Effects of pre-dissolved hydrogen on the precipitation and growth kinetics of gadolinium hydride on polycrystalline gadolinium surfaces

The kinetics of precipitation of hydride growth centers (GCs) as well as the growth velocities of the individually growing precipitated hydride 'patches' on polycrystalline gadolinium surfaces have been the subject of some recent studies. Hot stage microscopy coupled with a CCD camera enabled the evaluation of the precipitation rate functions and growth velocities at various reaction conditions and pre-treatments of the reacting surfaces. One of these pre-treatments has been to dissolve hydrogen into the near-surface zone prior to the hydriding step, by pre-exposing the surface to low pressure hydrogen under conditions that no growth centers are yet formed. Such a treatment has a most significant deleterious effect on the kinetics of precipitation and growth of the subsequent hydriding step. These observations contradict the intuitive expectation of an opposite trend, i.e. enhanced hydriding kinetics due to the presence of the pre-dissolved hydrogen that is already available at the reacting zone. Possible mechanisms for such a hydrogen induced 'passivation' effect are proposed.