The interaction of chromate species with aluminium supporting air-formed and anodic films—II. Impedance studies

Abstract

The impedance behaviour of aluminium, supporting either an air-formed or an anodic film, has been monitored during immersion in chromate/dichromate solution. The impedance was obtained over a wide range of frequency (10 kHz–10 mHz) with particular attention being paid to the low frequency region (1 Hz–10 mHz). At high frequencies ( > 10 Hz) the impedance response is from the film dielectric; any changes with time are associated generally with the penetration and chemically induced disaggregation of the alumina film by Cr 6+ species. The low frequency behaviour largely reflects local processes proceeding at flaws in the films. Specifically, the impedance behaviour at 0.1 Hz has been examined to assess reaction rates during the immersion period and differences in the rates are revealed for aluminium supporting air-formed films and anodic alumina films. The variation in the reaction rates evident during the immersion period is attributed to the type and geometry of persistent flaws in the respective specimens and the related effect of thinning of the anodic alumina film during immersion in the chromate solution. It is evident that Cr 6+ species are reactive to alumina films, with chromate species encouraging healing of anodic sites on aluminium supporting an air-formed film and blocking of cathodic sites on aluminium supporting an anodic alumina film.