Presentation Passivation Behavior of SS304 Stainless Steel and F82H Steel in Subcritical Tritiated Water

小柳津, 誠  ,  磯部, 兼嗣  ,  林, 巧

SS304 stainless steel will be widely used in fusion reactor, and F82H steel, one of low activation ferritic/martensitic steel has been developed as a structural material for blanket. Corrosion of these metals is an important issue for the establishment of safety and soundness of fusion reactors. It has recently been reported that the passivation of SS304 stainless steel was inhibited in tritiated water solution at ambient temperature, resulting in corrosion enhancement, and that the coexistence of tritium and oxygen in solution enhanced the corrosion [1]. Furthermore, it has been also reported that the effects of tritium on corrosion of F82H steel at ambient temperature was much higher than that of SS304 stainless steel [2]. However, cooling water temperature of water cooled solid breeder blanket system where F82H steel will be used is so far set from 553 to 598 K. Therefore in the present study, the effects of tritium on the passivation behavior of SS304 stainless steel and F82H steel in subcritical water are studied.Mirror finished SS304 stainless steel and F82H steel with their size of 5x5x1mm were used as samples. Each sample was set in each quartz test tube (upper side open) to avoid extrinsic effects. The test tubes with samples were set in an autoclave. The test tubes and autoclave were filled by pure water with tritium concentration of <0.5 Bq cm-3 or approximately 20 MBq cm-3. Gas of Ar or 10% O2 in Ar was purged the autoclave for more than 12 hours to equilibrate the dissolved oxygen concentration of the water in not only autoclave but test tubes. Thereafter, the autoclave was sealed and heated up to each set temperature of 523, 573 and 623 K in 30 min. The total heating time was set to be 7 hours. After heating, the oxide films of the samples were measured by means of X-ray photoelectron spectroscopy (XPS) system with Ar+ sputter gun.It was indicated from the XPS measurements for the oxide films of SS304 stainless steels formed in subcritical water at 623 K with dissolved oxygen in its concentration of approximately 4 g-O2 m-3 with and without tritium that no significant effects of tritium could be found, while it has been already reported in previous study that the oxide film formed at ambient temperature in tritiated water solution with dissolved oxygen declared the effect of tritium as the formation of thicker oxide film with more ambient interface between oxide film and bulk of SS304 stainless steel [1]. Therefore, it was implied that the effects of tritium on the passivation behavior of SS304 stainless steel in subcritical water would be less significant than those formed at ambient water solution. In the presentation, the difference of the effect of tritium on the passivation behavior formed in subcritical water and in ambient temperature water solution will be discussed in detail with the results for F82H steel.[1] M. Oyaidzu et al, Nuclear Materials and Energy 9 (2016) 508-511[2] M. Oyaidzu et al, 25th IAEA Fusion Energy Conference Proceedings (2014) 643
18th International Conference on Fusion Reactor Materials

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