The H migration characteristics were determined, at temperatures of between 700 and 1400K, by using H pressures ranging from 0.013 to 4.26Pa. The steady-state permeability, at temperatures greater than 900K, could be described by the expression:

P (torr liter/cm2 s torr½) = 1.6 x 10-5 exp[-30.6(kJ/mol)/RT]

At temperatures below 1000K, there was evidence of surface impedance and the results indicated that permeation decreased with decreasing temperature; unlike the volume-controlled high-temperature permeability. At temperatures greater than 1100K, the H diffusivity could be described by the expression:

D (m2/s) = 4.4 x 10-8 exp[-12.8(kJ/mol)/RT]

At lower temperatures, the diffusivity decreased below the extrapolated values of this expression. This behavior was again attributed to the effect of surface impedance. An isotope effect was detected in the diffusivities and permeabilities of H and D (table 95). The permeation constants were associated with a negative enthalpy.

Hydrogen Permeation and Diffusion in Niobium. R.Sherman, H.K.Birnbaum: Metallurgical Transactions A, 1983, 14[2], 203-10