Impact of grain-boundaries (GBs) on hydrogen diffusion and trapping mechanisms was investigated in a large range of grain size in nickel. Both aspects depend mainly on the nature of GBs. We illustrate the effects of the Random and Special boundaries on the different defects and trapping sites stored in the GBs, and their consequences on the 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. The predominance of one phenomenon depends on several parameters, such as the grain size, features associated to GBs like percolation, energy and excess of free volume.
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