ELECTROCHEMICAL STUDIES OF COATINGS AND THIN FILMS
DISSERTATION
Presented in Partial Fulfillment of the Requirements for
the Degree Doctor of Philosophy in the Graduate School of
The Ohio State University
By
Jiho Kang, M.S.
* * * * *
The Ohio State University
2006
Dissertation Committee: Approved by
Dr. Gerald S. Frankel, Adviser
Dr. Rudolph G. Buchheit ______
Dr. Suliman A. Dregia Adviser
Graduate Program in Materials Science and Engineering
ABSTRACT
This dissertation reports findings on three different but related topics.
Determination of cathodic kinetics for Al-containing phases is essential to characterize the corrosion behavior of high strength Al alloys. However, the current density measured from a potentiostat can be different than the true cathodic current because anodic dissolution occurs during cathodic polarization of Al alloys and a potentiostat only senses the net current. Therefore, it is necessary to use a nonelectrochemical measurement, such as Eletrochemical Quartz Crystal Microbalance (EQCM) technique. EQCM was used on thin film compositional analogs of S phase (Al2CuMg) particle, which is an important intermetallic particle commonly found in Al alloys, to evaluate the true cathodic current density. Mass and current measured simultaneously by the EQCM technique during cathodic polarization in chloride solutions showed that the true cathodic current density was much larger than the measured net current density. These observations, along with local pH increases measured with a micro pH electrode during cathodic polarization, indicated that the S phase film was undergoing cathodic corrosion. The mass loss rate for
S phase was reduced in solutions containing chromate or vanadate. The effect of chromate was much less than vanadate, suggesting that chromate exhibited stronger inhibition effect on S phase than vanadate. This was partly explained by the local pH
ii increase near the film surface in the vanadate-containing solution whereas there was no
pH change in the chromate-containing solution.
In principle, it should be possible to apply the EQCM technique to determine kinetic parameters, e.g., diffusivity of water. However, little research has been performed
to relate this information to delamination and subsequent corrosion of the substrate under the coatings. Therefore, it is interesting to use EQCM for investigating water uptake in organic coatings, delamination, and corrosion on coated Al electrode. Apparent mass increases were measured in the coated samples on the Au-deposited quartz immersed in water. For a similar sample containing an Al layer instead of Au, apparent mass increases were observed after the completion of initial water uptake, indicating the formation of a corrosion product (or oxide layer) underneath coating. Electrochemical Impedance
Spectroscopy (EIS) measurements also indicated changes during the same time period that were consistent with the formation of a corrosion product (or oxide layer).
EIS cannot accurately sense the initial degradation of protective coatings as they are just starting to fail because the low frequency impedance is typically higher than the input impedance of the EIS system for reasonably-sized samples. Changes in corrosion resistance of these good coatings cannot be sensed until a significant degradation occurs.
Therefore, it is interesting to investigate other evaluation techniques to assess the early stage of organic coating failure. Potentiostatic Pulse Testing (PPT), which involves the application of potential steps instead of sine waves, holds promise for the evaluation of these protective coatings. Current transients collected from dummy cells and real coated samples were fitted to an exponential decay function to evaluate the values of equivalent
iii circuit parameters. PPT only provided values for only some of the circuit elements,
whereas EIS revealed most values of the assumed circuit elements. However, it was difficult to know a priori whether to use a one- or two-time-constant model for EIS data obtained from the real coated panels. Fast Fourier Transform (FFT) analysis was used to
transform the data generated in the time domain and compare the data measured by the
EIS. The impedance spectra from the Fourier transforms was over only a part of the
frequency range accessible by the EIS, but the spectra from the two methods exhibited
reasonably good agreement.
iv
Dedicated to my wife, Jiyun and my son, Gerald
v
ACKNOWLEDGMENTS
I would like to express my heartfelt thanks to my advisor, Dr. Jerry Frankel, who has given me intellectural input, encouragement, profound guidance, excellent advice, and support over past five years. I deeply appreciate not only his knowledge for research but also his enthusiasm which made this thesis possible, and his patience in correcting both my stylistic and scientific errors. I would like to thank specially to Dr. Rudy
Buchheit and Dr. Suliman Dregia for their helps as academic advisory committee for comments and suggests on my research.
During this time, an enormous number of friends and colleagues in which a single page of thanks would not suffice gratefully assisted me both professionally and privately.
I would like to give special thanks to Dr. Patrick Leblanc, Dr. Nick Birbilis, Dr. Eiji
Tada, Mr. Younghoon Baek, Mr. Ron Clason and many other FCC members. I owe a special thanks to Dr. Sehoon Yoo, Dr. Chonghoon Lee, and Mr. Huyong Lee for their assistance in synthesizing thin film samples. I also thank Ms. Jingyu Shi, Mr. Hong Jin
Kim and Dr. Myoung-Gyu Lee for preparing coated and FIB cross-sectioned samples as well as assisting computer modeling. I would like to thank Mr. Anthony Lutton for ICP-
AES, Dr. Steve Goss for SIMS, Mr. John Grant for AES, and Mr. Mariano Iannuzzi for
SKPFM analysis. I want to give thanks to Dr. Pistorius and Dr. Granata for the PPT
vi work. I am grateful to Ms. Dena Bruedigam, Ms. Susan Meager, Ms. Chris Putnam, and
Mr. Mark Cooper for their administrative help and assistance.
I would personally like to thank the members of the Korean Students Association
in the Department of Materials Science and Engineering at The Ohio State University.
Thanks also go to the members of the Korean Mission of Lane Avenue Baptist Church
for their love and prayer, especially to Pastor Jae K. Chun. At this moment, I can not skip
to mention my dear friends’ names in Korea, Jihoon, Sejin and Bumjin, for being my
invisible and sincere supports. In addition, I really appreciate all of those who deserve
mention by name but are not named here.
I sincerely thank my parents and parents-in-law from my inmost heart and feelings for their endless love and care. Without their sacrificing love and encourage-
ment, I could not be here.
Most of all, I would like to give my wholehearted thanks to my loving wife, Jiyun
for her emotional supports and devoted love. I also thank my beloved son, Gerald
(Chavin) who has been joy to me after his birth. And I also want to share this with my
baby who will be born in year of 2006!
Lastly, I wish to thank God who created and saved me.
vii
VITA
November 24th, 1973...... Born – Pusan, Korea
Febuary,1998 ...... B.S. Metallurgical Engineering, Yonsei University, Seoul, Korea
Febuary, 2000 ...... M.S. Materials Science and Engineering, Korea Advanced Institute of Sci. & Tech., Taejon, Korea
March, 2003...... M.S. Materials Science and Engineering, The Ohio State University, Columbus, Ohio, U.S.A.
PUBLICATIONS
1. Jiho Kang and G. S. Frankel, "Potentiostatic Pulse Testing for Assessment of Early Coating Failure.” Z. Phys. Chem. 219, p. 1519 (2005).
FIELDS OF STUDY
Major Field: Materials Science and Engineering, Electrochemistry
viii
TABLE OF CONTENTS
ABSTRACT...... ii ACKNOWLEDGMENTS ...... vi VITA...... viii TABLE OF CONTENTS...... ix LIST OF TABLES...... xii LIST OF FIGURES ...... xiv
CHAPTER 1 ...... 1 REFERENCES ...... 4
CHAPTER 2 ...... 5 2.1. CORROSION OF AL ALLOYS WITH SEVERAL INTERMETALLIC PARTICLES...... 6 2.1.1. Corrosion Properties of the Intermetallic Particles in the Heat Treatable Al Alloys...... 7 2.1.1.1. Al2Cu Particles ...... 7 2.1.1.2. Al2CuMg Particles...... 9 2.1.1.3. AlCuFeMn Particles and Other Cu Containing Particles...... 11 2.1.1.4. Al3Fe Particles and Other Particles with Mn and Si...... 12 2.1.1.5. MgZn2 Particles with Different Cu and Al Content ...... 13 2.1.1.6. Al2CuLi Particles...... 13 2.1.1.7. Al3Zr Particles ...... 14 2.1.2. The Cathodic Corrosion on Aluminum Alloys...... 15 2.1.2.1. Cathodic Corrosion of Pure Al [53] ...... 15 2.1.2.2. Oxygen Reduction Reaction (ORR)...... 18 2.1.3. Summary...... 19 2.2. ORGANIC COATINGS FOR CORROSION PROTECTION...... 20 2.2.1. Paint Components...... 20 2.2.1.1. Binders...... 21 2.2.1.2. Pigments ...... 21 2.2.1.3. Other Additives ...... 22 2.2.1.4. Solvents ...... 22 2.2.1.5. Multilayer Coatings...... 23 2.2.2. Corrosion of Metal under Organic Coatings ...... 24
ix 2.2.2.1. Water and Oxygen Permeation and Underfilm Corrosion Initiation 24 2.2.2.1.1. Transport of O2...... 25 2.2.2.1.2. Transport of Water ...... 26 2.2.2.1.3. Transport of Ions ...... 26 2.2.2.2. Wet Adhesion ...... 27 2.2.2.3. Blistering ...... 27 2.2.2.4. Cathodic Delamination...... 29 2.2.2.5. Anodic Undermining...... 30 2.2.2.6. Filiform Corrosion...... 31 2.2.2.7. Early Rusting...... 34 2.2.2.8. Frash Rusting...... 35 2.2.3. Summary...... 35 2.3. CHARACTERIZATION TECHNIQUES...... 36 2.3.1. Open Circuit Potential Monitoring...... 36 2.3.2. Polarization Resistance Measurement...... 36 2.3.3. Electrochemical Noise Analysis (ENA)...... 37 2.3.4. Electrochemical Impedance Spectroscopy (EIS) ...... 38 2.3.5. Coating Capacitance Measurement ...... 40 2.3.6. Breakpoint Frequency Measurement...... 41 2.3.7. Potentiostatic Pulse Testing (PPT) ...... 43 2.3.8. Quartz Crystal Microbalance (QCM)...... 43 2.4. KEY UNRESOLVED ISSUES ...... 47 2.4.1. Cathodic Reactivity of Phases in Al Alloys ...... 47 2.4.2. Corrosion of the Organic Coated Al Alloys ...... 48 2.5. OBJECTIVE OF DISSERTATION ...... 48 REFERENCES ...... 50
CHAPTER 3 ...... 65 3.1. INTRODUCTION ...... 66 3.2. EXPERIMENTAL...... 69 3.3. RESULTS AND DISCUSSION...... 72 3.3.1. Thin Film Characterization...... 72 3.3.2. Characterization of Thin Film Samples after OCP Exposure in Aerated 0.5 M NaCl Solution...... 73 3.3.3. Electrochemical Behavior of S Phase Thin Films and AA2024-T3..... 74 3.3.4. Electrochemical Quartz Crystal Microbalance Measurements ...... 77 3.3.5. Calculations of True Cathodic Current Density ...... 80 3.3.6. Components of True Cathodic Current Density...... 84 3.3.7. Local pH Measurements on S Phase Films ...... 87 3.4. CONCLUSIONS ...... 88 REFERENCES ...... 90
CHAPTER 4 ...... 109 x 4.1. INTRODUCTION ...... 109 4.2. EXPERIMENTAL...... 111 4.3. RESULTS AND DISCUSSION...... 113 4.3.1. Mass and Potential Changes of Uncoated and Coated Samples on the Au-Deposited Quartz ...... 113 4.3.2. Mass and Potential Changes of Coated Samples on Al-Deposited Quartz 116 4.3.3. EIS Measurements and Analysis ...... 117 4.4. CONCLUSIONS ...... 122 REFERENCES ...... 123
CHAPTER 5 ...... 135 5.1. INTRODUCTION ...... 136 5.2. EXPERIMENTAL...... 138 5.3. RESULTS AND DISCUSSION...... 140 5.3.1. EIS Measurements on Dummy Cells...... 140 5.3.2. PPT Measurements on Dummy Cells...... 141 5.3.3. Computer Simulation for Early States of Coating Failure ...... 143 5.3.4. EIS and PPT Measurements for Real Coated Samples ...... 144 5.3.5. Frequency Domain Analysis ...... 145 5.3.6. Comparison of EIS and PPT...... 146 5.4. CONCLUSIONS ...... 148 REFERENCES ...... 150
CHAPTER 6 ...... 164 6.1. CONCLUSIONS ...... 164 6.2. FUTURE WORK ...... 166
BIBLIOGAPHY ...... 169
xi
LIST OF TABLES
Table Page
2.1: Summary of the intermetallic phases commonly found on Al alloys...... 58 2.2: The composition of paints [79]...... 59 2.3: Summary of the several measurements to characterize the protective properties of organic coatings...... 60
3.1: Thin film deposition parameters...... 93 3.2: SEM-EDS analysis at sites indicated in Figure 3.4a ∼3.4d...... 93 3.3: Concentrations of species in solution after the OCP and potentiostatic polarization exposures measured by ICP-AES for S phase in 0.5 M NaCl...... 94 3.4: Values of mass change rate, dm/dt, at OCP, corrosion current density calculated from corr the mass change rate assuming cation ratios given in Table 3.3, im , and corrosion corr current density calculated from extrapolation of the polarization curves, ip . ic, true was calculated based on the consideration of the Cu release and replating...... 94 3.5: Values of net current density measured by the potentiostat, inet, mass change rate dm/dt, the current density calculated from the mass change rate assuming cation ratios given in Table 3.3, im, inet – im, and true cathodic current density with consideration of the Cu release and replating, ic ,true. All measurements were at -1050 mVSCE...... 95
4.1: Main deposition conditions for Al thin films...... 126 4.2: Fitted values using Circuit 1 (0.2 ∼ 6 hr) shown in Figure 4.7a...... 126 4.3: Fitted values using Circuit 2 (6 ∼ 264 hr) shown in Figure 4.7b...... 127 4.4: Fitted values using Circuit 3 (96 ∼ 264 hr) shown in Figure 4.7c...... 127
5.1: Component values of actual dummy cells in the Type II configuration...... 151 5.2: Component values of virtual dummy cells used for computer simulations...... 151 5.3: Circuit element values determined by fitting EIS spectra measured on Type II dummy cells to a Type I equivalent circuit...... 152 5.4: PPT fitted values of the dummy cells using a first order exponential decay function...... 152 5.5: Several values of circuit parameters measured from the potentiostat and picoammeter...... 153
xii 5.6: PPT fitted values of the simulated cells with different ratios of the defect area using a first order exponential decay function...... 153 5.7: Analysis of data from real coated panel. EIS data fitted to Type I and Type II circuits and PPT data fitted to Type II circuit...... 154
xiii
LIST OF FIGURES
Figure Page
2.1: Schematic representation of the SCC in aged AA2024 [28]. Reprinted from K. Urushino and K. Sugimoto, Corrosion Science 19, p. 225 (1979) with permission from Elsevier...... 61 2.2: Experiments of Leidheiser et al. to prove the way in which K+ diffuse to blisters [76]. Reprinted from W. Funke, Ind. Eng. Chem. Prod. Res. Dev. 24, p. 343 (1985) with permission from American Chemical Society...... 61 2.3: Mechanism of blistering by underrusting [76]. Reprinted from W. Funke, Ind. Eng. Chem. Prod. Res. Dev. 24, p. 343 (1985) with permission from American Chemical Society...... 62 2.4: Mechanism of neutral blistering [76]. Reprinted from W. Funke, Ind. Eng. Chem. Prod. Res. Dev. 24, p. 343 (1985) with permission from American Chemical Society...... 62 2.5: Filiform corrosion on Al – the head is the primary anodic site and the back of the head (the beginning of the tail) is the primary cathodic site [77]...... 63 2.6: (a) Model for the impedance of a polymer coated metal and (b) theoretical impedance spectra for a degraded polymer coating [141]. Reprinted from F. Mansfeld, Journal of Applied Electrochemistry 25, p. 187 (1995) with permission from Springer Science and Business Media...... 63 2.7: Schematic representations of (a) the shear vibration of an AT-cut quartz resonator and (b) the transverse shear wave in a quartz crystal with excitation electrodes and a composite resonator comprising the quartz crystal, electrodes, and a thin layer of a foreign material [152]...... 64
3.1: Schematic diagram of the EQCM setup. Note that the micro pH electrode part was only installed for some experiments in which the pH was measured...... 96 3.2: XRD patterns of the as-deposited S phase film and the bulk Al2CuMg (PDF#65- 2501) [49]...... 97 3.3: (a) Positive and (b) negative SIMS depth profiles of the as-deposited S phase film. Note that the depth per cycle is different for the positive and negative profiles...... 98 3.4: Various surface morphologies with immersion times in the aerated 0.5 M NaCl solution. It is noted that (e) ∼ (h) were taken after 30 min. of OCP exposure followed by cathodic application during another 30 min. All scale bars are 20 μm...... 99
xiv 3.5: (a) Positive and (b) negative SIMS depth profiles of the S phase film after 30 min. OCP exposure in the aerated 0.5 M NaCl solution...... 100 3.6: SIMS line scans (x: distance, y: signal intensity) of (a) Al, (b) Cu, (c) Mg and (d) Si on pits found in several sputtered layer. The line scans are taken from the positions indicated on the associated maps. All scale bars are 10 μm...... 101 3.7: (a) OCP and (b) polarization behaviors of S phase in the deaerated, aerated 0.5 M NaCl solution (pH 5.98) with addition of 10-4 M of chromate (pH 6.60), 0.1 M of vanadate (pH 6.29 adjusted), and 10-4 M of NaOH (pH 9.89)...... 102 3.8: (a) OCP and (b) polarization behaviors of AA2024-T3 in the aerated 0.5 M NaCl solution (pH 5.98) with addition of 10-4 M of chromate (pH 6.60), 0.1 M of vanadate (pH 6.29 adjusted), and 10-4 M of NaOH (pH 9.89)...... 103 3.9: (a) OCPs and (b) mass changes for S phase thin film analogs during OCP exposure in the 0.5 M NaCl solution without and with NaOH additions. (c) OCPs and (d) mass changes in the 0.5 M NaCl solution with chromate and vanadate additions. 104 3.10: (a) Net current densities and (b) mass changes for S phase thin film analogs during the potentiostatic polarization of -1050 mVSCE in the 0.5 M NaCl solution without and with NaOH additions. (c) Net current densities and (d) mass changes in the 0.5 M NaCl solution with chromate and vanadate additions...... 105 3.11: Schematic for net currents on the measured polarization curves obtained in the 0.5 M NaCl solution...... 106 3.12: AES depth profiles at two different locations after potentiostatic treatments performed at -1050 mVSCE...... 107 3.13: Local pH profiles under OCP and cathodic polarization of the S phase thin film analogs in different solutions...... 108
4.1: The schematic of EQCM...... 128 4.2: (a) Potential, (b) frequency changes with respect to time during 2 hr of exposure of the Au-deposited quartz with coating. Uncoated data was also presented in (a) and (b)...... 129 4.3: (a) Potential and (b) frequency changes of the coated Al sample up to 300 hr of exposure in water. Data at short time of exposure is also presented...... 130 4.4: (a) Potential, (b) frequency changes of coated samples on the Al-deposited quartz up to 24 hr exposure in the aerated 0.5 M NaCl solution...... 131 4.5: Bode plots of the coated Au sample up to 720 hr of exposure in aerated 0.5 M NaCl solution...... 132 4.6: (a), (b) Bode, and (c) Nyquist plots of the coated Al sample up to 300 hr of exposure in water...... 133 4.7: (a) Circuit 1, (b) Circuit 2, and (c) Circuit 3 used for equivalent circuit modeling. 134
5.1: Type I equivalent circuit with one-time-constant...... 155 5.2: Type II equivalent circuit with two-time-constant...... 155 5.3: The impedance data for dummy cells indicated in Table 5.1...... 156 5.4: Typical current transients for dummy cells A ∼ C indicated Table 5.1 with 30 pA of current range and 1 or 100 Hz of sampling rate...... 157 xv 5.5: EIS spectra with different ratios of the defect area indicated in Table 5.2...... 158 5.6: Simulated current transients for simulated cells with different ratios of the defect area indicated in Table 5.2...... 159 5.7: The impedance variations for real coated panel with immersion times...... 160 5.8: (a) Measured current transient after application of –10 mV of the potential pulse during 30 sec and +10 mV next 30 sec on the 4 days immersed samples (b) curve fitting to a first order exponential decay function on the highlighted region of the (a)...... 161 5.9: Variations of the equivalent circuit parameters, such as (a) Rd, (b) Cdl, and (c) Rct. 162 5.10: Impedance spectra from EIS measurements and Fourier transforms during (a) 1.5, (b) 4.0, and (c) 17 days of immersion...... 163
xvi
CHAPTER 1
INTRODUCTION
This dissertation reports findings on three different but related topics, each of
which will be introduced in this chapter.
A lot of research activities concerning corrosion and corrosion protection of high
strength Al alloys have been performed because this is a critical issue in aircraft structure
applications. It is known that the localized corrosion of Al alloys is mostly induced by
locally developed galvanic cells due to the inhomogeneous distribution of the
intermetallic particles [1, 2]. It should also be noted that the Oxygen Reduction Reaction
(ORR) on the local cathodes as a result of the galvanic coupling increases the local pH of the solution near these intermetallic particles and allows the surrounding Al matrix to dissolve due to OH- attack [3]. Therefore, determination of cathodic kinetics for Al-
containing phases is essential to characterize the corrosion of the high strength Al alloys.
If there is any anodic dissolution during cathodic polarization of Al alloys, the net current
density measured from the potentiostat would be different than the true cathodic current
because electrochemical measurements made using a potentiostat only sense the net
current. Therefore, it is necessary to use a nonelectrochemical measurement, such as 1 Eletrochemical Quartz Crystal Microbalance (EQCM) measurements on thin film
compositional analogs of intermetallic particles [4] in order to evaluate the true cathodic
current density [5, 6].
Corrosion protection such as organic coatings has long been used to protect
metals from corrosion. The major function of an organic coating in providing corrosion
protection to a metallic substrate is to serve as a barrier to reactants, such as water,
oxygen, and ions by adhering to the metal surface [7, 8]. Because all organic coatings are
permeable to these species to some extent [9], knowledge of the transport of water,
oxygen and ion through the coatings is important to better understand the corrosion that
occurs under the organic coatings. In principle, it should be possible to apply the EQCM
technique to investigate water uptake, ion permeation, disbonding of the coating, and
corrosion initiation [10, 11]. However, little research has been performed with respect to the corrosion delamination of a coating on Al (or Al alloys), which then corrodes after water uptake.
For protective intact coatings, or coatings that are just starting to fail,
Electrochemical Impedance Spectroscopy (EIS) cannot accurately predict the initial degradation of the coated layer because the low frequency impedance is typically higher than the input impedance of the EIS system for reasonably-sized samples [12].
Furthermore, since a good paint film usually exhibits a purely capacitive response, changes in corrosion resistance cannot be sensed until a significant degradation occurs.
Therefore, it is necessary to propose other new evaluation technique for assessment of early coating failure.
2 This dissertation consists of six chapters. Chapter 1 is the current chapter giving the brief description of background.
Chapter 2 reviews the literature regarding cathodic corrosion associated with
ORR on individual intermetallic particles. The chapter addresses different types of corrosion found on the coated metal, and summarizes possible techniques to characterize the protective properties of organic coatings. In the end of the chapter, the key unresolved issues and the objectives of this study are given.
Chapters 3 through 5 contain the details of the technical findings of this dissertation with individual introduction, experimental, results & discussion, and summary sections essentially as stand-alone papers. Chapter 3 describes corrosion of S phase thin film analogs. The mass change rates measured by EQCM are presented as function of the pH and addition of inhibitors in aerated 0.5 M NaCl solution.
Chapter 4 describes water uptake in polyurethane coating on Au and Al. Mass changes for the coated samples on the Au and Al-deposited quartz immersed in water are presented.
Chapter 5 describes Potentiostatic Pulse Testing (PPT) for assessment of early coating failure. The application and limitations of PPT from simulated cells composed of known circuit values and from real coated panels are presented.
Finally, chapter 6 summarizes the findings of this work and makes conclusions on the electrochemical studies on coatings and thin films.
3
REFERENCES
1. R. G. Buchheit, Journal of the Electrochemical Society 142, p. 3994 (1995). 2. N. Birblis and R. G. Buchheit, Journal of the Electrochemical Society 152, p. B140 (2005). 3. M. B. Vukmirovic, N. Dimitrov, and K. Sieradzki, Journal of the Electrochemical Society 149, p. B428 (2002). 4. T. Ramgopal, P. Schmutz, and G. S. Frankel, Journal of the Electrochemical Society 148, p. B348 (2001). 5. Y. Baek and G. S. Frankel, Journal of the Electrochemical Society 150, p. B1 (2003). 6. G. S. Frankel, J. Kang, and Y. Baek, in the Conference Paper of the CORROSION NACExpo 2003, San Diego, CA, paper number 03394 (2003). 7. H. Leidheiser, Corrosion 38, p. 374 (1982). 8. H. Leidheiser, Corrosion 39, p. 189 (1983). 9. J. H. W. de Wit, Corrosion Mechanisms in Theory and Practice, in Inorganic and Organic Coatings, P. Marcus and J. Oudar, editor(s), p. 595, Marcel Dekker, New York, NY (1995). 10. B. Muller, I. Forster, and W. Klager, Progress in Organic Coatings 31, p. 229 (1997). 11. K. Noda, J. Park, A. Nishikata, and T. Tsuru, Proceedings of the Symposium on Advances in Corrosion Protection by Organic Coatings III, in QCM Study on Water Absorption in Organic Coatings, I. Sekine, M. Kendig, D. Scantlebury, and D. Mills, editor(s), p. 172, The Electrochemical Society, Pennington, NJ (1997). 12. P. C. Pistorius, in the Conference Paper of the 14th International Corrosion Council, Capetown, S.A., published on CD-ROM (1999).
4
CHAPTER 2
LITERATURE REVIEW
Corrosion of Al alloys has been a critical issue in material selection not only for structural applications but also for specific functional purposes. A lot of research activities concerning localized corrosion of Al alloys have been performed in the areas of pitting, crevice corrosion, intergranular corrosion, exfoliation, and Stress Corrosion
Cracking (SCC), and so on. The effects of intermetallic particles that result in microstructural heterogeneity have been also a main topic of study in corrosion of Al and
Al alloys. Furthermore, corrosion protection such as protective coatings containing inhibitors, chromate conversion coating, and anodizing has been widely investigated in order to retard corrosion for long-term service. Attention has also been given to the
proper monitoring of coating failure.
This chapter reviews cathodic corrosion associated with the Oxygen Reduction
Reaction (ORR) on individual intermetallic particles, different types of corrosion found on coated metals, and possible techniques to characterize the protective properties of
organic coatings. In the end of this chapter, some key unresolved issues that are major
concerns in this study for coatings and thin films are also stated. 5
2.1. CORROSION OF AL ALLOYS WITH SEVERAL INTERMETALLIC
PARTICLES
The intentional control of the heterogeneous microstructure of commercial Al
alloys has been used to produce desirable mechanical properties, but such heterogeneous
microstructure, which mainly results from the intermetallic particles and impurities that
are unavoidably introduced during melting, solidification from the melt and uncompleted
heat treatments, makes the alloys more susceptible to localized corrosion. For example,
localized corrosion, such as pitting and alkaline attack of Al based precipitation age
hardened alloys which are usually categorized into 2xxx (Al-Cu-Mg), 6xxx (Al-Mg-Si),
7xxx (Al-Zn-Mg), and 8xxx (Al-Li) series alloys, is mostly induced by locally developed
galvanic cells due to the inhomogeneous distribution of the specific alloying elements
associated with intermetallic particles [1, 2]. Therefore, it is important to characterize the
individual activities of intermetallic particles to better understand the corrosion of the Al
alloys. Another critical aspect of Al alloy corrosion is that the ORR on the local cathodes
as a result of the galvanic cell configuration increases the local pH of the solution near
these intermetallic particles and allows the surrounding Al matrix to dissolve due to OH- attack.
6 2.1.1. Corrosion Properties of the Intermetallic Particles in the Heat Treatable Al
Alloys
2.1.1.1. Al2Cu Particles
Data extracted from the literature shows that the Open Circuit Potential (OCP) of
Al2Cu (θ phase) particle ranges from –0.590 to –0.700 V for aerated and deaerated
solutions with chloride concentrations ranging from 0.2 to 1.0 M [1]. It has also been
reported by many other researchers that the θ phase is cathodic to the Al matrix [1, 3, 4].
Scully et al. reported that the OCP of the θ phase is much higher than that of pure Al
from pH 2 to 12 [5]. In particular, they showed that the θ phase supports cathodic
electron transfer reactions at enhanced rates relative to pure Al due to the presence of
metallic Cu in the oxide film, whereas pure Al hardly supports the ORR because the film
on pure Al is mostly insulating oxide [5]. This unoxidized metallic Cu in the passive film
was confirmed by the angle resolved X-ray Photoelectron Spectroscopy (XPS)
observation in another study [6]. Rüdiger and Köster found mixtures of Al2O3 needles and Cu oxide, not metallic Cu, on the surface of the θ phase after one hour of anodic polarization in 0.1 N NaOH solution [7].
Mazurkiewicz and Piotrowski observed the effect of Cu on the θ phase that was anodically polarized in solutions with various ranges of pH [8]. A peak associated with a change in the oxidation state of Cu in the anodic polarization curve was shown, indicating the occurrence of a redox reaction of metallic Cu on the electrode [8]. They reported that the anodic polarization behavior of the θ phase in the range of acidic and
- neutral pH is controlled by the presence of Al hydroxide, while it is controlled by AlO2 7 2- and CuO2 hydroxyl ions in the range of alkaline pH [8]. This is also illustrated in
Pourbaix diagrams which are the thermodynamic phase stability plots, showing oxidizing power and acidity of the solution [9, 10].
It should be noted that this noble OCP of the θ phase with respect to the Al matrix plays a significant role in determining the susceptibility to the localized corrosion on Al alloys. Scully et al. postulated that the θ phase may raise the potential of the Al matrix sufficiently to promote pit initiation [6]. Galvele and Micheli found that intergranular corrosion of Al-4%Cu occurs when a Cu-depleted zone along the grain boundaries due to the precipitation of the Cu-rich θ phase forms, lowering the breakdown potential of the
Cu-depleted zone with respect to that of the Al matrix [3]. The susceptibility of this Al-
Cu alloy to intergranular corrosion due to the θ phase has also been reported [11].
