*2.1. Zn Coatings*

Zinc coatings offer flexibility in fabrication and good affordability owing to their sacrificial property [22]. As a consequence, these coatings were fabricated by different methods to protect the bare metallic structures against deterioration and degradation upon their exposure in different corrosive environments. Among the different techniques, electrodeposition is simple, economic and versatile in producing uniform, adherent coatings with variable thickness at processing temperatures <100 ◦C. On the contrary, other techniques such as hot dip galvanization, ion vapor deposition techniques require high processing temperatures and expensive equipment to produce Zn coatings, and are relatively expensive in electrodeposition besides achieving uniformity in coatings. For instance, the cost to produce a 35-micron thick hot dip galvanized coating is \$1.76 /ft<sup>2</sup> in contrast to \$0.1/ft<sup>2</sup> [23,24] for the electrodeposited Zn coatings, signifying the techno-economic benefit of electrodeposition. When Zn coatings are exposed to aggressive environments such as coastal, marine which contain rich amounts of chlorides, sulfates, etc., their corrosion resistance is significantly influenced. The factors that influence the corrosion resistance of the zinc coatings obtained via electrodeposition method include: (i) applied current density, (ii) deposition temperature, (iii) electrolyte pH, (iv) mode of current deposition, (v) additives (grain refiners, brightening agents). For instance, Zhang et al. [25] showed that increasing the applied current density to an optimum value during the electrodeposition of zinc increased the nucleation density, cathodic current efficiency and most importantly, improved the grain refinement of the Zn. Grain refinement favors the nucleation while controlling the growth, resulting in a compact deposit. Such features delay the corrosion by reducing the contact area between the corrosive environment and the coating surface [25,26]. The same study has shown a deposit deterioration when the Zn deposition is performed beyond the optimum. Deposition temperature might play a role in (i) controlling the average size of the crystallite, (ii) energy consumption during the process, (iii) current efficiency. Tuaweri et al. [27] reported an increase in current efficiency when employing the acidic sulphate-based electrolyte while achieving a relatively low energy consumption (per unit mass of the deposit) by controlling the temperature between

40–45 ◦C. Increasing the temperature influenced the rate of deposition and crystal size reduction of the Zn deposit owing to their high cathodic reduction and increased nucleation density while controlling their growth. Increasing the pH of the electrolyte dictates the conductivity which significantly influences the hydrogen ion concentration at the cathode in addition to the electrodeposition of Zn. While lowering the pH favors the conductivity increase and facilitate good deposition, it acidifies the solution below a certain pH. Acidification of the electrolyte solution elevates the hydrogen ion concentration of the cathode and as a result, hydrogen evolution reaction dominates the Zn deposition, thereby affecting the overall deposition process and the corrosion resistance of the Zn deposit. As a consequence, significant works were carried out with different types of deposition media with different pH such as acid chloride [28], acid sulphate [29], mixed bath (chloride and sulphate, sulfate–gluconate) [30,31], alkaline zincate baths [32] and acetate baths [33]. Amongst them, acid sulphate was demonstrated to perform better in terms of plating, non-toxic nature and wide operating current density ranges [34].

Many recent studies reported that the modes of deposition, direct current (DC), pulse mode (PC), pulsed cycle reversal mode (PCR), influence the structure and indeed, the corrosion resistance of the Zn deposits. Results from [35–37] showed that deposition of Zn via pulse mode resulted in more compact thinner deposits with (i) less porosity, (ii) better corrosion resistance than the direct current mode, with PCR being predominant. The key advantage with the PCR mode of deposition is that it facilitates the formation of Zn deposits with nano-grains and contributes to better hardness and corrosion resistance than the Zn deposited by other modes of deposition. Wasekar et al. [35] demonstrated this approach by depositing Zn employing different deposition techniques, DC, PC, PCR, and correlated this with the formation of corrosion products on Zn surface. The authors observed that different corrosion products were formed when Zn was deposited using different deposition modes. A compact ZnO was reported to be formed from the corrosion of Zn deposited from PCR. On the contrary, corrosion of the Zn deposited from the other two modes (DC, PC) was shown to form zinc hydroxy-chloride, a highly porous corrosion product. Obtaining Zn deposits with grains in the nanometer range via the PCR mode of deposition was demonstrated to be the key in achieving better corrosion protection properties with high hardness. Such a morphology might facilitate the formation of ZnO film easily via the controlled diffusion mechanism occurring through the grain boundaries.

