Modes of Deposit-Induced Accelerated Attack of MCrAlY Systems at 1100 °C

Abstract

The reaction of cast NiCoCrAlY alloys with oxide–sulfate deposits in CO2–H2O–O2 was studied at 1100 °C. The minimum Al concentration needed to form an external Al2O3 scale was increased compared with deposit-free exposures, as Al2O3-forming compositions transitioned to internal Al2O3 and external Cr2O3 growth in the presence of certain deposits. Model deposits were used to investigate the role of each constituent in the complex reaction morphology observed with an industrial fly-ash. Two main modes of degradation were identified, which involved Al2O3 dissolution in molten Na silicate and solid-state Al2O3 reaction with CaO. Both led to enhanced Al consumption and promoted non-selective oxidation. Additions of Al2O3 or SiO2 decreased the CaO reactivity due to the formation of aluminates or silicates, while Na2SO4, on the contrary, enhanced the degradation by providing rapid mass transport in the molten state, and reduced alloy/scale adherence. A systematic study of the role of phase fractions and phase compositions in the γ-(Ni,Co) + β-(Ni,Co)Al metal system is reported, with the aim of providing guidance in coating design. In particular, high γ fractions and Cr concentrations, which offer optimal hot corrosion resistance, were most susceptible to degradation by oxide–sulfate deposits.