SUMMARYThis paper focuses on the comparison of ac electrograining of aluminium in hydrochloric and nitric acid. Several aspects are considered: the final morphology obtained as a function of the current density and the acid concentration for both systems, the development of the morphology as a function of the applied charge density, the formation of the etch products and the anodic current response obtained in the acid solutions containing chloride or nitrate anions. It is concluded that for both systems, in a current density—acid concentration diagram three different regions with characteristic morphologies can be recognised. Some remarkable differences are noted between the two systems. At relatively high concentrations, hydrochloric acid leads to a highly deep-etched surface while more unattacked surface areas are obtained in nitric acid. The development of the morphology as a function of the applied charge, the current density and the acid concentration is very complex and different for the two systems. Convection has an effect on the location of the regions of the different morphologies, meaning that the reactor type may have a major influence on the graining morphology obtained. Electrochemical measurements, represented in the form of anodic chronoamperometric diagrams, show that the basic mechanism of anodic pitting is different for the two types of anions.
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