Permeation of Hydrogen through Tungsten and Molybdenum

Using ultrahigh-vacuum and mass-spectroscopic techniques, permeation of hydrogen through wrought-and vapor-deposited W tubes and through a drawn Mo tube has been measured for pressures between 10−6 and 200 torr and temperatures between 1050° and 2400°K. Permeation rates varied with pressure and temperature in a complex manner. Expressions for the permeation rate are derived on the assumptions that (a) the rate determining step of permeation is diffusion of hydrogen in the bulk of the sample, and (b) the rate of desorption of hydrogen atoms Da, the rate of recombinative desorption of hydrogen molecules Dm, and the concentration of dissolved hydrogen atoms are each determined by the concentration of hydrogen atoms adsorbed on the metallic surface. For the hydrogen–tungsten system, this analysis agrees with measured rates over the entire range of temperature and pressure, and the experimental data is consistent with Hickmott's values for: the rates Da and Dm; the sticking probability for hydrogen atoms; and the probability that an incident hydrogen molecule adsorbs as two atoms. For the hydrogen–molybdenum system, permeation is controlled by phase-boundary processes at low driving pressures. Measured permeation constants follow the equation P = P0exp (− QP / RT) above 1 torr. The results P0 = (1.5 ± 0.5) × 10−3torr·litercm−1·sec−1·torr−1 / 2 and QP = (31.5 ± 1) kcal / mole are found for W. For Mo, the corresponding numbers are (7.1 ± 2) × 10−4 and (21.5 ± 1).