Nitric Acid Corrosion of As-Received and Autoclaved Zircaloy-4 in All Zr-4 Nitric Acid Test Facility

Zirconium and its alloys are highly resistant to nitric acid corrosion and are considered as candidate materials for applications in the aqueous reprocessing of nuclear spent fuels. The present study highlights the long-term corrosion behavior of Zr-4 in boiling 11.5 M nitric acid, using a unique Zr-4 high-temperature high-concentration test facility. This test facility eliminates the surface contamination of Zr-4 by SiO2 in the conventional borosilicate glass apparatus. The corrosion-tested samples were characterized using XRD, GDOES and XPS techniques. Long-term corrosion testing of Zr-4 samples tested up to 15,000 h showed a low corrosion rate of < 0.1 mpy, with negligible attack on the surface. The autoclaved Zr-4 samples with and without different scratch patterns tested up to 10,000 h exhibited excellent corrosion resistance (< 0.01 mpy) in boiling liquid, vapor and condensate phases of HNO3 due to protective ZrO2 film. The U-bend samples of Zr-4 tested up to 10,000 h did not show susceptibility to stress corrosion cracking. XPS analysis revealed the presence of protective ZrO2 film, without silicon contamination. From the corrosion rate results, and nature of surface film it is clear that zircaloy-4 is a potential material for application in high-concentrated and high-temperature nitric acid environments.