The role of indium and gallium in the passivity breakdown of aluminium

The results of an investigation into the passivity breakdown of aluminium by indium and gallium are described and discussed. The onset of passivity breakdown of aluminium by indium ions in halide solutions was associated with the formation of an Al-In surface alloy which in turn depended on the reduction of In3+ and the incorporation of metallic indium into the aluminium matrix. In the case of the Al-0.4%In alloy, pit initiation occurred at the boundaries of the indium precipitates. But direct attack; of the indium precipitates occurred in sulphate solutions. The onset of passivity breakdown by gallium ions in solution was less efficient and activation was observed only following cathodic polarisation of pure aluminium in gallium-containing solutions, where gallium could be deposited from alkaline environments. The addition of 0.1% gallium to aluminium altered only slightly the passive behaviour, but the addition of 2.6% gallium lead to considerable activation of the system.