Diffusion processes in Ni-Pt alloys, at temperatures of between 500 and 720C, were studied by using transient electrochemical techniques (table 105). The latter method involved the use of a cell of the form: Ni|fused electrolyte+NiCl2|Ni-Pt. The pulses were both anodic and cathodic, under galvanostatic and potentiostatic conditions, and the interdiffusion coefficient was deduced by numerically integrating the diffusion equation. The calculations took account of specific features of the diffusion process, such as motion of the interface due to electrolysis (dissolution or deposition of Ni), a dependence of the diffusion coefficient upon the alloy composition, and the effect of thermodynamic properties of the Ni-Pt solid solutions. This electrochemical technique permitted a direct comparison to be made of the inward (reduction of Ni2+) and outward (oxidation of Ni) diffusion. It was pointed out that, since the diffusion rate due to a vacancy mechanism was proportional to the local vacancy concentration, there should be a marked enhancement of the out-diffusion rate. It was suggested that most of the electrochemically created vacancies coalesced at the surface and were not injected into the interior of the alloy. Changes in the number of vacancies were suggested to be restricted to the very first atomic layers. By varying the time-scale of the experiments, it was possible to study the diffusion process over mean surface thicknesses which ranged from 2 to 100mn. No significant variation in the diffusion coefficient occurred within this range. It was concluded that the diffusion rate in the surface layers was the same as that in the bulk; at least for penetration lengths that were greater than about 2nm.

F.Lantelme, A.Salmi: Journal of the Electrochemical Society, 1996, 143[11], 3662-9

Table 105

Interdiffusion in the Ni-Pt System