Direct observation of the anodic film on a sputter-deposited amorphous Al-W alloy

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Abstract
The formation of barrier-type anodic films on sputter-deposited amorphous Al-30W (at.%) alloy has been studied for the first time to elucidate ionic transport mechanisms across the film and associated interfaces. The film, grown with high Faradaic efficiency at 50 Am−2 to 150 V in aqueous 0.01 m ammonium pentaborate electrolyte at 293 K, comprises two main layers of relatively uniform composition and with flat, parallel and sharp interfaces: An outer layer, representing about 15% of the film thickness and composed of tungsten-free anodic alumina, and an inner layer containing both aluminium and tungsten in an amorphous structure based on groupings of Al2O3 and WO3. The two layers form as a consequence of the slower migration of tungsten species in the film compared with that of A13 + ions. The inner layer has, as expected on compositional grounds, a lower ionic resistivity than the outer layer; yet, contrary to predictions, film formation proceeds in a uniform manner, i.e. without current channelling. Aluminium and tungsten are incorporated into the film at the alloy-film interface in their alloying proportions within the limitations of the measurements.