AIR BAKE-OUT TO REDUCE HYDROGEN OUTGASSING FROM STAINLESS-STEEL

Hydrogen outgassing is the most significant factor limiting the attain ment of outgassing rates below 10(-12) mbar l s(-1) cm(-2) in stainles s steel vacuum systems. This limit turns out to be crucial in very lar ge vacuum systems, like the VIRGO vacuum tubes (2 tubes 1.2 m diam, 30 00 m length). Heating the raw material at 400 degrees C in air was sug gested as a money saving alternative to the classical vacuum heating a t 950 degrees C. We report the results of hydrogen content analysis pe rformed 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 hydrog en absorbed in the bulk stainless steel, while the presence of an oxid e layer does not reduce the hydrogen outgassing. (C) 1998 American Vac uum Society.