Passivity and its breakdown on zirconium in high temperature nitric acid

Abstract

The corrosion resistance of zirconium in nitric acid is affected by concentration and temperature of nitric acid as well as applied potential. The purpose of this work is to study passivity and its breakdown of zirconium in high temperature nitric acid. (1) The polarization curve of zirconium in nitric acid was divided into three characteristic potential regions, i.e., primary, secondary and tertiary potential regions. (2) The primary potential region was a passive potential one and passivity was maintained by ZrO2 film. (3) The ZrO2 passive film broke down owing to partial dissolutions of ZrO2 as ZrO2+ and Zr4+ at the potential above 1.3V (SCE) up to 2.8V in 3N HNO3. Oxygen evolution above 1.8V in this potential region contributes to the thickening of ZrO2 film. (4) Current density increased extremely in the tertiary potential region by flaking of ZrO2 film and high applied potential. There was no tertiary potential region in the polarization curve for zirconium in sodium nitrate solution, because the detrimental action of H+ ions against stability of ZrO2 film is negligibly small. (5) Zirconium had transgranular type stress corrosion cracking (SCC) in secondary potential region in HNO3.