The incorporation of metal ions into anodic films on aluminium alloys

Abstract

Abstract Anodic oxidation of a non-equilibrium Al-1·9 at.% W alloy has been studied in order to investigate the mechanism of incorporation of metal ions into the anodic film at the alloy-film interface. Initially a relatively tungsten-free anodic alumina film is formed on the alloy to a thickness of about 28 nm, corresponding to an anodizing voltage of about 23 V. During growth of the alumina film, tungsten atoms are accumulated in a ∼ 1·5 nm layer of alloy, just beneath the anodic film, that becomes enriched in tungsten to an average composition of about Al-25 at.% W. At this critical condition of the interface, tungsten and aluminium are then incorporated into the anodic film in their alloy proportions by a mechanism having similarities with the anodizing of layered valve metals, forming oxides of differing ionic resistivity, but on a nanoscale. Transient fingers, composed of relatively pure WO3 or a tungsten-enriched mixture of units of WO3 and A12O3, develop at discrete sites, of about 1–2 nm separati...