A study of barrier film growth on aluminium in solutions of film-promoting and aggressive ions using secondary ion mass spectrometry

Abstract

A Cameca Ion Analyser has been used to examine barrier-type anodic films grown on aluminium in neutral phosphate, chromate or phosphate/chromate solutions and in phosphate and chromate solutions containing relatively small amounts of chloride ion. Barrier layers grow at relatively high current efficiencies in phosphate solution, and are characterized by a rather wide phosphorus-containing layer adjacent to the oxide/solution interface. In chromate solutions, barrier layers grow at relatively low current efficiencies and contain little, if any, chromium resulting from the film growth process itself. Barrier layers grown in chromate-phosphate mixtures are generally typical of those grown in pure phosphate solutions, even at quite high chromate/phosphate concentration ratios. The presence of chloride ions in either phosphate or chromate solutions lowers the current efficiency of barrier film growth, and is associated with pitting of the aluminium substrate at the bases of flaws in the growing film, resulting in lateral undermining of the film. The results are explained in terms of a model for film development, involving growth partly by ionic migration and partly by a dissolution/precipitation process.