A model for atomic hydrogen-metal interactions - application to recycling, recombination and permeation

A model for the interaction of hydrogen with metals is discussed which includes the surface concentration and the surface potential and is based on the observation that molecular hydrogen is dissociated before it is chemisorbed at a metal surface. It is shown that an activation barrier for dissociation of molecular hydrogen at the metal surface leads to an enhanced solubility and increased permeation rate of hydrogen in a metal exposed to atomic hydrogen as compared to molecular hydrogen. The sticking probability of molecular hydrogen on clean transition metals has been measured in the past and shown to be of the order of unity. As no activation barrier is present no difference is expected between atomic and molecular hydrogen. Whenever large differences have been observed in the past they must be attributed to an impurity contamination of the metal surface. Substantial reductions of the sticking probability due to dissociation barriers have been found for metals contaminated with less than one monolayer of electronegative impurities. We compare our model with earlier descriptions of hydrogen metal interactions. A theoretical expression is derived for the ''recombination rate constant''. Furthermore we discuss implications of our model on hydrogen permeation (''super permeation'') as well as hydrogen pumping and pumping panels in tokamaks. (orig.)