IMPERMEABLE THIN Al2O3 OVERLAY FOR TBC PROTECTION FROM SULFATE AND VANADATE ATTACK IN GAS TURBINES
In order to improve the hot corrosion resistance of conventional YSZ TBC system (YSZ/CoNiCrAlY/Inconel 601), an overlay Al{sub 2}O{sub 3} was sprayed on the surface of TBC samples by high velocity oxy-fuel (HVOF) spray techniques. The TBC preparation in Japan was based on our technical requirement by plasma spray. Bond coat CoNiCrAlY and the YSZ was produced by low-pressure plasma spray and air plasma spray respectively. Hot corrosion tests were carried out on the TBC with and without Al{sub 2}O{sub 3} coating in molten salts mixtures (Na{sub 2}SO{sub 4} + 5%V{sub 2}O{sub 5}) at 950 C for 10h. The microstructures of TBC and overlay before and after exposure were examined by means of scanning electron microscopy (SEM), energy-dispersive X-ray spectrometer (EDX) and X-ray diffraction (XRD). It has been found that TBC reacted with V{sub 2}O{sub 5} to form YVO{sub 4}. A substantial amount of M-phase was formed due to the leaching of Y{sub 2}O{sub 3} from YSZ. Al{sub 2}O{sub 3} overlay coating sprayed by HVOF was dense, continues and adherent to the TBC even after exposure to the molten salts. As a result, overlay Al{sub 2}O{sub 3} coating can prevent the YSZ from the attack by molten salts containing vanadium and arrest the penetration of salts into the YSZ along porous and cracks in the YSZ TBC. Accordingly, the amount of M-phase formed in TBC with Al{sub 2}O{sub 3} overlay was significantly lower than that in conventional YSZ TBC system. In the next period, the hot corrosion tests of TBC with EB-PVD Al{sub 2}O{sub 3} coating under Na{sub 2}SO{sub 4} + 5%V{sub 2}O{sub 5} will be again performed at 950 C. However before hot corrosion tests, the post-annealing will be carried at 1273K for 1h in order to transform the as-sputtered {gamma}-Al{sub 2}O{sub 3} overlay to crystalline {alpha}-Al{sub 2}O{sub 3} overlay. In addition, the effect of coating thickness on corrosion resistance and the mechanisms of cracking of EB-PVD alumina layer during hot corrosion will be also investigated.
- Research Organization:
- National Energy Technology Laboratory (NETL), Pittsburgh, PA, Morgantown, WV, and Albany, OR (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- FC26-01NT41189
- OSTI ID:
- 800951
- Report Number(s):
- FC26-01NT41189-03; TRN: US200221%%1638
- Resource Relation:
- Other Information: PBD: 30 Jun 2002
- Country of Publication:
- United States
- Language:
- English
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