Interdiffusion in thin layers at the surface of nickel-platinum alloys

The transport processes during the formation or dissolution of nickel-platinum alloys are studied by using a polarization cell with a fused electrolyte. The transient responses to an electrochemical perturbation are analysed by digital simulation. The calculations include the effects of the interface motion, the change in the diffusion coefficient vs the alloy composition and take account of the thermodynamic properties of the alloys. At short time scales, it is shown that the transport of matter in atomic layers near the interface is more rapid than in the bulk of the alloy. The change in the composition of the very initial atomic layers is very rapid and gives rise to an accumulation process which favors the deposition of the electroactive metal at a potential much more positive than the deposition potential of the bulk metal. This mechanism leads to the same effect as the phenomenon named underpotential deposition.