Model for water outgassing from metal surfaces. II

Abstract

The outgassing rate from an electropolished stainless steel (type 304) surface is measured after the surface is exposed to H2O vapor at pressures (p0) of 10−3–10 Torr for exposure periods (t0) of 3–600 min. The system is kept at 310, 350, or 390 K during H2O exposures and subsequent outgassing measurements. Our data show that the outgassing rate is proportional to pn0, where n=0.3, 0.26, 0.25 for T=310, 350, 390 K, respectively. The adsorption rate, defined as the quantity of H2O being adsorbed during exposures per unit time, is proportional to 1/t0. Continuing the previous work [Li and Dylla, J. Vac. Sci. Technol. A 11, 1702 (1993)] on correlating the outgassing with the source distribution of sorbed gas, we prove that the practical expression for the outgassing rate, Q(t)=Q10/tα, can be derived from a H2O concentration falling as (1/x)2α−1 with the spatial variable x defined as the distance away from the surface. The observed dependence of the outgassing rate on the exposure conditions is consistent with the predictions of this simple model.