(Digital Presentation) Znni Electroplating from an Ethylene Glycol Based Non-Aqueous Electrolyte for Corrosion Protection

Abstract

ZnNi alloys containing Ni in the 15 - 25 % wt. range have recently attracted the attention of researchers due to their interesting corrosion properties [1]. These materials, when deposited in the form of thin films, find application in a large number of industrial fields: oil and gas, automotive and constructions. ZnNi is normally electrodeposited from aqueous solutions, specifically alkaline cyanide-free or acidic chloride baths. In these electrolytes, plating takes place following a typical anomalous mechanism where the less noble metal (zinc) deposits preferentially [2].Even though ZnNi plating from water based solution is already implemented at the industrial scale, alternative deposition approaches have been developed in the last few years to further improve ZnNi plating performances. In particular, the use of non-aqueous electrolytes has been investigated by virtue of their significant advantages over water based solutions: wider electrochemical windows, reduced side reactions and low vapor pressure. ZnNi electrodeposition has been attempted from a great variety of such novel baths, mainly including II and III generation ionic liquids [3, 4].The present work investigates the use of non-aqueous ethylene glycol based electrolytes for the deposition of ZnNi alloys containing Ni in the 15 - 25 % wt. range. The use of ethylene glycol as solvent introduces many advantages with respect to aqueous solutions: low oxygen contamination, high deposits purity and no pH dependance [5]. ZnNi codeposition from ethylene glycol baths containing ZnCl2 and NiCl2 was studied employing cyclic voltammetry and thin layers were deposited under potentiostatic control. Deposition potential was varied and its influence on final composition and morphology was determined. Compositional data evidenced the occurrence of a potential dependent anomalous codeposition mechanism analogous to the one reported in water based electrolytes. Addition of an additive (NH4Cl) to enhance surface morphology was attempted as well [4]. Phase composition was investigated using XRD and the results obtained demonstrated the presence of a metastable γ-ZnNi phase, which has already been observed in the case of ZnNi plated from alkaline aqueous solutions [6]. Finally, corrosion behavior of the ZnNi layers obtained from ethylene glycol was assessed and compared with equivalent coatings deposited from commercial ZnNi aqueous solutions.[1] M. Gavrila et al., Surf. Coat. Technol. 123 (2-3), 164-172 (2000)[2] G. Roventi et al., J. Appl. Electrochem. 30 (2), 173-179 (2000)[3] K. K. Maniam et al., Appl. Sci. 10, 5321 (2020)[4] S. Fashu et al., Trans. Nonferrous Met. Soc. China 25 (6), 2054-2064 (2015)[5] G. Panzeri et al., Electrochim. Acta 271 (1), 576-581 (2018)[6] S. Ieffa et al., Trans. Inst. Met. Finish. 94, 321-324 (2014)