SURFACE-ANALYSIS OF OXIDE FORMED DURING HEAT-TREATMENT OF AL-MG-SI ALLOYS

The surface oxides formed during oxidation of Al alloys for automobile body panels were analysed using electron spectroscopy for chemical an alysis, Auger electron spectroscopy, and energy dispersive X-ray spect rometry. Two commercial AA 6000 alloys were studied. The alloys were o xidised in dry air at room temperature, 300 degrees C, 400 degrees C, and 520 degrees C. The thickness and the compositions of the oxides fo rmed on the matrix and on the Si, Mg-2,Si, and Al-Fe-Si particles were determined. The surface oxides consist primarily of Al2O3 and MgO. Th e content of MgO increases with temperature. The activity of Mg is dec reased owing to formation of Mg2Si a the metal/oxide interface during oxidation. A model for the formation and growth of MgO nodules at high temperatures is proposed; during the initial stages of oxidation Al2O 3 is formed, and after prolonged exposure the oxide cracks and Mg evap orates. Magnesium oxide is formed on the surface by condensation and o xidation of Mg. One piece of evidence for this mechanism is that MgO i s found on the non-Mg containing Si and Al-Fe-Si particles. (C) 1996 T he Institute of Materials.