HYDROGEN DIFFUSION AND PERMEATION IN MICROCRYSTALLINE AND NANOCRYSTALLINE NICKEL

Electrochemical studies with nanocrystalline nickel (grain size almost -equal-to 100 nm) have shown that hydrogen permeation is remarkably la rger in comparison with microcrystalline nickel (grain size almost-equ al-to 2 mum). This is due to an increase of both hydrogen solubility a nd hydrogen diffusivity. The latter quantity increases by two orders o f magnitude when the hydrogen activity is enlarged. However, measureme nts of the time lag during transient permeation show that at very low hydrogen activities the diffusion coefficient can be smaller when comp ared to a microcrystalline sample. For small cathodic current densitie s (< 10 muA/cm2) all of the produced hydrogen is absorbed in atomic fo rm by the sample whereas for large current densities the overwhelming part of the electric charge is consumed for the formation of gaseous h ydrogen according to the Volmer-Tafel mechanism. The results are expla ined in the framework of a model used for hydrogen diffusion in defect ive and disordered materials.