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緩衝材中における炭素鋼の腐食挙動の実験的検討,2; 10年間浸漬試料の腐食生成物分析結果

Experimental study on corrosion behavior of carbon steel in buffer material, 2; Analysis of corrosion products on coupons immersed for 10 years duration

谷口 直樹; 川崎 学; 内藤 守正

Taniguchi, Naoki; Kawasaki, Manabu; Naito, Morimasa

緩衝材を模擬した圧縮ベントナイト中における炭素鋼の10年間の浸漬試験を行い、腐食生成物の観察,分析を実施するとともに、熱力学的な観点から腐食生成物の安定性,溶解・沈澱挙動を検討した。その結果は以下のようにまとめられる。(1)温度80$$^{circ}$$Cの条件では試験片表面に黒色$$sim$$黒灰色の腐食生成物皮膜が形成されており、試験片に強く密着していた。しかし、より温度の低い50$$^{circ}$$Cの条件では試験片に密着した皮膜は形成されていなかった。(2)海水系の試験溶液条件では、結晶性の腐食生成物として、おもにシデライト(FeCO$$_{3}$$), Fe$$_{2}$$(OH)$$_{2}$$CO$$_{3}$$など2価鉄の炭酸塩化合物が同定された。しかし、希薄な水溶液条件ではマグネタイト(Fe$$_{3}$$O$$_{4}$$)が同定され、炭酸塩化合物は認められなかった。これらの腐食生成物は、試験条件を想定して熱力学的に推定される腐食生成物と整合した。(3)腐食生成物皮膜中にはCa, S, Si, Alの存在が確認され、その分布状態は試験条件によって異なっていた。(4)圧縮ベントナイト中における炭素鋼の腐食速度は皮膜の溶解・沈澱挙動に関連している可能性が示唆された。(5)既往の腐食モデルを参考に、圧縮ベントナイト中における皮膜形成下での炭素鋼の腐食進展モデルの概念を提示した。

Immersion tests of carbon steel were performed in compacted bentonite simulating the buffer material for 10 years duration, and corrosion products formed on the coupon surface were observed and analyzed by various methods. And then, the stability and dissolution/precipitation behavior of corrosion products were discussed from the view point of thermodynamics. The results of this study are summarized as follows; (1) In the case of the test in 80 $$^{circ}$$C, black or dark gray corrosion product was formed on the coupons, and it was tightly adhered to the surface. By contrast, in lower temperature of 50 $$^{circ}$$C, no tight corrosion product was observed on the coupon surface. (2) Ferrous carbonate corrosion products such as siderite, FeCO$$_{3}$$ or Fe$$_{2}$$(OH)$$_{2}$$CO$$_{3}$$ were identified as the crystallized corrosion products in seawater type test solutions. In the test result in dilute test solution, magnetite, (Fe$$_{3}$$O$$_{4}$$) was identified and no carbonate compounds were found. The presence of these compounds was agreed with thermo dynamical aspects assuming the test conditions. (3) Ca, S, Si and Al were detected in corrosion products layer besides Fe, and their distribution patterns were dependent on the experimental conditions. (4) It was indicated that the corrosion rate of carbon steel in compacted bentonite was correlated to the behavior of dissolution/precipitation of the corrosion products. (5) A conceptual model of the corrosion propagation behavior of carbon steel in compacted bentonite was proposed with reference to the existing corrosion models.

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