The macroscopic H diffusion coefficients in homogeneous Pd1-xAgx alloys, obtained by performing electrochemical current-pulse time-lag measurements, revealed (for small H concentrations) a strong dependence upon the alloy composition. At Ag concentrations of up to 25%, the diffusion coefficient remained almost constant. It then fell, by about 3 orders of magnitude, to a minimum at 60%Ag before increasing again (with increasing Ag content) towards the value for pure Ag. The latter was almost the same as that for Pd. The results could be fitted satisfactorily, for the complete range of alloy compositions, by basing Monte Carlo simulations on a simplified model in which 2 different octahedral sites were assumed to exist having different H occupation probabilities (different H solubilities). At smaller Ag contents, the Ag atoms partially blocked the energetically favored diffusion paths in the Pd matrix; leading to roundabout diffusion. At high Ag concentrations, the Pd atoms acted as traps for H in a Ag matrix. The interplay of H transport, via 2 kinds of site with differing H solubilities, determined the shape of the curve for the macroscopic diffusion coefficient as a function of the alloy composition. If Ag was replaced by Ni or Cu, an analogous behavior was observed for the face-centered cubic phase region. Upon adding V or Nb to Pd, where both metals exhibited a higher H solubility than that of Pd, they acted as traps for H in the Pd matrix (like Pd in a Ag matrix).

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