Anodization of Aluminum in a Sodium Hydroxide Solution: Effect of pH on Chemical Dissolution

The chemical dissolution of anodic alumina film was investigated using the re-anodization technique. The formation of thick oxide films in an alkaline electrolyte is thought to be difficult to achieve due to the high pH value and high solubility of anodic alumina. The dissolution rate of anodic film was found to be strongly affected by the pH of the solution used for chemical dissolution; the film dissolved significantly faster in a sodium hydroxide solution having a high pH of 13.11 than in acidic solutions commonly used for anodization, such as sulfuric acid (pH 0.98) and phosphoric acid (pH 1.54). Nevertheless, the addition of glycerol to the sodium hydroxide solution effectively suppressed the chemical dissolution of the anodic film. The change in solubility of the anodic alumina film in solution was greatly affected by the change in the dissociation of the solute in solution. The results demonstrated that oxide films more than 10 μm thick were produced in an alkaline electrolyte by adding glycerol to the solution. The suppression of the chemical dissolution of alumina by the addition of alcohol has thus been shown to occur not only in acid solutions but also in alkaline solutions.