Hot corrosion of ceramic-coating materials for industrial/utility gas turbines

Furnace hot corrosion tests of yttria-stabilized zirconia (YSZ) and other candidate ceramic coating materials were run under combinations of temperature, salt deposits, and gaseous environments know to cause severe hot corrosion of state-of-the-art metallic coatings for industrial/utility gas turbines. Specimens were free-standing ceramic coupons and ceramic-coated IN 792. X-ray fluorescence and diffraction data on free-standing YSZ coupons showed surface yttrium loss and cubic-to-monoclinic transformation as a result of exposure to liquid salt and SO/sub 3/. Greater destabilization was observed at the lower of two test temperatures (704 and 982/sup 0/C), and destabilization increased with increasing SO/sub 3/ pressure and V-containing salt deposits. The data suggest that hot corrosion of YSZ can occur by a type of acidic dissolution of Y/sub 2/O/sub 3/ from the ZrO/sub 2/ solid solution. In spite of the greater surface destabilization at 704/sup 0/C, the bond coat and substrate of YSZ-coated IN 792 were not attacked at 704/sup 0/C but severely corroded at 982/sup 0/C. These results show that degradation of ceramic-coated metallic components can be more strongly influenced by the porosity of the microstructure and fluidity of the liquid salt than by the chemical stability of the ceramic coating material in the reactive environment. Other ceramic materials (SiO/sub 2/, Si/sub 3/N/sub 4/, ZrSiO/sub 2/, and mullite), concurrently exposed to the same conditions which produced significant destabilization of YSZ, showed no evidence of reaction at 704/sup 0/C but noticeable corrosion at 982/sup 0/C. Also, the high temperature corrosion was greater in air than in SO/sub 3/-containing gases. These trends suggest that hot corrosion of the silicon-containing ceramics was basic in nature, and such materials have potential for good resistance to chemical decomposition under the acidic conditions characteristics of industrial/utility gas turbines