Influence of Dissolved Oxygen and Temperature on Corrosion and Elution Behaviors of Si3N4 Ceramics in High-Temperature Demineralized Water

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Influence of dissolved oxygen and temperature on corrosion and elution behaviors of Si3N4 ceramics containing 5wt% MgO as sintering aid in high-temperature pure water were studied using the once-through type experimental loop. Exposure tests were carried out under two different dissolved oxygen concentrations of below 20μg/l and 8mg/l. Exposure temperature was elevated gradually from 120°C to 200°C and then to 275°C, where the indicated temperature was kept for 8 days. The results suggested that dissolved oxygen did not promote corrosion of Si3N4 ceramics, although the weight loss of specimen after exposure was influenced largely by temperature and the amount was increased by almost one order of magnunitude with temperature change. These were considered to be caused by the reaction of Si3N4 grain with H2O, of which rate was increased with temperature. The specimen exposed at 275°C was largely leached and its amount of weight loss attained up to 52mg/cm2 under low dissolved oxygen condition. Then a part of corrosion products remained on the surface and formed the porous corrosion film. In these processes Mg tended to be accumulated in the leached layer. On the other hand, Si3N4 grain was scarecely eluted at 120°C, but the grain boundary contaming Mg was selectively eluted to test solution.
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