| After the widespread adoption of wet flue gas desulfurization(WFGD)technology in the coal-fired plants,the SO2 in the flue gas is efficiently removed,while the low removal efficient of SO3 and other acid species along with temperature reduction and humidifying result in the easy condensation of flue gas on the low-temperature surface of FGD system and subsequent facilities to form concentrated acid droplet and give rise to serious acid dew point corrosion.As the rapid growth of renewable energy generation in recent year,the coal-fired plants frequently participate in deep peak shaving,consequently the lower temperature and flow velocity of flue gas in the FGD system and subsequent facilities during low-load running aggravate the condensation and acid dew point corrosion.Hence,it is of great significance to develop new materials and surface technology with excellent acid dew point corrosion resistance for corrosion protection and normal operation of FGD system and subsequent facilities.In the present study,the NiCr22Mo13W alloy powders with main composition similar to Hastelloy C22 alloy were adopted to fabricate laser cladding NiCr22Mo13W alloy coating on Q235 carbon steel through coaxial powder feeding technology.The phase constitution of laser cladding NiCr22Mo13W alloy coating mainly consisted of y-Ni solid solution containing Cr,Mo,Co,Fe,etc.,accompanied by small amount of carbides,and the microstructure mainly consisted of primary dendrites and secondary eutectics with Mo segregation and exhibited non-equilibrium solidification characteristic which depended on the constitutional supercooling and G/R ratio,in which G and R represented temperature gradient and solidification rate,respectively.The solidification from the bottom to the top of coating was planar,cellular,columnar,and equiaxed in sequence.The direction of solidification went along with the direction of the largest thermal flux density.Corrosion resistance,passive film property,and corrosion behavior of laser cladding NiCr22Mo13W alloy coating in simulated sulfuric acid dew point corrosion environment(50wt.%H2SO4 at 50℃-70℃)were investigated and compared with rolled plates of Hastelloy C22 alloy,TC4 titanium alloy,and sulfuric acid dew point corrosion-resistant ND steel.The effect of microstructure on corrosion characteristics of laser cladding coating was discussed.Laser cladding NiCr22Mo13W alloy coating performed similarly high corrosion resistance to Hastelloy C22 alloy,which was much higher than that of TC4 alloy and ND steel.The corrosion rate of the later two was approximately 28 times and 144 times of that of the laser cladding coating,respectively.The laser cladding NiCr22Mo13W alloy coating and Hastelloy C22 alloy exhibited similar passive behavior and formed passive film possessed similar bilayer structure consisting of Cr2O3-dominated compact inner layer and porous outer layer containing oxides of Mo and hydroxides of Ni and Cr,in which the inner layer dominated the protectiveness of passive film.The element segregation in the microstructure of laser cladding coating leaded to the lower content and heterogeneity of Cr2O3 in the passive film,resulting in the slightly weaker corrosion resistance and higher sensitivity to localized corrosion comparing to Hastelloy C22 alloy and consequent intergranular corrosion performing as the preferential dissolution of secondary eutectics.As the temperature increased,the compact inner layer in the passive film of laser cladding coating first thickened then thinned,resulting in the best and worse corrosion resistance at 60℃ and 70℃,respectively.While the compact inner layer in the passive film of Hastelloy C22 alloy monotonously thinned,and the transition of electrode reaction mechanism occurred at 70℃,jointly resulting in the degradation of corrosion resistance.For revealing the effect of other acid species in flue gas condensate,the corrosion resistance,passive film property,and corrosion behavior of laser cladding NiCr22Mo13W alloy coating in simulated mixed acid dew point corrosion environment(50wt.%H2SO4+2wt.%HCl+1.5wt.%HNO3+0.2wt.%HF mixed acid solution at 50℃-70℃)was investigated and compared with rolled plates of Hastelloy C22 alloy.The effect mechanism of NO3-,halide ions and temperature were discussed.Laser cladding NiCr22Mo13W alloy coating exhibited similar corrosion resistance and passive behavior as Hastelloy C22 alloy did,forming passive film with bilayer structure similar to that in sulfuric acid dew point corrosion environment.The oxidation and compacting effect of NO3-on both porous outer layer and compact inner layer promoted the protectiveness of passive film of the laser cladding coating,which suppressed the preferential dissolution of secondary eutectics and resulted in the higher corrosion resistance than that in sulfuric acid dew point corrosion environment.The complexing and penetrating effect of halide ions on the passive film aggravated the pitting corrosion,but the influence on the overall corrosion resistance was submerged.The effect of NO3-on the passive film and corrosion resistance of Hastelloy C22 alloy was neglectable,while the halide ions aggravated the grain boundary corrosion.The increase in temperature facilitated the complexing and penetrating effect of halide ions on the passive film,aggravating the pitting corrosion of laser cladding NiCr22Mo13W alloy coating and the grain boundary corrosion of Hastelloy C22 alloy.For revealing the effect of alloying elements,the alloy powders containing only Ni,Cr,and Mo element with different content of Cr and Mo(18wt.%-26wt.%and 10wt.%16wt.%,respectively)were designed in accordance with the relative content of these three elements(65:22:13)in Hastelloy C22 alloy,and laser cladding Ni-Cr-Mo ternary alloy coatings with different content of Cr and Mo were fabricated.The effect of Cr and Mo on the phase constitution,microstructure,and corrosion characteristics in sulfuric acid dew point corrosion environment were investigated.The phase of each laser cladding coating with different content of Cr and Mo mainly consisted of y-Ni solid solution containing Cr and Mo,while the microstructure mainly consisted of primary dendrites and secondary eutectics with Mo segregation.The increase of Cr and Mo both resulted in the more black micropores in the eutectics,and the increase of Mo resulted in the bigger volume of eutectics.The laser cladding coatings with different content of Cr and Mo exhibited similar preferential dissolution of primary dendrites and pitting corrosion initiated at micropores.The forming passive film possessed similar bilayer structure with more Cr2O3 and absence of CrO3 which contributed to the higher corrosion resistance than previous laser cladding NiCr22Mo13W alloy coating,and the laser cladding coating with 16wt.%Mo performed better corrosion resistance than Hastelloy C22 alloy did.The increase of Cr leaded to the higher compactness and protectiveness of inner layer,while the relative decrease of Ni leaded to the thinning of porous outer layer.The competition between the effects of two elements resulted in the degradation and then improvement of the corrosion resistance with the increase of Cr.The increase of Mo from 10wt.%to 13wt.%leaded to the thicker compact inner layer and less protective outer layer,while the increase from 13wt.%to 16wt.%leaded to the higher compactness and protectiveness of both inner layer and outer layer.In general,the increase of Mo resulted in the significant improvement of corrosion resistance of laser cladding Ni-Cr-Mo ternary alloy coating. |
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