Competitive deposition of electroless Ni-W-P coatings on mild steel via a dual-complexant plating bath composed of sodium citrate and lactic acid

Based on the fact that the deposition reactions for electroless Ni-W-P coatings depend on the participation of H2PO2−, we developed a competitive deposition process that H2PO2− was controllably assigned to W, P and Ni deposition by the concentration ratio of dual-complexant composed of sodium citrate (NaCit) and lactic acid (Lac), namely NaCit/Lac ratio, and NiSO4 concentration in plating bath. The impact of the ratio and NiSO4 concentration on the deposition rate, composition, microstructure, microhardness, adhesion, corrosion resistance and wear resistance of coating was studied. The results indicate that both factors affect W codeposition through changing the deposition rate of Ni. The bath at NaCit/Lac ratio of 1:1 and NiSO4 concentration of 30-40 g/L accelerates W deposition and Ni deposition, but decelerates P deposition, and thus incorporates more W (6.97-7.02 wt.%) but less P (4.87-5.03 wt.%) in coating. Furthermore, the crystalline structure, nodular morphology, microhardness and adhesion of coating also depend on both factors. The coating plated at different NaCit/Lac ratios and NiSO4 concentrations demonstrates various corrosion-resistant and wear-resistant performances.