1 Introduction

Zinc coatings obtained by hot dip method are currently one of the most effective and economical methods of protecting steel against corrosion. The demand for zinc coatings is steadily growing to include an increasingly specific range of products such as high-carbon iron alloys (Ref 1), high-strength bolts (Ref 2) or wires for high-speed drawing (Ref 3). Zinc coating industry consumes half of world's zinc production (Ref 4). Part of this metal is unreasonably consumed due to the uncontrolled production of excessively thick coatings on so-called reactive steels and insufficient zinc drainage from product surface. The production of too thick coatings is economically unjustified and contributes to an increase in zinc consumption.

The increase in the price of zinc observed in the last dozen years and its instability increased the intensity of work on the possibilities of reducing the consumption of this metal. The main areas of research and research and development (R&D) supported by industry organizations of the galvanizing industry in Europe and around the world (e.g., European General Galvanizers Association EGGA, American Galvanizers Association AGA) focus on the development of effective methods of reducing zinc consumption by improving the technological efficiency of the galvanizing process. It is mainly about limiting the unfavorable influence of the silicon content in steel on the thickness of the coating, reducing the amount of zinc ash and reducing the losses of zinc caused by its excessive discharge from the bath. All these goals were achieved through the appropriate selection of chemical composition of the galvanizing bath. Contemporary galvanizing baths are only alloyed baths, in which alloy additives are the most frequently chosen configuration from the group of five metals: Al, Ni, Pb, Bi and Sn. Aluminum and nickel are nowadays constant bath alloying elements and their beneficial and negative effects are well understood. However, an important additives used interchangeably to improve the fluidity of the bath are Pb, Bi and Sn. This primarily affects the quality of the coating appearance and improves zinc drainage from the product surface, which contributes to reducing zinc losses. The article presents benefits and limitations of the use of Pb, Bi and Sn additives in the galvanizing bath and their impact on coating quality, the effectiveness of galvanizing process and impact on natural environment.

2 Factors Determining the Quality of Zinc Coatings

The quality of hot dip galvanizing coatings is determined on the basis of the following criteria: external appearance of the coating, its thickness and corrosion resistance.

The external appearance of the coating primarily determines the esthetic value of the product, but it can also affect the corrosion resistance of the coatings. A typical zinc coating has a smooth surface and a bright and shiny appearance. However, in many cases coatings with a gray and dull appearance or with a pronounced surface roughness are obtained. The appearance of the coating surface depends mainly on the chemical composition of the steel and its influence on the formation of the diffusion layer in the coating. Zinc coatings may also have a uniform surface appearance, but may also have a zinc spangle which in many cases is considered a desirable esthetic effect.

The thickness of the coating depends primarily on the parameters of the galvanizing process and the chemical composition of the steel, but it can also be optimized by appropriate selection of the chemical composition of the bath. The low thickness of the coating reduces the period of the protective effect of the coating. However, the production of too thick coatings is economically unjustified, increasing costs and increasing the consumption of zinc. Traditionally, industrially produced zinc coatings have a thickness of about 40-120 µm. Galvanizing of reactive steels leads to even several times the thickness of the coating. The minimum coating thickness is specified by the PN EN ISO 1461 (Ref 5) standard, which specifies the requirements depending on the type and thickness of galvanized elements.

The main reason for producing zinc coatings is to protect steel surface against corrosion. The duration of the protective effect of the coating is determined by its thickness, but the corrosion resistance also depends on the structure of the coating. Being able to control the growth of the coating and control its structure allows the corrosion resistance to be predicted.

3 The Role of Alloying Additives in a Zinc Bath

The main purpose of introducing alloying additives into zinc bath is to improve the coating quality and reduce the consumption of zinc. The synergistic effect of alloying additives allows to reduce the amount of zinc ash, reduce the reactivity of steel and improve the processability of the bath. A modern factor bath contains Al, Ni and a group of metals that improve the fluidity of the zinc bath: Pb, Bi and Sn (Ref 6).

The addition of Al protects the surface of the zinc bath against excessive oxidation. Due to the higher chemical affinity of aluminum for oxygen than for zinc, a continuous Al2O3 barrier layer is formed on the surface of the bath, which protects the surface of the bath against further oxidation. This reduces the amount of zinc ashes. Already at a content of 0.005 wt.% Al in the bath, it is effectively protected against oxidation (Ref 7). Nowadays, the addition of Al is used as standard, which contributes to the reduction in the zinc consumption. The Al content in the bath should not exceed 0.01 wt.% (Ref 8) because its reaction with the flux produces toxic smoke containing AlCl3. Moreover, the moisture contained in flux creates conditions for Al2O3 formation on the steel surface, which causes discontinuities in coating (Ref 9).

