ANODIC FILM FORMATION ON HIGH-STRENGTH ALUMINUM-ALLOY FVS0812

Barrier-type film growth on the high strength aluminium alloy FVS0812 has been studied by a combination of transmission electron microscopy and Rutherford backscattering spectroscopy. The film is composed mainl y of amorphous anodic alumina, but is contaminated with iron species i ncorporated into the film from the alloy. The film may also be contami nated with silicon and vanadium species at levels below the detection limit of the present experiments. The contaminant species are primaril y incorporated locally into the firm during oxidation of Al-13(Fe, V)( 3)Si dispersoids and the resulting film material is of reduced resisti vity compared with anodic alumina of high purity. As a consequence of the presence of regions of film material of differing resistivities, t he film is of irregular thickness. The average thickness corresponds t o a nm/V ratio of about 1.3. Iron species incorporated into the film m igrate outwards at roughly 2.1 times the rate of Al3+ ions. The iron s pecies are not ejected in significant amounts to the electrolyte on re aching the film/electrolyte interface and hence, a thin layer of film material highly enriched in iron species develops at the film surface. The layer may also be enriched in vanadium species, if these are inco rporated into the film and migrate more rapidly than Al3+ ions. Enrich ment of iron, and possibly other alloying element atoms, is found in a thin layer of alloy immediately beneath the anodic film, paralleling enrichments of alloying element atoms found following anodic oxidation of other aluminium alloys. The enrichments at both the alloy/film and film/electrolyte interfaces do not appear to be continuous across the macroscopic surface of the specimens, probably due to the non-uniform ity of film growth on the two-phase substrate. The maxim um voltage fo r the selected conditions of anodizing was limited to 68 V as a result of oxygen generation at flaws which are present extensively in the an odic film.