A tracer investigation of chromic acid anodizing of aluminium

A tungsten tracer introduced into a sputtering-deposited aluminium substrate was employed to investigate pore development in anodic films formed at 3 mA cm(-2) in 0.25 M chromic acid electrolyte at 313 K. The anodized specimens were observed by transmission electron microscopy (TEM), with compositions of films determined by Rutherford backscattering spectroscopy (RBS). The anodic films were found to be similar in thickness to that of the aluminium layer consumed during anodizing and revealed the feathered pore morphology that is a characteristic of the electrolyte. The anodizing efficiency was similar to 45-48%, with tungsten tracer species, in addition to aluminium species, being lost to the electrolyte at the pore base. These findings, together with the relatively uniform distribution of tungsten species within the film, are consistent with field-assisted dissolution of the alumina playing a major role in the development of pores. The films contrast with those formed in phosphoric and sulphuric acid electrolytes, for which feathering of pores is absent, the tracer distribution is inverted and the film thickness exceeds that of the consumed metal, features indicative of the influence of material flow in pore development. Copyright (C) 2007 John Wiley & Sons, Ltd.