SYNTHESIS AND PASSIVE PROPERTY OF NANOCOMPOSITE Ni-WC COATING

The metallic matrix composite with ceramic nano-particles has a wide prospect in many applications due to its superior properties. The nanocomposite Ni-WC coating has been synthesized by using DC co-electrodeposition of Ni with WC nano-particles. Its hardness was measured by using ultra-micro hardness tester. Its corrosion and passive properties were investigated in 0.05 mol/L H3BO3+0.075 mol/L Na2B4O7 buffer solution with pH=9.0 by using potentiodynamic polarization measurement. As compared to pure nanocrystalline (nc) Ni, nanocomposite refined remarkably, with average grain size about 21 nm and the hardness increase of 80%, reaching about 651 HV. The corrosion current density i(Corr) is 1.29 x 10(-7) A/cm(2), approximately one magnitude order lower than that of nc Ni. With the similar passive film breakdown potential, nanocomposite exhibits a lower passivation potential E-p of 10 mV and a much lower passive current density i(p) of 1.79x10(-6) A/cm(2), about 1/7 that for nc Ni. According to the Mott-Schottky analysis together with point defect model, the passive film on the nanocomposite exhibits p-type semi-conducting behavior, similar to that on the nc Ni. The grain refinement of Ni is beneficial to the reduction of both the donor density and diffusion coefficient.