As stated above, it is important to describe the cathodic behavior of the θ phase even though it is difficult to exactly determine the cathodic kinetics using conventional electrochemical techniques. Recently, Baek and Frankel reported the cathodic activities of θ phase using the Electrochemical Quartz Crystal Microbalance (EQCM), simultaneously measuring mass changes of the electrode as well as current [12, 13]. It was found that the calculated true cathodic current density of the cathodically polarized θ phase is almost equal to the measured net current density, which is the sum of the dissolution and ORR rate shown from the potentiostat, indicating that the dissolution rate is very small [12, 13]. In addition, they presented data showing the effect of chromate on the cathodic reaction rate [12, 13]. A larger decrease in ORR rate, which is the dominant cathodic current, was observed on the θ phase than on Al and the Al matrix [12, 13]. 8
2.1.1.2. Al2CuMg Particles
It is known that approximately 60 % of the second phase particles in AA2024-T3
are Al2CuMg (S phase) particles, which are active with respect to the Al matrix [14]. The active S phases are preferentially attacked in a neutral chloride solution with respect to the Al matrix [14-18]. Al and Mg are selectively dissolved away from the S phases and
leave a Cu-rich remnant. This dealloyed S phase particle behaves as a cathode for ORR,
while the Al matrix surrounding the particle acts as an anode. ORR at the cathode sites
increases the local pH, which promotes the dissolution of the surrounding Al matrix as
- the soluble AlO2 anion. The further increased pH accelerates the preferential dissolution of the S phase and the consequent dissolution of the surrounding Al matrix.
Buchheit et al. introduced the concept of “Cu liberation and redeposition” to explain the pitting associated with S phases [14]. They found that there are two different types of pitting morphologies of the S phase: active pitting resulting from the anodic behavior of this phase, and the periphery pitting occurring at the matrix near the dealloyed S phase [14]. They reported that the periphery pitting occurs because the intact
Cu-rich remnants induce pitting in the matrix at their periphery, while other Cu-rich remnants decompose into Cu clusters and detach from the surface [14]. These detached
Cu clusters are then electrically isolated from the substrate and free to oxidize or redeposit on the alloy [14]. The redeposited Cu possibly acts as preferential sites for ORR
[14-16]. Cu around inclusions is often observed by large-scale corrosion tests [19-24].
9 Several researchers have reported different corrosion morphologies of the S
phase. Dimitrov et al. observed the morphology of dealloyed Al alloy in an alkaline
solution [17]. They found “a bicontinuous dealloyed morphology” as a result of applying
the potential above the “critical value” [25]. Suter and Alkire observed morphological differences between pit initiation at the adjacent matrix of the S phase below the critical potential and pit initiation underneath a dealloyed Cu-rich remnant above the critical
potential [26], as described by Buchheit et al. [14]. Guillaumin and Mankowski also
reported different types of corrosion morphologies in the polarization of AA2024-T351,
showing two different characteristic breakdown potentials [27]. The first one is related to
the selective dissolution of the S phase [27]. At this stage, the matrix adjacent to the
dealloyed S phase dissolves away [27]. The second noble breakdown potential is
associated with the dissolution of the matrix and grain boundaries [27]. This dissolution
is a complex form of pitting (matrix breakdown) and intergranular corrosion (fragile
grain boundaries due to the initial dissolution of the intergranular precipitates) [27].
Moreover, it has been reported that the S phase is responsible for the SCC of Al-Cu
alloys [28]. Figure 2.1 illustrates that intergranular SCC results from the dissolution of
the Cu-denuded zones along the grain boundaries.
Schmutz and Frankel observed the corrosion inhibition of the S phase by
introducing dichromate using a in-situ Atomic Force Microscopy (AFM) scratching [29].
Kolics et al. also found that dealloying of Mg, Cu enrichment, and redeposition on the S
phase are strongly inhibited by chromate additions [18].
10 Leard reported several significant findings on the dealloying of the S phase by using a microelectrochemical cell technique [30]. He suggested that the S phase exhibits
both passive and active behaviors [30]. It was noted that the passive behaviors are due to the development of a Cu-rich dealloyed layer by sub-critical polarization [30]. This sub- critical polarization enhances passivity and that passivity is destroyed by exceeding the critical potential [30]. He explained that this accounts for why some S phase particles are selectively dissolved under the free corrosion condition while others are not [30]. He concluded that the breakdown of the AA2024-T3 is closely related to the breakdown of the S phase [30] because of the coincidence of the breakdown potential for the S phase and that for AA2024-T3 as predicted by Buchheit et al. [15, 16, 31].
Unlike the research described above, Schmutz and Frankel reported that the S phase at the surface of a freshly-polished AA2024-T3 sample in air is noble with respect to the Al matrix by using Scanning Kelvin Probe Force Microscopy (SKPFM) [32]. They concluded that this nobility is due to the presence of a surface oxide film, which has an altered composition compared to the particles in the bulk [32]. However, they stated that the S phase shows the active dissolution in chloride solution after a certain induction time
[32, 33].
2.1.1.3. AlCuFeMn Particles and Other Cu Containing Particles
It is known that the AlCuFeMn phase is more noble than the Al matrix and acts as a cathode [1]. Suter and Alkire confirmed this by measuring the pitting potential of the
AlCuFeMn phase, and observing the corrosion morphology using a microelectrochemical
11 cell [26]. However, Schmutz and Frankel reported that the noble AlCuFeMn phase
increases the surface reactivity, and then exhibits nonuniform dissolution in chloride
solution [32, 33].
Rüdiger and Köster conducted polarization experiments on Al-Cu-Fe phases in a
strong acid solution, and found a porous Cu layer after polarization [7]. Studies
performed by Leard showed that local alkalinity development at noble Al7Cu2Fe and
Al20Cu2Mn3 phases results in the dissolution of these phases [30].
2.1.1.4. Al3Fe Particles and Other Particles with Mn and Si
Many researchers have found that the Al3Fe phase is more noble than the Al
matrix [23, 34-36], and increases the pitting susceptibility. Thus, it is likely that the local
pH increase which is due to the ORR causes the formation of pits around the phase,
similar to the θ phase. Golubev and Ronzhin reported that the Al3Fe phase is detrimental
because it lowers the overpotential for Hydrogen Evolution Reaction (HER), which is the
primary cathodic reaction in the acids [37]. They remarked that this phase can be passive,
however, as a result of the formation of Fe3O4 on the surface after the selective dissolution of Al [37], leading to a significant increase in the cathodic reaction rate [34].
It is known that the addition of Mn is not harmful to the corrosion resistance of Al
alloys, whereas the addition of Fe adversely affects its corrosion properties as described
above [34, 38, 39]. This is because the corrosion potentials of Al and Al6Mn or
Al6(Fe,Mn) are similar, effectively minimizing the galvanic potential difference between the matrix and intermetallic particles [34, 38].
12 Si is believed to be a beneficial component like Mn, even though the available
corrosion data are limited [35]. Silica on the surface of Si-containing phases is
responsible for their corrosion resistance [35, 40]. Nisancioglu and Lunder also pointed
out that Mn/Fe or Si/Fe ratios in the alloy are important because these ratios can
determine the cathode/anode area ratio between the particles [40]. This area ratio is
frequently discussed in galvanic coupling where the anodic and cathodic sites are
separated with different areas.
2.1.1.5. MgZn2 Particles with Different Cu and Al Content
The MgZn2 (η phase) particle, which is found in the 7xxx Al alloys, is active with
respect to the Al matrix [1, 41]. Ramgopal et al. found that the breakdown potential of
the η phase in the neutral and alkaline deaerated solution with chloride is associated with
Zn dissolution and is a function of the Cu content in the particles from their systematic observation of the electrochemical behavior of the η phase with different Cu and Al contents [42]. However, they suggested that the breakdown of the η phase is not directly
related to the pitting potentials of the 7xxx Al alloys because the pitting potential of the
7xxx Al alloys are well above the breakdown potentials of the η phase [42].
2.1.1.6. Al2CuLi Particles
Al-Li alloys have attracted considerable attention for the possibility of weight
saving due to their low density [43]. To better understand the mechanical as well as
electrochemical behaviors, numerous researches have been performed [44-47]. There are
13 several major intermetallic particles in this alloy system, such as metastable Al3Li (δ′ phase), AlLi (δ phase), Al2CuLi (T1 phase), and Al6CuLi3 (T2 phase). Buchheit et al.
conducted studies on the T1 phase, which exhibits a high current density in the anodic
polarization curve [46]. They investigated two kinds of pitting attack in AA2090 (Al-2Li-
3Cu): small pitting due to the selective dissolution of the T1 phase along the sub-grain
boundaries and the large pit formation around Al-Cu-Fe constituent particles [46, 47].
They claimed that this “constituent particle pitting” is developed by the acidic
environment where sub-grains face each other followed by the sub-grain boundary pitting
[46].
2.1.1.7. Al3Zr Particles
Al-Li alloys have two distinct characteristics: low density and high stiffness.
Therefore, they have been used as developed versions of the 2xxx Al alloys to save
weight [46, 47]. However, Al-Li binaries suffer from low ductility and toughness due
primarily to the severe strain localization from the cutting of coherent Al3Li (δ′ phase)
particles by dislocations, resulting in cracking along the boundaries [48]. Thus, several
alloying elements, for example Cu and Mg, have been introduced to reduce the solubility
of Li in Al in order to form additional precipitates capable of dispersing dislocations
more homogeneously [48]. Zr additions form refined coherent Al3Zr particles, which
make grain structure stabilize and disperse dislocation [48]. Little study has been done on
this particle except the work performed by Scully et al., presenting that the OCPs of the
Al3Zr particle are higher than those of pure Al from pH 2 to 12 [5].
14
2.1.2. The Cathodic Corrosion on Aluminum Alloys
During cathodic polarization of Al and Al alloys, the cathodic reactions, such as
ORR and water reduction, lead to hydroxide ion generation at the surface, which results in a local pH increase. This “alkalization” at the electrode has been reported by Vetter
[49]. The increased pH results in the hydration of the Al oxide [49]. This phenomenon, particularly during cathodic polarization, is known as cathodic corrosion, and has been a topic of study in the area of Al corrosion [50-55]. Van der Ven and Hoelmans found that there is little dependence on the cathodic current density with the cathodic corrosion rate of Al at lower temperatures because all OH- ions produced are not involved in the chemical dissolution of Al [50]. Takahashi et al. found that pitting during cathodic polarization of Al varies significantly with the type and thickness of oxide film [55].
In this section, basic reaction steps for the cathodic corrosion of Al are described.
In particular, ORR, which is the primary cathodic reaction in neutral environments, is emphasized.
2.1.2.1. Cathodic Corrosion of Pure Al [53]
Moon and Pyun reported that the cathodic corrosion of pure Al consists of the chemical dissolution of the oxide film at the oxide/solution interface and the simultaneous oxide film formation at the Al/oxide interface [53]. For the oxide film formation, they used the point defect model [56], which describes how the growth of the passive film is controlled by the transport of the oxygen and metal vacancies across the
15 film [53]. The passive film can be formed by an electrochemical reaction, which
generates oxygen vacancies at the Al/oxide interface,
2+ - 2AlAl(m) = 2AlAl(ox) + 2⋅(3/2) VO (ox) + 2⋅3e (2-1)
2+ + 3H2O + 3VO (ox) = 3OO(ox) + 6H (aq) (2-2)
+ - 2AlAl(m) + 3H2O = 2AlAl(ox) + 3OO(ox) + 6H (aq) + 6e (2-3)
2+ where AlAl(m) is the Al in the metal site; AlAl(ox) is the Al in the oxide film; VO (ox) is
+ the oxygen vacancy; OO(ox) is the oxygen in the oxide film; H (aq) is the hydrogen ion
in the aqueous solution, respectively [53]. Because the dissolution of the oxide film is
normally accelerated under anodic polarization, it is quite reasonable that the
electrochemical dissolution of the oxide film under the cathodic polarization is limited
[53]. Thus, they proposed that the dissolution of the oxide film under cathodic
polarization can be explained by the chemical dissolution of the oxide film due to the
attack by OH-:
- - Al2O3 (2AlAl(ox) + 3OO(ox)) + 2OH (ad) = 2AlO2 (aq) + H2O (2-4)
- - where OH (ad) is the adsorbed hydroxide ion, and AlO2 (aq) is the aluminate ion in the
aqueous solution, respectively [53]. The following reaction (2-5) shows the total anodic
16 portion of the cathodic corrosion reaction by combining the oxide formation and the chemical dissolution reaction [53]:
- - + - 2AlAl(m) + 2H2O + 2OH (ad) = 2AlO2 (aq) +6H (aq) + 6e (2-5)
They suggested the water reduction in neutral and alkaline solutions resulting in hydrogen evolution and hydroxide ion formation as a cathodic reaction:
- - 6H2O + 6e = 3H2 + 6OH (aq) (2-6)
and concluded that the overall cathodic corrosion reaction of Al can be written by combining the two reactions above [53]:
- - 2AlAl(m) + 2H2O + 2OH (ad) = 2AlO2 (aq) + 3H2 (2-7)
If the ORR (reaction (2-8)) is proposed as the main cathodic reaction, the overall corrosion reaction of Al during cathodic polarization can be described as follows [53]:
- - (3/2) O2 + 3H2O + 6e = 6OH(aq) (2-8)
- - 2AlAl(m) + 2OH (ad) + (3/2) O2 = 2AlO2 (aq) + H2O (2-9)
17 2.1.2.2. Oxygen Reduction Reaction (ORR)
The ORR has been extensively studied in the area of the electrochemical energy
conversion including H2/O2 fuel cells [57-60]. Previous work revealed that oxygen is
- reduced to peroxide anions on carbon electrodes (a two-electron process, HO2 ) or to
hydroxide (a four-electron process, OH-), including the decomposition of the peroxide intermediates in the case of the electrodes with a catalyst [61-64]. Furthermore, it was reported that the ORR is strongly dependent on the electrode materials, surface modifications, chemisorption of O2, its reduction products, chemisorbed anions, and the
pH of solutions [65]. However, Yang and McCreery concluded in their literature review
that the mechanism, the identification of intermediates, and the rate-determining step in
the ORR are not yet clear [65].
Several studies of the ORR on Cu have been performed [66-68]. These studies are
valuable for a fundamental understanding of the cathodic reaction behaviors on Cu
bearing Al alloys. King et al. investigated the ORR on Cu, and found different ORR
rates, depending on Cu the surface reactivity with various Cu(0) and Cu(I) concentrations
[66]. Brisard et al. also suggested that the ORR rate on Cu surface depends on available
Cu sites for the adsorption of O2 [67]. In addition, Kendig and Jeanjaquet confirmed that
Cr(VI), which is widely used for corrosion inhibition, inhibits ORR on Cu [68]. Clark et
al. also reported Cr(VI) inhibition of ORR on Cu, Pt, and glassy carbon electrodes [69,
70]. They concluded that a thin Cr(III) monolayer from the Cr(VI) reduction strongly
inhibits the ORR due to the occupation of active chemisorption sites, which retards
further electron transfer [70].
18 However, in the case of Cu containing Al alloys, the cathodic reaction takes place
mainly at local cathodes including Cu-rich intermetallic particles such as Al2Cu,
Al2CuMg phases and redeposited metallic Cu on the alloy surface [14-16]. The high pH
zones are developed on and near the local cathodes due to their strong cathodic activities, which results in further dissolution and dealloying of this alloy [14-16]. Leclere and
Newman observed a relatively small limiting cathodic current of AlCu solid solution alloys in unbuffered solution compared with buffered solution, because corrosion of the
Al by alkalinity generated due to the ORR increases anodic current [54]. Ilevbare and
Scully found an increase of the ORR rates on intermetallic particles such as Al-Cu, Al-
Cu-Mg, Al-Cu-Mn-Fe, and a decrease of the ORR rates on the chromate conversion coated intermetallic particles [71]. Clark et al. also reported that chromate reduces the observed cathodic current on AA2024-T3 showing inhibition of the ORR on Cu sites
[69]. However, it is still unclear that the chromate conversion coating acts as either a
barrier to an electronic transfer reaction or a diffusion barrier to block ORR [71].
2.1.3. Summary
During cathodic polarization of the Al and Al alloys, the cathodic reactions, such as the ORR, lead to hydroxide ion generation at the surface, which results in a local pH increase. This increased pH causes the hydration of the oxide of the Al and Al alloys.
Even though cathodic reaction is proposed as a controlling one for the cathodic corrosion
on Al alloy, it is still unclear how to define the exact behavior of ORR on Al alloys due to
19 the microstructural heterogeneity mainly created by intermetallic particles, which are summarized in Table 2.1.
2.2. ORGANIC COATINGS FOR CORROSION PROTECTION
Two different forms of corrosion protection are most commonly used in real applications. One is a protective coating that separates the environment from the sample, and the other is an inhibitor that alters the environment. Organic coatings act primarily as a physical barrier between the substrate and the corrosive environment, including reactants, water, oxygen and ions, but may also serve as a reservoir for inhibiting compounds that protect the surface from attack [10, 72, 73]. The properties of these organic coatings mainly depend on the possible flaws in the polymer network, pigments and other additives, the metal substrate, and the surface pretreatment (e.g., conversion layer) [72-77]. Insufficient protection by organic coating results in corrosion. There are several different types and mechanisms for failure of a protective organic coating.
This chapter first explains the components of organic coatings, such as binders, pigments, fillers, additives and solvent. General corrosion degradation of these organic coatings and typical corrosion failures of organic coatings on Al and Al alloys are reviewed thereafter.
2.2.1. Paint Components
Organic coatings consist of several basic constituents [72, 78-80]: binders, pigments and fillers, additives (such as dryers, hardeners, stabilizers, dispersion agents,
20 and surface activating compounds), and solvent. Table 2.2 shows the function of the main
components of paint [79].
2.2.1.1. Binders
A binder is the continuous polymeric phase that adheres to the substrate, holds the other substances in the coating together and provides a good adhesion with the outer layer
[80-82]. Because it acts as the primary physical barrier against aggressive species, coatings are usually classified according to the application of the binder [81].
2.2.1.2. Pigments
Organic pigments are usually powdered materials that are colloidal-dispersed and remain suspended in the binder after film formation [80, 83]. They act primarily as coloring, opacification, anti-corrosion and cost-saving agents in the coating [83].
Inorganic pigments, such as earth, mineral, synthetic inorganic, and metallic pigments [78], are partially used as anti-corrosion agents in the coating. Two different roles of these functional pigments for anti-corrosion have been reported: corrosion inhibition and sacrificial protection [80]. In principle, inhibitive inorganic pigments are agents that are able to reduce the corrosion rate by inhibiting the anodic or cathodic reaction. Inorganic pigments include compounds such as chromates, phosphates, silicates, borates, molybdates of various metals such as lead, zinc, calcium, aluminum, barium or strontium, etc. [78]. Recently, zinc phosphate has been an important pigment because the use of lead and chromates has been prohibited by legislation due to their toxicity [84, 85].
21 Sacrificial protection, typically applied to Fe substrates, is obtained by adding actual particles or flakes of Zn [84, 85].
There are several other supplementary pigments for properties such as hardness, resistance to abrasion, and weathering [83]. They include tale, mica, silica, clay, etc. [80,
83]. It is very important, however, to ensure that they are compatible with the binder, resistant to the environment, and cost-effective [80].
2.2.1.3. Other Additives
Other additives can be added in small quantities to modify some properties of coatings, such as the drying of the coating mixture, the hardening of the coating film, the stability of the mixture before application, the dispersion or prevention of sedimentation of pigments and fillers, and surface activation [80, 86].
2.2.1.4. Solvents
Solvents are volatile liquids that are added to the coating formulation to achieve a homogeneous and adequately viscous mixture before application [80, 87]. However, solvent free coatings have been under development because of the tightening of worldwide environmental legislation that forces coating industries to decrease levels of pollutant substances released into the atmosphere [88, 89].
22 2.2.1.5. Multilayer Coatings
In real systems, multilayer coatings are used [82]. The typical stacking sequence
is: substrate, conversion coating, primer, and topcoat layer. The lower conversion coating
and primer layers may improve adhesion or provide inhibition, while topcoats provide
appearance and resistance to specific environments [82]. Both the primer and the topcoat
can contain anti-corrosion pigments [82].
Conversion layers have been known to exhibit good adhesion to the organic layer
and excellent corrosion resistance against various corrosive environments. Leidheiser suggested six important characteristics about chromate which is widely used for
corrosion inhibition and conversion coating [90]:
- a reduction to Cr(III) forming a low solubility of Cr (III) oxide,
- a widely effective range of pH for real applications,
- film formation at the coating/substrate interface that does not reduce the
adhesion,
- a reservoir to repair defects,
- inhibition of anodic reactions,
- and inhibition of cathodic reactions
A common chromate bath contains an acid to produce a low pH environment and
fluoride ions to remove existing oxides [77]. Cr(III) hydroxides, which are generally
amorphous, nonporous and gel-like films, form from the Cr(VI) reduction reactions and
23 play a major role in corrosion prevention [77]. However, suppression or nonuniform growth of the chromate conversion coating due to the presence of intermetallc particles distributed locally was reported [91, 92].
A recent tendency based on environmental regulations has been to prohibit the use of chromate due to its toxicity. Even though considerable research on Cr-free conversion layers is actively being pursued [68, 93], it is very difficult to find effective alternatives for the chromate layers.
2.2.2. Corrosion of Metal under Organic Coatings
Leidheiser [72] and de Wit [77] reviewed seven different types of corrosion underneath organic coatings: wet adhesion, blistering, cathodic delamination, anodic undermining, filiform corrosion, early rusting, and flash rusting, any of which may be related to the others [77]. Water and oxygen permeation are required for corrosion [77], so transport of water, oxygen and ion through the coatings is also important to better understand the corrosion that occurs under the organic coatings.
2.2.2.1. Water and Oxygen Permeation and Underfilm Corrosion Initiation
Water can permeate all organic coatings to some extent [77]. Corrosion of coated materials occurs only after the adhesion between the coating and the substrate is broken due to water or oxygen permeation and subsequent electrochemical reactions within the water thin layer [77]. There are three major driving forces for water permeation [77]:
24 - a concentration gradient through the polymer during immersion or exposure to a
humid atmosphere,
- osmotic pressure due to impurities or corrosion products at the interface between
the metal substrate and the coating,
- and capillary forces in the coating due to poor curing, improper solvent
evaporation, bad interaction between binder and additives, or entrapment of air
during application.
Hence, corrosion of the coated metals is effectively prevented when these driving forces
are reduced [77]. The basic transport mechanisms of O2, water and ions are described
below.
2.2.2.1.1. Transport of O2
There is some disagreement about how fast O2 migrates through organic coatings.
It was reported in German sources [94, 95] that O2 permeability is the rate-controlling
factor for the corrosion of organic coatings because the consumption of O2 determined
from the corrosion on bare carbon steel is much higher than the O2 permeability
determined from several painted films [82]. Mayne reported that O2 permeation,
however, is not the rate-controlling factor in the corrosion process because the observed
rate of O2 permeation is higher than the rate of consumption of water and O2 in the corrosion of uncoated steel [96, 97].
25 2.2.2.1.2. Transport of Water
There is common agreement that water permeability is faster than that required
for corrosion of the bare substrate [97, 98]. Because water is essential for the initiation of corrosion, corrosion prevention can be obtained successfully by increasing the coating thickness, using a coating with low porosity, and applying multilayer coatings [99].
2.2.2.1.3. Transport of Ions
It is widely known that the permeation of ions is much slower than the permeation of water [98]. Ruggeri and Beck claimed that the diffusion coefficient of ions through polyurethane is so low that under most conditions the transport of ions through paint can be neglected [100]. They also added that ionic conduction in polyurethane changes as the external electrolyte concentration changes [100]. Most cations selectively permeate
- 2- organic coating films, whereas permeation by anions, such as Cl or SO4 , is retarded [76,
101].
In summary, all the species mentioned above can be rate-limiting species in specific corrosion/coating environments. For example, when water and O2 permeate
through a paint film faster than ionic species, the diffusion of ions may be the rate-
limiting species. In this case, ions and the resulting corrosion products are built up at the
substrate/paint interface, leading to further adhesion problems and local blistering.
26 2.2.2.2. Wet Adhesion
Once water has reached the interface between the substrate and the coating, it may result in a loss of the coating’s adhesion, which is called wet adhesion. de Wit cited two different assumptions for the mechanism of adhesion loss [77]:
- chemical disbonding due to the chemical interaction of water molecules with
metal (metal oxide) and polymer,
- and mechanical disbonding due to forces caused by the accumulation of water
and osmotic pressures.
2.2.2.3. Blistering
Leidheiser proposed five mechanisms to explain blistering, which usually happens prior to corrosion [72]:
- osmotic pressure, which is considered as the most important mechanism
responsible for blister formation,
- volume expansion due to the swelling caused by water absorption,
- water penetration due to a potential gradient (electro-osmotic blistering),
- gas inclusions, air bubbles or volatile components that might be involved in film
formation,
27 - phase separation and local retention of coating components due to incomplete
film formation.
Besides these mechanisms described above, Funke also reported these other possible phenomena for blistering [76]:
- OH- ions increase pH locally due to the reduction reaction of the permeated
water. Even though the diffusion of these cations such as Na+ or K+ through the
organic films is very slow, cations migrate into the cathodic area to form NaOH
or KOH, which increases the osmotic pressure (alkaline blistering, Figure 2.2).
- Fe(OH)2, instead of NaOH, can produce a local corrosive environment under the
coating defects. Because this compound prohibits the transport of oxygen, area
coated with Fe(OH)2 increasingly lack oxygen and become anodically polarized.
This growing anodic character stimulates the formation of cathodic areas adjacent
to it. Blistering develops at the cathodic area (blistering by underrusting, Figure
2.3).
- Adsorbed water on the intact organic coating in the solution without alkali cations
is reduced, which produces a differential aeration cell. The center of the blister
becomes anodic, and the peripheral zone becomes cathodic. Electroneutrality
may be achieved by the dissociation of water, involving proton migration along
the interface. Blistering develops at the cathodic area (neutral blistering, Figure
2.4).
28
2.2.2.4. Cathodic Delamination
The high pH due to the cathodic reactions may cause loss of adhesion as a result
of dissolution of the metal at the interface or chemical disintegration of the coating polymer at the interface [102]. This loss of adhesion due to effects of the cathodic reaction at the interface is known as cathodic delamination [102]. This type of delamination can occur in the presence or absence of an applied cathodic potential [77].
de Wit stated schematically how blistering propagates due to cathodic delamination under an intact or defective coating [77]. Under an intact coating, the reaction of metal cations and penetrated oxygen results in the formation of a
semipermeable oxide, which allows only permeation of water [77]. Due to this oxide, anodic reactions occur under the oxide, while cathodic reactions are forced to move to the
edge of the blister where oxygen may still permeate the coating [77]. The pH increases at
the edge of the blister, causing delamination and further growth of the blister [77].
Cathodic delamination under a coating with defects is very similar to that under
intact coatings [77]. Because some of the metal substrate is exposed to the environment,
metal dissolution occurs immediately, which results in the formation of corrosion
products [77]. These products, however, act like an imperfect oxide film, which prevents
further oxygen permeation by sealing the defect in the coating [77]. Like blister
propagation under an intact coating, anodic and cathodic sites are separated, and further
29 delamination at the edge of the blister occurs due to the differential aeration cell effect
[77].
Because cathodic delamination is due to the cathodic reaction in the tips, the OH- concentration is key to determining and preventing this delamination [77]. de Wit showed four different observations to demonstrate the effect of OH- on cathodic delamination
[77].
- The presence of impurity cations leads to a higher delamination rate because they
lead OH- inside of the edges.
- Unidirectional growth is observed because of the diffusion path of OH-.
- A low rate of delamination is expected on Al due to the poor electronic
conductivity of the Al oxide compared with Zn and steel [72], reducing the
number of electrons that reach the reaction site.
- The addition of a transitional metal, such as cobalt ions [103], which trap the
electrons in the oxide and prohibit further cathodic reactions, reduces the rate of
delamination.
2.2.2.5. Anodic Undermining
Anodic undermining is one of the corrosion reactions underneath an organic
coating showing a loss of adhesion caused mainly by anodic dissolution of the substrate
30 metal or its oxide [77]. This might happen in either defective sites or, in most cases,
corrosion sensitive sites under the coating [77].
2.2.2.6. Filiform Corrosion
Filiform corrosion is a special form of corrosion on coated metals [77, 104]. It is
characterized by a thread-like undermining [72, 76, 102] and generally occurs in humid,
chloride-containing environments. The typical conditions for filiform corrosion are [77]:
- high relative humidity,
- coating with water permeability,
- presence of contaminants such as salts,
- defects in the coating
Hoch reviewed filiform corrosion on steel, Mg, and Al and confirmed that several different pH zones developed by hydrolysis around the head of the filiform [74]. Ruggeri
and Beck also reported in their review of filiform corrosion that the front edge of the head
contains a solution of low pH and appears to be the most anodically active area [105].
de Wit used the differential aeration cell and hydrolysis to describe filiform corrosion [77]. Once the metal substrate with oxide is attacked, metal is exposed to the
aqueous solution [72, 77]. The differential aeration cell might occur due to the different
diffusion paths for oxygen or to the resulting corrosion products that block oxygen
31 transport [77]. This difference in oxygen concentration causes a potential difference and can result in a separation of anodic and cathodic reactions [72, 77]:
Al = Al3+ + 3e- (2-10)
- - O2 + 2H2O + 4e = 4OH (2-11)
The head of the filiform is supplied with water by osmotic action due to the high concentration of Al ions [72, 77]. Al ions are hydrolyzed, and the pH of these anodic sites decreases with time [72, 77]:
3+ 3+ Al + 6H2O = Al(H2O)6 (2-12)
3+ 2+ + Al(H2O)6 = Al(H2O)5(OH) + H (2-13)
2+ + + Al(H2O)5(OH) = Al(H2O)4(OH)2 + H (2-14)
+ + Al(H2O)4(OH)2 = Al(H2O)3(OH)3 + H (2-15)
The precipitation of final product Al(H2O)3(OH)3 accumulates in the back of the head area and acts as a poor membrane to separate anodic and cathodic sites [72, 77], as shown in Figure 2.5 [105]. This acidity stimulates more Al dissolution like pit propagation with its autocatalytic property [10, 77, 104].
There are two extremely different possibilities in filiform corrosion: whether water and oxygen can diffuse through the coatings or through the tail. In the case that oxygen and water diffuse from the porous tail to the head, Al3+ diffuses to the cathodic
32 region, and subsequently reacts at the cathode [105, 106]. Ruggeri and Beck verified oxygen diffusion through the porous tail [105]. Morita and Yashida found that sealing the tail immediately halts the process, whereas removing the seal reactivates it [106].
On the other hand, oxygen and water diffuse through the coating layer itself.
First, the microblister forms in the differential aeration cell locally developed [98]. Then, the blister starts to grow into a filiform [98]. The filiform continues growing because oxygen and water are provided continuously [98]. It is probable that the oxygen or water diffusion through the organic coating is favorable in the case of a small degree of crosslinking film [98]. It is very difficult, however, to explain why the reaction products are at the back of the head [77].
