Pre-oxidized HVOF Sprayed CoNiCrAlY Coating as An Environmental Barrier Against Ammonia Corrosion for Pure Ammonia Combustion Gas Turbines

Effective utilization of ammonia as a fuel in gas turbines poses several challenges. Ammonia corrosion is one of the critical issues for the materials of the turbine components. Therefore, prevention of ammonia-based degradation of bulk materials and thermally sprayed coatings is indispensable. The potential of an oxide layer formed on High Velocity Oxy-fuel Flame (HVOF) sprayed CoNiCrAlY coating by heat treatment in air in preventing ammonia corrosion was studied. A comparative corrosion assessment of Hastelloy X substrate, as-sprayed and pre-oxidized CoNiCrAlY coating was conducted under NH₃ environment from 500 °C to 800 °C. The degradation behaviours of the substrate and the coatings were elucidated. Hastelloy X was found to underwent near-surface nitridation, whereas significant internal nitridation of the as-sprayed coating was observed due to the inter-connected porosities and imperfect inter-splat boundaries. X-ray diffraction analysis showed CrN and γ´-(Fe,Ni)₄N phases in Hastelloy X, while the nitrided layer in CoNiCrAlY coating consisted of CrN, AlN, Ni₃N and Co₂N phases. On the other hand, no nitridation of the pre-oxidized coating was identified. The dense aluminium oxide layer formed on the surface of the coating effectively impeded nitrogen diffusion and consequent internal nitridation. Hence, pre-oxidized coatings may be useful for ammonia fueled gas turbines.