Effect of heat treatment onthe structure and corrosion of Ni-P-SiC coatings

Ni-P-SiC coatings were successfully deposited onto 45 steel substrates by using ultrasonic-electroless process.The effect of heat treatment on the structures and corrosion characteristics of the Ni-P-SiC coatings were investigated by scanning electron microscope(SEM),X-ray diffraction(XRD),Vickers hardness,and electrochemical impedance spectroscopy(EIS) analyses.The results indicate that the optimum grain diameters of Ni and SiC in the as-plated Ni-PSiC coatings are approximately 91.7 and 46.8 nm,respectively.When the heat-treatment temperature is increased to400 ℃,the coating crystallizes into Ni3 P and NixPy.After heat treatment at 600 ℃,the coating changes into NiO,Ni3P and NixPy.And the microhardnesses of Ni-P-SiC3,Ni-P-SiC6,and Ni-P-SiC9 coatings are 1 170.4,1 265.1 and 1313.6 HV,respectively.The lowest corrosion current density value is obtained for the coatings heat treated at 400 ℃ for1 h.