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Intergranular oxidation within crevice of austenitic stainless steel in high temperature water

高温高圧水中に浸漬したオーステナイトステンレス鋼のすき間内における粒界酸化

相馬 康孝; 加藤 千明; 上野 文義

Soma, Yasutaka; Kato, Chiaki; Ueno, Fumiyoshi

高温高圧水中(温度288$$^{circ}$$C,圧力8.5MPa,溶存酸素濃度32ppm,導電率1.2$$pm$$0.2$$mu$$S(25$$^{circ}$$Cにおける値))に浸漬(500h)した低炭素オーステナイトステンレス鋼のすき間内において粒界酸化が発生した。粒界酸化はすき間の開口部から一定の距離を有し、かつすき間幅が比較的小さい特定の箇所で発生した。粒界酸化部ではFeとNi成分が選択溶解し、Crリッチな酸化物が形成した。粒界酸化の最大浸食深さは500hで約50$$mu$$mであった。粒界酸化部周辺で形成された表面酸化皮膜層を電位-pH図を以て熱力学的に解析したところ、環境のpHは約3.2から3.4と推定された。粒界酸化部が起きた箇所に小型のセンサーを挿入し、すき間内の局所的な溶液導電率を測定した。測定した導電率を理論的に求めた導電率-pHの関係と比較した。その結果、すき間内ではバルクに比べ約100倍の導電率を示し、それに対応するpHは約3.5であった。以上の結果から、すき間内では溶液の酸性化が起き、それにより粒界酸化が発生したものと推定された。

Intergranular oxidation (corrosion) occurred within crevice of austenitic low-carbon stainless steel (solution treated, almost no applied stress) after immersion in high temperature water (288$$^{circ}$$C, 8.5 MPa, dissolved oxygen conc. 32 ppm, electrical conductivity: 1.2$$pm$$0.2$$mu$$S (measured value at 25$$^{circ}$$C)) for 500 h. The intergranular oxidation occurred at specific position within the crevice that is relatively distant from the crevice mouth with relatively low crevice gap. Both the grain boundary and grain matrix were oxidized. In the oxidized area, Fe and Ni were depleted and Cr was enriched compared to the matrix. Maximum penetration depth of the oxidation was approximately 50 $$mu$$m after 500 h. In order to understand potential-pH condition within the crevice, surface oxide layer was microscopically and thermodynamically investigated. Thermodynamic properties of the surface oxides near the intergranular oxidized area indicated lowered pH of approximately 3.2 to 3.4. In-situ measurement of local solution electrical conductivity was carried out using small electrodes (dia. 800 $$mu$$m) imbedded into the crevice former plate. The solution pH was estimated using theoretically calculated pH vs. electrical conductivity relationship. In the area where the intergranular oxidation occurred, the solution electrical conductivity was nearly 100 times higher than that of bulk water and which indicated lowered pH of approximately 3.5. The above results suggested that, in the high temperature and relatively high purity water, acidification occurs within crevice of stainless steels and such aggressive corrosion condition result in the intergranular oxidation.

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