Anodizing of aluminum in sulfuric acid and oxalic acid solutions with percarboxylic acid-based additive

Abstract

Anodic oxidation of a pure Al plate was carried out in aqueous solutions containing sulfuric acid (H2SO4), oxalic acid (H 2C2O4) and a percarboxylic acid-based additive at temperatures between 273 and 293K for 0.6 to 9 ks under a constant current density of 300 A·m-2 (3 A·dm-2). The microstructures and the mechanical properties of the obtained anodic oxide film were investigated to determine the optimum electrochemical condition. In the H2SO4 + additive bath, the film with a maximum thickness of approximately 100μm was formed at 293K for 7.2 ks. However, the surface hardness of the film decreased with increasing processing time and film thickness. To enhance the film hardness, the Al plate was anodized in the H 2SO4 + H2xC2O4 + additive bath at lower temperatures, which resulted in higher Vickers hardness of 400500 and excellent wear resistance for the thick film formed at 278K for 7.2 ks. The film formed at 278K in the H2SO4 + H2C 2O4 + additive bath had a typical cell structure with a nano-sized pore at the center of each cell. Compared with the film formed in the H2SO4 bath, the cells were almost uniform in size and the pore size in the cell was smaller. In addition, the occupation ratio of the pores in unit area of the film formed in the H2SO4 + H2C2O4 + additive bath was smaller than that of the film formed in the H2SO4 bath. Such microstructural features are thought to be responsible for the high hardness of the anodic oxide thick film. © 2012 The Ceramic Society of Japan.