Na₂SO₄ deposit‐induced hot corrosion of BN‐coated α‐SiC

Abstract

BN coated α-SiC was exposed to ~2.5 mg/cm 2 of Na 2 SO 4 for 0.75-144 hours between 700°C and 1100°C in a 0.1% SO 2-O 2 gaseous environment. A combination of SEM, EDS, ICP-OES, XRD, and optical profilometry was used to determine that BN induced a previously unobserved form of Na 2 SO 4 deposit-induced hot corrosion of SiC below the melting temperature of Na 2 SO 4 (T m = 884°C). Above the melting temperature of Na 2 SO 4 BN did little to alter the hot corrosion behavior of SiC, due to B 2 O 3 volatility. Substrate attack below the melting temperature of Na 2 SO 4 was found to result in rivulet recession features, whereas deep, localized pitting occurred above the melting temperature of Na 2 SO 4. Initially, the mixture of Na 2 SO 4 and BN led to the formation of a Na 2 O-and B 2 O 3-containing liquid silicate and accelerated corrosion which after prolonged exposure, resulted in the partial crystallization into a lath-like tridymite and cristobalite mixture. Prolonged exposure resulted in a flattening of SiC consumption rates after exposure for several hours at 1000°C and after several days at 800°C, resulting in decreased SiC oxidation/corrosion kinetics.