components of

Sanchem 1000 as desmutter in Process II, is reported to have a low Al etch rate while being efficient in removal of the oxide layer and intermetallic particles from the Al alloy surface [11]. After immersion for 3 min at 55oC in this desmutter, surface

roughening in the form of connected etch pits appeared on the matrix to an extent

comparable to the 5 min treatment in Smut-go NC bath.

Nuclei of TCP film were observed after 10 s immersion in the TCP bath for the

Process II samples. This is sooner than for Process I samples, but coverage of the metal

surface by these particles was not complete until 3 min immersion, which is slower than

after Process II. On various samples the surface was characterized by the presence of

abnormally large clusters of TCP particles. It was evident that zirconium oxide particles tend to form clusters, which is uncommon for hexafluzirconate coating [21-23].

However, such large clusters were not easily observed on TCP formed after process I.

The SEM investigation on a series of samples after various TCP immersion times indicates that the growth of TCP film on the aluminum surfaces was different likely due to the changes of surface morphology and chemistry after the two different pretreatments [6, 12, 18]. Previous work has shown that nucleation mechanism of chromate conversion coating was different due to the changes of surface microstructure after different acid pickling processes [19]. The SEM observations indicate that TCP

97 nucleated uniformly across the surface of samples it after Process I (Figure 4.7 (d)),

while after Process II nucleation occurred only at preferred locations (Figure 4.7 (b)).

When dipped in the TCP bath after Process II, it is possible that the dissolution of the aluminum surface was hindered by the layer of silicate residue from the alkaline degreaser [18]. Although the silicon content was largely reduced after desmutting according to the XPS results (Figure 4.4), there was still a small amount of silicon left compared to the surface after desmutting in Smut-go NC. If so, anodic dissolution of the aluminum matrix accompanied by hydrogen evolution reaction was only possible at those sites on the surface without silicate coverage. The cathodic reaction increased the local pH resulting in the precipitation of hydrated zirconia particles on the surface along with hydrated chromium oxide. Afterwards, the precipitation of zirconium oxide and chromium oxide continued in other sites of the alloy surface resulted in a complete coverage of TCP film.

In the case of samples after Process I, with numerous micro-sized active sites, the nucleation of TCP film occurred relative uniformly over a broad front resulting in quick coverage over the whole surface after 30 s immersion.

4.4.3. Effect of Pretreatments on the TCP coating Chemistry and Thickness

Longer immersion in the TCP bath resulted in increased thickness of TCP film

on the AA2024-T3 for both pretreatments as can be seen from the TEM cross-section images in Figure 4.12 ~ 4.16. For 2 min samples after both treatments, the inner layer, which is composed of aluminum oxide, was observed to be thicker than 5 min and 10 min samples. It is likely that longer immersion in the fluoride-containing TCP bath

98 resulted in continuous thinning of the aluminum oxide layer during the growth of TCP

film. The chromium and zirconium contents in the coating after the two pretreatments

were quite similar with ~10wt% chromium and ~40wt% zirconium. Thus it appears

that the duration of TCP conversion and different pretreatments had no effect on the Cr

and Zr content in the coating. However, the fluorine content in the coating after Process

II was relatively lower compared to Process I. It is possible that the bifluoride salts in

the Smut-go NC bath was left on the metal surface after desmutting and contributed to

the higher fluorine content.

4.5 CONCLUSION

The nucleation and growth of TCP films after these two pretreatments are

different. Based on the microscopic characterization by TEM and SEM, chemical

analysis by XPS as well as the electrochemical measurements, the following

conclusions are drawn:

(1) The nucleation of TCP film after Process II was faster compared to Process I.

(2) The growth of TCP following Process I started after 30 s immersion, which was

faster compared to more than 2 min after Process II.

(3) The size of the particles was around tens of nm which was smaller compared to

those following Process II.

(4) The abnormal round clusters were not easily observed after Process I.

(5) Homogeneous nucleation occurred on the sample surface after Process I, while

heterogeneous nucleation occurred after Process II.

(6) Longer immersion in the TCP bath resulted in increased thickness of TCP film for

99 both pretreatments.

(7) These two pretreatments prior to TCP result in similar surface conditions on

AA2024-T3.

Considering the small differences in the details of the TCP coating after the two

pretreatments, it is concluded that the use of a silicate-based degreasing bath prior to

TCP formation on the AA2024-T3 surface does not have a major influence on the TCP

film.

100

Table 4.1 Samples treated by Process I with different times of degreasing, desmutting and coating.

• Turco 6849, 15min • Turco 6849 15min • Smut-go 5min 1. Smut-go 1min 4. TCP 10S Turco 6849, 2. Smut-go 5min 5. TCP 30S 3. Smut-go 10min 6. TCP 60S 15min 7. TCP 2min 8. TCP 3min 9. TCP 5min 10. TCP 7min 11.TCP 10min

Table 4.2 Samples treated by Process II with different times of degreasing, desmutting and coating.

• Degrease, 2min, • Desmut, 3min 2 min, 1. TCP 10S Na SiO 2 3 3min, 2. TCP 30S HNO + Sanchem 1000 + DI water + Na2CO3 3 3. TCP 60S + DI water 4. TCP 2min 5. TCP 3min 6. TCP 5min 7. TCP 7min 8. TCP 10min

101

Figure 4.1. SEM micrograph of as-received AA2024-T3 surface.

102

Figure 4.2. SEM micrograph of AA2024-T3 surface after degreasing by Process I for 15 min. Two regions of the surface are shown.

103

Figure 4.3. SEM micrograph of AA2024-T3 surface by Process II after degreasing for 2 min.

104

1000 Degreasing,Process II Degreasing,Process I 800

Desmutting,Process II 600 Desmutting,Process I CPS 400

200

0 94 96 98 100 102 104 106 108 110 Binding Energy (eV)

Figure 4.4. Si 2p spectra of surfaces after different pretreatments.

105

(a)

(b)

106 Figure 4.6. SEM micrograph of AA2024-T3 surface by Process II after desmutting for 3 min.

107

(a) (b)

(c) (d) Figure 4.7. SEM micrograph of AA2024-T3 surface after given treatment (a) Process I 10 s, (b) Process I 30 s, (c) Process II 10 s, (d) Process II 30 s.

108

(a) (b) Figure 4.8. (a) SEM micrograph of TCP on AA2024-T3 after Pretreatment II 30s. (b) EDX profile.

109

(a) (b)

(c) (d) Figure 4.9. SEM micrograph of AA2024-T3 surface after given treatment (a) Process I 60 s, (b) Process I 2 min, (c) Process II 60 s, (d) Process II 2 min.

110

(a) (b)

(c) (d) Figure 4.10. SEM micrograph of AA2024-T3 surface after given treatment (a) Process I 3 min, (b) Process I 5 min, (c) Process II 3 min, (d) Process II 5 min.

111

(a) (b)

(c) (d) Figure 4.11. SEM micrograph of AA2024-T3 surface after given treatment (a) Process I 7 min, (b) Process I 10 min, (c) Process II 7 min, (d) Process II 10 min.

112

Pt layer Au layer

TCP

Al matrix

(a)

100

Al Al 80 Au Au Cr F 60 O Zr

wt% O 40 Zr F 20 Cr

0 20 30 40 50 60 70 80 90 100 110 Position (nm)

(b) Figure 4.12. 2 min TCP on matrix of AA2024-T3 after Process I, (a) Transmission electron micrograph, (b) Nano-EDS line profiles.

113 Pt layer

Au layer TCP

Al matrix

(a)

100

Al 80 Au O Zr Al 60 Au Cr F wt% O 40 Zr

Cr 20 F

0 0 1020304050607080

Position (nm) (b) Figure 4.13. 2 min TCP on matrix of AA2024-T3 after Process II, (a) Transmission electron micrograph, (b) Nano-EDS line profiles.

114 Pt layer Au layer TCP

Al matrix

(a) 100

Zr Al 80 Au Al Au 60 Cr F O wt% 40 Zr O Cr 20 F

0 0 102030405060708090100110

Position (nm) (b) Figure 4.14. 5 min TCP on matrix of AA2024-T3 after Process I, (a) Transmission electron micrograph, (b) Nano-EDS line profiles.

115

TCP

TCP 10 min

(a)

80 Pt Al 60 Zr Al Cr F 40

wt% O O Pt Cr Zr 20 F

0 20 40 60 80 100 120 140 160 180 Position (nm)

(b) Figure 4.15. 10 min TCP on matrix of AA2024-T3 after Process I, (a) Transmission electron micrograph, (b) Nano-EDS line profiles.

116

(a) 100

80 Al Au Al 60 Au Cr t% F

w O O 40 Zr Cr Zr

20 F

0 0 102030405060708090100 Position (nm)

(b) Figure 4.16. 10 min TCP on matrix of AA2024-T3 after Process II, (a) Transmission electron micrograph, (b) Nano-EDS line profiles.

117

105 -80

|Z| Process I Process I -70 |Z| Process II Process II

-60

10 (degrees) Theta -50

-40 |Z|(Ohms) -30 103

-20

-10

102 0 10-2 10-1 100 101 102 103 104 105 Frequency (Hz) Figure 4.17. Bode plots obtained by EIS on the TCP coated AA2024-T3 after Process I and Process II

118

REFERENCES

1 . I.J. Polmear, Light Alloys Metallurgy of the Light Metals, Third ed., , London: Arnold, 1995.

