Evaluation of the Hydrothermal Corrosion of SiC and the Viability of Mitigation Coatings to Protect SiC Across Typical LWR Chemistry and Temperature Regimes

An important challenge in the development of ceramic matrix composite SiC for accident tolerant fuel cladding applications is corrosion under normal operating conditions. The present study has investigated the chemical kinetics of chemical vapor deposited SiC with and without neutron irradiation. Four commercial coating varieties (physical vapor deposited TiN, CrN, Cr, and multilayer Cr/CrN) were deposited on CVD SiC as potential corrosion-mitigation coatings and examined in parallel with uncoated SiC. Samples were exposed to high purity compressed water between 288 and 350°C with 1-4wppm added O₂ or 0.15-3wppm H₂. Additionally, samples were exposed in the MIT Nuclear Reactor PWR-simulating water loop at 300°C in three locations corresponding to neutron damage with radiolysis, reduced radiolysis, and pure water only. The coatings were also exposed to radiation damage in Ar.Pure, high resistivity CVD SiC was found to corrode in the active regime (no passivating SiO₂ film is formed).The reaction kinetics of CVD SiC was estimated to corrode with activation energy of 61 kJ/mol and 106 kJ/mol and a standard entropy of activation of -240 kJ/mol and -27 kJ/mol, with respect to water and aqueous oxygen, respectively. The overall corrosion rate in mg/cm²s can be described by, Rate = (0.1458)/(1+SA) T(1.09(1 − 10^(−3)T)[O_2]e^((−1.275 10^4)/T) + 7.91 10^(−6)e^((−7.39 10^3/T ), with T in kelvin and [O₂] in wppm. Polishing was found to have a significant effect on the observed corrosion rate in oxygen. Radiation fields increased the corrosion of SiC mildly and high resistivity SiC is predicted to recede less than 4µm in 5 years.The coatings investigated in this work were not protective. TiN oxidizes and spalls, possibly due to interface destabilization. This was exacerbated by radiation damage and radiolysis products. The multilayer Cr/CrN cracked under irradiation in Ar and spalled in the presence of water. A monolithic CrN coating was more protective in the absence of irradiation and cracked less under irradiation in Ar. Cr was protective in deoxygenated conditions but quickly oxidized and subsequently spalled in the presence of oxygen. Radiation damage caused extensive cracking and ~0.2% void swelling in the Cr coating. More compressive residual stresses, higher purity, and higher coating ductility are recommended for future coating generations.

This document is based on accepted, submitted, and in-progress publications in peer-reviewed journals