窒素含有オーステナイトステンレス鋼の電気化学的挙動

抄録

The effect of nitrogen on localized corrosion resistant properties and repassivation behavior of nitrogenbearing 316L stainless steels was investigated in the 3.5%NaCl, 0.1kmol m^-3 Na₂SO₄ and 0.5kmol m^-3 H₂SO₄ solutions. Interstitial nitrogen in the steel is converted to ammoniacal state with the efficiency of 100% when the metal dissolves inside a crevice. It has been proven that the molar amount of dissolved ammonia was equivalent to the molar amount of nitrogen contained in a dissolved portion of the steel. The result is in agreement with the report where it suppresses the decrease in pH, leading to the conditions for repassivation, since conversion of nitrogen into ammonia is a proton-consuming process. Neither nitrate-N (NO₃-N) nor nitrite-N (NO₂-N) was detected by chemical analysis although the measured potential lies in the stability region of NO₃-N in the E-pH diagram. A scratching electrode method was applied for evaluating the repassivation kinetic of nitrogen-bearing 316L stainless steels in 0.1kmol m^-3 Na₂SO₄. The technique developed in this study allowed us to measure the recovery process of passive film immediately following the exposure of bare metal surface. The repassivation process was faster with increasing dissolved nitrogen content in the steel.