Reduction of stainless-steel outgassing in ultra-high vacuum

The outgassing rate of a 2 mm thick stainless-steel sheet was measured in ultra-high vacuum at constant pressure to avoid the readsorption occurring in a `rate of pressure rise' determination. The rate was typically 10-12 torr l. cm-2 sec-1 and about 99% or more of the gas was hydrogen. Since stainless steel usually contains large amounts of hydrogen and the diffusion coefficient of hydrogen in stainless steel is high, it was suspected that the hydrogen diffuses to the surface from the interior of the metal and is released into the vacuum. Calculations show that the observed outgassing rate could be explained by such a process and should be reduced by several orders of magnitude by a high-temperature treatment. The effects of residual hydrogen in the treatment furnace and hydrogen permeation from the atmosphere are also considered in these calculations. The greater the thickness of the metal, the higher the temperature has to be. Measurements are in reasonable agreement with these calculations.