Characterization of corrosion resistance of electrodeposited Zn-Ni Zn and Cd coatings

The corrosion behavior of electrodeposited Zn-Ni alloy coatings was studied with a view to replace Zn and Cd coatings. A three electrode electrochemical cell was utilized to understand corrosion of the coatings in 3.5% NaCl solution by measuring the open circuit potentials (OCPs), polarization curves and electrochemical impedance spectroscopy (EIS). EIS was performed at intermittent time intervals to understand the extent of dezincification in Zn-Ni alloys and to determine the polarization resistance change with respect to time. Coatings were examined with SEM and XRD to correlate the corrosion behavior with the surface/interface morphology and residual stresses and to understand the nature of corrosion products. The effect of heat treatment and trivalent chrome passivation on the corrosion behavior of Zn-Ni coatings is also reported. Zn-Ni coatings studied here have higher polarization resistance, lower corrosion currents and the corrosion products offer additional protection in increasing the life of the coatings compared with Zn and Cd coatings. Based on these results Zn-Ni coatings are an attractive alternative to Cd and Zn coatings.