Degradation behaviour of HVOF sprayed CoNiCrAlY coating in high-temperature ammonia environment towards its applicability in ammonia fueled gas turbines

Abstract

Corrosive nature of ammonia may restrict its widespread application as an alternative carbon-free fuel in gas turbines. Hence, understanding of the ammonia related degradation of thermally sprayed coatings used in the turbine components is essential. This work deals with the degradation behaviour of High-Velocity Oxy-Fuel (HVOF) sprayed CoNiCrAlY coating in ammonia environment at temperatures from 500 °C to 800 °C. The coatings are exposed to 10 % NH₃ gas flow for 4 h. A gradual increase in weight gain of the specimens is identified with the temperature due to diffusion of nitrogen. The two phase (γ-Co,Ni and β-NiAl) structure of the coating near the surface has changed to an internal nitrided layer consisting of CrN/Cr₂N, AlN, γ-Co/Ni, and β-NiAl phases after the ammonia exposure test, resulting in reduction of the availability of oxide formers such as atomic Cr and Al near the surface. The rise in temperature has led to a linear increase in the thickness of the nitrided layer from around 4.5 μm–9 μm. Stress assisted diffusion of Co and Ni from the nitrided layer to the surface through the diffusional creep mechanisms followed by reaction with the atomic nitrogen at the surface to form Ni₃N and Co₂N phases has resulted in the formation of Co/Ni depleted region near the coating surface. The mechanism of degradation and effects of alloying elements are elucidated in detail. Such nitridation of CoNiCrAlY coating may deteriorate its strength and oxidation resistance, necessitating the development of environmental barrier for impeding ammonia corrosion.