Accelerated diffusion of hydrogen along grain boundaries in nickel

Accelerated diffusion of hydrogen was observed at room temperature in polycrystalline nickel samples with the electrochemical permeation technique. A larger hydrogen Aux was measured in annealed samples with 25 mu m grain size whereas faster hydrogen diffusion was observed at the early stage of the permeation process in samples with a larger grain size (150 mu m). The large flux in the small grain material is attributed to the contribution of the grain boundaries to the rotal hydrogen Aux. The influence of grain boundaries is limited to the near room temperature range as shown by comparison of permeation data obtained at 293 and 373 K. When compared to lattice diffusivity, the increase in the diffusion coefficient of hydrogen (computed with Fick's solutions for intergranular diffusion or from the permeation flux at steady state) due to short path diffusion of hydrogen along grain boundaries is of the order of 2-7. Furthermore, short circuit diffusion is probably minimized by trapping effects. Copyright (C) 1996 Acta Metallurgica, Inc.