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 H 2 PO 2 − , we developed a competitive deposition process that H 2 PO 2 − 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 NiSO 4 concentration in plating bath. The impact of the ratio and NiSO 4 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 NiSO 4 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 NiSO 4 concentrations demonstrates various corrosion-resistant and wear-resistant performances. [ABSTRACT FROM AUTHOR]