Physico-chemical characterisation of native air-formed oxide films on Al-Mg alloys at low temperature. Influence of water

Natural oxide films on two cold-rolled Al-Mg alloys were studied using seve ral techniques (XPS, SIMS, ToF-SIMS, GDOS, SEM, TEM, Wetting measurements). The aim was to characterise and compare the oxide layers formed on the two materials in order to better understand the influence of the thermomechani cal history. The experimental tools used allowed the determination of the i nfluence of the annealing process on the nature and structure of the oxides . The hydration and Bronsted behaviour were also investigated. The thicknes s of the layer, the amount of magnesium oxide and its dissolving resistance in water are strongly modified by the annealing. The oxide film formed on the annealed material is twice as thick as the one on the non-reheated mate rial. The layer on the annealed material contains more magnesium oxide and contains the crystalline forms:beta-Al2O3 and bayerite (Al(OH)(3)). On the contrary, the oxide film formed on the as-rolled material seems to be amorp hous. The magnesium oxide is less soluble in water in the annealed film tha n in the non-reheated one. Furthermore, aluminium and magnesium oxides were found to be hydroxylated on both alloys, and the layer surfaces to behave like a Bronsted meaning base. (C) 1999 Kluwer Academic Publishers.