Kinetic study of the hafnium-hydrogen reaction

The kinetics of hafnium hydride formation were studied utilizing conventional rate measurements (Sieverts system) combined with metallographic examinations of partially hydrided samples. The rate measurements were performed at 700 TorrH₂ over a temperature range 200-550 °C. Two types of hafnium samples (polycrystalline and crystal bar) were compared. The progression of the massive stage of the reaction is characterized by a contracting-envelope morphology with a constant hydride front velocity. The anisotropy in the reaction front velocity regarding different crystalline orientations of the metal is small, resulting in similar results for the different types of hafnium. The temperature dependence of the front velocity obeys an Arrhenius-type relation over the temperature range 250-450 °C, with an apparent activation energy of 0.50 ± 0.05 eV. Considering a diffusion-controlled model, a diffusion activation barrier of about 0.4 eV is evaluated, which agrees with the average reported value for the diffusion of hydrogen in hafnium hydride. At temperatures above about 500 °C, deviations from the Arrhenius relation are displayed, possibly owing to a change of mechanism.