The effect of sodium citrate as a complex agent on the corrosion properties of the electroless Ni-P coating

Abstract

Electroless Ni-P coatings have a wide range of industrial applications due to their surface properties especially for their corrosion resistance. The research presented investigates the influence of sodium citrate concentration, as a complexing agent, on the surface and corrosion properties of the Ni-P coating layer. The deposition rate (DR) at different sodium citrate concentrations was calculated. The results show that the maximum deposition rate of about 15 µm h-1 was obtained at the lowest sodium citrate concentration of 0.05 M. Surface profiles were used to show the effect of citrate concentration on the final coated surfaces and the average surface roughness (Ra) of the coated samples was calculated. The results show that the citrate concentration has an adverse effect on the deposition rate (DR) and the surface roughness (Ra). Electrochemical testing in aerated artificial seawater was used to monitor the corrosion properties of the coated and uncoated samples. The Ni-P coated samples, when compared to the uncoated steel sample, show a nobler Ecorr and lower corrosion current density (icorr) with an inhibition efficiency range from 86 to 94%. The corrosion properties of coated samples were greatly affected by the citrate concentration in the plating bath. It was found that the increase of citrate concentration results in an increase of coating P content and decrease the porosity involved in coating, which consequently maximizes the corrosion protectability of the coating.