2 . S. Wernick, R. Pinner, P.G. Sheasby, The surface treatment and finishing of aluminum and its alloys, 5th ed., Ohio : ASM International ; Teddington : Finishing, 1987.

3 . J.E. Hatch, Aluminum: properties and physical metallurgy, American Society for Metals, Metals Park, Ohio, 1984.

4 . W.C. Nickerson, E. Lipnickas, TriService Corrosion Conference Proceedings, (2003).

5 . A. Iyer, W. Willis, S. Frueh, W. Nickerson, A. Fowler, J. Barnes, L. Hagos, J. Escarsega, J. La Scala, S. Suib, Plating and Surface Finishing, (May 2010) 32-42.

6 . A.E. Hughes, R.J. Taylor, K.J.H. Nelson, B.R.W. Hinton, L. Wilson, Mater. Sci. Technol., 12 (1996) 928-936.

7 . R.G. King, Treatment and Finishing of Aluminium Pergamon Press, Headington Hill Hall, Oxford OX3 0BW, England, 1988.

8 . G.J. Cormier, ASM Handbook, Surface Engineering, Alkaline Cleaning, Volume 5.

9 . Henkel Turco 6849 Aqueous Alkaline Degreaser Technical Process Bulletin No.238832.

10 . W. Nickerson, Personal communication, (2011).

11 . T.G. Harvey, A.E. Hughes, S.G. Hardin, T. Nikpour, S.K. Toh, A. Boag, D. McCulloch, M. Horne, Appl. Surf. Sci., 254 (2008) 3562-3575.

12 . A.E. Hughes, G. Theodossiou, S. Elliott, T.G. Harvey, P.R. Miller, J.D. Gorman, P.J.K. Paterson, Mater. Sci. Technol., 17 (2001) 1642-1652.

119 13 . R.G. Buchheit, A.E. Hughes, Chromate and Chromate-Free Conversion Coatings, ASM Handbook: Fundamentals, Testing, and Protection, 2003.

14 . J.E. Hatch, Materials Park, OH: ASM International, (1984) 354.

15 . Henkel, Turco 6849, Aqueous Alkaline Degreaser, Material Safety Data Sheet, ID: 238832.

16 . Henkel Turco liquid Smut-go NC MSDS ID:239357DLP55G.

17 . J.I. Goldstein, Scanning electron microscopy and x-ray microanalysis, 3rd ed., Kluwer Academic/Plenum Publishers, New York, 2003.

18 . S.K. Toh, A.E. Hughes, D.G. McCulloch, J. duPlessis, A. Stonham, Surf. Interface Anal., 36 (2004) 1523-1532.

19 . P. Campestrini, E.P.M. van Westing, J.H.W. de Wit, Electrochim. Acta, 46 (2001) 2553-2571.

120

CHAPTER 5

Assessment of Active Corrosion Inhibition of Trivalent Chrome

Process Coatings on AA2024-T3

5.1 INTRODUCTION

Chromate conversion coatings (CCCs) have long been employed in the surface finishing process for AA2024-T3 and other metal alloys for their excellent ability to resist localized corrosion and to promote paint adhesion. “Self-healing” is the predominant feature of CCCs, in which substrate metal exposed at a scratch or defect in the CCC is protected by the Cr(VI) species leached from the coating [1-3]. However, due to the toxic and carcinogenic effects of hexavalent chromium compounds on the environment and human health, there have been increasingly stringent legislations regarding their use and waste disposal. Consequently, significant efforts have focused on chromate-free corrosion inhibitor systems that can provide comparable performance

[4].

The trivalent chrome process (TCP) developed at NAVAIR is currently one of

121 the leading non-chromate conversion coatings on the market and has been shown to

provide excellent corrosion protection and paint adhesion in standardized tests [5]. The

TCP process is a drop-in replacement for hexavalent chromate treatments and it

contains no Cr(VI) in the coating bath and resulting film. The TCP coating systems are

similar to commercial fluorozirconate/fluorotitanate-based conversion coatings, but

also contain Cr(OH)3 and Cr2O3, which improve the coating to provide better protection

[6]. There are currently seven Cr(VI)-free products that meets the requirements

according to Class 1A of MIL SPEC MIL-DTL-5541F, Chemical Conversion Materials

for Coating Aluminum and Aluminum Alloys [7-8]. These seven different products are

all based on Cr(III) [8].

Self-healing or active corrosion inhibition exhibited by CCCs is considered to

involve three steps: 1) release of Cr(VI) into a corrosive environment, 2) transport of

Cr(VI) through the aqueous phase to a non-coated or damaged area, and 3) reduction of

soluble chromate to insoluble and protective chromium hydroxide [1, 5, 9-15]. The self healing property of CCC along with the excellent protectiveness of chromium hydroxide coatings on light alloys makes CCC a highly anticorrosion coating [1, 3, 5,

16-17].

The Cr(VI) storage in CCC has been thought to be on the order of 10-7 mol/cm2 depending on the coating weight and thickness [1]. Spectroscopic analysis indicated that exposure of CCC to water or salt solution causes the covalent Cr(III)-O-Cr(VI) bond in the matrix of Cr(III)/Cr(VI) mixed oxide to break, as described by the following equation [1]:

- 2- + Cr(III)-O-Cr(VI)O3 (s) + H2O→Cr(III)-OH(s) + Cr(VI)O4 (aq) + H (aq) (1)

122

The Cr(VI) released from a CCC into an aqueous surface film or bulk solution can

reach a level of approximately 10-4 M [1, 5]. The low solution concentration of Cr(VI) anticipated in most environments helps the release of Cr(VI) from a CCC [5]. It is proposed that Langmuir-like adsorption behavior is followed during the release of chromate from a CCC, as it depends on solution ionic strength and the ratio of CCC surface area to solution volume [1]. In addition, it is also shown in Equation 1 that the

Cr(III)/Cr(VI) equilibrium is pH dependent with chromate uptake at low pH and release

2- - at high pH. This Cr(VI) in solution, usually in the form of CrO4 or HCrO4 , can

migrate to damaged locations in the coating and interact with exposed bare metal.

One approach to study self healing of protective coatings is the artificial scratch

technique [3]. In this technique, a bare, untreated surface representing bared metal at

the base of a scratch is exposed in close proximity to a coated surface, separated by a

thin layer of electrolyte. This separation allows for independent assessment of the

performance of the bare and coated surface. Analysis of the untreated surface also

provides evidence of transport because inhibitor species found on the untreated surface

must have migrated from the coating.

The goal of this work is to use the artificial scratch technique to assess and

quantify the self healing properties of TCP on AA2024-T3.

5.2 EXPERIMENTAL PROCEDURES

Samples. Aluminum alloy 2024-T3 sheets (nominal chemical composition of

Al, 3.8-4.9%Cu, 1.2-1.8%Mg, 0.30-0.9%Mn, 0.5%Fe, 0.5%Si, 0.25% Zn, 0.10%Cr,

123 0.15%Ti and 0.15% other elements [18]) with a thickness of 2 mm (ALCOA) were cut

from a commercial sheet stock.

The samples were treated by (i) degreasing for 15 min at 55oC in an alkaline

solution (15 vol% Turco 6849), (ii) deoxidation for 5 min at room temperature in an acid solution (20 vol% Liquid Turco Smut-go NC) (iii) conversion coating for 5 min at room temperature in a commercial bath (AlodineTM 5900S). The recommended contact

time for immersion is 5 ~ 9 min [19]. These solutions are all products of Henkel Corp.

The samples were rinsed with DI water after each step. After the last rinse they were

blown dried with compressed air at room temperature and exposed to lab air for at least

1 h prior to use if not mentioned otherwise. Samples that were treated with only steps (i)

and (ii) were termed “nonTCP”.

Artificial Scratch Cell. A schematic of the artificial scratch cell is shown in

Figure 5.1 (a) and a picture of the assembled cell is shown in Figure 5.1 (b). The

standard configuration of the scratch cell used a nonTCP sample situated above a

TCP-coated sample separated by an o-ring 4.8 mm in thickness and 40 mm in diameter.

The nonTCP sample was always placed above the TCP-coated sample to eliminate the

condition of inhibitor transport by gravity alone. A Pt wire attached to a Pt mesh

counter electrode (CE) was inserted through a hole in the o-ring. The CE was stiff

enough that it stayed suspended in the cell without making contact to either Al alloy

electrode. The samples were fastened together with bolts through holes drilled at the

edges. Another hole 6.2 mm in diameter was drilled through the top nonTCP sample to

facilitate insertion of a saturated calomel reference electrode (SCE) for electrochemical

measurements. The edge of this hole was sealed with red lacquer to prevent crevice

124 corrosion. Some cells used two nonTCP plates as a control.

The cell interior volume defined by the o-ring and the two Al alloy plates was

filled with corrosive solution. The solutions used in the artificial scratch cell were

0.5 M NaCl and dilute Harrison’s solution (0.05 wt% NaCl + 0.35wt% (NH4)2SO4), which is much less aggressive [20]. The solutions were prepared with reagent grade chemicals and deionized water with a resistivity of 18.2 MΩ•cm.

