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高温水クレビス模擬環境におけるFe-Cr-Ni合金の電気化学的性質

Electrochemical behavior of Fe-Cr-Ni alloys in simulated crevice condition in high temperature water

相馬 康孝   ; 加藤 千明   

Soma, Yasutaka; Kato, Chiaki

沸騰水型軽水炉においてはステンレス鋼すき間内部に外界の不純物イオンが濃縮し、酸性化することから、そのような環境(クレビス環境)におけるステンレス鋼の電気化学的性質を知ることは重要である。本研究は、クレビス模擬環境をバルクスケールで再現し、Fe-Cr-Ni合金の電気化学的性質に及ぼすCr濃度の影響を主体に調査した。クレビス環境を模擬した、温度288$$^{circ}$$C、Cl濃度2$$times$$10$$^{-4}$$ mol/dm$$^{-3}$$、pH約4.5、溶存水素濃度10ppbの水中で、Fe-20Ni-xCr (x=16.4, 23, 26)の分極曲線を計測した。その結果、-400mVにおける活性溶解のピーク電流、および-50mVにおける不働態保持電流密度はCr濃度x=16.4, 23、および26%の試験片でそれぞれおおよそ13.8, 15.9, 10.0 $$mu$$Acm$$^{-2}$$、および18.4, 8.5, 8.5 $$mu$$Acm$$^{-2}$$であった。いずれもx=26の電流値が若干低いが、本環境では分極曲線に及ぼす明確なCr濃度依存性は見られないと判断された。

It is important to understand the electrochemical properties of stainless steel in environment created within crevice of stainless steel in high temperature water (crevice environment). This is because acidification and concentration of impurity ions occur in the crevice environment and this is common inside the stress corrosion crack. In this study, we reproduced the crevice environment in bulk scale and investigated mainly the effect of Cr concentration on the electrochemical properties of Fe-Cr-Ni alloys. Polarization curves of Fe-20Ni-xCr (x=16.4, 23, 26) were measured in water with a temperature of 288$$^{circ}$$C, a Cl concentration of 2$$times$$10$$^{-4}$$ mol/dm$$^{-3}$$, a pH value of about 4.5, and a dissolved hydrogen concentration of 10 ppb. The peak currents of active dissolution (at -400 mV) and passive current density (at -50 mV) for specimens with Cr concentrations x = 16.4, 23, and 26% were approximately 13.8, 15.9, 10.0 $$mu$$Acm$$^{-2}$$, and 18.4, 8.5, 8.5 $$mu$$Acm$$^{-2}$$, respectively. Although the current values of x=26 were slightly lower in both cases, it was concluded that there was no clear dependence of the polarization curve on Cr concentration in this environment.

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