Oxidation of Aluminum Particles in the Presence of Water

Oxidation of spherical aluminum powder was investigated in mixed argon-oxygen-steam atmospheres by thermogravimetric measurements at heating rates between 1 and 20 K/min and up to 1100 degrees C. The observed oxidation behavior in the presence of steam differs markedly from oxidation in dry oxygen. Oxidation in steam is complete near 1000 degrees C vs 1500 degrees C in dry oxygen. Furthermore, in steam, a stepwise weight change is observed at the melting point of aluminum, while no such step can be distinguished in dry oxygen. The complete oxidation observed at a lower temperature in steam as compared to dry oxygen is explained by the stabilization of the gamma polymorph of the surface oxide in the presence of water so that a denser and slower growing alpha-alumina does not form until higher temperatures. Experiments in mixed oxygen/steam oxidizers showed that the size of the oxidation step observed upon aluminum melting only correlates with the concentration of steam in the atmosphere. This may be interpreted as the effect of transient porosity, the degree of which is controlled by the steam concentration, or the surface oxide stressed by the expanding melting metal core may behave as a semipermeable membrane where hydrous species have significantly higher diffusion rates than oxygen. A clear distinction cannot be drawn, and further research is warranted. Preliminary results on isoconversion processing of the oxidation kinetics are presented.