The cells were kept in sealed containers with high relative humidity to avoid the evaporation of electrolyte from the hole in the top panel. They were periodically removed from the container and a reference electrode was inserted in the top hole for each electrochemical measurement.

In some artificial scratch cells, the two surfaces were electrically shorted by copper tape during exposure, which is the condition of real scratches through a coating.

In some other cells, the area of the top nonTCP panel was reduced by application of corrosion protective tape leaving a hole with diameter of 3 mm and making the area ratio of TCP to nonTCP surfaces equal to 177. Otherwise for the regular artificial scratch cell, the area ratio of TCP to nonTCP surfaces is 1.03.

Electrochemical Impedance Spectroscopy (EIS). EIS measurements were periodically performed on both the nonTCP and TCP coated surfaces of the artificial scratch cells to evaluate the corrosion resistance of TCP coating and its active corrosion protection behavior. A Gamry Reference 600 potentiostat was used to apply a 10 mV signal around the open circuit potential at frequencies ranging from 105 to 10-2 Hz.

For shorted artificial scratch cells, the electrical connection of two surfaces by copper

125 tape was removed during EIS tests and reconnected after the tests were finished.

Inductively Coupled Plasma - Optical Emission Spectrometry (ICP-OES).

TCP coated samples were exposed to 20 ml dilute Harrison’s solution for various times in a Gamry PTC1 cell with an exposed area of 7.35 cm2, as shown in Figure 5.2. The

exposed electrolyte was analyzed by ICP-OES for the presence of chromium and other dissolved species that might have leached out from the TCP coating. A Perkin-Elmer

Optima 3000DV instrument at the Trace Element Research Laboratory of OSU was used for this analysis. The exposed solution was acidified with nitric acid and the content was determined with part-per-billion sensitivity.

X-ray Photoelectron Spectroscopy (XPS). The artificial scratch cells were opened after different exposure times and specimens with dimension 1 x 1 cm were cut from the nonTCP side. XPS measurements were conducted to examine the presence of chromium species on the nonTCP surfaces and their oxidation states. Before the signal was collected, the sample surfaces were etched by Ar gas for 120 s to remove contaminants from handling. A Kratos AXIS Ultra spectrometer controlled by a

VISION data system was used with a mono-chromatic Al Kα X-ray source operated at

1486.6 eV and 150 W. Typical operating pressures were around 2x10-8 Torr. All spectra

collected were calibrated by the carbon 1s peak at 284.6 eV.

Scanning Electron Microscopy (SEM) and Energy Dispersive

Spectrocopy (EDS). TCP coated AA2024-T3 samples with dimensions of 10 x 10 x 2

mm were exposed to dilute Harrison’s solution and then rinsed with deionized water

126 prior to SEM analysis. A Phillips Electronics model XL-30F Environmental SEM with

a field emission gun was used to examine the corrosion morphology. A secondary

electron detector was used to image the coating with a 15 kV accelerating voltage.

5.3 RESULTS

5.3.1 Visual Observation of Corrosion Protection in Artificial Scratch Cells

Artificial scratch cells were opened after various exposure times (7, 14 and

21 days) and examined for evidence of corrosion. Figures 5.3 (a) and (b) show the two sides of a cell exposed to 0.5 M NaCl for 21 days. The TCP-treated AA2024-T3 sheet still exhibited the pale tan color characteristic of TCP, Figure 5.3 (a). The

nonTCP side of the cell, Figure 5.3 (b), exhibited attack, but considerably less than

the two sides of the control cell comprising two nonTCP samples, Figures 5.3 (c) and

(d).

5.3.2 Inhibitor Release and Surface Morphology

ICP-OES results obtained from analysis of dilute Harrison’s solution after exposure to TCP-coated samples are shown in Figures 5.4 and 5.5, where concentrations of dissolved species are plotted as a function of exposure time. Figure

5.4 shows the concentrations of Cr, Cu and Zr ions. The ion concentrations were low and constant for the first 3 days. After 5 days the Cu and Cr ion contents increased.

The chromium concentration was about 0.3 μM during the first days and then increased,

127 reaching 2.7 μM after 10 days. It stayed around 3 μM until a sharp increase to 7.5 μM was observed for the 60-day-exposure sample. For the whole exposure time, the zirconium content in the exposed solution stayed just above the detection limit of the equipment, ~0.05 μM, which suggests that the zirconium was insoluble and remained in the coating. The concentration of released copper increased to 0.7 μM upon exposure, and then increased again after around 5 days to around 3 μM, where it stayed with some fluctuation for the duration of the exposure. The Al ion concentration is shown in

Figure 5.5. A spike in concentration was observed during the first days of exposure reaching a maximum concentration of 5 μM. During the following 15 days of exposure, the concentration of Al remained at about 2 μM. An increase to 5 μM was found for the 30 days exposure sample, after which the Al ion concentration increased steeply to

175 μM at 50 and 60 days.

5.3.3 Chromium Transport in the Artificial Scratch Cell Experiments

The surfaces of nonTCP samples from artificial scratch cells exposed after 7,

14 and 28 days were studied in XPS to investigate the presence and oxidation states of chromium species. Typical spectra are shown in Figure 5.6. After exposure in 0.5 M

NaCl, XPS measurements indicated a trace amount of chromium species on the nonTCP surface, as shown in Figure 5.6 (a). This might explain the remarkable corrosion inhibition exhibited by the nonTCP panel from visual inspection test, as shown in Figure 5.3 (b).

Similarly, after exposure in dilute Harrison’s solution, which is much less aggressive, XPS analysis also confirmed the presence of chromium species on the

128 surface of the nonTCP AA2024-T3 surface, Figure 5.6 b. The binding energy of the Cr

2p peak shifted toward higher values with time, indicating that the composition of the

chromium species on the nonTCP surface changed during exposure in the artificial

scratch cell. For comparison, XPS examination was also performed on the freshly

prepared non TCP surface and no traceable chromium signal was detected. Fitting of

the XPS spectra, as shown in Figure 5.7, indicated that these species are mainly Cr(III), but that a small portion of Cr(VI) could be present.

The aluminum signals from the untreated sample surfaces were also collected and the typical spectra are shown in Figure 5.8. For the 0 day sample, which was not exposed to the electrolyte, Al 2p peaks were observed at 75.91eV and 72.8 eV, representing the Al3+ and Al metal, respectively. The spectrum of the sample exposed

for 7 days exhibited an increase in the intensity of the Al3+ peak and a decrease in the Al metal peak. After 14 days exposure, the Al metal peak was only a slight bump and after

28 days this peak was not observed. The binding energy of the Al3+ peak showed a

similar trend as the Cr peak in that the binding energy shifted with time to higher values.

This shift suggests the formation of AlOOH and then Al(OH)3 with time.

5.3.4 EIS Characterization of TCP Coatings in the Artificial Scratch Cell

Impedance data were acquired periodically from the artificial scratch cells

exposed to the dilute Harrison’s solution. Bode magnitude and phase angle plots for the

TCP-treated and nonTCP surfaces are shown in the following figures.

The first electrolyte used in the artificial scratch cell was 0.5 M NaCl. The

TCP-treated side, Figure 5.9, started out with a low frequency impedance (which

129 should be close in magnitude to the polarization resistance) that was almost two orders

of magnitude higher than the nonTCP side, Figure 5.10. With time, the polarization

resistance decreased, but it remained about 10 times greater even after 20 days

exposure. However, the polarization resistance of the nonTCP sample was similar to a

control surface where both sides of the cell were nonTCP samples, Figure 5.11. This

implies that there was no significant corrosion inhibition provided by the nearby TCP

surface to the nonTCP surface. Note that these results are not in agreement with those

from visual inspection as shown in Figure 5.3, which indicated inhibition in 0.5 M NaCl.

Therefore, dilute Harrison’s solution, which is much less aggressive than the 0.5 M

NaCl solution, was used for the following experiments.

The Bode magnitude and phase angle plots from the TCP coated side of the artificial scratch cell exposed to the Harrison’s solution are shown in Figure 5.12.

Typical spectra collected from eight different exposure times are plotted. The low frequency impedance value of the TCP coated surface was 2 x 106 ohm⋅cm2 after 24 h.

The low frequency impedance increased steadily with time to 5 x 106 ohm⋅cm2 after

528 h exposure. After 692 h, the impedance value decreased suggesting commencement of attack of the TCP surface. The phase angle at the medium frequency range (0.1~100 Hz) took the value of -80°.

Figure 5.13 shows the Bode magnitude and phase angle plots from the nonTCP

side exposed in the artificial scratch cell. The initial coating resistance value was

comparatively lower at 6 x 105 ohm⋅cm2. A slight increase occurred before the

impedance value decreased back to the original value.

The phase angle spectra were different from those shown in the Figure 5.12 for

the TCP coated surface. The phase angle for the nonTCP surface was slightly more

130 negative (-85°) in the medium frequency range (0.1~100 Hz). It reached -10° at 10-2 Hz and then increased again at lower frequencies. The high frequency impedance in the

Bode plots, which is proportional to the ohmic resistance, fluctuated during exposure. It is possible that resistive corrosion product formed in the electrolyte but then changed in nature during the long term exposure.

As a comparison, EIS measurements were also made on the nonTCP treated surface in a nonTCP control cell, as shown in Figure 5.14. The low frequency impedance values fell into the range of 3 x 105 ohm⋅cm2 ~ 4 x 105 ohm⋅cm2. The phase

angle magnitude reached the value of -85° in the medium frequency range.

