**3. Recent Developments**

#### *3.1. Zn and Zn–Alloy Deposition in Ionic Liquids*

Though there are significant works on the deposition of Zn and Zn-alloys on various substrates from aqueous solutions, there are certain challenging issues that remain to be critically solved. All the zinc alloys with an alloying metal from the iron group (nickel, iron, cobalt) are obtained under so called anomalous codeposition; that is, with preferential deposition of the less noble zinc. One of the possible reasons for the anomalous codeposition is the formation of zinc hydroxide followed by its adsorption on the surface during the hydrogen evolution reaction. This will hinder the reduction of respective alloying metal ions (Ni2+) and control the overall alloy (Zn-Ni) composition as the high surface activity of zinc ions facilitates the easy replacement, inhibition of Ni ions and its nucleation, growth in the case of Zn–Ni alloy deposition. This combined with the hydrogen evolution affected the quality of the deposit drastically in terms of (i) visual appearance, (ii) crack formation, (iii) adhesion, (iv) brittleness, (v) throwing power, (vi) structural properties, (vii) corrosion resistance behavior. Besides, the optimization of the deposition parameters (current density, temperature, mode of deposition, bath agitation), electrolyte conditions (pH, concentration), additives are added either as complexing agents or levelers or brighteners or their combination which favor the anomalous deposition, formation of passivation layers (resulting due to corrosion). These shall circumvent the brittleness of the deposit by controlling the hydrogen embrittlement due to the evolution reaction and reduce the crack formation, thereby delaying the corrosion. Though there are significant research works carried out on the structural-property relation of the Zn–alloys, scientific understanding of the structural features (morphology, crystal size and orientation, alloy composition) in relation to their corrosion behavior has not been fully established. For instance, it is known that the rate of Ni deposition in the Zn–Ni alloy is hindered by the formation of zinc hydroxide. From the studies of [60,116], it was shown that the pH value measured near to the cathode surface doesn't form zinc hydroxide, indicating that the hindrance in the electroreduction of Ni ions did not occur via the hydroxide formation mechanism. The combination of Zn deposition and monolayer formation during the underpotential deposition and high overpotential of Ni resulted in anomalous Zn–Ni

deposition with controlled inhibition of the Ni ions' nucleation and growth process. Due to the ever-growing demand across a range of engineering and structural market applications, in deposition of Zn and Zn–alloys in non-aqueous electrolyte media, (i) ionic liquids and (ii) deep eutectic solvents were identified as an alternative technical competitive approach. Ionic liquids (ILs) are composed of single organic cation and an inorganic/organic anion while deep eutectic solvents (DESs) contain a combination of cations, anions. These media exhibit similar physical properties but differ in terms of synthesis, chemical properties. By employing ILs, it is possible to (i) eliminate the hydrogen gas liberation, (ii) tailor the redox properties, (iii) achieve the desired physical, chemical properties, (iv) control nucleation characteristics [117–121]. Their large electrochemical windows in combination with their good physico-chemical properties, thermal stabilities, low vapor pressures make them versatile for electrodeposition of Zn and Zn-alloys, enhance the coatings' corrosion resistance performance [122]. ILs offer an ideal alternative for the electrodeposition of Zn and its alloys such as Zn–Ni, Zn–Fe, Zn–Co, Zn–Mn in two ways. First is the hydroxide suppression mechanism that is responsible for the formation of anomalous deposits can be eliminated, and second is the elimination of hydrogen liberation owing to the absence of water in the non-aqueous bath [123]. The motivation for using ILs in Zn-alloy deposition such as Zn–Mn is due to (i) solution instability in aqueous media, (ii) low current efficiency, (iii) poor deposit morphology. Poor quality deposits and low current efficiencies arise in the case of Zn–Mn alloy coatings because these require higher negative potentials to reduce Mn, which results in drastic hydrogen gas liberation at the cathode [124]. One key benefit in using ILs is their ability to tune redox potentials via the metal speciation and promote better co-deposition of metal alloys: Zn–X (X: Ni, Co, Fe, Mn) without the need for a complexing agent, unlike aqueous electrolytes [125–128]. Since Zn and Mn co-deposits have a large difference in their redox potentials, employing an IL with a high electrochemical window shall favor the metals' co-deposition owing to their better tailoring properties. Table 3 shows the corrosion parameters obtained from the electrochemical characterization of the Zn and Zn–alloys deposited from ionic liquids.


**Table 3.** Table showing the corrosion parameters for the deposition of Zn and Zn-X alloys (X = Ni, Mn etc.) in ionic liquids (ILs).

1 Linear potentiodynamic polarization (LPP) conducted in 0.1 M NaNO3 (or) 3 wt % NaCl solution; EIS: Electrochemical impedance spectroscopy. 2 NaOAc: EG–Sodium Acetate: Ethylene Glycol. 3 Zn–Mn(0.4–*X*) indicate 0.4 M of ZnCl2 + *X* M MnCl2·4H2O in the electrolyte solution containing choline chloride: Urea in the molar ratio 1:2. *X*: 0.7–1.4 [70]. 4 Zn–Mn(1–*Y*) indicate 0.1 M of ZnCl2 + *Y* M MnCl2·4H2O in the electrolyte solution containing choline chloride: Urea in the molar ratio 1:2. *Y*: 0.1; 0.3 [128].

