High temperature vanadate corrosion of yttria-stabilized zirconia coatings on mild steel

Abstract

The molten vanadate corrosion of three different 8 wt.% yttria- stabilized zirconia (YSZ) coatings on mild steel at 700, 800 and 900 °C was investigated using coupons deposited with 2.5, 5 and 10 mg cm -2 of NaVO 3. The NaVO 3 induced a tetragonal-to-monoclinic destablization of the zirconia structure, which could be followed quantitatively by X-ray diffraction on the basis of the ratio of intensities of the monoclinic 11 1 peak to the tetragonal 111 peak. Crystals of YVO 4 were also identified on the YSZ surface, confirming reaction between Y 2O 3 and V 2O 5. Addition of SO 3 to the furnace air increased the destabilization rate because of higher V 2O 5 activity via the reaction 2NaVO 3+SO 3=Na 2SO 4+V 2O 5 but ultimate total destabilization was not necessarily greater. Penetration of oxygen through the NiCrAlY bond coat produced corrosion pits in the mild steel substrate at 800 and 900 °C, with oxide extrusion from the pits causing spalling of the overlying YSZ coating. Complete destabilization of YSZ at 700 °C failed to produce spalling, but perhaps because of the low 700 °C temperature and minimal thermal shock and/or thermal cycling used. This work provides insight into YSZ corrosion by molten vanadate, and establishes methodology for laboratory prescreening of the corrosion resistance of prototype stabilized zirconia coatings for engines and other applications.