Outgassing Characteristics of Electropolished Stainless Steel

To keep abreast of the exceptionally rapid advances of leading industries, technologies need to be developed for an Extreme-High Vacuum (XHV) process. Because of its outstanding corrosion resistance, strength, and outgassing characteristics, SUS304(L) and 316(L) stainless steel are used for ultra-high vacuum equipments. Also, electropolishing technique has been adopted by industries because it makes the surface roughness as small as possible by minimizing the actual areas. This is a future trend of XHV equipment. The outgassing characteristics of the electropolished surface of stainless steel and effects of gas contents and inclusions of steel itself on the outgassing rate controlled by diffusion have been studied. SUS316L stainless steels, which have various contents of gas and the amounts of nonmetallic inclusions, have been melted in various conditions and rolled to plates. The sample of 8 mm×8 mm×1 mm were prepared from these plates, and all surfaces were finally electropolished. The amounts of outgassing gas by thermal desorption and the outgassing rate of H2, H2O, CO, CO2 after baked at 1123 K in a vacuum of 10−7 Pa were measured by a quadruple mass spectrometer. The molecular amount of H2 of all the samples was larger by 10 to 102 times, than those of H2O, CO, CO2 and there was no correlation between the H2 molecular amounts of outgassing by heating and the H2 content in steel. But the outgassing rate of H2 was reduced with decreasing both the H2 content and the amounts of nonmetallic inclusions. Especially, the outgassing rate of H2 was attributed to the inclusions because the hydrogen was found to be entrapped by them on the electropolished surface. Clean steel which has been produced in special cleaned melt, showed 2 orders of magnitude lower in rate than normal steel. The outgassing rates of CO and CO2 were reduced with decreasing C content in steel.