Relation between grain size and hydrogen diffusion coefficient in an industrial Pd-23% Ag alloy

The diffusion mechanism of hydrogen in an industrial cold worked Pd-23% Ag alloy is investigated according to its grain size. The microstructural para meters are determined by X-ray diffraction analysis and the diffusion param eters are determined using the galvanostatic electrochemical permeation tec hnique. Two diffusion mechanisms are displayed. For apparent grain size bel ow 40 nm the hydrogen atoms diffuse in the grain boundaries. For grain size above 100 nm, an intergranular diffusion mechanism occurs. For intermediat e grain size both mechanisms coexist. Furthermore, the diffusion in the gra in boundaries is not faster than the diffusion in the bulk, as generally ob served for pure samples. This difference is due to interactions between dif fusing hydrogen and impurities in the grain boundaries. (C) 1999 Elsevier S cience B.V. All rights reserved.