| Electroless nickel-phosphorus(Ni-P) alloys reinforforced with nanoparticles are prepared by adding nanoparticles into plating bath. The incorporation of nanoparticles into Ni-P matrix can provide excellent functional properties. In this paper, Ni-P-SiO2 composite plating was developed by co-depositing nano-sized power SiO2 during electroless plating, and their growth mechanism, micro-structure, crystallization dynamics, enhanced corrosion resistance and condensation heat transfer characteristics were investigated systematically. The following are the main conclusions and innovation achievements:(1). The values of△GdepΘ,△SdepΘand△HdepΘwere calculated by application of Bockris equation into the deposition reaction of Ni-P. The results showed that the process was spontaneous and endothermic, which was consistent with the reaction condition of electroless plating. After the effects of different plating parameters on the electroless Ni-P deposition were given through experiments, the plot of coating temperatures versus deposition rates was made according to Arrhennius equation. The surface activation energy calculated using the slope of the plot was 35.36 KJ/mol. In addition, the kinetic equation was presented as: v=9.91·107·[Ni2+]0.81·[H2PO2-]0.20·[L]10.70·[H+]-0.10·exp(-35360/RT).(2). Nanoparticles codeposition played a major role in the growth of Ni-P matrix. Composite plating's growth model was epitaxial island with nanoparticles adsorbing on the substrate acted as nucleating centers, which didn't affect crystal's optimal nucleation and nodules's growth during electroless plating Ni-P alloys. During electroless composite plating, the deposition of nickel phosphor alloy would wrap the nano-particle. Numerous nucleating centers were helpful to decrease the dimension of nodules. This resulted in refinement in microstructure and disappearance of nodules in the deposit.(3). Crystallization behavior of nano-sized SiO2 particles reinforced composite coatings were investigated according to the dynamical model of amorphous alloys. The activation energy of composite coating was calculated as 222.3 kJ/mol, while it was 254.28 kJ/mol for the Ni-P coating. It indicated that the SiO2 particles had an influence on the crystallization behaviors of composite coatings. The crystallization temperature was lowered due to the existence of SiO2 particles. After heat treatment, Ni-P and Ni-P-SiO2 coatings transformed to be the mixture of crystal nickel and Ni3P. Because of the existence of second phase particles, microhardness of the electroless Ni-P alloy enhanced with SiO2 particles greatly increased. After annealing at 400℃, microhardness value of Ni-P-SiO2 composite coatings approached as high as HV1001118, while it approached HV1001027 for electroless plating.(4). The corrosion behavior of the composite coatings in 5%H2SO4 and 3.5%NaCl solutions had been investigated. The results showed that the incorporation of SiO2 nanoparticles into Ni-P matrixes increased the corrosion potential, surface resistance, greatly improved the corrosion resistance properties of the coatings. Dewpoint corrosion resistance property of the Ni-P-SiO2 composite plating was evaluated by the experiments. The results showed that the corrosion resistance of carbon steel is increased by 25 times after coated with composite platings. And composite plating was applied on the furnace tubes for industrial practice and the tubes succeeds in sustain dewpoint corrosion during two years'operation.(5). In this paper, surface free energy of carbon steel, electroless Ni-P and Ni-P-SiO2 were investigated by measuring contact angles of water on the three surfaces. The contact angle of water on the composite plating is 110°, greatly higher than that of carbon steel (40°) and electroless plating (74°). The incorporation of nanoparticles into Ni-P matrixes reduces the total surface energy of the coating. Condensation heat transfer characteristics of steam on vertical plates with Ni-P and Ni-P-SiO2 platings were investigated experimentally and theoretically. Stable dropwise condensation was achived on composite platings for its low surface energy. Compared with film condensation, the heat transfer coefficient for the composite platings was improved 3-5 times at the same subcooling temperature.
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