Deposition Behavior And Corrosion Resistance Of Electroless Ni-W-P Coatings Plated In Dual-complexant Bath

The ternary electroless Ni-W-P coating exhibits broad engineering applications for its notably enhanced corrosion-and wear-resistance;however,the deposition of alloying element W relies on Ni-induced codeposition process.To realize Ni-W codeposition,similar deposition potential between both is crucial,and adding appropriate complexants is an effective way to improve the codeposition.Compared with the mono-complexant bath,the dual-complexant one not only favors the incorporation of W into coating,but also can improve the deposition rate and the bath stability.In the present work,therefore,sodium citrate(NaCit)and lactate(Lac)dual-complexant bath was used to systematically investigate the effect of the concentration ratio between NaCit and Lac(NaCit/Lac ratio)as well as the concentration of NiSO4,NaH2PO2,Na2WO4 on the chemical composition,phase,surface morphology,thickness,microhardness and binding force of the Ni-W-P coating;and besides,based on the fact that the deposition reactions during electroless plating of Ni-W-P coatings depend on the reduction of H2PO2-,the competitive deposition behavior of Ni,W and P influenced by NaCit/Lac ratio was studied,while the corrosion-resistance and wear-resistance of electroless Ni-W-P coatings with various mass ratios between W and P(W/P ratios)were characteriazed.The surface morphology,chemical composition and phase composition of electroless Ni-W-P coating were investigated by SEM,EDS,XRD,respectively.The hardness and binding force were determined by microhardness tester and automatic scratch tester.The corrosion resistance of Ni-W-P coatings with various W/P ratios was studied by electrochemical method and neutral salt spray test to discuss the corrosion behavior.The tribological behavior of Ni-W-P coatings was investigated under dry condition by using a home-made ball-on-disk tribometer.The results are described as follows:as NaCit/Lac ratio increases,W content and W/P ratio increase and then decrease,but P content is on the contrary;and meanwhile,with an increase of NiSO4 concentration,W content and W/P ratio also exhibits a parabola-like tendency,but P content shows a consistently declining trend.The maximum W content(7.02 wt.%)and W/P ratio are obtained at NaCit/Lac ratio 1:1 and NiSO4 concentration 40 g/L.The deposition rate of Ni-W-P coating is consistent with W content,and that the crystalline characteristic of Ni phase and microhardness are influenced by W/P ratio;when the W/P ratio is 1.74,the coating presents the strongest crystalline characteristic and highest microhardness(754.8 HV0.02).The homogeneity of Ni particles improves and then deteriorates due to NaCit/Lac ratio increasing from 1:5 to 5:1,while the uniform size and distribution of Ni particles can be achieved at NaCit/Lac ratio of 3:1;furthermore,NiSO4 concentration had a significant impact on the surface morphology.The electrochemical testing indicates that the Ni-W-P coating prepared at the ratio of 3:1 reveals the highest corrosion potential(-0.3112 V);and at the same time,the minimum corrosion current density(0.382 ?A·cm-2).In addition,there is no obvious corrosion product after 720 h salt spray test,and the corrosion rate is rather small,while the EDS spectrum also demonstrates the lack of O and Fe.The coefficient of friction and wear rate decrease and then increase as variation of NaCit/Lac ratio;and furthermore,the excellent wear resistance is achieved at NaCit/Lac ratio 1:1;but when NiSO4 concentration increases to 40 g/L,the wear resistance can be further enhanced.In the NaCit-Lac dual-complexant bath with constant NaH2PO2 concentration,the change of NaCit/Lac ratio and NiSO4 concentration influence the formation of complex ions of Ni2+ and WO42-,and thus affect the rate of deposition reactions for Ni,W,and P during electroless plating,thus realizing compositional control of alloying element in coating.