Electroless Ni-P Plating of Carbon Steel via Hot Substrate Method and Comparison of Coating Properties with those for Conventional Method

Abstract

In this study, Ni-P electroless coating was deposited on carbon steel by a new method called SLHS (substrate local heating system) which makes a higher rate of deposition possible without the risk of decomposition of electroless baths. The effects of pH and temperature on plating rate, composition, surface morphology, hardness and corrosion of the coating in SLHS condition (Tsub = 190 °C, Tbath = 80 °C) were investigated. In addition, the impact of heat-treatment at 400 °C for 1 h on hardness, morphology and microstructure was also studied. Samples prepared by this method were characterized by Scanning Electron Microscopy/Energy-Dispersive x-ray analysis, Light Microscope and x-ray diffraction. They were then submitted to Vickers microhardness and tribological tests. The deposition rates of electroless nickel (EN) coating were first estimated by weight gain method and then by light and scanning electron microscopy. Electrochemical Impedance Spectroscopy (EIS), Tafel polarization and salt spray tests were then used to evaluate the corrosion properties of the coatings. The study shows that maximum deposition rates for conventional and SLHS samples were approximately 20 µm/h and 32 µm/h, respectively. This increase in the plating rate reduces the phosphorus level by nearly 1.5 wt.% for SLHS sample. The corrosion resistance of SLHS sample is improved in comparison to the conventional one.