DUPLEX STRUCTURE AND CRYSTALLIZATION OF A NODIC ALUMINA FILMS

The structure and the formation behavior of crystalline anodic films f ormed on aluminum in the mixture of boric acid/ammonium berate solutio n have been investigated. The film is composed of a boron-containing o uter 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 decrease s with increasing current density. A porous cell structure is develope d 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 r egions; i.e., the center top region, the upper interface region betwee n amorphous and crystalline phases, and the lower half region of the c rystals.