Mork Family Department of Chemical Engineering and Materials Science 2007 Student Symposium Evaluation of the Corrosion Resistance of Evaluation of the Corrosion Resistance of Anodized Aluminum Anodized Aluminum Samples Using Samples Using Electrochemical Impedance Spectroscopy (EIS) Electrochemical Impedance Spectroscopy (EIS) Yuelong Huang, Esra Kus and Prof. Florian Mansfeld Corrosion and Environmental Effects Laboratory (CEEL) The Mork Family Department of Chemical Engineering and Materials Science University of Southern California, Los Angeles, CA 90089-0241 I. Introduction I. Introduction The purpose of this study is to evaluate the corrosion resistance of anodized layers produced by new anodizing and sealing process using electrochemical impedance spectroscopy (EIS). The scanning electron microscope (SEM) was employed to determine the surface structure and the thickness of the anodized layers. The impedance spectra were analyzed using an appropriate equivalent circuit (EC). There was very little change in the impedance for all test samples during exposure period. The results of these tests suggest that anodized Al 6061 treated in the new processes was very corrosion resistant during extended exposure time periods. Anodized Aluminum (Al 6061) Samples Anodized Aluminum (Al 6061) Samples 1. R&D Tank Mixed Acid two samples tested in 3.5wt% NaCl for 14 days 2. Type III two samples tested in 3.5wt% NaCl for 14 days 3. Production Tank three samples tested in 3.5wt% NaCl for 365 days Anodized aluminum interface model Anodized aluminum interface model Equivalent circuit for the analysis of the impedance spectra for anodized Al samples. C po : Capacitance of the outer porous layer C b : Capacitance of the inner barrier layer R po : Pore resistance. R b : Polarization resistance of the barrier layer Z po : Constant phase element (CPE) to account for the variations of properties of the pores. Cpo Cb ~~ Rs Rpo Zpo Rb Aluminum Alloy Barrier Layer Porous Layer II. Experimental Methods and Results II. Experimental Methods and Results Impedance spectra were collected at the open-circuit potential (OCP) in a three-electrode cell. A stainless steel electrode was used as the counter electrode and a SCE as the reference electrode. The impedance spectra were obtained with a BSA-Zahner IM6 unit using a frequency range between 1 MHz to 1 mHz with an ac amplitude of 10 mV. The EIS data were analyzed using the ANODAL software of ANALEIS. The spectra for all test panels for a given treatment were almost identical and did not change much with exposure time (Fig. 1-3). The almost identical capacitance values at the highest and lowest frequencies suggest that the thickness of the outer porous layer and the inner barrier layer, respectively, are almost the same for the all tested samples. The SEM pictures revealed that the coating thickness was about 63 μm (Fig. 4). The capacitance of the porous layer C po for the all test samples was very stable during the entire exposure period (Fig. 5 and 6). The pore resistance R po had very high values for all test samples (Fig. 7 and 8). These results indicate that the anodizing process was very reproducible and that the sealing process was very effective. EIS Results EIS Results III. Conclusions III. Conclusions ● The impedance spectra for the samples that were anodized in the R&D tank mixed acid were very stable during 14 days exposure. ● The impedance spectra for the type III samples suggest that oxide layers are more complex than those for the other samples. The impedance spectra did not change significantly during the 14 days exposure time which suggests that these anodized surfaces were very corrosion resistant. ● The impedance spectra for the production tank samples were very stable for an exposure time of 365 days showing that these surfaces had very high corrosion resistance. ● The very high and stable values of the pore resistance R po for all three processes indicate that the newly developed anodizing and sealing processes were very effective. Acknowledgements Acknowledgements Funding by Lam Research Corporation and numerous helpful discussions with Dr. Hong Shih are greatly appreciated. References References 1. F. Mansfeld, G. Zhang and C. Chen, Plating and Surface Finishing, 84 , 72 ( Dec. 1997). 2. F. Mansfeld, L. T. Han, C. C. Lee and G. Zhang, Electrochim. Acta 43, 2933 (1998). 3. F. Mansfeld, H. Shih, H. Greene, C.H. Tsai, ASTM STP 1188 (1993) 37. 4. Dr. Hong Shih presentation. Cross Section of the anodized layer Cross Section of the anodized layer 5 7 9 11 13 15 0 50 100 150 200 250 300 350 Time (days) C po (nF) 0907-073 0907-005 0907-107 -3 -2 -1 0 1 2 3 4 5 6 1 2 3 4 5 6 7 8 -3 -2 -1 0 1 2 3 4 5 6 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 -3 -2 -1 0 1 2 3 4 5 6 1 2 3 4 5 6 7 8 -3 -2 -1 0 1 2 3 4 5 6 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 -4 -3 -2 -1 0 1 2 3 4 5 6 1 2 3 4 5 6 7 8 9 -4 -3 -2 -1 0 1 2 3 4 5 6 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 5 7 9 11 13 15 0 2 4 6 8 10 12 14 16 Time (days) C po (nF) Type3 TY3 D2Lot3 D2Lot2 5 6 0 2 4 6 8 10 12 14 16 Time (day) LogR po (ohm) D2Lot3 D2Lot2 TY3 Type3 5.5 6 0 50 100 150 200 250 300 350 Time (days) LogR po (ohm) 0907-107 0907-005 0907-073 Fig. 1. Bode-plots for the R&D tank mixed acid sample for an exposure period of 14 days Fig. 3. Bode-plots for the production tank sample for an exposure period of 365 days Fig. 2. Bode-plots for the type III sample for an exposure period of 14 days Fig. 7. Time dependence of R po for R&D tank mixed acid and type III samples Fig. 5. Time dependence of C po for R&D tank mixed acid and type III samples Fig. 8. Time dependence of R po for production tank samples Fig. 6. Time dependence of C po for production tank samples Fig. 4. SEM image of the production tank sample