Oxidation of different Al-Mg alloy powders in oxygen

Alloys of aluminum and magnesium are widely used in pyrotechnics as well as in other energetic formulations. They were also explored recently as metal additives in oxygen generators and as materials capable of reacting with water for hydrogen production. Thus, mechanisms of low-temperature, heterogeneous oxidation of such alloys by both oxygen and water are of interest. Understanding such mechanisms enables one to model both their aging and initiation in various devices. In this work, both commercial atomized spherical Al-Mg alloys and mechanically alloyed Al-Mg powders were oxidized in oxygen using thermo-gravimetry (TG). Fully and partially reacted powders were recovered and characterized using scanning electron microscopy and x-ray diffraction. Voids grow within oxidized alloy powders for both atomized and mechanically alloyed powders. Two oxidation steps were identified for both spherical Al-Mg alloys and mechanically alloyed powders. Magnesium was first oxidized selectively, producing MgO. Spinel MgAl₂O₄ was produced during the second step. 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. Apparent activation energies for both oxidation steps were obtained as a function of the thickness of the growing oxide layer. The switchover between oxidation steps occurs when the oxide thickness exceeds 1.2-1.5 μm.