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

pulse plating is an effective means of controlling the microstructure and composition of electrodeposits because it can be used to improve current distribution and modify mass transport. Thus, different problems like hydrogen evolution and uneven deposits caused by mass transport may be overcome and pH local changes minimized. Therefore, recently this method 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 optimum hardness, good corrosion resistance, properly ductility and high thermal stability[1-5]. Previous studies [2] showed that, the corrosion resistance of Ni–W alloys generally increased with W content (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 a Ni–W alloy with high resistance to corrosion. Effects of tungstate ion concentration, average current density and duty cycle on the properties of the coatings were studied. Ni–W alloy specimens were prepared by electrodeposition using a pulse rectifier on cupper sheets. The plating bath contained 147 g/l Na3C6H5O7·2H2O, 26 g/l NH4Cl, 26.3 g/l NiSO4·6H2O, 20-60-80 g/l Na2WO4·2H2O. The optimized coating was tested at 2 two different current densities and duty cycles to study the effect of pulse parameters on the properties of deposits. The coatings were characterized by Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Analysis (EDAX) and Vicker’s micro hardness tester. Tafel Polarization and electrochemical Impedance methods were used to evaluate the corrosion resistance behavior of the coatings in 0.5M NaCl solution. The SEM studies revealed the surface of the all coatings had cauliflower-like structure and crack-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) 30 0 1000 2000 3000 4000 5000 6000 7000 8000 0 2000 4000 6000 8000 -Z (ohm cm2) Z (ohm cm2) 20 g/l 60 g/l 80 g/l and maximum corrosion resistance compared to the other coatings obtained in the 20 or 80 g/l concentration of tungstate. Also with decreasing the current density in the constant duty cycle in the 60 g/l tungstate concentration bath, microhardness decreased (637 HV) and the current corrosion increased (from 5.115 μA/cm2 to 5.583 μA/cm2). Increasing duty cycle in the constant current density decreased corrosion resistance of the coating to 5.868 μA/cm2. As conclusions, beside bath tungstate concentration, pulse current density was the important factor to define chemical composition, hardness and corrosion resistance of the Ni–W coating.