The addition of Ni allows to limit the reactivity of the steel caused by the Si content in the Sandelin range (0.03-0.12 wt.% Si). The noticeable effect of the Ni content in the bath is achieved by keeping the Ni content at 0.04 wt.% up to 0.06 wt.% (Ref 10). Below 0.04 wt.% Ni in the bath, its action is ineffective. At higher nickel contents, spherical Γ2 phase (Fe6Ni5Zn89) precipitates may appear. According to the Fe-Ni-Zn equilibrium system, the Γ2 phase is stable at Ni content above 0.06 wt.% (Ref 10). Dissolution of the ζ phase at the interface with liquid zinc leads to supersaturation of this area with iron. As a result, with the content of Ni in the bath above 0.06 wt.% particles of the Γ2 phase is precipitated. These particles, called "floating dross," are formed in the liquid bath in the immediate vicinity of the diffusion layer of the coating. As a result, they are easily torn out when the product is withdrawn from the bath, causing defects on the surface of the coating and increasing the amount of zinc drawn from the bath. The formation of Γ2 phase particles also consumes nickel for their creation (Ref 10, 11). Nickel, although it is a permanent additive, requires strict control of the content in the bath and refilling to the optimal level.

Lead, bismuth and tin are the third group of alloying additives, used interchangeably to improve the drainage of liquid zinc from the surface of products. Lead lowers the surface tension of liquid zinc and improves the fluidity of the bath. The zinc bath has the best fluidity with a Pb content in the range of 0.4-0.5 wt.% (Ref 12). However, lead is harmful to human health and the environment. The use of this additive in a zinc bath is limited in many cases (Ref 13). Accordingly, Pb is increasingly removed from the bath. Lead is replaced by bismuth and tin which are not harmful to the environment. The addition of 0.1 wt.% Bi in the bath causes a similar intensity of liquid zinc runoff from the surface of the product as approx. 1 wt.% Pb (Ref 14). Its required content in the bath is therefore 10 times lower compared to the addition of Pb. Its concentration in the bath is optimal at the level of 0.05-0.1 wt.%. In order to intensify the influence of Bi in the bath, Sn (Ref 15) is often used as the additive. The optimal content of this additive is not specified, and the standard EN ISO 1461 (Ref 5) excludes this metal from limiting the total content of alloying additives in the bath of not more than 1.5 wt.%.

In addition to their beneficial effects, the additions of Pb, Bi and Sn also cause unfavorable phenomena. Bi (Ref 16) and Sn (Ref 17) may promote the occurrence of the phenomenon of liquid metal-assisted cracking (LMAC), which leads to cracking of the material in the liquid zinc. It is believed that these metals, due to limited solubility in liquid zinc and lack of solubility in Fe-Zn intermetallic phases, concentrate at the solid phase—liquid zinc boundary. Their presence in the liquid state favors grain boundary penetration and the accumulation of large amounts of these metals at the crack tip (Ref 18, 19). Numerous studies have shown that Pb, Bi and Sn, as well as alloys of these metals in combination with steel, show a high tendency to the occurrence of the LMAC phenomenon (Ref 20,21,22,23,24). The German Directive DASt-Richtlinie 022 (Ref 25) thus limits the Sn content to 0.1 wt.% and (Pb + 10Bi) up to 1.5 wt.%. These restrictions, although not obligatory in other countries, are applied in many galvanizing plants in Europe.

4 Influence of Pb, Bi and Sn Additives on the Quality of the Coating

4.1 The Appearance of the Coating

The appearance of zinc coating depends on the crystallization conditions of the outer layer of the coating. This layer, the solution of Fe in Zn-η, is in fact the alloy layer of the galvanizing bath. The chemical composition of the galvanizing bath affects the conditions of crystallization of the outer layer. The presence of Pb, Bi and Sn additives in the bath changes the crystallization conditions of the outer layer, which results in the characteristic spangle appearance. Figure 1 shows appearance of zinc coating obtained during air cooling of steel plates (100 × 50 × 2 mm) in a pure zinc bath and baths containing optimal contents of Pb (0.4 wt.%), Bi (0.05 wt.%), Sn (0.3 wt.%) and Bi + Sn (0.005 and 0.3 wt.%, respectively).

