Preparation And High Temperature Properties Of Composite Enamel Coatings On Cr25Ni20Si2 Substrate

It is indicated that the refractory steel will be early inactivation under conditions of hot corrosion in practice. Thereby, the protective coating is necessary for prolonging its service life. In this study, the enamel with Ni interlayer coating on refractory steel had been prepared .The high temperature oxidation behavior and mechanism of enamel coating on Cr25Ni20Si2 substrate with or without Ni interlayer were investigated in air at 900℃, and the hot corrosion behavior and mechanism of enamel coating on Cr25Ni20Si2 substrate with or without Ni interlayer were investigated in molten salt (75%Na2SO4+25%NaCl ) (mass fraction)at 800℃.The composite enamel coatings with higher quality and stronger adhesion were obtained successfully in this thesis. And the optimal process parameters of enamel coating are the thick of controlling nickel coating 5μm, the first firing process 1000℃×5min, and the second 950℃×20min.The results of high temperature oxidation resistance at 900℃showed that the oxidation resistance of enamel coatings and composite enamel coatings were obviously superior to Cr25Ni20Si2 alloy. The major products of alloy is the compact Cr2O3 film after being oxidized at 900℃for 100h, which lead to excellent high temperature oxidation resistance. Either the enamel coating with or without Ni-coating had not been exfoliated after oxidized for 100h at 900℃.As the structure of the coating was dense, it can hold the oxygen from diffusing in the substrate.The results of hot corrosion in molten salt (75% Na2SO4+25% NaCl ) at 800℃(mass fraction) indicated that the ability of hot corrosion resistance could be improved 30% with the enamel coating, while the ability of hot corrosion resistance of the composite enamel coating can be improved 100% above,compared with Cr25Ni20Si2 alloy.The disastrous corrosion was conformity with sulfuration-alkalescent melt manner, which being corroded for 44h in melting salt. And the corrosion products were FeS,Cr2O3,Fe2O3,CrS,MnCr2O4. As the nickel-free enamel coating fall off directly from the alloy, it lost the protective ability. As the nickel-free enamel coating would fall off directly after being corroded for 54h, the matrix lost protection which induced catastrophic corrosion and had no incubation period. While the composite enamel become invalidate slowly in manner of physical reducing of the enamel thickness. The enamel coating became thin after being corroded for 80h in the molten salt. XRD analysis showed that the phase of the composite enamel after corrosion were still SiO2,NaAl(SiO3)2.And the thin coating also had effective corrosion resistance.The Ni interlayer can enhance the adhesion of enamel on the substrate, prevent the enamel coating from exfoliate in the melten salt, which ensure constantly offer of the ability of hot corrosion resistance for the substrate. So, the hot corrosion resistance of the enamel-Ni composite coating was obviously superior to the enamel coating.The enamel composite coating with Ni interlayer prepared in this study has excellent performance and higher adhesion, which effectively improved the ability of high temperature oxidation resistance and hot corrosion resistance for substrate alloy.