Hydriding kinetics of Zircaloy-4 in hydrogen gas

We have studied the hydrogen absorption kinetics of Zircaloy-4 massive samples at 623 K and at an initial hydrogen gas pressure of 30 bar, using the volumetric technique. The specimens were Ar etched and Au-Pd covered (50 nm thick) prior to measurement in order to obtain uniform hydride fronts during hydriding. The resulting hydride phase has a face center tetragonal lattice (a=0.497 nm, c=0.485 nm; ε phase) and composition ZrHy with y=2.0±0.1. During the absorption process three successive kinetic stages could be identified before saturation: (i) an initial stage with low and constant hydrogen uptake rate, limited by diffusion through an oxygen-rich region in the Au-Pd/Zry-4 interface, (ii) an stage where the hydride fraction grows as t5/2, and (iii) a final stationary state with constant hydrogen absorption rate where the hydride front advances at 26 μm min-1. We have discussed the relationship between these stages and the diffusion processes (intra and inter-granular), the nucleation and growth of hydride platelets, and the process of crack formation. (orig.)