*2.2. Zn-Alloy Coatings*

Though zinc coatings have proven to be acting as a sacrificial layer to protect the ferrous substrates from corrosion, they readily undergo rapid corrosion within a short period of time which significantly impact the overall performance and durable life of the coatings over the period of time depending on its interaction with the type of environment. To enhance the corrosion performance in a harsh environment such as marine, Zn is alloyed with iron group metals, namely cobalt (Co), nickel (Ni), iron (Fe), introduced during the last three decades, with an intention to impart additional functional properties and match the industry market requirements [4,18,53]. Few of these include hardness, uniformity, deformability, weldability, paintability, corrosion and wear resistance. With the ever changing demands from automotive, aerospace, fastener, building and frame and marine industry, active research in the field is being pursued [54]. An exhaustive research has been conducted for many years to explore the possibility of replacing the toxic cadmium coatings with similar corrosion resistant zinc–nickel alloy coatings [10,55–58]. It was demonstrated that Zn–Ni alloys with Ni content of 12 to 15 wt.% possessed excellent corrosion resistance properties with longer corrosion protection life, reduced corrosion rate while retaining the primary sacrificial anodic behavior. Numerous studies were conducted to support the fact that incorporating Ni in the Zn–Ni alloys enhances the corrosion resistance of the overall coatings [53,59–61]. Besides, studies with varying Ni contents concluded that Zn–Ni alloys tend to become nobler with increasing Ni content and tend to lose their sacrificial property (with respect to steel) when the deposit contains above 30 wt.% Ni. Such Zn–Ni coatings transit from active to passive owing to their increasing nobler character, show cathodic behavior and favor the corrosion of bare ferrous steel substrates. Incorporation of Ni could slow down the dissolution rate of Zn when present in the range of 12 to 15 wt.%, retarding the dissolution of zinc and delaying the corrosion of bare ferrous steel substrate. Zn–Ni coatings with 12–15 wt.% Ni are known as " γ"-phase coatings and exhibit the best corrosion resistance [62]. Despite their excellent corrosion resistance, Zn–Ni alloy coatings lack two properties: (i) phosphatability and (ii) paintability, rendering them weak in coating applications. As a consequence, zinc–iron (Zn–Fe) alloys were introduced and studied extensively on the deposition from chloride, sulfate (with moderate pH) alkaline baths [54,63] and extended to Zn–Co coatings. While electrodeposited Zn–X (X: Ni, Co, Fe, Mn) have gained significant attention, development of Zn–Mn alloys with Mn contents varying from 10 to 40 wt.% paced up rapidly. Alloying Zn with Mn (10 to 40 wt.%) could facilitate the formation of an insoluble passive barrier layer, which enhances the protective ability of the coatings, and impart better corrosion properties [64]. However, Zn–Ni alloy coatings are reported to be corrosion resistant amongs<sup>t</sup> the other alloy coatings such as Zn–Fe, Zn–Co, Zn–Mn in a marine environment, with good mechanical properties, and considered as a potential alternative to toxic Cd coatings [65]. As the industry interest is shifting towards the development of lightweight materials, automotive industries shed some light on the development of Zn–Mn electrodeposits on base substrates such as aluminum (Al), magnesium (Mg). As the potentials of electrodeposited Zn–Mn alloys are in close proximity with reactive substrates: Al, Mg, they tend to serve more actively as a sacrificial anode and justify their ability to protect the surface from corrosion. Zn–Mn coatings offer excellent steel corrosion protection due to their good synergy, passive corrosion product layer, that are formed in corrosive environments [66] despite the fact that Mn is a thermodynamically less noble character than Zn. The synergistic effects can be attributed to the protective ability of Zn–Mn alloy deposits combined with the insoluble passive corrosion product layer. Obtaining Zn–Mn alloys by electrodeposition needs complexation because zinc and manganese have reversible potentials different by more than 0.4 V [67]. This motivated the scientific community to study Zn–Mn alloy electrodeposition, and previous results have shown that the coatings with increased Mn content offer salient benefits such as: (i) passive layer formation comprising oxides of Mn and Zn salts, (ii) monophasic structure. The formation of a compact, insoluble passive layer will not only control the anodic dissolution [68], but also favor the inhibition of dissolved oxygen reduction at the cathode [69].

