Hot-Dip Galvanizing Process and the Influence of Metallic Elements on Composite Coatings

Round 1

Reviewer 1 Report

I reviewed the manuscript entitled “Hot-Dip Galvanizing Process and Influence of Metallic Elements on the Composite Coating”. The work carried out in the manuscript is interesting However, in my opinion, several aspects should be modified or detailed more in-depth prior to publication, thus major modifications are advised. Here is a list of main comments:

Please consider all the comments below and highlight the changes.

- This work's innovation and importance are not clearly highlighted in the abstract, introduction, and conclusions. Please work on this and prove to us why this work is valuable.

- The abstract is well-written and provides a clear overview of the study. However, consider adding a brief sentence summarizing the key results at the beginning of the findings section to enhance clarity.

- The length of the paper is excessive. Rather than utilizing an excessive amount of textual content, it is strongly recommended to utilize tables to present the information. This format offers a clearer and more accessible presentation for readers, facilitating easier comprehension and review.

- The paper covers a wide range of topics related to hot-dip galvanizing, the organization of the content could be refined more. Consider structuring the paper into distinct sections with clear headings to facilitate easier navigation and comprehension for readers.

- The abstract mentions the incorporation of additional metallic elements into the zinc bath to form alloy coatings, but the subsequent sections primarily focus on the hot-dip galvanizing process itself. It would be better to provide a more in-depth analysis of the composition and performance of alloy coatings, as well as their effectiveness in enhancing corrosion resistance compared to pure zinc coatings.

- In the section discussing cooling techniques, could you elaborate on any potential drawbacks or limitations of water quenching compared to alternative cooling methods? Are there specific scenarios where alternative methods might be more suitable?

-Regarding passivation methods, can you provide more detailed insights into the mechanisms underlying both chromium and non-chromium passivation techniques? How do these mechanisms contribute to the effectiveness of each method in mitigating white rust formation?

-Could you provide additional information on the practical implementation of the research findings? For example, are there specific recommendations or guidelines for optimizing temperature, time, cooling techniques, and passivation methods in industrial settings based on the insights gained from this study?

-The quality of the figures needs to be improved. For instance, Figures 5e and 5f require enhancement.

Author Response

Response to reviewers

First, we deeply appreciate the editor and the reviewers for your hard work on our manuscript. From your comments, we learned a lot of new professional knowledge, experimental skills, etc. Below you will find our point-by-point response to reviewers’ comments (Responses are marked in blue, and sentences newly inserted into the manuscript and the Supplementary Information are marked in red):

*********************************************************************

Reviewer #1: I reviewed the manuscript entitled “Hot-Dip Galvanizing Process and Influence of Metallic Elements on the Composite Coating”. The work carried out in the manuscript is interesting However, in my opinion, several aspects should be modified or detailed more in-depth prior to publication, thus major modifications are advised. Here is a list of main comments:

Please consider all the comments below and highlight the changes.

RESPONSE: We greatly appreciate your recognition of our work, and also thanks so much for your insightful and constructive comments. It is our great pleasure to be favorably commented on by such a world-renowned expert in the field of hot-dip galvanizing. To address your comments and concerns, please see the point-by-point response attached below.

1. This work's innovation and importance are not clearly highlighted in the abstract, introduction, and conclusions. Please work on this and prove to us why this work is valuable.

RESPONSE: We express our gratitude to the reviewer for your constructive suggestions. In response to your recommendations, we underscore the innovation of this article and its significant value to personnel in the hot-dip galvanizing industry and researchers in this field in the abstract and conclusion sections.

The current reviews in the field of hot-dip galvanizing primarily addresses the types, properties and evaluation systems of coatings [e.g., Frontiers in Materials, 2020, 7: 1-19; e.g., Surface and Coatings Technology, 2015, 262: 210-215]. However, they lack a comprehensive discussion on the hot-dip galvanizing process. This article introduces specific issues in the current process in greater detail, offering effective theoretical and technical guidance for those entering the hot-dip galvanizing industry, and also suggests some viable research directions for researchers in this field. The content added to the abstract is as follows：

The objective is to offer a more comprehensive introduction to those new to the field of hot-dip galvanizing, and to provide theoretical insights for addressing production issues.

The content added to the conclusion and prospect is as follows：

In summary, this article offers both theoretical and technical insights to those involved in the hot-dip galvanizing industry, as well as potential research directions for those in the field. The latest research efforts aim to enhance the performance and cost-effectiveness of coatings, aiming to better meet the diverse requirements for coating materials across various sectors.

2. The abstract is well-written and provides a clear overview of the study. However, consider adding a brief sentence summarizing the key results at the beginning of the findings section to enhance clarity.

