IMPORTANCE OF THE SURFACE OXIDE LAYER IN THE REDUCTION OF OUTGASSING FROM STAINLESS-STEELS

Temperature dependence of outgassing was measured for differently surface treated type 316L stainless steel chambers in the temperature range of 25-330 degrees C. Based on the temperature dependence and outgassing characteristics of major outgassing species, the mechanism of outgassing from the stainless steel surface is discussed and the technique to produce an extremely low outgassing surface is indicated. The following are the important findings. (1) At temperatures below 250 degrees C, the outgassing species is predominantly hydrogen, which is released by diffusive outgassing from the bulk of the stainless steel. (2) At temperatures above 250 degrees C, the contribution from water and carbon monoxide outgassing becomes significant because of the enhanced surface reaction to generate these gas species. (3) The surface oxide layer formed by the oxidation in air is predominantly iron oxide and appears to serve as a more effective diffusion barrier for hydrogen outgassing compared with the mixed iron and chromium oxide layer formed on the fully degassed surface. (4) The oxidation reduces carbon contaminants on the surface which are a possible source of carbon containing outgassing species. (C) 1995 American Vacuum Society.