Dependence of outgassing rate on surface oxide layer thickness in type 304 stainless steel before and after surface oxidation in air

Abstract

Surface changes in an outgassing measurement system were investigated using X-ray photoelectron spectroscopy (XPS) from the system's fabrication through evacuation cycles following oxidation in air. Specimens for surface analysis were prepared using the same material, surface treatments and prebaking conditions as used for the outgassing measurement system itself. They were put into the outgassing measurement system to undergo an evacuation process which included oxidation in air at 200 °C for 3 h. They were taken out of the system one by one at different steps of the evacuation process and analyzed. The principal results are as follows. (1) In an oxygen-penetrated layer of a few nanometers, Cr concentration and the total amount of Cr surpass those of Fe, although the thickness of the layer and the ratio Cr Fe at the surface are changed. (2) Oxide layers of Cr are twice as thick as those of Fe. (3) The thicker the oxide layer is, the lower the outgassing rate after baking. (4) Cr makes the predominant contribution to a barrier function at the stainless-steel surface that prevents hydrogen from diffusing into the vacuum. (5) The oxidation in air causes changes in chemical composition and oxidation state at the surface, although it does not affect the outgassing rates before baking.