Study on the high-temperature corrosion behavior of superheater steels of biomass-fired boiler in molten alkali salts' mixtures

Alkali sulfates and chlorides may result in rapid corrosion attack of the materials during the process of biomass-fired power generation. The 9-mm-thick 12Cr1MoV superheater tube in biomass-fired boiler was cut into samples by wire cutting. Hot corrosion behaviors of the materials in molten NaCl-50% KCl, KCl-50% K2SO4, NaCl-50% Na2SO4, and Na2SO4-50% K2SO4 at 923K were investigated by the weight change kinetics, scanning electron microscopy, as well as energy-dispersive spectroscopy. The weight gain curve was studied by electronic balance after each experiment cycle at 12, 24, 48, and 60 h. Compared to the microstructures of the corrosion surface and cross section of each material, concentrated pores and cracks were observed on the samples in molten NaCl-KCl, indicating that chlorides induce severest corrosion by active oxidation of chlorine in all cases. There is an extensive decrease in corrosion rates when mixing chlorides and sulfates together, where active oxidation was inhibited by the sulfates. It is interesting to note that sodium (Na) is not as active and detrimental as contents of potassium (K) to the corrosion of 12Cr1MoV steels, which results in the slightest corrosion effects by NaCl-Na2SO4 both on the surface and cross section of the specimens. Therefore, appropriate adjustment on the mixtures of chlorides and sulfates helps to improve the corrosion resistance of 12Cr1MoV.