# Progress in Electrodeposition of Zinc and Zinc Nickel Alloys Using Ionic Liquids
## 1 Introduction  
The metallization of a surface is of great technological importance. Several techniques have been developed and used industrially to produce protective coatings. One such technique is 'electrolating', also known as metal electrodeposition when a metal is electroplated. Some of the benefits of such coatings include a smooth finish while providing corrosion resistance as well as a lower cost compared with some other types of coatings. The electrodeposition of corrosion-resistant coatings is an important manufacturing process, particularly in the automotive and aerospace sectors, to name a few. Corrosion and wear resistance are traditional areas that still receive significant attention. Intensive research efforts continue towards finding and developing electroplating solutions on new lightweight alloy composites for aerospace applications. Cadmium (Cd) coatings have been used as a sacrificial protection layer for steel components and parts in many industrial applications, including the aerospace sector, owing to its position in the galvanic series when compared with steel. However, its carcinogenic and toxic nature is a cause of concern and its use is being restricted as per the Registration, Evaluation, Authorisation, and Restriction of Chemicals (REACH) regulations and under the Restriction of Hazardous Substances-2 (RoHS 2: Directive 2011/65/EU) [1]. Zinc (Zn) and Zinc-nickel (Zn-Ni) alloys are considered to be more environmentally friendly and as having the potential to replace Cd coatings, and offer excellent corrosion resistance against the protection of ferrous substrates such as fasteners, automobile bodies, and aerospace parts, owing to the position of zinc in the galvanic series and its sacrificial protection to steel [2]. While the major demand for the Zn and Zn-Ni alloy deposits comes from the automotivesector [3], Zn-Ni alloy deposits are considered to be a potential alternative to Cd plating in the aerospace sector. The use of Zn-Ni alloy coatings in aerospace and defence applications is increasing at a fast rate and their use presents major challenges to the surface coatings industries, by requiring a completely different approach to that used for coating metals. Few automotive and aerospace companies define their own specifications for the Zn and Zn-Ni alloy deposits, keep them for their internal purposes, while few publish them. Nevertheless, the overall objective is to employ Zn-alloy deposits as a replacement to Cd coatings for sacrificial protection of steel against corrosion. Electrodeposition of Zn and Zn-alloys (containing Ni) is generally carried out from aqueous cyanide acid baths, alkaline cyanide-free baths, and acid chloride baths [4], which are toxic and corrosive. Despite their attractive properties such as high solubility to metal salts, high conductivity, and low viscosity, industrial zinc platings suffer from common problems such as hydrogen embrittlement and low coulombic efficiency [5]. In addition, the spent process water must be treated before recycling and reuse, which require the addition of chemical agents and will contribute to the recycling costs. Furthermore, certain complexing agents that are added to improve the Zn and Zn-Ni deposit properties are subjected to anodic oxidation, and will result in the formation of breakdown products during the electrolysis of Zn-Ni aqueous plating solution, which may pose environmental hazards. Moreover, it reduces the electrolyte lifetime (bath life) and requires frequent disposal followed by fresh preparation of the solution, and increases the production costs. In such a context, one of the major issues in the electroplating sector lies in compliance with legislation such as REACH in the European Union and similar regulations in other countries, which seek to limit the use of hazardous complexing agents and metal precursors in aqueous electrolytes.