Rainbow Field Test of Coatings for Hot Corrosion Protection of Gas Turbine Blades and Vanes: I — Blade Coatings

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Abstract
A rainbow field test sponsored by the Electric Power Research Institute (EPRI) under contract RP2465-1 was performed to evaluate the comparative hot corrosion resistance of commercially available coatings for gas turbine blades and vanes. The 10,307-hr field test was carried out on a Solar Turbines Incorporated Centaur T-4000 gas turbine operating on a lower grade liquid fuel at the Favianca site of the Owens-Illinois, Inc. glass manufacturing facility in Valera, Venezuela. This paper reviews the results of an evaluation of the performance of three modified aluminides, three MCrAlY overlays, and one duplex NiCoCrAlY/ZrO2-2OY2O3 overlay applied as coatings to the first-stage MAR-M421 and IN-738LC rotor blades, Visual and metallographic examination and remnant coating thickness measurements established that the MCrAlY overlay coatings were generally more effective than a Cr-aluminide and two Pt-aluminides protecting the first-stage blades. Individual differences between the various coatings were established. A remnant coating thickness index (RCTI) was defined to express coating survival and protectiveness quantitatively. The results of blade airfoil temperature estimates were correlated with the hot corrosion morphology.

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