Surface analysis of oxide formed during heat treatment of Al–Mg–Si alloys

Abstract

The surface oxides formed during oxidation of Al alloys for automobile body panels were analysed using electron spectroscopy for chemical analysis, Auger electron spectroscopy, and energy dispersive X-ray spectrometry. Two commercial AA 6000 alloys were studied. The alloys were oxidised in dry air at room temperature, 300°C, 400°C, and 520°C. The thicknesses and the compositions of the oxides formed on the matrix and on the Si, Mg₂Si, and Al–Fe–Si particles were determined. The surface oxides consist primarily of Al₂O₃ and MgO. The content of MgO increases with temperature. The activity of Mg is decreased owing to formation of Mg₂Si at 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 Al₂O₃ is formed, and after prolonged exposure the oxide cracks and Mg evaporates. Magnesium oxide is formed on the surface by condensation and oxidation of Mg. One piece of evidence for this mechanism is that MgO is found on the non-Mg containing Si and Al–Fe–Si particles.

MST/3348