Electroless nickel–phosphorus coatings to protect steel reinforcement bars from chloride induced corrosion

Electroless nickel–phosphorus (ENP) alloy coatings deposited on steel surfaces provide a high degree of protection against corrosion in comparison to the bare steel exposed in chloride-contaminated simulated concrete pore solution (SPS). Electrochemical AC (alternating current) impedance spectroscopy (EIS) and DC (direct current) polarization techniques, scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies have been used to study the kinetics and mechanism of corrosion protection and to characterize corrosion products. The pH of the deposition solution of coating plays very significant role on corrosion resistance of the deposited coatings. It is observed that ENP deposited at pH of 4.00 attains a very slow rate of corrosion from very beginning of their exposure in the test electrolyte, indicating that surface of the coating attains passivity in the solution. The coatings deposited at higher pH, however, show an increasing rate of corrosion with time. Medium phosphorus content (about 8%) coating is more impervious and protective in comparison to high phosphorus coating (16%). The heat treatment of deposited coatings has deleterious effect on their corrosion resistance. X-ray diffraction studies show that the medium phosphorus electroless nickel (MEPEN) is microcrystalline in nature and forms Ni2O3 and Ni5P2 passive films on its surface.