Model for the outgassing of water from metal surfaces

Numerous measurements of outgassing from metal surfaces show that the outgassing obeys a power law of the form Q=Q10t−α, where α is typically near unity. For unbaked systems, outgassing is dominated by water. This work demonstrates that α is a function of the water vapor exposure during venting of the system, and the physical properties of the passivation oxide layer on the surface. An analytic expression for the outgassing rate is derived based on the assumption that the rate of water diffusing through the passivation oxide layer to the surface governs the rate of its release into the vacuum. The source distribution function for the desorbing water is assumed to be a combination of a Gaussian distribution centered at the interior surface driven by atmospheric exposure, and a uniform concentration throughout the bulk. We have measured the outgassing rate from a clean stainless-steel (type 304) chamber as a function of water exposure to the chamber surface from <1 to 600 monolayers. The measured outgassing rate data show that α tends to 0.5 for low H2O exposures and tends to 1.5 for high H2O exposures as predicted by the model.