The rate of hydrogen release out of clean metallic surfaces

The theory of hydrogen permeation through metals is extended from the domain of high upstream H2 pressures p1, where diffusion limits the permeation flux phi, down to that of low p1 where hydrogen penetration and recombination at the surface are rate determining. In the high p1 case, as known, measurements yield the bulk parameters Ksub(s) and D(solubility and diffusion constants). At the low and intermediate pressures discussed, values of ksub(s) and ksub(r) can be determined: these phenomenological rate constants characterize respectively the dissociative solution and recombinative H2 release processes. A dimensionless number W' related to surface and bulk parameters allows one to evaluate the transition pressure from high to low p1 domains (W' =1). The variations of phi with p1 (stationary case) and with time (nonstationary evolution), as calculated and observed in the experimental device described here, are in good agreement. In the high p1 domain the results are close to data from the literature. (Auth.)