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Photographs included in the original manuscript have t>een reproduced xerographically in this copy. Higher quality 6" x 9" black and white photographic prints are availat)le for any photographs or illustrations appearing in this copy for an additional charge. Contact UMI directly to order. Bell & Howell Information arxl Learning 300 North Zeeb Road, Ann Aitx>r, Ml 48106-1346 USA 800-521-0600 UMT FORMATION AND FUNCTION OF CHROMATE CONVERSION COATING ON AIRCRAFT ALUMINUM ALLOY PROBED BY VIBRATIONAL SPECTROSCOPY DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Lin Xia, B.S. ***** The Ohio State University 2000 Dissertation Committee: Approved by Professor Richard L. McCreery, Adviser Professor Gerald S. Frankel Adviser Professor Terry L. Gustafson Department of Chemistry UMI Number 9971664 UMI UMI Microform9971664 Copyright 2000 by Bell & Howell Information and Leaming Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. Bell & Howell Information and Leaming Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor. Ml 48106-1346 ABSTRACT A Chromate Conversion Coating (CCC) is currently one of the most effective methods for protecting aluminum alloys from corrosion. Its unique “self-healing” property has been proved to be critical in corrosion prevention. During the formation process, Cr^' is “stored” in the CCC films. Under in-field conditions, most of the Cr^‘ can leach out and diffuse to local defects, and stop corrosion. However, the involvement of highly toxic Cr'^' makes CCC system environmentally hazardous. In order to find less- toxic alternatives, the formation and protection mechanisms of CCC must be imderstood. Formation and function of CCC film are the focus of this study, and vibrational spectroscopy was chosen due to its superior structural sensitivity. First, the structure of CCC film was characterized. The structural similarity between CCC film and a synthetic Cr-mixed-oxide was found, and certain tests were conducted on the bulk synthetic powder which were not feasible on the thin film. All of the structural studies indicated that CCC film is mainly a Cr'“-hydroxide gel layer, which adsorbs Cr'^'-oxy species through Cr"'-0-Cr'^' chemical bonds. Further analysis revealed the reversible Cr'"-Cr'^’ adsorption-desorption equilibrium, and a mathematical model ("Langmuir" model) was established to explain the Cr^' storage-release mechanism quantitatively. In addition, the function o f Fe(CN)6^ , an additive in the coating solution, was studied. The results indicate that Fe(CN)6^' mediates the slow reaction between A1 and Cr^', and the mediation mechanism can be illustrated as below: Fe(CN)6^-+Al = Fe(CN)6'‘‘+ t______ I, Fe(CN)6"' + Cr^' = Fe(CN)6^'+ Cr'" In general, the formation of CCC is mediated by Fe(CN)6^', thus A1 reduces Cr^' quickly and generates Cr‘"-hydroxide on the alloy surface. The nascent Cr'"-hydroxide is chemically active enough to form chemical bonds with Cr^' from the solution, through Cr'"-0-Cr^' bonding. Such Cr”‘-0-Cr'^* structure can form and break up reversibly according to the Langmuir model, providing mobile Cr'^' for in-field protection. Ill To the memory o f my father. To my mother, my husband. and my fam ily.. IV ACKNOWLEGMENTS The course of pursuing a dream is never easy. From that elementary school in central China to the laser lab in Newman Wolfrom, each step of this journey had been made possible by the help from a lot of people. The first person I'd like to say "thank you" to is my advisor. Dr. Richard L. McCreery. His support, encouragement, and enthusiasm had made this thesis possible. His guidance and wisdom are essential in the completion of this project. Meanwhile, he provided a fnendly environment for studying and doing research, and that really helped me in appreciating the joy of scientific research. My sincere gratitude also goes to Dr. Gerald S. Frankel, for his informative and inspiring discussions, and also for his help in several other aspects. The scientific conversions with Dr. Martin Kendig and Dr. Eiji Akiyama were also very exciting. I want to thank Dr. Terry L. Gustafson for his kindness and help, and Dr. Gordon Rankes for his suggestions at the early stage of my research. McCreery group’s members, who had been a very important part of my life during the past few years, deserve more than a “thank you ”. The former group members, Tze-Chi Kuo, HHY, Kristen Frost, Jun Zhao, Yi-Chun Liu, Ken Ray, Pei-Hong Chen, and Angle Horn, had been very kind to me and offered a lot of help, especially TC, who had taught me how to use almost all the spectrometers and software in the lab. Special thanks also go to Jeremy Ramsey and Stacy Du Vail, for their friendship, kindness, and understanding. I will always remember the witty conversions and the sparkling discussions with them. I appreciate the help from the other current group members. Bill McGovern (whose vivid mimic always made me laugh), Srikanth Ranganathan (a real smart gentleman), Ilson Steidel (who is very kind and hardworking). Bill Clark (who had released me from the strenuous “hammer-bandage” procedure), and Jingya Wu (whose happiness always brighten up the sky over everyone). The new girls, Belinda Hurley, and Yang-Yan Hu, also offered a lot of help and are sincerely appreciated. I’m obliged to my mother, my brothers, and sisters. Without their endless support and sacrifice, all these would be impossible. I’m very grateful for everything they had done for me. Finally, but not the least, I want to thank my husband, Hai-Ying, who had been with me through good and bad times, his presence in my life makes all the hard work meaningful. VI VITA January 27, 1971 ..................................... Bom - Shanghai, China 1993........................................................... B.S. Chemistry, Fudan University, Shanghai, China 1993-1995 ............................................... Graduate Research and Teaching Assistant, Fudan University, Shanghai, P. R. China 1995-present ............................................ Graduate Teaching and Research Assistant, The Ohio State University PUBLICATIONS 1. L. Xia, and R. L. McCreery, “Structure and Function of Ferricyanide in the Formation of Chromate Conversion Coatings on Aluminum Aircraft Alloy”, J. Electrochem. Soc., 146(10), 3696-3701(1999) 2. L. Xia, R. L. McCreery, “Chemistry of a Chromate Conversion Coating on Aluminum Alloy AA2024-T3 Probed by Vibrational Spectroscopy”, J. Electrochem. S'oc., 145(9), 3083-3089 (1998) 3. Kang-Nian Fan, Wen-Ning Wang, Lin Xia, Shi-Lin Pan, “Studies on the Harmonic Force Field and Vibrational Spectra of y-picolinic Acid and Its Isotopic Derivatives with Ab Initio Calculations”, Huaxue Xuebao 53(7), 637-44 (1995) (Chinese) FIELDS OF STUDY Major Field: Chemistry VII TABLE OF CONTENTS page Abstract............................................................................................................................ ii Dedication ....................................................................................................................... iv Acknowledgments ........................................................................................................... v V ita................................................................................................................................... vii List of Tables .................................................................................................................. xii List of Figures ................................................................................................................. xiv List of symbols and abbreviations ............................................................................... xix Chapters: 1. Introduction ............................................................................................................. 1 1.1 General introduction ................................................................................... 1 2 Fundamental corrosion science .................................................................. 4 1.2.1 Mixed potential theory and polarization resistance method ... 4 1.2.2 “Autocatalytic” pitting mechanism ............................................. 8 vm 1.3 Application of spectroscopic techniques in corrosion chemistry studies.......................................................................................................... 8 1.4 Brief introduction to Chromium