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1. Introduction
Electrodeposited chromium coatings are widely used in up-to-date industry because they ensure such important service properties such as corrosion and wear resistance, high microhardness and attractive surface appearance (Dennis and Such, 2015). Chromium coatings are generally electrodeposited from aqueous plating baths containing compounds of hexavalent chromium, which are extremely dangerous to the environment and people’s health. It is known that European Union adopted Regulation No 1907/2006 which forbade or severely limited the use of Cr(VI) compounds in functional and decorative electroplating. Therefore, the development of efficient alternatives to hexavalent chromium electroplating is a very important task in surface engineering and applied electrochemistry (Protsenko and Danilov, 2014).
Trivalent chromium electrodeposition is considered as a possible and attractive option to resolve this problem (Bikulčius et al., 2017; Danilov et al., 2011; Liang et al., 2017; Protsenko et al., 2011; Protsenko and Danilov, 2009; Protsenko and Danilov, 2014). Despite a number of attempts to replace Cr(VI) plating processes by Cr(III) electrodeposition, there are still a number of issues which are mainly associated with a very complicated solution chemistry of Cr(III) complexes in water solutions (Surviliené et al., 2006). In this context, chromium electroplating baths on the basis of deep eutectic solvents (DESs) seem to be a promising alternative to “common” technologies.
DESs are a new generation of room temperature ionic liquids; they are eutectic mixtures of various organic and inorganic components (Smith et al., 2014). The melting point of a DES becomes lower than those of its individual constituents due to the formation of hydrogen or coordination bonds between particles in the liquid mixture. It is important that DESs show an ionic character as concerns their transport properties, just like any ionic liquids (Abbott et al., 2003, 2004, 2007). Thus, the physicochemical characteristics and service properties of DESs are identical to those of “common” ionic liquids, whereas DESs are cheaper, more available and environmentally acceptable (Smith et al., 2014).
DESs can be successfully used in various electrochemical processes, especially in electrodeposition and electroplating (Abo-Hamad et al., 2015; Abbott and McKenzie, 2006; Abbott et al., 2008, 2013). A number of works have been published, in which some aspects of chromium electrodeposition from DESs-based electrolytes were considered (Abbott