Anodizing of aluminium in borax solution

SUMMARYThe growth of anodic films on aluminium in disodium tetraborate, so-called alkaline anodizing, has been examined by transmission electron microscopy of plan views of stripped anodic films and ultramicrotomed sections of the anodic film attached to the aluminium substrate. For the conditions selected, initial film growth proceeds at relatively high efficiency under galvanostatic anodizing conditions; during the following constant voltage period, porous anodic film results. The latter arises through penetration of electrolyte species into the alumina film, with eventual focus of the field in such regions, establishing conditions for field assisted dissolution, probably thermally enhanced, and development of a porous anodic film. The cell walls adjacent to the pores have a feathered appearance, which results from mechanical disruption of the film material at the pore base under the high field.