1.Introduction
The aluminium industry exists because of the universal presence of a protective layer of oxide 2 nm thick on the exposed surface of aluminium and its alloys. This impervious oxide layer prevents the diffusion of oxygen inside the aluminium matrix and also allows some deformation of the substrate without rupturing. How- ever, the oxide film found on the surface of aluminium alloys is impervious, continuous and self-healing only at room temperature or at higher temperatures in the initial stages of oxidation. As the film thickness increases with temperature and time, cracks appear and further oxidation of aluminium alloys is possible .In fact, aluminium alloys are highly resistant to oxidation at temperatures up to 200 C. At higher temperatures, the oxidation rate increases and when the temperature exceeds 350 C, internal oxidation of A1-Mg alloys containing appreciable amounts of magnesium may take place in an atmosphere containing both oxygen and water. This process is reported to be more significant at temperatures above 480C . Other alloys of aluminium are reportedly less susceptible to internal oxidation.
In the relevant literature, the internal oxidation of copper alloys , silver alloys , Co-Cr alloys , Ni-A1 alloys and iron-based alloys , etc., and its mechanism have been reported. On the other hand, only possibilities are encountered for internal oxidation in A1-Mg alloys and microstructural evidence of the phenomenon is not available. It has been reported that in A1-Mg alloys containing up to 8 wt % Mg, progressive heating up to 400C and at 500C can cause the formation of a duplex oxide film consisting of a thick outer layer of MgO over an inner compact layer of 7-A1203. Powder photographs of a stripped film formed at 500 C on an A1-3 wt % Mg alloy showed the presence of MgA1204 . Leontis and Rhines and Markworth have also demonstrated the presence of MgA1204 in the surface oxide films on high A1-Mg alloys. The oxi- dation of Al-(1 to 14%)Mg alloys at temperatures of 600 to 1100 C revealed that a protective amorphous film on the metal surface crystallized to magnesiumoxide and magnesium aluminate, accompanied by a sudden increase in the oxidation rate .