Effect of deposit composition and thermal cycling parameters on oxide- and sulfate-induced hot corrosion of CoNiCrAlY HVOF coatings

This work employs burner rig testing to understand the modes of degradation of a CoNiCrAlY coated superalloy under thermo-cyclic service condition upon exposure to oxide, oxide-sulfate and sulfate deposits. The mixed oxide and oxide-sulfate deposits adhered to the Al₂O₃ TGO without reacting, likely due to the decomposition of sulfates and sequestration of reactive oxides into silicates. In contrast, sulfate-only deposits readily react with the TGO, forming less-protective calcium aluminates for all cycling conditions. While the TGO on the specimen heat treated isothermally remained protective, increased cycling frequency led to reaction product delamination depleting the Al-reservoir and oxidizing other coating elements.