PHASE-COMPOSITION OF ANODIC OXIDE-FILMS ON TRANSITION-METALS - A THERMODYNAMIC APPROACH

The phase composition of anodic oxide films on transition metals is an alysed using thermodynamic calculations. The model of an oxide film re presents a two-layer system MkOt/MmOn, where the outer layer MkOl is t he highest oxide, and the inner layer MmOn is the lower oxide of the m etal. The changes of the Gibbs energy Delta G(r) are considered for th e reactions of oxidation and reduction at the MkOl/MmOn interface and it is shown that for Ti, Nb, Mo, and W the former reaction predominate s, resulting in the formation of the highest oxides: TiO2, Nb2O5, MoO3 , and WO3. In the case of vanadium, the reaction of reduction proceeds preferentially, indicating the possibility of the lower oxide formati on, namely VO2. It is concluded that anodic films on Nb, Ti, Mo and W mainly consist of the corresponding highest oxides of these metals, an d NbO2, Ti2O3, MoO2 or WO2 layers are relatively thin, while in the fi lms on vanadium the vanadium dioxide layer may be rather thick and onl y a very thin film near the outer boundary is close to the V2O5 stoich iometry. These results are in agreement with the experimental data on anodic oxidation of transition metals. (C) 1998 Elsevier Science S.A. All rights reserved.