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低酸素濃度下での模擬地下水の飽和した圧縮ベントナイト中における炭素鋼の腐食挙動

Corrosion behavior of carbon steel in compacted bentonite saturated with simulated groudwater under anaerobic condition

谷口 直樹; 川崎 学; 内藤 守正

Taniguchi, Naoki; Kawasaki, Manabu; Naito, Morimasa

低酸素の濃度下、圧縮ベントナイト中において10年間に渡る炭素鋼の浸漬試験を行った。XRD, XPSにより、ほとんどの条件で腐食生成物として2価鉄の炭酸塩化合物が確認された。炭酸塩濃度の高い溶液中では10年間の試験期間を通して他の条件よりも腐食量は小さくなった。また、50$$^{circ}$$Cの条件では初期の腐食速度は80$$^{circ}$$Cよりも小さいが、数年後にはむしろ大きくなった。これら炭酸塩濃度と温度による長期的な腐食速度への影響は鉄炭酸塩の沈殿・溶解挙動が関連していると推察された。また、初期腐食量と皮膜の保護性の関係を調べると、高炭酸塩溶液を除き初期腐食量の大きいほど皮膜の保護性も高くなる傾向が認められた。実験結果の外挿により炭素鋼の長期的な腐食量を推定すると、推定値の範囲と考古学的鉄製品の腐食量の範囲はおおむね一致した。

Immersion tests of carbon steel were performed in compacted bentonite for 10 years duration under anaerobic condition. The ferrous carbonates were identified as the corrosion product by XRD and XPS analysis in almost test cases. The amount of corrosion in high carbonate concentration was smaller than those of other test conditions throughout the test periods. Although the corrosion rate at 50$$^{circ}$$C was initially smaller than that at 80$$^{circ}$$C, it resulted in larger value after several years. Such the effects of carbonate content and temperature on the long-term corrosion rate seemed to be correlated to the behavior of dissolution/precipitation of iron carbonate. Additionally, the correlation between initial corrosion amount and the protectiveness of corrosion product film was examined. Except for high carbonate condition, as the corrosion amount at early stage of immersion was larger, the corrosion product film tended to become more protective. The long-term corrosion depth was estimated by the extrapolation of the laboratory test results. The range of the estimated value was well agreed with that of archaeological analogue data.

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