By using a low-pressure permeation technique, an investigation was made of the permeation of H through α-Pd. The observed permeability at pressures ranging from 2.9 x 10-5 to 1.7 x 10-3cmHg, and temperatures ranging from 300 to 709K, was governed by an activation energy of 3.745kcal/mol. The fact that the permeability exhibited a square-root dependence upon the pressure and a reciprocal dependence upon the thickness was taken to be evidence that bulk diffusion, rather than surface reaction, was the rate-controlling process. The permeability data were combined with known solubility data to show that the H diffusivity obeyed:

D (cm2/s) = 4.74 x 10-3 exp[-5.745(kcal/mol)/RT]

There was no effect due to sub-structural defects over the temperature range which was used. On the basis of the permeability data, coupled with grain-size measurements, it was concluded that the ratio of grain boundary diffusivity to bulk diffusivity was less than 105 over the present temperature range.

S.A.Koffler, J.B.Hudson, G.S.Ansell: Fall Meeting, Metallurgical Society of AIME, October 1968