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..........................................................................