THE EFFECTS OF INDIUM PRECIPITATES ON THE ELECTROCHEMICAL DISSOLUTIONOF AL-IN ALLOYS

The electrochemical behaviour of Al-0.4%In alloy, pure aluminium and p ure indium in chloride, sulphate, molybdate and chromate solutions was investigated. The presence of indium precipitates, although having li ttle effect on the electrochemical behaviour of the alloy in chloride solutions, provided defective centres where attack initiated. These pr ecipitates were dislodged from the surface during dissolution of the a luminium indium solid solution in the chloride electrolyte. The behavi our of the alloy in sulphate solutions was dominated by the availabili ty of indium precipitate surface regions. The indium precipitates were attacked by sulphate anions through the formation of reactive complex species in a manner similar to that observed with pure indium. This r esulted in an apparent activity of the alloy in sulphate solutions eve n though the aluminium-indium solid solution phase remained passive. T he effects of indium precipitates on the dissolution of the alloy were evident, also, on addition of dichromate to the chloride test solutio n. The breakdown potentials of both indium and aluminium were shifted in the noble direction but the alloy appeared to become more active. T his observation was attributed to the effects of localized galvanic ce lls set up between the second-phase indium precipitates and the alumin ium indium solid solution.