The effect of grain refining and phosphides formation on the performance of advanced nanocomposite and ternary alloy coatings on steel

This paper is a continuation of our investigation into the designing of high performance anti-corrosion nanocomposite and ternary alloy Ni–P coatings via electroless deposition technique. It discusses the effect of nickel grain refining and phosphides formation on the coating performance over steel. Hard chromate coating has been reported as the most efficient widespread conversion coatings for many metallic substrates. However, the waste containing toxic chromate has many limitations due to the environmental consideration and health hazards. In this paper, newly developed (Ni–P–W) ternary alloy coatings and nano-scattered alumina (Ni–P–Al2O3) composite coatings were prepared by electroless deposition technique on low carbon steel. The optimum conditions under which such coatings can provide the highest micro-hardness and anti-corrosion characteristics to the steel substrate were determined. The coating performance was investigated using electrochemical impedance spectroscopy and polarization measurements. Ni–P–W alloy coatings showed the highest hardness and anti-corrosion properties compared with Ni–P–Al2O3 and Ni–P. Surface examination of Ni–P–W and Ni–P–Al2O3 using XRD, SEM-EDS and macroscopic images revealed grain refining of Ni grains. A protective layer of nickel phosphide and tungsten phosphide was formed over steel substrate due to Ni–P–W. While a protective layer of nickel phosphide enriched with aluminum oxide was formed due to Ni–P–Al2O3. The aim of this article is to deepen the current understanding of corrosion and protection of steel substrate in chloride containing environments and to provide a base for future research work in this field. ► Advanced Ni–P–Al2O3 nanocomposite and Ni–P–W ternary alloy coatings have been developed. ► The superior corrosion resistance of Ni–P–W is due to Ni– and W–phosphides formation. ► The corrosion resistance of Ni–P–Al2O3 is due to Ni–phosphide Al-oxide film formation. ► The corrosion resistance is due to the grain refining of Ni and decreasing the voids.