#### *3.2. Superhydrophobic Zn and Zn–Alloy Coatings*

In recent times, superhydrophobic coatings are considered a beneficial approach for corrosion protection of metallic structures for a variety of applications such as aerospace, marine, oil and gas and so on. Superhydrophobic surfaces are usually formed with a combination of low surface energy materials and rough microstructures. To create superhydrophobic surfaces to resist against corrosion, it is important to create rough microstructures [135,136]. On one hand, the rough microstructure surfaces trap the air within them when they are in contact with water, acting like an additional barrier and retard the corrosion rate on aircraft and ship surfaces. On the other hand, they exhibit self-cleaning, anti-fouling, anti-icing/de-icing properties which enable them to be suitable potential

candidates for protecting pipelines and other surfaces that are exposed to the marine environment besides corrosion [137]. Low surface energy substances (generally organic-based) are often added directly to the electrolyte solutions to achieve superhydrophobic coatings. With the addition of low surface energy materials to the electrolyte solution containing the metal ions, they tend to react with the functional groups of these substances during electrodeposition and form a coating with low surface energy and high-water angle on the cathode surface. The key advantage of such an addition is that superhydrophobicity can be obtained without the need for any surface modification after electrodeposition [138]. The high-water contact angle will directly influence the reaction between the corrosive species and the bare metallic substrates (generally mild steel) and prolong the life of the coatings by lessening their reaction time. On increasing surface hydrophobicity, it is possible to limit the metals' interaction with corrosive species, such as water and other ions such as Cl<sup>−</sup>, SO4<sup>2</sup><sup>−</sup>, CO2, etc., and reduce the corrosion rate of the coatings deposited. For organic anticorrosive coatings, incorporating a superhydrophobicity property would impede the diffusive mass transport of water molecules and enhance the coating's protectiveness against corrosion of underlying metallic structures for longer periods [139]. In cases such as oil and gas, these coatings seem to be an economical solution to control the corrosion and fouling in pipelines for transporting oil and gas related products such as natural gas liquid products and liquid propane via subsea, and also, they have a high tendency to be used over different substrates [140].

Considering the likelihood of obtaining several surface morphologies with varied roughness and different microstructures, electrochemical deposition is considered to be the most versatile in terms of simplicity, scalability and cost effectiveness. Table 4 lists the Zn coatings prepared by electrodeposition that exhibit superhydrophobic properties. The most widely preferred mode to obtain a metallic coating is via the electrochemical deposition at the cathode. It is also possible to achieve coatings with superhydrophobic, corrosion resistance properties by anodic electrodeposition. For instance, Wang et al. [141] performed anodic electrodeposition and obtained a superhydrophobic coating on metallic zinc anode surfaces from the solution containing zinc tetradecanoate with platinum as the cathode. A corrosion resistant superhydrophobic Zn layer was formed on the zinc anode substrate by one-step potentiostatic deposition at 30 V for 2 h and room temperature. The authors demonstrated the possibility of obtaining the superhydrophobic coatings by oxidizing the Zn to Zn2+ initially, which resulted in the formation of a superhydrophobic Zn deposit film by combining with tetradeconate on the anode surface. Corrosion test results of the superhydrophobic Zn coatings showed an enhancement in corrosion protection of the substrate. The behavior of the air medium that is trapped between the pockets of the superhydrophobic surface was shown to be similar in the action of a dielectric film in a parallel plate type pure capacitor. Such a configuration would improve the corrosion resistance life of the substrate through circumventing the metallic pathway between the substrate and the electrolyte.


