Mechanism of surface-limited release of hydrogen from oxidized metals

Abstract

Analysis has been made of the time dependence of surface recombination and release of deuterium from helium-bubble traps in iron with both clean and oxidized surfaces. In both cases second-order kinetics are obeyed over a large range of time scales, including short times when the traps are saturated. We show this is possible only if, in both cases, release is limited by escape from surface sites which have the same energy as the traps. We therefore propose that the oxide represents an additional barrier, but does not alter the energies of sites from which recombination takes place relative to solution sites. We further show that for both iron and stainless steel the prefactor for the recombination rate is about the same for a bare surface and for an oxide layer. Thus effects of the barrier are mostly on the activation enthalpy of release.