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FORMATION and BREAKDDOWN of CHROMATE CONVERSION COATINGS on Al-Zn-Mg-Cu 7X75 ALLOYS

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FORMATION and BREAKDDOWN of CHROMATE CONVERSION COATINGS on Al-Zn-Mg-Cu 7X75 ALLOYS FORMATION AND BREAKDDOWN OF CHROMATE CONVERSION COATINGS ON Al-Zn-Mg-Cu 7X75 ALLOYS DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Yuhchae Yoon, M.S. * * * * * The Ohio State University 2004 Dissertation Committee: Dr. Rudolph G. Buchheit, Adviser Approved by Dr. Gerald S. Frankel ____________________________ Dr. Michael J. Mills Adviser Graduate Program in Mat. Sci. & Eng. Dr. Philip J. Grandinetti ABSTRACT The objective of this study was to characterize the formation and breakdown of chromate conversion coatings (CCCs) on aluminum alloys Al-Zn-Mg-Cu 7075 and 7475 with a focus on the effect of alloy temper, alloy purity and selected coating processing variables. Overall, results consistently pointed to a slight temper effect. Conversion coated AA7475-T7 was significantly more corrosion resistant than conversion coated AA7475-T6. In AA7075, there was only a slight difference in corrosion resistance between the two tempers. This was attributed to the effect of constituent particles on coating formation and breakdown, which are present to a much greater extent in AA7075 than in AA7475. The difference in the corrosion resistance between the T6 and T7 tempers in the coated and uncoated conditions is about the same suggesting that the origin of any “temper effect” in conversion coated materials is ultimately due to the intrinsic change in corrosion susceptibility of the alloy itself. Thicker coating formed on AA7475-T7 has the effect of increasing corrosion resistance, which could be associated with the 860 cm-1 Raman intensity band. Studies were also conducted with alloys in retrogression and reaged tempers and the W temper. Results with these tempers were mixed and no general conclusions could be drawn. In terms of electrochemically derived measures of corrosion resistance (electrochemical impedance, and pitting potential ii measurements), the magnitude of the temper effect was about the same as the effect due to the purity difference between AA7075 and AA7475. The temper effect was less significant than effects due to increasing coating time from 1 to 3 minutes, withholding certain substrate precleaning steps, or withholding key ingredients from the coating bath. Scanning probe microscopy, scanning Kelvin probe force microscopy (SKPFM) and scanning electron microscopy were used to characterize coating formation in the vicinity of constituent intermetallic particles (IMPs) present in the alloys. Coatings formed on IMPs exhibited different morphologies and were much thinner that coatings formed on the matrix. Coatings were thinnest on S phase particles, and in post-coating exposure to aggressive chloride environments coating breakdown was almost always associated with these particles. The difference in coating thickness between particles and the surrounding matrix was established within the first tens of seconds of coating. The thickness differential was then observed to remain constant for the remainder of the coating immersion time. Although coatings were thinner on IMPs, they did confer some level of protection. SKPFM measurements showed that the difference in the Volta potential measured between the matrix and the particles was strongly reduced by the conversion coating suggesting that intact coatings reduced any tendencies for microgalvanic coupling. Thin film analogs of two important intermetallic compound particles, η (MgZn2) and S (Al2CuMg), were synthesized and analyzed using quartz crystal microbalance weight change measurements, potentiodynamic polarization and Raman spectroscopy. MgZn2 compounds were prepared with different levels of Cu to simulate the compositional change that occurs when 7X75 alloys are artificially aged. Results showed iii that these phases are electrochemically active and the conversion coatings reduce activity only slightly. The coatings that do form on these compounds are thin and may have lower hexavalent Cr contents than coatings that form elsewhere. With increasing amounts of Cu dissolved in the η phase, coatings are even thinner and possibly more defficient in Cr6+. Multi-element electrode array experiments were used to electrochemically characterize coating formation and breakdown. Coating formation was characterized by an episode of measurable electrochemical activity on the array lasting for about 30 seconds. This was followed by a period of little measurable electrochemical activity. During the episode of electrochemical activity, individual electrode elements in the array exhibited transients that were predominantly