Corrosion fatigue of refractory materials with corrosion resistance to boiling nitric acid

沸騰硝酸に耐食性を有するリフラクトリー材料の腐食疲労

本岡 隆文; 木内 清

Motooka, Takafumi; Kiuchi, Kiyoshi

沸騰硝酸溶液中で優れた耐食性を示すリフラクトリー金属のうち、ジルコニウム,ニオブ,Ti-5Ta合金の腐食疲労進展挙動を沸騰3規定硝酸と室温大気中で調べた。ジルコニウムとニオブは、硝酸中では、腐食疲労により亀裂進展が加速した。一方、Ti-5Ta合金では亀裂進展は加速しなかった。Ti-5Ta合金の破断面には、両環境とも疲労の痕跡が認められた。ニオブでは、大気中では疲労破面、硝酸中では腐食疲労破面が観察された。ジルコニウムは、疲労と脆性破面が混在する特徴的な形態を示した。亀裂進展速度と破面様相の材料間の異なりは、結晶構造,応力腐食割れ感受性,耐食性の異なりと関連している。

Refractory metals such as Zr, Nb and Ti alloys with the excellent corrosion resistance in boiling nitric acid have been selected for structural materials of spent fuel reprocessing equipment. On this study, the fatigue crack growth rate of these materials was investigated by load control tests as a function of the stress intensity in boiling nitric acid and in air at room temperature. The fracture surface was observed by scanning electron microscopy. The crack growth rate of Zr and Nb was enhanced in boiling nitric acid compared with that in air at room temperature. Acceleration effect due to corrosion fatigue was not observed in the crack growth of Ti-5Ta alloy. The fracture surfaces of Ti-5Ta alloy showed the ductile striation in both environments. On the other hand, the fracture surfaces of Nb showed the ductile striation in air and brittle striation in nitric acid. The anodic dissolution was interpreted to the acceleration factor on corrosion fatigue of Nb. The fracture surfaces of Zr in nitric acid showed both brittle and ductile fracture with fluted pattern due to SCC.