THE INCORPORATION OF METAL-IONS INTO ANODIC FILMS ON ALUMINUM-ALLOYS

Anodic oxidation of a non-equilibrium Al-1.9 at.% W alloy has been stu died in order to investigate the mechanism of incorporation of metal i ons into the anodic film at the alloy-film interface. Initially a rela tively tungsten-free anodic alumina film is formed on the alloy to a t hickness of about 28 nm, corresponding to an anodizing voltage of abou t 23 V. During growth of the alumina film, tungsten atoms are accumula ted in a similar to 1.5 nm layer of alloy, just beneath the anodic fil m, that becomes enriched in tungsten to an average composition of abou t Al-25 at.% W. At this critical condition of the interface, tungsten and aluminium are then incorporated into the anodic film in their allo y 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 Al2O3, develop at discrete sites, of about 1-2 nm separation, along the interface and pe netrate the higher-resistivity, labile alumina. The sites are regions of alloy of higher than average tungsten concentration, probably assoc iated with tungsten-rich clusters of a critical size. Owing to the lim ited thickness of the enriched layer of alloy, individual fingers are restricted to lengths of similar to 5 nm. Comparatively pure alumina i s formed at sites along the alloy/film interface between the fingers a t which the critical size of cluster has not been reached. By this mec hanism, tungsten is incorporated discontinuously into the anodic film at local sites, with alternating local depletion of tungsten in the en riched layer by formation of the finger from a tungsten-rich cluster o f critical size, and subsequent enrichment of tungsten, to redevelop c lusters, by oxidation of aluminium. The average rate of incorporation of tungsten is constant since fingers form continually along the inter face which, through the incorporation mechanism, is finely roughened. By inference, the flow of ionic current normal to the alloy-film inter face fluctuates at particular sites along the interface during incorpo ration of tungsten into the film. The incorporated tungsten migrates o utwards in the anodic film, under the influence of the electric held, at an average rate of about 37% of the rate of Al3+ ions. The initiall y 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 O2-/OH- ions and tun gsten species, within the film. As a result of the initial development of the tungsten-rich layer at the alloy/film interface, prior to inco rporation of tungsten into the film, the distribution of tungsten in t he film is dependent upon the film thickness. For relatively thick fil ms, formed to voltages much greater than 23 V, the outer similar to 30 % of the film thickness is composed of relatively pure alumina, while the inner similar to 70% of the film is composed of a mixture of units of Al2O3 and WO3.