The electrograining of aluminium in hydrochloric acid—I. Morphological appearance

The application of alternating current to aluminium in hydrochloric acid electrolyte results ultimately in the formation of a high density of hemispherical pits, which merge to produce a relatively uniformly pitted and convoluted surface topography. Through variation of the electrograining frequency and anodic or cathodic charge, it is evident that the hemispherical pits propagate through the development of a high population density of cubic pits on the positive half-cycle, which form the wall of the hemispherical pit. The size of the cubic pits decreases with increase in frequency, leading to relatively smooth hemispherical pits. On the negative half-cycle, etch products deposit as highly porous hydrated alumina over the relatively uniformly pitted aluminium surface.