Chromic acid anodizing of aluminium

Summary

Chromic acid anodizing of aluminium has been studied by high resolution scanning electron microscopy and energy dispersive X-ray microanalysis. In the initial stages of anodizing, aluminium oxide protrusions from on the outer surface similar to those formed on etching in either mixed chromic-sulphuric acid or in the FPL etch solution. On non-planar surfaces the protrusions form preferentially on ridges and merge laterally to form a continuous deposit along the ridges. When the barrier layer has grown to its voltage determined thickness, the rate of ejection of aluminium ions into solution falls, the growth of protrusions stops, the porous layer develops and dissolution of the protrusions and outer surface of the porous layer takes place as anodizing proceeds.

To maintain pore formation perpendicular to the surface, new pores initiate by branching above concave surfaces and terminate above convex surfaces and ridges. The shape of the surface at the start of anodizing and also the change in shape in the initial stages determine the spatial arrangement of the pores. High resolution scanning electron micrographs show clearly the high degree of pore branching and the irregular spatial distribution of pores formed in chromic acid anodizing compared with the more regular and narrower pores formed on anodizing in sulphuric acid. The hardness, abrasion resistance, flexibility and adhesive bonding properties of the anodic oxide formed in chromic acid are related to the observed structure.