Zn–Mn alloys with monophasic structure are reported to hinder the local corrosion cell formation that generally originates in dual phase structure, indicative of better corrosion resistance in the former [37]. Fashu et al. [70] demonstrated that crystal size influences the behavior of Zn–Mn alloy deposits during corrosion, with a smaller size showing the best results. Claudel et al. [71] demonstrated that Zn-Mn alloys with Mn contents up to 30 wt.% could be achieved on steel substrates by pulse plating with a faradaic efficiency up to 90% in contrast to 65% efficiency by direct current. Additionally, the deposits obtained were pore-free and homogeneous when pulse plating was employed. Obtaining a small crystal size with high Mn content is difficult to achieve, as increasing the Mn content could aid the increase in crystal size of the monophasic Zn–Mn alloys and affect its corrosion resistance. Bucko et al. [72] observed such a phenomenon while depositing Zn–Mn alloys and concluded that incorporation of a high amount of Mn in the Zn–Mn alloy and monophasic structure are not the only conditions that enhance corrosion resistance. There are certain factors that affect the corrosion behavior of Zn–alloy coatings. Deposition temperature is a parameter which influences the metal–alloy electrodeposition process, and has the capability to tailor the corrosion resistance, structural characteristics (micro/nano), mechanical properties and alloy composition of Zn–alloy coatings. Beheshti et al. [73] conducted an experimental study on the effect of deposition temperature in relation to the structural properties, phase composition and corrosion behavior of Zn–Ni alloy electrodeposits on API 5L X52 low carbon steel using an aqueous chloride bath. The deposition temperature was varied from 25–70 ◦C and the corrosion behavioral study was conducted via electrochemical characterization techniques: linear polarization resistance, immersion method using 3.5 wt.% NaCl solution, analyzed in relation to the surface morphology. The study demonstrated that the Zn–Ni alloy electrodeposited via chronopotentiometric (constant current) method at 25 ◦C exhibited a compact and dense morphology with good uniformity, less crack and highest corrosion resistance. Additionally, Ni content was reported to be within the range of 12 to 15 wt.%. Increasing the temperature beyond 25 ◦C resulted in an increase in Ni content, decreased the uniformity, compactness of the deposits and the corrosion resistance. This was attributed to the formation of more cracks in the Zn–Ni coatings with increasing temperature due to the internal stress resulting from hydrogen embrittlement, indicative of predominant hydrogen evolution reaction. Hydrogen evolution reaction is a cathodic reaction commonly observed in aqueous electrolyte media which competes with the electrochemical reduction reaction between Zn/Ni, facilitates the hydrogen to diffuse inside the coatings, resulting in a brittle deposit inducing crack. Additionally, deposition of Zn–Ni at higher temperatures shall increase the Ni content in the alloy, making the deposit nobler than the ferrous steel substrate. Zn–Ni deposits shall then lose their sacrificial ability, thereby accelerating the corrosion of the underlying less noble ferrous steel substrate. Therefore, optimizing the deposition temperature was shown to be an important parameter in improving the properties of Zn–Ni alloys in aqueous solutions such as (i) corrosion resistance, (ii) mechanical (crack formation control), (iii) phase composition, (iv) structural (uniformity, compactness).

Alloying Zn with cobalt (Co) in low contents (<3 wt.%) are considered a potential alternative to the conventional Zn–Ni systems owing to their (i) less noble character than steel, (ii) better corrosion protection properties than Zn coatings [74,75]. In addition, their possibility to achieve the desirable surface finishing properties such as brightness, decorative aspects with low Co contents (1–3 wt.%) in contrast to high Ni wt.% (12–18 wt.%) in Zn-Ni makes it an economically viable candidate to replace the toxic Cd coatings. Significant works has been reported from past 2–3 decades from different electrolytes such as (i) acidic-chloride, (ii) alkaline-sulfate, (iii) cyanide, and shown that the deposition follows an anomalous type similar to Zn–Ni. Among them, Zn–Co with Co content in the range of 1 wt.% was shown to exhibit superior corrosion resistance and is widely accepted by the various industry segments (automotive, marine, sanitary) [76,77]. One of the major hurdles with the current chemistries is the presence of carcinogenic compounds as cyanides, complexing agents which pose human threats, environmental challenges. Replacing the electrolyte with acetate ones has shed some light on these alloy coatings and is shown to produce Zn–Co alloys with good corrosion protection properties. Selvaraju and Thangaraj [78] fabricated Zn–Co alloys via direct current electrodeposition on mild steel substrates and studied the influence of current density in relation to the corrosion resistance of the Zn–Co coating. It was demonstrated that Zn–Co deposited at 4 A dm−<sup>2</sup> exhibited (i) better coverage with good throwing power, (ii) hardness with high corrosion resistance and (iii) reduced corrosion rate. The authors attributed the enhancement in corrosion resistance to the texture, morphology obtained with the acetate-based electrolyte and demonstrated its techno-commercial capability to replace the currently used electrolytes.

