Degradation of free-standing air plasma sprayed CoNiCrAlY coatings by vanadium and phosphorus pentoxides

Use of alternative and/or low-cost fuels such as syngas, petcoke and coal/petcoke blend in gas turbine engines requires a thorough understanding in high temperature degradation of protective coatings. Deleterious combustion by-products can deposit, adhere, melt and degrade the protective coatings and underlying structural substrates. In this investigation, degradation of air plasma sprayed (APS) free-standing CoNiCrAlY coatings in contact with two different corrosive oxide contaminants, namely V2O5 and P2O5 were examined at high temperature. Different degradation mechanisms were observed from V2O5 melt interaction with CoNiCrAlY at 700 °C and 900 °C. At 700 °C, formation of chromium–aluminum orthovanadate (Cr,Al)VO4 was observed with no evidence of severe degradation. However, at 900 °C, extensive dissolution of CoNiCrAlY constituents by V2O5 was found with reaction products such as nickel–cobalt orthovanadate (Ni,Co)3(VO4)2 and (Ni,Co)(Al,Cr)2O4 spinel. Interaction of P2O5 melt with CoNiCrAlY at 350 °C for 2 h revealed the extensive consumption of constituents through formation of polyphosphate compounds such as (Ni,Co)(PO3)2 and (Cr,Al)(PO3)3.