Local Corrosion of Anode in Low Temperature Hard Anodizing Process of Aluminum

Experiments were carried out to determine the anodizing condition of low temperature hard anodizing process, under which a uniform oxide coating was formed, and also to examine the mechanism of local corrosion of anode.Commercial aluminum sheets such as 99.99% Al, 1100, 2017, 4043 and 5052 were used as specimens. Each sulfuric acid of 1, 10, 20, and 40%, containing aluminum sulfate equivalent to each 1/10 of the acid in concentration, was used as an electrolyte. The current density was 0.1-8.0amp./dm2 and the temperature of the bath was kept constant at 0.5°C for each electrolyte. The surfaces of anodized specimens were observed on microscope, electron microscope and scanning electron microscope, and their roughness was determined. In conclusion, it was preferable that electrolysis would be carried out in the acid of high concentration under low current density for the purpose of preventing local corrosion of the anode.When anodizing was performed under high voltage in a bath, in which solvent attack against the anodic coating is relatively weak, current passed preferentially through weak points of barrier layer of the coating to provoke the propagation of pittings or burnings of the anode.