EFFECT OF SULFATE ANIONS ON TUNNEL ETCHING OF ALUMINUM

Electrochemical etching of hard aluminium foil was studied at 100 degr ees C in NaCl solutions without and with Na2SO4 in concentrations up t o 1.0 M. Addition of Na2SO4 resulted in an increase in electric capaci tance, in refinement of etch configuration and in an increase in tunne l density per unit volume. A decrease in the number of pits from which the tunnels grew also occurred. The capacitance increased with increa sing concentration of Na2SO4 up to about 0.35 M, and then decreased. S ulphate ions depressed the formation of pits on the outer oxide-covere d surface, but enhanced the growth of the pits and the formation of tu nnels from the pits. It is suggested that the retardation of pit nucle ation and the acceleration of tunnel growth in the presence of SO42- i ons can be explained by a partial replacement of Cl- ions from the oxi de and metal surface, respectively. Smaller diameter tunnels may be du e to the formation of Al-2(SO4)(3) which can, in part, replace more ag gressive AlCl3, and to an easier formation of a passivating film on th e tunnel walls owing to their slower dissolution in the presence of Al -2(SO4)(3).