High temperature alkali corrosion kinetics of low expansion ceramics

Abstract

The gaseous alkali corrosion kinetics of silicon carbide, silicon nitride, alumina, mullite, cordierite, transformation toughened zirconia, and aluminum titanate were systematically investigated from 950°C to 1100°C in dry air containing 1.0 +/- 0.1 vol% alkali. The linear reaction rates observed at all temperatures indicate that the alkali corrosion of Si,N, and SiC is interfaced-controlled with an activation energy of 199 and 104 kJ/mol, respectively. The overall reaction in these materials involves a complex absorption-dissolution-oxidation process and the rate-controlling step appears to be the interfacial oxidation of Si,N, or SiC to SiO,. Linear alkali reaction rates were observed for alumina, resulting in the formation of sodium aluminate which occurs in a relatively planar fashion at low temperatures and in beta-alumina at higher temperatures with an experimental activation energy of 200 kJ/mol. The beta alumina forms in laths along the grain boundaries such that the fast ion conduction plane is perpendicular to the sample surface. Linear reaction rates were also observed for mullite at all temperatures with an experimental activation energy of 182 kJ/mol, resulting in the formation of sodium aluminate and carnegieite. The alkali reaction rate of cordierite is linear at low temperatures and parabolic at higher temperatures, apparently due to the change in reaction layer morphology from a highly porous to a more dense structure. The observed alkali reaction rate of transformation toughened zirconia is linear at all temperatures with an experimental activation energy of 229 kJ/mol. The fragile alkali reaction layer which formed on aluminum titanate resulted in a very large dimensional change (approximately 25%) for short times at 950°C. The reaction products which formed from long isothermal heat treatments of cordierite, zirconia, and calcia stabilized zirconia powders mixed with sodium or potassium carbonate were investigated. Observed phases as a function of composition, temperature and time are presented. Subsolidus phase equilibria in the soda-alumina-titania system were investigated and a tentative subsolidus ternary phase diagram for the low soda portion of this system is presented.