Comparative Study on the High-Temperature Corrosion Mechanisms of HVOF- and APS-Sprayed MCrAlY Coatings in a NaCl–Na2SO4 Environment

This study systematically compares the corrosion resistance and failure mechanisms of NiCoCrAlY coatings prepared by Atmospheric Plasma Spraying (APS) and High-Velocity Oxygen Fuel (HVOF) spraying in a high-temperature NaCl/Na2SO4 molten salt environment at 900 °C. Through microstructural characterization, phase analysis, and corrosion kinetics, the performance differences and corrosion evolution of the two coatings are revealed. The results show that the APS coating, due to its lamellar structure and higher porosity, facilitates the rapid penetration of molten salts through capillary action, accelerating the diffusion of Cl- and SO42-. During corrosion, the Cr2O3 oxide layer on the surface of the APS coating thickens locally to 22.535 μm. However, due to internal stresses, cracks propagate, leading to the peeling of the oxide layer and the formation of a loose structure. In contrast, the HVOF coating, benefiting from its dense structure, forms a continuous and uniform Cr2O3-Al2O3 composite oxide layer on the surface, effectively inhibiting the penetration of corrosive media. The study finds that sulfur (S) elements accumulate along the coating-substrate interface in the APS coating, triggering sulfidation reactions, while the continuous diffusion of aluminum (Al) in the HVOF coating promotes dynamic repair of the sub-surface Al2O3 layer. The research confirms that the HVOF process exhibits superior long-term service performance in high-temperature molten salt environments, providing a theoretical basis for the selection of MCrAlY coating processes.