STUDIES ON THE PASSIVATION OF ALUMINUM IN CHROMATE AND MOLYBDATE SOLUTIONS

Results on the electrochemical behaviour of aluminium in aggressive ch loride solutions containing molybdate and dichromate anions are presen ted and discussed. An ennoblement in the pitting potential of aluminiu m was observed in dichromate-containing solutions and in non-acidic mo lybdate-containing solutions. It was proposed that reduction of the mo lybdate and dichromate species occurred at flawed regions in the passi ve film, but that the larger polymeric molybdate species, formed in ac id solutions, could not bc accommodated at the flawed regions. An oxid ation peak, observed at -1200 to -1400 mV(SCE), in the molybdate-conta ining solution was attributed to the growth of a mixed aluminium-molyb denum passive layer. The complete passivation of an aluminium electrod e, undergoing meta-stable pitting, was achieved by the introduction of dichromate anions into the test solution. This was attributed to a tr ansformation process whereby chloride-containing aluminium hydroxy com plexes were converted into hydrated aluminium oxides.