Effect of sulphate impurity in chromic acid anodizing of aluminium

The formation of porous anodic alumina during potentiostatic anodizing of aluminium for times up to 900 s has been investigated using two grades of chromic oxide reagent. The anodic films were examined by ion beam analysis and scanning and transmission electron microscopies. Sulphate impurity in the chromic acid led to incorporation of sulphate into the anodic film, a lower current density, a smaller cell size and less feathering of the pore walls. In addition, the efficiency of film formation was increased. The sulphate concentration in the film was greatly enhanced relative to the electrolyte.