Corrosion of SiC, Si 3N 4 and AIN in molten K 2SO 4K 2CO 3 salts

Abstract

Silicon carbide, silicon nitride and aluminium nitride ceramics were immersed in potassium sulphate-potassium carbonate metls in air or in a nitrogen atmosphere at 1013–1200°C to examine their corrosion behaviour. Aluminium nitride ceramics exhibited great resistance to corrosive attack from potassium salts under these conditions, exhibiting only a slight weight loss. Silicon carbide ceramics dissolved slowly in molten potassium carbonate (K 2CO 3), but rapidly dissolved in both potassium sulphate (K 2SO 4) and 50 mol% K 2SO 4-50 mol% K 2CO 3 melts. The weight loss of silicon carbide in these potassium salt melts was larger in air than in nitrogen. Silicon carbide ceramics reacted with K 2SO 4 melt quantitatively and the stoichiometry of K 2SO 4/SiC was 0·8. On the other hand, the weight loss of silicon nitride ceramics was appreciable in all potassium salt metls. The reaction between Si 3N 4 and K 2SO 4 or K 2CO 3 proceeded quantitatively and the stoichiometry of K 2SO 4/Si 3N 4 and K 2CO 3/SI 3N 4 was 1·6 and 3·5, respectively. The presence of oxygen accelerated the reaction between Si 3N 4 and K 2CO 3, but greatly retarded the reaction between Si 3N 4 and K 2SO 4, due to the formation of a protective film. The oxidation mechanisms of Si 3N 4 and SiC with K 2SO 4 and K 2CO 3 melts are discussed on the basis of the experimental results.