Effect of oxidizing ions on corrosion rates of stainless steel in simulated solutions for reprocessing plants

再処理プラント模擬液中におけるステンレス鋼の腐食に及ぼす酸化性イオンの影響

本岡 隆文; 山本 正弘 

Motooka, Takafumi; Yamamoto, Masahiro

硝酸中でのステンレス鋼の腐食挙動は通常短時間試験で調査され、試験中のステンレス鋼の腐食速度の時間変化挙動にはあまり着目されてこなかった。本論文では、硝酸中でのステンレス鋼の腐食速度の時間依存性を調査した。SUS304ULCステンレス鋼の腐食試験を、Cr(VI)又はV(V)含有沸騰硝酸溶液を用いて行った。硝酸濃度は38M、初期イオン濃度は0.051g/Lである。240時間の試験中、試験液は更新せずステンレス鋼のサンプルは48時間ごとに更新した。得られた結果は以下の通りであった。(1)Cr(VI)又はV(VI)含有硝酸ではステンレス鋼の腐食は加速した。高濃度硝酸及び高イオン濃度ほど腐食速度は高かった。(2)Cr(VI)含有沸騰硝酸では、8M硝酸の場合、急激に腐食速度は低下した。一方、V(V)含有沸騰硝酸では腐食速度はほとんど低下しなかった。(3)腐食速度の時間依存性は、ステンレス鋼の腐食電位と良い相関を示した。硝酸中でのステンレス鋼の腐食速度は、Cr(VI), V(V)等が含有された場合の酸化還元平衡により決定される。

The corrosion behavior of stainless steel in nitric acid solutions has been usually investigated by short time corrosion tests. The change in corrosion rate with time has been not focused in these tests. In this report, the time-dependency of corrosion rate of stainless steel in nitric acid solution was investigated. Corrosion test of SUS304ULC stainless steel was conducted in boiling nitric acid solution containing Cr(VI) or V(V). The concentration of nitric acid solutions was ranged from 3 to 8 M, and the concentration of Cr(VI) or V(V) was ranged from 0.05 to 1 g/L. The samples of stainless steel were changed every 48 hours without renewal of the nitric acid solution during 240 hours. The obtained results were as follows; (1) The presence of Cr(VI) or V(V) in nitric acid solution accelerated the corrosion rate of stainless steel. The higher corrosion rate was observed in higher nitric acid concentration and greater content of Cr(VI) or V(V). (2) In boiling nitric acid solution with Cr(VI), the corrosion rate rapidly decreased in 8 M nitric acid solution. While in nitric acid solutions with V(V), the corrosion rate was little decreased. (3) The time-dependency of corrosion rates had a good correlation with corrosion potentials of stainless steel. Thus, the corrosion rate of stainless steel in nitric acid solutions with either Cr(VI) or V(V) would be controlled by corrosion potential.