A different approach that has been identified to improve the corrosion resistance of Zn deposits is the introduction of additives in the electrolyte. Additives can be organic or inorganic and they greatly influence the corrosion resistance of Zn deposits by modifying their structural characteristics, such as (i) surface composition, morphology (microstructure), (ii) grain size, (iii) crystal orientation, texture via controlling the reduction of metal ions [38–40]. They usually ge<sup>t</sup> adsorbed to the substrate that is being deposited via the non-bonding electron pairs present in nitrogen, Sulphur, oxygen, hydrophilic groups and (i) enhance the rate of nucleation while controlling the grain growth, (ii) aid the formation of fine, compact, refined deposit. One of the key advantages in obtaining a compact deposit via employing additives is the formation of crystallographic planes with closed packed structure. This contributes to the overall improvement in corrosion resistance of the Zn deposit [41,42]. For instance, Mouanga et al. [43] demonstrated an increase in the intensity of Zn crystal plane (1 1 2) with the addition of urea as an additive in a chloride-based zinc electrolyte. The study focused on the influence of 3 additives: (i) urea, (ii) thiourea, (iii) guanidine (which has same molecular structure but different electron pairing groups: oxygen, Sulphur and nitrogen), and studied the corrosion behavior in relation to the structural characteristics of the deposit. The study concluded that corrosion test results (performed by polarization, weight loss) showed an increase in the corrosion resistance for the Zn deposited in the presence of urea. This was attributed to the presence of oxygen in the molecular structure of urea to function as an effective additive. Though it has been demonstrated that the radical with more free electrons interacted more effectively with the metal substrate in controlling the morphology of the final deposit, the molecular weight of

the additive influences its adsorption capability. Ballesteros et al. [44] observed that the molecular weight of an oxygen group containing radical poly-ethylene glycol (PEG) had a significant influence on the final quality of the Zn deposit. When PEG with a molecular weight in the order of <10<sup>4</sup> was introduced into the Zn deposition bath, the results showed a greater adsorption of Zn(II) ions with the substrate than the ones containing the higher molecular weight PEG (>104). Issues were shown to occur on the addition of PEG with molecular weights >104, which decreased the number of oxygen pair electrons that can form an effective bond with the additive and affected the adsorption characteristics of the Zn(II) ions. The influence of additives towards improving the corrosion resistance properties of the deposits could be correlated with their ability to increase nucleation rate while retarding growth. Employing an additive may result in a higher cathodic overpotential than the non-additive containing electrolytes. High cathodic overpotential tends to increase the formation of new nuclei, increasing its nucleation rate, utilizing the free energy, thereby inhibiting the growth of Zn [45]. In general, the contribution from adding an additive (mostly organic) towards improvement in the corrosion resistance property of electrodeposited Zn coatings can be related to either of the following or their combination:


Electrodeposited Zn is composed of Zn with a hexagonal closed packed (hcp) structure with different crystallographic orientations representing different planes: basal, pyramidal, prismatic. These planes differ in terms of their packing density and significantly influence the corrosion rate. Zn crystals possessing low-index basal plane texture such as (0 0 1) possess high packing density and were reported to be significantly corrosion resistant relative to other orientations and different planes [46]. Based on the published literature, it was identified that promoting the presence of (0 0 2) basal plane via the additives contributed to the corrosion resistance property more effectively than the other crystal planes. For instance, Chandrasekar et al. [37] obtained a more compact Zn deposit with (0 0 2) as the dominant facet by employing polyvinyl alcohol (PVA) as the additive, and demonstrated a significant increase in the corrosion resistance. In this context, it is important to consider the influence of surface roughness over the crystal plane texture. Lowering the surface roughness results in a deposit with fine grain size which lowers the corrosion rate by providing a lower contact area between the deposit surface and the corrosive environment, indicating the predominant influence of grain refining over the crystal orientation/texture. Grain refining achieved via the addition of additives will produce a coating that accelerate the formation of ZnO passive films via the diffusional mechanism and elevate the corrosion resistance. Table 1 lists the most commonly used organic additives that are employed during the Zn deposition in different deposition media along with their functional role. These additives were demonstrated to be contributing towards the enhancement of corrosion protection by imparting additional functionalities to the deposit.

Besides many functions, additives such as thiourea [43] can also influence the compositional change in the Zn deposit with fine grains when added to the electrolyte. Despite its attractive grain refining property, such an addition incorporates sulfur in the deposit which made the neighboring regions anodic and decreased the corrosion resistance. Almeida et al. [47] performed a detailed investigation by studying the influence of glycerol on the corrosion resistance of the electrodeposited zinc obtained via the galvanostatic mode. Glycerol exhibit similar characteristics to urea, coumarin wherein the oxygen atoms double bonded with carbon act as radicals (free unpaired electrons) and favor the adsorption of the organic additive in the Zn deposit. Physical characterization revealed that addition of glycerol played the role of a grain refiner but decreased the intensity of (0 0 2) basal planes similar to the observations made by Chandrasekar et al. [37] and Nayana et al. [48] when the combinations of piperonal +PVA [37], cetyltrimethyl ammonium bromide (CTAB) + veratraldehyde (VV), formic acid (FA) + cyclohexylamine (CHA) [45] are employed as

additives. However, the electrochemical test results showed that these coatings possessed the best corrosion resistance. The authors ascribed this to the predominance of grain size over the texture by demonstrating the results from microhardness, surface measurements. An increase in compactness due to the grain refining was shown to exhibit better corrosion resistance despite the decrease in (0 0 2) basal plane.

**Table 1.** Table listing the additives that have been employed to improve the corrosion resistance property and impart addition functional properties to the Zn deposit.


1 Functional roles are listed based on the conclusions reported by the references mentioned in the table.
