Electroplating Nickel And Zinc-nickel Alloy And Their Corrosion Resistance Properties Study

Due to the low hydrogen brittleness, ease of processing, high corrosion resistance,weldability, the nickel and nickel alloy has received great attention for potentialapplications e.g. shielding materials for aluminum and irons.Semi-bright nickel and zinc-nickel alloy platings were electroplated on the surface ofaluminum alloys and A3steel by constant current method. Kinetic parameters of nickelelectrodeposition, such as activation energy, charge transfer coefficient and the exchangecurrent density were studied, and the electric crystal nucleation mode was proposed. Thecorrosion resistance was investigated in3.50%NaCl solution at30°C and an optimumplating conditions of nickel and zinc-nickel alloy platings were obtained. The Effect of Ce³⁺or Y³⁺on the corrosion resistance of zinc-nickel alloy plating was also studied by adding acerium or yttrium salt in the plating solution. In this study we used linear sweepvoltammetry, A.C. Impedance, chronoamperometry, XRD and SEM to study theelectrodeposition process or characterize the final platings.The results show that （1） A higher activation energy （106.34kJ/mol） is obtained whenthe electrodeposition was conducted in boric acid electrolyte at an overpotentia of30mV.The current density is found to be2.51×10^-4A/cm²and the charge transfer coefficient was0.514/n. It is indicated that an instantaneous nucleation process occurs at a highoverpotential, while continuous nucleation take place at low potential;（2） The major factorsthat influences the corrosion resistance of nickel or zinc-nickel alloy platings aretemperature and current density;（3） Appropriate amount of Ce³⁺（0.80g/L） or Y³⁺（（0.60g/L）） added to the plating solution helps to produce fine crystals in the crystallizationprocess, and make zinc-nickel alloy plating dense and uniform which gives a highercorrosion resistance. The experiments shows zinc-nickel sample with Ce³⁺added in theelectrolyte shows a better corrosion resistance than that of Y³⁺added;（4） The same amountof Ce³⁺or Y³⁺can reduce the initiating passivation current density and the maintainingpassivity current density remarkably, and make the initiating passivation potential shiftednegatively.（5） The preferred orientation of crystal plane in zinc-nickel alloy remainsunchanged before and after corrosion in NaCl （3.5%） aqueous solution, however, thetexture coefficient for every crystal plan changes a little bit. Therefore, addition of Ce³⁺andY³⁺are favorable for the growth of crystal planes with low Miller index.