Air bake-out to reduce hydrogen outgassing from stainless steel

Abstract

Hydrogen outgassing is the most significant factor limiting the attainment of outgassing rates below 10−12 mbar l s−1 cm−2 in stainless steel vacuum systems. This limit turns out to be crucial in very large vacuum systems, like the VIRGO vacuum tubes (2 tubes 1.2 m diam, 3000 m length). Heating the raw material at 400 °C in air was suggested as a money saving alternative to the classical vacuum heating at 950 °C. We report the results of hydrogen content analysis performed on stainless steel samples submitted to different treatments, and also the measurement performed on a prototype tube (1.2-m-diam, 48-m-long). We concluded that air bake-out drives out most of the hydrogen absorbed in the bulk stainless steel, while the presence of an oxide layer does not reduce the hydrogen outgassing.