Anodization of Aluminum in Moderate-to-High Concentrations of Sulfuric Acid

Sulfuric acid is widely used as an electrolyte for aluminum anodization in both industrial and academic applications. Industrial anodization typically employs sulfuric acid concentrations of 1-2.5 M, whereas academic studies, particularly those on the self-ordering of pore arrangements, often use low-concentration sulfuric acid (≤0.3 M). Although commercially available sulfuric acid (96-98%) has a concentration of approximately 18 M, the anodization behavior of aluminum in highly concentrated sulfuric acid (≥3 M) has received little attention. In this study, the effects of sulfuric acid concentration (1.5-12 M) on the voltage-time response during constant-current anodization, the structure of the resulting anodic film, and film formation efficiency were systematically investigated. The results showed that these properties exhibited nonmonotonic concentration dependence, with similar trends observed at both low and high sulfuric acid concentrations. This characteristic behavior was attributed to changes in the electrical conductivity of the electrolyte associated with the concentration-dependent dissociation of sulfuric acid. Furthermore, anodization in 12 M sulfuric acid resulted in increased sulfate incorporation into the anodic film, accompanied by increased film thickness and an apparent improvement in film formation efficiency.