Deposition and Electrochemical Corrosion Behavior of Electroless Ni-P/Ni-W-P Duplex Plating on Mild Steel Substrate

Abstract

Abstract:The emergence of electroless Ni-P based ternary coatings and consequence duplex deposition possibilities, now offer more excellent corrosion and functional properties in acidic and alkaline environment. In this study, medium and high phosphorus Ni-P and duplex Ni-W-P coatings were produced and sequentially deposited on mild steel using plating baths with 10 g/L and 25 g/L of sodium hypophosphite as reducing agent (RA) with composition of the resulting deposits to be 91.5 Ni: 8.5 P and 87.6 Ni: 12.4 P, respectively. From the field-emission scanning electron microscope (FE-SEM) examination, the deposit morphology was found to change from nodular with some surface porosity to become more dense and smoother after heating treatment between 200-600 °C. The cross-section reveals firm adhesion of the as-plated and the merging of the top duplex layers of the heat treated sample as a result of tungsten diffusion. The potentiodynamic polarization test shows that the coating demonstrated significant improvement of corrosion resistance in 3.5% NaCl solution with optimum performance at 600 °C. The noted improved corrosion resistance of the coating came as a result of annealing which strengthened the layer and broaden its application.