The macroscopic hydrogen diffusion coefficient D in homogeneous Pd1-xAgx alloys obtained by electrochemical current pulse time-lag measurements showed for small hydrogen concentrations a strong dependence upon the alloy composition. For small silver concentrations up to 25% the diffusion coefficient remained nearly constant. It then fell by about three orders of magnitude, reaching a minimum value at 60%Ag before increasing again with Ag content towards the value for pure silver; which was nearly the same as that for Pd. The results could be fitted satisfactorily for the complete range of alloy composition by Monte Carlo simulations on the basis of a simplified model. In this model two different octahedral sites were assumed to exist showing different hydrogen occupation probabilities (i.e. different hydrogen solubilities). At smaller silver contents the silver atoms partly block the energetically favored diffusion paths in the Pd matrix leading to a round-about way diffusion. At high silver concentrations, Pd atoms acted as traps for hydrogen in a silver matrix. The interplay of the hydrogen transport via two kinds of occupation sites with different hydrogen solubilities determines the shape of the curve for the macroscopic diffusion coefficient as a function of the alloy composition. If Ag was substituted by Ni or Cu an analogous behavior was observed for the fcc phase region. When alloying V or Nb to Pd - both metals had a higher hydrogen solubility compared to Pd - these metals acted as traps for hydrogen in the Pd matrix similar to Pd in a silver matrix.

Hydrogen Diffusion in Palladium Based FCC Alloys. H.Barlag, L.Opara, H.Züchner: Journal of Alloys and Compounds, 2002, 330-332, 434-7