| The problems of dew point corrosion are often encountered in the industrial production of economizer in industrial kilns. Due to the presence of sulfur, carbon, nitrogen and other elements in industrial fuel, flue gas usually contains the corresponding oxides. These oxides react with water vapor in the flue gas and generate acid vapors. When the acid steam meets the outer wall of economizer, whitch is cooled down to dew point,it will condense into acid droplets to cause severe corrosion of the economizer wall, which shortened service life. Therefore, the prevention of dew point corrosion is the key to improve the service life of economizer. As a result of industrial use of carbon steel as economizer, Q235 carbon steel was used as substratein this paper. A layer of Ni-Cu-P coating was plated on the substrate's surface by electroless plating to improve its corrosion resistance. Two step method was used to prepare more thick coating to improve the protective effect of coating on substrateand the corrosion mechanism of the coating was studied.In this research,the electroless plating formula and process conditions were determined by orthogonal test, the surface morphology of the coating was observed by SEM and the surface element content of coating was analyzed by EDS. The structure of the coating and compound form of the surface elements were analyzed by XRD and XPS.The effects of plating time on the properties of the coatings were studied by means of porosity test,hardness test and contact angle test.The corrosion behavior of the coatings was studied by corrosion immersion test and potentiodynamic polarization curves test.The main results are as follows:The chemical plating formula and process conditions used in this researchwere: NiSO4 25g/L?CuSO40.9g/L?NaH2PO2 35g/L?Na3C6H5O745g/L?NaCH3COO 15g/L and thiourea lmg/L, the temperature of plating soluting was 84 ?, the pH value of the plating solution was about 6, and the plating time was 1.5 h.The contents of Ni, Cu and P in the coating were 78.72%, 7.56% and 13.72% respectively. Cu and P formed solid solution with Ni, which made the coating amorphous structure. Among them, P and some Ni exchanged electrons to form covalent bonds, while Cu was in the form of elementary substances.Electroless plating of Ni-Cu-P on the surface of Q235 carbon steel(Ni-Cu-P/Q235)can significantly improve the corrosion resistance of Q235carbon steel in simulated flue gas condensate, 5% H2SO4, 10% H2SO4 and 20%H2SO4at 80 DEG C. In the four kinds of medium, the corrosion rates of Ni-Cu-P/Q235 were 1/9, 1/166, 1/165 and 1/466 times as much as Q235 carbon steel respectively; corrosion resistance was superior to ND steel, and the corrosion rates were 1/6, 1/136, 1/139 and 1/351 times as much as ND steel respectively. In three concentrations of H2SO4 solution (5%, 10% and 20%) at 80 DEG C, the corrosion rates were 1/8, 1/10, and 1/121 times as much as ND steel respectively, and corrosion resistance wasalso superior to ND steel.Four kinds of coatings were prepared by two-step method, in which the first stage plating time was 1.5 h, and the two stage plating time was 0.25 h,0.5 h, 1 h and 1.5 hrespectively.Bath composition and condition remained unchanged. The two-step method can effectively prolong the time when the coating can protect the substrate.Among the four kinds of coatings, when the two stage plating time was 1.5 h, the coating had the best properties. The coating thickness was43.8 ?m, the coating porosity was 0, the coating hardness was 656.1 Hv which was the minimum value, the corrosion rate was the lowest in 5% H2SO4 solution at 80 DEG C, andthe bubbling time (418 h)of the coating was 3.9 times as much as that of N-1.5h (106 h) prepared by one-step method.When the two stage plating time was 1.5 h, the coating that was immersed in the 5% H2SO4 solution at 80 DEG C was changed from beige to black, then from black to red, and finally from red to black. Among them,beige was the original color of the coating; the change from beige to black was due to the selective dissolution of Ni on the surface of the coating during the soaking process; the change from black to red was due to the continuous dissolution of Ni on the coating surface, resulting in the enrichment of Cu; the change from red to black was because of the corrosion of the second coating,and the Ni of the first layer coating begun to dissolve.Before the first layer corrosion, the corrosion rate of the coating changed slowly,and the coating still played a protective role on the substrate... |
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