RESPONSE: Firstly, we express our gratitude to the reviewer for their affirmation of the abstract and for their suggestions for revising the conclusion section. Following your suggestion, we have revised the conclusion section by adding a summary of the main content of this article at the beginning, including an introduction to each link in the hot-dip galvanizing process, as well as common problems and corresponding solutions; In addition, a series of research progress and application of hot-dip galvanized alloys were introduced. The revised content in the conclusion section is as follows：

Hot dip galvanizing and its alloys are currently one of the most widely used metal anti-corrosion technologies. This article briefly summarizes its development history and introduces its main processes, including alkaline washing, acid washing, water washing, auxiliary plating, galvanizing, cooling, passivation, etc. At the same time, some problems in each process are pointed out and corresponding solutions are discussed. Finally, the effect of adding Al, Mg, Sn, Pb, Bi and rare earth metal elements in hot-dip galvanizing is introduced.

3. The length of the paper is excessive. Rather than utilizing an excessive amount of textual content, it is strongly recommended to utilize tables to present the information. This format offers a clearer and more accessible presentation for readers, facilitating easier comprehension and review.

RESPONSE: We offer our sincere thanks to the reviewer for your valuable and helpful suggestions. In response to your recommendations, we have condensed the abstract, Galvanizing section, conclusion, and other sections of this article, 2.5 Galvanizing section and 2.7 Passivating section are summarized in the form of tables, as shown in Table 1 and Table 2:

Table 1. The actual range of temperatures for hot-dip galvanizing.

Table 2. The chemical composition of the sol-gel studied.

4. The paper covers a wide range of topics related to hot-dip galvanizing, the organization of the content could be refined more. Consider structuring the paper into distinct sections with clear headings to facilitate easier navigation and comprehension for readers.

RESPONSE: We thank the reviewer for their constructive suggestions on the structure of this paper. We have divided the 2.2. Picking section in the manuscript into 2.2.1. Problems encountered in the picking process and 2.2.2. Methods for waste acid treatment for introduction; refine the 2.7. Passivation section into 2.7.1. Chromate passion and 2.7.2. Non-chrome passion, which are introduced sequentially.

5. The abstract mentions the incorporation of additional metallic elements into the zinc bath to form alloy coatings, but the subsequent sections primarily focus on the hot-dip galvanizing process itself. It would be better to provide a more in-depth analysis of the composition and performance of alloy coatings, as well as their effectiveness in enhancing corrosion resistance compared to pure zinc coatings.

RESPONSE: Thank you for the constructive suggestions from the reviewer. In response to your suggestions, when introducing alloy coatings, we first introduced the properties of the alloy elements themselves and compared the alloy coatings with pure zinc coatings. The specific content is as follows:

(1) 4.2. Effect of Adding Mg element: Magnesium (Mg) appears in silver-white and belongs to the hexagonal crystal system. Its relative atomic mass is 24.34 and its melting point is 650 ℃.

Concurrently, it causes the surface texture of the galvanized layer to become notably thick and rough, leading to a milky-white appearance and diminished adhesion.

(2) 4.3. Effect of adding Sn element: Tin (Sn) has a silver-gray hue, belongs to the tetragonal crystal system, has a relative atomic mass of 118.7, and a melting point of 231.84 ℃. It is a low-melting-point metal. The addition of an appropriate amount of tin to the hot-dip galvanizing bath can significantly improve the surface condition of the coating, enhance the zinc effect, and make the surface smoother. In actual production, it was observed that the addition of 5% tin to the zinc solution can inhibit the ultra-thick growth of the coating on high silicon (mass fraction greater than 0.3%) active steel. The delta (δ) layer in the galvanized layer of high silicon steel becomes thicker and more compact, while the zeta (ζ) layer significantly thins and shifts from a loose blocky structure to aligned columnar crystals. The thickness of the iron-zinc alloy layer in the resulting coating is only about 60 μm after 3-5 minutes of immersion in zinc, which can reduce by 20% compared to the case without tin addition.

6. In the section discussing cooling techniques, could you elaborate on any potential drawbacks or limitations of water quenching compared to alternative cooling methods? Are there specific scenarios where alternative methods might be more suitable?

RESPONSE: Thank you for your insightful comment. The modified content is as follows: During the water cooling process of hot-dip galvanizing, the primary task is to maintain the cooling water temperature at 50-80 ℃ in order to minimize the temperature difference between the galvanized part and the cooling water. This helps prevent uneven contraction due to the difference in expansion coefficients between zinc and iron, which can lead to cracking and ultimately affect the quality and corrosion resistance of the galvanized layer. Once the workpiece is immersed in the water, it should be cooled rapidly through shaking and swinging. In particular, long and tubular parts should be lifted at an angle to prevent deformation. After cooling, it is essential to thoroughly remove all surface and internal water from the workpiece to avoid carrying moisture into the next passivation process.

