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. 2025 Mar 7;10(10):9809–9823. doi: 10.1021/acsomega.4c08303

The Effect of Zinc Bath Formulation on the Corrosion Resistance of Galvanized Steel: A Short Review

Abdelhalim Chaouki †,‡, Meriem Ben Ali ‡,*, Khadija El Maalam ‡, Khalil Aouadi ‡,§, Ismail Benabdallah †, Abdelouahed El Fatimy †, Sanae Naamane ‡
PMCID: PMC11923840  PMID: 40124069

Abstract

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In recent years, hot-dip galvanizing techniques have become a promising method for surface treatment in many industries. This treatment dramatically extends the life of metal parts by protecting them from corrosive elements such as water, salt, air, and chemicals. The electrochemical, mechanical, and aesthetic properties of hot-dip galvanized coatings are closely related to the bath formulation and the control parameters. This review discusses the effect of additive elements and control parameters on corrosion resistance, mechanical properties, and coating appearance. A comparative study between alloyed and standard zinc coatings is carried out based on the state of the coating after exposure in natural sites to accelerated corrosion tests or electrochemical tests.

Introduction

Our society is using more and more galvanized steel in a variety of applications, such as the automotive industry and building materials.1,2 The global galvanized steel market accounted for USD 163.1 billion in 2022 and is estimated to achieve a market size of USD 321.4 billion by 2032, growing at a CAGR of 7.2% from 2023 to 2032.3 Due to its low manufacturing cost and excellent mechanical and thermal properties, steel is available in various shapes and sizes. However, corrosion of steel is a persistent problem. This is an electrochemical process in which iron is consumed by oxidation due to corrosive elements such as oxygen and chloride.4−6 Unfortunately, the iron oxide that forms on the surface has no protective effect against corrosion due to its porosity and poor adhesion. This results in direct economic losses through permanent replacement of corroded materials or indirect losses through loss of performance or maintenance costs.7 In this sense, corrosion inhibitors, cathodic protection, and coatings are the most used techniques to reduce steel corrosion.8

Corrosion inhibitors are chemical compounds that protect metals from corrosion. They can delay or prevent the corrosion reaction by modifying the metal surface or the corrosive environment through the adsorption of ions and molecules to the metal surface, reducing the diffusion of reagents to the metal surface.9 Cathodic protection involves creating a negative electrical potential on the metal surface to prevent corrosion by polarizing the metal with an external electrical current. There are two main methods:10 galvanic cathodic protection, which uses a sacrificial metal that will corrode in place of the protected metal, and impressed current cathodic protection, which uses an external current source to polarize the metal.

Using coatings to protect steel from corrosion is widespread in various sectors.11 These coatings establish a physical barrier between the steel and the corrosive environment, providing adequate protection, which is less effective in highly corrosive environments. One of the most effective inorganic coatings is obtained by the hot-dip galvanizing technique, which consists of coating the metal surface with a layer of zinc by dipping it in a bath of molten zinc at a temperature of 450 to 490 °C.12 The molten zinc bonds with the metal surface to form a layer of metallic zinc that acts as a barrier against oxidation. This technique is developed and improved while modifying and adjusting the galvanizing process, controlling parameters,13 and introducing additive elements. Numerous additives were examined, with particular attention paid to the incorporation of Mg and Al as we have demonstrated in our previous work,6 as well as Si and Ti,14−16 Cu17 and other additional elements. In this literature review, we will make an in-depth study of the different stages of hot-dip galvanizing, the impact of various control parameters, and the effects of additive elements in the zinc bath on the quality of the coating. This study sheds light on the nuanced interaction between zinc bath composition and the corrosion performance of galvanized steel. Understanding this relationship has immense practical value, given the significant dependence on galvanized steel in many applications, and provides crucial information for industries striving to optimize product durability, reduce maintenance costs and improve structural integrity.

1. Hot Dip Galvanizing Process

Depending on the needs and applications, the hot-dip galvanizing process can be carried out continuously or discontinuously. To understand the production mechanism of hot-dip galvanized metal coatings, both methods are discussed below.

1.1. Continuous Hot-Dip Galvanizing

Continuous hot-dip galvanizing is the most economical method for large-scale production of galvanizing sheet metal; the United Kingdom first adopted this process in the 1930s, and it is now widely used in automotive sheet production worldwide.18 It is limited only to sheet, strip, and wire; the coated sheet is then cut or sheared and formed into a final part. The process is wholly automatized and involves the immersion of the steel in zinc liquid for a very short time to obtain the coated product. Adding about 0.15% aluminum in the zinc bath sufficiently slows the reaction between zinc and iron to prevent the formation of brittle Fe–Zn alloys that cause deformation of the coating when the sheet is formed. The Industrial processes (Figure 1) start with a cold rolled steel sheet, which is welded into a continuous strip and continuously degreased and pickled; it is then heat treated in a furnace where the material is wholly annealed to remove oxides potentially formed on the surface. The thickness of the zinc coating in a continuously galvanized strip varies from 10 to 35 μm.19,20 It can be controlled by the speed of the production line and by adding certain additives, such as aluminum and rare earth elements, to the Zinc bath. Indeed, for a minimum thickness requirement, increasing the production line speed and the addition of these additives leads to a slow reaction rate of the Zn–Fe, which allows a reduction in the thickness. F. Ozturk reported in the “Hot-Dip Galvanizing Process” (2017) that the production line speed can achieve 200 m/s.21

Figure 1.

