History effects in anodic oxidation of Al-W alloys

Stepped-current experiments are applied to the study of barrier film formation on an Al-6.5 at.% W alloy prepared by magnetron sputtering. These reveal transient changes in the composition and/or the density in local regions of the anodic film immediately following changes in the current density between 0.1 and 10 mA cm-(2). Specifically,following an increase and a decrease in the current, bands of film material approximately 6 nm thick are generated at, or near, the alloy-film interface, which are, firstly, respectively enriched and depleted in tungsten species and/or, secondly, of enhanced and reduced density respectively. There is also evidence that similar bands are generated simultaneously at, or near, the film-electrolyte interface. Further, either the density or the composition of film material formed at the alloy-film interface, or a combination of these factors, depends upon the current, with material retaining to some extent its original property as the film develops at the new current. Possible reasons for the occurrence of these features are considered.