SYNTHESIS AND PASSIVE PROPERTY OF NANOCOM-POSITE 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 icorr is 1.29×10 −7 A/cm, approximately one magnitude order lower than that of nc Ni. With the similar passive film breakdown potential, nanocomposite exhibits a lower passivation potential Ep of 10 mV and a much lower passive current density ip of 1.79×10 −6 A/cm, 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.