Fig. 1
figure 1

Photographs of external appearance of the zinc coating obtained in: (a) zinc bath and in the bath with the addition of: (b) Pb (0.4 wt.%), (c) Bi (0.05 wt.%), (d) Sn (0.3 wt.%) and (e) Bi + Sn (0.05 wt.%, 0.3 wt.%); low-silicon steel, temperature 450 °C, time 3 min

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Spangles are formed when the liquid zinc reaches a temperature of 419 °C, followed by nucleation and crystal growth. This leads to the exposure of a coarse-grained structure on the surface of the coating and, under favorable conditions, a dendritic one. Under the same cooling conditions in the bath containing Bi (Fig. 1c), a particularly large spangle is formed, in which the presence of Sn can reveal a distinct dendritic structure (Fig. 1e). On the surface of the coating obtained in the bath containing Pb (Fig. 1b) and Sn (Fig. 1d), a more regular coarse-grained structure is observed, although the effect of Sn addition is less intense and the obtained structure is finer.

The presence of lead reduces the interfacial energy at the solid/liquid interface during solidification of the outer coating layer (Ref 26, 27). This increases the energy barrier to heterogeneous nucleation. Moreover, lead is insoluble in solid zinc (Ref 28). This causes its precipitation during solidification at the grain boundaries and inside them, on the coating surface and on the cross section of the outer layer (Fig. 2). Inhibition of nucleation due to the presence of lead increases spaces between adjacent nucleation, enabling formation of larger grains. Bi and Sn are expected to have a similar effect, albeit with different intensity. Bi (Ref 29) and Sn (Ref 30) show no solubility in solid state in Zn. Solidification of the outer layer of "pure" zinc occurs under conditions of heterogeneous nucleation. Therefore, this coating has a particularly fine-grained structure that is barely visible to naked eye (Fig. 1a).

Fig. 2
figure 2

SEM images and EDS mapping: (a) on the surface and (b) on the cross section of the coating obtained in the Zn + 0.4Pb bath; low-silicon steel, temperature 450 °C, time 12 min

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Figure 3 compares the differences in the appearance and drainage of liquid zinc containing the Pb, Bi and Sn additions observed on the surface of a 170 × 100 × 2 mm galvanized steel mesh with a mesh size of 15 × 6 mm. The addition of 0.4 wt.% Pb (Fig. 3b), 0.05 wt.% Bi (Fig. 3c) and 0.3 wt.% Sn (Fig. 3d) gives a similar intensity of liquid zinc drainage from the mesh surface and is much better compared to the "pure" zinc bath (Fig. 3a). The best liquid zinc drainage is demonstrated by a coating obtained in a bath containing the total addition of Bi + Sn (Fig. 3e).

Fig. 3
figure 3

Photographs of external appearance of the steel mesh galvanized in: (a) "pure" zinc bath, and the zinc bath with the addition of: (b) Pb (0.4 wt.%), (c) Bi ( 0.05 wt.%), (d) Sn (0.3 wt.%) and (e) Bi + Sn (0.05 wt.% + 0.3 wt.%); low-silicon steel, temperature 450 °C, time 3 min

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Visual observations of the intensity of liquid zinc drainage from the surface of the meshes are presented in the graph of the average mass gain of the meshes after galvanizing (Fig. 4). Average mass gain represents the amount of zinc that is removed from the bath on the mesh surface. Additions of Pb, Bi and Sn reduce the amount of zinc drawn on the surface of the product from the bath and it is less sensitive to temperature changes in the range used in the industrial process from 440 to 460 °C. This keeps the bath temperature at the lower end of the range, which is beneficial in terms of extending the life of the galvanizing kettle. At the same time, the products are easier to clean and do not have stains and lumps. This allows to shorten the time of product finishing and reduce zinc losses caused by removing zinc stains. Therefore, the presence of one of these metals in the bath is desirable and improves the economic performance of the galvanizing process.

