Microstructure and Corrosion Resistance of NiCr-Based Coatings in Simulated Coal-Fired Boiler Conditions

Abstract

The corrosion behavior of NiCr-based coatings including 45CT, NiCrBSi, and Inconel 718 was studied under simulated coal-fired boiler conditions. It was found that these arc-sprayed coatings applied to TP347 boiler steel were comprised of Cr1.12Ni2.88 compound. Hot corrosion tests were performed to study the corrosion resistance of the coatings in simulated flue gases and synthetic fly ashes at 650 °C. The composition (wt.%) of synthetic ashes was 92.9fly ash–0.1NaCl–2.0Na2SO4–5.0CaSO4. The 45CT coating had optimal corrosion resistance while the NiCrBSi coating suffered from severe corrosion according to the experimental results. During corrosion tests, Cr2O3 and Ni3S2 were formed on the coating surfaces due to the oxidation and corrosion of the coatings. A protective Cr2O3 scale acted as a corrosion barrier, which led to the decrease in the corrosion rates. The Cr2O3 scale would lose protectiveness due to its dissolving caused by molten NaCl salt. The results indicated that compounds such as SiO2, CaO, and Al2O3 from the synthetic fly ashes were covered on the coating surfaces. Sulfur was found to be the most corrosive agent, substantially increasing corrosion rates. The mechanisms of S-induced corrosion and dissolving of Cr2O3 scale for NiCr-based coatings are discussed.