Direct and pulsed current electrodeposition of Zn-Mn coatings from additive-free chloride electrolytes for improved corrosion resistance

Highlights: • Cyclic voltammetry experiments of the additive-free, chloride-based Zn-Mn electrolytic bath were performed to observe its redox behaviour. • Optimum current density range for electrodeposition was obtained through potentiostatic studies and Hull cell electrodeposition experiments. • Zn-Mn alloy coatings were obtained through direct and pulsed current electrodeposition and their microstructure, composition and corrosion properties were studied and compared. • Electrochemical impedance spectroscopy was performed on selected DC and PC deposited coatings to study and compare their corrosion mechanisms. • PC deposited coatings at high pulse frequency and low pulse duty cycle showed remarkable improvement in corrosion resistance as compared to DC coatings. -- Abstract: Electrodeposited Zn-Mn alloy coatings are known to provide excellent corrosion resistance to steel due to their ability to form compact corrosion products when exposed to corrosive media. However, obtaining good quality coatings using conventional direct current (DC) electrodeposition from additive free baths is challenging. Pulsed current (PC) deposition has been reported for various Zn based alloy systems to obtain coatings with superior corrosion resistance properties. In this work, Zn-Mn coatings containing up to 9.2 wt. % Mn were electrodeposited on steel from additive-free chloride electrolytes using both DC and PC electrodeposition. Cyclic voltammetry, potentiostatic measurements, Hull cell experiments and SEM-EDS studies indicated good quality DC deposited Zn-Mn coatings in the current density range of 40-60 mA cm⁻² with clustered plate morphology at low and cauliflower morphology at higher current densities. PC deposited coatings showed increased Mn contents and improved corrosion rates. The coating deposited at a low duty cycle and a high frequency showed a fine and compact microstructure with the highest Mn content of 9.2 wt. %. XRD studies showed a complete change in phase composition from a monophasic η-Zn-Mn coating with DC to a monophasic ε-Zn-Mn with PC deposition. Electrochemical impedance analysis revealed porous corrosion products in DC coating, whereas the PC coating formed a compact corrosion product which inhibited the dissolution of Zn and Mn, thus providing higher resistance to corrosion. XRD analysis of the samples after corrosion confirmed the presence of zinc hydroxy chloride (ZHC) as a corrosion product. In summary, PC electrodeposited Zn-Mn coatings provided superior corrosion resistance than those obtained through DC.