Abstract
Aluminum sheets (99.99%) were anodized in various carboxylic acid solutions containing a small amount of sulfuric acid, and the surfaces of the oxide films and of the substrates separated from the films were observed by SEM. Observations revealed that the morphorogy of anodic oxide films depended upon the field-assisted film dissolving action of the carboxylic acids, chemical dissolving action on aluminum, and local field-assisted dissolving action of the dilute sulfuric acid contained in the solution. The addition of sulfuric acid in carboxylic acids, which are commonly used for the formation of barrier type films, caused formation of a local abnormally grown porous film. This sort of film was produced also in a dilute sulfuric acid bath. In monocarboxylic acids, the addition of sulfuric acid ceased pittings, resulting in formation of abnormally grown films. But in the case of polyoxymonocarboxylic acid, such as gluconic acid, which has a very strong chemical dissolving action, the addition of 0.5g/l sulfuric acid could not be effective to inhibit pittings. In the other carboxylic acids that are used for formation of porous type films, addition of sulfuric acid produced colored porous films which developed into darker abnormally grown films. In this case, the cell dimensions of porous films were proportional to the bath voltage which could be lowered by the addition of sulfuric acid.