Funke proposed a different mechanism for filiform corrosion occurring in a relatively high humidity environment to understand why filiform corrosion grows periodically, consequently produces the segmented structures, and finally is replaced by blistering [76]. According to his theory, the external cathodic area that results from the locally concentrated OH- grows until it meets the head of the filiform [76]. After meeting
each other, metal cations migrate to the new head front and OH- ions go into to the opposite direction [76]. Koehler emphasized that the contaminants can provide ions that promote the unwanted corrosion processes [107]. Service temperature and the pretreatment given to the metal substrates have also been reported as important factors for filiform corrosion [108].
Schmidt and Stratmann measured the local potentials of the active head and the back of the head in filiform corrosion on AA2024-T3 using a SKPFM [109]. The active
33 head as the anode and the back of the head as the cathode where the ORR occurs are distinctly separated due to the formation of an oxygen concentration cell [109]. This separation was also confirmed by Lebozec et al. [110]. In addition, Lebozec et al. reported Al hydroxide gel formation due to Al and hydroxyl ions generated by the anodic reaction and the ORR, respectively [110]. Mol et al. reported that the susceptibility to filiform corrosion varies with the solute atom, and Cu has a detrimental effect on the filiform corrosion on Al alloys [111]. Huisert also mentioned that it is important to control the reactions at the intermetallic particles because cathodic reactions take place predominantly on them [112]. Some work on the effect of mechanical grinding procedures has shown that filaments typically follow the grinding direction [74, 113].
2.2.2.7. Early Rusting
Early rusting typically occurs in latex coatings [72, 77]. During the latex coating, water evaporates from the coating with latex paricles coalescing [72, 77]. Then, latex particles are in contact with each other and they are deformed and hardened as a result of surface tension and capillary forces [72, 77]. Early rusting occurs when water-soluble salts diffuse through the coating before the coating is completely hardened [72, 77].
There are three major conditions that lead to early rusting [72, 77]:
- a thin latex coating (up to 40 μm ),
- a low substrate temperature,
- and a high humidity condition.
34
2.2.2.8. Frash Rusting
The appearance of brown rust stains on a blasted steel surface immediately after applying a water-based primer is called flash rusting [72, 77]. This results from the remaining contaminants after the application of blast cleaning, because the steel or ceramic grit produce crevices or local galvanic cells to activate the corrosion process as soon as the water-based primer is applied to the surface [72, 77]. This possible adverse effect on the corrosion performance is remarkable because blast cleaning is known as the conventional surface treatment for corrosion protection [72, 77].
2.2.3. Summary
In the case of filiform corrosion, it is not harmful to the metallic substrate itself, but it causes delamination of the coating such that the coating cannot protect the metallic substrate anymore. The separation of anodic and cathodic site [105], the differential aeration [72, 77, 105], pH difference [74, 105], periodical growth behavior [76, 77], and other effects such as pretreatments [74, 113] are understood. However, there are still unresolved issues concerning the diffusion path for water or oxygen [98, 105, 106] and the exact role of cathodic intermetallic particles providing cathodic reactions [111, 112], the substrate and surface pretreatment dependence on the filiform corrosion of Al alloys
[74, 113].
35 2.3. CHARACTERIZATION TECHNIQUES
There are several different approaches to characterize the protective properties of
organic coatings. In this section, open circuit potential monitoring, polarization resistance
measurement, Electrochemical Noise Analysis (ENA), Electrochemical Impedance
Spectroscopy (EIS), Potentiostatic Pulse Testing (PPT), and Quartz Crystal Microbalance
(QCM) are discussed for undertaking this purpose.
2.3.1. Open Circuit Potential Monitoring
Among the electrochemical techniques available for corrosion study, open circuit
potential monitoring [114-117] is the simplest and least expensive of all. The principle of
this technique is that a shift towards more negative potentials is a sign of active corrosion
development [118], whereas more positive potentials indicate formation of a film, or passivation. Because this technique is not an accelerated test [114], it may take a
considerable amount of time after the initial immersion. Moreover, the true identity of the
potential measured is occasionally ambiguous [114].
2.3.2. Polarization Resistance Measurement
Walter reviewed several electrochemical methods for coating performance and
concluded that measuring the polarization resistance is more useful than doing potential
measurement with time [115]. In the polarization resistance measurement, potentials
applied are normally oscillated within several tens of millivolts against OCP in order not
36 to perturb the system [115]. Corrections should be done, however, for ohmic potential
drop due to the presence of the organic coating [114, 115].
2.3.3. Electrochemical Noise Analysis (ENA)
“Noise” is a general term used to describe the fluctuating behavior of a physical
variable with time. The fluctuation in the electrochemical potential or current is referred
to as the electrochemical potential or current noise. This electrochemical noise has
various origins, such as charge flow due to diffusion or electrochemical reactions, film
deterioration, destruction and recovery at the metal and solution interface, and gas
evolution during corrosion reactions [119].
There are several papers that show the possibility of the Electrochemical Noise
Analysis (ENA) on characterization of localized corrosion [120-122]. Some other papers
introduce the ratio between the standard deviation of the current, σ(I(t)) and the potential,
σ(V(t)), which is called noise resistance, Rn according to Ohm’s law, for evaluating
coating performance [123-128].
σ (V( t )) R ( t ) = (2-16) n σ ( I( t ))
Every time-domain function has a counterpart in the frequency-domain by Fourier transformation [129]. The counterpart of the autocorrelation function, which is a function
of the time shift variable, is called the Power Spectral Density (PSD) [129]. This PSD
spectrum indicates how the sequence’s power or energy is distributed in the frequency- 37 domain [129]. The most commonly employed method to produce the PSD of a random
signal is the Fast Fourier Transform (FFT) algorithm [129]. An alternative method,
known as the Maximum Entropy Method (MEM), has also been used widely [129]. From
this analysis, it is also possible to obtain the spectral noise resistance, which is the ratio
between the spectral distributions of potential and current [125, 128],
VFFT ( f ) Rsn ( f ) = (2-17) I FFT ( f )
where VFFT(f) and IFFT(f) are the FFTs of potential and current fluctuations, respectively.
Mansfeld and Lee suggested that noise resistance might be closely related to the
polarization resistance determined by impedance measurement [125]. They found that
there is a slope change in plot of log Rsn versus log f as the coating degrades [128].
However, this quantitative analysis in the PSD for describing corrosion behaviors is still
controversial due to the lack of data reproducibility [119].
2.3.4. Electrochemical Impedance Spectroscopy (EIS)
Electrochemical Impedance Spectroscopy (EIS) is one of the most frequently used
methods for providing information on the performance of organic coatings on metals. A survey done by Murray [117] shows that publications dealing with EIS for the testing of coatings are the largest portion of the coatings literature.
The impedance can be determined by applying a potential of a certain frequency and measuring the corresponding current [10]. According to Ohm’s law, the ratio 38 between the potential, V(t) and current, I(t) is called as the impedance, Z(ω), at the chosen frequency, ω [10].
V( t ) V sinωt Z(ω ) = = o = Z′(ω ) + jZ′′(ω ) (2-18) I( t ) I o sin(ωt +θ )
The phase, θ accounts for the shift of the current with respect to the potential [10]. As
shown above in Equation (2-18), the impedance is given by a complex number with a real, Z′(ω) and imaginary, Z′′(ω) component [10]. A spectrum can be obtained by varying the frequency of the applied signal [10]. There are two plots commonly used for
impedance spectra: Nyquist plots, Z′(ω) versus Z′′(ω), and Bode plots, log |Z| and log θ versus log f [10]. An EIS spectrum can be analyzed by fitting it to the response of an equivalent electrical circuit composed of resistors, capacitors, and inductors so as to determine various characteristic properties of the coating [10].
As the coating degrades, the real part of the impedance decreases as a result of the formation of conductive paths due to the penetration of corrosive species as well as water uptake [99, 130-133]. In addition, Walter and Deflorian et al. showed the capacitance changes as coating degrades [134-136].
EIS has several limitations, such as the cost of the EIS instrumentation and the time needed to perform the tests [137]. Moreover, EIS has limitations as a tool for early indication of coating failure [137]. For protective coatings, such as intact coatings, the low frequency impedance is typically much higher than the input impedance of the EIS
39 system because all EIS systems have an input impedance limit above which they cannot
make accurate measurements [137]. Furthermore, it is very difficult to characterize a
particular coating delamination, for example, cathodic delamination and blistering [138].
2.3.5. Coating Capacitance Measurement
Coating capacitance is very sensitive to water absorption [139]. The dielectric
constant of polymer coatings is usually less than 10, whereas for water at 20 oC it is approximately 80 [139]. Consequently, water uptake results in coating capacitance increase according to the Equation (2-19) [139]:
εε A C = o (2-19) c d
-14 where εo is the permittivity of free space (8.854 × 10 F/cm), ε is the dielectric constant
of the polymer, d is the coating thickness, and A is the exposed area of the electrode.
Using this coating capacitance, it is also possible to estimate the amount of water
absorbed into the coatings [140]. The volume fraction of absorbed water (Xv) is given by
[140]:
⎛ C ⎞ log⎜ ⎟ ⎜ C ⎟ X = ⎝ o ⎠ (2-20) V log()ε H 2O
40 where C is the capacitance at a specific time, Co is the capacitance before any water uptake, and εH2O is the dielectric constant of water. This analysis assumes a uniform
absorption of water, thus it might not sense a local breakdown or delamination [132].
2.3.6. Breakpoint Frequency Measurement
As shown in Figure 2.6, a transition occurs from a capacitive region (slope = -1 due to the coating capacitance, Cc) to a resistive region (slope = 0 due to the pore
resistance, Rd) as the frequency decreases [141]. This critical frequency is called the
breakpoint frequency (fb) [141-144]. It can also be obtained by the simple mathematical
treatment that follows [141-144]:
1 1 = = Rd (2-21) jωCc j2πfbCc
1 fb = (2-22) 2πRd Cc
The breakpoint frequency has been used to estimate the defect area [141-144].
Because the delaminated area increases as corrosion propagates further, the pore and
coating resistances decrease, whereas the double layer capacitance increases [141-144].
Thus, the pore resistance and the coating capacitance, considering the defect area, Ad,
which changes with time, are given by [141-144]:
41 o Rd ρd Rd = = (2-23) Ad Ad
εε ()A − A εε A C = o d ≅ o (2-24) c d d
o where Rd is the unitary value of the pore resistance. If the defect is assumed to be a simple cylindrical pore in the coating with area, Ad, and a length equal to the thickness of the coating, d, then, the breakpoint frequency (fb) can be obtained as follows [141-144]:
1 1 Ad d Ad fb = = ⋅ = K1 ⋅ (2-25) 2πRd Cc 2π ρd εε o A A
where ρ is the intrinsic coating resistivity, and K1 is a constant that depends on the dielectric constant of the coating.
Because standard exposure testing and visual inspection are usually slow
(hundreds of days) and subjective (visual inspection) [145], the breakpoint method has been used as a rapid and quantitative alternative. However, the breakpoint method has been severely criticized [146, 147]. The method assumes that ρ and ε are independent of d and do not change with exposure time, which are not always true. Furthermore, Kendig et al. argued that there is no Rd decrease in a free film, which is 100 % disbonded, whereas Rd decreases as coatings degrade [146]. In the case of the intact organic coatings, it is difficult to find the breakpoint frequency in the measurable frequency range when the
42 pore resistance and the coating resistance are same order in the two-time-constant model
[147].
2.3.7. Potentiostatic Pulse Testing (PPT)
Pistorius reported that Potentiostatic Pulse Testing (PPT) is very useful for the early detection of the breakdown in intact paint coatings, which are free of visually apparent damage and have film resistances of the order of GΩ⋅cm2 [147]. He applied a square pulse of 0.1 V ∼ 2.0 V to a sample in a two-electrode cell with the reference electrode also acting as the counter electrode, and measured the resulting current transients [147]. The components of an assumed equivalent circuit could be calculated from such a transient as Granata and Kovaleski proposed [148]. Other studies have used this technique, for example, investigation of oxide growth kinetics [149], characterization of coated films [150], and frequency analysis obtained by Fourier or Laplace transforms
[151].
2.3.8. Quartz Crystal Microbalance (QCM)
The Quartz Crystal Microbalance (QCM) is an “inverse” piezoelectric devices capable of measuring very small mass changes on or adjacent to its oscillating electrode surfaces [152]. The application of an alternating potential across the quartz crystal causes vibrational motion with the amplitude parallel to the crystal surface [152, 153]. This vibrational motion of the quartz crystal results in a transverse acoustic wave that propagates back and forth in a direction perpendicular to the crystal surface, as shown in
43 Figure 2.7 [152]. The standing wave condition can be achieved when the thickness of the quartz, including the deposited electrodes, is a multiple of a half wavelength of the acoustic wave [152]. The resonant frequency, fo, of the acoustic wave in the fundamental mode is given by Equation (2-26)
ν tr N fo = = (2-26) 2tq tq
where νtr (= fo × λ) is the transverse velocity of sound in 10 MHz AT-cut quartz (3.340 ×
5 10 cm · Hz), tq is the resonator thickness, and N is the frequency constant [152]. The resonator thickness is related to its mass by
M tq = (2-27) ρ q A
3 where ρq is the density of quartz (2.648 g/cm ), A is the piezoelectric active area, and M is the mass of the resonator [152]. Combining Equations (2-26) and (2-27) yields
ρ N f = q (2-28) o m
where m = M/A (g/cm2) [152]. If the acoustic velocity and density of a foreign layer on the electrode are identical to those in quartz, the thickness change of the foreign layer is
44 identical to the thickness change of the quartz crystal [152]. Thus, a fractional change in thickness results in a fractional change in the resonant frequency [152]. If a foreign mass,
ΔM, is attached uniformly to one of the electrode area, A, and Δm = ΔM/A, then,
ρ N f + Δf = q (2-29) o m + Δm
⎛ ⎞ ⎛ ⎞ ρ N f f ⎜ Δm ⎟ ⎛ f 2 ⎞⎜ Δm ⎟ Δf = q − f = o − f = − o ⎜ ⎟ = −⎜ o ⎟⎜ ⎟ m + Δm o ⎛ Δm ⎞ o m ⎜ Δm ⎟ ⎜ ρ N ⎟⎜ Δm ⎟ ⎜1 + ⎟ ⎜ 1 + ⎟ ⎝ q ⎠⎜ 1+ ⎟ ⎝ m ⎠ ⎝ m ⎠ ⎝ m ⎠
(2-30)
where Δfo is the change of the reasonant frequency associated with the attachment of the foreign mass [152]. When m >> Δm, Equation (2-30) yields
1 2 −6 2 Δf ≅ − ⋅ fo Δm = −2.261×10 fo Δm = −C f Δm (2-31) ρq N
where Cf is the sensitivity factor [152]. This relationship was first obtained by Sauerbrey
[154]. According to the Sauerbrey equation, the mass change of the quartz crystal is directly proportional to frequency shift as long as the total thickness of the electrode and foreign layer is less than 2 % of the thickness of the quartz crystal due to the assumptions made to derive this equation [152, 153].
45 In the last decade, the QCM has been used to study electrolyte adsorption [155,
156], metallic deposition [157-159], mass changes (or thickness changes [160-162]) accompanying ion and solvent movement, passive film growth kinetics [163-167], and corrosion inhibition [168, 169]. Kouznetsov et al. found that the voltammetry and mass curves from the QCM make it possible to estimate the reaction species by analyzing the ratio of effective molecular weight to the number of electrons exchanged [155]. Baba et al. investigated the adsorption process of polyelectrolytes using a QCM [156]. Nomura and coworkers used QCM for the study of the electrodeposition of Cu and Ag in the early
1980s [157-159]. Landolt and coworkers published several papers on QCM studies of passive alloys such as Fe-Cr [161, 162], Fe-Cr-Mo [161], Cr [165], AISI 304 stainless steel [166], reporting that QCM can measure the in-situ thickness of the passive film, predict the selective dissolution in the passive film, and study the anodic film growth. Lin and Hebert studied the properties of the cathodic film on Al during cathodic polarization
[164]. There was work on QCM application to multicomponent alloys using magnetron sputtering technique [167]. Corrosion inhibition in different electrolyte solutions was studied using QCM, presenting the frequency decreases due to the strong adhesion onto the film [168, 169]. Several research groups performed the QCM work in the battery area
[170-172].
During cathodic corrosion of Al, the partial current generated by Al oxidation is directly consumed by partial cathodic reactions [12]. Therefore, the net cathodic current density measured by electrochemical techniques is possibly reduced by the anodic dissolution rate of Al [12]. QCM has been used for measuring the mass loss rate that has
46 been converted into an anodic dissolution rate and the true cathodic current density that has been calculated from the difference between the measured mass loss rate and net current density [12]. Thus, it has been possible to separate the true cathodic and anodic reaction rates [12, 13]. A few investigations have also been performed to show the possibility that QCM can be used to study organic coating failures [13, 173].
Even though the QCM has been extremely useful in clarifying many electrochemical processes, the effects of liquids [173, 174] on the response of the QCM and the limitations imposed by these effects are not widely appreciated.
2.4. KEY UNRESOLVED ISSUES
2.4.1. Cathodic Reactivity of Phases in Al Alloys
Besides the complexities of the ORR such as its mechanism, the identification of intermediates, and the rate-determining step in the ORR, the characterization of ORR on each intermetallic particle should be clarified to better understand the corrosion behaviors of Al alloys. Baek and Frankel investigated the cathodic behavior of the θ phase with
QCM [12]. They concluded that the apparent net current of the cathodically polarized θ phase results from a cathodic reaction, such as the ORR [12]. However, there has been little effort to see the cathodic activity of the S phase, which is approximately 60 % of the second phase particles in AA2024-T3 [14].
47 2.4.2. Corrosion of the Organic Coated Al Alloys
It is essential to understand water, oxygen and ion transport through the coatings, because this is the first step for the water uptake followed by further coating delamination. In principle, it should be possible to apply the Electrochemical Quartz
Crystal Microbalance (EQCM) technique to investigate in-situ detection and measurement of water uptake in organic coatings, delamination, and corrosion at the coating/metal interface. Several reports have shown that EQCM is a useful technique in determining kinetic parameters, e.g., diffusivity of water [173, 175-178]. However, little research has been performed to relate this information to delamination and subsequent corrosion of the coated samples.
As summarized in Table 2.3, most EIS cannot accurately sense the early stage of intact organic coating failure because the impedance measured usually exceeds the instrumental input impedance [137, 147]. Capacitance measurement hardly provides any information concerning corrosion failure initially occurring at local defect sites [132].
The breakpoint frequency method is also limited when the system has one-time-constant or two-time-constant with almost the same pore resistance as the coating resistance [146,
147]. Thus, the development of an accelerated test or a corrosion monitoring technique for protective intact coatings, or coatings that are just starting to fail is required.
2.5. OBJECTIVE OF DISSERTATION
Thin film analogs [12, 42] are introduced to characterize S phase intermetallic particle. Basic electrochemical testing of S phase thin film analogs, which are deposited
48 by the sputtering technique under ultra high vaccumn, can be performed. The EQCM is used in this study to investigate the reactivity of S phase thin film analogs under the cathodic polarizations in various environments with or without inhibitors. This approach allows dissolution and cathodic currents to be distinguished by measuring the mass change and the current, simultaneously. Furthermore, it can provide the relationship between the cathodic activities and the inhibition effect of chromate and other alternative inhibitors by comparing the true cathodic current densities.
As described above, EQCM can be used to investigate in-situ detection and measurement of water uptake in organic coatings, delamination, and corrosion at the coating/metal interface. In this thesis, water uptake in polyurethane coating on Au and Al is monitored using EQCM in order to evaluate the capabilities of EQCM for in-situ detection and measurement by presenting the changes of the OCP and frequency of the coated samples.
In addition, PPT is employed as a technique to assess the early stage of organic coating failure. Data treatment using Fourier transforms is also shown to improve the analysis.
49
REFERENCES
1. R. G. Buchheit, Journal of the Electrochemical Society 142, p. 3994 (1995). 2. N. Birblis and R. G. Buchheit, Journal of the Electrochemical Society 152, p. B140 (2005). 3. J. R. Galvele and S. M. de De Micheli, Corrosion Science 10, p. 795 (1970). 4. K. Sugimoto, K. Hoshino, M. Kaeyama, and Y. Sawada, Corrosion Science 15, p. 709 (1975). 5. J. R. Scully, T. O. Knight, R. G. Buchheit, and D. E. Peebles, Corrosion Science 35, p. 185 (1993). 6. J. R. Scully, D. E. Peebles, A. D. Romig, D. R. Frear, and C. R. Hills, Metallurgical Transactions A 23A, p. 2641 (1992). 7. A. Rüdiger and U. Köster, Journal of Non-Crystalline Solids 250-252, p. 898 (1999). 8. B. Mazurkiewicz and A. Piotrowski, Corrosion Science 23, p. 697 (1983). 9. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE, Houston, TX (1974). 10. D. A. Jones, Principles and Prevention of Corrosion, Prentice Hall, Upper Saddle River, NJ (1996). 11. I. L. Muller and J. R. Galvele, Corrosion 17, p. 179 (1977). 12. Y. Baek and G. S. Frankel, Journal of the Electrochemical Society 150, p. B1 (2003). 13. G. S. Frankel, J. Kang, and Y. Baek, in the Conference Paper of the CORROSION NACExpo 2003, San Diego, CA, paper number 03394 (2003). 14. R. G. Buchheit, R. P. Grant, P. F. Hlava, B. Mckenzie, and G. L. Zender, Journal of the Electrochemical Society 144, p. 2621 (1997). 15. R. G. Buchheit, L. P. Montes, M. A. Martinez, J. Michael, and P. F. Hlava, Journal of the Electrochemical Society 146, p. 4424 (1999). 16. R. G. Buchheit, M. A. Martinez, and L. P. Montes, Journal of the Electrochemical Society 147, p. 119 (2000). 17. N. Dimitrov, J. A. Mann, M. Vukmirovic, and K. Sieradzki, Journal of the Electrochemical Society 147, p. 3283 (2000). 18. A. Kolics, A. S. Besing, and A. Wieckowski, Journal of the Electrochemical Society 148, p. B322 (2001). 19. G. S. Chen, M. Gao, and R. P. Wei, Corrosion 52, p. 8 (1996). 20. M. Gao, C. R. Feng, and R. P. Wei, Metallurgical and Materials Transactions A 29A, p. 1145 (1998). 50 21. C. -M. Liao, J. M. Olive, M. Gao, and R. P. Wei, Corrosion 54, p. 451 (1998). 22. R. P. Wei, C. -M. Liao, and M. Gao, Metallurgical and Materials Transactions A 29A, p. 1153 (1998). 23. C. -M. Liao and R. P. Wei, Electrochimica Acta 45, p. 881 (1999). 24. H. M. Obispo, L. E. Murr, R. M. Arrowood, and E. A. Trillo, Journal of Materials Science 35, p. 3479 (2000). 25. K. Wagner, S. R. Brankovic, N. Dimitrov, and K. Sieradzki, Journal of the Electrochemical Society 144, p. 3545 (1997). 26. T. Suter and R. C. Alkire, Journal of the Electrochemical Society 148, p. B36 (2001). 27. V. Guillaumin and G. Mankowski, Corrosion Science 41, p. 421 (1999). 28. K. Urushino and K. Sugimoto, Corrosion Science 19, p. 225 (1979). 29. P. Schmutz and G. S. Frankel, Journal of the Electrochemical Society 146, p. 4461 (1999). 30. R. R. Leard, The Electrochemical Behavior of Copper Bearing Intermetallics and the Corrosion of AA2024-T3, Master Thesis, The Ohio State University, Columbus, OH (2001). 31. C. Blanc, B. Labelle, and G. Mankowski, Corrosion Science 39, p. 495 (1997). 32. P. Schmutz and G. S. Frankel, Journal of the Electrochemical Society 145, p. 2285 (1998). 33. P. Schmutz and G. S. Frankel, Journal of the Electrochemical Society 145, p. 2295 (1998). 34. O. Lunder and K. Nisancioglu, Corrosion 44, p. 414 (1988). 35. K. Nisancioglu, Journal of the Electrochemical Society 137, p. 69 (1990). 36. O. Seri and M. Imaizumi, Corrosion Science 30, p. 1121 (1990). 37. A. I. Golubev and M. N. Ronzhin, Electrochemical and Corrosion Behavior of Aluminum-Base Binary Alloys and Intermetallic Compounds, in Corrosion of Metals and Alloys, N. D. Tomashov and E. N. Mirolyubev, editor(s), p. 48, Israel Program for Scientific Translations, Jerusalem, Israel (1966). 38. W. A. Anderson and H. C. Stumpf, Corrosion 36, p. 212 (1980). 39. M. Zamin, Corrosion 37, p. 627 (1981). 40. K. Nisancioglu and O. Lunder, Significance of the Electrochemistry of Al-Base Intermetallics in Determining the Corrosion Behavior of Aluminum Alloys, in Aluminum Alloys Their Physical and Mechanical Properties, E. A. Starke and T. H. Sanders, editor(s), p. 1125, West Midlands, U.K. (1986). 41. E. Mattsson, L. -O. Gullman, L. Knutsson, R. Sundberg, and B. Thundal, Br. Corros. J. 6, p. 73 (1971). 42. T. Ramgopal, P. Schmutz, and G. S. Frankel, Journal of the Electrochemical Society 148, p. B348 (2001). 43. D. B. Williams, Microstructural Characteristics of Al-Li Alloys, in Alumium- Lithium Alloys, T. H. Sanders and E. A. Starke, editor(s), p. 89, AIME, New York, NY (1981). 44. J. M. Silcock, Journal of the Institute of Metals 88, p. 357 (1959-60).
51 45. M. K. Tosten, A. K. Vasudevan, and P. R. Howell, Metallurgical and Materials Transactions A 19A, p. 51 (1988). 46. R. G. Buchheit, J. P. Moran, and G. E. Stoner, Corrosion 46, p. 610 (1990). 47. R. G. Buchheit, J. P. Moran, and G. E. Stoner, Corrosion 50, p. 120 (1994). 48. H. L. Fraser, Lecture Notes from MSE 655, The Ohio State University (2001). 49. K. J. Vetter, Electrochemical Kinetics - Theoretical and Experimental Aspects, Academic Press, New York, NY (1967). 50. E. P. G. T. Van de Ven and H. Koelmans, Journal of the Electrochemical Society 123, p. 143 (1976). 51. J. Radosevic, M. Kliskic, P. Dabic, R. Stevanovic, and A. Despic, Journal of Electroanalytical Chemistry 277, p. 105 (1990). 52. A. R. Despic, J. Radosevic, P. Dabic, and M. Kliskic, Electrochimica Acta 35, p. 1743 (1990). 53. S. -M. Moon and S. -I. Pyun, Corrosion Science 39, p. 399 (1997). 54. T. J. R. Leclere and R. C. Newman, Journal of the Electrochemical Society 149, p. B52 (2002). 55. H. Takahashi, K. Fujiwara, and M. Seo, Corrosion Science 36, p. 689 (1994). 56. C. Y. Chao, C. F. Lin, and D. D. Macdonald, Journal of the Electrochemical Society 128, p. 1187 (1981). 57. A. J. Appleby, Journal of Electroanalytical Chemistry. 357, p. 117 (1993). 58. P. S. D. Brito and C. A. C. Sequeira, Journal of Power Sources 52, p. 1 (1994). 59. S. Strbac and R. R. Adzic, Electrochimica Acta 41, p. 2903 (1996). 60. R. R. Adzic, Electrocatalysis, in Recent Advances in the Kinetics of Oxygen Reduction, J. Lipkowski and P. N. Ross, editor(s), p. 197, Wiley-VCH, New York, NY (1998). 61. E. Yeager, P. Krouse, and K. V. Rao, Electrochimica Acta 9, p. 1057 (1964). 62. R. J. Taylor and A. A. Humffray, Journal of Electroanalytical Chemistry 64, p. 63 (1975). 63. R. J. Taylor and A. A. Humffray, Journal of Electroanalytical Chemistry 64, p. 85 (1975). 64. A. Appel and A. J. Appleby, Electrochimica Acta 23, p. 1243 (1978). 65. H. Yang and R. L. McCreery, Journal of the Electrochemical Society 147, p. 3420 (2000). 66. F. King, M. J. Quinn, and C. D. Litke, Journal of Electroanalytical Chemistry 385, p. 45 (1995). 67. G. Brisard, N. Certrand, P. N. Ross, and N. M. Markovic, Journal of Electroanalytical Chemistry 480, p. 219 (2000). 68. M. W. Kendig and S. Jeanjaquet, Journal of the Electrochemical Society 149, p. B47 (2002). 69. W. J. Clark, J. D. Ramsey, R. L. McCreery, and G. S. Frankel, Journal of the Electrochemical Society 149, p. B179 (2002). 70. W. J. Clark and R. L. McCreery, Journal of the Electrochemical Society 149, p. B379 (2002).
52 71. G. O. Ilevbare and J. R. Scully, Journal of the Electrochemical Society 148, p. B196 (2001). 72. H. Leidheiser, Corrosion 38, p. 374 (1982). 73. H. Leidheiser, Corrosion 39, p. 189 (1983). 74. G. M. Hoch, A Review of Filiform Corrosion, in Localized Corrosion, R. W. Staehle, B. F. Brown, J. Kruger, and A. Agrawal, editor(s), p. 134, NACE, Houston, TX (1974). 75. E. L. Koehler, Corrosion Under Organic Coatings, in Localized Corrosion, R. W. Staehle, B. F. Brown, J. Kruger, and A. Agrawal, editor(s), p. 117, NACE, Houston, TX (1974). 76. W. Funke, Ind. Eng. Chem. Prod. Res. Dev. 24, p. 343 (1985). 77. J. H. W. de Wit, Inorganic and Organic Coatings, in Corrosion Mechanisms in Theory and Practice, P. Marcus and J. Oudar, editor(s), p. 581, Marcel Dekker, New York, NY (1995). 78. P. Nylen and E. Sunderland, Modern Surface Coatings - A Textbook of the Chemistry and Technology of Paints, Varnishes, and Lacquers, Interscience Publishers, London, U.K. (1965). 79. R. Lambourne, Paint Composition and Applications - A General Introduction, in Paint and Surface Coatings: Theory and Practice, R. Lambourne, editor(s), p. 23, Ellis Horwood Limited, Chichester, U.K. (1987). 80. Z. W. Wicks, F. N. Jones, and S. P. Pappas, Organic Coatings Science and Technology, Wiley-Interscience, New York, NY (1999). 81. J. Bentley, Organic Film Formers, in Paint and Surface Coatings: Theory and Practice, R. Lambourne, editor(s), p. 41, Ellis Horwood Limited, Chichester, U.K. (1987). 82. G. Grundmeier, W. Schmidt, and M. Stratmann, Electrochimica Acta 45, p. 2515 (2000). 83. J. F. Rolinson, Pigments for Paint, in Paint and Surface Coatings: Theory and Practice, R. Lambourne, editor(s), p. 111, Ellis Horwood Limited, Chichester, U.K. (1987). 84. T. K. Ross and J. Wolstenholme, Corrosion Science 17, p. 341 (1977). 85. B. Muller, I. Forster, and W. Klager, Progress in Organic Coatings 31, p. 229 (1997). 86. R. A. Jeffs and W. Jones, Additives for Paint, in Paint and Surface Coatings: Theory and Practice, R. Lambourne, editor(s), p. 212, Ellis Horwood Limited, Chichester, U.K. (1987). 87. R. Lambourne, Solvents, Thinners, and Diluents, in Paint and Surface Coatings: Theory and Practice, R. Lambourne, editor(s), p. 194, Ellis Horwood Limited, Chichester, U.K. (1987). 88. N.S. Allen, C. J. Regan, R. McIntyre, B. W. Johnson, and W. A. E. Dunk, Progress in Organic Coatings 32, p. 9 (1997). 89. Y. Nakayama, Progress in Organic Coatings 33, p. 108 (1998). 90. H. Leidheiser, Journal of Coating Technology 53, p. 29 (1981).