anodic, predominantly cathodic or mixed. In subsequent potentiodynamic polarization of these coated electrodes in a chloride environment there was a weak dependence of the pitting potential on the character of the current transient with electrodes exhibiting cathodic transients having slightly more noble pitting potentials than electrodes that had anodic or mixed transients. iv Dedicated to My loving wife, Chimi Woo, My beloved son, John Hyunsung Yoon, My parents and parents-in-law, And Elohim v ACKNOWLEDGMENTS I would like to express my heartfelt thanks to my advisor, Dr. Rudy Buchheit, who has given me profound guidance, inspiring motivation, excellent advice, and support over past five years. I deeply appreciate not only his knowledge for research but also his demeanor as a scholar. I would like to thank specially to Dr. Jerry Frankel and Dr. Michael Mills for their helps as academic advisory committee for comments and suggests on my research. I would like to give special thanks to all members of Fontana Corrosion Center: Dr. Patrick Leblanc, Dr. Valerie Laget, Dr. Christian Pagila, Dr. Thodla Ramgopal, Dr. Weilong Zhang, Dr. Wenping Zhang, Dr. Qingjiang Meng, Mr. Rob Fecke, Mr. Ryan Leard, Mr. Younghoon Baek and many current FCC members. I owe a special thanks to Jiho Kang who gave me much help for EQCM, flash evaporation instrument, and computer and also to Ms. Belinda Hurley from Department of Chemistry for Raman Spectroscopy. I also thank to Ms. Cindy Flores, Ms. Christine Putnam, Ms. Dena Bruedigam, and Ms. Susan Meager for their administrative help and assistance. To all of the faculty and staff of the Materials Science and Engineering I wish to give my thanks for their teaching and help. vi I wish to extend my thanks to all church members of the Korean Mission Lane Ave Baptist Church and to my friends in the Christian Graduate Student Alliance for their love and prayer, especially to Pastor Chun and Bob Trube. Thanks also go to the staffs of the Korean Students Abroad in USA. 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 couldn’t be here. Most of all, I would like to give my wholehearted thanks to my loving wife, Chimi who has been with me from freshman year at Yonsei University as life partner for her emotional supports and devoted love. I also thank my beloved son, Johnny who has been joy to me after his birth and forever. Lastly, I wish to thank God who created and saved me. vii VITA July 16, 1970........................................................Born – Pusan, Korea 1996......................................................................B.S. Metallurgical Engineering, Yonsei University, Seoul, Korea 1998......................................................................M.S. Metallurgical Engineering, Yonsei University, Seoul, Korea 1999 - present ......................................................Graduate Research Associate The Ohio State University 2002......................................................................M.S. Materials Science and Engineering, The Ohio State University PUBLICATIONS 1. Y. Yoon, V. Laget, and R.G. Buchheit. "The Effect of Artificial Heat Treatment on the Chromate Conversion Performance of Al-Zn-Mg-Cu alloys,” p.363, Tri- Service Corrosion, San Antonio, TX (2002). 2. Y. Yoon and R.G. Buchheit, “The Formation and Breakdown of Chromate Conversion Coatings in Al-Zn-Mg-Cu alloy using Electrode Arrays,” Proceedings of the 204th Meeting of the Electrochemical Society, Philadelphia, PA (2003). FIELDS OF STUDY Major Field: Materials Science and Engineering viii TABLE OF CONTENTS page Abstract............................................................................................................................... ii Acknowledgments.............................................................................................................. vi Vita…. ..............................................................................................................................viii List of Tables .................................................................................................................... xii List of Figures.................................................................................................................. xiv Chapters: 1. Introduction.................................................................................................................. 1 2. Literature review.......................................................................................................... 8 2.1 7xxx Aluminum Alloys ......................................................................................... 9 2.1.1 Historical Development of 7xxx series ..........................................................
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