The Permeation of Hydrogen Isotopes Through Structural Metals at Low Pressures and Through Metals with Oxide Film Barriers

The permeation and the pressure dependence of the permeation of hydrogen isotopes through metals and oxidized metals were studied at temperatures from 300 to 800°C and at pressures of 10^-3 Torr to 1 atm. Such knowledge is important to tritium management in both fusion and fission nuclear reactors. An adequate basis for predicting the permeation of hydrogen at very low pressures has not previously been established; therefore, the two complementary objectives of this study were (a) to determine the pressure dependence of hydrogen permeation through materials of which steam generators might be built, and (b) to determine whether an oxide film might serve as a tritium permeation barrier. The metals studied included nickel, Type-304 L stainless steel, Hastelloy N, Incoloy 800, Croloy T9, Croloy T22, and Type-406 stainless steel. Deuterium, rather than normal hydrogen, was used as the permeating gas in order to achieve high sensitivity in the mass spectrometric analyses. At a given temperature, the permeation rate of deuterium through metals that are substantially free of oxide films was found to proceed with a half-power pressure dependence in accordance with the relationship

where J is the permeation flow rate, K is a constant, and P₁ and P₂ are the upstream and downstream gas pressures, respectively.

The rates of the permeation of deuterium through oxidized metals were usually lower than through unoxidized metals and the observed pressure dependence was frequently greater than the half-power. For some alloys, a reduction in permeation rate by a factor of a hundred or more at 1-atm pressure was observed; the reduction at low pressures was even greater. The observations made are consistent with considerations of the chemical stability of the oxide and of the presence of cracks or other imperfections in the oxide film.