Figure 1

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Diagram of the continuous hot-dip galvanizing process.

1.2. Discontinuous Hot-Dip Galvanizing

The Discontinuous hot-dip galvanizing process is used after the manufacturing of parts to protect the steel against corrosion; these parts are immersed in a zinc bath to be galvanized. Immersion galvanizing is suitable for all shapes of parts, but it is necessary to adjust the bath’s dimensions to fit the parts. Unlike the continuous process, which involves immersing the steel for a very short time, the discontinuous process requires more extended periods of immersion.21 Before galvanization, the preparation step of the part’s surface is essential to obtain a pure metallic surface. It is ensured by chemical pretreatment, which involves the immersion of the parts in several baths.

For industrial parts, surfaces are always contaminated by organic products, such as oils and greases, or metallic, such as oxides and carbides. Therefore, it is essential to treat contaminated metal surfaces by immersion in baths containing chemical cleaning solutions.22 The steps of the hot-dip galvanizing process are shown in Figure 2(a) and are detailed below.

Figure 2.

Figure 2

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(a) Stages of discontinuous hot-dip galvanizing, (b) Principle of chemical pickling with HCl. (c) Different rinsing methods. (d) Immersion of fluxed steel in the zinc bath.

The basic operations of chemical pretreatment are degreasing, pickling, and fluxing. Each operation is followed by rinsing to prevent the transfer of chemicals between the treatment baths, which would lead to their deterioration. Chemical pretreatment baths must resist extreme corrosion stresses; in modern galvanizing plants, these baths are made of welded polypropylene, which presents a high chemical resistance.

1.2.1. Degreasing

This first step’s objective is to remove grease, oil, lubricants, and traces of previous handling that have accumulated on the part’s surface. These contaminants prevent perfect wetting in the pickling bath.23

Degreasing is one of the operations that determines the quality of the galvanization. It can be done in the aqueous phase or by solvent.

1.2.1.a. Degreasing in Aqueous Phase

this step is done by immersing the parts in a bath containing a solution of carbon tetrachloride (TCC) or a hot alkaline solution (NaOH) maintained at a temperature between 40 and 60 °C.24 After this step, the parts that emerged in the liquid solvent must be cleaned to remove the supernatant fat and foams that will be redeposited on the parts when leaving the bath. Acid degreasing, usually carried out with phosphoric acid at room temperature, is becoming increasingly well-known.24 Acidic degreasing baths initiate the pickling process so that the grease formed does not dissolve in the tub but precipitates to the bottom of the tank as sediment. These are removed at regular intervals.

1.2.1.b. Solvent Degreasing

Solvent degreasing can be performed with one of the following components: Trichloroethylene (C2HCl3), Perchloroethylene (C2Cl4), and Trichloroethane (CHCl3). These elements are in a gaseous state at very low temperatures, and the vapor condenses on the part to dissolve the dirt. These elements are less expensive and more effective for very greasy parts, but they present a significant source of toxicity.

1.2.2. Pickling

The purpose of pickling is to remove oxide layers formed at high temperatures during subsequent thermal or mechanical treatments, such as Fe2O3, and to eliminate the compounds generated during the degreasing operation. Pickling is performed by immersing the parts in a hydrochloric acid (HCl) solution, water diluted from 33% to 50%, at room temperature.25 This concentration range will have a low risk of embrittlement on the metal.

To increase the pickling efficiency, a proportion of ferrous chloride FeCl2 is added to the bath, ensuring a better dissolution of the scale covering the surface of the parts to be galvanized. To understand the mechanism of HCl pickling, it is essential to examine the different layers of oxides formed on the surface, as shown in Figure 2(b).

The scale contains three layers of iron oxides. A very long stripping time is required when the layer is predominantly magnetite. In cases where the Wustite (FeO) layer is predominant, the pickling time to obtain a clean metal surface can be very short. The wustite layer dissolves first because the ferrous oxide is the most soluble in the pickling solution.

1.2.2. r.1
1.2.2. r.2

The reaction of pure iron with hydrochloric acid can be present, leading to hydrogen production. The latter causes harmful effects on the steel and any coating applied to it afterward.

1.2.2. r.3
1.2.3. Rinsing

This step avoids transferring the elements produced in the pickling step to the fluxing step and then to the zinc bath. Furthermore, it stops the chemical reactions by dilution and removes the supernatant grease and foams that can redeposit on steel. This step can be done by various methods, as shown in Figure 2(c):

  • Dead rinse: tank without clean water supply, periodically emptied;

  • Current rinsing: tank continuously supplied with clean water;

  • Cascade rinsing: association of several rinsing tanks with circulation of the part in counter-current to the water;

  • Eco rinsing: used before and after the treatment bath.

1.2.4. Fluxing

This intermediate step aims to protect the surface with a flux film, which prevents oxidation and ensures good wettability for the molten zinc. The flux bath consists of zinc chloride and ammonium chloride in the form of double salt ZnCl2, 2(NH4Cl), with a concentration between 300 and 700 g/L. The immersion of the part in this bath forms a thin protective layer against surface oxidation (Figure 2(d)), which will be melted once the part is immersed in the galvanizing bath, thus ensuring excellent contact and promoting the iron–zinc metal reaction. The flux thin film is decomposed according to the following reaction:26

1.2.4. r.4

The NH3 and HCl products dissolve the oxides according to the following reactions:

1.2.4. r.5
1.2.4. r.6

Chemically, ammoniacates are complexes (FeCl2, NH3, and ZnCl2) called burnt salts. They give rise to ashes that float on the zinc surface.

