T Rabizade - MSc Student
S. R. Allahkaram - Associate Professor and Member of Center of Excellence in High Performance Ultra Fine Materials
A. Zarebidak - PhD Student School of Metallurgy and Materials Engineering, University College of Engineering

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.

Electroless plating; Ni-P coating; Nano-crystalline coating; Vacuum heat treatment; Micro-hardness; X-ray diffraction; Corrosion resistance