Corrosion Behavior of Zr-0.75Sn-0.35Fe-0.15Cr-xNb Alloys in Oxygen-Containing Super-Heated Steam at 400 °C

Abstract Due to a compact layout design of water-cooled small modular reactors (SMRs), no deaeration facilities is generally used, resulting in a high dissolved oxygen (DO) lever in the primary loop water. Therefore, the effect of DO on the corrosion behavior of zirconium alloys as fuel cladding materials is one of the issues that need attention in SMRs. In this study, the corrosion tests of Zr-0.75Sn-0.35Fe-0.15Cr-xNb (x = 0, 0.15, 0.3, 0.5, 1.0, wt%) alloys were carried out in super-heated steam with deaeration, 300 ppb DO and 1000 ppb DO at 400 °C/10.3 MPa using static and dynamic autoclaves. The microstructure and valence state of elements of the alloys and oxide films were characterized using TEM and XPS. Results show that DO accelerates the corrosion of Zr-0.75Sn-0.35Fe-0.15Cr-xNb alloys and the accelerated corrosion effect increases with the increase of Nb in the alloys and the DO content in the steam. Both the Nb in the alloys and the DO in the steam promote the oxidation of Fe to Fe2+ and Fe3+, and Cr to Cr3+, which will increase the number of defects to promote the diffusion of O2− vacancies, leading to accelerating the corrosion of the alloys. Compared with the deaerated environment, the transition layer at the oxide/metal interface formed in the DO environment is thinner, which provides less time for stress release during oxidation to promote the generation of cracks and accelerate the corrosion of zirconium alloys.