History effects in anodic oxidation of Al-W alloys

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