1.2.5. Drying

After the fluxing step, the flux applied to the parts must be dried to avoid zinc spatter when the load is immersed in the galvanizing bath. Immediately after drying, the parts should be absorbed in the molten zinc.

1.2.6. Galvanization

After the surface treatment phase, the surface of the parts can be galvanized. There is not only a simple entrainment of the zinc that solidifies on the surface of the steel but also chemical diffusion reactions between the zinc and the iron. This reaction occurs while the steel is immersed in a zinc bath, forming several zinc–iron alloy layers. These layers differ in iron and zinc content, with increasing zinc content and decreasing iron content while getting closer to the surface of the coating. The layers are designated by Greek letters, as shown in the iron–zinc phase diagram (Figure 3(a)).

Figure 3.

Figure 3

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(a) Binary phase diagram of the Fe–Zn system for the Zn-rich region. (b) Intermetallic layers present in conventional hot-dip galvanized steel (zinc alone).

The Zn–Fe alloy coating forms at a temperature higher than the melting point of zinc (419.4 °C) but lower than the melting point of iron (1538 °C). Thus, in batch hot-dip galvanizing at temperatures of about 450 °C, the equilibrium conditions for a peritectic transformation (one liquid and one solid phase are transformed into a new solid phase) or for a peritectoid transformation (two solid phases are converted into a new solid phase) can only occur during the so-called “high-temperature galvanizing”.

The intermetallic sublayers formed in the conventional hot-dip galvanizing process (zinc bath only) are indicated by Greek letters, Γ (Fe3Zn10), Γ1 (Fe5Zn21), δ (FeZn10), ζ (FeZn13) and η (Zn). The properties and position of each sublayer are presented in Figure 3(b) and in Table 1.

Table 1. Characteristics of Intermetallic Layersa.
Phase Chemical symbol Crystalline structure Wt % iron At. % iron Hardness
Eta (η) Zn Hexagonal 0 0 Very hard
Zeta (ζ) FeZn13 Monoclinic 5–6.2 6.7–7.2 Hard
Delta (δ) FeZn10 Hexagonal 7–10 8.5–13.5 Brittle
Gamma (Γ1) Fe5Zn21 FCC 15.8–23.5 18.5–23.5 Hard and brittle
Gamma (Γ) Fe3Zn10 BCC 20.2–31 23.2–31 Brittle
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a

Reprinted with permission from ref (27). Copyright 2022 Springer Nature.

The coating formed by these underlayments provides a sacrificial protective barrier against corrosion. This is related to the zinc element, which corrodes first, thus protecting the underlying steel in a corrosive environment.

In continuous hot-dip galvanizing, the steel substrate is annealed before being immersed in the molten zinc bath. This annealing step is crucial, as it promotes the formation of a uniform oxide layer that improves the adhesion of the zinc coating. The coating microstructure consists of a layer of intermetallic compounds, mainly composed of δ and ζ phases, which form due to the rapid diffusion of zinc into the iron substrate during the short exposure time to molten zinc.28,29 This results in a thinner coating with a more refined microstructure, which is advantageous for applications requiring precise coating thickness and uniformity.30,31 In contrast, the discontinuous hot-dip galvanizing process involves a longer immersion time, resulting in thicker coatings. The longer exposure time facilitates the development of a more complex microstructure with a more variety of intermetallic phases, including the formation of the γ-phase, which can improve the mechanical properties of the coating.28,32

2. Control Parameters

Several studies have shown that the coating quality obtained by galvanizing depends not only on the condition of the steel surface but also on the steel core and the galvanizing process. Several control parameters are applied to ensure the effectiveness of the coating, such as the type of steel, the temperature of the bath, the immersion time, the speed of immersion/immersion, and the composition of the bath. In this section, we will study the role and effect of each parameter and their influence on coating quality.

2.1. Steel Grade

Structural steels contain a small amount of silicon, used as a Deoxidizer in the steelmaking process. Silicon is also an inexpensive alloying element for strengthening steels.33 However, its presence in the steel during the hot-dip galvanizing process impacts the characteristics of the resulting coating. Indeed, silicon most likely affects the hot-dip galvanizing reactions by influencing the diffusion of zinc in the steel and the diffusion of iron in the coating, and this diffusion results in the thickening of the zinc coating. In addition, the silicon content influences the thickness of the undercoats of the coating, as shown in Table 2 and Figure 4.34 In fact, each phase layer has its own growth kinetics, which is determined by silicon content and immersion time. A compact and continuous zinc layer with Γ, δ, ζ, and η layers is apparent on the sample with a silicon content of less than 0.01% (Figure 4(a)). Between 195 and 1200 s, there is a significant difference in the microstructure of the zinc layer and the thickness of the δ layer (Table 2). After 195 s, ζ phase growth reached its maximum, while δ phase growth took over. The coating layer with an immersion time of 1200 s had a greater thickness than the ζ layer. A thick layer was produced at immersion times of 195 and 1200 s of silicon content in the Sandelin range (Si 0.06% and 0.11%) (Figure 4(b,c)). The coating on the sample with a silicon content of 0.30% is characterized by a thin layer δ and a thick layer δ + ζ with a floating ζ (Figure 4(e), Table 2). Coating growth with immersion times of 195 and 1200 s was strongly influenced by the silicon content of the steel (Figure 4(f)).