Besides the water-cooling method, there exists an industrial air-cooling method in which galvanized steel is placed in the air to naturally cool down. Despite the similarity in the phase composition of coatings achieved through both air and water cooling, the water-cooling method suffers from significant issues of water resource wastage and large space requirements compared to air cooling. On the other hand, the air-cooling method typically maintains the relative integrity of the coating, whereas water cooling method tends to result in fragmentation of the outer layer structure. Nevertheless, it's noteworthy that during the uncontrolled cooling process in the air, dendrite growth and thickening of the interdendritic regions occur, leading to an increase in coating thickness by approximately 25%.

In practical industrial production, in order to enhance production efficiency, it is typically required that galvanized steel can be rapidly cooled to facilitate timely subsequent processing and storage. In this crucial step, the water-cooling method stands out due to its unique advantages, which are unparalleled compared to air-cooling. Consequently, in actual production processes, most enterprises still prefer to adopt the water-cooling method to achieve rapid cooling of steel.

7. Regarding passivation methods, can you provide more detailed insights into the mechanisms underlying both chromium and non-chromium passivation techniques? How do these mechanisms contribute to the effectiveness of each method in mitigating white rust formation?

RESPONSE: We express our gratitude to the reviewer for raising highly professional questions. In 2.7 Passivating section, we have included detailed mechanisms for both chromium-containing and chromium-free passivating in mitigating the formation of white rust. Detailed supplementary content is provided below:

Although there has been a large amount of research on traditional chromate conversion coatings and many valuable insights have been proposed, the detailed process and mechanism of their inhibition of corrosion in galvanized coatings are not yet fully understood, and only the basic core reactions can be summarized. Currently, scholars generally believe that the formation of chromate conversion coatings is actually an oxidation-reduction reaction between Cr (VI) ions and the base metal (Zn). This reaction process can be briefly described as follows: first, the zinc on the coating surface dissolves into an acidic solution containing Cr (VI), and then undergoes an oxidation-reduction reaction with Cr (VI). Cr (VI) is reduced to Cr (III), forming a precipitate attached to the coating surface.

Current research suggests that the precipitation in chromate coatings is a decisive factor in inhibiting zinc layer corrosion and reducing white rust formation. While soluble components (including Cr (VI)) are considered to only provide supplementary protection. Even if soluble Cr (VI) has completely detached from chromate conversion coatings, the overall corrosion resistance can still be maintained at a high level. However, another group of researchers hold different opinions, as they found that soluble Cr (VI) can move to scratches and other local damage areas of the coating, and bring effective re passivation, which can also be demonstrated by the self-healing performance of the chromate conversion film. Therefore, they believe that soluble Cr (VI) plays an important role in anti-corrosion, and the anti-corrosion effect actually depends on the content of Cr (VI) remaining in the coating (or adsorbed on the coating) after chromate treatment.

The current chromium-free passivation technology primarily involves inorganic salts and organic compounds. Among inorganic salts, molybdate is the most extensively studied, as molybdenum and chromium are both members of Group VIA in the periodic table. Its compounds can form a molybdate passivation film with zinc oxides, thereby reducing the formation of white rust. Phytic acid, which has been extensively studied in organic compounds, is an effective metal integrator. When it complexes with metals, it forms multiple integrating rings, ensuring the stability of the complexes over a wide pH range. A dense, single-molecule organic protective film formed by these complexes can effectively prevent O₂ and other substances from entering the metal surface, thereby improving anti-corrosion performance and reducing the occurrence of white rust.

8. Could you provide additional information on the practical implementation of the research findings? For example, are there specific recommendations or guidelines for optimizing temperature, time, cooling techniques, and passivation methods in industrial settings based on the insights gained from this study?

RESPONSE: We express our gratitude to the reviewer for posing a question of significant practical value. In practical industrial production, temperature and time are critical factors influencing the quality of the final coating. The following provides specific optimization recommendations for these factors:

(1) Galvanizing: The optimal temperature range fo r hot-dip galvanizing is 430~460 ℃, and exceeding 475 ℃ will lead to the rapid formation of undesirable iron-zinc intermetallic compounds. As the temperature of the zinc melt varies, the thickness ratio of each phase layer also changes. Therefore, in actual galvanizing production, a temperature of 440~465 ℃ is often used. Within this range, the iron dissolution of steel parts and zinc pots is relatively low, which helps to reduce the production of zinc slag.

