Effect of neptunium ions on corrosion of stainless steel in nitric acid solution

硝酸溶液中のステンレス鋼の腐食に与えるネプツニウムの影響

本岡 隆文; 木内 清

Motooka, Takafumi; Kiuchi, Kiyoshi

ネプツニウム含有硝酸溶液中でのステンレス鋼の腐食挙動を研究した。SUS304Lステンレス鋼を用いたネプツニウム含有硝酸溶液中での腐食試験を、浸漬と伝熱状態で実施した。ステンレス鋼の腐食速度を重量減量測定と金属イオンの定量分析から求めた。腐食面の形態は走査型電子顕微鏡で観察した。また、腐食の加速機構を電気化学及び分光測定法を用いて調べた。硝酸溶液中のステンレス鋼の腐食は、ネプツニウムの添加によって加速した。粒界優先の腐食形態が観察され、浸漬状態と比較して伝熱状態で腐食は激しかった。分極測定よりネプツニウム存在下ではカソード分極が減少しカソード電流が増加すること、分光測定より沸騰硝酸中ではNp(V)がNp(VI)に酸化することを明らかにした。ネプツニウム含有硝酸溶液中の腐食加速機構として、Np(V)の再酸化機構を提案した。

A corrosion of a stainless steel in nitric acid solution containing Np has been studied. Using SUS304L stainless steel, corrosion tests in boiling neptunium nitrate solution were conducted under immersion and heat-transfer condition. By the weight loss measurement of stainless steel and the quantitative analysis of metallic ions dissolved in solution, the corrosion rates were obtained. The surface morphology was observed by scanning electron microscopy. The acceleration mechanism was investigated by electrochemistry and spectrophotometry. The corrosion rate was accelerated by addition of neptunium in nitric acid. Preferential intergranular corrosion was observed. The corrosion was enhanced under heat-transfer condition compared to immersion condition. In polarization measurements, the cathodic over-voltage was decreased; the cathodic current was increased by addition of Np. Spectrophotometric measurements showed that Np(V) oxidized to Np(VI) in boiling nitric acid. The corrosion acceleration mechanism in nitric acid solution containing Np suggested the re-oxidation of Np(V).