An Investigation on Effects of Electroless Nickel- Phosphorous Coating and Heat Treatment on Corrosion Resistance of Carbon Steel

The autocatalytic (electroless) deposition of Ni–P coating is a well-known commercial process that has found numerous applications because of its excellent anticorrosive, wear, magnetic, solderable properties, etc. It is a barrier coating,protecting the substrate by sealing it off from the corrosive environments, rather than by sacrificial action. In the present investigation Ni–P nano-crystalline coatings were prepared by electroless deposition from electroless nickel bath with sodium hypophosphite as reducing agent. Post-deposition vacuum heat treatment effects on coating properties were studied. X-ray diffractometry (XRD), scanning electron microscopy (SEM),energy dispersive analytical X-ray (EDAX), potentiodynamic polarization were utilized to analyse the temperature dependence of the microstructure, composition, mechanical and corrosion properties of the coatings. X-ray diffraction (XRD) studies indicated that the as-plated deposits were nano-crystalline, i.e. 39 nm. The heat treatment of the coatings increased their grain size and a mixture of polycrystalline Ni and Ni3P alloy occurred at around 400°C. The highest microhardness was achieved for the samples annealed at 600°C for 15 min because of formation of an inter-diffusional layer. Corrosion resistance of the films was evaluated in 3.5% sodium chloride solution via potentiodynamic polarization and electrochemical impedance (EIS) methods. Lower corrosion current density values were obtained for the coatings heat treated at 400°C for 1hour.