Microstructure and corrosion resistance of ERNiCrMo-13 and NiCrBSi coatings in simulated coal-fired boiler conditions: The effect of fly-ash composition

This work is devoted to the study of microstructure, corrosion behavior, and deposition formation of ERNiCrMo-13 and NiCrBSi coatings in simulated coal-fired boiler environments. It was found that these coatings applied to TP347 boiler steel by means of arc spraying were comprised of Cr1.12Ni2.88 compound. The hot corrosion tests were performed in simulated flue gases and synthetic fly ashes at 900 degrees C. The composition (wt%) of synthetic ashes was 75 fly ash + (25 - x) Na2SO4 + x NaCl with varying levels of Na2SO4 and NaCl (x = 0, 3.25, 7.99, and 25). The experimental results showed that NiCrBSi coatings had better corrosion resistance than ERNiCrMo-13 coatings. The corrosion kinetic of the coatings was composed of two stages including a rapid mass gain and a reduction of the mass gain rate due to selective oxidation (stage I) and a mass loss or a larger mass gain rate due to dissolving of oxide scales or deposition of fly ashes (stage II). The high level of NaCl in the fly ashes was responsible for the non-parabolic rate law of the corrosion kinetics, the accelerated corrosion of the coatings, and the internal oxidation of the substrates. The severe corrosion of ERNiCrMo-13 and NiCrBSi coatings occurred in the condition with a high content of NaCl. During corrosion tests, the different compounds such as NiO, Cr2O3, CrO2, NiSi2, and Fe3Cr2Si3O12 were formed on the coating surfaces. The protective Cr2O3 and/or CrO2 scales formed due to the selective oxidation acted as a corrosion barrier in the initial stage, and will lose the protection when the reaction between chromium oxides and NaCl occurs. The oxides and salts were found to be covered on the coatings due to the deposition of the fly ashes. The mechanisms of deposition formation, Cl-induced corrosion, and internal oxidation under the simulated coal-fired boiler conditions are discussed.