| According to actual problems that there were serious erosion, corrosion and resulting in major economic losses caused on the boiler tubes (water wall,superheater, reheater, economizer tube) of coal-fired thermal power plants at home and abroad, low-cost FeMnCrAl/carbide series coatings have been studied to resolve the surface temperature corrosion and erosion problems using high velocity arc spraying process in this paper. In this study, low-cost FeMnCrAl/carbide series coatings were successfully prepared using Mn, Cr, Al, Cr3C2, Ni-coated WC, Ni-coated Cr3C2, etc cored wires together with high velocity arc spraying for surface protection of the boiler tubes. New arc-spraying materials with good cost-effective development of coal-fired boiler tubes were studied. This study content is an energy-saving protection issues, having great practical value and academic significance.FeMnCr/Cr3C2 coating, FeMnCrAl/Cr3C2 coating and FeMnCrAl/Cr3C2-Ni9Al coating were deposited on 20 steel substrates by the high velocity arc spraying. The coatings characteristically consisted of successive layers contained metal crystalline phases, oxide phases, un-melted particles and pores. Compared with that of FeMnCr/Cr3C2 coating, FeMnCrAl/Cr3C2 coating and FeMnCrAl/Cr3C2-Ni9Al coating significantly had less un-melted particles, oxide content and porosity, possessed good interfacial bonding and microstructure between coatings and substrate, no coarse pores and cracks.Hardness, bond strength, cohesive strength and morphology of tensile fracture of FeMnCr/Cr3C2 coating, FeMnCrAl/Cr3C2 coating and FeMnCrAl/Cr3C2-Ni9Al coating have been studied. The results show that compared with that of 20 steel, FeMnCr/Cr3C2 coating and FeMnCrAl/Cr3C2 coating possess high average hardness,2-3 times than that of 20 steel. FeMnCrAl/Cr3C2 coating and FeMnCrAl/Cr3C2-Ni9Al coating exhibit higher bonding strength between coatings and substrate than that of FeMnCr/Cr3C2 coating. The average cohesive strength of FeMnCrAl/Cr3C2 coating is 174.5MPa, higher than that of FeMnCr/Cr3C2 coating.Characteristics of tensile fracture of coating are characteristics of quasi-cleavage fracture of metal crystalline phases and brittle fracture of oxide phases, characteristics of separation and disengagement of "laminations" and characteristics of un-melted particles integral putout.Thermal shock resistances of FeMnCr/Cr3C2 coating, FeMnCrAl/Cr3C2 coating and FeMnCrAl/Cr3C2-Ni9A1 coating have been analyzed. Compared with that of FeMnCr/Cr3C2 coating and FeMnCrAl/Cr3C2 coating, FeMnCrAl/Cr3C2-Ni9Al coating possesses the best thermal shock resistance, being no exfoliation phenomenon after the number of thermal shock cycles 32. Failure mechanism of thermal shock of arc-sprayed coatings includes cracks initiating and propagating along the oxide phases and pores. Cracks, pores and interfaces between "laminations" are "fast track" of oxygen diffusion within coatings, leading to high temperature oxidation occurred in the coating, and secondary cracks formed and propagated in the new sites of coatings. Pores in the coatings may hinder crack propagation. However, if the shape of pores was irregular, cracks would be activated and propagated along the large curvature of pore tip. On the other hand, oxygen penetrating to the combination interface of coatings and substrate caused oxidation during thermal shock test, leading to coating failure.High temperature oxidation and hot corrosion contrast tests of arc-sprayed coatings have been studied. The results show that FeMnCr/Cr3C2 coating and FeMnCrAl/Cr3C2 coating have better oxidation resistance and hot corrosion resistance than that of 20 steel. FeMnCrAl/Cr3C2 coating possess the best oxidation resistance and hot corrosion resistance.Erosion tests of FeMnCr/Cr3C2 coating, FeMnCrAl/Cr3C2 coating and 20 steel were performed in an erosion tester at different impact angles. Compared with that of 20 steel, FeMnCr/Cr3C2 coating and FeMnCrAl/Cr3C2 coating possess lower average erosive rate than that of 20 steel, and FeMnCrAl/Cr3C2 coating exhibiting the best erosion resistance under all impact angles. Erosion mechanisms of the coatings as follows:Failure mode at low impact angles is ploughing and cutting, accompanying with plastic deformation and brittle desquamation under secondary impacting. Failure mode at high impact angles is indenting, brittle cracks and desquamation, accompanying with fatigue cracking off and desquamation after excessive plastic deformation. Failure mode at intermediate impact angles is multi-transition status.In order to improve wettability and binding between carbide and metal matrix crystalline phases, Ni-coated WC and Ni-coated Cr3C2 powder were prepared by ultrasonic-assisted electroless plating process without conventional sensitization and activation treatments at room temperature. WC and Cr3C2 powders were first pretreated by immersing into an aqueous solution of hydrofluoric acid (HF) and ammonium fluoride (NH4F). The Ni layers on the powders had cell structure with dense, uniform distribution. The nucleation. growth and aggregation mechanisms of Ni layers as follows:1) the reactants in the plating solution were adsorbed on catalytic activity surfaces of powders and happened oxidation-reduction reaction.2) The growth and aggregation mechanisms of Ni particles after nucleation are "stripy" Ni-cells grew up, bend, bifurcated, and aggregated through "linear" approach, then wounding into a "cellular" structure, like "wrapping wool clusters" in the life. Finally "cellular" structure Ni particles grew up and merged into a layer. Compared with that of FeMnCrAl/WC coating and FeMnCrAl/Cr3C2 coating, Ni coated WC and Cr3C2 powders improve the bond strength and cohesive strength of coatings. Hardness of FeMnCrAl/Ni-coated WC coating and FeMnCrAl/Ni-coated Cr3C2 coating are similar to those of FeMnCrAl/WC coating and FeMnCrAl/Cr3C2 coating. Ni coated WC powder improves the erosion wear resistance of FeMnCrAl/Ni-coated WC coating. Erosion mechanism of FeMnCrAl/Ni-coated WC coating is similar to that of FeMnCrAl/WC coating.FeMnCrAl/carbide series coatings had been used to heat surfaces of water wall and the screen tubes as protective coatings.After more than six months to run, the tests results show that FeMnCrAl/carbide series coatings play very good protective effects. The coatings may be expected to use more than 2 years.
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