High Temperature Sulfur Corrosion Behavior Of Ni-Cr-Mo Laser Cladding Coating

With the wide application of low NOx combustion technology in(ultra)supercritical coal-fired power plants in China,the water wall of pulverized coal boiler suffers severe high temperature corrosion due to local reductive atmosphere combustion environment,which seriously affects the operation safety and reliability of the unit.Therefore,aiming at the low NOx combustion corrosion environment of pulverized coal boiler,it is of great significance to develop a new anti-corrosion technology of water wall tube for the safe,efficient and environmental protection operation of(ultra)supercritical coal-fired power plants.In this paper,the H2S corrosion environment in low NOx combustion atmosphere of pulverized coal boiler is simulated by high temperature tube furnace.The corrosion atmosphere is N2-0.2vol.%H2S-0.1vol.%O2.The high temperature corrosion behaviors of TP91 steel,Hastelloy C22 alloy and Hastelloy C22 laser cladding coating in high temperature corrosion atmosphere were studied.Then,the influence of Cr,Si and Fe content on the high temperature sulfur corrosion performance of Ni-Cr-Mo laser cladding coating was studied.The phase composition and element distribution of corrosion products were analyzed,and the corrosion mechanism of Ni-xCr-Mo,Ni-Cr-Mo-xSi and(Ni-CrMo)-xFe laser cladding coating was obtained.This study provided theoretical basis for the application of Ni-Cr-Mo laser cladding coating in low NOx combustion environment and has important reference value for developing a new type of anti-corrosion material for coalfired boiler.The high temperature corrosion behavior of TP91 steel,Hastelloy C22 alloy and Hastelloy C22 laser cladding coating in simulated low NOx combustion environment was studied.The results show that the mass gain curves of TP91 steel,Hastelloy C22 alloy and Hastelloy C22 laser cladding coating are parabolic.The corrosion layer of the three materials is double layer structure,the outermost layer is loose sulfide layer,and the inner layer is dense corrosion product layer with protective effect.At 500℃,the corrosion rates of the three materials are slow.When the temperature is≥550℃,the corrosion rate increases sharply.However,the corrosion rate of Hastelloy C22 alloy and Hastelloy C22 laser cladding coating is still low,and the total mass gain is≤3.60mg/cm2.The corrosion resistance of Hastelloy C22 laser cladding coating is slightly better than that of Hastelloy C22 alloy and significantly better than TP91 steel in the simulated low NOx combustion environment.In order to study the influence of Cr element on the high temperature corrosion performance of Ni-Cr-Mo laser cladding coating,Ni-xCr-Mo laser cladding coating with different Cr content(x=10wt.%,15wt.%,20wt.%,25wt.%,30wt.%)was prepared.The corrosion behavior of Ni-xCr-Mo laser cladding coating in simulated low NOx combustion environment was studied.The results show that the high temperature corrosion resistance of Ni-xCr-Mo laser cladding coating increases with the increase of Cr content.The diffusion rate of Ni determines the corrosion resistance of the cladding layer.When Cr content is 10wt.%,the corrosion kinetics curve of the cladding layer shows a linear law,and the inner corrosion products can not effectively prevent the diffusion of Ni.When Cr content is≥15 wt.%,the inner corrosion products become denser,which can effectively slow down the outward diffusion of Ni,and the corrosion kinetics curve shows a parabolic law.In order to study the influence of Si element on the high temperature corrosion performance of Ni-Cr-Mo laser cladding coating,Ni-Cr-Mo-xSi laser cladding coating with different Si content(x=0wt.%,1wt.%,3wt.%,5wt.%)was prepared.The corrosion behavior of Ni-Cr-Mo-xSi laser cladding coating in simulated low NOx combustion environment was studied.The results show that a proper amount of Si(1 wt.%)can reduce the number of holes in the laser cladding coating,but excessive Si reduces the mechanical properties of the cladding layer,resulting in a large number of fine cracks at the interface.The laser cladding coating with 1 wt.%Si content has the best corrosion resistance,followed by the laser cladding coating without Si.When Si content is 3wt.%and 5wt.%,the corrosion products in the inner layer are discontinuous distribution,and it is difficult to form a dense corrosion product layer,resulting in the corrosion resistance of the cladding layer decreases.In order to study the influence of Fe diffusion on the high temperature corrosion behavior of Ni-Cr-Mo alloy cladding layer,(Ni-Cr-Mo)-xFe laser cladding coating with different Fe content(x=3.6wt.%,5.8wt.%,7.5wt.%,9.6wt.%)was prepared.The corrosion behavior of(Ni-Cr-Mo)-xFe laser cladding coating in simulated low NOx combustion environment was studied.The results show that the corrosion resistance of(Ni-Cr-Mo)xFe laser cladding coating decreases with the increase of Fe content,and the corrosion kinetics curve is parabolic.When Fe content is 9.6 wt.%,the corrosion kinetics curve is approximately linear.The inner layer of corrosion products of(Ni-Cr-Mo)-xFe laser cladding coating is composed of corrosion products of Cr,Mo and Fe,and Fe mainly exists in the form of Fe2O3,FeO and FeS.When Fe content reaches 9.6wt.%,corrosion products of Fe in the inner layer are too much,resulting in the cracking phenomenon of the inner layer,and the corrosion resistance of the cladding layer decreases seriously.In order to verify the application of Ni-Cr-Mo laser cladding coating in the actual coal-fired power plants with low NOx combustion environment and high sulfur coal condition,this paper used laser cladding technology to prepare Ni-Cr-Mo-Si laser cladding coating(the content of Si is 1wt.%)with thickness of 0.65-0.85mm on the surface of water wall tube,and installed in a coal-fired power plant.Application study shows that Ni-CrMo-Si laser cladding coating has no obvious corrosion on the surface,and it has a good protection effect on the water wall tube.