Electrochemical behaviour of high phosphorus electroless Ni–P–Si3N4 composite coatings

Abstract

A high phosphorus electroless nickel bath was used to prepare plain Ni–P and composite coatings containing submicrometre size silicon nitride particles. Deposits were characterised for their composition, morphology and electrochemical behaviour. Codeposition of particles in a Ni–P matrix has not influenced the phosphorus content (10 wt-%). Surface morphology of plain Ni–P deposits was smooth; the composite deposits became slightly rough with small nodules due to particle incorporation. Cross-sectional examination of composite coating revealed that the particles were uniformly distributed throughout the thickness of the coating. Potentiodynamic polarisation and electrochemical impedance studies were carried out in 3·5 wt-% sodium chloride solution in non-deaerated condition. Potentiodynamic polarisation studies showed that the corrosion current density value obtained for composite coatings is lower than that for plain Ni–P coatings. Electrochemical impedance spectroscopy studies showed that the coating resistance of the composite coating is higher than that of plain Ni–P coating. This was further confirmed by SEM analysis of corroded samples.