Barrier-layer formation and its control during hydrogen permeation through Ti-24AI-11Nb alloy

Hydrogen transport through Ti-24Al-11Nb (at. pct) alloy has been measured using ultrahigh vacuum permeation techniques over the temperature range of 500 °C to 900 °C and a hydrogen pressure range of 20 to 1333 Pa. The hydrogen uptake behavior of the alloy is influenced by even very small amounts of contaminants in the hydrogen gas. It is shown that contaminants such as CO and H₂O are released from the walls of the vacuum system by the incoming hydrogen gas due to collision-induced desorption and that these species form a barrier layer. In the presence of the barrier layer, the hydrogen-permeation behavior of the alloy has two apparent temperature regimes: a high-temperature regime where the barrier layer has minimal effect, and a low-temperature regime where the barrier layer inhibits the hydrogen uptake. A physical model to explain this behavior is presented. It is further shown that the effect of the barrier layer can be minimized by maintaining dynamic flow conditions in the upstream chamber. Under these conditions, the transport process is diffusion-limited: the permeability has a weak temperature dependence, but the diffusivity has a strong temperature dependence.