The low frequency impedance (impedance magnitude at 0.01 Hz) for surfaces in

the artificial scratch cells and control cells as a function of exposure time were

compiled, as shown in Figure 5.15. The low frequency impedance data were collected

from six different samples and statistical analysis was carried out. The plots show the

average value of the low frequency impedance and the error bars are the standard

deviation. The initial values of the polarization resistance for the TCP and nonTCP

surfaces as well as the nonTCP sample in the control cell were all similar, in the range

of 0.5 – 1.2 x 105 ohm⋅cm2. Within the first 24 hours of exposure, the low frequency

impedance of the nonTCP surface in the artificial scratch cell increased to 3.5 x 105 ohm⋅cm2, which was only slight lower than the TCP surface and almost twice that of the

nonTCP control surface, as shown in Figure 5.15 (a). After 24 hours, there was a large

increase in the low frequency impedance of the TCP surface to 1.2 x 106 ohm⋅cm2 within 168 hours. It remained stable at ~ 106 ohm⋅cm2 through the 700 hours of

exposure, as shown in Figure 5.15 (b). For the nonTCP surface, the low frequency

131 impedance value slightly increased to 5 x 105 ohm⋅cm2 after 72 hours exposure and they stayed at 4 x 105 ohm⋅cm2 through the whole test period. The low frequency

impedance of the nonTCP control remained at about 2 x 105 ohm⋅cm2, half the value of

that for the nonTCP sample.

Equivalent circuit modeling was used to determine the surface characteristics of

TCP and nonTCP surfaces in the artificial scratch cells, as shown in Figure 5.16. The

TCP coating acts as a dielectric layer and is represented by Ccoat (coating capacitance).

As the TCP coating is exposed, defects that appear in the coating are represented by Rpo

(pore resistance). At the interface of the coating and the Al substrate, the double layer capacitance Cdl appears parallel to the polarization resistance Rcorr. The solution

resistance is represented by Rs.

Figure 5.17 shows the polarization resistance of TCP and nonTCP surfaces in

5 2 artificial scratch cells. The Rcorr of TCP surface increases from 3 x 10 ohm⋅cm to 1 x

6 2 10 ohm⋅cm after 200 hours exposure. And for the nonTCP surface, Rcorr stays at 3.5 x

105 ohm⋅cm2 after a initial decrease to 3 x 105 ohm⋅cm2. As a comparison, the

polarization resistance of nonTCP control remains at ~ 2 x 105 ohm⋅cm2. The coating

capacitance is shown in Figure 5.18. The TCP surface has a stable capacitance value ~

3.5 μF/cm2. For the nonTCP surface, the initial capacitance is 5 μF/cm2 and after 168

hours the value decreases to 4.5 μF/cm2. For the control surface, the capacitance was

stable at about 4.5 F/cm2 until 400 hours after which it increased to 5.8 μF/cm2. The coating capacitance of nonTCP and control samples started out with similar values; the increase in the capacitance of the control sample was possibly associated with degradation of the surface.

132 5.3.5 Artificial Scratch Cell Tests with Electrical Connection and Different Area

Ratio

To better simulate real scratches, in which defects are electrically shorted to the TCP coating, the two panels in the artificial scratch cells were connected by copper

tape during exposure. This type of artificial scratch cell is termed “shorted” in the

following figures. The non-shorted cell is termed “open”. Furthermore, the nonTCP

treated surfaces in some of the artificial scratch cells were partly covered by corrosion

protective tape to change the area ratio (AR) of TCP to nonTCP surface.

Figure 5.19 shows the open circuit potentials collected for different surfaces in

the artificial scratch cells mentioned above. In the open artificial scratch cells, the

corrosion potential for nonTCP surfaces (nonTCP/TCP open, AR=1.03) increased from

-710 mV SCE after 100 h to about -650 mV SCE after 300 h. The OCP for TCP coated

surface (nonTCP/TCP, open, AR=1.03) decreased from -520 mV SCE to -600 mV SCE.

The behavior of the nonTCP surface in the shorted cells with area ratio equal to 1.03

was different than the open cell, The OCP for the nonTCP surface (nonTCP/TCP, shorted, AR=1.03) stayed at -570 mV SCE during the first week of exposure and then increased. For the shorted cell with area ratio equal to 177, the corrosion potential for the TCP surface (nonTCP/TCP, shorted, AR=177) exhibited a similar trend as in the

open cell with a value initially at -520 mV SCE that stabilized at -550 mV SCE.

However, for the nonTCP surface in the shorted cell (nonTCP/TCP, shorted, AR=177), the OCP value remained at about -560 mV SCE which was 100 mV positive to the nonTCP surface in the open cells. Overall, the TCP surfaces in all types of artificial scratch cells exhibited OCP values of about -550 mV SCE in the first 300 h of exposure; while the electrical shorting and higher area ratio both raised the corrosion potential

133 values of the nonTCP surface by 100 mV.

The low frequency impedance values at 0.01 Hz for TCP and nonTCP surfaces

in different types of artificial scratch cells were plotted as a function of time, as shown

in Figure 5.20. At the lower part of this figure, the nonTCP surface in the shorted cell

with area ratio 177 had the lowest impedance value, about 2x105 ohm⋅cm2, which was

even lower than the nonTCP surface in the control cell. It appeared that the TCP surface

in the same cell had the similar impedance value as the TCP surface in the regular cell,

about 1x106 ohm⋅cm2. In the case of the shorted cell with area ratio 1.03, both the

nonTCP and TCP coated surface exhibited impedance value of about 4x105 ohm⋅cm2,

which was only comparable to the nonTCP surface in the regular cell.

5.4 DISCUSSION

5.4.1 Corrosion Protection of TCP Coated Surface

Zirconium based coatings have been reported to enhance the protectiveness of

paints but provide only modest corrosion resistance on metallic substrates in the

unpainted state [6, 21]. By the addition of small amount of chromium species, the TCP

coating is reported to have comparable corrosion performance to the chromate

conversion coatings [6]. The photographs shown in Figure 5.3 (c) demonstrate that the

TCP coating provided significant corrosion protection to the AA2024-T3 substrate

without a paint overcoat in an environment containing high chloride concentration after three weeks exposure. This visual observation agrees with published results [22]. It was also reported that the impedance of TCP-coated AA7075-T6 substrate reached 1x106

134 ohm⋅cm2 while the value for TCP-coated AA2024-T3 was much lower [6].

The excellent corrosion protection demonstrated by TCP coating probably resulted from the dense structure of the zirconium/chromium oxide, which provided

barrier protection. The performance can also be attributed to the chromium addition

into the coating system.

5.4.2 Active Corrosion Protection by Self-Healing

The uniqueness of the artificial scratch cell is that it can provide clear evidence of the self-healing property of a coating system. A lower corrosion rate of the nonTCP surface in the cell relative to a control cell can only be attributed to the inhibitor released from the coating nearby. Zhao and coworkers first used the artificial scratch

cell to explore the self healing property of chromate conversion coating by Raman spectroscopy and XPS [5, 23]. Chromate was found to be released from the conversion coating and migrate to a nearby uncoated surface resulting in the enhancement of the corrosion resistance of the uncoated surface.

The ICP-OES characterization of the electrolyte exposed to the TCP coating for different exposure time proved that the chromium species in the TCP coating was released into the aqueous environment while zirconium was barely detected, as shown in Figure 5.4. The experimental fact that the trivalent chromium can be released from the TCP coating provides evidence for one of the requirements of self-healing, i.e. release of active species. Evidence for a second requirement of self-healing, transport to a nearby bare surface, was provided by the XPS analysis, Figure 5.6, which showed the presence of chromium on the surface of the nonTCP sample. Visual observation

135 shows that the nonTCP surface was well protected by the nearby TCP coating in the artificial scratch cell, as shown in Figure 5.3 (d). In contrast, severe corrosion was observed on the control nonTCP surfaces.

Despite the clear evidence of Cr release, transport and protection, the EIS data from the artificial scratch cell provided evidence of only minor corrosion inhibition of the nonTCP surface. For instance, the low frequency impedance for the nonTCP surface was about 4x105 ohm⋅cm2, which is about twice that found for control experiments with two nonTCP surfaces. Since the low frequency impedance is considered to be proportional to the polarization resistance, the higher value for the nonTCP surface indicates marginally lower corrosion rate, probably resulting from the chromium released from the nearby TCP coating. A rapid increase in the impedance value was observed during the first 24 hour exposure for the nonTCP surface suggesting an increase in corrosion resistance which might be attributed to chromium release and deposition during this time.

The values of coating capacitance for artificial scratch cells also suggested mild active corrosion protection. The increase in the capacitance of control surface might result from attack of the nonTCP surfaces. However, for the nonTCP surface, the capacitance value decreased slightly from initial 5 μF/cm2 to 4.5 μF/cm2, which suggested slight active corrosion inhibition was provided by the TCP coated surface across the electrolyte.

When the samples in the artificial scratch cell were electrically shorted, the impedance value for the nonTCP surface did not change much. Interestingly, when the

TCP/nonTCP area ratio was increased from 1.03 to 177, the low frequency impedance decreased to a lower value than the control sample ~2x105 ohm⋅cm2. It was expected

136 that the smaller exposed nonTCP area would benefit from having relatively more

chromium for protection. The shorting might have an adverse effect on the corrosion

resistance during the exposure which balanced the resistance enhancement by larger

area ratio.