53 91. W. R. McGovern, P. Schmutz, R. G. Buchheit, and R. L. McCreery, Journal of the Electrochemical Society 147, p. 4494 (2000). 92. G. M. Brown and K. Kobayashi, Journal of the Electrochemical Society 148, p. B457 (2001). 93. R. L. Twite and G. P. Bierwagen, Progress in Organic Coatings 33, p. 91 (1998). 94. K. Baumann, Plaste Kautsch 19, p. 455 (1972). 95. K. Baumann, Plaste Kautsch 19, p. 694 (1972). 96. J. E. O. Mayne, Anti-Corrosion October, p. 3 (1973). 97. J. E. O. Mayne, The Mechanism of the Protective Action of Paints, in Corrosion, L. L. Shreir, editor(s), p. 15:24, Newnes-Butterworths, London, U.K. (1976). 98. G. W. Walter, Corrosion Science 26, p. 27 (1986). 99. G. Grundmeier and A. Simoes, Corrosion Protection by Organic Coatings, in the Manuscripts for Publication. 100. R. T. Ruggeri and T. R. Beck, Corrosion Control by Organic Coatings, in The Transport Properties of Polyurethane Paint, H. Leidheiser, editor(s), p. 62, NACE, Houston, TX (1981). 101. W. Funke, Blistering of Paint Films, in Corrosion Control by Organic Coatings, H. Leidheiser, editor(s), p. 29, NACE, Houston, TX (1981). 102. J. J. Ritter and J. Kruger, Studies on the Subcoating Environment of Coated Iron Using Qualitative Ellipsometric and Electrochemical Techniques, in Corrosion Control by Organic Coatings, H. Leidheiser, editor(s), p. 28, NACE, Houston, TX (1981). 103. H. Leidheiser and I. Suzuki, Journal of the Electrochemical Society 128, p. 242 (1981). 104. M. G. Fontana, Corrosion Engineering, McGraw-Hill, New York, NY (1986). 105. R. T. Ruggeri and T. R. Beck, Corrosion 39, p. 452 (1983). 106. J. Morita and M. Yoshida, Corrosion 50, p. 11 (1994). 107. E. L. Koehler, Corrosion 33, p. 209 (1977). 108. A. Bautista, Progress in Organic Coatings 28, p. 49 (1996). 109. W. Schmidt and M. Stratmann, Corrosion Science 40, p. 1441 (1998). 110. N. Lebozec, D. Persson, A. Nazarov, and D. Thierry, Journal of the Electrochemical Society 149, p. B403 (2002). 111. J. M. C. Mol, B. R. W. Hinton, D. H. Van der Weijde, J. H. W. de Wit, and S. Van der Zwaag, Journal of Materials Science 35, p. 1629 (2000). 112. M. Huisert, Electrochemical Characterisation of Filiform Corrosion on Aluminium Rolled Products, Ph. D. Thesis, TU Delft, Delft, Netherlands (2001). 113. W. H. Slabaugh, W. Dejager, and S. E. Hoover, Journal of Paint Technology 44, p. 76 (1972). 114. J. Wolstenholme, Corrosion Science 13, p. 521 (1973). 115. G. W. Walter, Corrosion Science 26, p. 39 (1986). 116. H. Leidheiser, Journal of Coating Technology 63, p. 21 (1991). 117. J. N. Murray, Progress in Organic Coatings 31, p. 225 (1997). 118. U. Steinsmo and E. Bardal, Corrosion 48, p. 910 (1992).
54 119. J. Kang, A Study on the Metastable Pitting Behavior of Austenitic Stainless Steel by the Electrochemical Noise Analysis, Master Thesis, Korea Advanced Institute of Science and Technology, Taejon, Korea (2000). 120. K. Hladky and J. L. Dawson, Corrosion Science 21, p. 317 (1981). 121. K. Hladky and J. L. Dawson, Corrosion Science 22, p. 231 (1982). 122. U. Bertocci, Journal of the Electrochemical Society 127, p. 1931 (1988). 123. C-T. Chen and B. S. Skerry, Corrosion 47, p. 598 (1991). 124. F. Mansfeld, L. T. Han, and C. C. Lee, Journal of the Electrochemical Society 143, p. L286 (1996). 125. F. Mansfeld and C. C. Lee, Journal of the Electrochemical Society 144, p. 2068 (1997). 126. U. Bertocci, C. Gabrielli, F. Huet, and M. Keddam, Journal of the Electrochemical Society 144, p. 31 (1997). 127. U. Bertocci, C. Gabrielli, F. Huet, M. Keddam, and P. Rousseau, Journal of the Electrochemical Society 144, p. 37 (1997). 128. F. Mansfeld, L. T. Han, C. C. Lee, C. Chen, G. Zhang, and H. Xiao, Corrosion Science 39, p. 255 (1997). 129. Alan V. Oppenheim, Ronald W. Schafer, and John R. Buck, Discrete-Time Signal Processing, Prentice-Hall, Inc., Upper Saddle River, NJ (1999). 130. G. W. Walter, Corrosion Science 26, p. 681 (1986). 131. I. Dehri and M. Erbil, Corrosion Science 42, p. 969 (2000). 132. J. R. Scully, Journal of the Electrochemical Society 136, p. 979 (1989). 133. S. Lin, H. Shih, and F. Mansfeld, Corrosion Science 9, p. 1331 (1992). 134. G. W. Walter, Corrosion Science 32, p. 1059 (1991). 135. G. W. Walter, Corrosion Science 32, p. 1085 (1991). 136. F. Deflorian, L. Fedrizzi, S. Rossi, and P. L. Bonora, Electrochimica Acta 44, p. 4243 (1999). 137. Jiho Kang and Gerald S. Frankel, Z. Phys. Chem 219, p. 1 (2005). 138. D. H. Van der Weijde, E. P. M. Van Westing, and J. H. W. de Wit, Electrochimica Acta 41, p. 1103 (1995). 139. F. Bellucci and L. Nicodemo, Corrosion 49, p. 235 (1993). 140. R. E. Touhsaent and H. Leidheiser, Corrosion 28, p. 435 (1972). 141. F. Mansfeld, Journal of Applied Electrochemistry 25, p. 187 (1995). 142. F. Mansfeld and C. H. Tsai, Corrosion 47, p. 958 (1991). 143. R. Hirayama and S. Haruyama, Corrosion 47, p. 952 (1991). 144. H. P. Hack and J. R. Scully, Journal of the Electrochemical Society 138, p. 33 (1991). 145. R. G. Buchheit, Lecture Notes of MSE 881, The Ohio State University (2001). 146. M. W. Kendig, F. Mansfeld, and C. H. Tsai, Corrosion 47, p. 964 (1991). 147. P. C. Pistorius, in Proceedings of the in Proceedings of the 14th International Corrosion Council, Capetown, S.A. published on CD-ROM (1999). 148. R. D. Granata and K. J. Kovaleski, Evaluation of High-Performance Protective Coatings by Electrochemical Impedance and Chronoamperometry, in Electrochemical Impedance: Analysis and Interpretation, ASTM STP 1188, J. R. 55 Scully, D. C. Silverman, and M. W. Kendig, editor(s), p. 450, ASTM, Philadelphia, PA (1993). 149. P. Meisterjahn, U. Konig, and J. W. Schultze, Electrochimica Acta 34, p. 551 (1989). 150. O. Genz, M. M. Lohrengel, and J. W. Schultze, Electrochimica Acta 39, p. 179 (1994). 151. J. L. Gilbert, Journal of Biomedical Materials Research 40, p. 233 (1998). 152. M. D. Ward, Principles and Applications of the Electrochemical Quartz Crystal Microbalance, in Physical Electrochemistry: Principles, Methods, and Applications, I. Rubinstein, editor(s), p. 296, Marcel Dekker, New York, NY (1995). 153. D. A. Buttry, The Quartz Crystal Microbalance as an In-Situ Tool in Electrochemistry, in Electrochemical Interfaces: Modern Techniques for In-Situ Interface Characterization, H. D. Abruna, editor(s), p. 542, VCH, New York, NY (1991). 154. G. Z. Sauerbrey, J. Physik 155, p. 206 (1959). 155. D. Kouznetsov, A. Sugier, F. Ropital, and C. Fiaud, Electrochimica Acta 40, p. 1513 (1995). 156. A. Baba, F. Kaneko, and R. C. Advincula, Colloids and Surfaces A: Physicochemical and Engineering Aspects 173, p. 39 (2000). 157. T. Nomura and O. Hattori, Analytica Chimica Acta 115, p. 323 (1980). 158. T. Nomura and M. Iijima, Analytica Chimica Acta 131, p. 97 (1981). 159. T. Nomura and M. Okuhara, Analytica Chimica Acta 142, p. 281 (1982). 160. A. Frignani, M. Fonsati, C. Monticelli, and G. Brunoro, Corrosion Science 41, p. 1217 (1999). 161. P. Schmutz and D. Landolt, Corrosion Science 41, p. 2143 (1999). 162. P. Schmutz and D. Landolt, Electrochimica Acta 45, p. 899 (1999). 163. M. R. Deakin and O. R. Melroy, Journal of the Electrochemical Society 136, p. 349 (1989). 164. C-F. Lin and K. R. Hebert, Journal of the Electrochemical Society 141, p. 104 (1994). 165. C. -O. A. Olsson, D. Hamm, and D. Landolt, Journal of the Electrochemical Society 147, p. 2563 (2000). 166. C. -O. A. Olsson and D. Landolt, Journal of the Electrochemical Society 148, p. B438 (2001). 167. E. Juzeliunas, K. Leinartas, M. Samuleviciene, A. Sudavicius, P. Miecinskas, R. Juskenas, and V. Lisauskas, Journal of Solid State Electrochemistry 6, p. 302 (2002). 168. D. Jope, J. Sell, H. W. Pickering, and K. G. Weil, Journal of the Electrochemical Society 142, p. 2170 (1995). 169. J. Telegdi, A. Shaban, and E. Kalman, Electrochimica Acta 45, p. 3639 (2000). 170. H. Yang, K. Kwon, T. M. Devine, and J. W. Evans, Journal of the Electrochemical Society 147, p. 4399 (2000).
56 171. S. H. Park, J. Winnick, and P. A. Kohl, Journal of the Electrochemical Society 148, p. A346 (2001). 172. M. Taguchi and H. Sugita, Journal of Power Sources 109, p. 294 (2002). 173. K. Noda, J. Park, A. Nishikata, and T. Tsuru, QCM Study on Water Absorption in Organic Coatings, in Proceedings of the Symposium on Advances in Corrosion Protection by Organic Coatings III, I. Sekine, M. Kendig, D. Scantlebury, and D. Mills, editor(s), p. 172, The Electrochemical Society, Pennington, NJ (1997). 174. S. Bruckenstein and M. Shay, Electrochimica Acta 30, p. 1295 (1985). 175. Cody M. Berger and Clifford L. Henderson, Polymer 44, p. 2101 (2003). 176. Katherine E. Mueller, William J. Koros, Yvonne Y. Wang, and C. Grant Willson, SPIE 3049, p. 871 (1997). 177. Asoka Weerawardena, Calum J. Drummond, Frank Caruso, and Malcolm McCormick, Langmuir 14, p. 575 (1998). 178. Yongxiang Xu, Chuanwei Yan, Jie Ding, Yanmin Gao, and Chunan Cao, Progress in Organic Coatings 45, p. 331 (2002). 179. R. Grauer and E. Wiedmer, Werkstoffe Korrosion 31, p. 550 (1980).
57 Nobility Phase Structure with respect Remarks to matrix ♦ Supporting cathodic reaction rate Al2Cu Tetragonal Noble [5] (θ phase) [20, 48] [1, 3-8, 11-13] ♦ Net current density ≈ True cathodic current density [12, 13] Active ♦ Cu liberation and redeposition [14] Al CuMg Orthorhombic [14-21, 24-31] 2 ♦ Passive & active behavior [30] (S phase) [20, 48] or Noble [32, 33] ♦ Active and noble [32, 33] Active ♦ Active and noble [32, 33] [1, 26] AlCuFeMn ? or Noble ♦ Others: Al-Cu-Fe Al7Cu2Fe and [32, 33] Al20Cu2Mn3 [7, 23, 30] ♦ Fe: detrimental effect [34-37, 179] Al3Fe Monoclinic Noble ♦ Adding Mn: not harmful [34, 38, (β phase) [41] [23, 34-36] 39] ♦ Adding Si: not harmful [35, 40]
MgZn2 Hexagonal Active ♦ Breakdown of the η phase ≠ Pitting (η phase) [48] [1, 41] potentials of the 7xxx Al alloys [42]
Al2CuLi Hexagonal Active ♦ Found the selective dissolution of (T1 phase) [44] [46, 47] the T1 phase [46]
♦ OCPs of the Al Zr particle are Tetragonal Noble 3 Al Zr higher than that of pure Al from pH 2 3 [41] [5] to 12 [5].
Table 2.1: Summary of the intermetallic phases commonly found on Al alloys.
58 Paint Components Typical function
Vehicle Polymer ♦ Provides the basis of continuous (continuous or Resin film sealing or the surface protection. phase) (Binder)
Solvent ♦ Provides a homogeneous and
or Diluent adequately viscous mixture.
Pigment ♦ Provide minor components e.g., (discontinuous Additives catalysts, driers, flow agents. phase)
Primary ♦ Provides opacity, color, and other pigment (fine optical or visual effects.
particle organic ♦ May be included for anti-corrosive or inorganic) properties in primers.
Table 2.2: The composition of paints [79].
59 Technique Advantage in general Disadvantage in using for coating
♦ Simple ♦ Long time to predict the failure OCP ♦ Inexpensive [114]
♦ Relatively faster than OCP ♦ Ohmic potential drop due to the measurement R presence of the coating layer [114, p ♦ Not disturbing the system due to 115] applying small potential variations ♦ Data reproducibility, especially ♦ Easy in PSD analysis such as slopes ENA ♦ Possible to compare with data [119, 128], cut-off frequencies from EIS (Bode plot) [119] ♦ Inadequate for intact organic ♦ Acquiring the value of equivalent EIS coating with huge impedance due to circuit elements assumed the instrumentation of most EIS
♦ Easily obtaining the amount of ♦ Not a representative tool for C c water absorbed detecting a local delamination [132]
♦ Limited in one-time-constant or fb ♦ Estimating the detective area two-time-constant with almost the same Rd and Cc Value [147]
♦ Predicting early stage of coating ♦ Difficult to resolve each current PPT failure decay if currents decay with a ♦ Inexpensive compared to EIS number of time constants [137]
♦ Measuring mass change very ♦ Requiring specimen as a thin EQCM sensitively film [152, 153] ♦ Possible sensing water uptake
Table 2.3: Summary of the several measurements to characterize the protective properties of organic coatings.
60
Figure 2.1: Schematic representation of the SCC in aged AA2024 [28]. Reprinted from K. Urushino and K. Sugimoto, Corrosion Science 19, p. 225 (1979) with permission from Elsevier.
Figure 2.2: Experiments of Leidheiser et al. to prove the way in which K+ diffuse to blisters [76]. Reprinted from W. Funke, Ind. Eng. Chem. Prod. Res. Dev. 24, p. 343 (1985) with permission from American Chemical Society.
61
Figure 2.3: Mechanism of blistering by underrusting [76]. Reprinted from W. Funke, Ind. Eng. Chem. Prod. Res. Dev. 24, p. 343 (1985) with permission from American Chemical Society.
Figure2.4: Mechanism of neutral blistering [76]. Reprinted from W. Funke, Ind. Eng. Chem. Prod. Res. Dev. 24, p. 343 (1985) with permission from American Chemical Society.
62
Figure 2.5: Filiform corrosion on Al – the head is the primary anodic site and the back of the head (the beginning of the tail) is the primary cathodic site [77].
Figure 2.6: (a) Model for the impedance of a polymer coated metal and (b) theoretical impedance spectra for a degraded polymer coating [141]. Reprinted from F. Mansfeld, Journal of Applied Electrochemistry 25, p. 187 (1995) with permission from Springer Science and Business Media.
63
Figure 2.7: Schematic representations of (a) the shear vibration of an AT-cut quartz resonator and (b) the transverse shear wave in a quartz crystal with excitation electrodes and a composite resonator comprising the quartz crystal, electrodes, and a thin layer of a foreign material [152].
64
CHAPTER 3
CORROSION OF S PHASE THIN FILM ANALOGS
S phase (Al2CuMg) particles are known to play an important role in the corrosion of high strength Al alloys. However, it is difficult to assess the electrochemical properties of these small particles separate from the matrix. In this work, thin film analogs of S phase were prepared to have large areas for testing. The structure and composition of the thin films were verified by X-Ray Diffraction (XRD) and Scanning Electron Microscope
(SEM) with Energy Dispersive Spectroscopy (EDS). The corrosion potential in chloride solution initially increased rapidly to a maximum value and then exhibited a continuous decrease as a result of localized corrosion. Mass and current were measured simultaneously by the Electrochemical Quartz Crystal Microbalance (EQCM) technique during cathodic polarization, showing that both the mass loss rate and true cathodic current density were much larger than the measured net current density. These observations, along with local pH increases measured with a micro pH electrode during the cathodic polarization, indicate that the S phase film was undergoing cathodic corrosion. The true cathodic current density and mass loss rate for S phase were reduced in chloride solutions containing 10-4 M of chromate or 0.1 M of vanadate. The mass 65 change in the presence of chromate was greatly reduced. This effect was much greater than in the presence of vanadate, suggesting that chromate exhibited stronger inhibition effect on S phase than vanadate. This was partly explained by the local pH increase near the film surface in the vanadate-containing solution whereas there was no pH change in the chromate-containing solution.
3.1. INTRODUCTION
High strength Al alloys such as AA2024-T3 and AA7075-T6 contain a variety of intermetallic particles that are known to play critical roles in the alloy corrosion mechanism [1, 2]. Some of the Cu-rich particles such as Al2Cu (θ), Al7Cu2Fe and
Al20Cu2Mn3 are clearly cathodic with respect to the matrix and are active cathodes. S phase (Al2CuMg) is another important intermetallic particle commonly found in these alloys. Owing to its high Mg concentration, this compound can be very reactive and is often the site of localized corrosion initiation [3-5]. The Open Circuit Potential (OCP) of large S phase grains has been measured in chloride solution and found to be active with respect to the Al matrix [1]. However, Scanning Kelvin Probe Force Microscope
(SKPFM) measurements on as-polished AA2024-T3 indicate that S phase particles are more noble than the matrix in that condition [6, 7]. Subsequent reaction of the particles can further change their galvanic relationship with the matrix.
Clearly it is important to sort out the reaction sequence associated with S phase particles in high strength Al alloys. The following scenario can be constructed from the published evidence. The oxide formed on the surface of these particles upon exposure to
66 air is essentially Al oxide with some small amounts of Cu and Mg [6, 7]. It is likely that such an oxide would form on particles at the surface of a real component that was inadvertently scratched in service if the environments were not aggressive. This film provides temporary protection and the particles are initially noble relative to the matrix because of this oxide. During exposure to an aggressive chloride solution, the potential difference between these particles and the matrix decreases with time, indicating a degradation in the protectiveness of the oxide film [6, 7]. Eventually the particles activate and aggressive corrosion is initiated. This is the expected behavior from consideration of the steady-state OCP of a bulk analog [3-5]. These particles are attacked by a dealloying process in which the Mg and Al are preferentially dissolved and the surface becomes enriched in Cu [3-5]. Such a Mg-depleted S phase particle might then act as a cathode like the other Cu-containing intermetallic particles and enhance corrosion in the nearby matrix. The porous Cu residue can release Cu nonfaradaically. The very fine Cu particles can then disperse, reach their own corrosion potential, and dissolve with oxygen reduction. The dissolved Cu ions can then plate out on the Al matrix, creating large active cathodes that in turn cause further attack [3-5].
The active cathodes, both intermetallic particles and replated Cu, drive the potential in the noble direction owing to their ability to reduce oxygen at high rates. This leads to the initiation of localized corrosion at susceptible sites. There is another important aspect to the reactions at these cathodes. The locally high rate of Oxygen
Reduction Reaction (ORR) leads to local increases in pH in the regions near the particles
[8]. The high pH results in increased dissolution of the neighboring Al matrix as the
67 - soluble AlO2 anion, and surface enrichment of solute Cu. Through this and the Cu replating process, the surface fraction of local cathodes can increase well beyond the initial value, resulting in a surface very susceptible to further attack.
Considerable research on the corrosion of S phase has been performed, focusing on Cu liberation and redeposition [3, 5, 8, 9], morphological observations before and after exposure [3, 10-16], pit initiation [17-20], local pH changes [8], and measurements of the electrochemical variables using micro cell experiments [14, 21].
Numerous studies in recent years also have focused on corrosion inhibition of Al alloys by chromates [22-31]. However, the effect of chromate on S and other intermetallic particles is not well understood. Determination of cathodic kinetics for Al- containing phases is challenging because of the influence on the passive film of the local pH increase generated by the ORR during the cathodic polarization (e.g., cathodic corrosion [32]). Electrochemical measurements made using a potentiostat only sense the net current. If there is any anodic dissolution during cathodic polarization, the net current density measured from the potentiostat will be different than the true cathodic current
[32, 33]. Therefore, it is necessary to carefully differentiate the rate of the anodic and cathodic reactions for an accurate analysis.
The Electrochemical Quartz Crystal Microbalance (EQCM) can measure simultaneously sub-monolayer mass changes for a thin film electrode and electrochemical signals. The EQCM actually measures the changes in resonant frequency of the quartz crystal substrate, which can be converted to mass change using the
Sauerbrey equation [34]. According to this equation, the mass change of the quartz
68 crystal is directly proportional to frequency shift as long as the total thickness of the electrode and foreign layer is less than 2 % of the thickness of the quartz crystal [35-38].
Applications of the EQCM in corrosion studies, however, have been limited because it requires samples to be in the form of thin films.
Thin film compositional analogs of intermetallic particles have been used for the electrochemical characterization of specific intermetallic particles [33, 39, 40]. Since thin films can have very different structures than bulk phases, the use of thin film analogs requires an assumption that the influence of chemical composition on the electrochemical behavior is much stronger than the effects of any structural difference between the thin film analog and the real particles.
In this study, sputter-deposited thin film S phase analogs were studied. The cathodic and mass loss rates were separated using the EQCM technique. The mass change rates were recorded as functions of the pH and addition of inhibitors in aerated
0.5 M NaCl solution. Local pH measurements using a micro pH electrode were made to assess the local pH changes.
3.2. EXPERIMENTAL
S phase thin films were deposited on (100) Si wafers and 10 MHz AT-cut quartz crystals by the DC magnetron sputtering technique using an Al2CuMg alloy target from
Plasmaterials, Inc. Prior to deposition, the substrates were ultrasonically cleaned with trichloroethylene, acetone, methyl alcohol, and deionized water. The base pressure and operation pressure were 2.0 × 10-7 and 5.0 × 10-3 torr, respectively. It has been reported
69 that Al thin films deposited with a base pressure of 2.0 × 10-7 torr or better behave very similarly to the bulk material [41]. The main deposition conditions used for this study are summarized in Table 3.1. The thickness of the samples was measured by a Dektak3 ST surface profiler. S phase thin films were deposited on both sides of the quartz crystal in a typical “lollipop” pattern [33]. The film on the topside was less than 0.5 μm thick and acted as the working electrode. The film on the backside of the quartz crystal was around
0.1 μm thick and served as the electrical connection to the EQCM oscillator. X-Ray
Diffraction (XRD), Scanning Electron Microscope (SEM) with Energy Dispersive
Spectroscopy (EDS), Secondary Ion Mass Spectroscopy (SIMS), and Auger Electron
Spectroscopy (AES) were used to characterize the structure and composition of the thin films.
Electrochemical tests were carried out in aerated 0.5 M NaCl (pH 5.98), deaerated
0.5 M NaCl, and aerated 0.5 M NaCl with 10-4 M NaOH (pH 9.84). In other experiments,
-4 inhibitors were added to aerated 0.5 M NaCl: 10 M Na2CrO4⋅4H2O (pH 6.60) or 0.1 M
NaVO3 (pH 6.29 adjusted with HCl). A Pt mesh electrode and a Saturated Calomel
Electrode (SCE) were used as counter electrode and reference electrode, respectively.
Samples were exposed at OCP for 30 min prior to potentiodynamic polarization at a scan rate of 1 mV/s using a Gamry Instruments PC4-FAS1 potentiostat. Anodic and cathodic portions of the polarization curves were measured in separate experiments on different samples both starting from the OCP.
An Elchema EQCN-900 EQCM was used for this study. The quartz crystal with thin films deposited on both sides was placed in a Teflon crystal holder that exposed the
70 topside to the solution and the other side to air. Electrical contact between the thin film electrode and wire leads was made by colloidal silver paint, which was isolated from the solution by silicone resin. This cell design and configuration were similar to those used by Schmutz and Landolt [42]. The crystal holder was placed horizontally into an electrochemical cell as shown in Figure 3.1. The currents, potential, and mass-related frequency change were recorded with the VOLTSCAN program provided by Elchema.
All EQCM experiments were performed in a Faraday cage to minimize electromagnetic interference from the surroundings. Each EQCM experiment was repeated several times to verify its reproducibility. To determine the exact experimental sensitivity factor of the
EQCM system, calibration was performed by electrodeposition of Cu on a Au deposited quartz crystal at -626 mVSCE in 0.05 M CuSO4 + 0.05 H2SO4.
A W/WO3 electrode [43-47] was used for local pH measurement during cathodic polarization of S phase. A 1.6 mm diameter W wire was electropolished in 0.5 M NaOH solution at a current density of ∼1 A/cm2 for several minutes to thin the wire to a diameter of about 100 μm. This wire was mounted into a glass tube with epoxy and the end was polished flat. After rinsing in deionized water, the wire was immersed in concentrated
HNO3 for at least 12 h to oxidize the surface. The experimental setup for the micro pH electrode measurement is included in Figure 3.1. A capillary connected to an SCE electrode was coplanar with the end of the W/WO3 electrode to minimize IR drops. The electrode was calibrated in several ranges of pH using pH buffer solutions. A window in the electrochemical cell permitted observation of the position of the electrode, the end of which was slowly moved toward the sample using a micrometer. The pH electrode was
71 moved toward the sample surface until it just touched, and then moved away from the surface by a distance estimated to be about 100 μm. The pH measurement was made at this distance.
3.3. RESULTS AND DISCUSSION
3.3.1. Thin Film Characterization
S phase films were successfully sputter-deposited with good adhesion on Si and blank quartz wafers. Figure 3.2 shows the XRD pattern of the as-deposited S phase film on a Si wafer without subsequent annealing. Despite attempts to remove the Si substrate peaks, the sharp peak around 33° was probably the Kα of Si (200). This peak probably originated from rediffraction by {111} planes of x-rays diffracted by {111} planes [48].
The overall XRD pattern of the synthesized S phase was quite different than the reference for bulk Al2CuMg (PDF#65-2501) [49] shown in Figure 3.2, presenting a peak broadening and a shift of the diffraction lines to new 2θ positions. These changes are probably due to a lack of crystalline structure of the as-deposited film [50] and residual stress in the thin films [50]. It can be concluded that the structure of the S phase thin film is different than that of bulk S phase and probably also different than S phase particles in
Al alloys. As mentioned above, the use of these films to represent the behavior of S phase particles relies on the similarity of the composition and the expectation that compositional effects dominate structural effects in determining the corrosion behavior.
SEM-EDS characterization on the as-deposited S phase indicated a composition of 53% Al, 22% Cu, and 24% Mg, which is close to the composition of S phase particles
72 found in Al alloys. Compositional depth profiles of the as-deposited film were performed using SIMS. As can be seen in Figure 3.3, uniform distributions of Al, Cu, Mg and a low level of O were observed by the positive and negative SIMS, respectively.
3.3.2. Characterization of Thin Film Samples after OCP Exposure in Aerated 0.5
M NaCl Solution
Figure 3.4 shows SEM images of the S phase thin film analogs deposited on Si wafers after various times of OCP immersion in aerated 0.5 M NaCl solution, with or without subsequent cathodic polarization. With increasing OCP immersion time, the S phase thin film exhibited localized corrosion and local Cu enrichment. EDS analysis was used to assess the local composition, and is summarized in Table 3.2. After 3 min, bright spots 5 ∼ 15 μm in diameter became visible in the secondary electron images, Figures
3.4b and 3.4c. These spots were enriched in Cu as shown in Table 3.2 (sites 2 and 5).
Since EDS is not a surface-sensitive technique, these values do not reflect the real enrichment of Cu on the surface. After 5 min, corrosion products formed on the surface in regions between the Cu-enriched bright spots, Figure 3.4c. SEM-EDS indicated that the product was Al oxide or oxyhydroxide with some content of Cl. Figure 3.4d shows the locations of the analyzed spots for the Cu enriched area (site 8) and the unattacked area (sites 6 and 7). After cathodic polarization, more corrosion product was evident on the surface, Figures 3.4e ∼ 3.4h. Fibrous particles were found both after OCP exposure and after cathodic polarization as shown in Figures 3.4c ∼ 3.4e. Discontinuous clusters of
73 particles were observed after the application of -1050, -1150, or -1250 mVSCE, Figures
3.4f ∼ 3.4h.
