Corrosion behavior of electroless Ni–P alloy coatings containing tungsten or nano-scattered alumina composite in 3.5% NaCl solution

The corrosion protection performance of electroless deposited nickel–phosphorus (Ni–P) alloy coatings containing tungsten (Ni–P–W) or nano-scattered alumina (Ni–P–Al2O3) composite coatings on low carbon steel was studied. The effect of heat treatment on the coating performance was also studied. The optimum conditions under which such coatings can provide good corrosion protection to the substrate were determined after two weeks of immersion in 3.5% NaCl solution. Electrochemical impedance spectroscopy (EIS) and polarization measurements have been used to evaluate the coating performance before and after heat treatment. The Ni–P–W coatings showed the highest surface resistance compared with Ni–P–Al2O3 and Ni–P. The surface resistance of Ni–P–W coatings was 12.0 × 104 Ω cm2 which is about the double of the resistance showed by Ni–P–Al2O3 (7.00 × 104 Ω cm2) and twenty times greater than the surface resistance of Ni–P (0.78 × 104 Ω cm2). XRD analysis of non-heat-treated samples revealed formation of a protective tungsten phosphide phase. Heat treatment has an adverse effect on the corrosion protection performance of tungsten and alumina composite coatings. The surface resistance decreased sharply after heat treatment.