Effect of bath composition and operating parameters on deposit character and corrosion behaviour of Zn-Ni alloy

Abstract

Electrodeposited Zn-Ni alloys are extensively used as protective coatings for steel substrates and hence the studies on the factors which enhance corrosion resistance are of considerable significance. The present work details the optimization of acid chloride bath for bright Zn-Ni alloy over mild steel and study of the parameters which influence Ni content in the deposit. Use of sulphanilic acid and gelatin was found to show significant effect on brightness of the deposit. The effect of molar ratio of Ni⁺²/Zn⁺² in the bath on limiting current density of nickel deposition was emphasized. Under no conditions of bath compositions and operating parameters studied, the change in codeposition behaviour from anomalous to normal type was observed. The wt. %Ni in the deposit was found to be the independent factor of its corrosion resistance. The photomicrograph of electroplates confirmed that superior corrosion resistance is due to good surface morphology. The effect of bath composition, current density (c.d.), pH and temperature on appearance, hardness and corrosion resistance of deposits were studied and discussed. Corrosion behaviour of electroplates has been studied by Tafel's extrapolation method. Electrochemical impedance spectroscopy analysis revealed that superior corrosion resistance of Zn-Ni alloy coatings at optimized current density is due to barrier resistance at the interface of deposit and medium. Formation of n-type semiconductor film at the interface was confirmed by Mott-Schottky plot. Addition of small amount of cadmium chloride did not increase corrosion resistance.