Dear Dr. Nancy Payne,

outgassing rates depends on materials and treatments.

For unbaked metals, water vapour is the leading outgassed molecule. Its outgassing rate is inversely proportional to the pumping time. It is not strongly dependent on treatment and grade. Venting to air cancel the effect of previous gas release.

For baked metals, the outgassing process is dominated by hydrogen. It depends on the nature of the material and, strongly, on the applied thermal treatment. Venting to air does not recharge hydrogen into solid metal used for vacuum chambers.

Best regards, Pierluigi Traverso

The intended pressure is 1.3 x10-14 atm. Using the Ellingham diagram for oxides you can estimate the dissociation temperatures of the possible oxides present on the stainless steel surface at 1.3x10-14 atm.

The oxide film on stainless steel would be a Cr-rich spinel layer. However, Ellingham diagram lists only simple oxides. The dissociation temperature of Fe2O3 to Fe3O4 + O2 will be about 600 C, NiO will dissociate at about 800 C, Fe3O4 will dissociate to FeO at about 900 C, FeO will dissociate at 1100 C, and Cr2O3 will dissociate at about 1450 C. These are all approximate values.

Therefore, as long as you do not heat to more than ~500 C, you may not dissociate the surface oxides. Desorption of species sitting on the oxide layer such as moisture, air etc is a different story because the oxide film will have a larger surface area than the geometric area.

I analyze steel and oxide layers on top of it down to a pressure of 5*10^-8Pa=5*10^-10mbar. My machine can not reach lower pressures. I do not see any outgasing.

But if you analyze industrial samples in UHV, you must sometimes clean the sample surface chemically before analyzing. For example I clean it with THF, isopropanol and ethanol. This is removing the organic contaminations, but lets the oxides intact.