SCC of Zirconium and Its Alloys in Nitric Acid

Stress corrosion cracking (SCC) of zirconium Grades 702 (commercially pure zirconium), 704 (a Zr-1.5 Sn alloy), and 705 (a Zr-2.5 Nb alloy) in nitric acid has been studied by constant strain techniques, optical microscopy, scanning electron microscopy, and electrochemical techniques. Zirconium and its alloys, except welded Grade 705, had high SCC resistance in 70% HNO3 up to the boiling point. By avoiding high sustained tensile stresses, unwelded Grades 702 and 704 had very long time-to-failure, if not infinite, in 90% HNO3 at 30 C. The failure was transgranular cleavage or quasicleavage fracture. No zirconium hydride was detected by the anodizing method. Electrochemical results show that zirconium and its alloys exhibit passive-to-transpassive transition over a broad range of acid concentrations and electrochemical potentials.