Electrochemical impedance spectroscopy and indentation studies of pure and composite electroless Ni-P coatings

Electroless Ni-P coatings offer excellent corrosion and wear resistance and ability to withstand acidic and salt solutions. Medium and high phosphorus Ni-P coatings were produced using plating baths with 10 g/L or 25 g/L of sodium hypophosphite as reducing agent (RA) with composition of the resulting deposits to be 91.5 Ni: 8.5 P and 87.6 Ni: 12.4 P, respectively. From field-emission scanning electron microscope (FE-SEM) examination, the deposit morphology was found to change from nodular with surface porosity and cracks to dense, smooth upon increasing the RA content. Addition of nanostructures such as nanoparticles of alumina (Al2O3) or silicon carbide (SiC) or multi-walled carbon nanotubes (CNT) into Ni-P matrix, at low loading levels, was investigated for their effect on corrosion resistance and hardness of Ni-P-Al2O3, Ni-P-SiC and Ni-P-CNT composite coatings. Electrochemical impedance spectroscopy (EIS) studies in 4 wt.% NaCl solution revealed 91.5 Ni: 8.5 P coating to offer much superior corrosion resistance than 87.6 Ni: 12.4 P coating even after immersion for 42 days. Among all composite coatings, however, Ni-P-Al2O3 produced from 1.0 g/L Al2O3 in plating solution exhibits higher impedance values at low and intermediate frequencies. Nyquist plots for different frequencies were analyzed for comparison between different composite coatings. Microhardness tests indicate higher hardness value of 8.46 GPa for Ni-P-SiC coating as compared to 7.42 GPa for pure Ni-P coating.