THE SYNERGISTIC INTERACTION BETWEEN INDIUM AND ZINC IN THE ACTIVATIONOF ALUMINUM IN AQUEOUS-ELECTROLYTES

The results of an investigation into the activation of aluminium elect rodes, by indium, zinc. and indium and zinc salts, in halide solutions are described and discussed. The presence of indium, or indium and zi nc salts, but not zinc salts, in halide solutions, caused the initiati on of areas of localized attack on the aluminium test electrode at pot entials more active than those observed for aluminium in halide soluti ons. Indium and zinc salts added simultaneously to the test solution a cted in a synergistic fashion; much shorter induction periods, prior t o the onset of localized attack, were observed, higher dissolution rat es and significantly fewer repassivation events were evident when comp ared to the activation of aluminium by indium salt. These results are explained in terms of an increased rate of cracking and rupture of the protective surface by the deposited zinc leading to the transient exp osure of bare aluminium and accelerated deposition of indium with subs equent modification of the surface to enhance the adsorption of chlori de. The higher dissolution rates observed arise as a result of the rep eated cycle of cracking and healing steps which allow rapid and contin ued reaction of aluminium with adsorbed chloride.