Structure of Anodic Layers on the (100) Face of Aluminum

Abstract

The structure of coherent anodic layers grown on the {100} faces of aluminum in tartaric acid—ammonia electrolytes has been studied by replicas and transmission electron microscopy. At pH 5.5 these layers are pore-free to the limit of our sensitivity (10–15 Å) and faithfully replicate the original surface formed during electropolishing. The hillock structure, characteristic of perchloric acid polishing baths, can be considerably reduced by electropolishing in a nitric acid—methanol bath at −20°C. The metal—oxide interface in this instance is featureless to within 20 Å. Anodic layers grown at pH 2.0 or 10.5 are highly porous and an electropolishing action takes place. An ultrastructure with characteristic spacing 50 Å is observed on surfaces polished in perchloric acid mixtures. The structure is also seen on the electrolyte interface of the anodic layer. It is proposed that this structure is not an artifact of the shadowing, and that it may correspond to particles of aluminum hydroxide formed during growth of the anodic layer.