Anodizing of aluminium in borax solution

The growth of anodic films on aluminium in disodium tetraborate, so-called alkaline anodizing, has been examined by transmission electron microscopy o f plan views of stripped anodic films and ultramicrotomed sections of the a nodic film attached to the aluminium substrate. For the conditions selected , initial film growth proceeds at relatively high efficiency under galvanos tatic anodizing conditions, during the following constant voltage period, p orous anodic film results. The latter arises through penetration of electro lyte species into the alumina film, with eventual focus of the field in suc h regions, establishing conditions for field assisted dissolution, probably thermally enhanced, and development of a porous anodic film. The cell wall s adjacent to the pores have a feathered appearance, which results from mec hanical disruption of the film material at the pore base under the high fie ld.