Porous anodic film formation on aluminium

Abstract

The development of porous anodic films on aluminium in the major forming acids has been established^1–5. However, little information has been provided to explain the markedly different rates of steady-state film growth for anodizing in constant voltage conditions in these acids, with the anodic film cell parameters dependent on the cell voltage¹. The regular cellular morphology of the steady-state films and the substructure developed in the forming acids are revealed in Fig. 1. The characteristic parallel-sided pores, at the centres of approximately hexagonal cells, pass normal to the macroscopic metal surface but are separated from it by the barrier layer. Here, from considerations of the film material substructure and the distribution of species within the film derived from the forming acid, it is postulated that the likely variations in the local field strengths across the barrier layers can explain the steady-state anodizing behaviour observed in the different acids in constant voltage conditions.