**Table 4.** Table showing the list of Zn coatings with superhydrophobic properties prepared by electrodeposition.

1 DES: Deep eutectic solvent consisting of chloine chloride:ethylene glycol (1:2).

In a study by Wang et al. [148], the zinc-laurylamine superhydrophobic complex film with corrosion resistant properties was obtained on a zinc substrate via the same anodic electrodeposition route. The corrosion resistance of the deposited film was investigated in a simulated marine environment. The results showed that the superhydrophobic film coating was corrosion resistant with a protection efficiency of ≥99% [149]. Obtaining structures similar to Micropogonias Undulatus scales on the coatings via electrodeposition could result in micro patterns with superhydrophobicity. Such micro patterns exhibit the similar skin surface topographical features that are observed with marine creatures (sharks and fish) [150]. Considering the advantages with electrodeposition in obtaining structures on various geometries from simple to complex, it can be assumed that such a pattern is achievable. Inclusion of micropatterns similar to the topographical features of marine creatures (e.g., Micropogonias Undulatus-like scales) is expected to boost the physical properties and contribute to the enhancement in the corrosion resistance of the mild steels [151]. A number of scientists and researchers have leveraged the benefits of zinc coatings fabricated by electrodeposition in improving the corrosion resistance [11]. Li et al. [144] fabricated a crater-like Zn structure on an X90 steel pipe surface with superhydrophobic coating via 2 steps: (i) galvanostatic electrodeposition in sulfate electrolyte followed by (ii) chemical modification using perfluorooctanoic acid (PFOA). Contact angle measurement data showed a stable value of ~150 ◦C even after exposure to air for 80 days and the superhydrophobic coatings demonstrated good quality with self-cleaning properties and air stability. In addition, these coatings were shown to play a dual role acting as self-cleaning coatings on the one hand and exhibiting cathodic protection on the other hand, thereby enabling a double protection to the bare metal substrate. Imparting superhydrophobic properties to Zn coatings shall overcome the limitations of short corrosion life that are commonly observed with conventional Zn coatings under high humid conditions such as coastal and marine environments. Such a surface can resist the formation of a moisture film owing to its small tilt angle or high-water contact angle, which makes it difficult to hold the water molecules. Polyakov et al. [145] aimed at investigating the possibility of forming superhydrophobic Zn coatings and estimating their corrosion protection ability under salt spray chamber conditions, using 0.5 M NaCl test solution. Attempts were made by modifying the electrochemical pretreatment of carbon steel surface prior to deposition followed by 2-stage treatment in obtaining the Zn coatings with superhydrophobic properties. The 2-stage treatment involved the potentiostatic deposition of Zn dendrites from sulfate–acetate-based electrolytic solution followed by treatment with stearic acid (hydrophibising/surface energy reducing agent). The results showed that employing such an electrochemical pretreatment will play a vital role in preserving the superhydrophobic properties of the obtained coatings as the pretreated surface via galvanostatic method provides a polymodal surface with adequate roughness for creating

an anti-wetting surface. Additionally, corrosion test results from salt spray, 0.5 M NaCl confirmed that the coatings can withstand severe corrosion owing to the formation of a gas interlayer on the superhydrophobic coating surface which acted like an insulator or dielectric film, thereby preventing the Zn dissolution. This was justified by evaluating the average value of the wetting angle for the superhydrophobic coated with Zn/Stearic acid that was shown to be ≥151◦ after 148 h of exposure in the salt spray chamber. The authors also identified that ultrasonication of Zn coatings with stearic acid specimens had a positive influence on improving the superhydrophobic properties, preserving it for a long duration while possessing excellent corrosion resistance.

Subsequently, research has shifted towards the deposition of Zn, Zn–alloys with ferromagnetic iron group metals such as Fe, Co from ionic liquids resulting in the formation of superhydrophobic coatings. For instance, Li et al. [147] utilized the advantages associated with DESs, a class of ILs, in obtaining nanostructured deposits and synthesized hierarchical Zn structures via two-step electrodeposition from choline chloride: ethylene glycol-based DESs on copper-based substrates. It was observed and shown that the Zn structural coating was mainly composed of a combination of micro-slices containing pure, uniform, dense nano-concaves of Zn and zinc–stearate. Designing a superhydrophobic coating with microand nano-structural combinations was demonstrated to be highly adherent to the substrate and a promising potential solution. While nano-concaves generate van der Waals' forces and strong negative pressure, micro-slices control the surface wettability and the degree of super hydrophobicity. Development of such a unique structure shall not only endow the Zn-based coatings with high surface roughness but also with low surface energy and can be employed for applications such as self-cleaning, anti-icing and so on. Chu et al. [152] demonstrated the formation of Zn–Co alloy coating with superhydrophobic properties on AM60B magnesium alloy via electrodeposition from choline chloride-based ionic liquid and subsequent surface modification employing stearic acid as the surface energy reducer. The coating so obtained displayed improved corrosion resistance behavior and immersion test results. Additionally, the superhydrophobic coating exhibited high stability in aqueous solution and could maintain the rough surface textures even after mechanical destruction, indicative of mechanical scratch resistance. Development of superhydrophobic surfaces on lightweight metal alloy substrates such as Al, Mg provide a water-repellent surface and prevent the permeation of water into the substrate, thereby enhancing the corrosion performance of the coatings. Additionally, the scanning electrodeposition technique was developed recently, where the electrodeposition process takes place by holding the substrate stationary while the anode nozzle is kept in motion [153]. Such a technique shall overcome the difficulty associated with the plating complex shaped part such as cargo restraints (marine), propeller shaft housings (marine), wing flap bearings (aerospace). The unique structure and surface composition are expected to bestow the resulting Zn-based coatings on lightweight materials such as Al and Mg with several desirable properties. These include: (i) high surface roughness, (ii) low surface energy, (iii) reduced water-contact surface, (iv) flexibility for use in various applications, and they show a grea<sup>t</sup> potential in developing smart materials for corrosion protection of metallic parts in marine, aerospace, oil and gas subsea lines against chemical, mechanical, biological, physical corrosion causing agents.

#### **4. Cost Considerations and Future Challenges**
