Outgassing reduction of type 304 stainless steel by surface oxidation in air

Abstract

The effects of surface oxidation in air on the outgassing reduction from stainless steel are investigated. The outgassing measurement system of a throughput method is fabricated from a type 304 commercial-grade stainless steel. It is mirror-finished with buff- and electropolishing before the prebaking at 450 °C for 30 h. An evacuation cycle including 18-h baking at 200 °C is repeated four times to bring the chamber into the thoroughly degassed state before oxidizing in the atmosphere at 200 °C for 3 h. Then, four evacuation cycles including 200 °C baking are conducted. An outgassing rate of 6.1×10−11 Pa m3/s m2 is obtained at the thoroughly degassed state. The lowest value of 1.6×10−11 Pa m3/s m2 is attained in the first evacuation after the oxidation in air. A hydrogen peak is significantly reduced to be lower than a carbon monoxide peak. In the following three evacuations, the outgassing rate is maintained below 3×10−11 Pa m3/s m2, half of that in the thoroughly degassed state. These results confirm that the effect of the oxidation in air on the outgassing reduction is significant and is maintained during many evacuation cycles. The outgassing reduction is caused by the reduction of hydrogen. This may be due to the strengthening of a barrier function of an oxide layer on the stainless steel surface that prevents hydrogen from diffusing out of the metal and escaping into the vacuum.