Phase composition of anodic oxide films on transition metals: a thermodynamic approach

Abstract

The phase composition of anodic oxide films on transition metals is analysed using thermodynamic calculations. The model of an oxide film represents a two-layer system M k O l /M m O n , where the outer layer M k O l is the highest oxide, and the inner layer M m O n is the lower oxide of the metal. The changes of the Gibbs energy Δ G r are considered for the reactions of oxidation and reduction at the M k O l /M m O n interface and it is shown that for Ti, Nb, Mo, and W the former reaction predominates, resulting in the formation of the highest oxides: TiO 2, Nb 2O 5, MoO 3, and WO 3. In the case of vanadium, the reaction of reduction proceeds preferentially, indicating the possibility of the lower oxide formation, namely VO 2. It is concluded that anodic films on Nb, Ti, Mo and W mainly consist of the corresponding highest oxides of these metals, and NbO 2, Ti 2O 3, MoO 2 or WO 2 layers are relatively thin, while in the films on vanadium the vanadium dioxide layer may be rather thick and only a very thin film near the outer boundary is close to the V 2O 5 stoichiometry. These results are in agreement with the experimental data on anodic oxidation of transition metals.