5.4.3 Possible Mechanism of Active Corrosion Protection

Based on the evidence presented above, it is clear that TCP can provide some amount of active corrosion inhibition. However, the mechanism for active corrosion protection by

TCP should be different than the CCC mechanism because the Cr(III) ion is generally not regarded to be mobile. Zhao et al. stated that active corrosion protection of CCCs was accomplished by the following three steps: 1) release of hexavalent chromium species from the chromate conversion coating, 2) migration to a defect site and 3) reduction to insoluble Al/chromium precipitate [5]. The active corrosion protection by

CCCs is very strong. The Cr(VI) reservoir in CCC has been thought to be on the order of 10-7 mol/cm2 depending on the coating weight and thickness [1], and Cr(VI) is released into aqueous solution to a level of approximately 10-4 M [10-11]. This low concentration can reduce to form a monolayer of Cr(III) oxide or hydroxide on a bare

Al alloy surface that is extremely protective in that it stifles both the oxygen reduction reaction and further Cr(VI) reduction and results in a large reduction in the corrosion rate of the Al alloy [5, 13, 24-25].

The active protection provided by TCP is much more subtle. It was shown

above that Cr species are released into solution contacting TCPs and can deposit on a

137 nearby bare Al surface. The dissolution and transport might be connected to transient

formation of Cr(VI). Swain and coworkers recently provided evidence that Cr(VI) can

form on TCP surfaces during oxygen reduction [26]. They suggested that peroxide

intermediate formed during the oxygen reduction reaction oxidizes Cr(III) to mobile

Cr(VI) ions [26]. Then a protective film forms on damaged regions by migration and

reduction of the Cr(VI) ions.

Another possibility is that Cr(III) is released non-faradaically into the solution,

diffuses to the bare area and deposits. For instance, the ICP-OES results showed the

existence of chromium species in the electrolyte exposed to the TCP coating but no Zr

species. It is possible that certain amount of organometallic Cr(III) species in the TCP bath is incorporated into the mixed Zr-Cr oxide during coating formation and can desorb from the coating into the electrolyte. It is also possible that Cr(III) hydroxide disintegrates from the mixed Zr-Cr oxide film due to the exposure to aggressive ions in the electrolyte such as Cl-. The dilute Harrison’s solution contains ammonia at a pH of

about 5.8, which might promote the release of Cr(III) from the TCP into solution in the

3+ form of [Cr(NH3)6] which is soluble and mobile [27].

5.5 CONCLUSION

The artificial scratch cell was characterized by EIS and XPS for clear assessment of various aspects of active corrosion inhibition.. Elemental analysis of the exposed electrolyte of TCP surface was conducted by ICP-OES. Following conclusions are suggested:

138 (1) TCP treatment can greatly improve the corrosion resistance of AA2024-T3 to

sustain the long time exposure to simulated corrosive environments.

(2) After static exposure in dilute Harrison’s solution, chromium species was detected

in the exposed electrolyte suggested the capability of chromium release from the

TCP coating. However, the zirconium release was not detected.

(3) In the artificial scratch cell, on the uncoated sheets, chromium was found on the

surface which provided evidence that TCP can provide some degree of active

corrosion inhibition for Al alloys.

(4) EIS data showed the polarization resistance of uncoated surface was stable at 4x105

ohm•cm2, which was twice as much as uncoated controls, indicated active

corrosion inhibition was provided by the TCP coating to the near by uncoated

surface.

139

(a)

(b)

Figure 5.1 (a). Schematic drawing of artificial scratch cell, (b). Photographs of shorted artificial scratch cell.

140

Figure 5.2. Photographs of leaching cells.

141

(a) (b)

Figure 5.3. Photographs of artificial scratch cell sheets exposed to 0.5 M NaCl; (a) & (b) TCP-treated and corresponding bare surface exposed for 21 days; (c) & (d) bare surface controls exposed for 14 days.

142

7 Cr Cu 6 Zr M)

μ 5

3 Concentration ( Concentration 2

0 0 102030405060 Exposure time (days)

Figure 5.4. Inhibitor released after static exposure in the dilute Harrison’s solution by ICP-OES.

143

180

160

140 Al 120 6

M) 5 μ 100 4 0-20 days

80 3

60 2

1 Concentration ( 40 0 048121620 20

0 0 102030405060 Exposure time (days)

Figure 5.5. Aluminum content after static exposure in the dilute Harrison’s solution by ICP-OES.

144 1750

1700

14 d 1650

1600 Intensity (CPS)

1550

1500 595 590 585 580 575 570 Binding Energy (eV)

(a)

1900

1800 28 d

7 d 1700

1600 Intensity (CPS) 14 d

1500

fresh 1400 595 590 585 580 575 570 Binding Energy (eV)

(b)

Figure 5.6. XPS spectra measured from untreated AA2024-T3 surface in the artificial scratch cell: (a) after 14 days in 0.5M NaCl; (b) after 0, 7, 14 and 28 days in dilute Harrison’s solution.

145

A: CrOOH Cr 2p3/2

2- B: CrO4

Cr 2p1/2

A A

B B

Figure 5.7. Cr 2p spectra of uncoated surface of the artificial scratch cell after 14 days exposure.

146

2000

7 d 1500

14 d

1000 28 d Intensity (CPS)

500 fresh

0 84 82 80 78 76 74 72 70 68 Binding Energy (eV)

Figure 5.8. Al 2p spectra of nonTCP surface of the artificial scratch cell after different exposure time.

147

7 10 -100

106 -80

105 -60 ) Theta (degree) 2 104 -40 3

|Z| (Ohmcm 10

-20 102

101 0

0 10 20 10-3 10-2 10-1 100 101 102 103 104 105 Frequency (Hz)

|Z| 24 h Phase 24 h

|Z| 72 h Phase 72 h

|Z| 168 h Phase 168 h

|Z| 216 h Phase 216 h

|Z| 288 h Phase 288 h

|Z| 360 h Phase 360 h

|Z| 480 h Phase 480 h

|Z| 720 h Phase 720 h

Figure 5.9. Bode plots by EIS for the TCP treated surface in the artificial scratch cell containing 0.5 M NaCl.

148

6 10 -80

105 -60

104 ) Theta (degree) 2 -40

103

|Z| (Ohmcm -20 102

0 101

0 10 20 10-3 10-2 10-1 100 101 102 103 104 105 Frequency (Hz)

|Z| 48 h Phase 48 h

|Z| 72 h Phase 72 h

|Z| 168 h Phase 168 h

|Z| 216 h Phase 216 h

|Z| 120 h Phase 120 h

|Z| 288 h Phase 288 h

|Z| 360 h Phase 360 h

|Z| 480 h Phase 480 h

|Z| 624 h Phase 624 h

|Z| 720 h Phase 720 h

Figure 5.10. Bode plots by EIS for the nonTCP treated surface in the artificial scratch cell containing 0.5 M NaCl.

149

5 10 -80

-60 104 ) Theta (degree) 2 -40

103

|Z| (Ohm cm |Z| (Ohm -20

102 0

1 10 20 10-3 10-2 10-1 100 101 102 103 104 105 Frequency (Hz)

|Z| 24 h Phase 24 h

|Z| 48 h Phase 48 h

|Z| 72 h Phase 72 h

|Z| 120 h Phase 120 h

|Z| 168 h Phase 168 h

|Z| 216 h Phase 216 h

|Z| 288 h Phase 288 h

|Z| 360 h Phase 360 h

|Z| 480 h Phase 480 h

Figure 5.11. Bode plots by EIS for the control nonTCP treated surface in the artificial scratch cell containing 0.5 M NaCl.

150

107

-80 106

-60 105 Theta (degree) Theta ) 2

104 -40 |Z| (Ohmcm 103 -20

102 0

101 10-3 10-2 10-1 100 101 102 103 104 105

Frequency (Hz)

Z 24h P 24h

Z 72h P 72h

Z 168h P 168h

Z 216h P 216h

Z 288h P 288h

Z 456h P 456h

Z 528h P 528h

Z 692h P 692h

Figure 5.12. Bode plots by EIS for the TCP treated surface in the artificial scratch cell containing dilute Harrison’s solution.

151 106

-80

105

-60 Theta (degree) Theta

) 4

2 10

-40

103 |Z| (Ohmcm

-20

102

101 10-3 10-2 10-1 100 101 102 103 104 105

Frequency (Hz)

Z 24h P 24h

Z 72h P 72H

Z 96H P 96H

Z 168H P 168H

Z 216h P 216h

Z 288h P 288h

Z 360h P 360h

Z 456h P 456h

Z 528h P 528h

Z 696h P 696h

Figure 5.13. Bode plots by EIS for the nonTCP surface in the artificial scratch cell containing dilute Harrison’s solution.

152

106

-80

105

-60 Theta (degree)

) 4

2 10

-40

103 |Z| (Ohmcm

-20

102

101 10-3 10-2 10-1 100 101 102 103 104 105

Frequency (Hz)

Z 24h P 24H

Z 96h P 96h

Z 168h P 168h

Z 240h P 240h

Z 312h P 312h

Z 408h P 408h

Z 480h P 480h

Z 648h P 648h

Figure 5.14. Bode plots by EIS test for the nonTCP surface in the control cellcontaining dilute Harrison’s solution.

