Corrosion behavior of Ni-based coating containing spherical tungsten carbides in hydrochloric acid solution

A Ni-based alloy coating with 30wt.% spherical tungsten carbide particles was prepared through plasma transferred arc welding on 42CrMo steel. The composition and microstructure of the coating were examined through X-ray diffraction and scanning electron microscopy with energy-dispersive spectrometry. The corrosion behaviors of the coating compared to the Ni coating without tungsten carbide particles and to the bare substrate in a 0.5mol/L HCl solution were presented through polarization curves, electrochemical impedance spectroscopy (EIS) measurements and long-term immersion tests. The results demonstrated that the composite coating microstructure comprised Ni matrix, Ni-rich phase, tungsten carbide particles, W-rich phase and Cr-rich phase. The polarization curves and EIS measurements presented that a passivation film, which mainly included Ni, Cr, Fe and W oxides, was formed in the composite coating that protected the substrate from corrosion by HCl solution. In the immersion tests, a micro-galvanic reaction at the new-formed phases and Ni matrix interface caused severe pit corrosion and Ni matrix consumption. The debonding of Ni-rich and W-rich phases could be observed with the immersion time extension. The tungsten carbide particles and Cr-rich phase were still attached on the surface for up to 30days.