#### *2.3. Zn and Zn–Alloy Composite Coatings*

To enhance the strength, durability of zinc-based coatings for their application in harsh conditions, metal nanoparticles with better chemical stability than the matrix are often incorporated. These additions promote the development of microstructures with a uniform lower number of surface defects, facilitate the formation of stable passivation film with good adherence and resist further corrosion attack. Such coatings are referred to as composite coatings. Unlike alloys, composites are made from two or more different materials, which are physically distinct from each other by certain boundary/interface and contain 2 phases: (i) a continuous matrix phase, (ii) an insoluble reinforcement phase, bonded in such a way as to form a solid material. Alloys are obtained through a combination of two or more materials (metallic, non-metallic) which form a homogeneous solid solution at a certain temperature. Composites are reinforced materials that are tailored to either enhance the existing properties of a coating or impart additional functional properties that might be required for a specific application. Studies of composite coatings are shown to possess improved corrosion, mechanical properties than the traditional metallic coatings [79,80]. Therefore incorporating the metal nanoparticles into the metallic coatings broadens their range of applications and is reported to perform good while minimizing the addition of hazardous chemical agents (complexing, organic chelating) with elimination of chrome passivation. This could reduce (i) the environmental impact, (ii) economics, and as a result, these composite coatings are encouraged to substitute for the cyanide-based aqueous Zn/Zn–alloy coatings [81–83]. The development of Zn, Zn–alloy composite coatings by electrodeposition is motivated by the sacrificial ability of Zn in protecting bare steel against corrosion, thereby making it attractive to fabricate advanced novel matrix composite coatings with improved surface properties in oil and gas, marine, automotive, aerospace, etc. [84,85]. These applications are quite demanding with ever changing market dynamics, and hence, the Zn-based composite coatings technology attracts significant interest.

Zn-composites have been demonstrated as technically competitive in comparison to the Zn coatings in harsh corrosive environments such as marine, coastal, their overall corrosion protection life is mitigated by the early formation of their corrosion products [80]. It is important to optimize the conditions to obtain a composite coating with improved particle dispersion and microstructure as the quality of the composite based deposit is dependent on the deposition conditions besides particle loading, concentration and the way of particle incorporation [86–88]. Tuaweri et al. [88] studied the influence of applied current density, deposition time, particle concentration, agitation in relation to the current efficiency, deposit characteristics of Zn–SiO2 composite coatings. The results showed that Zn–SiO2 composite coatings displayed a higher cathode current efficiency at low current densities, SiO2 concentration of 26 g L−<sup>1</sup> under an agitated condition. With a further increase in time, Zn dendrites were shown to face certain struggle in building up through the dense SiO2 layer, indicative of predominant dense SiO2 as the top layer. Tuaweri and Ohgai [88–90] investigated the effect of time, current density on the composite growth, thickness and studied in relation to the increase in weight, thickness, microstructural characteristic of the Zn-SiO2 deposit. It was shown that the composite thickness and its growth was not significantly affected on varying the current density. Though the coating became thicker with deposition time, cracks were reported to be growing with time. Such

a composite is prone to rapid corrosion owing to the rapid transport of corrosive species through the cracks formed at the surface. In order to achieve good composite coatings with enhanced properties, it is necessary to optimize not only the deposition conditions but also control the particle dispersion and distribution. Incorporation of particles (metal oxides, ceramics, borides, nitrides, carbides, etc.) into the matrix might tend to impede the grain growth, structural characteristics which subsequently shall result in the formation of small-sized crystals containing the microstructures [91–93].

By dispersing them in the Zn matrix, the defect prone regions of the composite coatings such as pores, gaps, microholes, crevices, etc., which represent the corrosion active defective sites, ge<sup>t</sup> covered up and form a compact layer, acting like a physical barrier in separating the corrosive species from the metal matrix [92,94]. Praveen et al. [95], Punith et al. [96] and Rekha et al. [97] reported on the corrosion performance of Zn composites containing nano-sized carbon particles. The test results conducted employing 3.5 wt.% NaCl electrolyte solution as the corrosive media showed that the Zn metal dissolution in the matrix took place at a steady rate in comparison to the Zn metal coatings and at higher anodic potentials. Zirconia (ZrO2) is reported to exhibit high hardness and thermal stability with excellent wear resistance and a similar coefficient of thermal expansion to that of iron [98]. Considering the advantages of ZrO2, Vathsala et al. [91] and Setiawan et al. [99] studied the influence on the corrosion resistance of Zn by incorporating ZrO2 nanoparticles in the Zn metal matrix. The results demonstrated that incorporating ZrO2 (i) influenced the kinetics of the electrode reactions, (ii) favored the formation of a stable passivation layer, (iii) enhanced the corrosion protection of the composite coatings. To improve the coatings' corrosion performance, it is necessary to optimize and establish the particle loading/incorporation. For instance, Malatji et al. [100] demonstrated that addition of Al2O3, SiO2 to the Zn metal matrix beyond an optimum concentration of 5gL−1, resulted in the formation of agglomerates, decreasing the corrosion performance of the composite coatings significantly. Incorporating the agglomerates in the metal matrix could promote the initiation of surface defect sites, chemical heterogeneities in the final composite coatings that will directly (or indirectly) contribute to the overall corrosion degradation performance.

