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The corrosion behavior and strength degradation of alumina ceramics with 99%, 99.9%, and 99.99% Al2O3 were studied in water at 300°C and 8.6 MPa for 1 to 10 d. The weight loss in alumina ceramics was mainly attributed to the dissolution of SiO2 and Na2O grain‐boundary impurities. Intergranular corrosion proceeded in the alumina ceramics by preferential attack at the grain boundaries. The extent of the strength reduction for corroded alumina ceramics was related to the impurity level in the alumina ceramics.

The aqueous corrosion behavior of hot isostatically pressed Si,N4 (HIPed-Si,N,) without additives was studied under hydrothermal conditions at 300°C and 8.6 MPa (86 atm). The accelerated weight loss in the HIPed-Si3N, was attributed to uniform thinning of the specimen accompanied by dislodgement of Si,N, grains from the substrate due to preferential attack at grain boundaries. Enhanced attack at grain boundaries was due to the presence of amorphous SiO, from impurities in the starting powder.