The hydriding kinetics of massive ZrCo

The kinetics of hydride phase development in massive samples of ZrCo was in vestigated at temperatures between 25 and 300 degreesC and for hydrogen pre ssure ranged from 1 bar down to the absorption equilibrium pressures. Metal lographic examinations of partially hydrided samples have shown that a hydr ide 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 con stant for given pressure and temperature, provided a proper vacuum annealin g is performed prior to exposure. Under relatively high pressure (i.e., whe re P/Peq(T) much greater than 1), the velocity approaches a saturation (pre ssure independent) asymptotic value (U-saturation(T)), whereas, at the rela tively low pressure regime (i.e., where P/P-eq(T) approximate to 1), the pr essure dependence of the front velocity is linear obeying a pressure functi on (P/P-eq(T) - 1), with a slope U-0(T). The activation energies obtained f or the pressure independent constants U-0(T), and U-saturation(T) are about the same, 45.4 +/-0.1 kJ/mol. The bulk velocities measured in this work we re compared to the surface growth velocities measured previously. It has be en found that far from equilibrium, the bulk and surface velocities are muc h the same. Similar results were found for the bulk front velocities in wel l-annealed massive uranium samples and ZrCo, under similar experimental con ditions. Some conclusions were drawn concerning the hydriding mechanisms in both systems. (C) 2001 Elsevier Science B.V. All rights reserved.