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

The electrochemical behaviour of Al-0.4%In alloy, pure aluminium and pure indium in chloride, sulphate, molybdate and chromate solutions was investigated. The presence of indium precipitates, although having little effect on the electrochemical behaviour of the alloy in chloride solutions, provided defective centres where attack initiated. These precipitates were dislodged from the surface during dissolution of the aluminium indium solid solution in the chloride electrolyte. The behaviour of the alloy in sulphate solutions was dominated by the availability 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 resulted in an apparent activity of the alloy in sulphate solutions even though the aluminium-indium solid solution phase remained passive. The effects of indium precipitates on the dissolution of the alloy were evident, also, on addition of dichromate to the chloride test solution. The breakdown potentials of both indium and aluminium were shifted in the noble direction but the alloy appeared to become more active. This observation was attributed to the effects of localized galvanic cells set up between the second-phase indium precipitates and the aluminium indium solid solution.