高濃度硝酸塩溶液中における金属イオン種の再酸化挙動に関する硝酸熱力学モデルを用いた検討

The Consideration of re-oxidation behavior of metal ions in high concentration nitrate solution by thermodynamic model of nitric acid

加藤 千明   ; 本岡 隆文; 山本 正弘 

Kato, Chiaki; Motooka, Takafumi; Yamamoto, Masahiro

高濃度硝酸塩を含む硝酸水溶液中では、硝酸塩由来のNO濃度が高いほどCr(VI)等の酸化性イオンの再酸化能が高まりステンレス鋼の腐食が加速することが報告されている。これまでに開発してきた沸騰硝酸水溶液の熱力学モデルを用いて、沸騰硝酸水溶液中の金属イオン種の原子価状態を定量的に計算し実測値との比較を行った。さらに、その応用として、硝酸水溶液中でのCr(VI)の再酸化挙動に及ぼすNO濃度の影響を検討した。その結果、最適化された沸騰硝酸熱力モデルは、硝酸の酸化還元電位と金属イオン種の再酸化挙動を定量的に評価可能であり、高濃度沸騰硝酸中ではCr(VI)が生成可能で有ることを熱力学的に示すことができた。

It is reported that high concentration NO ion accelerated corrosion of stainless steel in nitric acid solutions including the high concentration nitrate due to reoxidation of oxidizability ions such as Cr(VI). The valence state of oxidizability ions in a boiling nitric acid aqueous solution was calculated quantitatively by developed thermodynamics model of boiling nitric acid solutions and comparison with the measured value and calculated value was conducted. As the applications of the thermodynamics model, the influence of the NO concentration in high concentration nitrate solutions accelerated re-oxidation of oxidizability ions such as Cr(VI) was investigated. As a result, optimized thermodynamics model of boiling nitric acid solutions is able to calculate REDOX potential of nitric acid solution and evaluate quantitatively the valence state of oxidizability ions in a boiling nitric acid aqueous solution.