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CORROSION PROTECTION PROVIDED by TRIVALENT CHROMIUM PROCESS CONVERSION COATINGS on ALUMINUM ALLOYS By

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CORROSION PROTECTION PROVIDED BY TRIVALENT CHROMIUM PROCESS CONVERSION COATINGS ON ALUMINUM ALLOYS By Liangliang Li A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of Chemical Engineering – Doctor of Philosophy 2013 ABSTRACT CORROSION PROTECTION PROVIDED BY TRIVALENT CHROMIUM PROCESS CONVERSION COATINGS ON ALUMINUM ALLOYS By Liangliang Li High strength aluminum alloys are widely used in aviation and aerospace industries because of their desirable strength/weight ratio that results from the alloy addition ( e.g. , Cu, Fe etc ). However, this alloy addition causes pitting corrosion of the aluminum surrounding those intermetallic inclusions. One efficient way to inhibit the corrosion is through the protective coating system that contains the topcoat, primer, and conversion coating. The conversion coating is in direct contact with the alloy surface and is expected to provide both good corrosion protection and adhesion. The chromate conversion coating (CCC) has been widely used in aviation/aerospace industry and provides excellent active corrosion protection and adhesion to aluminum alloys. Unfortunately, the Cr(VI) is toxic and chromate is a carcinogen. Therefore, a trivalent chromium process (TCP) coating was developed as a drop-in replacement of CCC. This dissertation focuses on a fundamental understanding of the formation mechanism, chemical structure, and basic electrochemical properties of the TCP coating on three high strength aluminum alloys: AA2024-T3, AA6061-T6, and AA7075- T6. The formation of the TCP coating is driven by an increase in the interfacial pH. The coating is about 50-100 nm thick and has a biphasic structure consisting of a 3- ZrO 2/Cr(OH) 3 top layer and an AlF 6 /Al(OH) 3 interfacial layer. The coating contains hydrated channels and or defects. The TCP coating provides good corrosion protection to all three aluminum alloys evidenced by e.g. , increased Rp, decreased icorr , and higher Epit for the coated alloys than uncoated samples. However, the protection by TCP is increased for AA2024-T3 < AA7075-T6 < AA6061-T6. The protection mechanisms include (i) reducing oxygen reduction kinetics, (ii) partially blocking the mass transfer of dissolved O 2 and ions to the metal, and (iii) insulating the electron transfer through the non-conductive oxide layer. The TCP coating (Cr(III)/Zr-based) provides better corrosion protection compared to other non-Cr alternatives of CCC, e.g. , Ti/Zr- and Zn/Zr-based conversion coatings, Another significant finding of this work is that the TCP coating provides some 2- active corrosion protection by transiently forming Cr(VI) species ( e.g. , CrO 4 ). The formation involves two steps: (i) O2 diffuses through the defects to the Cu-rich intermetallic sites where it is reduced to H 2O2; (ii) H2O2 oxidizes nearby Cr(III) to Cr(VI). The insights gained from this fundamental study provide information to improve the corrosion protection provided by TCP. For example, the coating contains defects through which dissolved O 2 and ions can still diffuse to the underlying metal. Therefore, its corrosion protection can be improved by (i) compressing the coating structure and (ii) reducing the alloy sites where defects form. The former is achieved by optimizing the curing temperature and time and the corrosion resistance is increased by ~4×; the latter is through optimization of the deoxidation/desmutting time and solution and the corrosion protection property is improved by ~5×. ACKNOWLEDGEMENTS My success in graduate school was made possible by the combined effort of a number of people. I would like to first thank my advisor, Dr. Greg Swain. He continually and convincingly conveyed a spirit of adventure in regard to research and an excitement as to teaching. He provided me with a research environment that encouraged me to solve scientific problems. His understanding and patience supported me through my research. I have been awarded the Dissertation Completion Fellowship, which would have been impossible without his advising and mentoring. I enjoyed working with him and am sincerely grateful for him and his guidance. I would also like to thank my committee professors, Dr. Scott Barton, Dr. Lawrence Drzal, and Dr. Andre Lee. They provided professional guidance and generous advice. I also thank several other great professors, Dr. Gerald Frankel and Dr. Rudy Buchheit from The Ohio State University (OSU), Dr. Mark Orazem from University of Florida, Dr. Wei Lai from Michigan State University (MSU) for their generous advice, persistent encouragement, and insightful comments. I would also like to take this opportunity to thank Dr. Robert Ofoli who was my first advisor in graduate school. Without his understanding