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炭酸塩水溶液及び人工海水における炭素鋼の腐食挙動に及ぼす材料中不純物元素の影響

Effect of impurity elements in metal on the corrosion behavior of carbon steel in carbonate aqueous solution and synthetic seawater

谷口 直樹; 鈴木 宏幸*; 内藤 守正

Taniguchi, Naoki; Suzuki, Hiroyuki*; Naito, Morimasa

腐食現象は材料と環境の相互作用であり、地層処分環境における炭素鋼オーバーパックの腐食挙動は環境因子の影響だけでなく、材料因子による影響を受ける可能性がある。本研究では炭素鋼中に一般的に含まれる主要な不純物であるC, Si, Mn, P, Sに着目し、炭酸塩水溶液と人工海水を用いて、これらの元素が電気化学的挙動と低酸素濃度下での腐食速度に及ぼす影響を調べた。その結果は以下のようにまとめられる。(1)0.01M炭酸塩(pH10)溶液中での不動態化電流,不動態保持電流に及ぼす不純物元素の影響は小さいことが確認された。(2)Si濃度が比較的高い0.73%と0.97%の条件では不動態皮膜の破壊やアノード溶解の促進が観察された。(3)0.01M炭酸塩(pH10)溶液の飽和した緩衝材共存下では不動態化せず、アノード分極挙動への不純物元素の影響も小さいことが確認された。(4)人工海水中,低酸素濃度下での腐食速度は不純物元素濃度が大きいほど腐食速度は大きくなる傾向があり、P, Mnによる影響が比較的大きくなった。(5)Si, Mn, Pの添加による腐食速度増加はカソード反応である水素発生反応の促進によるものと推察された。

Corrosion of metal is an interaction between the material and the environment, so that the corrosion behavior of carbon steel overpack might be affected by not only the environmental factors but also the material factors. In this study, the effect of general impurities in carbon steel such as C, Si, Mn, P and S on the electrochemical behavior and the corrosion rate were studied using carbonate aqueous solution and synthetic seawater. The experimental results were summarized as follows; (1) The effect of the impurities on the critical passivation current density, $$I_{p}$$ and the passive current density, $$I_{pass}$$ were small in 0.01M carbonate aqueous solution at pH10. (2) Breakdown of passive film and increase in anodic dissolution were observed in the tests for high Si condition of 0.73% and 0.97%. (3) In buffer material saturated with 0.01M carbonate aqueous solution, no passivation was observed and the effect of impurities on the anodic polarization behavior was small. (4) The corrosion rate of carbon steel in seawater was increased with the concentration of impurities. Among the impurity elements, the effects of P and Mn were relatively large. (5) It was inferred that the increase in corrosion rates in synthetic seawater by the addition of Si, Mn and P was promoted by the activation of hydrogen evolution reaction as a cathodic reaction.

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