Corrosion of Zirconium in the Context of the Spent Nuclear Fuel Reprocessing Plant

Abstract : Zirconium is used as material for various applications in spent nuclear fuel reprocessing plants involving concentrated nitric acid medium under highly corrosive oxidizing conditions. In such conditions, zirconium can show two different oxidation regimes as a function of potential. At high potentials, the oxidation current is relatively important (few $\mu$A/cm$^2$) and constant with time. A poorly protective black oxide layer (ZrO$_2$) is formed on the surface, which has a thickness of few micrometers. At low potentials, the oxidation current decreases with time and progressively tends to low values (few nA/cm$^2$). This protective effect has been attributed to the formation/growth of a nanometric oxide layer (ZrO$_2$) on the surface. The thickness evolution of this layer has been simultaneously estimated as a function of time by in-situ electrochemical methods (Electrochemical Impedance Spectroscopy (EIS) and coulometry) and by ex-situ X-ray Photoelectron Spectroscopy analyses (XPS). These experimental results have been successfully compared with an anodic oxide layer growth model based on the high field regime. The addition of small amounts of fluorine in nitric acid (a few tenths of millimoles per liter) modifies notably the corrosion processes. Three phases are observed in the corrosion mechanism:(1) a first incubation phase where the corrosion rate is negligible, (2) a second phase with a sharp increase of the corrosion rate (the SEM observations show a process of nucleation / growth /coalescence of pits) and (3) a third phase with a progressive decrease of the corrosion rate (corresponding to a generalized corrosion mechanism by an oxidation/complexation process limited by the supply of the reactive species)