Corrosion resistance enhancement of Ni–P electroless coatings by incorporation of nano-SiO2 particles

Composite coatings were prepared using hypophosphite reduced electroless nickel bath containing 7 g/L SiO2 nano-particles at pH 4.6 ± 0.2 and temperature 90 ± 2 °C. Deposition rate for SiO2 nano-composite coatings was 10–12 μm/h. The amount of SiO2 nano-particles co-deposited in the Ni–P matrix was around 2 wt.%. The analyzes of coating compositions, carried out by Energy Dispersive Analysis of X-ray (EDAX), showed that plain Ni–P and Ni–P/nano-SiO2 deposits contained around 8 wt.% phosphorus. The X-ray diffraction (XRD) pattern of Ni–P/nano-SiO2 coating was very similar to that of plain electroless Ni–P coating, whose structure was also amorphous. Scanning electron microscopy (SEM) morphology of the surface deposits revealed that some agglomeration occurred because of the absence of surfactant. Electrochemical impedance spectroscopy and polarization tests showed that addition of nano-SiO2 particles demonstrated significant improvement of corrosion resistance of Ni–P coatings in salty atmosphere.