Fig. 4
figure 4

Average mass gain of the mesh after galvanizing in baths with various alloy additives

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Pb, Bi and Sn lower the surface tension of liquid zinc. This results in better zinc drainage from the surface of the product. The surface tension of pure zinc at 435 °C is 770 dynes/cm, while the surface tension of a zinc bath with 0.4 wt.% Pb drops to 600 dynes/cm (Ref 31). Also the addition of 0.05-0.1 wt.% Bi in the bath reduces the surface tension of the liquid zinc alloy at 450 °C from about 750 dynes/cm to 620-650 dynes/cm (Ref 15). Thus, a similar effect of lowering the surface tension can be obtained with the concentration of lead in the bath at the level of 0.4-0.5 wt.%. The lower the surface tension, the faster the liquid zinc flows and the easier it can be removed before it crystallizes on the surface of the product.

4.2 Thickness of Coatings

The growth kinetics of coatings obtained on low-silicon steel in industrial baths containing optimal contents of Pb, Bi and Sn as well as Al and Ni additives are shown in Fig. 5. Additions of Pb, Bi and Sn reduce the thickness of the coatings. In the scope of the immersion time from 3 to 12 min, in which the industrial galvanizing process is carried out, it is possible to obtain coatings with a thickness of 45-80 μm, meeting the requirements of EN ISO 1461 (Ref 5). The course of the growth kinetics is close to parabolic, and the additions of Pb, Bi and Sn clearly reduce the intensity of the coating thickness increase. In such conditions, control of the coating thickness is easier.

Fig. 5
figure 5

Kinetics of growth of coatings obtained in industrial baths with Pb, Bi and Sn additions; temperature 450 °C

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The influence of these additives on the thickness of the coatings is confirmed by tests of coatings obtained on low-silicon steel in baths containing only the addition of Pb or Bi or Sn in a different concentration range of these metals (Ref 32,33,34,35). Figure 6 compares the thickness of the coatings obtained in baths containing these metals in the concentration ranges that have been in the past or are currently used in industrial practice. As can be seen, the coatings obtained in the baths containing Pb, Bi and Sn showed a lower thickness than the coating obtained in the "pure" Zn bath. Increasing the lead content to 1.2 wt.%, which is the saturation state in liquid zinc and has been widely used in industrial processes in the past, does not significantly alter the thickness of the coating. Increasing the content of Bi in the bath to 0.3 wt.% and Sn to 0.9 wt.% causes a decrease in the thickness of the coatings in proportion to the content of these metals in the bath.

Fig. 6
figure 6

Thickness of coatings obtained on low-silicon steel in Zn baths with different contents of Pb, Bi and Sn; temperature 450 °C, immersion time 3 min

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Lead, bismuth and tin mainly affect the formation of the outer layer of the coating. Sebisty and Edwards (Ref 36) showed that lead has no effect on the growth of the diffusion layer of the coating obtained on low-carbon steel. According to Reumont and Perrot (Ref 37), the addition of Pb does not change the phase composition of the coatings or the morphology and growth kinetics of the layers of intermetallic phases Γ, δ1 and ζ forming the diffusion layer of the coatings. Pistofidis et al. (Ref 38) showed that the addition of 2 wt.% Bi also does not change the phase composition of the diffusion layer and the morphology of the Γ, δ1 and ζ phases. Although Wang et al. (Ref 39) observed an increase in the layer thickness of the δ1 + ζ phase when the Bi content in the bath was 0.5 wt.%. In turn, Katiforis et al. (Ref 40) argue the addition of tin to 3 wt.% also does not affect the structure of the diffusion layer of the coating obtained on low-silicon steel. However, studies have shown that Bi and Sn can reduce the reactivity of steels containing Si in liquid zinc. Avettand-Fènoël et al. (Ref 41) observed that the content of 1-5 wt.% Sn reduces the thickness of the diffusion layer of coatings on Sandelin's and hyper-Sandelin's steel. Gilles et al. (Ref 42) noticed that reducing the influence of Si in steel on the growth of the diffusion layer is possible with the content of at least 2.5 wt.% Sn in the bath. The limitation of the growth of the diffusion layer on reactive steels is explained by the precipitation of Bi and Sn due to their lack of solubility in the intermetallic phases Γ, δ1 and ζ. These metals, with their high content in the bath, form a thin barrier layer on the boundary of the ζ phase layer with the Zn bath, which hinders the diffusion of iron and zinc (Ref 43). Fratesi et al. (Ref 44), however, claim that the addition of Bi in the bath has no effect on the growth of the coating on low-silicon steels.

The decrease in the thickness of the coating on low-silicon steel, observed in Fig. 5 and 6, should therefore be associated with a decrease in the thickness of the outer layer of the coating, which confirms the beneficial effect of these metals on improving the removal of liquid zinc from the surface of the product.

