MORPHOLOGY AND STRUCTURE OF OXIDE-FILMS FORMED ON MGAL ALLOYS BY EXPOSURE TO AIR AND WATER

Morphology and structure of oxide films formed naturally on magnesium- aluminum alloys by exposure to humid air and water have been investiga ted by use of ultramicrotomed cross sections by transmission electron microscopy and compared to the properties of films formed on pure magn esium. Similar to pure magnesium, the initial air-formed film is amorp hous and dense. The film formed in water has a three-layered structure consisting of a hydrated inner layer, a dense intermediate layer simi lar in structure to the air-formed film, and a top layer with platelet -like morphology. With increasing aluminum content of the alloy, all l ayers become dehydrated and enriched in aluminum oxide. Furthermore. t he innermost and outermost layers decrease in thickness. These changes are particularly significant when the aluminum content of the alloy i s increased above 4%, which corresponds to about 35% Al in the innermo st layer. This threshold is characterized by a significant improvement in the corrosion resistance. The improvement is attributed to the alu mina component of this inner Layer becoming the dominating: factor in determining the passivity of the surface, presumably by forming a cont inuous skeletal structure in an amorphous mixture of aluminum and magn esium (hydr-)oxides.