Abstract
Stainless steel is the most frequently used material for manufacturing of vacuum chambers and components. Recombination was shown to be a rate-limiting mechanism in kinetics of hydrogen outgassing at low concentration. Unfortunately, published data of recombination rate coefficient, K L , are poorly correlated with surface composition. This makes numerical modelling uncertain and it is difficult to compare the efficiency of different pre-treating methods; vacuum annealing, air oxidation, etc. Thin walled stainless steel test cells (wall thickness 0.15 mm, V=0.16 l, A=460 cm 2) were processed in situ by moderate vacuum bake-out. Pressure measurements in the cells by a spinning rotor gauge (SRG) started at least 24 h after they were sealed-off. Gas accumulation time was typically a few days. The hydrogen outgassing rate was determined from the slope of the total pressure rise. The range of q out below 2×10 -14 mbar l s -1 cm -2 was routinely obtained at room temperature. Additionally, two different pre-treatment methods were applied, hydrogen annealing and oxidation in air, but no evident influence to final outgassing rate was observed. Surface composition was analysed by AES before and after pumping in all cases. Correlation of these data and the resulting outgassing rates are discussed.
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