There are significant works reported on the Zn-alloy composite coatings: composite coatings prepared from Zn–Co [101,102], Zn–Ni [63,80], Zn–Fe [103] were identified to be excellent candidates for corrosion protection. Among them, Zn–Ni composite coatings attracted predominant interest owing to the chemical stability of Ni and mechanical properties [63]. Zn–Ni composite coatings incorporated with metal oxides (Al2O3, SiO2, TiO2, ZrO2) and carbides (SiC) were formulated to enhance (i) corrosion resistance, (ii) good adhesion, (iii) hardness, (iv) wear resistance, (v) crack free surface [81,85]. For instance, incorporating Al2O3 in the Zn–Ni matrix with uniform distribution was shown to (i) minimize surface defects, (ii) achieve smaller crystallites, (iii) improve the grain growth, compactness in the final deposit [104]. It was also demonstrated that incorporating the metal oxide particles along with its size, influenced the crystallite size of the deposited Zn–Ni composite. Besides, the addition of second phase metal oxide particles shall also influence the Ni content in the electrodeposited Zn–Ni composite. Works from [104,105] demonstrated the addition of metal oxide particles: Al2O3, SiO2 influenced the Ni content which increased up to 12.3 wt.% with a significant decrease in Zn up to 87.7 wt.% in the final Zn-Ni composite. Furthermore, inclusion of metal oxide particles into the Zn-Ni matrix shall influence the morphological features of the resultant composite coating. [106] reported on the morphological transitions of the Zn–Ni composite coating from spherical nodular like to cauliflower type morphology when Al2O3 was added into the matrix. Corrosion test results from [107] showed that the addition of Al2O3 particles in the concentration range of5gL−<sup>1</sup> to the deposition electrolyte solution yielded a deposit which displayed (i) reduced corrosion currents, (ii) increased polarization resistance. These results in combination with the data obtained by [83,108] conclude in the fact that the Al2O3 imparts a corrosion inhibiting effect on the corrosion of the composite matrix owing to low electronic

conductivity, thereby perturbing the corrosion current when present. Similarly, the Zn–Ni composite matrix consisting of SiO2 nano-sized particles was reported to be possessing excellent corrosion resistance when tested in 3 wt.% NaCl solution [100,105]. Table 2 shows the list of Zn and Zn-alloy composites that showed significant progressive developments in the past 10 years and have been focused upon in the recent reviews [109].


**Table 2.** Table listing the Zn and Zn–alloy composites that have been developed in the past 10 years.

Considering the significant advances in the utilization of TiO2 for development of Zn–Ni–TiO2 composites to achieve better corrosion resistant, mechanical properties, Anwar et al. [82] studied the corrosion behavior analysis of the Zn–Ni–TiO2 composite deposited via galvanostatic mode on mild steel substrates. Deposition was performed from citrate-based baths containing nano-sized TiO2 as these baths are identified to be stable in nature and comparison was made with chloride (non-citrate)-based bath. The authors observed that the Zn–Ni–TiO2 deposits from citrate-based electrolyte yielded the following: (i) formation of compact coatings, (ii) small sized crystals, (iii) uniform texture, (iv) reduced hydrogen evolution, (v) good corrosion resistance. In addition, they revealed the corrosion products that are formed on the γ-phase Zn–Ni composites upon exposure to seawater environment when conducted in laboratory. Their study demonstrated that ZnO (zincite), Zn(OH)2 (Wulfingite), Zn5(CO3)2(OH)6(hydrozincite), Zn5(OH)8Cl2 (simonkolleite) are the predominant products that are formed due to the corrosion which is aligned with the data proposed by Leygraf et al. [21]. Figure 2 shows the sequence of corrosion products that are formed on the Zn–Ni–TiO2 composite surface upon exposure to corrosive media over a period of time recorded up to 72 h. It was shown that the immersion time had significantly influenced the composition of the corrosion products with simokolleite being the predominant. Though the initial corrosion resistance was shown to be lower, there was a significant increase in corrosion resistance on increasing the exposure time beyond 24 h owing to the formation of the robust, compact corrosion product layers (hydrozincite, simonkolleite). The authors observed the conversion of simonkolleite back to hydrozincite, and attributed this to the unstable nature of hydrozincite and demonstrated its conversion back to simonkolleite, which is represented as a reversible loop between hydrozincite and simonkolleite in Figure 2.

**Figure 2.** Figure depicting the schematic sequence of corrosion products that are formed on the surface of Zn-Ni-TiO2, as reported in [82].
