A study of the mechanism of A.C. electrolytic graining of aluminium in hydrochloric and nitric acid

a.c. Electrolytic graining or electrograining employs an alternating voltage or current on aluminium electrodes in a suitable electrolyte, e.g. hydrochloric or nitric acid, to produce uniformly pitted and convoluted surfaces for lithography. The resultant morphology or the electrograined surface is dependent upon the particular electrical, solution and substrate conditions employed. Here, current understanding of electrograining of aluminium in hydrochloric acid is reviewed and compared with graining in nitric acid. Electron micrographs revealed that during electrograining large hemispherical pits are formed in both electrolytes. However, close examination of the hemispherical pits formed in hydrochloric acid showed that they are composed of a large number of fine cubic pits. Conversly, these crystallographic pits are not observed after treatment in nitric acid; instead a hemispherical pit with a characteristic dimension is found. Such morphological appearances have been studied as a function of graining frequency and current density. In addition to the formation of pits, a non-uniform film of etch products is formed, masking completely the real surface morphology. This etch film is deposited largely as a gel of water and amorphous Al(OH)3, aluminium particles and some chloride or nitrate ions. The formation of this etch film is explained by a cathodic filming process on the aluminium electrode. Surface alterations by treatment in both electrolytes are explained in terms of the applied a.c. signal.