High temperature alkali corrosion of SiC and Si3N4

Abstract

Abstract The high temperature alkali corrosion kinetics of SiC and Si 3 N 4 were investigated from 950°C to 1100°C in dry air containing 0.98 vol.% alkali. For both materials linear reaction rates were observed at all temperatures indicating that the alkali corrosion of SiC and Si 3 N 4 is surface reaction controlled with activation energies of 104 and 199 kJ/mol, respectively. The average composition of the sodium silicate reaction layer was (Na 2 O)·(SiO 2 ) 2.22 for Si 3 N 4 and (Na 2 O)·(SiO 2 ) 2.55 for SiC as determined by atomic absorption analysis. The overall corrosion reactions are complex involving absorption, decomposition, dissolution, diffusion and surface reaction processes, but the rate-controlling step in the corrosion process for both materials appears to be the interfacial oxidation of SiC or Si 3 N 4 to SiO 2 . A comparison of the reaction kinetics and mechanism between the oxidation of each material with and without alkalis present is presented.