Corrosion Behavior of MgO-ZrO<sub>2</sub> and CaO-ZrO<sub>2</sub> Sintered Bodies in Molten Fluoride Salts

Abstract

The corrosion behavior of MgO-ZrO2 and Ca O-ZrO2 sintered bodies was studied in molten fluoride and fluorosilicate salts. Most samples used for a corrosion test were commercially available cubic stabilized zirconia (CSZ) and specimens made in our laboratory. The CSZ containing MgO and CaO readily corroded in molten fluoride salts. However, the sintered body of ZrO2 containing Y2O3 was superior to other ZrO2 ceramics in regard to the corrosion resistance. The corrosion depth of Mg-CSZ, Ca-CSZ and 3 Y-PSZ depended on the ionic radius of alkaline metal cations of the fluoride salts. The phase transformation of various CSZ ceramics from cubic to monoclinic phase, which occurred on the corrosion site, was considered to be caused by dissolution (or elution) of the stabilizer from the sintered body to the salt. The corrosion of CSZ in hexafluorosilicate salt was estimated to be caused by decomposition of a fraction of the fluorosilicate salt to fluoride and silicon fluoride. The corrosion depth in hexafluorosilicate salt was smaller than that caused by the fluoride salt in CaO-ZrO2 and MgO-ZrO2 sintered bodies in contrast to the Y2O3-ZrO2 sintered body. The magnitude of the phase transformation of CSZ depended upon the kind of stabilizer and the grain size of the cubic phase.