The oxidation of rolled and heat treated Al-Mg alloys

Abstract

The formation of oxide films on two production grade Al-Mg alloys (0.8wt% and 2.5 wt% Mg) during heat treatment after cold rolling, in the range 350–600°C, in either dry or moist flowing air has been studied. Quantitative Auger electron spectroscopy, in conjunction with argon ion sputtering, has been used to obtain composition-depth profiles through the oxide layers, with concomitant weight gain measurements. The effects on the kinetics and thermodynamics of the oxidation process by changing the bulk magnesium content, the heat treatment and the humidity of the environment have been ascertained. The measurements are consistent with the following oxide growth mechanism. In the cold rolled state a very thin self-healing amorphous film of A1 2O 3 exists. During heat treatment oxide crystallites nucleate and the thickness increases by grain boundary diffusion of aluminium and magnesium to the free surface. The difference in diffusivity of the species ensures that the surface becomes magnesium-rich. An island MgO film forms on the surface while Al 2O 3 in the film is reduced by the outwardly diffusing magnesium to form the spinel MgAl 2O 4. Eventually free aluminium can exist within the oxide. The MgO islands join to form an aluminium-free surface. The kinetics of oxidation and the morphology and composition of the oxide can be controlled by the humidity during heat treatment, probably because of the incorporation of hydroxyl ions. During dry storage at 60°C no significant changes occur in the oxide film but samples stored in moist conditions exhibit a marked reduction in the magnesium atom fraction of the surface, and magnesium-free surfaces can be produced. The implications of the surface layer composition of Al-Mg alloys to joining technology are discussed.