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

The results of an investigation into the activation of aluminium electrodes, by indium, zinc. and indium and zinc salts, in halide solutions are described and discussed. The presence of indium, or indium and zinc salts, but not zinc salts, in halide solutions, caused the initiation of areas of localized attack on the aluminium test electrode at potentials more active than those observed for aluminium in halide solutions. Indium and zinc salts added simultaneously to the test solution acted in a synergistic fashion; much shorter induction periods, prior to the onset of localized attack, were observed, higher dissolution rates and significantly fewer repassivation events were evident when compared 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 exposure of bare aluminium and accelerated deposition of indium with subsequent modification of the surface to enhance the adsorption of chloride. The higher dissolution rates observed arise as a result of the repeated cycle of cracking and healing steps which allow rapid and continued reaction of aluminium with adsorbed chloride.