Table 2. Layer Thicknesses δ and ζ with Immersion Time 1200 s.
Si content Thickness of δ (μm) Thickness of ζ (μm)
<0.01 44 40
0.06 33 168
0.11 1 849
0.17 26 192
0.30 41 467 mixed phases (δ + ζ)
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Figure 4.

Figure 4

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Microstructure of zinc coating after different immersion times (195 and 1200 s): (a) Si < 0.01%, (b) Si 0.06%, (c) Si 0.11%, (d) Si 0.17%, (e) Si 0.30%. (f) Coating thicknesses at different substrate silicon contents. Reprinted with permission from ref (34). Copyright 2016 Estonian Academy of Sciences.

In 1940, Robert W. Sandelin et al.35 studied the effect of silicon content in galvanized steel; they concluded that when the silicon content is higher than 0.03 wt %, the thickness of the coating increases significantly as a result of the change in the structure of the diffusion layers. This behavior is known as the Sandelin effect. Later, John J. Sebisty continued Sandelin’s research and found that for the silicon content range from 0.03% to 0.14%, the zinc thickness follows a Gaussian distribution and reaches a maximum of 215 μm at 0.09% Si. However, as Figure 5(a) shows, the coating thickness increases again when the silicon content exceeds 0.2%, as the coating becomes loose and excessively thick due to the transport of the silicon contained in the steels into the coating.36

Figure 5.

Figure 5

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Effect of (a) silicon and (b) phosphorus on coating thickness, (c) variation in zinc thickness based on the combined phosphorus and silicon content, and (d) Fe–Zn layer formation as a function of silicon content. Panel d reprinted with permission from ref (34). Copyright 2016 Estonian Academy of Sciences.

In the case of the hot-dip galvanizing process, it is generally known that phosphorus, like silicon, increases the rate of dissolution of iron in molten zinc and thus favors the formation of a thicker coating, whose structure is dominated by the zeta phase (ζ).37 The level of phosphorus in the steel greatly influences the thickness of the obtained coating, as shown in the orange curve of Figure 5(b). A significant thickness can be received for a phosphorus content of about 0.1%; this formed coating is brittle due to the large mass of zinc on the steel part.

According to the Sandelin effect, three categories are distinguished by combining the effect of silicon and the effect of phosphorus, as shown in Figure 5(c). Each category has different properties, as indicated in Table 3. The composition range comprising 0.1% and 0.2% of silicon +2.5% of phosphorus is not recommended because of its unpredictable consequences, including the formation of flaking and increased surface brittleness. It is important to note that the C category significantly increases coating thickness and a dull overall appearance. Figure 5(d) shows that Sandelin steel is characterized by a high reaction rate for ζ phase formation. Expansion of the δ phase is limited. Although the ζ phase predominates in the coating, the growth of the δ and ζ phases is linked to the coating growth of Sebisty steel. The coating exhibits a mixed δ + ζ phase when the steel has a high silicon content (Si 0.30%).

Table 3. Properties of the Three Sandelin Categories.
  Aspect Mechanical resistance Coating mass Adherence Application
Category A Excellent Excellent Standard Excellent Esthetic and anticorrosion research
Category B Good Very good Standard Good Anticorrosion research
Category C Average Good Stronger Variable Optimum protection for aggressive environments
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In most cases, batch-produced galvanized steel with a carbon content of up to 0.2 wt % is intended for welded construction. For these steels, the carbon content has no significant effect on the zinc layer’s formation, composition, and thickness.38 However, a high carbon content (>0.3 wt %) can increase the rate of the iron dissolution reaction in the zinc melt and consequently increase the overall thickness of the coating through the growth of the zeta phase (ζ). Such a coating is brittle and easily detaches from the steel.39 It should also be mentioned that free carbon, precipitated at the grain boundaries, can cause hydrogen corrosion during the pickling phase.40

2.2. Bath Temperature

The effect of bath temperature on the coating is significant. Indeed, the temperature influences the kinetics of the reaction between liquid zinc and iron. The thickness of the coatings varies considerably over the temperature range from 450 to 530 °C, as shown in the Figure 6.(a). The effect on the thickness of the intermetallic layers as shown in Figure 6.(b), which indicates the increase in the thickness of Γ layer and δ layer with the increase of the zinc bath temperature.41 In the temperature range between 450 and 480 °C, the temperature of the zinc bath does not affect the number of intermetallic layers. Still, it does influence the morphology and thickness of the sublayers. When the temperature is between 450 and 460 °C, the morphology of the coatings is relatively compact, and the ζ and η layers are formed on top of the δ layer. Often, the zinc bath is kept at this range.40 Above 470 °C, the morphology of the δ and ζ phases changes significantly with the variation of the zinc bath temperature. In the temperature range of 470–480 °C, the compact structure changes to an incomplete structure with a porous character, and the boundary between the ζ and δ layers is undefined.41 Above the temperature of 480 °C, the fragmentation of the alloy layer occurs and stimulates the penetration of zinc into the metal, which leads to the formation of a considerable amount of trash that degrades the quality of the coating.11 For this reason, the lowest possible temperature with suitable zinc viscosity should be used to minimize ash and trash, save fuel, and avoid damage to the zinc vessel. The coating thickness reached a maximum (80–90 μm) at the temperature of 530 °C and thus provided corrosion protection for an extended period. However, a poor surface appearance is obtained due to dross formation on the surface.40

Figure 6.