SIMS depth profiling was performed on a sample exposed in the aerated 0.5 M
NaCl solution for 30 min at OCP. No significant difference was found in positive SIMS depth profiles before (Figure 3.3a) and after exposure (Figure 3.5a) because the window size used for depth profiling was large relative to the actual pitted area where Cu was believed to be enriched. However, the O signal was greatly enhanced at the surface in the negative SIMS depth profile from the exposed sample, Figure 3.5b, indicating that considerable corrosion product was produced during the immersion, even on the area that appears unattacked. Figure 3.6 shows line scans in the region of pits taken from 50 μm by
50 μm SIMS maps after a couple of initial sputtering cycles from the surface of the sample. The line scans were generated by analysis of the data after the completion of the
SIMS sputtering, and clearly show local Cu enrichment at the pits along with Al and Mg and depletion. The pits seemed to have penetrated to the substrate because the Si signal was also strong over the pits.
3.3.3. Electrochemical Behavior of S Phase Thin Films and AA2024-T3
The OCP trends and polarization curves of the S phase thin film analogs on Si wafers are presented in Figure 3.7. The base solution was aerated 0.5 M NaCl. Figure
3.7a shows that the OCP for S phase in the base solution first increased, and then decreased and stabilized at about -870 mVSCE. As shown above, the sample pitted during the first few min at OCP, so the potential increase was associated with localized
74 dissolution of Mg and Al. The subsequent potential decrease then occurred as corrosion products formed on the surface. The polarization curve for 0.5 M NaCl solution also indicates spontaneous formation of pits at open circuit, as the current increased rapidly at potentials above OCP. In fact, the OCP was probably pinned close to the breakdown potential owing to the relative nonpolarizibility of the pitting reaction.
In deaerated 0.5 M NaCl, the initial OCP increase was not observed, but the final potential was only 100 mV lower than in the aerated condition and pits were also observed on the sample surface, which looked similar to the sample exposed to aerated solution, shown in Figure 3.4d. Figure 3.7b shows that the cathodic portion of the polarization curve was very similar to that for the aerated condition because the open- circuit/pitting potential was close to the potential for water reduction. As a result, there was only a small range in the aerated cathodic polarization curve where oxygen reduction could be observed, and deaeration had only a small effect. The breakdown potential was slightly lower and the dissolution current was slightly higher relative to the aerated case.
The addition of 10-4 M chromate decreased the OCP by an amount similar to that of deaeration, but it had a large effect on the cathodic and anodic portions of the polarization curve. The cathodic current at a given potential was decreased by a factor of
20 ∼ 40 in the presence of chromate, and the anodic polarization curve exhibited stable passivity with a low passive current density. No pits were observed on the surface of the sample after OCP exposure and anodic polarization to -700 mVSCE.
Both the anodic and cathodic currents were also reduced in a solution containing
0.1 M vanadate, although not as much as in the solution containing 10-4 M chromate. The
75 anodic part of the curve was inhibited more than the cathodic part, resulting in an increase in the OCP. Again, no pits were observed on the surface of the sample after OCP exposure and anodic polarization to -700 mVSCE.
The curve measured in the aerated 0.5 M NaCl solution adjusted to pH 10 with addition of NaOH was included in Figure 3.7 to represent the alkaline environment that might be generated near the electrode surface as the result of cathodic reactions. The open-circuit/pitting potential was about 150 mV lower than in the neutral solution. Even though the solution was aerated, there was no region where oxygen reduction dominated the cathodic reaction because of the very low breakdown potential.
The OCP trends and polarization curves for AA2024-T3 in same solutions are given in Figure 3.8 for comparison. The values of OCP measured in the aerated 0.5 M
NaCl solutions without or with addition of vanadate, and NaOH were very close to each other after 30 min exposure and considerably higher than for the thin film S phase. The
OCP in the chromate-containing solution was 250 mV less than that in the other conditions. For all solutions except the one containing chromate, the anodic portions of the polarization curves were almost identical, and the curve for the chromate solution almost overlapped the others at the high potentials, Figure 3.8b. Interestingly, the
AA2024-T3 samples pitted in the chromate and vanadate-containing solutions, whereas the thin film S phase samples did not (at least at potentials up to -700 mVSCE). Cathodic limiting current densities were dramatically reduced by adding inhibitors, as shown in
Figure 3.8b. However, 10-4 M of chromate had a stronger inhibition effect on AA2024-T3 than 0.1 M of vanadate, as was the case for S phase thin films. Cathodic behavior on the
76 AA2024-T3 in 0.5 M NaCl solution was quite similar to that in pH 10 solution, suggesting that the local alkalinity generated near the ORR sites played an important role in controlling the whole cathodic process for AA2024-T3.
3.3.4. Electrochemical Quartz Crystal Microbalance Measurements
EQCM was used to measure simultaneously mass change and OCP, or cathodic current under potential control, for S phase thin film analogs sputter-deposited on 10
MHz AT-cut quartz crystals. Figure 3.9 shows OCP and mass change as a function of time for S phase thin film analogs in aerated 0.5 M NaCl solution without or with additions of chromate, vanadate or NaOH. The potential trends were almost identical to those for the samples deposited on Si wafers, as shown in Figure 3.7a. In the base 0.5 M
NaCl solution, the mass increased initially and then decreased continuously with time,
Figure 3.9b. This is especially interesting in light of the observation of pitting during the first 5 min, followed by corrosion product formation, Figure 3.4b and 3.4c. The rate of mass increase during the first 5 min was almost constant, at about 0.93 ng/cm2s.
Apparently, corrosion product formed either locally as a result of pitting or on the rest of the surface must dominate weight loss associated with pitting. The rate of mass loss observed after the first 5 min was about -0.88 ng/cm2s. For comparison to electrochemical measurements, this mass loss rate can be converted to an equivalent anodic current density, im, by:
77 ⎛ dm ⎞ ⎜ ⎟ dt i = −n∗F ⎝ ⎠ (3-1) m M ∗
where n* is the average valence of dissolution, M* is the average atomic weight of the dissolved material, and F is Faraday’s constant. n* and M* were determined using the concentration of species in solution after the OCP exposure in 0.5 M NaCl solution, as measured by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES),
Table 3.3. It is assumed that all corrosion products are soluble in the solution, so the molar ratio can be used for the estimation of the ratio of the dissolved species after measurement. The molar ratio of Al : Cu : Mg ions in solution was 52.9 : 0.6 : 46.5.
Assuming that these elements dissolved in the 3+, 2+, and 2+ oxidation state respectively, the values of n* and M* are 2.53 equiv/mol and 25.96 g/mol, respectively. Using these
corr values, the corrosion dissolution current density at OCP, im , associated with the steady state mass loss rate was 8.27 μA/cm2, Table 3.4. This rate is considerably higher than the dissolution rate predicted by extrapolation of the anodic and cathodic portions of the
corr 2 polarization curve, ip , 0.3 ∼ 2 μA/cm , respectively. This observation will be discussed further below. Solution analysis was not performed for other solutions. However, assuming the same cation ratios, the calculated corrosion rate from the mass change was also much higher than the rates from extrapolation of the polarization curve for the solution with NaOH added, Table 3.4. In the vanadate and chromate solutions, the mass change during the OCP exposure was extremely low, Figure 3.9d.
78 S phase thin films deposited on quartz crystals were cathodically polarized in the various solutions described above at the potential of -1050 mVSCE after the 30 min OCP exposure. Even though the measured net currents were cathodic (negative), the mass decreased and reached a constant rate of mass loss in the 0.5 M NaCl solution equal to -
2.19 ng/cm2s, Figure 3.10b. This value is higher than that observed at OCP in this solution, even though the applied potential was almost 200 mV lower. The rate of mass
2 loss at -1050 mVSCE in the pH 10 0.5 M NaCl solution, -2.58 ng/cm s, was close to that in the neutral 0.5 M NaCl at the same potential. No significant mass change was observed in chromate-containing solution as shown in Figure 3.10d. The mass decreased with time in the vanadate-containing solution, initially at a rate of -2.01 ng/cm2s and then at a much slower rate of -0.41 ng/cm2s after 20 min.
Metallic dissolution under net cathodic conditions occurs through the chemical dissolution of a soluble surface oxide film. Assuming the thickness of oxide film is constant in the steady state condition, dm/dt can be directly related to a current density,
* * im, using Equation (3-1). The values of n and M were determined from the ICP-AES data as described above, but using the changes in solution concentration measured after the polarization treatment, Table 3.3. The values for n* and M*, 2.79 equiv/mol and 26.42 g/mol, respectively, were slightly different during cathodic polarization than during the
OCP exposure. It was assumed that n* and M* in the other solutions were the same as in
0.5 M NaCl. The values of im are summarized in Table 3.5. Again, the lowest rate was for the chromate-containing solution.
79 3.3.5. Calculations of True Cathodic Current Density
It is of interest to determine the total true cathodic current density and the rates of the individual cathodic reactions. The true cathodic density can be assessed from the measured current density and the mass loss rate. However, various factors need to be considered. A conversion of mass loss rate into a current density according to Equation
(3.1) results in a value that includes both faradaic and nonfaradaic components:
im = iF + iNF (3-2)
where iF is the current density associated with faradaic dissolution and iNF is the equivalent current density associated with nonfaradaic release of material. The true cathodic current density at any potential, ic true, is the difference between the measured or net current density, inet, and the faradaic dissolution current density, iF:
ic,true = inet − iF (3-3)
A variety of cathodic reactions might occur on the surface of the S phase thin film and each should be considered. The standard cathodic reactions in aqueous systems are possible, i.e. oxygen reduction (if dissolved oxygen is present, as was the case in these experiments) and hydrogen evolution. It is also possible for copper ions in solution to replate onto the alloy surface and for the reduction of other reducible ions in solution if
80 present, such as chromate. Therefore, the true cathodic current density, which is a sum of all of the cathodic reactions occurring on the surface, can be expressed as:
i = i + i + i 2+ + i 6+ 3+ = i − i + i (3-4) c,true O2 H 2 Cu −Cu Cr −Cr net m NF
where iO2, iH2, iCu2+-Cu, and iCr6+-Cr3+ are the cathodic current densities associated with
ORR, HER, Cu replating and chromate reduction, respectively.
The values of inet and im have been measured in the above experiments. The extent
to which the quantity inet − im represents ic, true depends on the magnitude of iNF. Note
that at OCP inet = 0, so ic,true = −iF = −im + iNF . The effect of nonfaradaic material loss and the possible magnitudes of the Cu replating and chromate reduction reactions will be considered in turn presently.
The ICP-AES data presented above indicate that the content of Cu ion in solution both after OCP exposure and cathodic polarization was much lower than the 22 at% in the S phase thin film analog. This suggests that either Cu was enriched preferentially on the surface as a result of Al and Mg dealloying, or that Cu was released from the surface and then replated back (The possibilities of the formation of insoluble compounds or plating on the counter electrode were not considered. Cupric should be very soluble, and the Pt counter electrode was at a much higher potential than the working electrode both at
OCP and during cathodic polarization of the sample).
As described in the introduction, Cu can be released from dissolving S phase as the result of the dealloying of Al and Mg, and the subsequent physical separation from 81 the sample of portions of a remnant Cu sponge-like structure. During the OCP exposure in 0.5 M NaCl, a steady state mass loss rate of -0.88 ng/cm2s was observed. Assuming that this mass loss rate represents congruent loss of the S phase elements and that all of the Cu was released nonfaradaically into solution, the current calculated from the mass
corr loss, im , would be larger than the faradaic dissolution component by iNF = 1.18
2 2 μA/cm . This worst-case assumption results in ic,true = −im + iNF = -7.09 μA/cm , Table
3.4. Making the same assumptions, the current calculated from the steady state mass loss
2 rate of -2.19 ng/cm s during the polarization at -1050 mVSCE in 0.5 M NaCl would be
2 larger than the faradaic dissolution component by iNF = 2.95 μA/cm , resulting in
2 ic,true = inet − im + iNF = -24.55 μA/cm , Table 3.5. If Cu were enriched on the surface by dealloying rather than release and redeposition, iNF = 0 and im would be an accurate assessment of iF.
As described above, some assumptions were involved in the determination of n* and M* from the ICP-AES data. To check the effect of those assumptions on the calculation of dissolution current density from the mass loss rate, different scenarios were tested. If, as described above, Cu is either released nonfaradaically or is not released because of dealloying, it can be assumed that only Al and Mg dissolve. In that case, n* and M* would be 2.67 equiv/mol and 26.09 g/mol, respectively. The value of im at OCP
2 would then be 8.69 μA/cm . The value of ic,true = −im + iNF would be -7.51 or -8.69
μA/cm2 assuming nonfaradaic release or dealloying, respectively.
82 The OCP was far below the reversible potential for Cu dissolution, but a scenario was examined involving congruent faradaic dissolution, in which the elements (including
Cu) dissolve according to their concentration in the thin film. With this assumption, n* and M* would be 2.50 equiv/mol and 35.45 g/mol, respectively, and the resulting im calculated with these values for the OCP exposure in 0.5 M NaCl is 5.99 μA/cm2. In this
2 case, iNF = 0 so im = iF and ic, true = - im = -5.99 μA/cm . These values are still higher than the corrosion current density obtained by Tafel extrapolation of the polarization curve,
corr 2 ip = 0.3 ~ 2 μA/cm .
The same scenarios can be addressed for the condition of cathodic polarization at -
1050 mVSCE. Considering nonfaradaic release and the possible ranges for n* and M*
2 results in possible ic, true values in the range of -20.09 ∼ -27.50 μA/cm . Like the situation at open circuit, these values are much higher than the net current density measured by the potentiostat.
The difference between the calculated true cathodic current densities for S phase in 0.5 M NaCl solution and the net measured current densities indicate that interpretation of cathodic polarization curves for Al-containing alloys must be performed with care.
The observations can be explained with the help of a schematic Evans diagram qualitatively describing the electrode kinetics, Figure 3.11. This figure shows the measured polarization curves for S phase in 0.5 M NaCl and schematic representations of the partial anodic and cathodic polarization curves. The data points at OCP and -1050 mVSCE represent the ranges for the ic, true and im considering nonfaradaic release and the possible ranges for n* and M*. Note that the modified curves illustrate the average values
83 of the ic, true and im. As described above, the current density associated with the rate of mass loss of thin film S phase at OCP in 0.5 M NaCl was much higher than the extrapolation of the curves to the OCP. For this to be so, the true cathodic current density and the passive current density below the open-circuit/pitting potential must both have been high, similar in magnitude, and relatively independent of potential. As a result, the measured currents were much less than the true currents. At a controlled cathodic potential, the local environment changed, which further altered the electrochemical kinetics. The cathodic reaction generated a local alkaline environment near the electrode surface (evidence for which will be given below), and the anodic polarization curve changed as a result of this pH increase. The passive film became soluble and the passive current density increased significantly, as is shown in Figure 3.11 by the bend in the curve representing the passive current density.
3.3.6. Components of True Cathodic Current Density
As described in Equation (3-4), the cathodic current density could contain components related to oxygen reduction, hydrogen evolution, Cu replating, and chromate reduction. For the solutions containing inhibitor, the measured cathodic current densities were close to the calculated true cathodic current densities as summarized in Table 3.5.
Chromate and vanadate inhibited both the anodic and cathodic reactions on S phase.
However, chromate present at only 10-4 M was a much more effective inhibitor than the
0.1 M vanadate addition. This is partly explained by the local pH changes which will be discussed later. The true cathodic current density was also strongly reduced by chromate,
84 probably due to the presence of a Cr3+ film, approximately a monolayer in thickness [28,
29, 33], which blocks ORR sites [24, 25]. This layer also reduced the anodic dissolution rate, as has also been found at anodic potentials [51]. It has been also reported that chromate inhibits Mg dissolution, and retards Cu enrichment on the S phase [26].
However, the amount of the current density associated with reduction of chromate seemed to be small. Clark and McCreery showed that the reduction of chromate is self- limiting and only a Cr3+ monolayer forms [29]. The charge associated with this monolayer formation (on a Cu electrode) was found to be 1.2 mC/cm2 and the time for monolayer formation was on the order of seconds [29]. As shown in Figure 3.10c, the current measured for the S phase thin film in chromate-containing solution was essentially zero during the 30 min treatment. It is likely that the Cr3+ monolayer formed during the exposure at open circuit prior to the beginning of cathodic polarization.
Therefore, the current associated with chromate reduction was probably minimal in the steady state condition.
It is also of interest to assess the amount of Cu replated during OCP immersion and potentiostatic exposure. The Cu signal measured by SIMS mapping performed after
OCP exposure, Figure 3.6, was concentrated locally around pits, and disappeared rapidly during sputter etching. Figure 3.12 shows AES depth profiles at several locations after potentiostatic treatments performed at -1050 mVSCE. These AES depth profiles were produced by linear least squares fitting to improve the signal-to-noise of the profiles and to separate different chemical states of elements. Rather than using the peak-to-peak height within a window (just two data points), linear least squares fitting uses a linear fit
85 for the entire spectrum in the window (utilizing all data points) [52, 53]. Linear least squares fitting also assumes that a reference spectrum can be used as a basis spectrum, and this is the case in most practical applications [52, 53]. The basis spectrum can be obtained by summing spectra from selected portions of the depth profile where the composition is not changing. Figure 3.12a and 3.12b show the SEM morphologies of the surface at low and high magnification, respectively. The small areas (about 5 μm × 5 μm) for analysis within the large sputtered area were selected to generate depth profiles of the different regions under the same sputter procedures. Two AES depth profiles from different sites are shown in Figure 3.12c and 3.12d, indicating the presence of Al oxide with different Cu contents. Cu was depleted at the surface of site 2, a region of apparent localized attack. At site 1, a nearby region with no evidence of attack, there is a slightly higher Cu concentration of the surface. The presence of the Cu on the surface was evident. However, it is very difficult to quantify how much Cu was released and replated.
Note that two different Al signals associated with the different chemical states are represented in the AES depth profiles, Al oxide (Al(O)), and metallic Al (Al(m)).
The largest possible effect of Cu replating can be determined by assuming that Cu was released congruently into solution, either nonfaradaically or by dissolution, and then all replated back onto the electrode surface. The maximum current density associated
2 with this process at OCP is equal to the maximum iNF calculated above, or 1.18 μA/cm .
The discussion above accounts for the possibility of nonfaradaic release of Cu associated with dealloying. However, it is possible that there is some other mechanism for nonfaradaic release of material, such as grain fallout. It should be emphasized that
86 there is no evidence for such phenomena. However, another form of nonfaradaic release of material would bring yet closer the current calculated from the mass loss and the net current.
3.3.7. Local pH Measurements on S Phase Films
To understand the effects of the chromate and vanadate inhibitors, local pH was measured using micro pH electrode measurements. Figure 3.13 shows the local pH changes on the electrode surface in 0.5 M NaCl and 0.5 M NaCl with inhibitors. The first part of the exposure was at OCP, and then a potential of -1050 mVSCE was applied. The pH measured at OCP was about 7 for all solutions, which is close to the bulk solution pH values. During the cathodic polarization, the pH became alkaline with time in 0.5 M
NaCl solution, which is in good agreement with the work of Vukmirovic et al. [8]. In the case of inhibitor-containing solution, the changes of pH were not as significant as the case in 0.5 M NaCl solution. However, local pH in the vanadate-containing solution was higher than that in the chromate-containing solution.
A simple calculation using Fick’s first law enables an estimation of the local pH near the electrode surface:
nFD − {(C − ) − (C − ) } i = OH OH solution OH surface (3-5) L δ
87 -5 2 where n for ORR is 4 equiv/mol, F is Faraday’s constant, DOH- (5.30 × 10 cm /s [54]) is
- - the diffusion coefficient of OH , (COH-)surface and (COH-)solution are the concentration of OH at the electrode surface and bulk solution, and δ is the thickness of the diffusion layer at the electrode surface. Assuming a diffusion layer thickness of 0.05 cm for natural convection conditions [4], the pH electrode was located 100 μm above the sample or 20
% of the way from the surface to the edge of the diffusion layer. For the 0.5 M NaCl
2 solution, the true cathodic current density was -27.50 μA/cm and (COH-)solution = 9.55 ×
10-9 M (bulk pH of 5.98), so the estimated concentration of OH- at the location of the micro pH electrode was 5.38 × 10-5 M, or pH 9.73. This is in good agreement with local pH measurements as shown in Figure 3.13.
3.4. CONCLUSIONS
Thin film analogs of S phase were studied using Electrochemical Quartz Crystal
Microbalance (EQCM) at Open Circuit Potential (OCP) and under cathodic polarization in 0.5 M NaCl with additions of chromate, vanadate and NaOH. The following conclusions can be made:
• During immersion at OCP in the chloride solution, the S phase film exhibited
pitting corrosion and local Cu enrichment. The polarization curve indicated
spontaneous formation of pits at open circuit.
• Corrosion current density measured by EQCM was higher than that obtained
by extrapolation of the anodic and cathodic portions of the polarization curve.
True cathodic current density at -1150 mVSCE was much higher than the net
88 current density measured by the potentiostat, so the measured polarization
curves were misleading.
• The net current density and the current density measured from the mass loss
rate for S phase were reduced in chloride solutions containing 10-4 M of
chromate or 0.1 M of vanadate. The mass change in the presence of chromate
was greatly reduced. This effect was much greater than in the presence of
vanadate, suggesting that chromate exhibited stronger inhibition effect on S
phase than vanadate.
• Local pH measurement revealed the development of high pH zone near the S
phase under the cathodic polarization in 0.5 M NaCl solution.
• Local pH on S phase in the vanadate-containing solution was higher than that
in the chromate-containing solution.
89
REFERENCES
1. R. G. Buchheit, Journal of the Electrochemical Society 142, p. 3994 (1995). 2. N. Birblis and R. G. Buchheit, Journal of the Electrochemical Society 152, p. B140 (2005). 3. R. G. Buchheit, R. P. Grant, P. F. Hlava, B. Mckenzie, and G. L. Zender, Journal of the Electrochemical Society 144, p. 2621 (1997). 4. R. G. Buchheit, L. P. Montes, M. A. Martinez, J. Michael, and P. F. Hlava, Journal of the Electrochemical Society 146, p. 4424 (1999). 5. R. G. Buchheit, M. A. Martinez, and L. P. Montes, Journal of the Electrochemical Society 147, p. 119 (2000). 6. P. Schmutz and G. S. Frankel, Journal of the Electrochemical Society 145, p. 2285 (1998). 7. P. Schmutz and G. S. Frankel, Journal of the Electrochemical Society 145, p. 2295 (1998). 8. M. B. Vukmirovic, N. Dimitrov, and K. Sieradzki, Journal of the Electrochemical Society 149, p. B428 (2002). 9. H. M. Obispo, L. E. Murr, R. M. Arrowood, and E. A. Trillo, Journal of Materials Science 35, p. 3479 (2000). 10. K. Urushino and K. Sugimoto, Corrosion Science 19, p. 225 (1979). 11. K. Wagner, S. R. Brankovic, N. Dimitrov, and K. Sieradzki, Journal of the Electrochemical Society 144, p. 3545 (1997). 12. V. Guillaumin and G. Mankowski, Corrosion Science 41, p. 421 (1999). 13. N. Dimitrov, J. A. Mann, M. Vukmirovic, and K. Sieradzki, Journal of the Electrochemical Society 147, p. 3283 (2000). 14. T. Suter and R. C. Alkire, Journal of the Electrochemical Society 148, p. B36 (2001). 15. G. O. Ilevbare, O. Schneider, R. G. Kelly, and J. R. Scully, Journal of the Electrochemical Society 151, p. B453 (2004). 16. O. Schneider, G. O. Ilevbare, J. R. Scully, and R. G. Kelly, Journal of the Electrochemical Society 151, p. B465 (2004). 17. G. S. Chen, M. Gao, and R. P. Wei, Corrosion 52, p. 8 (1996). 18. C. -M. Liao, J. M. Olive, M. Gao, and R. P. Wei, Corrosion 54, p. 451 (1998). 19. C. -M. Liao and R. P. Wei, Electrochimica Acta 45, p. 881 (1999). 20. P. Leblanc and G. S. Frankel, Journal of the Electrochemical Society 149, p. B239 (2002).
90 21. R. R. Leard, The Electrochemical Behavior of Copper Bearing Intermetallics and the Corrosion of AA2024-T3, Master Thesis, The Ohio State University, Columbus, OH (2001). 22. P. Schmutz and G. S. Frankel, Journal of the Electrochemical Society 146, p. 4461 (1999). 23. G. O. Ilevbare, J. R. Scully, J. Yuan, and R. G. Kelly, Corrosion 56, p. 227 (2000). 24. G. O. Ilevbare and J. R. Scully, Corrosion 57, p. 134 (2001). 25. G. O. Ilevbare and J. R. Scully, Journal of the Electrochemical Society 148, p. B196 (2001). 26. A. Kolics, A. S. Besing, and A. Wieckowski, Journal of the Electrochemical Society 148, p. B322 (2001). 27. M. W. Kendig and S. Jeanjaquet, Journal of the Electrochemical Society 149, p. B47 (2002). 28. W. J. Clark, J. D. Ramsey, R. L. McCreery, and G. S. Frankel, Journal of the Electrochemical Society 149, p. B179 (2002). 29. W. J. Clark and R. L. McCreery, Journal of the Electrochemical Society 149, p. B379 (2002). 30. D. Chidambaram, C. R. Clayton, and G. P. Halada, Journal of the Electrochemical Society 151, p. B151 (2004). 31. M. Kendig and C. Yan, Journal of the Electrochemical Society 151, p. B679 (2004). 32. T. J. R. Leclere and R. C. Newman, Journal of the Electrochemical Society 149, p. B52 (2002). 33. Y. Baek and G. S. Frankel, Journal of the Electrochemical Society 150, p. B1 (2003). 34. G. Z. Sauerbrey, J. Physik 155, p. 206 (1959). 35. D. A. Buttry, The Quartz Crystal Microbalance as an In-Situ Tool in Electrochemistry, in Electrochemical Interfaces: Modern Techniques for In-Situ Interface Characterization, H. D. Abruna, editor(s), p. 542, VCH, New York, NY (1991). 36. M. D. Ward, Principles and Applications of the Electrochemical Quartz Crystal Microbalance, in Physical Electrochemistry: Principles, Methods, and Applications, I. Rubinstein, editor(s), p. 296, Marcel Dekker, New York, NY (1995). 37. D. N. Furlong, Quartz Crystal Microbalance, in Modern Characterization Methods of Surfactant Systems, B. P. Binks, editor(s), p. 485, Marcel Dekker, New York, NY (1999). 38. A. R. Hillman, The Electrochemical Quartz Crystal Microbalance, in Encyclopedia of Electrochemistry, A. J. Bard, M. Stratmann, and P. R. Unwin, editor(s), p. 233, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany (2003). 39. T. Ramgopal, P. Schmutz, and G. S. Frankel, Journal of the Electrochemical Society 148, p. B348 (2001).
91 40. G. S. Frankel, J. Kang, and Y. Baek, in the Conference Paper of the CORROSION NACExpo 2003, San Diego, CA, paper number 03394 (2003). 41. G. S. Frankel, J. R. Scully, and C. V. Jahnes, Journal of the Electrochemical Society 143, p. 1834 (1996). 42. P. Schmutz and D. Landolt, Corrosion Science 41, p. 2143 (1999). 43. M. Ashraf-Khorassani and R. D. Braun, Corrosion 43, p. 32 (1987). 44. L. B. Kriksunov, D. D. Macdonald, and P. J. Millett, Journal of the Electrochemical Society 141, p. 3002 (1994). 45. L. T. Dimitrakopoulos, T. Dimitrakopoulos, P. W. Alexander, D. Logic, and D. B. Hibbert, Analytical Communications 35, p. 395 (1998). 46. F. Saito, Y. Nagashima, L. Wei, Y. Itoh, A. Goto, and T. Hyodo, Applied Surface Science 194, p. 13 (2002). 47. E. Tada, K. Sugawara, and H. Kaneko, Electrochimica Acta 49, p. 1019 (2004). 48. D. B. Williams and C. B. Carter, Transmission Electron Microscopy, Plenum Press, New York, NY (1996). 49. ICDD Database. 50. B. D. Cullity, Elements of X-Ray Diffraction, Addison-Wesley Publishing Company, Inc., Reading, MA (1978). 51. J. D. Ramsey and R. L. McCreery, Corrosion Science 46, p. 1729 (2004). 52. W. F. Stickle and D. G. Watson, J. Vac. Sci. Technol. A 10, p. 2806 (1992). 53. W. F. Stickle, The Use of Chemometrics in AES and XPS Data Treatment, in Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy, D. Briggs and J. T. Grant, editor(s), p. 377, IM Publications and SurfaceSpectra Limited, West Sussex and Manchester, U.K. (2003). 54. A. Yu Simaranov, T. I. Sokolova, A. I. Marshakov, and Yu. N. Mikhailovskii, Protection of Metals 27, p. 329 (1991).
92 Base pressure < 2 × 10-7 torr Working pressure ∼ 5 × 10-3 torr Pre-sputtering time 300 sec Flow rate of Ar 39.5 sccm DC power ∼ 400 W DC bias voltage ∼ 620 V Deposition time 420 sec Deposition rate ∼ 1 nm/sec Film thickness < 500 nm
Table 3.1: Thin film deposition parameters.
Al (atomic %) Cu (atomic %) Mg (atomic %) Site 1 53 22 25 Site 2 50 28 22 Site 3 54 22 24 Site 4 53 22 25 Site 5 49 33 18 Site 6 55 22 23 Site 7 55 22 23 Site 8 48 32 20
Table 3.2: SEM-EDS analysis at sites indicated in Figure 3.4a ∼3.4d.
93 Before immersion After OCP After polarization Al (μg/cm2) 0 9.24 18.75 Cu (μg/cm2) 0 0.24 0.12 Mg (μg/cm2) 0 7.32 9.61
Table 3.3: Concentrations of species in solution after the OCP and potentiostatic polarization exposures measured by ICP-AES for S phase in 0.5 M NaCl.
corr corr dm/dt im ic, true ip [ng/cm2s] [μA/cm2] [μA/cm2] [μA/cm2]
0.5 M NaCl -0.88 8.27 -7.09 0.3 ∼ 2 with Chromate -0.04 0.38 0.1 ∼ 0.25 with Vanadate -0.07 0.66 0.6 ∼ 1 with NaOH -1.12 10.53 -9.35 0.15 ∼ 2
Table 3.4: Values of mass change rate, dm/dt, at OCP, corrosion current density corr calculated from the mass change rate assuming cation ratios given in Table 3.3, im , and corr corrosion current density calculated from extrapolation of the polarization curves, ip . ic, true was calculated based on the consideration of the Cu release and replating.
94 inet dm/dt im inet – im ic, true [μA/cm2] [ng/cm2s] [μA/cm2] [μA/cm2] [μA/cm2]
0.5 M NaCl -5.19 -2.19 22.31 -27.50 -24.55 with Chromate -0.14 -0.08 0.82 -0.96 with Vanadate -2.66 -0.41 4.18 -6.84 with NaOH -5.21 -2.58 26.29 -31.50 -28.55
Table 3.5: Values of net current density measured by the potentiostat, inet, mass change rate dm/dt, the current density calculated from the mass change rate assuming cation ratios given in Table 3.3, im, inet – im, and true cathodic current density with consideration of the Cu release and replating, ic ,true. All measurements were at -1050 mVSCE.
