Synergistic effects of Si and Y on corrosion behavior of cast cladding steels by pre-laying Y powder for nuclear applications in static liquid LBE

Abstract

Ferritic/martensitic (F/M) steels with (9–12)wt.% Cr are potential candidates for structural applications in Lead-cooled Fast Reactors (LFR) due to their low solubility in liquid Lead-Bismuth Eutectic (LBE) coolant. How to improve corrosion resistance of 9-12Cr F/M steels has been a challenge. Here we cast three steels by pre-laying yttrium (Y) powder on the bottom of mold to obtained Y-bearing steels. Precursory work on this family of Y-bearing steels has previously highlighted them with the longer steady state creep stage and the lower minimum creep rate at 650 °C under 120 MPa, which has motivated their study under more extreme conditions, such as corrosion behavior in liquid LBE. We find a synergistic effect of Si and Y on both optimizing structure and reducing thickness of oxide scale at various oxygen concentrations, exposing temperatures and test times. The oxide scale changed from dual layers to single spinel layer, the thickness reduced from 36 μm to 4 μm, dissolution corrosion and PbBi penetration were inhibited. Low diffusion coefficient of ions in SiO2 and Y segregation on grain boundaries of steels hinder the outward diffusion of cations, and strong affinity between Y and O leads to the formation of Y2O3 preferentially, acting as nucleation sites of SiO2 and Cr2O3, accelerating a dense and continuous oxide scale. This work enables the development of structure steels with high corrosion resistance for various high temperature application, including boiler steel, gas turbine, and heat exchanger tubes.