Whitening of aluminum surface by anodizing at high current density in sulfuric acid solution

In this study, we examined a method for whitening of aluminum surface by anodizing in sulfuric acid solution, with the aim of industrializing the process. Anodic oxidation in sulfuric acid solution at 2.5 Adm-2 for 120 s achieved whitening of the aluminum surface, but localized burning occurred. When anodizing was performed at low current density after anodizing at higher current density for short time (2.5 Adm-2 for 4.5 s), white anodic film was successfully obtained without burning. Cross-sectional SEM (scanning electron microscopy) observation of white film revealed that the etching by pretreatment created an uneven surface, and the pore branching created an uneven metal-oxide interface. The fluctuating value of the film thickness obtained by length measurement using SEM were 0.2-0.3 μ m. This value is sufficient to whiten the anodic film due to increase of diffusion reflection at the film surface and the metal-oxide interface. These results indicate that the difference in peak voltage between the first and second anodizing resulted in the formation of a branched pore structure and the achievement of film whitening. © 2023 The Japan Institute of Light Metals.