| China is a country which rich in coal,but short of oil and gas,with a coal reserves of 600 billion tons,ranking third in the world.The coal is very suitable for development and utilization.Therefore,thermal power in China has a very obvious cost advantage and position is very important.Heat-resistant steel T91 is the material of heat resistant parts in coal-fired power plant and it is formed on the basis of 9Cr-lMo steel by adding V,Nb,N and other trace elements.The design service temperature is 600-650?.But,on the one hand,Because sublimation substances contained in coal-fired ash often produce hard ash on the heating surface of high-temperature coal-fired boilers in power plants,Thereby,oxidation and corrosion of heat-resistant steel T91 are produced;on the other hand,thermal power plants are also developing towards high parameters.The maximum actual wall temperature of coal-fired boilers has exceeded 600?.Heat-resistant steel T91 has been unable to reach its original design life,Therefore,it is urgent to improve the oxidation and corrosion resistance of heat-resistant steel T91.It aims at this limitation in this work.Three kinds of AlSi coatings with thickness of 10 micron 5%,10%and 20%Si on heat-resistant steel T91 were deposited by vacuum arc ion plating,and then three kinds of AlSi diffusion coatings were formed by thermal diffusion at 550 ? and vacuum argon charging for 40 hours.The oxidation behavior of three coatings at 600,700 and 800 ? in air and the corrosion behavior of 75%Na2SO4+25%NaCl salt film in air at 650 and 750 ?was studied by discontinuous weighing method.The results show that:1)Preparation of Coatings:AlSi diffusion coating was successfully prepared on the surface of heat-resistant steel T91 by vacuum ion plating and vacuum heat treatment at 550?.No transverse crack on coating surface,Cross section morphology shows that there is no longitudinal crack in the coating,and it bonds well with the base alloy.Coating thickness is about 10?m.XRD phase diagram shows that the coating is mainly composed of Als6Fei4 and Al9FeSi3 phase.2)Oxidation resistance of coatings:The oxidation of heat-resistant steel T91 and three kinds of AlSi diffusion coatings were obvious after 200 h at 600,700 and 800 ? in air.However,according to the kinetic curve,the weight gain of the three kinds of Al-Si diffusion coatings were much less than that of the matrix,Therefore,they were protective to the matrix.According to XRD,cross-section morphology,cross-section line scanning and energy spectrum,the protective oxide film is Al2O3.Therefore,three kinds of AlSi diffusion coatings can protect the matrix well,improve its oxidation resistance at high temperature,and improve its service temperature.3)Corrosion resistance of coatings:The corrosion of heat-resistant steel T91 and three kinds of AlSi coatings were obvious after 100 h at 650 and 750 ? in air under the salt film.XRD phase diagram,cross section morphology,cross section element line scanning and cross section component analysis showed that thick corrosion products were formed on the surface of the matrix.The main components was Fe2O3,and a little Cr2O3,FeS,and the oxide film is loose and easy to fall off.The thickness increases greatly with the increase of temperature.However the three kinds of AlSi diffusion coatings formed a protective layer of AL2O3 at both temperatures,which prevented the further corrosion of the matrix and improved the corrosion resistance of the matrix. |
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