Air bake-out to reduce hydrogen outgassing from stainless steel

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 degrees C in air was suggested as a money saving alternative to the classical vacuum heating at 950 degrees 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. (C) 1998 American Vacuum Society.