Electrolyte contamination effects of the high temperature deposition of barrier anodic films on aluminium in aqueous solutions

The formation of barrier alumina films in aqueous electrolytes is known to proceed anomalously at temperatures above 40°C and to result in unstable films. In contrast with this the advantage of the high temperature anodization is proved by industrial experience. In order to elucidate the reasons for these discrepancies the galvanostatic anodization of aluminium in a boric acid solution at 90°C was studied. Some impurities of very low concentrations in the electrolyte were found to become active at high temperature and to cause the appearance and enlargement with the film growth of corrosion spots with a high electronic conduction. The shape, size and number of these spots turned out to be associated with the nature and concentration of the active impurities. These impurities are incorporated in the film during anodization and the electrolyte is thus ‘adapted’ by elimination of the impurities carried away by the oxide. The anodization in such ‘adapted’ electrolytes was found to proceed normally and to give high-resistance anodic films. Some practical application aspects of these findings are also discussed.