95
Figure 3.1: Schematic diagram of the EQCM setup. Note that the micro pH electrode part was only installed for some experiments in which the pH was measured.
96
Figure 3.2: XRD patterns of the as-deposited S phase film and the bulk Al2CuMg (PDF#65-2501) [49].
97
(a)
(b)
Figure 3.3: (a) Positive and (b) negative SIMS depth profiles of the as-deposited S phase film. Note that the depth per cycle is different for the positive and negative profiles.
98
(a) as-deposited (b) after 3 min.
(c) after 5 min. (d) after 30 min.
(e) after -950 mVSCE application (f) after -1050 mVSCE application
(g) after -1150 mVSCE application (h) after -1250 mVSCE application
Figure 3.4: Various surface morphologies with immersion times in the aerated 0.5 M NaCl solution. It is noted that (e) ∼ (h) were taken after 30 min. of OCP exposure followed by cathodic application during another 30 min. All scale bars are 20 μm.
99
(a)
(b)
Figure 3.5: (a) Positive and (b) negative SIMS depth profiles of the S phase film after 30 min. OCP exposure in the aerated 0.5 M NaCl solution.
100
(a) (b)
(c) (d)
Figure 3.6: SIMS line scans (x: distance, y: signal intensity) of (a) Al, (b) Cu, (c) Mg and (d) Si on pits found in several sputtered layer. The line scans are taken from the positions indicated on the associated maps. All scale bars are 10 μm.
101
(a)
(b)
Figure 3.7: (a) OCP and (b) polarization behaviors of S phase in the deaerated, aerated 0.5 M NaCl solution (pH 5.98) with addition of 10-4 M of chromate (pH 6.60), 0.1 M of vanadate (pH 6.29 adjusted), and 10-4 M of NaOH (pH 9.89).
102
(a)
(b)
Figure 3.8: (a) OCP and (b) polarization behaviors of AA2024-T3 in the aerated 0.5 M NaCl solution (pH 5.98) with addition of 10-4 M of chromate (pH 6.60), 0.1 M of vanadate (pH 6.29 adjusted), and 10-4 M of NaOH (pH 9.89).
103
(a) (b)
(c) (d)
Figure 3.9: (a) OCPs and (b) mass changes for S phase thin film analogs during OCP exposure in the 0.5 M NaCl solution without and with NaOH additions. (c) OCPs and (d) mass changes in the 0.5 M NaCl solution with chromate and vanadate additions.
104
(a) (b)
(c) (d)
Figure 3.10: (a) Net current densities and (b) mass changes for S phase thin film analogs during the potentiostatic polarization of -1050 mVSCE in the 0.5 M NaCl solution without and with NaOH additions. (c) Net current densities and (d) mass changes in the 0.5 M NaCl solution with chromate and vanadate additions.
105
Figure 3.11: Schematic for net currents on the measured polarization curves obtained in the 0.5 M NaCl solution.
106
(a) overall image (b) local image
(c) site 1 (d) site 2
Figure 3.12: AES depth profiles at two different locations after potentiostatic treatments performed at -1050 mVSCE.
107
Figure 3.13: Local pH profiles under OCP and cathodic polarization of the S phase thin film analogs in different solutions.
108
CHAPTER 4
WATER UPTAKE IN POLYURETHANE COATING ON AU AND AL
Water uptake in a polyurethane coating on Au and Al was monitored using an
Electrochemical Quartz Crystal Microbalance (EQCM). Apparent mass increases were measured in the coated samples on the Au-deposited quartz immersed in water. For a similar sample containing an Al layer instead of Au, apparent mass increases were observed after the completion of initial water uptake, indicating the formation of a corrosion product (or oxide layer) underneath coating. Electrochemical Impedance
Spectroscopy (EIS) measurements also indicated changes during the same time period that were consistent with the formation of a corrosion product (or oxide layer).
4.1. INTRODUCTION
The major function of corrosion protective organic coatings is to serve as a physical barrier between the metallic substrate and reactants, such as water, oxygen, and ions [1, 2]. However, the barrier nature of a coating becomes degraded with time because all organic coatings are permeable to these species to some extent [3]. It is well known that corrosion of coated metals occurs only after the adhesion between the coating and the 109 substrate is broken due to water or oxygen permeation and subsequent electrochemical reactions within the thin layer of water [4, 5]. An understanding of water, oxygen and ion transport through the coatings is critical for a study of the corrosion that occurs under organic coatings [6-9]. Corrosion prevention can be obtained successfully by increasing the coating thickness, using a coating with low porosity, and applying multilayer coatings where pores are not matched geometrically [10].
Electrochemical Impedance Spectroscopy (EIS) is one of the main electrochemical methods for evaluating the performance of coatings on metals [4, 5, 11-
17]. The complex impedance generated by EIS is often analyzed by fitting to the response of an equivalent electrical circuit composed of passive elements such as resistors, capacitors and inductors, thereby determining various characteristic properties of the coating [18]. However, it is sometimes not easy to select the appropriate equivalent circuit to use in the fitting process [18].
In the last decade, the Quartz Crystal Microbalance (QCM) has been used as an experimental method providing information on processes such as electrolyte adsorption
[19, 20], metallic deposition [21-23], ion and solvent movement [24-26], passive film growth kinetics [27-31], and corrosion inhibition [32, 33]. In principle, it should be possible to apply the Electrochemical QCM (EQCM) technique to investigate water uptake, ion permeation, disbonding of the coating, and corrosion initiation [34, 35].
According to the Sauerbrey equation [36], the mass change of a quartz crystal, including thin layers deposited on it, is directly proportional to frequency shift as long as the total
110 thickness of the electrode and foreign layers is less than 2 % of the thickness of the quartz crystal:
2 2 f o Δm Δf = − = −C f Δm (4-1) A μq ρ q
11 2 where, A is the electrode area, μq is the shear modulus of quartz (2.947 × 10 dyn/cm ),
3 ρq is the density of quartz (2.648 g/cm ), fo is the resonant frequency of the quartz and Cf is the sensitivity factor.
Several reports have shown that EQCM is a useful technique for determining kinetic parameters, e.g., diffusivity of water [35, 37-40]. However, little research has been performed to relate this information to delamination and subsequent corrosion of the substrate under the coatings. The objective of this study is to use EQCM and EIS on coated Al electrode to investigate water uptake in organic coatings, delamination, and corrosion at the coating/metal interface.
4.2. EXPERIMENTAL
AT-cut Au-deposited (0.1 μm) or blank quartz crystals were used for the tests. Al thin films were deposited on the blank 10 MHz AT-cut quartz crystals by DC magnetron sputtering using a 99.99 % Al target. The base pressure and operation pressure were 2.0 ×
10-7 and 5.0 × 10-3 torr, respectively. It has been reported that Al thin films deposited with a base pressure of 2.0 × 10-7 torr or better behave very similarly to the bulk Al [41].
111 The main deposition conditions used for this study are summarized in Table 4.1. Thin films were deposited on both sides of the quartz crystal in a typical “lollipop” pattern [42,
43]. The films on the topside and backside were both around 0.2 μm thick and acted as the working electrode and the electrical connection to the EQCM oscillator, respectively.
The electrodes were coated using a typical aircraft polyurethane topcoat formulated from Desmophen 650A-65 PMA resin, Desmodur N75 BA/X hardener, and
Desmorapid PP catalyst supplied by Bauer. It was necessary to apply only a thin layer of the coating because of the limitations imposed by EQCM. The viscosity of the mixture and thus the thickness of the coating were controlled by dilution with Methyl Ethyl
Ketone (MEK). A spin coater was used to deposit the coating formulation on the Au or
Al-deposited quartz. Spin coating was done at a rotation speed of 1,100 rpm for 60 sec.
To increase coating thickness, spin coating was repeated on the coated samples after curing at 80 oC for 8 hr in air. The thickness was measured by a Dektak3ST surface profiler.
An Elchema EQCN-900 EQCM was used for this study. The coated samples were fixed in a in a Teflon crystal holder that exposed the topside to the solution and the other side to air. Electrical contact between the coated thin film electrode and wire leads was made by colloidal silver paint, which was isolated from the water by silicone resin. This cell design and configuration were described elsewhere [25]. The crystal holder was placed horizontally into an electrochemical cell as shown in Figure 4.1. The currents, potential, and mass-related frequency change were recorded by the VOLTSCAN program provided by Elchema. All measurements were conducted in a Faraday cage to minimize
112 the interference from the surroundings. To determine the exact experimental sensitivity factor of the EQCM system, calibration was performed by electrodeposition of Cu on a
Au-deposited quartz crystal at -626 mVSCE in 0.05 M CuSO4 + 0.05 H2SO4.
The Gamry FAS1 potentiostat was used to perform impedance measurements. A
10 mV AC voltage with respect to the Open Circuit Potential (OCP) was applied over the frequency range 105 ∼ 10-2 Hz during the immersion to water. The impedance data were fitted using commercial programs.
4.3. RESULTS AND DISCUSSION
4.3.1. Mass and Potential Changes of Uncoated and Coated Samples on the Au-
Deposited Quartz
Figure 4.2 shows the potential and frequency changes for uncoated and polymer coated Au-deposited samples with various coating thickness. The potential of the uncoated sample decreased to 200 mVSCE during the water immersion. The frequency changed within about 1 s as a result of the rapid introduction of the water environment, as shown in Figure 4.2b. Immersion of a QCM into a liquid is known to modulate the vibration frequency owing to coupling of the fluid near the surface with the crystal [44,
45]. After the initial rapid change, the frequency of the Au-deposited quartz without polymer coating was quite constant. Measurement of the frequency was started immediately after water was injected to minimize the data loss at the very initial stage.
Since Au is noble and inert, it is reasonable that no further mass change of the uncoated sample occurred.
113 The potential of the coated Au samples took time to stabilize as shown in Figure
4.2a, but there was no trend with coating thickness. The frequency change of the coated
Au samples was quite different than the uncoated Au samples, as shown in Figure 4.2b.
The frequency changed within about 4 s of introduction of the water, which is rapid but not as quickly as for the uncoated case. After the initial frequency change, it then continued to change slowly over hours, and the time required to reach steady state increased with increasing thickness. Using the Sauerbrey equation [36], the frequency change can be expressed as a mass change. The decrease in frequency with time is associated with an increase in mass.
Based on the literature, the diffusivity of water in polyurethane coatings should be on the order of 10-13 m2/sec [35, 37-40]. It is therefore expected that the time for water to diffuse into a coating of thickness on the order of a couple of μm should be several seconds. This corresponds well with the time of the fast frequency change for the coated samples. However, detailed analysis of the EQCM data is quite difficult for several reasons. The fact that the frequency for the coated sample did not exhibit an immediate jump upon immersion in water as it did for the uncoated sample indicates that there was no coupling of the water with the quartz crystal oscillation for the coated sample. Only as the water was absorbed into the coating did the frequency change. It was expected that coatings with thickness of 2 μm or less would be able to couple with the crystal [44, 45].
The fact that the coated sample was not immediately affected by contact with water also brings into question if the EQCM was able to fully sense the water in the coating. The changes of the frequency of the coated samples upon immersion were only slightly higher
114 than those of the uncoated sample, Figure 4.2b, and there was a very small effect of coating thickness on this change. It is expected that the amount of water taken up into the coating would scale with the coating thickness. Since the apparent mass change increased only slightly as the coating thickness increased from 0.5 to 2 μm, it is likely that the
EQCM was not sensitive to the full amount of water in the coating. This is probable due to the presence of the coating layer which had already absorbed water immediately after water introduction. Again, only a couple of seconds needed for water molecule to arrive at the interface between the coating layer (with a couple of μm thickness) and quartz substrate. Thus, the quartz vibration, which was not fully coupled with coating layer, would be slow down as the coating layer became saturated with water. The amount of the frequency change, however, can be different between the uncoated and coated samples because of the presence of the coating layer which contained water molecules.
Furthermore, the slow change of frequency with time after the initial fast change was probably the result of swelling or relaxation of the polymer matrix and a resulting increase in the amount of water in the coating layer. Quantification of this change is complicated by the limited sensitivity of the crystal to the water in the coating layer.
Therefore, EQCM can sense the change of the frequency due to the water uptake in the very thin coating layer. However, it is likely that the EQCM does not sense the full change of the frequency associated with the water uptake throughout the coating layer.
115 4.3.2. Mass and Potential Changes of Coated Samples on Al-Deposited Quartz
Figure 4.3 shows potential and frequency changes for a polymer-coated Al- deposited quartz sample exposed in water with short and long time scales. This experiment with 1.9 μm of coating thickness was repeated 3 times, and the results were all similar. As can be seen in Figure 4.3a, the potential decreased to approximately -500 mVSCE after 15 hr. Over larger times, the potential decreased to the range of -1000 mVSCE. A frequency decrease, which is associated with mass increase according to the
Sauerbrey equation [36], was observed with time, Figure 4.3b. Two frequency plateaus are seen in Figure 4.3b. The first stage, which is enlarged in Figure 4.3b, was associated with the initial water uptake and then coating relaxation. This is similar to the coated Au samples, as shown in Figure 4.2b. The frequency then decreased further after 100 hr of exposure. This stage was related to the formation of a corrosion product (or oxide layer) underneath coatings, which will be discussed below. After reaching the second plateau, frequency was found to increase slightly (associated with a decrease in mass). This is connected with a portion of the edge of the coating film delaminated [35], which was more evident in the experiments performed in the Cl-containing solution.
Figure 4.4 shows potential and frequency changes for a polymer-coated Al- deposited quartz sample exposed to aerated 0.5 M NaCl solution. As can be seen in
Figure 4.4a, the potential stabilized at a value of about -900 mVSCE. During the first 3 hr of exposure the frequency decreased, which was associated with mass increase due to water uptake followed by coating relaxation, Figure 4.4b. After 3 hr of immersion, the frequency increased, indicating a mass decrease. It was easily observed after the
116 measurement that a portion of the edge of the coating film was delaminated, which allowed more rapid attack of the Al coated substrate [35]. This delamination was quite fast compared to that in the water; the sample is more susceptible to coating delamination in Cl-containing solution than in water.
4.3.3. EIS Measurements and Analysis
Figure 4.5 shows the EIS results for a polymer-coated Au-deposited quartz sample (the exposed area was about 0.256 cm2) exposed in aerated 0.5 M NaCl solution.
The samples for these tests were different than those used in the EQCM experiments.
However, the coating recipe was identical and the coating thickness was about 1.9 μm.
There was no significant change in the EIS spectra up to 720 hr of immersion, except the small decrease in capacitance. This is probably caused by the swelling or increase in coating thickness associated with coating relaxation. It is noted that the initial water uptake happened too fast to be captured by EIS measurements, indicating that the coating was saturated with water quickly and no further reaction occurred.
Figure 4.6 shows the EIS results for a polymer-coated Al-deposited quartz samples exposed in water. A plateau at high frequencies is observed for samples exposed less than 24 hr. The plateau value was initially about 106 Ω, and it then decreased with time. The SCE reference electrode remained immersed in the solution during the whole exposure period, as was the case for the EQCM measurements also. The high frequency plateau is likely a reflection of the solution ohmic resistance, Rs, which decreased with time owing to release of ions from the reference electrode. The downward bending of the
117 impedance at the highest frequencies for the shortest exposure times is likely a reflection of the boundary of accurate measurement for the Gamry FAS1 impedance system [46].
Ignoring these artifacts at high frequency, the impedance spectra were fitted well by an one-time-constant equivalent circuit, shown in Figure 4.7a, up to 1.5 hr of immersion. A
Constant Phase Element (CPE) was used instead of a perfect capacitor for better fitting.
The impedances of a perfect capacitor and CPE are [47]:
1 Z = (4-2) C i ⋅ω ⋅C
1 Z = (4-3) CPE T (i ⋅ω)P
where C, T and P are capacitance, CPE constant and exponent, respectively. When P approaches 1, then the CPE resembles a perfect capacitor [47]. The true capacitance can be extracted from the CPE values using the equation below [48]:
1 (1−P) C = T P R P (4-4)
where R is the resistance that is in parallel with the CPE. The fitted values and calculated capacitances using the equivalent circuit shown in Figure 4.7a are summarized in Table
4.2, which shows that all the values up to 1.5 hr of immersion were similar. As mentioned above for the Au substrate, the initial water uptake happened too fast to affect the EIS
118 measurements. The parallel equivalent circuit components were considered to represent the coating parameters Rd (defect or pore resistance) and CPEc (coating CPE), respectively [16]. The coating capacitance (Cc) is related to the coating thickness, d, and dielectric constant, ε, according to:
A C = ε ⋅ε ⋅ (4-5) c o d
-14 where εo is the vacuum permittivity (8.854 × 10 F/cm) and A is the area. The content of water in the coating at the very short time exposure can be calculated from the frequency change performed by EQCM. Assuming the capacitance measured during the first 1.5 hr of exposure is the coating capacitance after water uptake, the coating capacitance at t = 0
(Co) can be obtained from the Brasher and Kingsbury Equation [49] using that the amount of water absorbed is approximately 0.5 wt%.:
⎛ 0.5×log80 ⎞ ⎜ logC − ⎟ ⎝ 100 ⎠ −9 Co =10 =1.497 ×10 (F) (4-6)
From calculation above, the dielectric constant of the coating can be calculated to be
12.5, which is higher than the reported value of about 8.8 [50]. This is probably because
EQCM can not fully sense the frequency change associated with the water uptake throughout the coating layer.
119 The value Rd increases after 6 hr of immersion and an equivalent circuit with one time constant can no longer be fitted to the data successfully. It is interesting that the low frequency impedance increased after 6 hr and 24 hr exposures as shown in Figure 4.6a. In general, impedance of a coating exposed to water decreases with time because all organic coatings are permeable to water to some extent [3]. To fit the data, it is necessary to use an equivalent circuit with a second RC constant as shown in Figure 4.7b. It is considered that this time constant reflects some loss of adhesion between coating and metal substrate, with the introduction of the circuit elements Rct and CPEdl, which describe the charge transfer resistance and double layer capacitance at the delaminated interface, respectively. The fitted values using the two-time-constant equivalent circuit shown in
Figure 4.7b are presented in Table 4.3. The values of Rct at 144 and 264 hr were not obtainable; unrealistically high values were reported by the fitting program. Nonetheless, the value of total resistance, Rd + Rct, is seen to decrease with time during the period of 6
∼ 96 hr.
The data from the EQCM test on the polymer-coated Al-deposited quartz sample showed a second decrease in frequency after 100 hr of exposure. Similarly, the impedance spectra after 96 hr of immersion were different than the earlier spectra. The low frequency impedance decreased steadily with time and the phase angle increased significantly. To analyze the data at larger times, a circuit with a third time constant as shown in Figure 4.7c, was used. It is considered that the mass changes are associated with the formation of a corrosion product (or oxide layer) under the polymer coating. This
120 circuit contains the elements CPEl and Rl representing this layer between the elements for the polymer coating and the electrochemical interface.
The values of the Cc decreased with time during the period up to 24 hr and then were relatively constant as shown in Table 4.3. This decrease in Cc is likely caused by the swelling or increase in coating thickness associated with coating relaxation. The capacitance of the corrosion product layer (Cl) increased and the resistance (Rl) decreased with time, Table 4.4, indicating that this layer was not protective. Similarly, Cdl increased and Rct decreased with time, which might be associated with an increase in corrosion rate with time.
From the frequency difference between the two plateaus and assuming that the density of Al hydroxide is 2.4 g/cm3, the thickness of the corrosion product layer was calculated to be 27 nm on average. Assuming that the dielectric constant of Al hydroxide is 2.2, the capacitance associated with a corrosion product layer of this thickness can be calculated to be about 14 nF, which is slightly greater than the values of the Cl in Table
4.4. Again, EQCM can not accurately sense the frequency change associated with the water uptake throughout the coating layer.
In summary, three different equivalent circuit models were selectively chosen for fitting the EIS data. After 96 hr of immersion, a circuit containing an extra time constant was required. The circuit elements associated with this time constant are consistent with the formation of a corrosion product (or oxide layer), as is indicated by the apparent mass increase after 100 hr of exposure measured by EQCM.
121 4.4. CONCLUSIONS
Water uptake in polyurethane coatings was successfully observed using in situ
Electrochemical Quartz Crystal Microbalance (EQCM) and Electrochemical Impedance
Spectroscopy (EIS). The following conclusions can be made:
• Coated samples on the Au-deposited quartz exhibited an apparent mass
increase with time over a period of hours.
• Quantification of the frequency change associated with the mass change due
to the water uptake was complicated by the limited sensitivity of the crystal to
the water in the coating layer.
• For a coated sample on the Al-deposited quartz, apparent mass increases were
observed after the completion of initial water uptake, indicating the formation
of a corrosion product (or oxide layer) underneath coating.
• Additional circuit elements fitted well to the EIS data of the coated samples
on the Al-deposited quartz, supporting the interpretation of the apparent mass
increase after 100 hr of exposure measured by EQCM as being associated
with the formation of a corrosion product (or oxide layer).
122
REFERENCES
1. H. Leidheiser, Corrosion 38, p. 374 (1982). 2. H. Leidheiser, Corrosion 39, p. 189 (1983). 3. J. H. W. de Wit, Inorganic and Organic Coatings, in Corrosion Mechanisms in Theory and Practice, P. Marcus and J. Oudar, editor(s), p. 581, Marcel Dekker, New York, NY (1995). 4. E. P. M. van Westing, G. M. Ferrari, and J. H. W. de Wit, Corrosion Science 34, p. 1511 (1993). 5. E. P. M. van Westing, G. M. Ferrari, and J. H. W. de Wit, Corrosion Science 36, p. 979 (1994). 6. J. E. O. Mayne, Anti-Corrosion October, p. 3 (1973). 7. J. E. O. Mayne, The Mechanism of the Protective Action of Paints, in Corrosion, L. L. Shreir, editor(s), p. 15:24, Newnes-Butterworths, London, U.K. (1976). 8. G. W. Walter, Corrosion Science 26, p. 27 (1986). 9. R. T. Ruggeri and T. R. Beck, The Transport Properties of Polyurethane Paint, in Corrosion Control by Organic Coatings, H. Leidheiser, editor(s), p. 62, NACE, Houston, TX (1981). 10. G. Grundmeier and A. Simoes, Corrosion Protection by Organic Coatings, in the Manuscripts for Publication. 11. C. Perez, A. Collazo, M. Izquierdo, P. Merino, and X. P. Novoa, Progress in Organic Coatings 36, p. 102 (1999). 12. C. Perez, A. Collazo, M. Izquierdo, P. Merino, and X. P. Novoa, Progress in Organic Coatings 37, p. 169 (1999). 13. S. Duval, M. Keddam, M. Sfaira, A. Srhiri, and H. Takenouti, Journal of the Electrochemical Society 149, p. B520 (2002). 14. B. Liu, Y. Li, H. Lin, and C. -N. Cao, Corrosion 59, p. 817 (2003). 15. J. M. Hu, J. Q. Zhang, and C. N. Cao, Progress in Organic Coatings 46, p. 273 (2003). 16. F. Wong and R. G. Buchheit, Progress in Organic Coatings 51, p. 91 (2004). 17. Jin-Tao Zhang, Ji-Ming Hu, Jian-Qing Zhang, and Chu-Nan Cao, Progress in Organic Coatings 51, p. 145 (2004). 18. Jiho Kang and Gerald S. Frankel, Z. Phys. Chem. 219, p. 1519 (2005). 19. A. Baba, F. Kaneko, and R. C. Advincula, Colloids and Surfaces A: Physicochemical and Engineering Aspects 173, p. 39 (2000). 20. D. Kouznetsov, A. Sugier, F. Ropital, and C. Fiaud, Electrochimica Acta 40, p. 1513 (1995). 123 21. T. Nomura and O. Hattori, Analytica Chimica Acta 115, p. 323 (1980). 22. T. Nomura and M. Iijima, Analytica Chimica Acta 131, p. 97 (1981). 23. T. Nomura and M. Okuhara, Analytica Chimica Acta 142, p. 281 (1982). 24. A. Frignani, M. Fonsati, C. Monticelli, and G. Brunoro, Corrosion Science 41, p. 1217 (1999). 25. P. Schmutz and D. Landolt, Corrosion Science 41, p. 2143 (1999). 26. P. Schmutz and D. Landolt, Electrochimica Acta 45, p. 899 (1999). 27. M. R. Deakin and O. R. Melroy, Journal of the Electrochemical Society 136, p. 349 (1989). 28. E. Juzeliunas, K. Leinartas, M. Samuleviciene, A. Sudavicius, P. Miecinskas, R. Juskenas, and V. Lisauskas, Journal of Solid State Electrochemistry 6, p. 302 (2002). 29. C-F. Lin and K. R. Hebert, Journal of the Electrochemical Society 141, p. 104 (1994). 30. C. -O. A. Olsson, D. Hamm, and D. Landolt, Journal of the Electrochemical Society 147, p. 2563 (2000). 31. C. -O. A. Olsson and D. Landolt, Journal of the Electrochemical Society 148, p. B438 (2001). 32. D. Jope, J. Sell, H. W. Pickering, and K. G. Weil, Journal of the Electrochemical Society 142, p. 2170 (1995). 33. J. Telegdi, A. Shaban, and E. Kalman, Electrochimica Acta 45, p. 3639 (2000). 34. B. Muller, I. Forster, and W. Klager, Progress in Organic Coatings 31, p. 229 (1997). 35. K. Noda, J. Park, A. Nishikata, and T. Tsuru, QCM Study on Water Absorption in Organic Coatings, in Proceedings of the Symposium on Advances in Corrosion Protection by Organic Coatings III, I. Sekine, M. Kendig, D. Scantlebury, and D. Mills, editor(s), p. 172, The Electrochemical Society, Pennington, NJ (1997). 36. G. Z. Sauerbrey, J. Physik 155, p. 206 (1959). 37. Cody M. Berger and Clifford L. Henderson, Polymer 44, p. 2101 (2003). 38. Katherine E. Mueller, William J. Koros, Yvonne Y. Wang, and C. Grant Willson, SPIE 3049, p. 871 (1997). 39. Asoka Weerawardena, Calum J. Drummond, Frank Caruso, and Malcolm McCormick, Langmuir 14, p. 575 (1998). 40. Yongxiang Xu, Chuanwei Yan, Jie Ding, Yanmin Gao, and Chunan Cao, Progress in Organic Coatings 45, p. 331 (2002). 41. G. S. Frankel, J. R. Scully, and C. V. Jahnes, Journal of the Electrochemical Society 143, p. 1834 (1996). 42. Y. Baek and G. S. Frankel, Journal of the Electrochemical Society 150, p. B1 (2003). 43. Jiho Kang, G. S. Frankel, S. H. Goss, J. T. Grant, and R. A. Mantz, in preparation (2006). 44. D. A. Buttry, The Quartz Crystal Microbalance as an In-Situ Tool in Electrochemistry, in Electrochemical Interfaces: Modern Techniques for In-Situ
124 Interface Characterization, H. D. Abruna, editor(s), p. 542, VCH, New York, NY (1991). 45. M. D. Ward, Principles and Applications of the Electrochemical Quartz Crystal Microbalance, in Physical Electrochemistry: Principles, Methods, and Applications, I. Rubinstein, editor(s), p. 296, Marcel Dekker, New York, NY (1995). 46. Gamry Instruments, Can I Trust my EIS Measurements? - from EIS Accuracy Contour Plots, www.gamry.com (2002). 47. J. R. Macdonald, Impedance Spectroscopy, John Wiley & Sons, New York, NY (1987). 48. G. J. Brug, A. L. G. van Eeden, M. Sluyters-Rehbach, and J. H. Sluyters, J. Elecroanal. Chem. 176, p. 275 (1984). 49. D. M. Brasher and A. H. Kingsbury, J. Appl. Chem 4, p. 62 (1954). 50. I. Diaconu and D. Dorohoi, J. of Optoelectronics and Advanced Materials 7, p. 921 (2005).
125 Base pressure < 2 × 10-7 torr Working pressure ∼ 5 × 10-3 torr Pre-sputtering time 300 sec Flow rate of Ar 39.5 sccm DC power ∼ 320 W Deposition time 1200 sec Deposition rate ∼ 0.15 nm/sec Film thickness < 200 nm
Table 4.1: Main deposition conditions for Al thin films.
0.2 hr 1 hr 1.5 hr 6 hr
Rs (Ω) - - - - Rd (GΩ) 1.75 1.79 1.82 6.36 -1 T1 (nΩ ) 1.34 1.39 1.34 0.96 CPEc P1 0.87 0.86 0.87 0.92 Cal. Cc (nF) 1.52 1.61 1.53 1.13
Table 4.2: Fitted values using Circuit 1 (0.2 ∼ 6 hr) shown in Figure 4.7a.
126 6 hr 24 hr 96 hr 144 hr 264 hr
Rs (Ω) - - - - - Rd (GΩ) 0.92 1.58 0.31 0.25 0.15 -1 T2 (nΩ ) 0.72 0.55 0.55 0.56 0.54 CPEc P2 0.95 0.97 0.98 0.97 0.98 Cal. Cc (nF) 0.70 0.55 0.52 0.54 0.51 Rct (GΩ) 12.4 7.00 2.15 - - -1 T3 (nΩ ) 0.70 1.50 3.88 6.28 9.53 CPEdl P3 0.67 0.69 0.80 0.89 0.54 Cal. Cdl (nF) 2.03 4.31 6.69 - -
Table 4.3: Fitted values using Circuit 2 (6 ∼ 264 hr) shown in Figure 4.7b.
96 hr 144 hr 264 hr
Rs (Ω) - - - Rd (GΩ) 0.28 0.22 0.17 -1 T4 (nΩ ) 0.55 0.54 0.55 CPEc P4 0.98 0.98 0.98 Cal. Cc (nF) 0.52 0.51 0.52 Rl (GΩ) 0.18 0.14 0.12 -1 T5 (nΩ ) 2.51 6.23 8.96 CPEl P5 0.66 0.73 0.89 Cal. Cl (nF) 1.64 5.99 9.07 Rct (GΩ) 2.38 1.38 1.05 -1 T6 (nΩ ) 1.44 6.14 34.6 CPEdl P6 1.08 0.90 1.03 Cal. Cdl (nF) 1.32 7.79 30.8
Table 4.4: Fitted values using Circuit 3 (96 ∼ 264 hr) shown in Figure 4.7c.
127
Figure 4.1: The schematic of EQCM.
128
(a)
(b)
Figure 4.2: (a) Potential, (b) frequency changes with respect to time during 2 hr of exposure of the Au-deposited quartz with coating. Uncoated data was also presented in (a) and (b).
