Permeation of hydrogen through tantalum: influence of surface effects

Gas driven permeation of hydrogen through Ta has been investigated at 900 K and p=50–5000 Pa. The permeation rate as a function of time has a peaked shape: after initial rise, it decreases for several hours, remaining approximately proportional to the gas pressure, which is the feature of the surface limited regime (SLR). Computations using an SLR model were performed. It was concluded that the Ta membrane becomes asymmetric due to the contamination of the inlet surface. The solubility S=(9.3±3.0)×1018 H cm−3 Pa−1/2 is in good agreement with data available from literature. The recombination and absorption coefficients K and Ka have been obtained in the ranges K=(6–500)×10−26 at−1 cm4 s−1 and Ka=(3–200)×1012 at cm−2 s−1 Pa−1, respectively. The values of Ka are very low. It has been concluded that long permeation experiments need special measures for purification of the working gas.