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高pH化した海水系地下水環境における炭素鋼の局部腐食進展挙動

Localized corrosion behavior of carbon steel in high-pH seawater-type groundwater environment

谷口 直樹; 鈴木 宏幸*; 内藤 守正

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

炭素鋼は高pH環境において不動態化し、条件によっては孔食,すきま腐食などの局部腐食を受けることが知られている。一般に局部腐食の進展速度は全面腐食の場合よりも大きく、炭素鋼オーバーパックに局部腐食が生じた場合には短期破損をもたらす可能性がある。孔食・すきま腐食は塩化物イオンに代表される不動態皮膜破壊型の化学種の共存下において発生することが知られている。処分環境では、海水系地下水のような塩化物イオンを含む地下水が人工バリア周辺に施工されるコンクリート構造物中のセメント材料と地下水が接触してそのpHが上昇し、炭素鋼オーバーパックに孔食やすきま腐食をもたらす場合などが想定される。本研究では海水系地下水の一例として幌延の模擬地下水を用い、コンクリートと接触させて高pH化させた条件で孔食,すきま腐食の進展挙動を調べた。その結果、孔食係数(最大腐食深さと平均腐食深さの比)は中性$$sim$$アルカリ性環境や種々の天然水環境で得られた過去のデータの範囲内にあることが確認された。実験データの極値統計解析によりオーバーパックにおける最大腐食深さを推定した結果、推定値はいずれの条件でも従来の孔食・すきま腐食進展に関する経験モデルにより算出される値を超えないことがわかった。

It has been known that carbon steel can be passivated in high pH environment and sometimes be attacked by localized corrosion such as pitting corrosion, crevice corrosion. If carbon steel overpack is attacked by localized corrosion, it is possible that the overpack may be penetrated in a short term, since the propagation rate of localized corrosion is larger than that of general corrosion in general. It has been known that the pitting corrosion and crevice corrosion are initiated in the presence of aggressive species for passive film represented by chloride ion. In repository environment, it is possible that the pH in groundwater containing chloride ion such as seawater type groundwater is raised by a contact with the cement material in concrete structures constructed around the engineered barrier system and then pitting corrosion or crevice corrosion is caused on the carbon steel overpacks. In this study, the propagation behavior of pitting corrosion and crevice corrosion was examined by immersion tests under air equilibrium condition using artificial groundwater at Horonobe as an example of seawater-type groundwater. As the results, the pitting factor (ratio of the maximum corrosion depth and average corrosion depth) were within the literature data obtained in neutral and alkaline water and in various natural water environments. The maximum corrosion depth of carbon steel overpack was predicted by extreme value statistical analysis of the experimental data, and it was confirmed that the predicted corrosion depths were not over the values calculated from current empirical models for propagation of pitting corrosion and crevice corrosion in every cases.

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