The amount of sulfate found in anodic coatings formed on pure aluminum anodes in sulfuric acid electrolytes increased as the current density and concentration of acid increased, but decreased as the temperature was raised. Chemical analysis showed that the coatings consisted substantially of Al 2 O 3 and SO 3 with practically no water. Factors affecting solubility of the coatings, such as heat developed within the coating, and time of immersion, were investigated. Rapid changes in current and voltage, which occurred in the first 15 sec after application of current, gave certain clues as to the possible mechanism of pore formation.
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Ralph B. Mason (1955) studied this question.