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 larger in comparison with microcrystalline nickel (grain size almost-equal-to 2 mum). This is due to an increase of both hydrogen solubility and hydrogen diffusivity. The latter quantity increases by two orders of magnitude when the hydrogen activity is enlarged. However, measurements of the time lag during transient permeation show that at very low hydrogen activities the diffusion coefficient can be smaller when compared to a microcrystalline sample. For small cathodic current densities (< 10 muA/cm2) all of the produced hydrogen is absorbed in atomic form by the sample whereas for large current densities the overwhelming part of the electric charge is consumed for the formation of gaseous hydrogen according to the Volmer-Tafel mechanism. The results are explained in the framework of a model used for hydrogen diffusion in defective and disordered materials.