Figure 6

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Effect of zinc bath temperature on (a) coating thickness, (b) intermetallic layer thickness, (c) potentiodynamic polarization curves in 3.5% wt NaCl, and (d) electrochemical kinetic data for hot-dip galvanized coatings. Panel a reprinted with permission from ref (12). Copyright 2007 Elsevier. Panels b–d reprinted with permission from ref (41). Copyright 2024 TechnoFit Academic Publishers LLC.

The impact of temperature on the morphology and thickness of sublayers affects the ability of coatings to resist corrosion. Figure 6(c) illustrates the influence of zinc bath temperature on the corrosion resistance of hot-dip galvanized steels. The characteristic polarization curves of the coatings, obtained in a 3.5% by-weight NaCl solution, show that increasing the temperature of the zinc bath increases the corrosion current density. In addition, an increase in the corrosion potential of the hot-dip galvanized coatings was observed as a function of the temperature of the zinc bath (Figure 6(d)).

2.3. Immersion Time

Coating thickness is significantly influenced by immersion time. The evolution of zinc coating thickness acquired during hot-dip galvanizing of cylindrical steel samples for two cooling modes (air-cooled and water-cooled) as a function of immersion time (t = 15, 30, 60, 180, and 300 s) is illustrated in Figure 7(a).44 For both cooling modes, zinc coating thickness increases with immersion time. Figure 7(b) shows X-ray diffraction patterns of hot-dip galvanized steel for different immersion times in the air cooling mode. XRD diagram shows how the interaction between zinc coatings and steel leads to changes in microstructure and phase transformation. The presence of ferrite (alpha iron: α-Fe) in the steel substrate is demonstrated, and various phases, including eta (η), zinc oxide (ZnO), zeta (ζ), delta (δ), and gamma (Γ), are found in the coatings. After 30 s of incubation, the gamma (Γ) phase was found to have formed. However, Hakim et al.42 found that an increase in immersion time greatly reduced the hardness of the coating at the same bath temperature (460 °C). The main cause of this drop in hardness is that as the immersion time increases, more mass is deposited on the component, so the hardness also increases. After the substrate has been immersed in the molten zinc bath for around 20 s, the alloying reactions begin. Traditionally, the hot-dip galvanizing procedure is kept to around 4 to 5 min for best results.40

Figure 7.

Figure 7

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(a) Evolution of zinc coating thickness as a function of immersion time and cooling modes. (b) X-ray diffraction patterns of steel samples hot-dip galvanized at 460 °C for different immersion times. Reprinted with permission from ref (43). Copyright 2025 Springer Nature.

2.4. Immersion Speed

For good galvanization, the immersion speed must be controlled to ensure the melting of the flux generated during the fluxing step; an uncontrollable immersion speed can generate unexpected intermetallic deposits. Indeed, a slow speed causes the merging of the flux on the surface of the part before immersion, thus leading to poor wetting and reoxidation since the part is bare. Meanwhile, a fast speed does not allow the melting of the flux, and therefore, it remains deposited on the surface and prevents intermetallic diffusion of Zn–Fe. The quality of a steel part galvanized at 450 °C for an immersion time of 1 min is best at immersion speeds of 3 m/min, 4 m/min, and 5 m/min.44

The steel substrate is removed from the bath when the reaction between the iron and zinc is complete; the thickness of the coating depends on the withdrawal rate. By convention, an optimum shrinkage rate of 1.5 m/min is maintained to form a bright zinc coating. If the shrinkage rate is too slow, a uniform layer of unalloyed zinc is formed, while an uneven coating is formed at a faster shrinkage rate. Therefore, the shrinkage rate should be controlled to regulate the coating’s nature, thickness, and surface finish.40

2.5. Cooling Speed

Cooling is usually performed in ambient air. Slow cooling leads to further diffusion of iron, which causes the increase of Fe–Zn alloys. This reaction can be stopped using water immersion. M. Xu et al. (2022) reported that the coating’s best corrosion resistance is shown at a cooling rate of 1 °C/s; this information is confirmed by the salt spray results and the electrochemical tests.45

3. Effect of Additives in the Galvanizing Bath

The zinc coating offers high corrosion resistance due to its dual protective effect: on the one hand, it acts as a barrier that mechanically protects the steel from the external environment; on the other hand, it acts as a galvanic protector due to its electrochemical potential, which is lower than that of the steel.

During the hot-dip galvanizing process, the chemical reaction that occurs between the steel part and the molten zinc in the bath results in the formation of different intermetallic sublayers such as Γ (Fe3Zn10), Γ1 (Fe5Zn21), δ (FeZn10), ζ (FeZn13) and η (Zn). during the anodic dissolution process in the aqueous solution, the coating begins to dissolve into adsorbed intermediate species (Znad+, Znad2+ and ZnOHad) to ultimately form corrosion products according to the reactions proposed by Cachet et al.46 as follows:

3. r.7
3. r.8
3. r.9

However, this zinc-based structure cannot meet corrosion resistance standards, especially in highly aggressive environments.47,48 Metallic elements can be added to the electroplating bath to optimize the mechanical and chemical properties of the coating.49−51 Nevertheless, this optimization is not straightforward and has several obstacles,52−54 making it a topic worthy of further study.

A comparative study of the coating performance with different additive elements is meaningful if the obtained coatings were performed and tested under the same conditions. This review will present the effect of the other elements added to the zinc bath.

