Grain size and grain-boundary effects on diffusion and trapping of hydrogen in pure nickel

The impact of grain size on hydrogen diffusion and trapping mechanisms has been investigated for a wide range of grain size of non-textured pure nickel. Both aspects depend mainly on the nature of grain boundaries (GBs). In particular, we illustrate the effects of random and special boundaries on the different defects and trapping sites stored in the GBs, and their consequences on hydrogen transport and segregation. The high-angle random boundaries are considered as disordered phase where the hydrogen diffusion is accelerated, while the special boundaries constitute a potential zone for hydrogen trapping due to the high density of trapping sites as dislocations and vacancies. The predominance of one phenomenon over the other depends on several parameters, such as the grain size, the probability of grain boundary connectivity, the grain boundary energy and the excess of free volume. In addition, our experiments confirm that hydrogen promotes vacancy formation probably in GBs.