Effect of lead oxide vapor on the strength of alumina

This effect of PbO vapor on the strength of alumina was studied to determine the corrosion mechanism. Characterization of the corroded samples was carried out using X-ray diffraction (XRD), scanning electron microscopy, strength and sound velocity measurements, weight and dimensional changes. The strength and sound velocity of alumina decreased when exposed to PbO but weight and volume increased. Thermodynamic modeling of the PbO-Al₂O ₃ system was carried out using the CALculation of PHAse Diagram method, which showed that three pseudo-binary compounds, PbAl₂O ₄, Pb₂Al₂O₅, and PbAl ₁₂O₁₉ can be formed, in agreement with prior work. It was postulated that PbAl₁₂O₁₉ is most likely to form when alumina is exposed to PbO at sintering temperatures and this was confirmed by XRD and energy-dispersive analysis. The PbAl₁₂O₁₉ phase does not form a surface layer but rather nucleates and grows as individual crystals embedded in the alumina surface, forming a more porous structure. This surface porosity then acts to concentrate applied stress, resulting in a lower strength. This strength degradation mechanism makes alumina kiln furniture more sensitive to thermal stresses with repeated use, explaining the observed cracking behavior.