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5496 |
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Friday, June 09, 2017, Hall 12 11:40 AM Power Generation - Steam |
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Effect of secondary carbide precipitation on chlorine high temperature corrosion of HVOF and HVAF sprayed Cr3C2-NiCrMoNb coatings |
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Davide Fantozzi* / Tampere University of Technology, Finland Ville Matikainen / Tampere Univeristy of Technology, Finland Mikko Uusitalo/ Valmet Technologies Oy, Finland Heli Koivuluoto/ Tampere Univeristy of Technology, Finland Petri Vuoristo/ Tampere Univeristy of Technology, Finland |
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Highly corrosion and wear resistant thermally sprayed chromium carbide (Cr3C2) based metal matrix composite (MMC) coatings are nowadays needed to allow traditional fluidised bed boilers (FBB) to be operated with ecological waste and biomass fuels. The large concentration of corrosive chlorides in the exhaust gases significantly limits the durability of such materials. Especially the carbide hard phase is known to be sensitive to the chlorine induced high temperature corrosion that usually takes place in modern boiler environments. The heat input of thermal spraying processes causes carbide dissolution in the matrix. This alters the coating structure and forms carbon saturated amorphous and nanocrystalline areas that will change the microstructure during service. This study analyses the effect of carbide dissolution in the metal matrix of MMC coatings and its effect on high temperature corrosion resistance. Four Cr3C2-NiCrMoNb coatings were thermally sprayed with high-velocity air-fuel (HVAF) and high-velocity oxygen-fuel (HVOF) spray processes in order to obtain microstructures with increasing amount of carbide dissolution in the metal matrix. HVAF spraying was used to obtain low melting degree of the particles while HVOF spraying was used to achieve the high melting degree condition. The specimens were heat treated in inert argon atmosphere at 700ÚC to induce secondary carbide precipitation. After heat treatment, self-standing coatings were covered with KCl and their corrosion resistance was screened with thermogravimetric analysis (TGA) up to 550ÚC. The corrosion behaviour and microstructural features were analysed and discussed. |