MODEL FOR WATER OUTGASSING FROM METAL-SURFACES .2.

The outgassing rate from an electropolished stainless steel (type 304) surface is measured after the surface is exposed to H2O vapor at pres SureS (p0) of 10(-3)-10 Torr for exposure periods (t0) of 3-600 min. T he system is kept at 310, 350, or 390 K during H2O exposures and subse quent outgassing measurements. Our data show that the outgassing rate is proportional to p0n, where n = 0.3, 0.26, 0.25 for T = 310, 350, 39 0 K, respectively. The adsorption rate, defined as the quantity of H2O being adsorbed during exposures per unit time, is proportional to 1/t 0. Continuing the previous work [Li and Dylla, J. Vac. Sci. Technol. A 11, 1702 (1993)] on correlating the outgassing with the source distri bution of sorbed gas, we prove that the practical expression for the o utgassing rate, Q(t) = Q10/t(alpha), can be derived from a H2O concent ration falling as (1/X)2alpha-1 With the Spatial variable x defined as the distance away from the surface. The observed dependence of the ou tgassing rate on the exposure conditions is consistent with the predic tions of this simple model.