(2) Water Cooling: During the water cooling process of hot-dip galvanizing, the primary task is to maintain the cooling water temperature at 50-80 ℃ in order to minimize the temperature difference between the galvanized part and the cooling water. This helps prevent uneven contraction due to the difference in expansion coefficients between zinc and iron, which can lead to cracking and ultimately affect the quality and corrosion resistance of the galvanized layer. Once the workpiece is immersed in the water, it should be cooled rapidly through shaking and swinging. In particular, long and tubular parts should be lifted at an angle to prevent deformation. After cooling, it is essential to thoroughly remove all surface and internal water from the workpiece to avoid carrying moisture into the next passivation process.

(3) Passivation: The workpiece should be slowly immersed in a passivation solution at 20-50 ℃. Workers should carefully place the workpiece in the center of the passivation tank to avoid collision with the tank wall. If necessary, hooks should be used to fix the workpiece to prevent shaking. Soak the workpiece in the passivation tank for about 2 minutes, and after the surface is fully passivated, remove it to ensure that the passivation solution flows back into the tank and does not drip onto the ground. Subsequently, the workpiece will be lifted to the trimming process and finely trimmed by professional operators.

9. The quality of the figures needs to be improved. For instance, Figures 5e and 5f require enhancement.

RESPONSE: Thank you for the reviewer's suggestions on the images. We have revised Figures 5e and 5f and improved the overall quality of other images in the manuscript. The new images are shown below:

Figure 1. Main process flow for hot-dip galvanizing technique

Figure 2. EIS analysis of the various hot-dip galvanized samples in 5 wt.% NaCl: (a) Nyquist and (b)Bode plots (Reproduced with permission from Ref. [67]); (c) Nyquist and  (d) Bode plots after 72 h immersion of PFGO-0.1 and PFmGO-0.1 versus immersion time. (Reproduced with permission from Ref. [68]); SEM images of sol–gel coating A after (e) 30 hours and (f) 36 hours under salt spray conditions. The micrograph of (g) sol–gel coating B and (h) sol–gel coating C with cross-sections after 24 hours of exposure to the salt spray test (Reproduced with permission from Ref. [70]).

Figure 3. (a) Nyquist plot and (b) Bode moduli of coating samples (Reproduced with permission from Ref. [71]); (c) Changes in the thickness of the reaction layer with and without ultrasonic vibration during hot-dip immersion as a function of time and (d) scanning electron micrograph (SEM) showing the reaction layer at the interface after hot-dip immersion with ultrasonic vibration for 10 s (Reproduced with permission from Ref. [73]); (e) Potential dynamic polarization curves of original and hot-dip aluminizing annealed sample and (f) SEM of the hot-dip aluminizing annealed sample (Reproduced with permission from Ref. [74]).

Figure 4. Development history of hot-dip galvanizing alloys

Figure 5. (a) Microhardness comparison of eta phase in coatings at 440 °C; (b) Tafel plots comparing coatings at 440 °C; (b) Tafel plots of different coatings made at 440 °C for comparison (Reproduced with permission from Ref. [79]); (c) Corroded Zn-55Al-1.6Si-1.5Mg alloy surface morphology after dissolving corrosion products (d) EIS-Nyquist plots of zinc alloy coated steels in 3.5 wt.% NaCl; (Reproduced with permission from Ref. [84]); Low-mag SEM images of Zn–Al–Mg alloy coatings with (e) 1 & (f) 5 wt.% Mg, cleaned for 30 s. (Reproduced with permission from Ref. [96]).

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript studies the hot-dip galvanizing process and the influence of metallic elements on the composite coating. The manuscript has serious flaws and should be extensively revised:

1- The English language of the manuscript should be revised with the help of an English native speaker.

2- The introduction and the state-of-the-art were not described well. Please improve your introduction and mention the other results and findings. Please also explain the other Zn coating and compositions by citing the following reference: 

https://doi.org/10.1016/j.elecom.2021.107169

3- The quality of the graphs is not good. Please replace them with the higher quality ones.

4- The basics of the corrosion tests and why the authors use mainly EIS were not explained clearly. Please improve your manuscript in this sense.

5- The texts in Fig. 5 are not clear and also there are too much. Please check the other papers in the journal and fix it. 

6- The conclusion section is very long and messy. Please make it shorter and write only the central findings of the manuscript.

The English language of the manuscript should be revised with the help of an English native speaker.