4.3 Corrosion Resistance

Corrosion parameters of coatings obtained in industrial baths containing Pb, Bi and Sn additives are presented in Table 1. The electrochemical parameters of the corrosion process in a solution of 3.5% NaCl of the coatings obtained in the baths containing Pb, Bi and Bi + Sn indicate an increase in the value of the corrosion current density (jcorr) and a slight shift of the corrosion potential (Ecorr) toward more negative values. The corrosion potential of the coatings obtained in the baths containing Pb, Bi and Bi + Sn additives are, respectively, − 777, − 779 and − 781 mV (vs. normal hydrogen electrode NHE). The negative potential of the coatings provides sacrificial protection of the steel; however, the lower potential of these coatings compared to the Zn-AlNi coating (− 768 mV) indicates a higher corrosion tendency of coatings obtained in baths containing Pb, Bi and Bi + Sn.

Table 1 Corrosion parameters of coatings obtained in industrial baths with Pb, Bi and Sn additions (electrochemical parameters (Ecorr) and (jcorr) obtained in 3.5% NaCl, NSS according to EN ISO 9227)
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The values (jcorr) are, respectively, 17.90 µA/cm2 for the coating obtained in the Zn-AlNiPb bath, 14.26 µA/cm2 for the coating obtained in the Zn-AlNiBi bath and 18.84 µA/cm2 for the coating obtained in the Zn-AlNiBiSn bath. They are higher than the value (jcorr) of the coating obtained in the Zn-AlNi bath (6.24 µA/cm2) (Table 1). Corrosion current density (jcorr) characterizes the amount of mass loss according to Faraday's law, so its higher value indicates a reduction in corrosion resistance.

Corrosion tests of coatings in neutral salt spray (NSS) test showed a continuous increase in their weight. After 1000 h exposure in a salt spray chamber without removing corrosion products, the coatings showed a final weight gain of 157.42 g/m2 for the coating obtained in the Zn-AlNiPb bath, 140.34 g/m2 for the coating obtained in the Zn-AlNiBi bath and 170.38 g/m2 for the coating obtained in the Zn-AlNiBiSn bath. In this corrosion test, the Zn-AlNi coating (96.09 g/m2) also showed the smallest weight gain. It can therefore be concluded that in the tested industrial baths, the additions of Pb, Bi and Sn are responsible for the reduction in the corrosion resistance.

In order to confirm the influence of Pb, Bi and Sn in the bath on the corrosion resistance of coatings, corrosion tests of coatings obtained in baths containing only the addition of Pb or Bi or Sn were carried out. Baths with a content of up to 1.2 wt.% Pb, up to 0.3 wt.% Bi and up to 0.9 wt.% Sn were selected for the tests. The coatings were produced at the temperature of 450 °C and the immersion time of 3 min. The NSS test was carried out in accordance with EN ISO 9227 (Ref 45) in a mist of 5% NaCl solution. During exposure in a salt chamber, corrosion products were removed from the surface of the samples in accordance with ISO 8407. The samples were immersed in a solution of ammonium chloride (NH4Cl) in distilled water (100 g/dm3) for 5 min at the temperature of 70 °C. After the samples were removed from the solution, they were dried. Corrosion products were removed in accordance with EN ISO 9227 after 24, 48, 96, 164, 240, 720 and 1000 h of exposure. Unit weight loss of samples after the end of NSS test is summarized in Fig. 7. On the basis of the obtained test results, it can be concluded that the presence of Pb, Bi and Sn additives in the bath causes greater corrosion losses of the coating compared to the coating obtained in the "pure" zinc bath. The unfavorable increase in the concentration of these additives, which reduces the corrosion resistance, is also clearly visible. Within the range of optimal concentrations of these metals, the lowest weight loss was noted for the addition of 0.05 wt.% Bi—88.15 ± 5.37 g/m2, while for the addition of 0.4 wt.% Pb it is 106.81 ± 6.14 g/m2, and for 0.1 wt. % Sn, it is 98.21 ± 5.34 g/m2. However, it can be expected that the addition of Bi has the strongest effect on corrosion losses. With its content in the bath of 0.3 wt.% Bi, the unit weight loss (154.10 ± 7.15 g/m2) was greater than that observed with much higher additions of Pb (1.2Pb-133.11 ± 6.62 g/m2) and Sn (0.9Sn—146.09 ± 6.29 g/m2) (Ref 45, 46).