129
(a)
(b)
Figure 4.3: (a) Potential and (b) frequency changes of the coated Al sample up to 300 hr of exposure in water. Data at short time of exposure is also presented.
130
(a)
(b)
Figure 4.4: (a) Potential, (b) frequency changes of coated samples on the Al-deposited quartz up to 24 hr exposure in the aerated 0.5 M NaCl solution.
131
Figure 4.5: Bode plots of the coated Au sample up to 720 hr of exposure in aerated 0.5 M NaCl solution.
132
(a)
(b)
(c) Figure 4.6: (a), (b) Bode, and (c) Nyquist plots of the coated Al sample up to 300 hr of exposure in water. 133 Rs CPEc
Rd
(a)
Rs CPEc
Rd CPEdl
Rct
(b)
Rs CPEc
Rd CPEl
Rl CPEdl
Rct
(c) Figure 4.7: (a) Circuit 1, (b) Circuit 2, and (c) Circuit 3 used for equivalent circuit modeling.
134
CHAPTER 5
POTENTIOSTATIC PULSE TESTING FOR ASSESSMENT
OF EARLY COATING FAILURE
Potentiostatic Pulse Testing (PPT) was investigated as a technique to assess the early stage of organic coating failure. Current transients collected from dummy cells and real coated samples were fitted to an exponential decay function to evaluate the values of equivalent circuit parameters. PPT only provided values for only some of the circuit elements, whereas Electrochemical Impedance Spectroscopy (EIS) revealed most values of the assumed circuit elements. However, it was difficult to know a priori whether to use a one- or two-time-constant model for EIS data obtained from the real coated panels. Fast
Fourier Transform (FFT) analysis was used to transform the data generated in the time domain and compare the data measured by the EIS. The impedance spectra from the
Fourier transforms was over only a part of the frequency range accessible by the EIS, but the spectra from the two methods exhibited reasonably good agreement.
135 5.1. INTRODUCTION
Water can permeate all organic coatings to some extent [1-3]. Corrosion of coated materials occurs only after the adhesion between the coating and the substrate is broken due to water and oxygen permeation and subsequent electrochemical reactions at the interface. Thus, it is very crucial to study how to water and other ionic species diffuse (or permeate) through the coatings.
Electrochemical Impedance Spectroscopy (EIS) is one of the most frequently used techniques for providing information concerning the performance of organic coatings on metals. The complex impedance generated by EIS is often analyzed by fitting to the response of an equivalent electrical circuit composed of passive elements such as resistors, capacitors and inductors, thereby determining various characteristic properties of the coating. The low frequency impedance is commonly used for predicting the coating protectiveness. However, EIS has several disadvantages for evaluating the coating performance, such as the cost of EIS instrumentation and the time needed to perform the tests. Furthermore, EIS has limitations as a tool for early indication of coating failure because all EIS systems have an input impedance limit, above which they cannot make accurate measurements [4]. For protective intact coatings, or coatings that are just starting to fail, the low frequency impedance is typically higher than the input impedance of the EIS system for reasonably-sized samples. A good paint film exhibits a purely capacitive response, and changes in corrosion resistance cannot be sensed until a significant degradation occurs. As a result, neither the true impedance nor the initial decrease in impedance associated with early stages of failure can be determined.
136 Potentiostatic Pulse Testing (PPT) has been reported to be useful for monitoring the early stages of degradation of paint coatings [4, 5]. A square pulse of 0.1 ∼ 2.0 V was applied to a coated sample in a two-electrode cell with the reference electrode also acting as the counter electrode [4]. Values of charge transfer (Rct), pore (or defect, Rd)
11 2 resistances on the order of 10 Ω⋅cm and double layer capacitance (Cdl) on the order of
10-11 F/cm2 have been successfully determined with this technique [4]. This range of impedance is well beyond the typical range of standard EIS equipment. However, it is not possible to get all of the circuit components simultaneously by PPT when the time constants are very different. On the other hand, the unavailable components (e.g. the solution resistance (Rs) and coating capacitance (Cc)) are typically not critical for the determination of the overall corrosion resistance or the early stages of coating degradation [4]. PPT can be sensitive and cost efficient if a picoammeter is used instead of a potentiostat, and PPT data can be very useful for fast prediction of coating performance at the early stage of the coating failure.
The selection of the equivalent circuit is very critical in the analysis of PPT data as is also the case for the analysis of EIS data. Martin et al. reported that the equivalent circuit with two-time-constant that is widely used in the analysis of EIS data from painted samples is not good for the analysis of damaged organic coatings [6]. They claimed that each critical frequency associated with the time constants mathematically depends on all circuit component values [6].
It is well known that the impedance spectrum may be obtained by Laplace or
Fourier transforms of transient data recorded in the time domain [7, 8], which allows
137 maximum usage of experimental information with minimum ambiguity [9]. Some work on galvanostatic pulse testing with or without data analysis in the frequency domain has been reported [8, 10, 11], but little work has been done on PPT followed by appropriate frequency treatment.
The objective of this study is to show the applications and limitations of PPT from simulated cells composed of known circuit values and from real coated panels. Data treatment using Fourier transforms is shown to improve the analysis.
5.2. EXPERIMENTAL
Two different dummy cell circuits, which are wired electrical circuits with real resistors and capacitors, were used for mimicking the response of a typical coating. Type
I, shown in Figure 5.1, has one-time-constant and Type II, shown in Figure 5.2, has two- time-constant. The selection of each component value was based on the literature [12,
13]. Table 5.1 shows the circuit component values for three different Type II dummy cells (identified as A ∼ C) with fixed values of Rs, Cc, and Rd, but varying values of Cdl and Rct. Another set of values of the assumed equivalent circuits, which are expected to represent the early stages of degradation of paint coatings, were also used (Table 5.2).
Due to difficulties in obtaining the real electrical circuit components with these values, only computer simulation of EIS spectra and PPT current transients was performed using commercial software.
The Gamry FAS1 potentiostat, which is specially designed for measurements of high impedance systems such as protective coatings, was used to perform both
138 impedance and PPT measurements. For impedance measurement on the dummy cells, a
10 mV AC voltage was applied over the frequency range 105 ∼ 10-2 Hz. Several I/E ranges (30 pA to 30 nA) and data sampling rates (1 ∼ 100 Hz) were chosen for capturing the current transients during the PPT measurements. PPT measurements also were performed using a Keithley 487 picoammeter/voltage source. The potential pulses were applied and the current transients were collected via GPIB communication by means of a
LabVIEW software program that was developed for the instrument control and data acquisition. The data collection with the picoammeter was limited to about 1 Hz because of limitations of the instrument. The PPT current transients measured with the picoammeter were compared with those from the Gamry FAS1 potentiostat.
The tested painted sample was a chromate conversion coated AA2024-T3 panel coated with a 2 mils thick strontium-chromate-containing epoxy primer and a 3 mils thick epoxy topcoat without chromate. An area of 1 cm2 was exposed to aerated 0.5 M NaCl for up to 40 days. A carbon electrode was employed as counter and reference electrode.
EIS measurements on the painted sample were performed using the Gamry FAS1 potentiostat with a 10 mV signal about the Open Circuit Potential (OCP) over the frequency range of 105 ∼ 5 × 10-3 Hz. PPT measurements on the painted sample were made using the Gamry FAS1 potentiostat. The potential pulse was 10 mV and the data sampling rate was 1000 Hz. The frequency of the potential variation was adjusted to ensure stabilization of the current at the end of each pulse. Values of circuit elements were calculated from the current transients. Commercial Fast Fourier Transform (FFT)
139 software was used to transform the data on from the time domain to the frequency domain.
5.3. RESULTS AND DISCUSSION
5.3.1. EIS Measurements on Dummy Cells
Figure 5.3 shows impedance spectra collected from the Type II dummy cells A ∼
C described in Table 5.1. The impedance in the low frequency region decreases as Rct decreases and Cdl increases. Even though the Type II dummy cells have two-time- constant, only one is observed in the Bode plots. If such data were collected from a physical system, it is likely that a Type I equivalent circuit would be used to fit the data, and they can be fitted well to such a circuit. The results of fitting the data to a Type I circuit are given in Table 5.3. R1 should represent Rs, but the fitted values do not match it well because the ohmic limit was not reached in the frequency range used in the experiments. The fitted values of R2 are close to the values of Rd + Rct, but are all about 5
∼ 15 % higher than the expected values. The values of C1 are very close to those of Cc, but it is not possible to determine the values of Cdl by fitting the data to a single time constant circuit. If a Type II equivalent circuit is used to fit the data (which were in fact generated from a Type II circuit), it is possible to distinguish Rd from Rct and to determine
Cdl.
In summary, EIS fitting is very powerful because it can reveal most values of the assumed circuit elements. However, it is not easy to know a priori which equivalent
140 circuit to use. Typically, the simplest circuit needed to fit the data well is employed, but a too-simple circuit might result in misinterpretation of the data.
5.3.2. PPT Measurements on Dummy Cells
The current transients were recorded during PPT testing of the Type II dummy cells A ∼ C with values indicated in Table 5.1 using different full scale current ranges and sampling rates. Figure 5.4 shows typical current transients for dummy cells A ∼ C with
30 pA of current range and 1 or 100 Hz of sampling rate. The complete current transient associated with the application of a voltage step across the Type II circuit should exhibit an initial decay from a high current to an intermediate current followed by a second decay to a lower steady state current. However, it is not possible to observe the complete current transient under any of the conditions employed. For most conditions, a single decay was observed, in particular at lower current ranges and higher sampling rates.
Table 5.4 presents averages for all sampling rates and current full scales of the component values determined by the fitting the transients to a first order exponential decay function:
−kt I = I steady + (I initial − I steady ) × e (5-1)
V V 1 ⎛ 1 1 ⎞ step step ⎜ ⎟ where Iinitial = , I steady = and k = ⎜ + ⎟ , respectively. It was Rd Rd + Rct Cdl ⎝ Rd Rct ⎠ possible to fit Equation (5-1) to the exponential decays, and the fitted values of Cdl, Rd
141 and Rct are relatively close to the real values, as can be seen in Table 5.4. The fitted Rd value is off by a factor of 3 for circuit A, and Cdl is off by factors of 2 and 0.5 for circuits
A and C, respectively.
The time constant associated with the expected first decay in current should be
-7 associated with the product RsCc or 10 s for the dummy cells A ∼ C. Limitations in data acquisition rate prevented access to this early part of the transients, although an increase in sampling rate from 1 to 100 Hz clearly improved the resolution of the measured transient. The current transient decay measured on the time scale of a few seconds is believed to the response from the RdCdl combination. It is concluded that the current transients from the inner circuit components (Rd is connected in series with Cdl and Rct which are connected in parallel) using appropriate current range and sampling rate can be obtained by PPT, and the values obtained by fitting show fairly good agreement with actual values.
PPT measurements were also made on dummy cell D from Table 5.1 using both a picoammeter and a potentiostat. Picoammeters are typically cheaper and can measure smaller currents, but have a smaller bandwidth than potentiostats. Table 5.5 shows values of each circuit component determined by fitting to the first order exponential decay function. All values from the picoammeter are about three times higher or lower than the actual values, and the values from the potentiostat are closer to the actual values because of the higher collection rate and the shorter time delay involved with the potential changes.
142 In summary, PPT fitting based on the current transients generated using the appropriate current range and sampling rate can determine the inner components of a
Type II equivalent circuit. PPT using a picoammeter is found to be suitable, but the values determined using a potentiostat are much closer to the actual values.
5.3.3. Computer Simulation for Early States of Coating Failure
To better understand how current transients decay with time in a system that is representative of the early stages of coating failure, computer simulation using commercial software was employed. The programs used for this work were ZVIEW by
Scribner Associates Inc. and SIMetrix Intro by Catena Software Ltd.
Figure 5.5 shows EIS spectra for Type II equivalent circuits E ∼ H from Table 5.2 representing samples with different ratios of defect area. With increasing defect area ratio, the impedance in the low frequency regions becomes limited by the value of Rct. The measured impedance changes from one- to two-time-constant, even though all simulations were made with two-time-constant equivalent circuits. Again it is difficult to determine the best equivalent circuit to model a spectrum.
Simulated PPT current transients were successfully obtained using the equivalent circuits defined in Table 5.2 and fitted to a first order exponential decay function as shown in Figure 5.6. After an initial current spike due to the presence of the RsCc combination in the simulated cells, all currents decay exponentially. The peak and base currents and the time to reach the stable current values all increase with increasing defect area ratio. Table 5.6 summarizes the values determined by fitting simulated current
143 transients to a first order exponential decay function. The fitted values of Rd, Cdl and Rct are very close to the original values of the simulated cells. Again, not all the circuit parameters can be evaluated by PPT.
5.3.4. EIS and PPT Measurements for Real Coated Samples
Figure 5.7 shows impedance spectra for the real coated samples as a function of time in aerated 0.5 M NaCl. The low frequency Bode magnitude was observed to decrease with time. The spectra are all well below the open leads measurement for the
Gamry FAS1 potentiostat, indicating that the measurements are not limited by the equipment. Initially, the impedance exhibits a largely capacitive response with only a slight deviation in the phase at frequencies below 0.1 Hz. With time, the low frequency impedance decreases and the data look like one-time-constant spectra. After 37 days of immersion there is an indication of a second time constant, probably due to degradation of the coating. This behavior was is predicted by the calculated spectra in Figure 5.5, which show the development of a second time constant with increasing defect area.
Based on these observations, the data were fitted to the Types I and II equivalent circuits and the fitted values are shown in Table 5.7. The Type II circuit with two-time-constant is not valid before 4 days of immersion because of the sudden increase in Rd from the two-time-constant model between 1.5 and 4.0 days of immersion (R2 ∼ (Rd + Rct)).
However, the best equivalent circuit in the middle time period is still not clear.
PPT measurements were also made on the real coated samples using a potentiostat to control the potential and capture the data, which was done at a rate of 1,000 Hz and the
144 current range set on autorange. The current transients were fitted to Equation (5-1) and the values of Rd, Cdl and Rct were extracted. Figure 5.8 shows the raw PPT current transient and an expanded version with the fitted curve for the coated panel after 4 days of immersion. The results of all PPT fitting are summarized in Table 5.7 and Figure 5.9.
EIS is not able to differentiate Rd from Rct in very early stages whereas PPT shows reasonable changes of the fitted values. The circuit values determined by EIS and PPT merge over time as shown in Figure 5.9.
5.3.5. Frequency Domain Analysis
One of the common methods of analyzing electrochemical current signals is to transform them into the frequency domain using Fourier or Laplace transforms. The inverse of the sampling period represents the lower frequency limit of the transformed data. The upper frequency limit is one half of the inverse of the sampling interval due to the Nyquist sampling theorem [14]. In this work, the measurements were taken at 0.001 second intervals and the sampling period was selected as 8.192 seconds (the total number of digitized data is 8,192). Thus, the lower and upper limits of the resolution were 0.122 and 500 Hz, respectively. The impedance is determined from the ratio of the Fourier transforms of the potential and current [15, 16]:
F[]E(t) Z( jω) = K′⋅ (5-2) F[]I(t)
145 where K′ is a constant that depends on the implementation of the Fourier transform [15].
K′ is easily defined as the relationship between the Fourier transformed values of input and output signal when the frequency approaches zero.
Figure 5.10 shows the impedance spectra for the coated panel at different times of immersion in aerated 0.5 M NaCl determined by both EIS and Fourier transforms of the
PPT data. The Fourier transforms reveal only a part of the frequency range accessible by the EIS method, but the spectra from the two methods exhibit reasonably good agreement.
The Fourier transformed data exhibit scatter in the frequency range from around 10 to
100 Hz. This scatter could be reduced and frequency range extended by changing the sampling period and the data sampling window.
5.3.6. Comparison of EIS and PPT
Both EIS and PPT involve the analysis of current responses to time-varying potential changes. The data from both techniques can be fitted to equivalent electrical circuits to quantify the response. EIS fitting is very powerful because it can reveal most values of the assumed circuit elements. However, it is sometimes not easy to select the appropriate equivalent circuit to use in the fitting process. Some of the spectra measured on real circuits with values given in Table 5.1, as well as some of the simulated responses for circuits with values given in Table 5.2, exhibited one-time-constant behavior even though the circuits used to generate the spectra have two-time-constant. Two different resistances (Rd and Rct) in the circuits representing samples with varying ratios of defect area could not be distinguished in the small defect area circuits, and they merged into a
146 single resistance (R2). Such behavior complicates the EIS analysis of real samples that would exhibit an increase in defect area with time. On the other hand, the values of Rd,
Cdl and Rct determined by fitting the simulated PPT current transients were very close to the original values of the simulated cells for all circuits. The analysis of the PPT data is simpler because of the limitations of the technique. The important resistances in the Type
II circuit can be determined and distinguished, so the main benefits of the PPT technique are that it is simple and capable. Furthermore, EIS requires a potentiostat and frequency response analyzer or lock-in amplifier. However, PPT can be sensitive and cost efficient if a picoammeter is used instead of a potentiostat. In this study, both a picoammeter and a potentiostat were used for PPT measurements. The potentiostat provided results much closer to the real values because the most sensitive picoammeters do not have the dynamic response required to make measurements at very short times. Higher frequencies than those achievable in this study are possible if faster data interfaces are used.
The main disadvantage of PPT is that it is not capable of resolving all of the components of the two-time-constant circuit. However, the outer circuit elements, Rs and
Cc, are typically not critical for the determination of the overall corrosion resistance or the early stages of coating degradation. It is also noted that the selection of the sampling rate and current range in PPT measurement is very important to fit the data. Thus, repetitive tests should be performed with different measuring conditions to get the best fitting to current responses. For example, the Cdl value after 26 days of immersion fitted by PPT analysis shown in Figure 5.9b appears to be out of line with the other data. This
147 was probably due to the fitting of a current transient that was not fully-decayed to a stable current.
5.4. CONCLUSIONS
Potentiostatic Pulse Testing (PPT) was investigated using dummy cells, computer simulation, and measurements on coated panels. Similar experiments were performed using Electrochemical Impedance Spectroscopy (EIS) for comparison. The following conclusions can be made:
• Equivalent circuit modeling of both PPT and EIS data requires the assumption
of a particular circuit configuration, and the selection of the most appropriate
equivalent circuit is critical.
• PPT only provides values for the inner circuit components of a typical
equivalent circuit used to model a flawed coating. The coating capacitance is
not accessible.
• EIS generates data over a wider frequency range than PPT and it is possible to
extract values for all components of the equivalent circuit. However, it is
difficult to know a priori whether to use a one- or two-time-constant circuit to
model experimental data. Owing to the limitations in the PPT, only a one-
time-constant model can be used, so the analysis is simplified.
• PPT measurements were made using both a potentiostat and a picoammeter.
The potentiostat provided results much closer to the real values.
148 • Owing to the complications associated with equivalent circuit selection, the
data generated by EIS analysis on real coated samples during the early stages
of immersion were probably less accurate than those from PPT.
• Frequency domain analysis of the PPT current transients generated impedance
spectra over only a part of the frequency range accessible by the EIS method,
but the spectra from the two methods exhibited reasonably good agreement.
149
REFERENCES
1. G. W. Walter, Corrosion Science 26, p. 27 (1986). 2. G. W. Walter, Corrosion Science 32, p. 1041 (1991). 3. N. L. Thomas, Progress in Organic Coatings 19, p. 101 (1991). 4. P. C. Pistorius, in the Conference Paper of the 14th International Corrosion Council, Capetown, S.A., published on CD-ROM (1999). 5. R. D. Granata and K. J. Kovaleski, Evaluation of High-Performance Protective Coatings by Electrochemical Impedance and Chronoamperometry, in Electrochemical Impedance: Analysis and Interpretation, J. R. Scully, D. C. Silverman, and M. W. Kendig, editor(s), p. 450, ASTM, Philadelphia, PA (1993). 6. F J. Martin, K. E. Lucas, and C. E. Bevans, in the Conference Paper of the 5th Workshop on Quantitative Methods for Evaluating of Paint Coating Performance, Myrtle Beach, SC, p. 2 (2001). 7. J. L. Gilbert, Journal of Biomedical Materials Research 40, p. 233 (1998). 8. Cui Lu and Yan Peiyu, Corrosion Science 42, p. 675 (2000). 9. Arthur A. Pilla, Journal of the Electrochemical Society 117, p. 467 (1970). 10. N. Birbilis, K. M. Nairn, and M. Forsyth, Electrochimica Acta 49, p. 4331 (2004). 11. K. E. Heusler, M. Krebs, and K. Nachstedt, Werkstoffe und Korrosion 36, p. 484 (1985). 12. H. P. Hack and J. R. Scully, Journal of the Electrochemical Society 138, p. 33 (1991). 13. F. Mansfeld, Journal of Applied Electrochemistry 25, p. 187 (1995). 14. Alan V. Oppenheim, Ronald W. Schafer, and John R. Buck, Discrete-Time Signal Processing, Prentice-Hall, Inc., Upper Saddle River, NJ (1999). 15. R. Alvarez and M. Sanchez, Corrosion Reviews 20, p. 69 (2002). 16. D. D. Macdonald and M. C. H. McKubre, Impedance Measurements in Electrochemical Systems, in Modern Aspects of Electrochemistry - No. 14, J. O'M. Bockris, B. E. Conway, and R. E. White, editor(s), p. 131, Plenum Press, New York, NY (1982).
150 dummy A B C D cell identification
Rs (KΩ) 0.10 0.10 0.10 0.1
Cc (nF) 1.01 1.01 1.01 1.1
Rd (GΩ) 1.17 1.17 1.17 5
Cdl (nF) 0.10 1.01 10.02 0.6
Rct (GΩ) 10.23 1.17 0.10 10
Table 5.1: Component values of actual dummy cells in the Type II configuration.
dummy E (10-9) F (10-8) G (10-7) H (10-6) cell identification (Ad/A)
Rs (KΩ) 0.012 0.012 0.012 0.012
Cc (nF) 2 2 2 2
Rd (GΩ) 1.3 0.13 0.013 0.0013
Cdl (nF) 0.0013 0.013 0.13 1.3
Rct (GΩ) 130 13 1.3 0.13
Table 5.2: Component values of virtual dummy cells used for computer simulations.
151 dummy A B C cell identification
R1 (KΩ) 0.015 0.014 0.015
C1 (nF) 1.038 1.037 1.019
R2 (GΩ) 12 2.112 1.256
Table 5.3: Circuit element values determined by fitting EIS spectra measured on Type II dummy cells to a Type I equivalent circuit.
A B C
values actual fitted actual fitted actual fitted
Rs (KΩ) 0.10 - 0.10 - 0.10 -
Cc (nF) 1.01 - 1.01 - 1.01 -
Rd (GΩ) 1.17 2.974 1.17 1.114 1.17 1.084
Cdl (nF) 0.10 0.199 1.01 1.104 10.02 4.764
Rct (GΩ) 10.23 8.255 1.17 1.089 0.10 0.184
Table 5.4: PPT fitted values of the dummy cells using a first order exponential decay function.
152 Values measured by PPT Values measured by PPT
(potentiostat) (picoammeter) values actual fitted actual fitted
Rs (KΩ) 0.1 - 0.1 -
Cc (nF) 1.1 - 1.1 -
Rd (GΩ) 5.0 4.698 5.0 2.804
Cdl (nF) 0.6 0.589 0.6 2.047
Rct (GΩ) 10 9 10 35
Table 5.5: Several values of circuit parameters measured from the potentiostat and picoammeter.
-9 -8 -7 -6 E (Ad/A = 10 ) F (Ad/A = 10 ) G (Ad/A = 10 ) H (Ad/A = 10 )
values actual fitted actual fitted actual fitted actual fitted
Rs (KΩ) 0.012 - 0.012 - 0.012 - 0.012 -
Cc (nF) 2 - 2 - 2 - 2 -
Rd (GΩ) 1.3 1.569 0.13 0.157 0.013 0.016 0.0013 0.0016
Cdl (nF) 0.0013 0.0011 0.013 0.011 0.13 0.107 1.3 1.070
Rct (GΩ) 130 135 13 14 1.3 1.368 0.13 0.137
Table 5.6: PPT fitted values of the simulated cells with different ratios of the defect area using a first order exponential decay function.
153
)
ct Ω 35 24 22 R 223 (G 6.333 0.578
dl C (nF) 0.125 0.189 0.284 0.404 0.869 0.398 )
d Ω R PPT – Type II Circuit (G 3.939 3.510 3.361 3.282 3.180 0.455 )
ct Ω 65 58 24 R 156 (G Type I and II circuits 7.109 0.549
dl C (nF) 0.018 0.020 0.362 0.300 0.312 0.522 )
d Ω R 18 10 (G 0.159 0.144 4.991 0.548
c C (nF) EIS – Type II Circuit 0.064 0.069 0.074 0.079 0.075 0.074 )
s Ω coated panel. EIS data fitted to R (K 2.711 2.690 2.862 2.730 2.846 2.909 )
2 Ω R 64 52 23 11 153 (G 1.001
1 C (nF) 0.073 0.077 0.081 0.085 0.079 0.077 )
1 Ω R EIS – Type I Circuit (K 2.830 2.820 2.811 2.716 2.834 2.897
0 17 26 37 1.5 4.0 Time PPT data fitted to Type II circuit. Table 5.7: Analysis of data from real Table 5.7: Analysis
Table 0.1: Analysis of data from real coated panel. EIS data fitted to Type I and Type II circuits and PPT data fitted to Type II circuit.
154 R1 C1
R2
Figure 5.1: Type I equivalent circuit with one-time-constant.
Rs Cc
Rd Cdl
Rct
Figure 5.2: Type II equivalent circuit with two-time-constant.
155
Figure 5.3: The impedance data for dummy cells indicated in Table 5.1.
156
(a) Cell A – 1 Hz (b) Cell A – 100 Hz
(c) Cell B – 1 Hz (d) Cell B – 100 Hz
(e) Cell C – 1 Hz (f) Cell C – 100 Hz
Figure 5.4: Typical current transients for dummy cells A ∼ C indicated Table 5.1 with 30 pA of current range and 1 or 100 Hz of sampling rate.
157
Figure 5.5: EIS spectra with different ratios of the defect area indicated in Table 5.2.
158
Figure 5.6: Simulated current transients for simulated cells with different ratios of the defect area indicated in Table 5.2.
159
Figure 5.7: The impedance variations for real coated panel with immersion times.
160
(a)
(b)
Figure 5.8: (a) Measured current transient after application of –10 mV of the potential pulse during 30 sec and +10 mV next 30 sec on the 4 days immersed samples (b) curve fitting to a first order exponential decay function on the highlighted region of the (a).
161
(a)
(b)
(c)
Figure 5.9: Variations of the equivalent circuit parameters, such as (a) Rd, (b) Cdl, and (c) Rct. 162
(a)
(b)
(c) Figure 5.10: Impedance spectra from EIS measurements and Fourier transforms during (a) 1.5, (b) 4.0, and (c) 17 days of immersion. 163
CHAPTER 6
CONCLUSIONS AND FUTURE WORK
6.1. CONCLUSIONS
Thin film analogs of S phase were studied using Electrochemical Quartz Crystal
Microbalance (EQCM) at Open Circuit Potential (OCP) and under cathodic polarization in 0.5 M NaCl with additions of chromate, vanadate and NaOH. During immersion at
OCP in the chloride solution, the S phase film exhibited pitting corrosion and local Cu enrichment. The polarization curve indicated spontaneous formation of pits at open circuit. Corrosion current density measured by EQCM was higher than that obtained by extrapolation of the anodic and cathodic portions of the polarization curve. True cathodic current density at -1150 mVSCE was much higher than the net current density measured by the potentiostat, so the measured polarization curves were misleading. The net current density and the current density measured from the mass loss rate for S phase were reduced in chloride solutions containing 10-4 M of chromate or 0.1 M of vanadate. The mass change in the presence of chromate was greatly reduced. This effect was much greater than in the presence of vanadate, suggesting that chromate exhibited stronger inhibition effect on S phase than vanadate. Local pH measurement revealed the 164 development of high pH zone near the S phase under the cathodic polarization in 0.5 M
NaCl solution. Local pH on S phase in the vanadate-containing solution was higher than that in the chromate-containing solution.
Water uptake in polyurethane coatings was successfully observed using in situ
Electrochemical Quartz Crystal Microbalance (EQCM) and Electrochemical Impedance
Spectroscopy (EIS). Coated samples on the Au-deposited quartz exhibited an apparent mass increase with time over a period of hours. However, quantification of the frequency change associated with the mass change due to the water uptake was complicated by the limited sensitivity of the crystal to the water in the coating layer. For a coated sample on the Al-deposited quartz, apparent mass increases were observed after the completion of initial water uptake, indicating the formation of a corrosion product (or oxide layer) underneath coating. Additional circuit elements fitted well to the EIS data of the coated samples on the Al-deposited quartz, supporting the interpretation of the apparent mass increase after 100 hr of exposure measured by EQCM as being associated with the formation of a corrosion product (or oxide layer).
• Potentiostatic Pulse Testing (PPT) was investigated using dummy cells,
computer simulation, and measurements on coated panels. Similar
experiments were performed using Electrochemical Impedance Spectroscopy
(EIS) for comparison. Equivalent circuit modeling of both PPT and EIS data
requires the assumption of a particular circuit configuration, and the selection
of the most appropriate equivalent circuit is critical. PPT only provides values
for the inner circuit components of a typical equivalent circuit used to model a
165 flawed coating. The coating capacitance is not accessible. EIS generates data
over a wider frequency range than PPT and it is possible to extract values for
all components of the equivalent circuit. However, it is difficult to know a
priori whether to use a one- or two-time-constant circuit to model
experimental data. Owing to the limitations in the PPT, only a one-time-
constant model can be used, so the analysis is simplified. PPT measurements
were made using both a potentiostat and a picoammeter. The potentiostat
provided results much closer to the real values. Owing to the complications
associated with equivalent circuit selection, the data generated by EIS analysis
on real coated samples during the early stages of immersion were probably
less accurate than those from PPT. Frequency domain analysis of the PPT
current transients generated impedance spectra over only a part of the
frequency range accessible by the EIS method, but the spectra from the two
methods exhibited reasonably good agreement.
6.2. FUTURE WORK
This study has opened up several issues for the electrochemical studies of coatings and thin films as future work.
• As indicated in Table 3.7, various corrosion current densities were calculated
based on the assumptions made regarding the values of n* and M*. Therefore,
it is necessary to acquire the accurate values of n*, M*, and the mass loss
rates, im, only associated with faradaic process on the electrode to accurately
166 use the EQCM technique for the thin film analogs. In addition, it is very
important to quantify how much Cu is released and replated during OCP
immersion and potentiostatic exposure.
• In this study, chromate present at only 10-4 M was a much more effective
inhibitor than the 0.1 M vanadate addition. Since predominant dissolved
species of V5+ depends on the concentrations and pH of the vanadate, the
effect of vanadate concentrations should be considered in determining the
inhibition power and comparing this with chromate.
