Corrosion-resistant metallic coating on silicon carbide for use in high-temperature water

To improve corrosion resistance of silicon carbide (SiC) in boiling water reactor (BWR) environments, corrosion-resistant coatings on SiC substrate are being developed. A metallic Ti coating with a Cr bonding layer is known to provide corrosion resistance in high-purity water with a dissolved oxygen concentration of 8.0 mg/l at 561 K, which is a simulated BWR-normal water chemistry (NWC) environment. In this paper, the mechanism of corrosion resistance was examined by evaluating the effects of heat treatment in the coating process on adhesive property and corrosion behavior in oxygenated high-purity water at high temperatures under unirradiated conditions. The Cr bonding layer was considered to contribute to lessening of thermal stress generated in the coating on the SiC substrate and to forming a rigid interface between Cr and SiC through diffusion and formation of chromium silicide. Furthermore, excessive heat treatment caused transformation of Ti to TiC and the possibility of coating disbondment. A TiO₂ film that formed on Ti surface was insoluble and this characteristic differed from Cr₂O₃, CrOOH and SiO₂ which were soluble in the BWR-NWC environment under the unirradiated condition. Thus, the TiO₂ film coat was expected to provide corrosion resistance.