Influence of chemisorbed hydrogen molecules on the H-permeation and recycling : model and equations

We have shown in a previous report that a "molecular chemisorption model" (MCM) is better adapted than the dissociative chemisorption model (DCM) of Lennard-Jones to describe the solubility and permeation behaviours of hydrogen in metals and alloys such as iron, nickel, copper, steels and Inconels. It was further shown that the MCM allows to account adequately for the measurements made in surface physics on hydrogen adsorption on these metals. A corollary of the MCM is that the (H$_{2}$)a admolecules should catalyse the recombinative release of hydrogen out of these lattice. This effect should strongly influence the permeation experiments made under simultaneous atomic and molecular exposures. Incident electrons should moreover dissociate the chemisorbed molecules when the electron energy is sufficiently high. This increases the fluxes of hydrogen penetrating into and permeating through the lattice ; it also decreases the surface coverage factor and the catalytic activity of the admolecules (H$_{2}$)aupstream. This report analyses these effects. It is shown that permeation measurements can provide valuable information on the kinetics and equilibrium properties of the adsorbed layer and that the behaviour predicted by the MCM is in several aspects surprising a priori. The latter is however observed, as shown by measurements which will be presented and discussed in another report