The variation in over-potential with time, on Fe membranes which were cathodically polarized in M/3 H2SO4, was found to be due to the formation of an adsorbed layer of H. This layer decayed with time but it could be removed by applying an alternating current. Anomalous time-lag curves, permeation maxima, and the formation of so-called barriers during the diffusion of electrolytic H were largely attributed to surface-adsorbed H. At 25C, the average value of the H permeability in Fe was 10-6cm/s, and the diffusivity was equal to 6 x 10-5cm2/s. On the basis of these data, it was concluded that the H which was retained in traps within the metal exceeded the concentration of H in the lattice by about one order of magnitude.

Diffusion Phenomena of Electrolytic Hydrogen in Pure Iron Membranes. S.Wach: British Corrosion Journal, 1971, 6[3], 114-8

Table 31

Diffusivity of H at 20-22C after Electrolysis of Fe in Vacuum at 20 to 50C