153

7x105

6x105

5 )

2 5x10 nonTCP/TCP

4x105 (Ohms cm 3x105 nonTCP/TCP 0.01Hz

|Z| 2x105 nonTCP control 1x105

0 2 4 6 8 101214161820222426 Time (hour)

(a)

1.4x106

1.2x106 ) 2 1.0x106

5 nonTCP/TCP

(Ohms cm (Ohms 8.0x10 0.01Hz 6.0x105 |Z| nonTCP/TCP 4.0x105 nonTCP/nonTCP 2.0x105

0 100 200 300 400 500 600 700 Time (hours)

(b)

Figure 5.15. Low frequency impedance at 0.01Hz for TCP and nonTCP surfaces in the artificial scratch cell and control cell containing dilute Harrison’s solution; (a) exposure within 24h, (b) exposure after 24h.

154

Figure 5.16. Equivalent circuit models for resistance and capacitance calculations of coated surface.

155

) 2 106 nonTCP/TCP

nonTCP/TCP Polarization Resistance cm (Ohms nonTCP/nonTCP control

105 0 100 200 300 400 500 600 700 exposure time (hour)

Figure 5.17. Polarization resistance for TCP and nonTCP samples in the artificial scratch cell and control cell containing dilute Harrison’s solution.

156

6 10-6

5.5 10-6 ) 2 nonTCP/nonTCP

5 10-6

4.5 10-6 nonTCP/TCP Surface Capacitance (F/cm Capacitance Surface

4 10-6

nonTCP/TCP

3.5 10-6 0 100 200 300 400 500 600 700 exposure time (hour)

Figure 5.18. Surface capacitance for TCP and nonTCP samples in the artificial scratch cell and control cell containing dilute Harrison’s solution.

157

-450

-500 nonTCP/TCP open, AR=1.03

nonTCP/TCP shorted, AR=177

-550 nonTCP/TCP shorted, AR=177 TCP/TCP open nonTCP/TCP shorted, AR=1.03 -600 nonTCP/TCP shorted, AR=1.03

OCP (mV vs SCE) vs (mV OCP -650 nonTCP/TCP open, AR=1.03

-700 nonTCP/nonTCP open

-750 0 50 100 150 200 250 300 350 exposure time (hour)

Figure 5.19. Open Circuit Potential collected for TCP coated and bare surfaces of artificial scratch cells containing dilute Harrison’s solution. “AR” is area ratio of TCP/nonTCP surface. “Open” is used for non-shorted cell. The underlines in the legend signify the surfaces on which the OCP values were measured, whereas the other surface in the cell is given after the slash.

158

1.2 106

nonTCP/TCP open, AR=1.03 nonTCP/TCP shorted, AR=177 1 106

) 8 10 2

6 105 (ohms cm nonTCP/TCP shorted, AR=1.03 0.01Hz Z 4 105 nonTCP/TCP open, AR=1.03 nonTCP/TCP shorted, AR=1.03 nonTCP/nonTCP open 2 105 nonTCP/TCP shorted, AR=177

0 50 100 150 200 250 300 350 time (hour)

Figure 5.20. Low frequency impedance at 0.01Hz for TCP and uncoated surfaces in the artificial scratch cell containing dilute Harrison’s solution.

159

REFERENCES

1 . L. Xia, E. Akiyama, G.S. Frankel, R.L. McCreery, J. Electrochem. Soc., 147 (2000) 2556-2562.

2 . F.W. Lytle, R.B. Greegor, G.L. Bibbins, K.Y. Blohowiak, R.E. Smith, G.D. Tuss, Corrosion Sci., 37 (1995) 349-369.

3 . M.W. Kendig, A.J. Davenport, H.S. Isaacs, Corrosion Sci., 34 (1993) 41-49.

4 . A. Nylund, Aluminum Transactions, 2 (2000) 121-137.

5 . J. Zhao, G. Frankel, R.L. McCreery, J. Electrochem. Soc., 145 (1998) 2258-2264.

6 . W.C. Nickerson, E. Lipnickas, TriService Corrosion Conference Proceedings, (2003).

7 . U.S. MILITARY SPECIFICATION, MIL-DTL-5541F, (2006).

8 . C. Matzdorf, Personal communication, (2011).

9 . H.A. Katzman, G.M. Malouf, R. Bauer, G.W. Stupian, Applications of Surface Science, 2 (1979) 416-432.

10 . P.L. Hagans, C.M. Haas, Surf. Interface Anal., 21 (1994) 65-78.

11 . T. Drozda, E. Maleczki, Journal of Radioanalytical and Nuclear Chemistry, 95 (1985) 339-349.

12 . K. Asami, M. Oki, G.E. Thompson, G.C. Wood, V. Ashworth, Electrochim. Acta, 32 (1987) 337-343.

13 . G.S. Frankel, R.L. McCreery, The Electrochemical Society Interface, 10 (2001) 34.

14 . L. Xia, R.L. McCreery, J. Electrochem. Soc., 145 (1998) 3083-3089.

160 15 . L. Xia, R.L. McCreery, J. Electrochem. Soc., 146 (1999) 3696-3701.

16 . F. Pearlstein, V.S. Agarwala, Non-Chromate Conversion Coatings for Aluminum Alloys in Corrosion Protection by Coatings and Surface Modification, PV 93-28 (1993) 199.

17 . Z. Yu, H. Ni, G. Zhang, Y. Wang, S. Dong, G. Zhao, Appl. Surf. Sci., 62 (1992) 217-221.

18 . J.E. Hatch, Aluminum: properties and physical metallurgy, American Society for Metals, Metals Park, Ohio, 1984.

19 . Henkel Alodine T5900 Technical Process Bulletin No. 239582.

20 . ASTM, ASTM G85-09 Standard Practice for Modified Salt Spray (Fog) Testing, in, ASTM.

21 . L.S. Sander, E.M. Musingo, U.S. Patent No. 4,921,552, (1 May 1990).

22 . C.A. Matsdorf, W.C. Nickerson, E. Lipnickas, NACE paper 2005-54. in NACE International, (2005).

23 . J. Zhao, L. Xia, A. Sehgal, D. Lu, R.L. McCreery, G.S. Frankel, Surface & Coatings Technology, 140 (2001) 51-57.

24 . W.J. Clark, J.D. Ramsey, R.L. McCreery, G.S. Frankel, J. Electrochem. Soc., 149 (2002) B179-B185.

25 . A. Sehgal, G.S. Frankel, B. Zoofan, S. Rokhlin, J. Electrochem. Soc., 147 (2000) 140-148.

26 . L. Li, G.P. Swain, A. Howell, D. Woodbury, G.M. Swain, Michigan State University, unpublished research.

27 . E. Nakayama, T. Kuwamoto, S. Tsurubo, H. Tokoro, T. Fujinaga, Analytica Chimica Acta, 130 (1981) 289-294.

161

CHAPTER 6

CONCLUSIONS AND FUTURE WORK

6.1 SUMMARY OF KEY RESULTS

In this study, Trivalent Chrome Process coatings (TCP) as possible replacements for Chromate Conversion Coatings (CCCs) were investigated to develop a better understanding of the coating nucleation and growth, composition and structure, effects of pretreatment on the coating performance, and mechanism of inhibition on aluminum alloys. The following conclusions were made through this study:

1. The morphology of the TCP treated aluminum surface revealed a dense layer

of rounded particles hundreds of nm in size, which is similar to CCCs. The

XPS spectra revealed the presence of extensive amounts of zirconium oxide

along with trivalent chromium compound, which indicated a mixed oxide

layer formed on the aluminum substrate after immersion in the AlodineTM5900

162 bath.

2. During TCP coating formation, the fluoride ions break down the passivating

Al oxide layer, which leads to the dissolution of aluminum metal accompanied

by a shift of the electrode potential to the negative direction. At this potential,

both the reduction of dissolved oxygen and hydrogen evolution occur.

Increase in the local pH is resulted by the above cathodic reactions. The

alkalization favors the hydrolysis and precipitation of a hydrated zirconia film

on the surface along with hydrated chromium oxide. Thus, the deposition of

the TCP coating induced by the local pH increase is fundamentally different

from the formation of CCCs.

3. The thickness of the TCP was different at different locations on the

heterogeneous microstructure of AA2024-T3 ranging from 40-120 nm

depending on the conversion time. Nano-EDS line profiling of 10 min TCP

formed on AA2024-T3 substrate revealed a coating thickness of 70 nm. The

TCP coating consists of a two layered structure, with zirconium-chromium

oxide in the outer layer and aluminum oxide or oxyfluoride at the

metal/coating interface.

4. The mud-crack morphology of TCP is always observed in the SEM

micrographs. Freshly prepared TCP coating formed on AA2024-T3 surface is

not defective. Aging under low humidity results in the loss of water from the

coating, and the resultant shrinkage leads to the crack formation. The high

vacuum condition in the traditional SEM dehydrates the water quickly;

consequently, mud-cracks are always observed as an artifact.

5. TCP treatment, which is considered to be protective barrier layer, can greatly

163 improve the corrosion resistance of AA2024-T3 to sustain the long time

exposure to simulated corrosive environments based on visual observation of

artificial scratch cells.

