Hydrothermal Corrosion of Magnesia-Pyrochlore Composites for Inert Matrix Materials

The corrosion behavior of MgO-Nd2Zr2O7 cercer composites was investigated under hydrothermal conditions at 300 degrees C and saturation pressure. It was found that corrosion resistance was microstructure dependent in terms of MgO contiguity and homogeneity. The inhomogeneous composites completely dissolved in less than 1 h showing poor hydration resistance similar to MgO. The homogeneous composites exhibited an improved hydration resistance and the corrosion was primarily limited to the surface. The normalized mass loss (NML) was proportional to the corrosion time and the NML rate increased with MgO volume fraction. The corrosion process followed an Arrhenius relationship and the apparent activation energy was calculated to be similar to 39-41 kJ/mol. It is suggested that the mass loss of the MgO-Nd2Zr2O7 composites is mainly governed by hydration of MgO and the NML rate is enhanced by contiguity of MgO. A desirable microstructure with enhanced corrosion resistance is viewed to have minimum contiguity of MgO, good homogeneity, and large MgO grain size (minimizing grain boundaries).