Outgassing in thin wall stainless steel cells

Abstract

Outgassing of hydrogen determines the ultimate pressure of a metal ultrahigh vacuum system, but also limits the long-term performance of sealed-off metal devices, where a thermal insulating grade of vacuum is required for a period of several years. Hydrogen concentration in stainless steel is related to equilibrium pressure by means of Sievert’s law, while the outgassing rate qout and equilibrium pressure are not related, particularly in the high vacuum range. Thin wall stainless steel cells (wall thickness 0.15 mm, AISI304 and AISI316) were used to test the relation between thermal treatment and the resulting room temperature outgassing rate qout. They were cleaned and baked in situ at ∼205 °C for ∼16 h. The pressure rise was recorded by a spinning rotor gauge after sealoff. The initial pressure rise slope at a H2 equivalent pressure of ∼10−5 mbar was in the order of qout≅1×10−14 mbar l H2/s cm2. During the measurements, which were performed at least 750, but even in excess of 3000 h, the outgassing rate started to decrease, which may be explained by its approaching equilibrium pressure. From the difference, the recombination rate coefficient KL was determined on the basis of a simple model. It lay in the range of KL (296 K)≅10−25–10−24 molecules H2 cm4/(atoms2 H s). The cells inner surfaces were analyzed by Auger electron spectroscopy. Their composition was rather typical for technical grade stainless steel covered by a native oxide layer. The corresponding KL values, which were not measured previously at room temperature, are matched with extrapolated reported values for an electropolished stainless steel surface. In addition, hydrogen annealing at 1030 °C was applied as a pretreatment of two of the cells without any noticeable effect on the final outgassing rate even if the inner surface was covered by a thick oxide layer. By increasing of the bakeout temperature, it was possible to prepare cells where qout was in the order of ≅1×10−15 mbar l H2/s cm2.