6. After static exposure in dilute Harrison’s solution, chromium species was

detected in the exposed electrolyte, which suggested the capability of

chromium release from the TCP coating. However, the zirconium release was

not detected.

7. In the artificial scratch cell, chromium was found on the nonTCP surface

which provided evidence that TCP can provide some degree of active

corrosion inhibition for Al alloys. EIS data showed the polarization resistance

of uncoated surface was rapidly increased in the first 24 hour exposure and

was stable at 4 x 105 ohm•cm2, which was twice as much as uncoated controls.

8. The pretreatment details affected the TCP film on AA2024-T3. The growth of

TCP following Process I (Henkel Chemicals) started after 30 s immersion,

which was faster compared to more than 2 min after Process II (silicate

treatment). The size of the particles was around tens of nm which was smaller

compared to those following Process II. The abnormal round clusters were not

easily observed after Process I. Uniform nucleation occurred on the sample

surface after Process I, while non-uniform nucleation occurred after Process II.

Longer immersion in the TCP bath resulted in increased thickness of TCP film

for both pretreatments. These two pretreatments prior to TCP result in similar

surface conditions on AA2024-T3.

164 6.2 FUTURE WORK

This project has exploited the use of TCP coatings as replacement for CCCs.

Some issues were raised that are of interest for future study.

1. The copper containing intermetallic particles was reported to result in a

non-uniform CCC coating on the surface of AA2024-T3, which had small

coating thickness above the IMC. The formation of TCP layer on the

heterogeneous microstructure of AA2024-T3, in particular the copper

containing particles might need to be further investigated.

2. The release of chromium species into the aqueous environment in a

continual immersion was observed, which might be responsible for the

self-healing property of TCP coating. To investigate the notion of

transient chromate formation during exposure to corrosive conditions will

help better understanding the mechanism of the inhibition. X-ray

Absorption Spectroscopy (XAS) can be used since it is very sensitive to

small amounts of Cr(VI) owing to the existence of a strong pre-edge peak.

3. EIS measurements revealed that AA7075-T6 is well protected by TCP

with corrosion resistance similar to CCCs while only poor protection to

AA2024-T3 was observed perhaps due to the Cu-rich intermetallic

particles on the surface. Coating performance has shown to be enhanced

after the removal of the Cu-rich layer by desmutting. Proper pretreatments

which not only remove the organic contaminants but also alter the surface

chemically to favor the formation of TCPs will enhance the effective of

corrosion protection of TCPs.

165

BIBLIOGRAPHY

C.F. Baes, R.E. Mesmer, The Hydrolysis of Cations, Robert E. Krieger Publishing Company, Inc, Malabar, Fl, 1986.

L. Xia, R.L. McCreery, J. Electrochem. Soc., 145 (1998) 3083-3089.

W. Zhang, R.G. Buchheit, Corrosion, 59 (2003) 356-362.

J.R. Waldrop, M.W. Kendig, J. Electrochem. Soc., 145 (1998) L11-L13.

X. Sun, R. Li, K.C. Wong, K.A.R. Mitchell, T. Foster, J. Mater. Sci., 36 (2001) 3215-3220.

D. Chidambaram, C.R. Clayton, G.P. Halada, J. Electrochem. Soc., 151 (2004) B151-159.

R.G. Buchheit, A.E. Hughes, Chromate and Chromate-Free Conversion Coatings, ASM Handbook: Fundamentals, Testing, and Protection, 2003.

W.C. Nickerson, E. Lipnickas, TriService Corrosion Conference Proceedings, (2003).

I.J. Polmear, Light Alloys Metallurgy of the Light Metals, Third ed., , London: Arnold, 1995.

R.W. Hinton, Metal Finishing, 89 (1991) 55.

P. O'Brien, A. Kortenkamp, Transition Metal Chemistry, 20 (1995) 636-642.

Fedral Register, 69 (October, 2004).

L. Xia, E. Akiyama, G.S. Frankel, R.L. McCreery, J. Electrochem. Soc., 147 (2000)

166 2556-2562.

A.E. Hughes, J.D. Gorman, P.J.K. Patterson, Corrosion Sci., 38 (1996) 1957.

D. Galjard, Finishing, 7 (1983) 34.

J.H. Nordlien, J.C. Walmsley, H. Qsterberg, K. Nisancioglu, Surface and Coating Technology, 153 (2002) 72-78.

C. Matzdorf, M. Kane, J. Green, U.S. Patent No. 6,375726, (2002).

J.E. Hatch, Aluminum: properties and physical metallurgy, American Society for Metals, Metals Park, Ohio, 1984.

H.A. Katzman, G.M. Malouf, R. Bauer, G.W. Stupian, Applications of Surface Science, 2 (1979) 416-432.

P.L. Hagans, C.M. Haas, Surf. Interface Anal., 21 (1994) 65-78.

T. Drozda, E. Maleczki, Journal of Radioanalytical and Nuclear Chemistry, 95 (1985) 339-349.

K. Asami, M. Oki, G.E. Thompson, G.C. Wood, V. Ashworth, Electrochim. Acta, 32 (1987) 337-343.

G.S. Frankel, R.L. McCreery, The Electrochemical Society Interface, 10 (2001) 34.

L. Xia, R.L. McCreery, J. Electrochem. Soc., 146 (1999) 3696-3701.

J. Zhao, G. Frankel, R.L. McCreery, J. Electrochem. Soc., 145 (1998) 2258-2264.

A. Nylund, Aluminum Transactions, 2 (2000) 121-137.

A. Iyer, W. Willis, S. Frueh, W. Nickerson, A. Fowler, J. Barnes, L. Hagos, J. Escarsega, J. La Scala, S. Suib, Plating and Surface Finishing, (May 2010) 32-42.

L.F. Mondolfo, Aluminum Alloys: Structure and Properties, London, U.K.: Butterworths, 1976.

W.F. Smith, Structure and Properties of Engineering Alloys, McGraw Book Company, 1981.

167 R.G. Buchheit, J. Electrochem. Soc., 142 (1995) 3994-3996.

R.G. Buchheit, R.P. Grant, P.F. Hlava, B. McKenzie, G.L. Zender, J. Electrochem. Soc., 144 (1997) 2621.

P. Leblanc, G.S. Frankel, J. Electrochem. Soc., 149 (2002) B239-B247.

R.G. Buchheit, L.P. Montes, M.A. Martinez, J. Michael, P.F. Hlava, J. Electrochem. Soc., 146 (1999) 4424-4428.

R.G. Buchheit, M.A. Martinez, L.P. Montes, J. Electrochem. Soc., 147 (2000) 119-124.

S. Wernick, R. Pinner, P.G. Sheasby, The surface treatment and finishing of aluminum and its alloys, 5th ed., Ohio : ASM International ; Teddington : Finishing, 1987.

A.E. Hughes, R.J. Taylor, K.J.H. Nelson, B.R.W. Hinton, L. Wilson, Mater. Sci. Technol., 12 (1996) 928-936.

R.G. King, Treatment and Finishing of Aluminium Pergamon Press, Headington Hill Hall, Oxford OX3 0BW, England, 1988.

G.J. Cormier, ASM Handbook, Surface Engineering, Alkaline Cleaning, Volume 5.

Henkel, Turco 6849, Aqueous Alkaline Degreaser, Material Safety Data Sheet, ID: 238832.

M.W. Kendig, R.G. Buchheit, Corrosion, 59 (2003) 379-400.

W. Nickerson, Personal communication, (2011).

G.M. Brown, K. Shimizu, K. Kobayashi, G.E. Thompson, G.C. Wood, Corrosion Sci., 34 (1993) 1045-1054.

J.A. Treverton, N.C. Davies, Metals Technology, 4 (1977) 480-489.

A.E. Hughes, G. Theodossiou, S. Elliott, T.G. Harvey, P.R. Miller, J.D. Gorman, P.J.K. Paterson, Mater. Sci. Technol., 17 (2001) 1642-1652.

Final NCMS Report 0273RE95, National Center for Manufacturing Sciences, (1995).

Toxicological Profile for Chromium, ATDSR/TP-88/10, , in: U.S.P.H.S. Agency for Toxic Substance, Washington, DC (Ed.), 1989.

168 F.A. Cotton, G. Wilkinson, Advanced inorganic chemistry 5th ed., New York : Wiley, 1988.

J.D. Ramsey, R.L. McCreery, J. Electrochem. Soc., 146 (1999) 4076-4081.

E. Akiyama, G.S. Frankel, J. Electrochem. Soc., 146 (1999) 4095-4100.

W.J. Clark, J.D. Ramsey, R.L. McCreery, G.S. Frankel, J. Electrochem. Soc., 149 (2002) B179-B185.

W. Clark, R.L. McCreery, J. Electrochem. Soc., 149 (2002) B379-386.

P. Schmutz, G.S. Frankel, J. Electrochem. Soc., 146 (1999) 4461-4472.

A. Kolics, A.S. Besing, A. Wieckowski, J. Electrochem. Soc., 148 (2001) B322-B331.

U.S. MILITARY SPECIFICATION, MIL-C-5541E, (1990).

M.W. Kendig, A.J. Davenport, H.S. Isaacs, Corrosion Sci., 34 (1993) 41-49.

