Low-Pressure Plasma-Spray Coatings for Hot-Corrosion Resistance

The hot corrosion of low-pressure plasma-sprayed coating, GT-29 (Co-29Cr-6Al-1Y) on a γ^′-strengthened nickel base superalloy, IN-738, was characterized at both 871 °C and 983 °C in a simulated gas turbine environment. The test results show that at 871 °C the dense, defect-free, low-pressure, plasma-sprayed GT-29 coatings provide good corrosion protection up to 8000 hr. The mechanism of protection was the formation of a dense, adherent Al₂O₃ scale underneath a mixed oxide scale; scale was maintained by a uniformly dispersed Al rich β(CoAl) reservoir. At 982 °C, the coating corrosion protection exceeded 5000 hours and also utilized a protective Al₂O₃ scale. Due to the higher test temperature, interdiffusion of O, S, Al, Co, and Cr was higher and internal sulfidation/oxidation of the coating occurred; however, the rates were much lower than uncoated IN-738. Field tests were run (13051 and 19842 hr) to evaluate the low-pressure, plasma-sprayed GT-29 coatings’ corrosion resistance on turbine buckets. Metallographic inspection of the field-tested coatings verified the simulated burner-rig test results and demonstrated that the low-pressure plasma process provides an attractive alternative to other coating processes for producing hot-corrosion resistant coatings.