*4.1. Economic Aspects*

Electrodeposited Zn and Zn–alloy coatings have been demonstrated as low cost, scalable solutions to minimize the surface degradation of mild steel parts, structures (of reasonable size) in the marine environment. Their corrosion protection abilities combined with additional functionalities gained by incorporation of nanoparticles make them technocommercially viable for such applications. While there were potential developments in the Zn, Zn–alloy coatings and composite coatings over the past decade, it is important to ge<sup>t</sup> an appreciation of the costs that are required for the fabrication of the material system. Figure 3 shows the cost comparison of the electrodeposited Zn, Zn–alloy and composite coatings produced by different electrolytes: less toxic citrate, acetate; non aqueous ILs [33,82,127,134,154], which were selected based on their significant developments over the past decade, the number of works published by scientific community, and that were demonstrated to be techno-commercially viable solutions as corrosion resistant coatings. As can be seen, the Zn, Zn–alloy deposition from ILs seems to be more expensive while the Zn, Zn–alloy composites deposited from the aqueous system seem to be economical and are in line with the conventional Zn, Zn-alloy deposits. In addition, certain relatively low-toxic aqueous electrolytes such as citrate, acetate are less expensive than the sulfate. Hopefully, these electrolytes could live up to their primary aptitude and offer real, ecofriendly solutions to numerous technical challenges that are currently being faced with the conventional chloride, sulfate systems. With the rapid advancement in the applications of ILs at a relatively large scale, it becomes obvious that the bulk production of ILs may increase and reduce the costs by up to ~1 USD per kg. Additionally, certain advantages such as recycling large fractions of plating solutions combined with the possibility of bulk scale production illustrate their economic competitiveness in producing high quality Zn, Zn–alloy deposits and are expected to make potential savings in costs [155]. With ever-growing composites, it becomes more important to understand the composite properties, its intended application and corrosion behavior in a particular service condition. Therefore, judicious selection of the electrolyte combined with the properties and the costs required must be put into consideration. Development of Zn and Zn–alloy composites from aqueous media have been demonstrated not only to be technically competitive but also economically viable and are expected to overcome the challenges associated with marine applications such as (i) corrosion resistance, (ii) wear resistance, (iii) frictional drag, (iv) fouling. Minimizing the costs of the coatings developed while meeting the performance requirements for marine applications remains of significant interest while their service life remains a grea<sup>t</sup> challenge.

**Figure 3.** Plot showing the cost comparison of different electrolytes that were developed for the deposition of Zn, Zn-alloys and Zn, Zn-alloy composites considering the initial make up of 1 L solution [33,82,127,134,154]. NaOAc-sodium acetate; HEAP-2-hydroxyethyl ammonium propionate.

## *4.2. Future Challenges*

Electrodeposition of Zn and its alloys, and composites, have significantly progressed, employing low toxicity aqueous electrolytes (e.g., citrate, acetate) and ILs over the past decade. These have shown promising applications with additional properties such as wear

resistance, superhydrophobicity and hardness in addition to corrosion resistance [63,78,81,119]. However, the globe continues to face challenges that are critical in materials deterioration, often resulting in failures of components during service. The conflicting choice of a compatible material envisioned for an explicit use and environmental hazards' control will be one of the key challenges that remains to be critically addressed by the researchers, scientists, industrial experts, etc., in the field. One of the cost-effective approaches to protect the metal parts that are being used in various applications ranging from automotive to marine is by choosing the right material and method. Electrodeposition of Zn and Zn-alloys have been demonstrated to fit the requirement owing to the sacrificial property offered by a relatively cheap Zn metal combined with the simple, cost effective method of fabrication. Electrodeposited Zn, Zn–alloy coatings combined with incorporation of nano-particles could not only protect surface degradation of mild steel in the marine environment but also impart the necessary additional properties such as superhydrophobicity, improved hardness, wear resistance, fouling resistance of mild steel components. As a consequence, these become the foundation for surface adhesion and corrosion property enhancement [11].

