On the Reaction Mechanism of MCrAlY Alloys with Oxide–Sulfate Deposits at 1100 °C

Abstract

The corrosion of γ-(Ni,Co) + β-(Ni,Co)Al NiCoCrAlY alloys exposed to CaO-rich, Na2SO4-containing deposits in air and CO2–H2O–O2 at 1100 °C was studied with a focus on the mechanisms governing the accelerated attack of γ-rich compositions. Internal oxidation of Al resulted from the breakdown of an initially formed Al2O3 scale, due to its reaction with the deposit. Compared to deposit-free conditions, this scale exhibited: (i) a delayed Al2O3 structure transition from metastable θ to stable α; (ii) finer grains; (iii) a reduced adherence to the metal, to the extent that a gap developed at the metal/oxide interface; and (iv) significant permeability to S, as well as N during air exposures. The occurrence of sulfidation in S-free gases reflected the establishment of an elevated S activity under the molten Na2SO4, due to the locally reduced $$p_{{{\text{O}}_{ 2} }}$$ . The factors affecting the Al2O3-scale microstructure were examined, as well as the associated consequences on Al consumption and on the transition to internal oxidation. The roles of alloy and gas compositions in the identified mechanisms were also discussed.