EXFOLIATION CORROSION KINETICS OF HIGH STRENGTH ALUMINUM ALLOYS DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in the Graduate School of The Ohio State University By Xinyan Zhao, M.S. ***** The Ohio State University 2006 Dissertation Committee: Dr. Gerald S. Frankel, Adviser Approved by Dr. Rudolph G. Buchheit ____________________________ Dr. Michael J. Mills Adviser Dr. Stanislav I. Rokhlin Graduate Program in Materials Science and Engineering ABSTRACT The objectives of this study were to quantitatively study localized corrosion, especially exfoliation corrosion (EFC), of high strength aluminum alloys and to investigate the mechanism of exfoliation corrosion with a focus on the effects of alloy temper, microstructure, relative humidity (RH) and mechanical stress. A new technique, Exfoliation of Slices in Humidity (ESH) was developed for the determination of exfoliation corrosion (EFC) susceptibility and quantification of EFC kinetics. Two AA7178 plates taken from the wingskin of a retired KC135 airplane were used as test samples. Slices of the plate were pretreated by potentiostatic polarization in chloride solution to develop localized corrosion sites. Subsequent exposure to high humidity after pretreatment of properly oriented and unconstrained samples resulted in the development of EFC at the edges of the slices. The EFC kinetics was determined by measuring the width of the central unattacked region of the samples. The ESH results were representative of the different EFC behavior of the two plates during outdoor exposure. These results show the capability of the ESH test to discriminate between plates of varying susceptibility and to determine EFC rates ii quantitatively. Optical microscopy and analytical TEM were used to investigate the effect of microstructure and local chemistry at grain boundary on EFC susceptibility. Alloys with more elongated grain shape are more susceptible to EFC and a high Zn content in the grain boundary precipitate free zone relates to a high susceptibility. The effects of RH, temper and applied stress on EFC kinetics of AA7178 were investigated by ESH tests. The critical RH for EFC propagation in AA7178 was found to be about 56% and the EFC kinetics increased with RH. The effect of temper in ESH test was consistent with published results from EXCO tests, but they provide a quantitative description of the temper effect. The effects of applied compressive and tensile stresses on EFC kinetics were studied using a four-point bending jig. Compression accelerated EFC significantly and tension reduced kinetics. An equation describing the effects of RH, stress, and time on EFC kinetics was developed based on the ESH results using Eyring model. In situ X-ray radiography was used to characterize intergranular and exfoliation corrosion in high strength Al alloys. The samples were either exposed to sodium chloride solution (NaCl) at a controlled potential or to high humidity after an electrochemical pretreatment in NaCl solution. Intergranular corrosion (IGC) growth rates in solution for samples encased in epoxy and exposed to solution were distributed over a range with an upper limit equal to the rates determined by the foil penetration technique. Unencased AA2024-T3 samples that were held at a controlled potential in solution exhibited IGC and then exfoliation corrosion. AA2024 and AA7178 samples encased in epoxy developed sharp intergranular fissures during exposure to 96% RH following an electrochemical pretreatment. Unencased samples given the same iii treatment exhibited exfoliation in the high humidity environment. In situ X-ray radiography of intergranular corrosion attack provides a wide range of intergranular corrosion kinetics including the fastest growing sites that can be detected by foil penetration technique. This method is a good approach for visualizing the EFC process. iv To my father, Tianhai Zhao and my mother, Fengqin Zhao v ACKNOWLEDGMENTS I would like to express my sincere thanks to my adviser, Dr. Gerald Frankel, for his guidance, inspiring motivation, and helps during the supervision of this project. I have been fortunate to study under his tutelage. Not only his knowledge, but also his passion towards aircraft corrosion has been inspired me a lot through 5 years of graduate study. Next, I would like to thank Dr. Rudy Buchheit, for his participation on my academic advisory committee and for comments and suggests on my research. I am also grateful for a lot of good comments and suggestions given by Dr. Michael Mills and Dr. Stanislav Rokhlin, both on my advisory committee. I would like to acknowledge funding from the Aging Aircraft Division of ASC in support of the Aeronautical Enterprise Structures Strategy with a contract through S&K Technologies. My special