Model for 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=Q{sub 10}t{sup -alpha}, where alpha is typically near unity. For unbaked systems, outgassing is dominated by water. This work demonstrates that alpha 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 alpha tends to 0.5 for low H{sub 2}O exposures and tends to 1.5 for high H{sub 2}O exposures as predicted by the model.
- Research Organization:
- Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States)
- Sponsoring Organization:
- USDOE Office of Energy Research (ER) (US)
- DOE Contract Number:
- AC05-84ER40150
- OSTI ID:
- 802126
- Report Number(s):
- CEBAF-PR-92-43; DOE/ER/40150-5052; TRN: US200223%%899
- Journal Information:
- Journal of Vacuum Science and Technology. A, Vacuum, Surfaces and Films, Vol. 11, Issue 4; Other Information: J. Vac. Sci. Technol. A 11, 1702 (1993); PBD: 1 Jun 1993; ISSN 0734-2101
- Country of Publication:
- United States
- Language:
- English
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