Thermal desorption kinetics of H2O and H2 from rapidly solidified Al-Zn-Mg alloy powders

Abstract

The thermal desorption of water and hydrogen from a rapidly solidified Al-Zn-Mg alloy powder was studied by means of temperature programmed desorption mass spectrometry (TPD-MS). By applying the Kissinger formalism, an activation energy of 137 ± 7 kJ/mol and a pre-exponential factor of 5.2 × 1011 s−1 were deduced for the primary broad water desorption peak, strongly conforming to the two pure phases of Al(OH)3, gibbsite and bayerite. Desorption of hydrogen is assigned to the oxidation of aluminum by water evolving during dehydration of the surface hydroxide layer upon heating. Overall, a complex mechanochemical mechanism is captured, with desorption profiles that are indicative of two types of oxidation processes: (1) fast, non-kinetic oxidation, initiated at ∼377 °C, followed by, (2) a slower, diffusion controlled oxidation event. The former may be viewed as a solid experimental evidence for the breakage of the surface oxide film, occurring at a relatively low temperature (<400 °C) during degassing. This breakage exposes either Al or other alloying elements to available water, which trigger in turn a fast oxidation reaction from which hydrogen is evolved.