Investigation on microstructure and hydrogen generation performance of Al-rich alloys

In order to study the effect of cooling rate on the microstructures and hydrogen generation performance of Al alloys, two ingots (20 g and 45 g) with a composition of 94 Al, 3.8 Ga, 1.5 In and 0.7 Sn (in mass%) were prepared by arc melting under high purity argon atmosphere, and a rod (10 g) with the same composition was cast in a vacuum chamber. The microstructures and phase compositions of the three samples were investigated by means of X-ray diffraction and scanning electron microscope with energy dispersed X-ray. The melting point of the grain boundary phase was measured using differential scanning calorimeter. Based on the structural analysis, samples with different but uniform grain sizes were cut from these alloys for H2 generation. The reactions of Al alloys with water were measured at different temperatures. The measured H2 generation rates were found to increase rapidly once the grain size was reduced below 50 μm. An isothermal kinetic model was employed to analyze the measured kinetic data so as to obtain kinetic parameters of reactions. The reaction order (n) for these alloys was found to be about 0.7. The activation energy (Ea) decreases with grain size d, i.e., 30% reduction of Ea as d was reduced from 258 to 23 μm. A mechanism of Al alloy corrosion in water was proposed.