A TEM INVESTIGATION OF NATURALLY FORMED OXIDE-FILMS ON PURE MAGNESIUM

The morphology and structure of oxide films formed naturally on pure m agnesium by exposure to humid air and water have been investigated by use of ultramicrotomed cross sections and TEM. The initial film formed immediately after exposing fresh surface by scratching in air is thin and dense, and it has an amorphous structure. In humid air, a hydrate d layer forms between the metal and the initial layer as a result of w ater ingress through the initial layer and metal oxidation. The film f ormed in water contains an additional top layer with platelet-like mor phology, formed by redeposition of sparingly soluble magnesium, which migrates outward also through the initial layer. The properties of the se layers and their significance in determining the corrosion behavior of magnesium are discussed. (C) 1997 Elsevier Science Ltd.