Reduction in hydrogen outgassing from stainless steels by a medium-temperature heat treatment

The authors carried out heat treatments, in-vacuum or in-air at 400°C, to reduce the hydrogen outgassing rate from stainless steels. An outgassing rate as low as 2× 10-14 Torr s-1 cm-2 was routinely achieved by a medium-temperature bakeout, but it took much longer time than reported to perform intensive thermal treatment. The result shows that the diffusion process governs degassing only at the early stage of degassing while the recombination limits outgassing at low concentrations. Air baked chambers had somewhat lower outgassing rates than in-vacuum baked chambers, suggesting that the surface oxide acts as a further barrier for H2 outgassing. However, the main effect may be attributed to the removal of mobile hydrogen through diffusion. The results showed that the ultralow outgassing rate can be reproducibly achieved for stainless steel chambers with the established heat treatment procedure. The study also showed that the ultralow outgassing property of a treated chamber can be restored by a low temperature (>150°C) postbakeout, after exposure to ambient air.