Effect of pH on the Distribution of Anions in Anodic Oxide Films Formed on Aluminum in Phosphate Solutions

Abstract

The composition of anodic oxide films anodically formed on aluminum in phosphate solutions with different pH's was examined by chemical sectioning in a sulfuric acid solution combined with impedance measurements, solution analysis, and electron microscopy. The oxide films were found to consist of two layers: an outer layer containing phosphate and an inner layer consisting of pure . With increasing pH of the anodizing solution, the thickness of the outer layer increases and the phosphate concentration in the outer layer decreases. In the chemical sectioning, the dissolution rate of the outer layer increases with anodizing solution pH, while the dissolution rate of the inner layer is slower and unaffected by the anodizing solution pH. For the outer layer, there is a linear relationship between the phosphate concentration and the sectioning rate in the sulfuric acid solution. The mechanism of film formation during anodizing is discussed in terms of Al3+ transport through the oxide.