Comparison of the A.C. electrograining of aluminium in hydrochloric and nitric acid

Homogeneous roughening of aluminium surfaces is carried out by A.C. electrograining in dilute acids, to produce uniformly pitted and convoluted surfaces. The morphology of A.C. electrograined aluminium surfaces is affected by electrolyte and graining conditions. By comparison of morphologies developed during treatment in hydrochloric and nitric acid, it is proved that the morphological building elements and their ordering are fundamentally different, leading to different final morphologies. The basic elements building up the morphology of surfaces treated in nitric and hydrochloric solutions are respectively a small flat walled hemispherical pit and a fine cubic pit. The effective diameter of the building elements increases, with increasing current density and decreasing graining frequencies. The susceptibility for pitting is higher in chloride solutions: pits are initiated at much lower potentials and at much higher rates; in addition the initiation of pits occurs much faster.