The incorporation of metal ions into anodic films on aluminium alloys

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 separation, along the interface and penetrate the higher-resistivity, labile alumina. The sites are regions of alloy of higher than average tungsten concentration, probably associated with tungsten-rich clusters of a critical size. Owing to the limited thickness of the enriched layer of alloy, individual fingers are restricted to lengths of ∼ 5 nm. Comparatively pure alumina is formed at sites along the alloy/film interface between the fingers at which the critical size of cluster has not been reached. By this mechanism, tungsten is incorporated discontinuously into the anodic film at local sites, with alternating local depletion of tungsten in the enriched layer by formation of the finger from a tungsten-rich cluster of critical size, and subsequent enrichment of tungsten, to redevelop clusters, by oxidation of aluminium. The average rate of incorporation of tungsten is constant since fingers form continually along the interface which, through the incorporation mechanism, is finely roughened. By inference, the flow of ionic current normal to the alloy-film interface fluctuates at particular sites along the interface during incorporation of tungsten into the film. The incorporated tungsten migrates outwards in the anodic film, under the influence of the electric field, at an average rate of about 37% of the rate of Al³⁺ ions. The initially non-uniform distribution of tungsten in the anodic film, due to the discrete nature of incorporation, is smoothed, but not eliminated, by the ionic transport processes, involving Al³⁺ and O^2-/OH⁻ ions and tungsten species, within the film. As a result of the initial development of the tungsten-rich layer at the alloy/film interface, prior to incorporation of tungsten into the film, the distribution of tungsten in the film is dependent upon the film thickness. For relatively thick films, formed to voltages much greater than 23V, the outer ∼30% of the film thickness is composed of relatively pure alumina, while the inner ∼ 70% of the film is composed of a mixture of units of A1₂O₃ and WO₃.