Ionization of atomic oxygen on the surface of a specimen in plasma anodization

The behavior of the species of oxidation dominating the oxide growth in plasma anodization of Al is investigated in this paper. The relationship between the current efficiency and the plasma parameters was observed under the different anodizing conditions with changing the construction of the discharge electrodes and the discharge current in order to clarify the factors contributing to the anodization. The formula of the current efficiency was tentatively derived from a rough model of the growth process of the oxide film with an approximation. The formula, in practice, was consistent with the experimental results. Consequently, it was suggested that the rate-limiting step in the oxide growth is due to the supply of the atomic oxygens from the plasma to the surface of the specimen and also to their ionization.