Anodic activation of aluminium containing small amounts of gallium and tin

• Trace element Ga does not cause passivity breakdown alone. It is stable in Al.
• Ga and Sn present together in Al cause high anodic activation in aqueous solution.
• Synergy is caused by Sn enhancing surface enrichment of Ga by dealloying.
• Liquid phase GaSn film forming on the surface destabilizes the passivating oxide.
• The presence of chloride is not needed for activation in the presence of GaSn film.

Anodic activation due to surface enrichment of Sn and Ga on AlGaSn model alloys, containing 50–1000 ppm Sn and 1000 ppm Ga in pure aluminium, as a result of heat treatment was investigated. Sn was enriched because of annealing at 300 °C, while Ga was stable in solid solution because of its high solubility in Al. Anodic polarization in 5% NaCl solution resulted in highly active surfaces in relation to the binary alloys AlGa and AlSn. Presence of segregated Sn at the surface catalysed dealloying, such that Ga was enriched on the corroding surface giving increased activation.