硫化物を含む人工海水における純銅の応力腐食割れ挙動に及ぼす電位と材質の影響

Effect of electrode potential and material grade on the behavior of stress corrosion cracking of pure copper in synthetic seawater containing sulfide

谷口 直樹 ; 川崎 学; 内藤 守正 

Taniguchi, Naoki; Kawasaki, Manabu; Naito, Morimasa

一般に、純銅は酸素を含まない天然水環境で熱力学的に安定であり、水の還元による腐食は生じない。しかし、硫化物を含む環境では熱力学的な安定性を失って硫化銅として腐食し、条件によっては応力腐食割れ(SCC)に対して感受性を示す。本研究では、硫化ナトリウムを含む人工海水中において純銅の低歪速度試験を実施し、応力腐食割れ感受性に及ぼす電位,材質の影響を検討した。その結果は以下のようにまとめられる。(1)低酸素濃度下での自然電位, Ecorrより+100mV及び+300mV貴な電位では、自然電位の場合よりもSCC感受性は低下した。しかし、Ecorrより700mV貴な電位では、Ecorr+100mV, Ecorr+300mVの場合よりもSCC感受性は大きくなった。(2)リン脱酸銅と無酸素銅を比べると、無酸素銅のほうが亀裂は発生しやすい傾向が認められたが、引張強さ,全伸びなど機械的特性に違いはなかった。(3)緩衝材共存下でのSSRTを行うため、試験片に装着する試験カラムを作成した。これを用いたリン脱酸銅に対する試験の結果、NaS濃度0.001M, -420mV vs. SCE(窒素雰囲気での自然電位に相当)において応力腐食割れの発生は認められなかった。

In general, copper is thermodynamically stable under anaerobic condition, so that corrosion due to water reduction can not occurr on copper. However, in the presence of sulfide, copper loses its thermodynamical stability and corrodes as copper sulfide, and is susceptible to stress corrosion cracking (SCC) depending on environmental condition. In this study, slow strain rate tests (SSRT) were performed in synthetic sea water containing sodium sulfide and the effect of electrode potential and the difference of material grade were examined. The results were summarized as follows; (1) The SCC susceptibilities at the electrode potentials of +100 mV and +300 mV above Ecorr (corrosion potential in anaerobic condition) were lower than that at a level of Ecorr. In case of Ecorr +700 mV, the SCC susceptibilities became larger than those in Ecorr +100 mV and Ecorr +300 mV. (2) In comparison of oxygen free copper and phosphorous deoxidized copper, the former tended to be allowed to crack initiation more readily, but the mechanical properties such as tensile strength and elongation were almost equal in both materials. (3) In order to apply the SSRT with buffer material, a test column attached to a specimen was developed. As the result of SSRT of phosphorous deoxidized copper using the column, no SCC was observed in 0.001 M - NaS at -420 mV vs SCE, which is the rest potential under anaerobic condition (nitrogen atmosphere) in buffer material.