The hydriding kinetics of massive ZrCo

The kinetics of hydride phase development in massive samples of ZrCo was investigated at temperatures between 25 and 300°C and for hydrogen pressure ranged from 1 bar down to the absorption equilibrium pressures. Metallographic examinations of partially hydrided samples have shown that a hydride layer is formed on the sample exterior, advancing into the bulk of the sample during the hydriding process. The front velocity was found to be constant for given pressure and temperature, provided a proper vacuum annealing is performed prior to exposure. Under relatively high pressure (i.e., where P/P_eq(T) ≫ 1), the velocity approaches a saturation (pressure independent) asymptotic value (U_saturation(T)), whereas, at the relatively low pressure regime (i.e., where P/P_eq(T) ∼1), the pressure dependence of the front velocity is linear obeying a pressure function (P/P_eq(T)-1), with a slope U₀(T). The activation energies obtained for the pressure independent constants U₀(T), and U_saturation(T) are about the same, 45.4±0.1 kJ/mol. The bulk velocities measured in this work were compared to the surface growth velocities measured previously. It has been found that far from equilibrium, the bulk and surface velocities are much the same. Similar results were found for the bulk front velocities in well-annealed massive uranium samples and ZrCo, under similar experimental conditions. Some conclusions were drawn concerning the hydriding mechanisms in both systems.