3.1. Zn–Al Formulation

The Zn–Al coatings have been extensively studied for their high corrosion resistance and mechanical properties, compared to standard zinc coatings, in atmospheric, saline, acidic and even concrete environments as reported by Yang et al.,55 after testing the samples Zn–4% Al, Zn–8% Al, Zn–12% Al, and Zn–6% Al immersed in an acid rain solution baths, it has been found that the corrosion rates of all samples decreased with increasing aluminum content in the bath formulation, this observation can be explained by the formation of corrosion product Zn6Al2(OH)16CO3-4H2O. The concentration of this product in the solution becomes progressively supersaturated and begins to precipitate above 8 wt % aluminum. In addition, the samples immersed in the solution for 5 days noted the appearance of minor pitting marks on the surface of the coatings containing 4 and 8 wt % aluminum. However, the pitting was less pronounced on the coatings containing 12 and 16 wt % aluminum than those containing 4 and 8 wt %.

A Zn–5 wt % Al coating of the GALFAN trade shows improved corrosion resistance over conventional zinc coating in a marine site. A significant corrosion product on the Zn–5% Al coating, subjected to the exposure conditions, is the layered double hydroxide Zn6Al2(OH)16CO3·4H2O. During exposure to a marine environment, the evolution of corrosion products for a conventional zinc coating56 and a Zn–Al coating57 is shown in Table 4.

Table 4. Evolution of Corrosion Products of a Zinc Coating and a Zinc–Aluminum Coating Exposed to a Long-Term Marine Environment.

3.1.

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A Zn–7 wt % Al coating was developed by Tachibana et al.58 and tested for ten years at two sites: the seaside of Shirahama and the typical rural area surrounded by the mountains of Aizuwakamatsu. Compared to zinc coating, the corrosion resistance is significantly improved due to the formation of protective corrosion products such as Zn6Al2(OH)16CO3·4H2O. This type of coating does not lead to the formation of zinc oxides, as aluminum has a higher affinity for oxygen than zinc. In addition, the Zn–7 wt % Al coating presents better adhesion than 100% zinc coating; this property is verified by a bending test with a bending parameter expressed as P = (thickness of the coated steel/width between the legs of the letter U) × 100%. The Zn–7 wt % Al coating elongated without exfoliating even at P = 40%, while the conventional zinc coating easily exfoliated from the steel substrate even at P = 10% (Figure 8). The good adhesion is probably due to the formation of a mixture of heterogeneous delicate phases of zinc, aluminum, and iron at the steel-coating interface.

Figure 8.

Figure 8

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Bending test of Zn coating and Zn–7% Al alloy coating. Reprinted with permission from ref (58). Copyright 2006 Elsevier.

Coating compositions with 0, 10, 20, 25, and 30 wt % aluminum were developed for hot-dip galvanized steel sheets. After being exposed to salt spray for 1900 h (Figure 9), the results show that galvanized steel without aluminum (100% Zn) has less corrosion resistance, while the coating with 10 wt % Al has the highest resistance.19

Figure 9.

Figure 9

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Digital photographs of hot-dip galvanized steel plates with 100% Zn and Al–Zn alloys after exposure in a salt spray chamber for 1900 h. Reprinted with permission from ref (50). Copyright 2020 Elsevier.

In an environment containing a porous concrete solution, a Zn–10 wt % Al coating also demonstrates significant corrosion protection compared with galvanized steel without aluminum (100% Zn) (Figure 10). The corrosion products formed in Zn–10 wt % Al coating, distinct from those produced by the zinc coating, are responsible for the increased corrosion resistance. The corrosion byproducts of both coatings contain zinc oxide (zincite) and hydrozincite phases. The corrosion products of the Zn–10 wt % Al coating, which are extremely strong and protective, include zincite, hydrozincite, and simonkoleite (Zn5(OH)8Cl2.H2O) phases. The corrosion products of the Zn coating contain the iron oxide phases goethite (α-FeOOH) and aragonite (β-FeOOH), which are less protective than simonkoleite.19 Aluminum added to the bath is localized at the steel-coating interface to form a Fe2Al5 compound.59

Figure 10.

Figure 10

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Digital photographs of 10AZ (Zn–10 wt % Al) and 100Z(100% Zn) coated rebar removed from mortar after 20 wet/dry treatment cycles. Reprinted with permission from ref (50). Copyright 2020 Elsevier.

3.2. Zn–Mg Formulation

Magnesium is added to the bath to increase the corrosion resistance by forming more stable corrosion products than those developed in the zinc coating. Indeed, Volovitch et al.60 found, after qualitative analysis of the corrosion products, that simonkoleite is stabilized on the Zn–Mg coating throughout the exposure to the atmospheric test. In contrast, it is only observed during the early stages of the Zn coating. Also, they demonstrated that this corrosion product is more compact than the other forms of Zn corrosion products observed and, therefore, acts as a more substantial barrier to the diffusion of corrosive elements such as oxygen.

Yao et al.61 showed that the corrosion resistance and microhardness increase significantly by adding 1, 2, 3, and 4 wt % Mg. However, adding 3 wt % Mg showed better corrosion resistance after testing the different coatings by immersion in salt water for 19 days (Figure 11(a,b)) and electrochemical test (Figure 11(c)).

Figure 11.

Figure 11

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Coatings (a) Zn, (b) Zn-3%Mg after salt water immersion test for 19 days. (c) Polarization curves of Zn and Zn–x% Mg coatings with x = 1, 2, 3 and 4%. Reprinted with permission from ref (61). Copyright 2014 Elsevier.

