Hydrogen diffusion in palladium based f.c.c. alloys

The macroscopic hydrogen diffusion coefficient D in homogeneous Pd1-xAgx alloys obtained by electrochemical current pulse time-lag measurements shows for small hydrogen concentrations a strong dependence on the alloy composition. For small silver concentrations up to 25% the diffusion coefficient remains nearly constant. Then it falls off drastically by about three orders of magnitude reaching a minimum value at 60% silver content before increasing again with rising silver content directly towards the value for pure silver, which is nearly the same as 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 are 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 act 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 is substituted by Ni or Cu an analogous behavior is observed for the f.c.c. phase region. When alloying V or Nb to Pd - both metals have a higher hydrogen solubility compared to Pd - these metals act as traps for hydrogen in the Pd matrix similar to Pd in a silver matrix. (C) 2002 Elsevier Science B.V. All rights reserved.