Studies on the Passivation of Aluminium in Chromate and Molybdate Solutions

Results on the electrochemical behaviour of aluminium in aggressive chloride solutions containing molybdate and dichromate anions are presented and discussed. An ennoblement in the pitting potential of aluminium was observed in dichromate-containing solutions and in non-acidic molybdate-containing solutions. It was proposed that reduction of the molybdate and dichromate species occurred at flawed regions in the passive film, but that the larger polymeric molybdate species, formed in acid solutions, could not be accommodated at the flawed regions. An oxidation peak, observed at −1200 to −1400 mV(SCE), in the molybdate-containing solution was attributed to the growth of a mixed aluminium-molybdenum passive layer. The complete passivation of an aluminium electrode, undergoing meta-stable pitting, was achieved by the introduction of dichromate anions into the test solution. This was attributed to a transformation process whereby chloride-containing aluminium hydroxy complexes were converted into hydrated aluminium oxides