The development of protective coatings against alkali-induced high-temperature corrosion is critical for extending the service life of components in aggressive environments such as boiler co-firing biomass. This study investigated the influence of Cr3C2-NiCr coating thickness on the corrosion performance of A516 carbon steel substrates exposed to alkali chloride vapor (NaCl + 55 wt% KCl) at 600 °C for 100 hours. Cr3C2-NiCr as the coating material was deposited by High Velocity Oxy Fuel (HVOF) spraying with approximate thicknesses of 150 μm and 20 μm. Their degradation behaviors were evaluated through mass change measurements, corrosion rate analysis, and detailed microstructural characterization using field emission-scanning electron microscopy (FE-SEM) equipped with electron dispersive spectroscopy (EDS), X-Ray Diffraction (XRD), and surface hardness testing. The thicker coating of 150 μm exhibited a significantly lower corrosion rate (0.15 mm/y during the first 20 hours) than the thinner coating of 20 μm, which experienced rapid degradation and visible spallation after 40 hours. XRD analysis revealed that the surface of the thicker coating was dominated by Cr2O3 and NiCr2O4. In contrast, the thinner coating formed a more complex oxide mixture consisting of Cr2O3, NiCr2O4, Fe3O4, and NaCrO2, indicating a severe corrosion attack to the thinner coating and substrate. The underlying mechanisms of alkali salt vapor corrosion for both coating thicknesses are explained in this paper, offering understanding into microstructural of Cr3C2-NiCr coating on A516 carbon steel in corrosive high-temperature environments. © Published under licence by IOP Publishing Ltd.