The polarization curves of the coatings (Figure 11(c)) showed that the Zn–3 wt % Mg coating has the lowest corrosion current density, which means good corrosion resistance compared to the other coatings. This property is related to the type of corrosion product formed on the surface of this coating. There are mainly two types of corrosion products of simonkoleite present on the surface of zinc coating: the hexagonal crystallized plate type and the flocculent type; the first type is simple to transform into other types of soluble corrosion products, making it an unstable corrosion product,60 this type is found in coatings with 1 and 2 wt % Mg. However, the fluffy kind of simonkoleite is observed only on the surface of the coating with 3 wt % Mg, and it remains stable and uniform.

3.3. Zn–Mn Formulation

Manganese is electrochemically more negative than zinc and iron; therefore, it can provide sacrificial protection to the steel. Studies have shown that Zn–Mn has better corrosion resistance than Zn coating. The mechanism behind the improved corrosion resistance of Zn–Mn is the formation of the hydrozincite product promoted by Mn. Grandhi et al.63 show that adding 0.4 wt % Mn can decrease the passive current density of Zn in the passive region by reducing the porosity of the film, which inhibits ion transport in the film.

Manganese forms the oxide Mn2O3 as a corrosion product, suppressing oxygen reduction at the cathode and thus slowing down the corrosion process.64 However, this oxide causes irregular coloration (Figure 12), which reduces the coating’s aesthetic quality. This is why aluminum has been used to prevent Mn oxidation, as its oxidation-free energy is more damaging than that of Zn and Mn.62

Figure 12.

Figure 12

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Zn–Mn coating with and without aluminum. Reprinted with permission from ref (63). Copyright 2021 Elsevier.

3.4. Zn–Al–Mg Formulation

A zinc-based metal coating with magnesium and aluminum additions provides excellent corrosion protection. Because of this, it is used in several applications, including rooftop and ground-mounted solar installations.64 In the following, we will investigate the effect of varying the percentages of Al and Mg on the properties of Zn–Al–Mg coatings.

Yao et al.65 prepared Zn–5 wt % Al–x wt % Mg coatings with x = 0.5, 1, 1.5, 2 wt %. They showed that the Zn grain size was refined, and the eutectic zones at the Zn grain boundaries increased with increasing Mg content, and the Mg tended to exist in the outer layer and thus improve the corrosion resistance. At the same time, Al is present in the inner layer and forms an intermetallic layer of Fe2Al5, which increases the hardness of the coating. The improvement of the properties of the Zn–Al–Mg coating (especially Zn–5 wt % Al–1.5 wt % Mg) compared to the standard galvanization is related to the formation of simonkoleite-flocculant type, which prevents the penetration of O2 and H2O inside the coating.

Lebozec et al.66 developed a Zn–2.7 wt % Al–1.5 wt % Mg coating and showed that the binary eutectic Zn/MgZn2 (BE) and the ternary eutectic Zn/MgZn2/Al (TE) are present on the surface. Thus, the small size of zinc grains increases corrosion resistance, as also confirmed by Cai.67 The reduced size of the zinc dendrites, which have cathodic zones, favors the preferential dissolution of the Mg-rich phase (eutectic) and the formation of a stable double hydroxide layer on the surface. However, there is a lower limit of zinc dendrite size beyond which the effect is no longer positive, and no improvement is observed. They also noted that grain size decreases with increasing cooling rates.

Figure 13 shows the distribution of zinc dendrites and eutectics in the cross-section. Compared to the cross-section of a conventional zinc coating, magnesium, and aluminum surround the zinc dendrite as eutectics, which is the reason for the increased corrosion resistance.

Figure 13.

Figure 13

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SEM images of coating cross-section (a) Zn–0.2% Al, (b) Zn–2.7% Al–1.5% Mg, and (c) Zn–1.5% Al–1.1% Mg. Panel a reprinted with permission from ref (68). Copyright 2009 Elsevier. Panel b reprinted with permission from ref (66). Copyright 2025 Elsevier. Panel c reprinted with permission from ref (67). Copyright 2025 Elsevier.

Schuerz et al.68 showed that the Zn–2 wt % Al–2 wt % Mg coating presents significant corrosion resistance compared to a pure zinc coating in a NaCl medium. Indeed, the Zn–2% Al–2% Mg coating shows no signs of corrosion even after 600 h, whereas a pure zinc coating starts to corrode after just 100 h of salt spray testing (Figure 14).

Figure 14.

Figure 14

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Zn and Zn–Al–Mg coatings before and after different periods of salt spray exposure. Reprinted with permission from (68). Copyright 2009 Elsevier.

