Studies of the Anodic Behavior of Aluminum

Abstract

In order to avoid solvent action by the electrolyte during anodic oxidation of aluminum, the electrolyte must have a high buffering capacity. With such an electrolyte the conversion of aluminum to aluminum oxide is essentially quantitative. The film thickness is proportional to the voltage drop across the film, and the proportionality constant does not depend on the length of time which the voltage is left applied. The film contains, as an essential part of its structure, a quantity of the anion from the electrolyte. The film appears to be nonstoichiometric, having excess aluminum during formation, the amount of excess decreasing with increased length of anodizing time. The coulombic transfer during film formation is quantitatively accounted for during formation by oxidation of aluminum to the trivalent state and liberation of oxygen gas. After the film growth has ceased, the current can be only partially accounted for by oxygen evolution, but it is thought that this is because of difficulty in nucleating oxygen bubbles.