Morphology and structure of water-formed oxides on ternary MgAl alloys

Oxides on ternary magnesium alloys MgAlZn and MgAlRE were investigated by transmission electron microscopy using ultramicrotomed film sections. These films have a three-layered structure, similar to pure Mg and binary MgAl alloys, characterized by a hydrated inner layer, a thin and dense intermediate region, and a platelet-like-outer layer. Zinc and rare-earth elements present in the two types of ternary alloys become incorporated in the oxide film so as to increase its stability in an aqueous environment, in particular by reducing hydration and increasing resistance to magnesium egress of the inner layer, which is responsible for the passivity of the surface. The apparent presence of trace amounts of rare-earth oxides in the film is particularly effective in improving passivity of the surface and, thereby, the corrosion resistance of MgAlRE alloys. The presence of aluminum together with rare-earth elements (RE) in the alloy is an essential factor in obtaining these results.