B.L. Hurley, R.L. McCreery, J. Electrochem. Soc., 150 (2003) B367-B373.

W. Zhang, B. Hurley, R.G. Buchheit, J. Electrochem. Soc., 149 (2002) B357-B365.

A.E. Hughes, R.J. Taylor, B.R.W. Hinton, Surf. Interface Anal., 25 (1997) 223-234.

P.L. Hagans, C.M. Haas, "Chromate Conversion Coatings," in Surface Engineering, in: Materials Park, ASM International, OH, 1987, pp. 405.

G.M. Brown, K. Shimizu, K. Kobayashi, G.E. Thompson, G.C. Wood, Corrosion Sci., 33 (1992) 1371-1385.

J.H. Osborne, Prog. Org. Coat., 41 (2001) 280-286.

J. A. J. Aldykiewicz, A.J. Davenport, H.S. Isaacs, J. Electrochem. Soc., 143 (1996) 147-154.

J. A. J. Aldykiewicz, H.S. Isaacs, A.J. Davenport, J. Electrochem. Soc., 142 (1995) 3342-3350.

J. Sinko, Prog. Org. Coat., 42 (2001) 267-282.

169 D.J. Arrowsmith, J.K. Dennis, P.R. Sliwinski, Trans. Inst. Met. Finish., 62 (1984) 117.

W.J. Wittke, Metal Finishing, 87 (1989) 24-26.

J.A. Treverton, M.P. Amor, J. Mater. Sci., 23 (1988) 3706-3710.

J.A. Treverton, M.P. Amor, Transactions of the Institute of Metal Finishing, 60 (1982) 92.

J.A. Treverton, N.C. Davies, Surf. Interface Anal., 3 (1981) 194.

F.W. Lytle, R.B. Greegor, G.L. Bibbins, K.Y. Blohowiak, R.E. Smith, G.D. Tuss, Corrosion Sci., 37 (1995) 349-369.

P. Campestrini, G. Goeminne, H. Terryn, J. Vereecken, J.H.W.d. Wit, J. Electrochem. Soc., 151 (2004) B59-B70.

G.P. Halada, C.R. Clayton, J. Electrochem. Soc., 138 (1991) 2921-2927.

D.C. Koningsberger, R. Prins, X-ray absorption : principles, applications, techniques of EXAFS, SEXAFS, and XANES, New York : Wiley, 1988.

M.Jaime Vasquez, G. P. Halada, C.R. Clayton, J.P. Longtin, Surf. Interface Anal., 33 (2002) 607-616.

A. Kolics, A.S. Besing, P. Baradlai, A. Wieckowski, J. Electrochem. Soc., 150 (2003) B512-B516.

M. Kendig, C. Thomas, J. Electrochem. Soc., 139 (1992) L103-L104.

A. Aballe, M. Bethencourt, F.J. Botana, M.J. Cano, M. Marcos, Mater. Corros., 53 (2002) 176-184.

A.E. Hughes, J.D. Gorman, P.R. Miller, B.A. Sexton, P.J.K. Paterson, R.J. Taylor, Surf. Interface Anal., 36 (2004) 290-303.

B.R.W. Hinton, D.R. Arnott, N.E. Ryan, Metals Forum, 7 (1984) 211-217.

G.O. Ilevbare, J.R. Scully, J. Electrochem. Soc., 148 (2001) B196-B207.

A. Sehgal, G.S. Frankel, B. Zoofan, S. Rokhlin, J. Electrochem. Soc., 147 (2000) 140-148.

170 D.R. Arnott, N.E. Ryan, B.R.W. Hinton, B.A. Sexton, A.E. Hughes, Appl. Surf. Sci., 22/23 (1985) 236-251.

C. Wang, F. Jiang, F. Wang, Corrosion, 60 (2004) 237-243.

S.T. Pride, J.R. Scully, J.L. Huston, J. Electrochem. Soc., 141 (1994) 3028-3039.

G.O. Ilevbare, J.R. Scully, J. Yuan, R.G. Kelly, Corrosion, 56 (2000) 227-242.

D.A. Jones, Principles and Prevention of Corrosion, 2 ed., NJ: Prentice-Hall, Inc, Upper Saddle River, 1996.

V. Laget, C.S. Jeffcoate, H.S. Isaacs, R.G. Buchheit, J. Electrochem. Soc., 150 (2003) B425-B432.

S.E. Dolan, U.S. Patent No. 5449415, (1995 ).

L.S. Sander, E.M. Musingo, U.S. Patent No. 4,921,552, (1 May 1990).

Nelson J. Newhard Jr., in: J. Henry Leidheiser (Ed.) Corrosion Control by Coatings, Proceedings of the Meeting on corrosion control by coatings, SCIENCE PRESS, Princeton, 1979, pp. 225.

S. Adhikari, K.A. Unocic, Y. Zhai, G.S. Frankel, J. Zimmerman, W. Fristad, Electrochim. Acta, (2010).

D.Narayan, US Patent No 5139586, (1992).

D.Y. Dollman, T.J. O'Grady, US Patent No 4191596, (1980).

L.Hallman, WO No 9012902 A1, (1990).

T. Schram, G. Goeminne, H. Terryn, W. Vanhoolst, P. Van Espen, Trans. Inst. Met. Finish., 73 (1995) 91-95.

G.G. T. Schram, H. Terryn, W. Vanhoolst, P. Van Espen, Trans. Inst. Met. Finish., 73 (1995) 91-95.

P.D. Deck, D.W. Reichgott, Metal Finishing, 90 (1992) 29-35.

F. Andreatta, A. Turco, I. de Graeve, H. Terryn, J.H.W. de Wit, L. Fedrizzi, Surface & Coatings Technology 201 (2007) 7668-7685.

171 L. Hallman, US Patent No 4992116, (1991).

O. Lunder, C. Simensen, Y. Yu, K. Nisancioglu, Surface & Coatings Technology, 184 (2004) 278-290.

A.S. Hamdy, M. Farahat, Surface & Coatings Technology, 204 (2010) 2834-2840.

F. Pearlstein, V.S. Agarwala, Plating and Surface Finishing, 81 (1994) 50-55.

F. Pearlstein, V.S. Agarwala, U.S. Patent No. 5304257, (1994).

Henkel Alodine T5900 Material Safety Data Sheet, ID:239583DLP470.

L. Li, G.P. Swain, A. Howell, D. Woodbury, G.M. Swain, Michigan State University, unpublished research.

X. Dong, P. Wang, S. Argekar, D.W. Schaefer, Langmuir, 26 (2010) 10833-10841.

C.A. Matsdorf, W.C. Nickerson, E. Lipnickas, NACE paper 2005-54. in NACE International, (2005).

E.N. Beck, TriService Corrosion Conference Proceedings, (2003).

P. Laha, T. Schram, H. Terryn, Surf. Interface Anal., 34 (2002) 677-680.

ASTM, ASTM G85-09 Standard Practice for Modified Salt Spray (Fog) Testing, in, ASTM.

Henkel Turco liquid Smut-go NC MSDS ID:239357DLP55G.

D. Chidambaram, G.P. Halada, C.R. Clayton, Appl. Surf. Sci., 181 (2001) 283-295.

Q.J. Meng, G.S. Frankel, Surf. Interface Anal., 36 (2004) 30-42.

S. Verdier, N. van der Laak, F. Dalard, J. Metson, S. Delalande, Surface & Coatings Technology, 200 (2006) 2955-2964.

A.L. Glass, Materials Protection, 7 (1968) 26-30.

Henkel Turco 6849 Aqueous Alkaline Degreaser Technical Process Bulletin No.238832.

172 T.G. Harvey, A.E. Hughes, S.G. Hardin, T. Nikpour, S.K. Toh, A. Boag, D. McCulloch, M. Horne, Appl. Surf. Sci., 254 (2008) 3562-3575.

J.E. Hatch, Materials Park, OH: ASM International, (1984) 354.

J.I. Goldstein, Scanning electron microscopy and x-ray microanalysis, 3rd ed., Kluwer Academic/Plenum Publishers, New York, 2003.

S.K. Toh, A.E. Hughes, D.G. McCulloch, J. duPlessis, A. Stonham, Surf. Interface Anal., 36 (2004) 1523-1532.

P. Campestrini, E.P.M. van Westing, J.H.W. de Wit, Electrochim. Acta, 46 (2001) 2553-2571.

U.S. MILITARY SPECIFICATION, MIL-DTL-5541F, (2006).

C. Matzdorf, Personal communication, (2011).

F. Pearlstein, V.S. Agarwala, Non-Chromate Conversion Coatings for Aluminum Alloys in Corrosion Protection by Coatings and Surface Modification, PV 93-28 (1993) 199.

Z. Yu, H. Ni, G. Zhang, Y. Wang, S. Dong, G. Zhao, Appl. Surf. Sci., 62 (1992) 217-221.

Henkel Alodine T5900 Technical Process Bulletin No. 239582.

J. Zhao, L. Xia, A. Sehgal, D. Lu, R.L. McCreery, G.S. Frankel, Surface & Coatings Technology, 140 (2001) 51-57.

E. Nakayama, T. Kuwamoto, S. Tsurubo, H. Tokoro, T. Fujinaga, Analytica Chimica Acta, 130 (1981) 289-294.

173