地層処分環境での炭酸塩濃度上限を想定した炭酸水素ナトリウム水溶液中における炭素鋼の応力腐食割れ感受性に及ぼす温度の影響

Influence of temperature on stress corrosion cracking susceptibility of carbon steel in sodium hydrogen carbonate aqueous solution assuming upper limit of carbonate concentration in geological disposal environment

高橋 里栄子*; 谷口 直樹 

Takahashi, Rieko*; Taniguchi, Naoki

地層処分環境での地下水中の炭酸塩濃度上限と想定される0.1-0.5mol/dmの条件を対象に温度をパラメータとしてNaHCO水溶液中で炭素鋼のSSRT試験を行った。伸び比など機械的特性への温度の影響は見られなかったが、破面率では303、323Kの低温でSCC感受性が高くなると判断された。その要因として低温での再不動態化の抑制が示唆された。また、低温で粒界型、高温で粒内型のSCCが支配的になる傾向がみられた。

Carbon steel is one of the candidate materials for overpacks in geological disposal of high-level radioactive waste, and is known to susceptible to stress corrosion cracking(SCC) depending on the condition in carbonate environment. In order to understand the influence of temperature on the SCC susceptibility of carbon steel, slow strain rate test (SSRT) of rolled steel were performed in NaHCO aqueous solution with varying temperature in the range of 303-393K for conditions of 0.1-0.5 mol/dm, which is assumed to be the upper limit of carbonate concentration in groundwater in a geological disposal environment. As the results, no obvious influence of temperature on mechanical properties such as fracture strain ratio and reduction area ratio were observed, but SCC susceptibility based on SCC fracture ratio increased at relatively low temperatures of 303K and 323K. It was suggested that the reason for the higher SCC sensitivity at lower temperatures was due to slower repassivation at lower temperatures. Regarding the type of SCC, intergranular SCC was dominant at low temperatures and tended to transition to intergranular SCC at higher temperatures. Transgranular SCC tended to be observed at lower potentials than those at which intergranular SCC was observed.