Effect of equal-channel angular pressing on pitting corrosion resistance of anodized aluminum-copper alloy
(1. Department of Materials Process Engineering, Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan;
2. Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University,
744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan;
3. Department of Applied Chemistry and Biochemistry, Faculty of Engineering, Kyushu Sangyo University,
2-3-1 Matsukadai, Higashi-ku, Fukuoka, 813-8503, Japan)
2. Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University,
744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan;
3. Department of Applied Chemistry and Biochemistry, Faculty of Engineering, Kyushu Sangyo University,
2-3-1 Matsukadai, Higashi-ku, Fukuoka, 813-8503, Japan)
Abstract: The effect of equal-channel angular pressing(ECAP) on the pitting corrosion resistance of anodized Al-Cu alloy was investigated by electrochemical techniques in a solution containing 0.2 mol/L AlCl3 and also by surface analysis. Anodizing was conducted for 20 min at 200 and 400 A/m2 in a solution containing 1.53 mol/L H2SO4 and 0.018 5 mol/L Al2(SO4)3∙16H2O at 20 ℃. Anodized Al-Cu alloy was immediately dipped in boiling water for 20 min to seal the micro pores present in anodic oxide films. The time required before initiating pitting corrosion of anodized Al-Cu alloy is longer with ECAP than without, indicating that ECAP process improves the pitting corrosion resistance of anodized Al-Cu alloy. Second phase precipitates such as Si, Al-Cu-Mg and Al-Cu-Si-Fe-Mn intermetallic compounds are present in Al-Cu alloy and the size of these precipitates is greatly decreased by application of ECAP. Al-Cu-Mg intermetallic compounds are dissolved during anodization, whereas the precipitates composed of Si and Al-Cu-Si-Fe-Mn remain in anodic oxide films due to their more noble corrosion potential than Al. FE-SEM and EPMA observation reveal that the pitting corrosion of anodized Al-Cu alloy occurs preferentially around Al-Cu-Si-Fe-Mn intermetallic compounds, since the anodic oxide films are absent at the boundary between the normal oxide films and these impurity precipitates. The improvement of pitting corrosion resistance of anodized Al-Cu alloy processed by ECAP appears to be attributed to a decrease in the size of precipitates, which act as origins of pitting corrosion.
Key words: aluminum-copper alloy; equal-channel angular pressing; anodizing; pitting corrosion