Abstract
During the growth of barrier-type anodic films on aluminium in aqueous electrolytes, electrolyte species, derived from electrolyte anions, are usually incorporated into an essentially alumina film. Here, use has been made of this phenomenon to investigate the behaviour of anions near the film surface that leads to apparent selective incorporation of electrolyte species during anodizing in mixed electrolytes. For this purpose films were formed in mixtures of different electrolytes and the resultant films analysed by Rutherford backscattering spectroscopy and nuclear reaction analyses. The film compositions support a model in which the total concentration of adsorbed anions at the film surface, and hence the consequent contamination of the film by electrolyte species, is determined by the double-layer charge density. However, the relative concentrations of the two electrolyte species in films formed in mixed electrolytes are dependent upon the kinetics and thermodynamics of interfacial processes which cause the compositions of the interfacial and bulk electrolyte to differ significantly. The principal selective processes are transport of anions across the diffusion layer, hydrogen ion-anion reactions in the diffusion layer, adsorption of anions on the film surface and formation of gel layers above the anodic film: Their consequence is apparent selective incorporation of electrolyte species into the film, most strikingly seen in electrolytes containing either phosphate or silicate ions which are particularly effective in excluding electrolyte species derived from other electrolyte anions.