Magnesium forms a MgZn2 phase; this phase corrodes before the pure zinc phase, which increases the strength of the coating. Aluminum is more reactive to iron than zinc; it is first localized at the steel/coating interface and forms a thin layer identified as an intermetallic compound Fe2Al5; this layer prevents the diffusion of zinc atoms into the iron, which reduces the thickness of the coating and stops the formation of sublayers in conventional galvanization. In addition, aluminum is found on the surface of the layer and forms alumina Al2O3, which increases the corrosion resistance. Duchoslav et al.69 showed that the high corrosion resistance of the Zn–2% Al–2% Mg coating is due to three main corrosion products: simonkoleite, hydrozincite, and a double-layer hydroxide (DLH) layer: the following reactions from these corrosion products:

3.4. r.10
3.4. r.11
3.4. r.12

After a long exposure, simonkoleite, a very dense and compact corrosion product,61 transforms into hydrozincite, which is less protective, has poor adhesion to the coating layer, and is characterized by a porous structure.70

The MgZn2 and Mg2Zn11 phases formed in the magnesium coating in the zinc bath increase corrosion resistance due to their lower anode current compared to the pure zinc phase.71

Several studies70,72−74 have characterized the Zn–6 wt % Al–3 wt % Mg coating under the trade name ZAM. These studies confirm that the corrosion resistance is better than the conventional Zn–0.2 wt % Al coating due to the alloying elements that improve the stability of corrosion products, such as simonkoleite, known for its compactness. However, the study of Zhang et al.74 suggests that the Fe–Al intermetallic layer consists of Fe4Al13 instead of Fe2Al5. In addition, other research has revealed that the Fe–Al intermetallic layer consists of FeAl3.75 This variation in the intermetallic phases observed can mainly be attributed to the immersion time75,76 or the temperature of the part before immersion in the galvanizing bath.77

3.5. Zn–Al–Si/Zn–Al–Mg–Si Formulation

In the 1960s, Bethlehem Steel developed a new type of coating based on aluminum and silicon Zn–55 wt % Al–1.5 wt % Si by weight under the commercial name GALVALUME. This type of coating has shown a higher corrosion resistance than the conventional coating.78,79 It has been used in severe environments such as acid rain or the presence of deicing salts.14,80 The poor protection of conventional zinc coatings in high-temperature environments (above 400 °C)81 has been overcome by this coating because of its high-temperature stability due to the aluminum rate (55 wt %), which increases the melting point of the coating. Consequently, this coating protects the steel against dry corrosion and provides excellent protection against wet corrosion.82,83

Adding Si to the Zn–Al–Mg bath is a promising method to improve corrosion resistance. Li et al.84 have explored this option by adding 0.1% Si to the Zn–6% Al–3% Mg coating; they showed that the low Si content significantly changes the coating microstructure. Indeed, there is a formation of a new Mg2Si phase on the surface and the presence of aluminum dendrites on the surface and in the middle of the coating instead of the steel/coating interface (Figure 15); the same conclusion was found by Yamada85 for a Zn–11% Al–3% Mg–0.2% Si coating. This homogeneous distribution of aluminum dendrites in the middle of the Zn/Al/MgZn2 ternary eutectic prevents the dissolution of zinc-rich phases by the formation of hydrated aluminum oxide (Al2O3-3H2O), which provides an excellent barrier against corrosion and consequently improves the coating performance. In addition, the presence of silicon prevents the transformation of Zn5(OH)Cl2·H2O into Zn5(CO3)2(OH)6.84

Figure 15.

Figure 15

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Cross-sectional microstructures of the (a) Zn–6% Al–3% Mg and (b) Zn–6% Al–3% Mg–0.1% Si coatings. Reprinted with permission from ref (84). Copyright 2015 Elsevier.

The work of Qian et al.86 indicated that all Si atoms are dissolved in the Fe2Al5 phase when the Si content in the alloy bath is shallow, and the Fe–Al–Si ternary system can be considered a pseudobinary system. When the Si content in the alloy bath is higher than 0.2 wt %, a ternary τ4 (Al3FeSi2) phase appears. This phase is composed of 53.5–46 at. % Al, 16 at. % Fe and 30.5–38 at. % Si.87

Conclusions

In summary, the exploration of the various studies on the issue of continuous or batch hot dip galvanizing has enabled us to identify several parameters for the success of the coating. Before steel galvanizing, the pretreatment step is necessary to ensure good adhesion. The bath temperature is used to increase the diffusion rate between the zinc and iron, which improves the thickness of the coating. The coating grows fastest at 480 °C in the continuous hot-dip galvanizing process. Increasing the immersion time also increases the thickness, but excessive exposure can cause coating embrittlement. The choice of immersion rate is also essential to ensure the flux is melted at the right time. A cooling rate of 1 °C/s provides the best corrosion resistance for the coating.

On the other hand, coating characteristics, such as appearance, thickness, and mechanical and electrochemical properties, are primarily influenced by the additive components. The low aluminum content at the steel/coating interface means there are only two sublayers, reducing the coating thickness and ensuring adhesion. An aluminum content of less than 8% is recommended. Zn–Al–Mg coatings offer better corrosion resistance and significantly higher hardness than zinc and Zn–Al coatings. Adding aluminum and magnesium allows the formation of binary and ternary eutectics, which significantly increase physical and chemical properties over conventional coatings by producing more protective corrosion products such as zinc hydroxide and simonkoleite. Silicon and manganese also increase corrosion resistance, except that manganese requires the addition of aluminum to avoid irregular coloration of the coating surface. Finally, it is essential to note that predicting the effects of a formulation without prior experience is complex, as the properties of the coating vary not only according to the percentages of the elements composing the formulation but also according to the control parameters applied in the hot-dip galvanizing process.

Acknowledgments

This work was supported by the APRD program launched by the Moroccan Ministry of Higher Education, Scientific Research and Innovation, the OCP Foundation, Mohammed VI Polytechnic University, and the National Center for Scientific and Technical Research, with the support of the Chair “Multiphysics and HPC” led by Abdelouahad El Fatimy at Mohammed VI Polytechnic University.

The authors declare no competing financial interest.

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