Abstract: The corrosion of steel fasteners exposed to marine environment for a long time seriously affects the service safety of marine engineering equipment and facilities. Electroplated alloy coating is a common protection method for fasteners, among which nickel-cobalt alloy coating has better corrosion resistance. In the study, SiO₂ particles with different particle sizes are added into Ni-Co plating solution, and by the method of electrodeposition, Ni-Co-SiO₂ composite is coated on 45 steel. Influences of SiO₂ particle size on surface morphology and microstructure of the composite coating are analyzed, and corrosion resistance of the composite coating in NaCl solution with mass fraction of 3.5% evaluated. Microhardness and friction coefficient of the composite coating are tested. The results show that with the increase of SiO₂ particle size in the bath, the distribution uniformity of SiO₂ on the surface of the composite coating increases first and then decreases. When the SiO₂ particle size is 70 nm, a more complete SiO₂ film is formed on the surface of the coating. Potentiodynamic polarization and electrochemical impedance spectroscopy tests show that the composite coating doped with 70 nm SiO₂ has the best corrosion resistance. With the increase of SiO₂ particle size in the bath, the hardness of composite coating decreases gradually, but its effect on friction coefficient is slight.