HYDROTHERMAL AND STEAM CORROSION OF SILICON CARBIDE FOR NUCLEAR APPLICATIONS

Abstract

Silicon carbide is a promising cladding candidate for water-cooled reactors due to its excellent high-temperature properties, such as good corrosion resistance and strong mechanical properties. In this study, we investigate the corrosion resistance of SiC ceramics produced through chemical vapor deposition and spark plasma sintering in hydrothermal environments. The SiC samples were exposed to corrosion in pressurized water (19 MPa) at 360°C in an autoclave, simulating the normal operating conditions of water-cooled reactors. Additionally, SiC performance was evaluated in steam at 1200°C-1400°C to simulate the loss of coolant accident scenario for water-cooled reactors. The impacts of CVD SiC growth orientation, sintering aid effects, and grain size on SiC corrosion behaviours were studied. For CVD SiC ceramics, we observed preferential corrosion at grain boundaries during the early stages of hydrothermal corrosion. At later stages, fracture of grains occurred on the SiC surface, contributing to an abrupt increase in weight loss. These fractures resulted from the transverse propagation of cracks caused by chemical corrosion. Moreover, fractures occurred earlier on surfaces parallel to the SiC growth direction, likely due to the larger average grain size. The corrosion depth of perpendicular surfaces was shallower, indicating a lower corrosion rate. XRD results suggest that 6H SiC corroded faster than 3C SiC, possibly due to the smaller grain size of 6H SiC and its preferential distribution at grain boundaries. For SPS samples, SiC with 3wt.% sintering aids demonstrates the best corrosion resistance, owing to the formation of a protective, intact oxide scale. The SPS SiC sample with 1wt.% SA exhibits the poorest corrosion resistance due to its high porosity. In the case of long-duration corrosion, corrosion pits form in SiC with 5wt.% and 10wt.% SA, resulting in substantial weight loss. A two-layered oxide structure develops on the surface of the SPS SiC samples. The top layer of this crystal-like phase is the re-precipitation of metal ions from the water solution during the cooling process for SiC with 1, 3, and 5wt% SA. When the SA content reaches 10wt.%, the top layer forms not only from the precipitation from the solution during cooling but also from the sample itself during corrosion due to the oxidation of YAG. It is confirmed that larger grains have better corrosion resistance due to the formation of a protective layer on their surface. The oxidation behavior of SPS SiC with varying sintering aid content at temperatures ranging from 1200°C to 1400°C in steam environments is also investigated. At 1200°C, no pores or porosity were observed on the oxide scale surface of SPS SiC, but a large number of needle-like or granular white oxides, primarily consisting of Y2O3, formed on the oxide surface. At 1300°C and 1400°C, pores formed across the oxide scale on SPS SiC, caused by the formation of gaseous reaction by-products. The outward diffusion of SA cations significantly lowered the activation energy of the parabolic process, accelerating the oxidation rate.

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philip Withers (Supervisor) & Ping Xiao (Supervisor)