Oxidation of differently prepared Al-Mg alloy powders in oxygen

• Voids form in the oxidized particles.
• Oxidation proceeds in two steps and occurs at the alloy-oxide interface.
• Both Al and Mg oxidize during both steps.
• Switch between oxidation steps is caused by formation of spinel, Al2MgO4.
• Apparent activation energy of reaction is found as a function of oxide thickness.

Powders of both commercial atomized spherical Al-Mg alloy and mechanically alloyed Al-Mg were oxidized in oxygen using thermo-gravimetry (TG). For both powders, the Al/Mg mass ratio was equal to 1. Fully and partially reacted powders were recovered and characterized using scanning electron microscopy and x-ray diffraction. Voids grow within oxidized alloy particles for both atomized and mechanically alloyed powders. Results were interpreted accounting for the measured particle size distribution for the spherical powder and distributing the TG-measured weight gain among the individual particle size bins. The reaction interfaces were always located at the internal surface of the oxide shell as determined by matching the oxidation dynamics for particles with the same sizes but belonging to powders with different particle size distributions. Thus, the reaction is always rate limited by inward diffusion of oxygen ions through the growing oxide shell. Two oxidation stages were identified for both materials. Both Al and Mg oxidize during both observed oxidation stages. The second oxidation stage is caused by formation of the spinel phase, most likely occurring at a threshold temperature. In the present measurements, the step in the oxidation rate, or switch between the oxidation stages, occurs when the oxide shell grows above a certain thickness of approximately 1.5 μm. The apparent activation energy during the first oxidation stage energy changes during the first oxidation stage suggesting that more than one reaction occur in parallel, e.g., causing formation of MgO and amorphous alumina. For the second oxidation step, controlled by diffusion of oxygen through spinel layer, the activation energy remains nearly constant around 185 kJ/mol.