**1. Introduction**

Steels are commonly used as structural materials in diverse fields (construction, marine, aerospace, automotive, mining) [1] as they possess interesting engineering properties such as (i) high tensile strength, (ii) melting point, (iii) hardness [2,3]. Among them, mild steel is one of the most widely used materials in various industrial applications such as automotive [4], oil and gas [5], marine (ship hull, naval architecture) [6], owing to its compatible functions and properties with diversified industrial functions. The formation of rust is one of the most widely recognized case of the corrosion, commonly observed with ferrous steel materials such as carbon steel, and can be visualized as a salt of the original metal with different phases (oxides, oxy-hydroxides) in reddish-brown color. The formation of rust scale affects the functional engineering properties of structures, materials besides appearance, strength and liquid, gas permeability (through pores), indicative of material deterioration by corrosion process [7].

Corrosion mitigation is an indispensable challenge, particularly in aggressive environments such as seawater, underground mining, aerospace, automotive to biomedical implants, etc. The annual global cost of corrosion is estimated to be around 2.5 trillion USD, which is ~3.4% of the world's gross domestic product (GDP) [8,9]. Corrosion protection of carbon steel and other low alloy steels has been a topic of interest for many years and are continuously being studied with more emphasis on identifying a suitable alternative to the conventional toxic cadmium coatings [10]. One of the recommended solutions to combat corrosion is to employ a metallic protective coating that can improve the corrosion

resistance of ferrous steel materials such as mild steel/carbon steel, low alloy steels, etc. [11]. These coatings protect the metal structures by acting either as a physical barrier or as a sacrificial coating [12]. Adherence of coating to the substrate surface and their internal strains are two key parameters that needs to be optimized in order to overcome in-service mechanical stresses such as vibration, friction, etc. [13]. An ideal coating should have higher corrosion resistance and pose minimal environmental threat. Proper selection of metal and its alloys as preferred coating material can be considered as a mitigation technique to combat severe corrosion. In order to choose an ideal metallic protective coating, it is important to consider the intended application and the exposure environment [11].

Considering the intended application and economics, zinc (Zn) is the most commonly used metal that is identified for corrosion protection due to its highly sacrificial nature with electrochemical potential less than that of the ferrous metals such as mild steel/carbon steel [14,15]. Zn is widely used to coat mild steel to prevent corrosion by at least 50%. Additionally, it is the fourth most common metal in use with annual production just below that of iron, aluminum and copper [16]. Moreover, Zn and the corrosion products of Zn are not as toxic as cadmium and are found to be most suitable for corrosion resistant coatings applications. Figure 1 lists the various applications in which zinc and its alloys are employed as protective coatings (either as composite or metallic layers) along with their primary requirements. Overall, zinc and zinc-alloy protective coatings cover a wide range of applications ranging from structural steelwork for buildings, offshore platforms and bridges with flat structures to nuts, bolts, sheet, wire, tubes.


**Figure 1.** Figure shows the applications of zinc and its alloys along with their functional requirements in various industry sectors.

> One of the key benefits in employing a Zn/Zn–alloy coating to protect the metal surfaces is that it offers a cathodic corrosion protection layer which dissolves and significantly delays the time until the substrate material can be attacked by the corrosive environment [17,18]. Generally, corrosion performance of Zn/Zn–alloy coatings are studied under different climatic conditions, regions depending on the nature of corrosivity and test condi

tions as per different standards. The primary objective of performing corrosion studies is to evaluate the coating durability when exposed to a certain corrosive environment.

When corrosion studies related to the Zn-coated structural materials are performed during their exposure to different corrosive environments, one can expect an initial mass increase. This initial mass increase can be generally ascribed to the corrosion products that are formed on the coating surface as a result of Zn corrosion followed by a decrease in mass indicating the corrosion products separation from coating surface [19]. Such a transition during the initial period of studies (between 1 to 3 years) are reported to be uneven and can occur either sooner or later depending on the presence of certain aggressive constituents (such as carbon dioxide, chloride, sulphates, nitrates), and their relative concentrations. Considering such a scenario, conducting long-term corrosion studies of Zn/Zn–alloy protective coatings under atmospheric conditions deserve significant attention [20] as they provide information on the corrosion products, processes and their formation mechanisms on the coated surface. Studies covering the atmospheric corrosion of zinc in both short and medium term have been published by different groups [21]. A consolidated review on the corrosion performance of the electrodeposited Zn, Zn–alloy coatings performed in different environments such as urban, rural, sea (natural, synthetic), microbial corrosion has been not covered so far. The number of articles that have been published on the zinc-based coatings for different applications in the past 10 years range from 1200–1700 every year (based on the data from scopus), signifying the importance of the field. This review will cover the progress on the recent developments in Zn, Zn–alloy, composite coatings, electrodeposited on different commonly used industrial substrates and their corrosion performance along with future challenges and economics.

#### **2. Corrosion Performance of Zinc and Zinc–Alloy Coatings**
