Whitening of Aluminum Surface by Anodizing at High Current Density in Sulfuric Acid Solution

Abstract

In this study, we examined a method for whitening an aluminum surface by anodizing it in a sulfuric acid solution to industrialize the process. Anodic oxidation in a sulfuric acid solution at 2.5 Adm⁻² for 120 s resulted in the whitening of the aluminum surface, but localized burning occurred. When anodization was performed at a low current density for a short time (2.5 Adm⁻² for 4.5 s) after a high current density, a white anodic film was successfully obtained without burning. Cross-sectional scanning electron microscopy (SEM) observation of the white film revealed that etching by pretreatment created an uneven surface, and pore branching created an uneven metal-oxide interface. The fluctuating value of the film thickness obtained by length measurements using SEM was 0.2–0.3 µm. This value is sufficient to whiten the anodic film because of increase in diffusion reflection at the film surface and the metal-oxide interface. These results indicate that the difference in the peak voltage between the first and second anodizing resulted in the formation of a branched pore structure and film whitening.

 

This Paper was Originally Published in Japanese in J. JILM 73 (2023) 273–278. Abstract, Table 1 and the caption of Fig. 6 were slightly modified.

Photographs of aluminum surface (a) before and (b) after anodizing at 2.5 Adm⁻² for 4.5 s followed by 0.25 Adm⁻² for 1200 s.