Dissolution Behavior of the Barrier Layers of Porous Oxide Films during the Initial Period of Anodizing of Aluminum

Abstract

Anodizing of Al was carried out in oxalic acid-oxalate solutions over the pH range from 1.7 to 6.4 at constant anodic potentials and variation of the anodizing current, the structure of the film and the amount of dissolved Al ions were observed with time. The current-time curve was divided into four successive periods of a, b, c and d in relation to the surface struc-ture of the film. In the initial period of anodizing (period a), the current falls nearly expo-nentially with time in accordance with the growth of the barrier layer. After a certain period of time the lowering of current ceases (period b) and the current begins to increase with time (period c) and finally reaches a steady value (period d), owing to the pore initiation and the growth of the porous layer. It was found that an appreciable amount of film dissolves even in the period a; the rate of dissolution is affected strongly by the anion species in the solution but scarcely changes with the pH of the solution. A high rate of dissolution observed at periods c and d increased remarkably with rise of anode potential and with fall of pH. This is explained in terms of the mechanism which assumes the formation of cation defects at the outermost part of the barrier layer and their interaction with protons in the solution. A new method of determining the barrier layer thickness of the porous type film was proposed.