The Electrochemistry Properties Of Ni–W Alloy Coatings Obtained By PulsePlating From Citrate Media

pulse plating is an effective means of controlling the microstructure and composition ofelectrodeposits because it can be used to improve current distribution and modify masstransport. Thus, different problems like hydrogen evolution and uneven deposits caused bymass transport may be overcome and pH local changes minimized. Therefore, recently thismethod have been received many attentions especially in plating of pure nickel and it’s alloys.Among them, Efforts to get plated Ni–W alloys were increased as they proved to have optimumhardness, good corrosion resistance, properly ductility and high thermal stability[1-5]. Previousstudies [2] showed that, the corrosion resistance of Ni–W alloys generally increased with Wcontent (up to 11 wt.%). Also it would generally seem that as W content in the alloy increases,grain size diminishes [2]. The objective of this work is to optimize the parameters to obtain aNi–W alloy with high resistance to corrosion. Effects of tungstate ion concentration, averagecurrent density and duty cycle on the properties of the coatings were studied. Ni–W alloyspecimens were prepared by electrodeposition using a pulse rectifier on cupper sheets. Theplating bath contained 147 g/l Na3C6H5O7·2H2O, 26 g/l NH4Cl, 26.3 g/l NiSO4·6H2O, 20-60-80g/l Na2WO4·2H2O. The optimized coating was tested at 2 two different current densities andduty cycles to study the effect of pulse parameters on the properties of deposits. The coatingswere characterized by Scanning Electron Microscopy (SEM), Energy Dispersive X-RayAnalysis (EDAX) and Vicker’s micro hardness tester. Tafel Polarization and electrochemicalImpedance methods were used to evaluate the corrosion resistance behavior of the coatings in0.5M NaCl solution.The SEM studies revealed the surface of the all coatings had cauliflower-like structure andcrack-free. The coatings were obtained from the bath with 60 g/l tungstate concentration,current density of 70 mA/cm2 and duty cycle of 10% show highest microhardness (719 HV)and maximum corrosion resistance compared to the other coatings obtained in the 20 or 80 g/lconcentration of tungstate. Also with decreasing the current density in the constant duty cyclein the 60 g/l tungstate concentration bath, microhardness decreased (637 HV) and the currentcorrosion increased (from 5.115 μA/cm2 to 5.583 μA/cm2). Increasing duty cycle in theconstant current density decreased corrosion resistance of the coating to 5.868 μA/cm2. Asconclusions, beside bath tungstate concentration, pulse current density was the important factorto define chemical composition, hardness and corrosion resistance of the Ni–W coating.

کلیدواژه ها:

Pulse Electrodeposition ، Ni-W alloy coatings ، Corrosion resistance ، Microstructurecharacterization microhardness