Experimental investigation of aluminum oxidation in water

The use of aluminum as an energetic component in pyrotechnics, solid fuel formulations, and propellants is motivated by its high gravimetric oxidation enthalpy. However, the rate of its oxidation is significantly lower than that of other, typically hydrocarbon-based, components of fuel formulations. As a result, aluminum combustion will take place in the combustion products of those components, i.e. H₂O and CO/CO₂. In order to design and optimize fuel formulations as well as the devices where these formulations are used, the thermodynamics and kinetics of the reaction of aluminum with the component gases need to be understood. This report shows results of oxidation of aluminum powders in water-argon gas mixtures at temperatures between 300 - 1100°C using thermogravimetry. In this environment, aluminum oxidizes in several distinct stages. Initial slow oxidation between 300 - 500°C is followed by stepwise oxidation of approximately 10 % near 550°C. At low heating rates (<5 K/min), the next sharp oxidation step of about 10 % is observed at the aluminum melting point of 660°C. This oxidation step shifts to higher temperatures at increased heating rates. The initial two oxidation steps are followed by one or more distinguishable spread-out oxidation reactions that terminate when the material is fully oxidized above 1000°C. The temperatures and degrees of oxidation achieved in the individual steps depend on the particle size, the heating rate, and the water concentration in the atmosphere. The results are comparable to previously published results of oxidation in oxygenargon mixtures, although there are significant differences. Most notably, the oxidation step associated with aluminum melting was not previously observed experimentally. Phase analysis will be conducted on fully and partially oxidized powders, and a mechanistic model for aluminum oxidation in water will be developed.