It is well known that Zn coatings are prone to atmospheric corrosion and result in the formation of different corrosion products depending on the nature of environment they are exposed to. The evolution of corrosion products on zinc surfaces when exposed to either sulfur-dominated or chloride-dominated environments are most commonly observed. Exposure to a sulfur-based environment might result in the formation of zinc hydroxysulfate (Zn4SO4(OH)6·nH2O) with different amounts of water depending on the moisture it is exposed to, followed by the formation of Zn4Cl2(OH)4SO4·5H2O. In aerosol dominated environments such as marine and offshore industries, the presence of high concentrations of NaCl or other chlorides favors the formation of Simonkolleite Zn5Cl2(OH)8·H2O with Gordaite (NaZn4Cl(OH)6SO4·6H2O) as the final product. The sequence of different corrosion products that are formed due to exposure to sulfur-rich and chloride-rich environment over the period is represented in the Figure 4 as shown below. In chloride rich environments such as marine, the aerosols are expected to have a high concentration of NaCl which initially form Zn5Cl2(OH)8·H2O. These will tend to form gordaite (NaZn4Cl(OH)6SO4·6H2O) by interacting with sulfate, whose emission comes from the biological activity (microorganisms, sulfate producing bacteria), droplets containing sulphate. This is reported by many works as the main reason for the frequent detection of gordaite as the final corrosion product from a chloride rich environment [156,157].

**Figure 4.** Figure showing the sequence of corrosion products that are formed on the Zn surface exposed to chloride and sulfate-based environment [21].

Considering the recent changes in the world's climatic conditions, it has been observed that the chloride-based environment is predominantly increasing relative to the sulfurbased environment [21]. Corrosion studies on Zn and Zn–alloy composites showed that the nanoparticle incorporation tend to lower the corrosion current, signifying a reduction in the corrosion rate for the coatings. The nanoparticles are thought to be capable of blocking the penetration path of the corrosive medium through the voids, gaps, crevices and holes within and on the surface of the coatings, besides incorporating additional functionalities such as brightness, uniformity, etc. In such cases, the transport of corrosive species to reach the metal substrate is further delayed, indicating improved corrosion

resistance. Most of the reports signify the corrosion protection offered by the Zn, Zn–alloy nanocomposite coatings was improved as compared to pure Zn, Zn–alloy coatings [80,109]. Though there are significant studies that reported on the atmospheric corrosion of Zn, especially on the development of corrosion products on their surface, such studies on the electrodeposited Zn, Zn–alloys from aqueous and non-aqueous media (such as ILs) remain scarce. Hence, it becomes very important to understand not only the chemical speciation of electrodeposited Zn, Zn–alloys surfaces developed from different electrolyte media such as aqueous, non-aqueous (ILs) in different environmental settings, but also from potential Zn–composite surfaces.

Different environments have different chemistries and include a variety of combination of anions such as Cl<sup>−</sup>, OH− and others found in marine environment. Understanding the nature of the zinc corrosion platina layer at different circumstances might require primary attention. The formation of porous or a compact ZnO layer either at the bottom or intermediate remains critical for the electrodeposited Zn, Zn-alloy metal structures and their composites fabricated using the less toxic citrate, acetate baths and ILs used in a marine environment. Surface methods such as scanning electron microscope (SEM) and X-ray diffraction (XRD) that are utilized for identification of corrosion products formed may not be a reliable indicator in determining the relationship between the corrosion performance of the layer formed and its full structural assembly. A prodigious deal of work is required in this area to produce a much superior understanding on the (i) formation of layered structures, (ii) conditions under which the various elements interact on the surface of electrodeposited Zn, Zn–alloys fabricated using the newly developed electrolytes. Finally, the role of each corrosion product layer towards the formation of a corrosion barrier needs to be elucidated with electrodeposited Zn, Zn–alloy and their composite coatings' surfaces [157]. This shall help to identify the environmental risks, service life of the coatings and contribute significantly when performing an assessment study such as life cycle assessment, atmospheric corrosion.

