Surface effects on hydrogen permeation

A new model on hydrogen permeation is proposed, considering absorption and desorption processes on the sample surfaces. Analytical solution, satisfying the flux continuity rather than either the concentration boundary conditions or the constant flux boundary conditions, is derived from the model. The diffusivity can be evaluated by fitting the whole permeation curve. Drift velocity through surface and drift velocity in bulk are introduced and their ratio determines the validity of the time-lag model. When the ratio of the drift velocity through surface over that in bulk approaches infinity, the proposed model is reduced to the time-lag one. The obtained results can predict the effects of temperature, sample thickness and energy barriers of absorption and desorption on the permeation process. The thickness effect occurred in using the time-lag model is well explained by the effects of absorption and desorption on the permeation process. Permeation experiments were conducted at room and high temperatures on fully- annealed-commercially-pure iron with two kinds of surface treatment, the group I samples with plasma cleaning and the group II samples without it. The experimental results show that the diffusivity evaluated by the new model is independent of sample thickness and surface treatment, while the diffusivity evaluated by the time-lag model varies two orders in magnitude. The experimental results confirm that the energy barrier for desorption is lower in the group I samples than that in the group II samples. However, the energy barrier for either group is higher than the activation energy of diffusion. Consequently, the ratio of drift velocity through surface to that in bulk increases with increasing temperature and makes the time-lag method appropriate at elevated temperatures.;In addition, the effects of isolated traps on hydrogen permeation were studied experimentally at room and high temperatures on Fe-(1 ∼ 4) wt.% Ni alloys. The energy barriers on the sample surfaces were obtained.