Masatsugu Arai

Effect of surface oxides on the permeation of deuterium through stainless steel

Abstract The temperature dependence of deuterium permeation was measured for differently surface-treated type 316L stainless steel specimens in the temperature range of 473–623 K. The factors investigated were thickness (less than a few hundred A) and composition (Fe oxide vs Cr oxide) of the surface oxide layers formed on the stainless steel surfaces. The oxidation was performed typically in a controlled oxygen partial pressure atmosphere. These produced either iron oxide or a mixture of iron and chromium oxide. The permeability of deuterium was reduced by the formation of oxide layers. The analysis shows that this reduction is achieved by the reduction of the diffusion coefficient. With the Cr-rich oxide, the permeability and diffusion coefficient decrease with increasing oxide thickness up to about 100 A, while with the Fe-rich oxide, they are almost independent of thickness. Furthermore the permeability and diffusion coefficient decrease with increasing Cr (Cr + Fe) ratio of the oxide layers. It may be concluded that the Cr-rich oxide is less penetrable than the Ferich oxide.

Effect of surface oxide layers on deuterium permeation through stainless steels with reference to outgassing reduction in ultra- to extremely high vacuum

Abstract Hydrogen is a dominant outgassing species from stainless steel vacuum chambers in ultra- to extremely high vacuum. Oxidation of stainless steel surfaces has been known to reduce the outgassing. The oxide layer formed on the stainless steel surface is expected to serve as a diffusion barrier for hydrogen diffusing from the bulk. In the present study the effects of oxidation on the outgassing rate, on hydrogen thermal desorption spectra, on surface contamination and on deuterium permeation and diffusion characteristics through the stainless steels are summarized. Discussion is given to the role of the surface oxide layer in outgassing reduction as well as to the effect of thickness and chemical compositions on reduction in hydrogen permeability and diffusion. The surface oxide formed in the present study appears to serve as the diffusion barrier for hydrogen and is found to decrease the deuterium permeability to 1 2 – 1 3 of the unoxidized one by lowering the diffusion coefficient. The most effective oxide is proposed to be Cr-rich oxide with 100–200A thickness.