High temperature oxidation behavior of plasma sprayed fly ash reinforced NiAl and NiCr coatings on inconel-600

High-temperature oxidation and hot corrosion resistance are critical requirements for metallic structural components deployed in medical waste incineration systems, where operating temperatures of 950–1100°C combine with aggressive combustion atmospheres rich in vanadates, alkali sulfates, and chlorine-bearing species. This study investigates the oxidation behavior of plasma-sprayed fly ash composite coatings incorporating NiAl and NiCr binder phases on Inconel-600 superalloy substrates, following 1000 h of cyclic thermal exposure in the secondary chamber of an operational medical waste incinerator. Seven sample configurations were evaluated: one uncoated baseline (S1), three NiAl-based coatings with fly ash contents of 60, 70, and 80 wt% (S2–S4), and three corresponding NiCr-based coatings (S5–S7). Coating performance was assessed through surface roughness profilometry, micro-Vickers hardness testing, gravimetric analysis, corrosion penetration rate measurements, scanning electron microscopy, and X-ray diffraction analysis conducted before and after oxidation exposure. NiAl-based coatings formed slow-growing, adherent alumina scales with weight gains of 1.2–2.5 mg/cm² and parabolic oxidation kinetics, while NiCr-based coatings developed chromia scales susceptible to catastrophic hot corrosion, exhibiting weight gains of 4.5–5.7 mg/cm² and paralinear kinetics driven by vanadate fluxing, sulfidation, and repeated spallation. Sample S2 with 60% fly ash and 40% NiAl achieved the lowest weight gain of 1.2 mg/cm² and corrosion rate of 4.4 mpy, performance comparable to commercial MCrAlY bond coat systems. XRD analysis confirmed the formation of non-protective CrVO₄, NaCrO₂, and CrS phases exclusively in NiCr samples, validating Type I hot corrosion as the dominant degradation mechanism. The overall performance ranking S2 > S3 > S4 > S1 > S5 > S6 > S7 demonstrates that the nature of the protective oxide scale governs oxidation resistance more decisively than fly ash concentration, establishing NiAl additions as essential for durable high-temperature protection in fly ash-laden combustion environments.