Duplex structure and crystallization of anodic alumina films

The structure and the formation behavior of crystalline anodic films formed on aluminum in the mixture of boric acid / ammonium borate solution have been investigated. The film is composed of a boron-containing outer layer and a boron-free inner layer with lens-like crystals being present at the interface. The transport number of Al³⁺ estimated from the thickness ratio of the outer to inner layers is 0.4 when the film is formed at the current density of 20 A·m^-2. It increases to 0.45 when the current density is 200 A·m^-2. The film thickness decreases with increasing current density. A porous cell structure is developed when the film is formed at a current density higher than 100 A·m^-2. Voids in the lens-like crystals are localized in three different regions; i.e., the center top region, the upper interface region between amorphous and crystalline phases, and the lower half region of the crystals.