Zirconia sol–gel coatings on alumina–silica refractory material for improved corrosion resistance

The corrosion resistance of a simple zirconia (ZrO2) sol–gel coating deposited from a highly dilute sol on an alumina–silica (Al2O3–SiO2) refractory crucible material to molten borosilicate glass was investigated. The bulk density and porosity of the Al2O3–SiO2 refractory substrate material measured by mercury intrusion porosimetery (MIP) were 3.3 g/ml and 38%, respectively. The porosity measurements were verified by electron microscopic image analysis, revealing that the porosity was appreciably decreased to a value of 18% for the ZrO2 coated refractory. The corrosion loss of an uncoated refractory in molten borosilicate glass held at 1400 °C for 168 h was 27%, as measured by ASTM C621-84. A lower corrosion loss of 16% was observed for ZrO2 coated refractory. Nanoscratch testing indicated strong adhesion of the zirconia film to the refractory materials. Microstructural analysis performed at the corroded interface regions by optical microscope and SEM/EDS suggested that the corrosion loss of the Al2O3–SiO2 refractory was governed by the decomposition of the SiO2 phase and diffusion of Al2O3 crystals into the molten borosilicate glass. The crystal phases present in the corroded interface regions were characterized using X-ray diffraction (XRD). The improved corrosion resistance of the ZrO2 coated refractory to molten borosilicate glass was correlated to the reduced surface porosity and may be aided by the presence of zircon (ZrSiO4) in the surface layer.