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Microstructural analysis of the corrosion of Al2O3 and Zro2 in frit melts
Dense alumina and zirconia crucibles manufactured in the laboratory by slip casting were tested for their resistance to corrosive attack by opaque and transparent frits between 1400-1500oC. Interface between the crucible and the frit was investigated by Scanning Electron Microscopy (SEM, Philips XL-30S FEG) equipped with EDS (Energy Dispersive Spectroscopy) unit, X-ray diffraction (XRD, Philips X.Pert Pro), and Optical Microscope (OM, Nikon Eclipse L150). Formation of a continuous band of zinc aluminate (gahnite) crystals was observed at the interface between the alumina crucible wall and the contained opaque and/or transparent frit melt. When opaque frit was used, isolated pockets of zirconia were present adjacent to the zinc aluminate band inside the frit. Deeper inside the frit incompletely dissolved zircon were observed. On the other hand zirconia crucible failed against both of the corrosive frits, resulting in complete penetration of frit species into zirconia crucible wall. Thermodynamic predictions based on the use of phase diagrams also produced similar conclusions with the practically observed results. Crucibles with 50 wt% zirconia and 50 wt% alumina were also manufactured and tested for their corrosion resistance against the frit. It was found that the alumina crucibles could be safely used for corrosion testing with minimal aluminum contribution to the frit melt.