Structure and corrosion behavior of Zn-Ni-Mn/Zn–Ni layered alloy coatings electrodeposited under various potential regimes

The Zn-Ni-Mn/Zn–Ni multi-layer coatings were electrodeposited under switching potential regime from an alkaline solution. In order to deposit Zn–Ni layers, more positive potential −1560 mV was used as first electrodeposition potential in each cycle. More negative potentials −1860 mV and −1980 mV was used as the second potential in the various potential regime. The pulse-electrodeposited samples showed a smoother surface compared to coatings electrodeposited at a constant potential. Also, potentiodynamic polarization and electrochemical impedance spectroscopy were used to study the corrosion resistance of these coatings. All coatings electrodeposited at constant potential showed passivation behavior, while such behavior was only seen in pulse-electrodeposited samples from bathes with Mn2+ concentration higher than 1.17 μM. Samples electrodeposited using −1860 mV as the second potential for 30 s in each cycle had more positive corrosion potential and lower corrosion current density compared to samples deposited at a constant potential, while samples electrodeposited under potential regimes with more negative second potential had more negative corrosion potential and higher corrosion current density. The electrochemical impedance curves showed that the coatings electrodeposited at constant potential had higher polarization resistance and the passive layer formed at their surface was more compact than the pulse-electrodeposited ones.