Corrosion protection and failure mechanism of ZrO2 coating on zirconium alloy Zry-4 under varied LiOH concentrations in lithiated water at 360 °C/18.5 MPa

Abstract

Maintaining a good corrosion resistance in primary coolant loop during normal operations is a prerequisite for ZrO2 as a protective coating on zirconium alloy cladding. Research on corrosion performance of ZrO2 coating in nuclear water chemistry is relatively deficient, and existing reports failed to provide an in-depth explanation for the failure causes of ZrO2 coating. Here, we proposed a detailed corrosion process of ZrO2 coating in lithiated water at 360 °C and 18.5 MPa based on experimental study and molecular dynamics simulation. The protective effect and failure mechanism of ZrO2 coating on Zry-4 under varied LiOH concentrations was further revealed. ZrO2 coating provided a favorable corrosion protection with the occurrence of localized corrosion at low LiOH concentrations. Factors influencing the corrosion resistance include pitting corrosion extension, enhanced Li+ permeation, short-circuit diffusion of O2– and ZrO2 phase transformation. In highly-concentrated LiOH solutions, intergranular corrosion, internal oxidation and perforation result in coating failure. Zr ions were released to the coating surface to form flocculent ZrO2 and ZrO2 clusters due to the strong diffusion and dissolution tendency of α-Zr in the Zry-4 substrate. This work can give some references to understand the service behavior of nuclear coatings under variable water chemistry conditions and promoting in-pile application of ZrO2 coating.