Kinetics of hydrogen absorption by scandium at high temperature

Hydrogen absorption by Sc has been investigated over wide ranges of temperature (790–1280 K) and pressure (10–150 mbar). The absorbed quantities of H were in agreement with those expected from p-x-T isotherms, available in the literature, only for temperatures higher than 1000 K, where the absorption curves could be fitted to a Johnson-Mehl-Avrami type of relationships. The gas–solid reaction was first-order and the reaction-rate-constant k exhibited an Arrhenius type of temperature dependence with an associated activation energy of 2.1 ± 0.1 eV (202 ± 10 kJ/mol·H2). The step controlling the absorption rate turned out not to be H diffusion in the bulk. Namely, the values of the apparent H diffusion coefficient deduced from absorption data were found to be some orders of magnitude smaller than expected from extrapolations of lower temperature anelastic and spin-lattice relaxation data. The absorption rate appears to be governed by H penetration through a sub-surface Sc layer containing a high concentration of interstitial oxygen, originated from the decomposition of surface oxides occurring between 900 K and 1000 K.