The behaviour of iron during anodic oxidation of sputtering-deposited Al–Fe alloys

Abstract Interfacial enrichment of iron atoms and iron ions has been observed at the alloy/film and film/electrolyte interfaces respectively during anodizing of sputtering-deposited, non-equilibrium Al–7.5 at.% Fe alloys. The formation of the anodic oxide involves initial oxidation of aluminium and enrichment of iron in a layer of alloy, about 2 nm thick, immediately beneath the oxide film. When the level of enrichment reaches about 3.8×10 19 Fe atoms m −2 , corresponding to an average composition of the enriched alloy layer of approximately Al–25 at.% Fe, both aluminium and iron species are incorporated into the film in the presence of the enriched layer. The incorporated iron species, namely Fe 3+ ions, migrate outward in the growing anodic film, about 2.0 times faster than Al 3+ ions. Consequently, the iron species eventually reach the film/electrolyte interface forming a thin, amorphous layer probably composed of Fe 2 O 3 . Oxygen is generated at, or near, the alloy/film interface subsequent to incorporation of iron species into the film. The general anodizing behaviour is similar to that reported previously for an Al–4 at.% Fe alloy, although in that case significant oxygen generation and void formation, which limits thickening of uniform films, is postponed to higher voltages, while the outer, iron-rich layer is composed mainly of crystalline FeOOH.