thanks go to Dr. Bahman Zoofan from Welding department for his kind help and corporation with the use of X-ray radiography facility. Dr. Bahman Zoofan has been spending many hours working with me and contributes a great amount of work to this effort. I also want to thank Mr. Andrew Bonifas for his input to this project. He helped and designed the four point bending apparatus for studying mechanical effect on exfoliation corrosion. I also wish to thank Mr. Jim Suzel at S&K vi Technology for providing the AA7178 samples and Dr. William Abbott of Battelle for performing the environmental exposure testings for these samples. I would like to thank all members of Fontana Corrosion Center: Dr. Weilong Zhang, Dr. Wenping Zhang, Dr. Qingjiang Meng, Dr. Xiaodong Liu, Dr. Tsai-Shang Huang, Ms. Hong Guan, and many current FCC members. I would also like to thank Mr. Henk Colijn, Mr. Cameron Begg and Dr. Lisa Hommel, who helped and trained me how to use TEM, FIB, SEM and XPS. I would like to thank Ms. Dena Bruedigam, Ms. Chris Putnam for their office help. I would like to acknowledge Mr. Steve Bright, Mr. Ken Kushner for their helps in preparing my experimental samples. I also wish to thank Dr. Suliman Dregia, Mr. Mark Cooper, Ms. Mei Wang, and Ms. Wendy Ranney for their kind help with my academic problems. Finally, I would like to thank my parents for their unwavering support and encouragement throughout my educational years. vii CURRICULUM VITA July 1999………………………………………B.S. Materials Physics, University of Science and Technology Beijing, China. March 2003……………………………………M.S. Materials Science and Engineering, The Ohio State University. 2000-present……………………………………Graduate Research Associate, Materials Science and Engineering, The Ohio State University. PUBLICATIONS 1. X.Zhao, G.S.Frankel, B.Zoofan, and S.I.Rokhlin, “In-Situ X-Ray Radiographic Study of Intergranular Corrosion in Al Alloys,” Corrosion, 59,1012-1018 (2003) 2. X.Zhao, G.S.Frankel, “The Visual Determination of Exfoliation Rate of Al Alloy Slices in Humidity”, The Journal of Corrosion Science and Engineering, Volume 6 Paper C134, 2003, The paper was presented at the conference “Corrosion Science in the 21st Century” held at UMST in July 2003. 3. X. Zhao, T. Huang, G. S. Frankel, B. Zoofan, and S. I. Rokhlin, " Intergranular Corrosion Morphology and Growth Kinetics In High Strength Aluminum Alloys," in Critical Factors in Localized Corrosion IV, S. Virtanen, P. Schmutz, and G. S. Frankel, eds, PV 2002-04, The Electrochemical Society, 2003. 4. T.Huang, X.Zhao, Gerald Frankel, B.Zoofan and S.Rokhlin, “Growth Kinetics of Intergranular and Exfoliation Corrosion in AA7178,” proceedings of Triservice Corrosion Conference, Las Vegas, 2003 viii 5. Tsai-Shang Huang, Xinyan Zhao, and G.S.Frankel, “Localized Corrosion Growth Kinetics in AA7xxx Alloys, “2005 Triservice Corrosion Conference Proceedings, 2005 6. G.S.Frankel, Tsai-Shang Huang, and Xinyan Zhao, “Localized Corrosion Growth Kinetics in AA7178,” P.Marcus, ed., Elsevier, 2005 FIELDS OF STUDY Major Field: Materials Science and Engineering ix TABLE OF CONTENTS Page ABSTRACT...................................................................................................................II ACKNOWLEDGMENTS ............................................................................................ VI VITA .......................................................................................................................VIII LIST OF TABLES .....................................................................................................XIII LIST OF FIGURES....................................................................................................XIV CHAPTERS: 1. INTRODUCTION ..................................................................................................1 2. LITERATURE REVIEW........................................................................................3 2.1 BASIC FORMS OF CORROSION OF ALUMINUM ALLOYS ........................................4 2.2 PITTING .............................................................................................................5 2.3 EXFOLIATION CORROSION..................................................................................6 2.3.1 The Mechanism of Exfoliation Corrosion .................................................6 2.3.2 Factors That Influence EFC....................................................................11 2.3.2.1 Effects of Alloying Elements on EFC..........................................11 2.3.2.2 Effects of The Grain Shape on Exfoliation Corrosion..................12 2.3.2.3 Effect of Heat Treatment on Susceptibility to Exfoliation Corrosion....................................................................................16