and support during my transferring group, I could not get the chance to work in my interested research fields. My sincere thanks also go to my past and current group members for their sharing information and experiences. Dr. Doo Young Kim and Dr. Matthew Fhaner encouraged and helped train me to become the scientist I am today. I would also send iv thanks to David Woodbury, Brandon Whitman, Dr. Xingyi Yang, Marion France, LeighAnn Tomaswick, Yan Zhu, Chen Qiu and Catharine Munson along with all other Swain Group member. I would like to thank my friends I made during my graduate school. I cannot gain the success without their support, understanding, and help. I thank Xinting Lin, Huang Wu, Ruiguo Yang, Yan Li, Pitichon Klomjit (OSU), Zhicao Feng (OSU), Chao Cheng, and Heyi Hu for their academic and personal help. I thank Dr. Stanley Flegler, Carol Flegler, Abigail Tirrell, and Per Askeland for their technical support. Finally, I would like to thank my family, especially my parents for their support and understanding when I had to decline family functions for work reasons. I cannot achieve so much without their encouragement and motivation. v TABLE OF CONTENTS LIST OF TABLES ........................................................................................................... x LIST OF FIGURES........................................................................................................ xii CHAPTER 1. INTRODUCTION AND BACKGROUND ................................................... 1 1.1 Aluminum Alloys ............................................................................................... 1 1.1.1 Designation system, chemical compositions, and mechanical properties .. 1 1.1.2 Applications ............................................................................................... 3 1.2 Corrosion and Corrosion Protection for Aluminum Alloys ................................. 4 1.2.1 Corrosion types .......................................................................................... 4 1.2.2 Corrosion protection methods .................................................................... 7 1.3 Chromate Conversion Coatings ...................................................................... 12 1.4 Non-Chromate Conversion Coatings .............................................................. 14 1.5 Metal Surface Pretreatment ............................................................................ 17 1.5.1 Cleaning/degreasing ................................................................................ 17 1.5.2 Etching ..................................................................................................... 17 1.5.3 Deoxidizing/desmutting ............................................................................ 18 1.6 Research Objective and Aims ......................................................................... 19 CHAPTER 2. EXPERIMENTAL SECTION ................................................................... 22 2.1 Reagents......................................................................................................... 22 2.2 Preparation of TCP-coated aluminum alloys ................................................... 23 2.3 Physical and Chemical Characterization ......................................................... 24 2.3.1 Raman Spectroscopy............................................................................... 24 2.3.2 X-ray Photoelectron Spectroscopy .......................................................... 24 2.3.3 Auger Depth Profiling ............................................................................... 25 2.3.4 Scanning Electron Microscopy / Energy-Dispersive X-ray Spectroscopy 25 2.3.5 Transmission Electron Microscopy .......................................................... 25 2.3.6 Atomic Force Microscopy ......................................................................... 26 2.3.7 Spectroscopic ellipsometry ...................................................................... 26 2.3.8 Contact angle measurement .................................................................... 27 2.4 Electrochemical Characterization .................................................................... 27 2.5 In situ pH Measurement .................................................................................. 36 2.5.1 Tungsten Microelectrode Preparation ...................................................... 36 2.5.2 OCP traces of the tungsten microelectrode in buffer solutions ................ 37 2.5.3 Calibration Curves of OCP vs. pH ........................................................... 38 2.6 Accelerated corrosion (degradation) tests ...................................................... 40 2.6.1 Full immersion test ..................................................................................
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