Fig. 7
figure 7

Unit weight loss of coatings obtained in baths with different Pb, Bi and Sn content after 1000 h; NSS test

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The reduction in the corrosion resistance of the coatings may be explained by the structure of the coatings obtained in baths containing Pb, Bi and Sn additives. As shown in Fig. 2, lead is located in the coating in the form of precipitates on the surface of the coating and in the outer layer. The presence of precipitates in the zinc matrix is of decisive importance for the corrosive action. Contact between two metals that differ in electrode potential: Pb (E°(Pb2+/Pb) = − 0.1262 V vs. standard hydrogen electrode SHE) and Zn (E°(Zn2+/Zn) = − 0.7618 V vs. SHE) (Ref 47) leads to the formation of corrosion cells in which the Zn matrix is the anode and is dissolved, while the precipitates containing Pb are in a rather passive state. The bath with the addition of Pb, however, is of less and less importance and is being withdrawn from industrial use due to the toxicity of lead. Its place is taken by the more and more commonly used baths with the addition of Bi and Bi + Sn. Figure 8 shows the microstructure of the coating obtained in a bath with the addition of Bi and the addition of Bi + Sn. As the cross section of the coating shows, Bi also locates in the form of precipitates in the outer layer (Fig. 8a). If, on the other hand, tin is also added to the bath together with Bi, precipitates containing both Bi and Sn are formed in the coating. An example of the precipitates on the surface of a coating containing Bi and Sn is shown in Fig. 8b. Bi and Sn also show higher electrode potentials than the zinc matrix (E°(Bi3+/Bi) = 0,308 V, E°(Bi+/Bi) = 0,5 V, E°(Sn+2/Sn) = -0.1375 V, vs. SHE) (Ref 47). Hence, a similar corrosive effect of these metals and an unfavorable effect on the corrosion resistance of the coatings can be expected. The particularly positive Bi potential may explain its intense influence on the corrosion losses of the coating at its much lower concentration in the bath compared to the addition of Pb and Sn. The consequence of the presence of more cathodic precipitates is the use of zinc renewal for the sacrificial protection of the precipitates themselves. It can also be concluded that the presence of precipitates on the surface of the coatings accelerates the corrosion processes already in the initial period of the coating`s operation. In addition, Pb, Bi and Sn reduce the thickness of the coating (Fig. 6). Reducing the thickness allows to meet the requirements of EN ISO 1461, but it shortens the protective effect of the coating.

Fig. 8
figure 8

SEM images and EDS analysis of coatings obtained in the bath containing Bi and Sn: (a) Bi precipitation in the outer layer of the coating (Zn-0.05Bi bath) and (b) Bi + Sn precipitation on the coating surface (Zn-0.05Bi0.3Sn bath); low-silicon steel, temperature 450 °C, time 12 min

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5 Conclusions

The instability of the zinc price and the increasing costs of producing zinc coatings determine the continuous reduction in the coating production costs while maintaining its high quality. The hot dip galvanizing process generates the highest losses of zinc among all manufacturing processes that process zinc and its alloys. The technological process produces a significant amount of zinc-rich waste (hard zinc, zinc ash). At the same time, the consumption of zinc is determined by its amount, which is discharged from the bath on the surface of the product. This is due to the production of excessively thick coatings on reactive steels, insufficient drainage of zinc from the surface of the product and the need to remove zinc drips from the finished product. The technological properties of the zinc bath are improved by the Pb, Bi and Sn alloy additions. They affect the appearance of the outer coating layer by creating a spangle effect on the surface and additionally improve the drainage of liquid zinc from the surface of the product. Finishing of products after galvanizing in baths containing these alloying additives is easier. The obtained coatings are characterized by a lower thickness, and the kinetics of coating growth allows for easier control of their thickness. For this reason, the presence of the additives Pb, Bi and Sn in the bath is beneficial and helps to reduce unjustified losses of zinc.

When selecting alloy additives, the negative effects of their impact should also be taken into account. Therefore, it is reasonable to eliminate the addition of lead from the bath due to its toxicity. An alternative to lead is currently the addition of Bi and Sn. However, the risk of their negative impact on the LMAC phenomenon, as well as on the corrosion resistance of coatings, requires minimizing their content. Content not exceeding 0.05 wt.% Bi and 0.1 wt.% Sn should be considered optimal in this respect.