Hydrogen Release Through Metallic Surface: The Role of Sputtering and of the Impurity Dynamics

A metal surface at elevated temperatures in vacuum is usually covered by a monolayer of nonmetallic impurities. Such a monolayer reduces by many orders of magnitude the rate constant, kr, of the recombinative release of absorbed hydrogen. Due to that, D/T recycling slows down, and D/T retention and permeation rise, by orders of magnitude at the metal interaction with energetic hydrogen particles. In reality, impurity coverage is resulting from a dynamic balance between impurity removal by physical and chemical sputtering and impurity supply both from the metal bulk due to surface segregation and from the outside, e.g., due to reactions with residual gases, etc. The question arises how and in what way does such impurity dynamics affect kr? Nb containing a solute O impurity was taken as an example for our experiments using a combination of ion beam and permeation techniques. Under our ultra-high vacuum conditions, the surface segregation of solute impurity was the only mechanism to restore the monolayer coverage subject to sputtering by energetic hydrogen. It is found that at elevated temperatures, sputtering typically can neither remove the impurity monolayer nor even noticeably increase the concentration of impurity vacancies in the coverage as compared to its equilibrium value. As a result, the rate of hydrogen reemission cannot be noticeably increased by sputtering until the concentration of solute impurity is significantly decreased over a definite zone beneath the surface. An interesting point is that the depth of this zone turns out to be the larger, the greater the concentration of solute impurity; e.g., it may stretch to a few tenths of millimeter in Nb at an O concentration of a few tenths of atomic per cent. Thus it is the metal purity that is expected to be the key factor determining the reemission, retention and permeation at the metal interaction with energetic hydrogen at elevated temperatures.