| As the advantages of its excellent hardness, high corrosion resistance, high abrasion resistance, good solderability and good magnetic-shielding ability, electroless Ni-P coating has been widely applied in practice. Recently, the most of electroless Ni-P alloy coating process was obtained at high temperature (85-95℃) and unsatisfied with high energy consumption, bad working environment and poor bath stability. To solve the problems of traditional high temperature electroless nickel plating process and further improve properties of Ni-P alloy coating, the phase composition, surface morphology, coating properties and deposition process of Ni-P-nano SiC electroless composite coating on brass surface in acidic nickel plating system at medium temperature (70 ℃) was studied with XRD, SEM, micro-hardness tester, electrochemical workstation and friction test machine, by using nickel sulfate as main salt, sodium hypophosphite as reducing agent, lactic acid and glacial acetic acid as composite complexing agent, ammonium sulphate as accelerating agent, nano-SiC particles as composite particles, dodecyl benzene sulfonate as surfactants and mechanical stirring.The results show that, molar ratio of Ni2+/H2PO2-, composite complexing agent concentration, accelerating agent concentration and dispersity of nano-SiC particles have significant effects on surface morphology, deposition rate and properties of electroless Ni-P-nano SiC composite coating, in which, the influence of composite complexing agent is the most. A good performance and adhered well with brass substrate Ni-P-SiC composite coating can be derived from the bath contained nickel when sulfate concentration is 25g/L, molar ratio of Ni2+/H2PO2- is 0.4, lactic acid concentration is 7.5 mL/L, glacial acetic acid concentration is 13.0 mL/L, ammonium sulphate concentration is 10g/L, SiC concentration is 5g/L, SDBS concentration is 40mg/L, the pH value of the bath is 5.3 under mechanical stirring of 250rpm at medium temperature. Ammonium sulfate (accelerating agent) weakens the P-H bonds in sodium hypophosphite molecular and enhances the activity of hypophosphite, which increases deposition rate of Ni-P coating. Compared with Ni-P alloy coating, composite coating not only improves the hardness, but also increases the amount of catalytic active site and inhibits grain growth, which finally forms a homogeneous and dense coating due to the incorporation of nano-SiC particles within Ni-P coating. Furthermore, diffuse distribution of nano-SiC particles inhibite the grain growth and the aggregation and coarsening of Ni3P in coating during heat treatment, which has a significant improvement in their abrasion resistance and corrosion resistance of composite coating. The scouring and blow grind effect of SiC particles on the coating surface are increased when excessive SiC particles are added to the bath, which not only makes the codeposition of Ni-P and SiC particles more difficult, but also reduces deposition rate and particle content of coating, and leads to an decrease in thickness, an increase in porosity and deteriorative performance. Therefore the addition of SiC particles should be maintained in a reasonable range. The suitable addition of SiC is 5g/L under the presented experiment condition. The results of immersion tests and electrochemical measurements show that, Ni-P-SiC composite coating on brass exhibites high corrosion resistance in 3.5%NaCl and 10%NaOH solution but low in 10%H2SO4 solution.In the initial stage of Ni-P-SiC electroless composite coating, active sites exhibit an non-uniform island-like structure distributed on the brass substrate, and grow as two dimensional clusters nucleation mode to form lamellar structures by layer upon layer on brass substrate. Followed, the subsequent coatings grow as columnar growth or cellular growth mode. Meanwhile, the SiC particles attracted on various ions are co-deposited in coating, and the uniform, continuous and dense Ni-P-SiC composite coating is formed finally. |
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