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高温水中におけるステンレス鋼すき間内溶液導電率に及ぼす微量塩化物および鋼中不純物の影響

Effect of impurities in steel and ppb level of chloride in bulk water on electrical conductivity of stainless steel's crevice solution in high temperature water

相馬 康孝   ; 小松 篤史 ; 加藤 千明   

Soma, Yasutaka; Komatsu, Atsushi; Kato, Chiaki

高温水中におけるステンレス鋼すき間内溶液導電率Kを知ることは、すき間腐食の発生メカニズムや応力腐食割れの先端部における溶解挙動を知る上で重要である。本研究では、温度288$$^{circ}$$C、圧力8MPaにおいて溶存酸素濃度を変化させた場合のKの応答挙動に及ぼす、バルク水中の微量塩化物イオン、およびステンレス鋼材中に含まれる不純物(CaS介在物の有無, リン, 硫黄含有量)の影響を調べた。CaS介在物を硝酸で除去した酸洗材、および鋼中リン, 硫黄濃度を低減したSUS316EHP鋼ですき間を形成してKを測定し、一般的なSUS316Lステンレス鋼(標準材)のそれと挙動を比較したところ、これらには顕著な差がないことが分かった。このことから、鋼材から溶出する不純物としてのリン, 硫黄はKの変化に大きな寄与を与えないことが分かった。バルク水に50ppbのCl$$^{-}$$を添加した環境で標準材のKの変化を測定したところ、Kの最大値は未添加の場合の2倍以上の値になった。このことから、微量塩化物はすき間の内部に泳動濃縮することが分かった。

In this study, the effect of impurities in steel and ppb level of chloride in bulk water on electrical conductivity of stainless steel's crevice solution (K) has been studied. Crevice specimens were made of as-polished Type-316L stainless steel (standard-SS), standard-SS exposed to 60% nitric acid to dissolve sulfur containing inclusions (acid-picked SS), and 316EHP steel in which sulfur and phosphorous concentration was decreased compared to standard-SS (EHP-SS). These crevice specimens were immersed into 561 K, 8 MPa water the K values were measured as a function of time with stepwise increase in dissolved oxygen levels. In addition, effect of 50 ppb Cl$$^{-}$$ added to bulk water was investigated using standard-SS crevice. The all of the standard-SS, acid-picked SS, and 316EHP showed similar K vs time curves. It can be concluded that impurities dissolved from the steel itself do not significantly contribute to the increase of K. The effect of 50 ppb Cl$$^{-}$$ on K vs time curve was obvious because maximum K value became more that 2 times larger than the solution without Cl$$^{-}$$ addition. This indicate that small concentration of impurities can be migrated into the crevice.

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