• It is likely that the EQCM does not sense the full change of the frequency
associated with the water uptake throughout the coating layer because
quantification of this change was complicated by the limited sensitivity of the
crystal to the water in the coating layer. Therefore, it is necessary to evaluate
the accurate sensitivity with different thickness of the coating layer for the
quantitative assessment of the kinetic of water uptake throughout the coating
layer.
• Equivalent circuit modeling was employed to explain the mass increase of the
polymer-coated Al-deposited quartz sample immersed in water, suggesting the
changes of the impedance of a corrosion product (or oxide layer) with time.
This can be much clearer if the state-of-art morphological observation using
an electron microscopy combined with the Focused Ion Beam (FIB) cross-
sectioning of the corrosion product (or oxide layer) underneath coatings can
be done.
167 • In this study, both a picoammeter and a potentiostat were used for PPT
measurements. However, the potentiostat provided results much closer to the
real values because the most sensitive picoammeters do not have the dynamic
response required to make measurements at very short times. Therefore, PPT
can be sensitive and cost efficient if higher frequencies than those achievable
in this study are possible by using faster data interfaces.
• The upper frequency limit of the FFT is calculated as one half of the inverse
of the sampling interval. However, these frequencies were not exactly same as
the frequencies obtained as presented in Figure 5.10. Therefore, it is necessary
to elaborate PPT technique on some of the data sampling restrictions for
proper FFT work.
168
BIBLIOGAPHY
Adzic, R. R., Electrocatalysis, in Recent Advances in the Kinetics of Oxygen Reduction, Lipkowski, J. and Ross, P. N., editor(s), p. 197, Wiley-VCH, New York, NY (1998).
Allen, N. S., Regan, C. J., McIntyre, R., Johnson, B. W. and Dunk, W. A. E., Progress in Organic Coatings 32, p. 9 (1997).
Alvarez, R. and Sanchez, M., Corrosion Reviews 20, p. 69 (2002).
Anderson, W. A. and Stumpf, H. C., Corrosion 36, p. 212 (1980).
Appel, A. and Appleby, A. J., Electrochimica Acta 23, p. 1243 (1978).
Appleby, A. J., Journal of Electroanalytical Chemistry. 357, p. 117 (1993).
Ashraf-Khorassani, M. and Braun, R. D., Corrosion 43, p. 32 (1987).
Baba, A., Kaneko, F. and Advincula, R. C., Colloids and Surfaces A: Physicochemical and Engineering Aspects 173, p. 39 (2000).
Baek, Y. and Frankel, G. S., Journal of the Electrochemical Society 150, p. B1 (2003).
Baumann, K., Plaste Kautsch 19, p. 455 (1972).
Baumann, K., Plaste Kautsch 19, p. 694 (1972).
Bautista, A., Progress in Organic Coatings 28, p. 49 (1996).
Bellucci, F. and Nicodemo, L., Corrosion 49, p. 235 (1993).
Bentley, J., Organic Film Formers, in Paint and Surface Coatings: Theory and Practice, Lambourne, R., editor(s), p. 41, Ellis Horwood Limited, Chichester, U.K. (1987).
Berger, Cody M. and Henderson, Clifford L., Polymer 44, p. 2101 (2003).
Bertocci, U., Journal of the Electrochemical Society 127, p. 1931 (1988).
169
Bertocci, U., Gabrielli, C., Huet, F. and Keddam, M., Journal of the Electrochemical Society 144, p. 31 (1997).
Bertocci, U., Gabrielli, C., Huet, F., Keddam, M. and Rousseau, P., Journal of the Electrochemical Society 144, p. 37 (1997).
Birbilis, N., Nairn, K. M. and Forsyth, M., Electrochimica Acta 49, p. 4331 (2004).
Birblis, N. and Buchheit, R. G., Journal of the Electrochemical Society 152, p. B140 (2005).
Blanc, C., Labelle, B. and Mankowski, G., Corrosion Science 39, p. 495 (1997).
Brasher, D. M. and Kingsbury, A. H., J. Appl. Chem 4, p. 62 (1954).
Brisard, G., Certrand, N., Ross, P. N. and Markovic, N. M., Journal of Electroanalytical Chemistry 480, p. 219 (2000).
Brito, P. S. D. and Sequeira, C. A. C., Journal of Power Sources 52, p. 1 (1994).
Brown, G. M. and Kobayashi, K., Journal of the Electrochemical Society 148, p. B457 (2001).
Bruckenstein, S. and Shay, M., Electrochimica Acta 30, p. 1295 (1985).
Brug, G. J., Eeden, A. L. G. van, Sluyters-Rehbach, M. and Sluyters, J. H., J. Elecroanal. Chem. 176, p. 275 (1984).
Buchheit, R. G., Journal of the Electrochemical Society 142, p. 3994 (1995).
Buchheit, R. G., Lecture Notes of MSE 881, The Ohio State University (2001).
Buchheit, R. G., Grant, R. P., Hlava, P. F., Mckenzie, B. and Zender, G. L., Journal of the Electrochemical Society 144, p. 2621 (1997).
Buchheit, R. G., Martinez, M. A. and Montes, L. P., Journal of the Electrochemical Society 147, p. 119 (2000).
Buchheit, R. G., Montes, L. P., Martinez, M. A., Michael, J. and Hlava, P. F., Journal of the Electrochemical Society 146, p. 4424 (1999).
Buchheit, R. G., Moran, J. P. and Stoner, G. E., Corrosion 46, p. 610 (1990).
170 Buchheit, R. G., Moran, J. P. and Stoner, G. E., Corrosion 50, p. 120 (1994).
Buttry, D. A., The Quartz Crystal Microbalance as an In-Situ Tool in Electrochemistry, in Electrochemical Interfaces: Modern Techniques for In-Situ Interface Characterization, Abruna, H. D., editor(s), p. 542, VCH, New York, NY (1991).
Chao, C. Y., Lin, C. F. and Macdonald, D. D., Journal of the Electrochemical Society 128, p. 1187 (1981).
Chen, C-T. and Skerry, B. S., Corrosion 47, p. 598 (1991).
Chen, G. S., Gao, M. and Wei, R. P., Corrosion 52, p. 8 (1996).
Chidambaram, D., Clayton, C. R. and Halada, G. P., Journal of the Electrochemical Society 151, p. B151 (2004).
Clark, W. J. and McCreery, R. L., Journal of the Electrochemical Society 149, p. B379 (2002).
Clark, W. J., Ramsey, J. D., McCreery, R. L. and Frankel, G. S., Journal of the Electrochemical Society 149, p. B179 (2002).
Cullity, B. D., Elements of X-Ray Diffraction, Addison-Wesley Publishing Company, Inc., Reading, MA (1978).
Deakin, M. R. and Melroy, O. R., Journal of the Electrochemical Society 136, p. 349 (1989).
Deflorian, F., Fedrizzi, L., Rossi, S. and Bonora, P. L., Electrochimica Acta 44, p. 4243 (1999).
Dehri, I. and Erbil, M., Corrosion Science 42, p. 969 (2000).
Despic, A. R., Radosevic, J., Dabic, P. and Kliskic, M., Electrochimica Acta 35, p. 1743 (1990).
Diaconu, I. and Dorohoi, D., J. of Optoelectronics and Advanced Materials 7, p. 921 (2005).
Dimitrakopoulos, L. T., Dimitrakopoulos, T., Alexander, P. W., Logic, D. and Hibbert, D. B., Analytical Communications 35, p. 395 (1998).
Dimitrov, N., Mann, J. A., Vukmirovic, M. and Sieradzki, K., Journal of the Electrochemical Society 147, p. 3283 (2000). 171
Duval, S., Keddam, M., Sfaira, M., Srhiri, A. and Takenouti, H., Journal of the Electrochemical Society 149, p. B520 (2002).
Fontana, M. G., Corrosion Engineering, McGraw-Hill, New York, NY (1986).
Frankel, G. S., Kang, J. and Baek, Y., in the Conference Paper of the CORROSION NACExpo 2003, San Diego, CA, paper number 03394 (2003).
Frankel, G. S., Scully, J. R. and Jahnes, C. V., Journal of the Electrochemical Society 143, p. 1834 (1996).
Fraser, H. L., Lecture Notes of MSE 655, The Ohio State University (2001).
Frignani, A., Fonsati, M., Monticelli, C. and Brunoro, G., Corrosion Science 41, p. 1217 (1999).
Funke, W., Blistering of Paint Films, in Corrosion Control by Organic Coatings, Leidheiser, H., editor(s), p. 29, NACE, Houston, TX (1981).
Funke, W., Ind. Eng. Chem. Prod. Res. Dev. 24, p. 343 (1985).
Furlong, D. N., Quartz Crystal Microbalance, in Modern Characterization Methods of Surfactant Systems, Binks, B. P., editor(s), p. 485, Marcel Dekker, New York, NY (1999).
Galvele, J. R. and Micheli, S. M. de De, Corrosion Science 10, p. 795 (1970).
Gamry Instruments, Can I Trust my EIS Measurements? - from EIS Accuracy Contour Plots, www.gamry.com (2002).
Gao, M., Feng, C. R. and Wei, R. P., Metallurgical and Materials Transactions A 29A, p. 1145 (1998).
Genz, O., Lohrengel, M. M. and Schultze, J. W., Electrochimica Acta 39, p. 179 (1994).
Gilbert, J. L., Journal of Biomedical Materials Research 40, p. 233 (1998).
Golubev, A. I. and Ronzhin, M. N., Electrochemical and Corrosion Behavior of Aluminum-Base Binary Alloys and Intermetallic Compounds, in Corrosion of Metals and Alloys, Tomashov, N. D. and Mirolyubev, E. N., editor(s), p. 48, Israel Program for Scientific Translations, Jerusalem, Israel (1966).
Granata, R. D. and Kovaleski, K. J., Evaluation of High-Performance Protective Coatings by Electrochemical Impedance and Chronoamperometry, in Electrochemical 172 Impedance: Analysis and Interpretation, ASTM STP 1188, Scully, J. R., Silverman, D. C. and Kendig, M. W., editor(s), p. 450, ASTM, Philadelphia, PA (1993).
Grauer, R. and Wiedmer, E., Werkstoffe Korrosion 31, p. 550 (1980).
Grundmeier, G., Schmidt, W. and Stratmann, M., Electrochimica Acta 45, p. 2515 (2000).
Grundmeier, G. and Simoes, A., Corrosion Protection by Organic Coatings, in the Manuscripts for Publication.
Guillaumin, V. and Mankowski, G., Corrosion Science 41, p. 421 (1999).
Hack, H. P. and Scully, J. R., Journal of the Electrochemical Society 138, p. 33 (1991).
Heusler, K. E., Krebs, M. and Nachstedt, K., Werkstoffe und Korrosion 36, p. 484 (1985).
Hillman, A. R., The Electrochemical Quartz Crystal Microbalance, in Encyclopedia of Electrochemistry, Bard, A. J., Stratmann, M. and Unwin, P. R., editor(s), p. 233, WILEY- VCH Verlag GmbH & Co. KGaA, Weinheim, Germany (2003).
Hirayama, R. and Haruyama, S., Corrosion 47, p. 952 (1991).
Hladky, K. and Dawson, J. L., Corrosion Science 21, p. 317 (1981).
Hladky, K. and Dawson, J. L., Corrosion Science 22, p. 231 (1982).
Hoch, G. M., A Review of Filiform Corrosion, in Localized Corrosion, Staehle, R. W., Brown, B. F., Kruger, J. and Agrawal, A., editor(s), p. 134, NACE, Houston, TX (1974).
Hu, J. M., Zhang, J. Q. and Cao, C. N., Progress in Organic Coatings 46, p. 273 (2003).
Huisert, M., Electrochemical Characterisation of Filiform Corrosion on Aluminium Rolled Product, Ph. D. Thesis, TU Delft, Delft, Netherlands (2001).
ICDD Database
Ilevbare, G. O., Schneider, O., Kelly, R. G. and Scully, J. R., Journal of the Electrochemical Society 151, p. B453 (2004).
Ilevbare, G. O. and Scully, J. R., Corrosion 57, p. 134 (2001).
Ilevbare, G. O. and Scully, J. R., Journal of the Electrochemical Society 148, p. B196 (2001).
173 Ilevbare, G. O., Scully, J. R., Yuan, J. and Kelly, R. G., Corrosion 56, p. 227 (2000).
Jeffs, R. A. and Jones, W., Additives for Paint, in Paint and Surface Coatings: Theory and Practice, Lambourne, R., editor(s), p. 212, Ellis Horwood Limited, Chichester, U.K. (1987).
Jones, D. A., Principles and Prevention of Corrosion, Prentice Hall, Upper Saddle River, NJ (1996).
Jope, D., Sell, J., Pickering, H. W. and Weil, K. G., Journal of the Electrochemical Society 142, p. 2170 (1995).
Juzeliunas, E., Leinartas, K., Samuleviciene, M., Sudavicius, A., Miecinskas, P., Juskenas, R. and Lisauskas, V., Journal of Solid State Electrochemistry 6, p. 302 (2002).
Kang, J., A Study on the Metastable Pitting Behavior of Austenitic Stainless Steel by the Electrochemical Noise Analysis, Master Thesis, Korea Advanced Institute of Science and Technology, Taejon, Korea (2000).
Kang, Jiho, Frankel, G. S., Goss, S. H., Grant, J. T. and Mantz, R. A., in preparation (2006).
Kang, Jiho and Frankel, Gerald S., Z. Phys. Chem. 219, p. 1519 (2005).
Kendig, M. W. and Jeanjaquet, S., Journal of the Electrochemical Society 149, p. B47 (2002).
Kendig, M. W., Mansfeld, F. and Tsai, C. H., Corrosion 47, p. 964 (1991).
Kendig, M. and Yan, C., Journal of the Electrochemical Society 151, p. B679 (2004).
King, F., Quinn, M. J. and Litke, C. D., Journal of Electroanalytical Chemistry 385, p. 45 (1995).
Koehler, E. L., Corrosion Under Organic Coatings, in Localized Corrosion, Staehle, R. W., Brown, B. F., Kruger, J. and Agrawal, A., editor(s), p. 117, NACE, Houston, TX (1974).
Koehler, E. L., Corrosion 33, p. 209 (1977).
Kolics, A., Besing, A. S. and Wieckowski, A., Journal of the Electrochemical Society 148, p. B322 (2001).
174 Kouznetsov, D., Sugier, A., Ropital, F. and Fiaud, C., Electrochimica Acta 40, p. 1513 (1995).
Kriksunov, L. B., Macdonald, D. D. and Millett, P. J., Journal of the Electrochemical Society 141, p. 3002 (1994).
Lambourne, R., Paint Composition and Applications - A General Introduction, in Paint and Surface Coatings: Theory and Practice, Lambourne, R., editor(s), p. 23, Ellis Horwood Limited, Chichester, U.K. (1987).
Lambourne, R., Solvents, Thinners, and Diluents, in Paint and Surface Coatings: Theory and Practice, Lambourne, R., editor(s), p. 194, Ellis Horwood Limited, Chichester, U.K. (1987).
Leard, R. R., The Electrochemical Behavior of Copper Bearing Intermetallics and the Corrosion of AA2024-T3, Master Thesis, The Ohio State University, Columbus, OH (2001).
Leblanc, P. and Frankel, G. S., Journal of the Electrochemical Society 149, p. B239 (2002).
Lebozec, N., Persson, D., Nazarov, A. and Thierry, D., Journal of the Electrochemical Society 149, p. B403 (2002).
Leclere, T. J. R. and Newman, R. C., Journal of the Electrochemical Society 149, p. B52 (2002).
Leidheiser, H., Journal of Coating Technology 53, p. 29 (1981).
Leidheiser, H., Corrosion 38, p. 374 (1982).
Leidheiser, H., Corrosion 39, p. 189 (1983).
Leidheiser, H., Journal of Coating Technology 63, p. 21 (1991).
Leidheiser, H. and Suzuki, I., Journal of the Electrochemical Society 128, p. 242 (1981).
Liao, C. -M., Olive, J. M., Gao, M. and Wei, R. P., Corrosion 54, p. 451 (1998).
Liao, C. -M. and Wei, R. P., Electrochimica Acta 45, p. 881 (1999).
Lin, C-F. and Hebert, K. R., Journal of the Electrochemical Society 141, p. 104 (1994).
Lin, S., Shih, H. and Mansfeld, F., Corrosion Science 9, p. 1331 (1992). 175
Liu, B., Li, Y., Lin, H. and Cao, C. -N., Corrosion 59, p. 817 (2003).
Lu, Cui and Peiyu, Yan, Corrosion Science 42, p. 675 (2000).
Lunder, O. and Nisancioglu, K., Corrosion 44, p. 414 (1988).
Macdonald, D. D. and McKubre, M. C. H., Impedance Measurements in Electrochemical Systems, in Modern Aspects of Electrochemistry - No. 14, Bockris, J. O'M., Conway, B. E. and White, R. E., editor(s), p. 131, Plenum Press, New York, NY (1982).
Macdonald, J. R., Impedance Spectroscopy, John Wiley & Sons, New York, NY (1987).
Mansfeld, F., Journal of Applied Electrochemistry 25, p. 187 (1995).
Mansfeld, F., Han, L. T. and Lee, C. C., Journal of the Electrochemical Society 143, p. L286 (1996).
Mansfeld, F., Han, L. T., Lee, C. C., Chen, C., Zhang, G. and Xiao, H., Corrosion Science 39, p. 255 (1997).
Mansfeld, F. and Lee, C. C., Journal of the Electrochemical Society 144, p. 2068 (1997).
Mansfeld, F. and Tsai, C. H., Corrosion 47, p. 958 (1991).
Martin, F J., Lucas, K. E. and Bevans, C. E., in the Conference Paper of the 5th Workshop on Quantitative Methods for Evaluating of Paint Coating Performance, Myrtle Beach, SC, p. 2 (2001).
Mattsson, E., Gullman, L. -O., Knutsson, L., Sundberg, R. and Thundal, B., Br. Corros. J. 6, p. 73 (1971).
Mayne, J. E. O., Anti-Corrosion October, p. 3 (1973).
Mayne, J. E. O., The Mechanism of the Protective Action of Paints, in Corrosion, Shreir, L. L., editor(s), p. 15:24, Newnes-Butterworths, London, U.K. (1976).
Mazurkiewicz, B. and Piotrowski, A., Corrosion Science 23, p. 697 (1983).
McGovern, W. R., Schmutz, P., Buchheit, R. G. and McCreery, R. L., Journal of the Electrochemical Society 147, p. 4494 (2000).
Meisterjahn, P., Konig, U. and Schultze, J. W., Electrochimica Acta 34, p. 551 (1989).
176 Mol, J. M. C., Hinton, B. R. W., Weijde, D. H. Van der, Wit, J. H. W. de and Zwaag, S. Van der, Journal of Materials Science 35, p. 1629 (2000).
Moon, S. -M. and Pyun, S. -I., Corrosion Science 39, p. 399 (1997).
Morita, J. and Yoshida, M., Corrosion 50, p. 11 (1994).
Mueller, Katherine E., Koros, William J., Wang, Yvonne Y. and Willson, C. Grant, SPIE 3049, p. 871 (1997).
Muller, B., Forster, I. and Klager, W., Progress in Organic Coatings 31, p. 229 (1997).
Muller, I. L. and Galvele, J. R., Corrosion 17, p. 179 (1977).
Murray, J. N., Progress in Organic Coatings 31, p. 225 (1997).
Nakayama, Y., Progress in Organic Coatings 33, p. 108 (1998).
Nisancioglu, K., Journal of the Electrochemical Society 137, p. 69 (1990).
Nisancioglu, K. and Lunder, O., Significance of the Electrochemistry of Al-Base Intermetallics in Determining the Corrosion Behavior of Aluminum Alloys, in Aluminum Alloys Their Physical and Mechanical Properties, Starke, E. A. and Sanders, T. H., editor(s), p. 1125, West Midlands, U.K. (1986).
Noda, K., Park, J., Nishikata, A. and Tsuru, T., QCM Study on Water Absorption in Organic Coatings, in Proceedings of the Symposium on Advances in Corrosion Protection by Organic Coatings III, Sekine, I., Kendig, M., Scantlebury, D. and Mills, D., editor(s), p. 172, The Electrochemical Society, Pennington, NJ (1997).
Nomura, T. and Hattori, O., Analytica Chimica Acta 115, p. 323 (1980).
Nomura, T. and Iijima, M., Analytica Chimica Acta 131, p. 97 (1981).
Nomura, T. and Okuhara, M., Analytica Chimica Acta 142, p. 281 (1982).
Nylen, P. and Sunderland, E., Modern Surface Coatings - A Textbook of the Chemistry and Technology of Paints, Varnishes, and Lacquers, Interscience Publishers, London, U.K. (1965).
Obispo, H. M., Murr, L. E., Arrowood, R. M. and Trillo, E. A., Journal of Materials Science 35, p. 3479 (2000).
177 Olsson, C. -O. A., Hamm, D. and Landolt, D., Journal of the Electrochemical Society 147, p. 2563 (2000).
Olsson, C. -O. A. and Landolt, D., Journal of the Electrochemical Society 148, p. B438 (2001).
Oppenheim, Alan V., Schafer, Ronald W. and Buck, John R., Discrete-Time Signal Processing, Prentice-Hall, Inc., Upper Saddle River, NJ (1999).
Park, S. H., Winnick, J. and Kohl, P. A., Journal of the Electrochemical Society 148, p. A346 (2001).
Perez, C., Collazo, A., Izquierdo, M., Merino, P. and Novoa, X. P., Progress in Organic Coatings 36, p. 102 (1999).
Perez, C., Collazo, A., Izquierdo, M., Merino, P. and Novoa, X. P., Progress in Organic Coatings 37, p. 169 (1999).
Pilla, Arthur A., Journal of the Electrochemical Society 117, p. 467 (1970).
Pistorius, P. C., in the Conference Paper of the 14th International Corrosion Council, Capetown, S.A., published on CD-ROM (1999).
Pourbaix, M., Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE, Houston, TX (1974).
Radosevic, J., Kliskic, M., Dabic, P., Stevanovic, R. and Despic, A., Journal of Electroanalytical Chemistry 277, p. 105 (1990).
Ramgopal, T., Schmutz, P. and Frankel, G. S., Journal of the Electrochemical Society 148, p. B348 (2001).
Ramsey, J. D. and McCreery, R. L., Corrosion Science 46, p. 1729 (2004).
Ritter, J. J. and Kruger, J., Studies on the Subcoating Environment of Coated Iron Using Qualitative Ellipsometric and Electrochemical Techniques, in Corrosion Control by Organic Coatings, Leidheiser, H., editor(s), p. 28, NACE, Houston, TX (1981).
Rolinson, J. F., Pigments for Paint, in Paint and Surface Coatings: Theory and Practice, Lambourne, R., editor(s), p. 111, Ellis Horwood Limited, Chichester, U.K. (1987).
Ross, T. K. and Wolstenholme, J., Corrosion Science 17, p. 341 (1977).
Rüdiger, A. and Köster, U., Journal of Non-Crystalline Solids 250-252, p. 898 (1999). 178
Ruggeri, R. T. and Beck, T. R., The Transport Properties of Polyurethane Paint, in Corrosion Control by Organic Coatings, Leidheiser, H., editor(s), p. 62, NACE, Houston, TX (1981).
Ruggeri, R. T. and Beck, T. R., Corrosion 39, p. 452 (1983).
Saito, F., Nagashima, Y., Wei, L., Itoh, Y., Goto, A. and Hyodo, T., Applied Surface Science 194, p. 13 (2002).
Sauerbrey, G. Z., J. Physik 155, p. 206 (1959).
Schmidt, W. and Stratmann, M., Corrosion Science 40, p. 1441 (1998).
Schmutz, P. and Frankel, G. S., Journal of the Electrochemical Society 145, p. 2285 (1998).
Schmutz, P. and Frankel, G. S., Journal of the Electrochemical Society 145, p. 2295 (1998).
Schmutz, P. and Frankel, G. S., Journal of the Electrochemical Society 146, p. 4461 (1999).
Schmutz, P. and Landolt, D., Electrochimica Acta 45, p. 899 (1999).
Schmutz, P. and Landolt, D., Corrosion Science 41, p. 2143 (1999).
Schneider, O., Ilevbare, G. O., Scully, J. R. and Kelly, R. G., Journal of the Electrochemical Society 151, p. B465 (2004).
Scully, J. R., Journal of the Electrochemical Society 136, p. 979 (1989).
Scully, J. R., Knight, T. O., Buchheit, R. G. and Peebles, D. E., Corrosion Science 35, p. 185 (1993).
Scully, J. R., Peebles, D. E., Romig, A. D., Frear, D. R. and Hills, C. R., Metallurgical Transactions A 23A, p. 2641 (1992).
Seri, O. and Imaizumi, M., Corrosion Science 30, p. 1121 (1990).
Silcock, J. M., Journal of the Institute of Metals 88, p. 357 (1959-60).
Simaranov, A. Yu, Sokolova, T. I., Marshakov, A. I. and Mikhailovskii, Yu. N., Protection of Metals 27, p. 329 (1991). 179
Slabaugh, W. H., Dejager, W. and Hoover, S. E., Journal of Paint Technology 44, p. 76 (1972).
Steinsmo, U. and Bardal, E., Corrosion 48, p. 910 (1992).
Stickle, W. F., The Use of Chemometrics in AES and XPS Data Treatment, in Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy, Briggs, D. and Grant, J. T., editor(s), p. 377, IM Publications and SurfaceSpectra Limited, West Sussex and Manchester, U.K. (2003).
Stickle, W. F. and Watson, D. G., J. Vac. Sci. Technol. A 10, p. 2806 (1992).
Strbac, S. and Adzic, R. R., Electrochimica Acta 41, p. 2903 (1996).
Sugimoto, K., Hoshino, K., Kaeyama, M. and Sawada, Y., Corrosion Science 15, p. 709 (1975).
Suter, T. and Alkire, R. C., Journal of the Electrochemical Society 148, p. B36 (2001).
Tada, E., Sugawara, K. and Kaneko, H., Electrochimica Acta 49, p. 1019 (2004).
Taguchi, M. and Sugita, H., Journal of Power Sources 109, p. 294 (2002).
Takahashi, H., Fujiwara, K. and Seo, M., Corrosion Science 36, p. 689 (1994).
Taylor, R. J. and Humffray, A. A., Journal of Electroanalytical Chemistry 64, p. 63 (1975).
Taylor, R. J. and Humffray, A. A., Journal of Electroanalytical Chemistry 64, p. 85 (1975).
Telegdi, J., Shaban, A. and Kalman, E., Electrochimica Acta 45, p. 3639 (2000).
Thomas, N. L., Progress in Organic Coatings 19, p. 101 (1991).
Tosten, M. K., Vasudevan, A. K. and Howell, P. R., Metallurgical and Materials Transactions A 19A, p. 51 (1988).
Touhsaent, R. E. and Leidheiser, H., Corrosion 28, p. 435 (1972).
Twite, R. L. and Bierwagen, G. P., Progress in Organic Coatings 33, p. 91 (1998).
Urushino, K. and Sugimoto, K., Corrosion Science 19, p. 225 (1979). 180
Ven, E. P. G. T. Van de and Koelmans, H., Journal of the Electrochemical Society 123, p. 143 (1976).
Vetter, K. J., Electrochemical Kinetics - Theoretical and Experimental Aspects, Academic Press, New York, NY (1967).
Vukmirovic, M. B., Dimitrov, N. and Sieradzki, K., Journal of the Electrochemical Society 149, p. B428 (2002).
Wagner, K., Brankovic, S. R., Dimitrov, N. and Sieradzki, K., Journal of the Electrochemical Society 144, p. 3545 (1997).
Walter, G. W., Corrosion Science 26, p. 27 (1986).
Walter, G. W., Corrosion Science 26, p. 39 (1986).
Walter, G. W., Corrosion Science 26, p. 681 (1986).
Walter, G. W., Corrosion Science 32, p. 1041 (1991).
Walter, G. W., Corrosion Science 32, p. 1059 (1991).
Walter, G. W., Corrosion Science 32, p. 1085 (1991).
Ward, M. D., Principles and Applications of the Electrochemical Quartz Crystal Microbalance, in Physical Electrochemistry: Principles, Methods, and Applications, Rubinstein, I., editor(s), p. 296, Marcel Dekker, New York, NY (1995).
Weerawardena, Asoka, Drummond, Calum J., Caruso, Frank and McCormick, Malcolm, Langmuir 14, p. 575 (1998).
Wei, R. P., Liao, C. -M. and Gao, M., Metallurgical and Materials Transactions A 29A, p. 1153 (1998).
Weijde, D. H. Van der, Westing, E. P. M. Van and Wit, J. H. W. de, Electrochimica Acta 41, p. 1103 (1995).
Westing, E. P. M. van, Ferrari, G. M. and Wit, J. H. W. de, Corrosion Science 34, p. 1511 (1993).
Westing, E. P. M. van, Ferrari, G. M. and Wit, J. H. W. de, Corrosion Science 36, p. 979 (1994).
181 Wicks, Z. W., Jones, F. N. and Pappas, S. P., Organic Coatings Science and Technology, Wiley-Interscience, New York, NY (1999).
Williams, D. B., Microstructural Characteristics of Al-Li Alloys, in Alumium-Lithium Alloys, Sanders, T. H. and Starke, E. A., editor(s), p. 89, AIME, New York, NY (1981).
Williams, D. B. and Carter, C. B., Transmission Electron Microscopy, Plenum Press, New York, NY (1996).
Wit, J. H. W. de, Inorganic and Organic Coatings, in Corrosion Mechanisms in Theory and Practice, Marcus, P. and Oudar, J., editor(s), p. 595, Marcel Dekker, New York, NY (1995).
Wolstenholme, J., Corrosion Science 13, p. 521 (1973).
Wong, F. and Buchheit, R. G., Progress in Organic Coatings 51, p. 91 (2004).
Xu, Yongxiang, Yan, Chuanwei, Ding, Jie, Gao, Yanmin and Cao, Chunan, Progress in Organic Coatings 45, p. 331 (2002).
Yang, H., Kwon, K., Devine, T. M. and Evans, J. W., Journal of the Electrochemical Society 147, p. 4399 (2000).
Yang, H. and McCreery, R. L., Journal of the Electrochemical Society 147, p. 3420 (2000).
Yeager, E., Krouse, P. and Rao, K. V., Electrochimica Acta 9, p. 1057 (1964).
Zamin, M., Corrosion 37, p. 627 (1981).
Zhang, Jin-Tao, Hu, Ji-Ming, Zhang, Jian-Qing and Cao, Chu-Nan, Progress in Organic Coatings 51, p. 145 (2004).
182