Superhydrophobic Zn coatings are considered as potential corrosion resistant coatings for marine applications and subsea pipelines in oil and gas, and incorporate several additional functions such as (i) self-healing, (ii) anti fouling, (iii) nominal involvement from external agents (e.g., UV light). There are limited studies on the Zn-based superhydrophobic coatings synthesized by electrodeposition and the corrosion performance of these coatings has only been evaluated for shorter periods in laboratory. Hence, there is an imminent necessity to fabricate Zn-based superhydrophobic coatings that maintain corrosion protection capabilities and other properties such as wear, self-cleaning for a long period. Long term exposure to severe corrosive conditions would represent the marine environment where structures, subsea pipelines (e.g., X60, X80, X90 pipelines) and parts coated with Zn (combined with a polymer) are mostly used. In addition, the development of Zn superhydrophobic coatings could be extended to protect the industry grade carbon steel substrates that are generally used for transporting oil, gas through the pipelines. With respect to the corrosion studies, validation of the indirect lab corrosion measurements via the electrochemical characterizations, weight loss measurements while considering the factors that control the electrochemical corrosion rate become critically important. There is a wide growing interest in the development of Zn, Zn–alloy composites from the newly developed electrolytes with different surface structures that offer superhydrophobic properties combined with tribological properties for marine applications [158]. Studies on the evolution of corrosion products of such surfaces on exposure to simulated marine environment could provide an understanding of the service life of the coating and help the engineers, scientists, manufacturers to decide the service life of the ship hull.

#### **5. Conclusions and Outlook**

This review covered the developments in the electrodeposition of Zn, Zn–alloys and their composite coatings produced from different electrolyte media ranging from low toxicity aqueous citrate to halide free acetate- based ionic liquids. These electrolyte media have shown a promising potential to develop coatings with improvement in their texture, morphology, phases in the case of Zn–alloys which showed good results in terms of corrosion resistance. Electrodeposited Zn, Zn–alloy composite coatings have been encouraged by the availability of newer and nanostructured materials such as TiO2, Al2O3, CeO2, etc., and have seen major progress for potential application in marine, automotive aerospace, heavy duty engineering and so on.

Some recent works on forthcoming applications such as Zn-superhydrophobic coatings offering improved corrosion resistance with mechanical and tribological properties have been reviewed. The review also covered the works that have been conducted to investigate the corrosion behavior of electrodeposited Zn and Zn–alloys in controlled laboratory environments. Recent developments in the electrodeposition of Zn and Zn–alloys using ionic liquids and composites with different nano-particle incorporations have been discussed. Despite the significant developments of the electrodeposition of Zn, Zn–alloys and Zn-composites using low toxicity aqueous electrolytes, halide free ILs, the evolution and roles of different corrosion products formed on such deposited surfaces in tropical marine environments still need a detailed investigation. Corrosion studies of the Zn–coatings produced by newly developed electrolytes in the tropical marine atmosphere could help the marine industry understand the challenges associated with the coatings. Understanding the relation between the tests performed at lab scale combined with the corrosion kinetics and chloride salt deposition rate remains of critical importance for predicting the corrosion behavior of the electrodeposited Zn-based coatings. Development of zinc-manganese (Zn–Mn) alloys have started receiving primary attention for their high corrosion resistance and formation of denser corrosion products and can be potential alternatives to the Zn–Ni, Co alloys. However, certain challenges associated with Zn–Mn electroplating such as the need for complexing agents and their corrosion behavior in real environments still require detailed investigation. In addition, studies of the formation of layered structures representing different corrosion products on the progressively developed Zn–alloy composites, Zn–alloys from ionic liquids and Zn-superhydrophobic coatings will be of greater significance. Considering future trends towards the development of superhydrophobic coatings, the combination of corrosion protection abilities along with mechanical, tribological studies should also be explored. To conclude, the role of each layer in creating a corrosion barrier needs to be explicated for the rapid pace of industrialization.

**Author Contributions:** K.K.M. wrote the draft paper and S.P. reviewed the draft and provided supervision. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie gran<sup>t</sup> agreemen<sup>t</sup> No.885793.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
