Hydrogen pumping with a hot martensitic steel membrane

Abstract Long-term maintenance of low pressure in vacuum thermal insulating devices is a difficult task. Even in completely tight metal envelopes, gas accumulation, due to outgassing of applied materials, represents the main problem. Among gases which are dissolved in constructional metals, hydrogen is the most harmful one, but it is also the only gas species that might permeate to the atmosphere. We present experimental work where a heated metal membrane made of martensitic steel, termed Eurofer, was studied as a selective hydrogen pump. It was built as an extended part of a tight UHV vacuum chamber. Initial hydrogen pressure was set in a wide range from 1.5 bar to 1 × 10 −3  mbar at temperatures from 100 °C to 400 °C. The observed hydrogen permeation flow was expressed as the specific pumping speed S A . At the initial pressure p 0  = 1 mbar it was, S A (400 °C) = 1.6 × 10 −6  L s −1  cm −2 . At low pressures, this attractive pump performance was overwhelmed by background outgassing of CO and CO 2 . At temperatures lower than 200 °C, the outgassing rate of CO and CO 2 was not detectable over the first 20 h and pumping due to hydrogen permeation continued into the 10 −4  mbar pressure range.