Enrichment of alloying elements beneath anodic oxides: investigation of Ta–1.5 at.% Cu alloy

Abstract In order to elucidate the factors controlling enrichment of alloying elements during anodic oxidation of alloys, and anodic oxidation behaviour of alloys generally, the influence of anodic film growth on a Ta–1.5 at.% Cu alloy has been examined by transmission electron microscopy, Rutherford backscattering spectroscopy and glow discharge optical emission spectroscopy. Contrasting with the behaviour of Al–Cu alloys, film growth proceeds without significant enrichment of copper in the tantalum alloy substrate, although a thermodynamic driving force for enrichment exists. Factors controlling the development of enriched alloy layers are considered, including influences of distributions of alloying element in the substrate, film structure and ionic transport in the anodic film. Comparisons of alloy systems suggest kinetic factors may be important, in particular the influence of film formation at the alloy/oxide interface on concentrations of vacancies and diffusion rates in the alloy layer immediately beneath the growing oxide film.