Development of a zinc-tin electroplating process from deep eutectic solvens based on choline chloride and ethylene glycol

In the current work, a new electroplating process to produce Zn rich Zn-Sn coatings with commendable corrosion resistance has been developed. The innovation is the use of deep eutectic solvents (DES) based on choline chloride and ethylene glycol. Cyclic voltammetry curves indicated that Zn and Sn were co-deposited from this DES electrolyte under distinct reduction potentials as separate Zn and Sn phases. The influences of electrolyte composition and plating parameters such as current density, temperature and stirring condition to the electrodeposition behaviour, coating appearance, composition and cathodic current efficiency have been well investigated. Sn content of the obtained Zn-Sn deposits increased with the Zn/Sn ratio or total concentration of ZnCl2 and SnCl2 salts in plating bath. An optimum process window was defined by the results of Taguchi parameter design. The addition of organic additives such as EDTE, HETDA-3Na, EDTA-2Na and especially EDTP helped to suppress the formation of dendrites and improve the qualities of Zn-Sn coatings. Pure Zn+Sn anode with separate current supply showed a better anodic behaviour in keeping a stable bath composition and coating qualities, than the ZnSn30 alloy anode and dimensional stable Ti/IrO2 anode. Smooth Zn-Sn coatings with Sn contents from 5 % to 40 % and coating thickness up to 40 µm were electroplated. The Zn-Sn coated steel is protected against corrosion by both the cathodic sacrificial effect of Zn and the barrier effect of Sn. In comparison to aqueous baths, the Zn-Sn coatings produced from DES baths showed a slightly improved corrosion resistance in salt spray corrosion test.