Author Response

Response to reviewers

First, we deeply appreciate the editor and the reviewers for your hard work on our manuscript. From your comments, we learned a lot of new professional knowledge, experimental skills, etc. Below you will find our point-by-point response to reviewers’ comments (Responses are marked in blue, and sentences newly inserted into the manuscript and the Supplementary Information are marked in red):

*********************************************************************

Reviewer #2: The manuscript studies the hot-dip galvanizing process and the influence of metallic elements on the composite coating. The manuscript has serious flaws and should be extensively revised:

1. The English language of the manuscript should be revised with the help of an English native speaker.

RESPONSE: We express our gratitude to the reviewer for your constructive suggestions. In response to your recommendations, we have asked colleague who speaks English to correct the details in this manuscript and highlighted the modifications.

2. The introduction and the state-of-the-art were not described well. Please improve your introduction and mention the other results and findings. Please also explain the other Zn coating and compositions by citing the following reference: https://doi.org/10.1016/j.elecom.2021.107169.

RESPONSE: Thank you for the valuable comments provided by the reviewer. We have revised the introduction section to incorporate some other latest achievements and list some components of zinc coatings and other coatings. The revised content is as follows:

To protect steel and extend its service life, the industry has developed various anti-corrosion technologies, including electroplating, painting, thermal spraying, surface chemical treatment, and physical vapor deposition. They mainly form different types of coatings on the surface of steel, such as Zn based alloys (ZnNi, Zn-Al, Zn-Al-Mg), Polymers (epoxy resin, polyurethane), etc. to protect the substrate and extend its service life.

3. The quality of the graphs is not good. Please replace them with the higher quality ones.

RESPONSE: Thank you for the reviewer's suggestions on the images. We have improved the overall quality of the images in the manuscript, and the new images is shown below:

Figure 1. Main process flow for hot-dip galvanizing technique

Figure 2. EIS analysis of the various hot-dip galvanized samples in 5 wt.% NaCl: (a) Nyquist and (b)Bode plots (Reproduced with permission from Ref. [67]); (c) Nyquist and  (d) Bode plots after 72 h immersion of PFGO-0.1 and PFmGO-0.1 versus immersion time. (Reproduced with permission from Ref. [68]); SEM images of sol–gel coating A after (e) 30 hours and (f) 36 hours under salt spray conditions. The micrograph of (g) sol–gel coating B and (h) sol–gel coating C with cross-sections after 24 hours of exposure to the salt spray test (Reproduced with permission from Ref. [70]).

Figure 3. (a) Nyquist plot and (b) Bode moduli of coating samples (Reproduced with permission from Ref. [71]); (c) Changes in the thickness of the reaction layer with and without ultrasonic vibration during hot-dip immersion as a function of time and (d) scanning electron micrograph (SEM) showing the reaction layer at the interface after hot-dip immersion with ultrasonic vibration for 10 s (Reproduced with permission from Ref. [73]); (e) Potential dynamic polarization curves of original and hot-dip aluminizing annealed sample and (f) SEM of the hot-dip aluminizing annealed sample (Reproduced with permission from Ref. [74]).

Figure 4. Development history of hot-dip galvanizing alloys

Figure 5. (a) Comparison of microhardness of eta phase in different coatings at 440 °C; (b) Tafel plots of different coatings made at 440 °C for comparison (Reproduced with permission from Ref. [79]); (c) Morphologies of the corroded coating surfaces of Zn-55Al-1.6Si-1.5Mg alloy after dissolving the corrosion products with 10 wt.% ammonium persulphate solution (d) EIS-Nyquist plots of different zinc alloy coated steels in 3.5 wt.% sodium chloride solution.; (Reproduced with permission from Ref. [84]); Low–magnification SEM micrographs of the reaction layer of the Zn–Al–Mg alloy coatings with (e) 1; (f) 5 wt.% Mg, obtained aftercleaning for 30 s. (Reproduced with permission from Ref. [96]).

4. The basics of the corrosion tests and why the authors use mainly EIS were not explained clearly. Please improve your manuscript in this sense.

RESPONSE: Thank you to the reviewer for posing such a professional question. In response to your suggestion, we have introduced the corrosion test：

Before delving into the research on passivation, it is necessary to first introduce some corrosion testing methods. There are various evaluation methods for the corrosion resistance of metal surface coatings, including routine testing, salt spray testing, damp heat testing, and electrochemical testing. Among them, electrochemical impedance spectroscopy (EIS) stands out as a particularly prominent method. It employs small-amplitude sinusoidal wave perturbation signals, exerting minimal impact on the system. EIS can accurately and rapidly obtain information such as coating capacitance and resistance, as well as the capacitance and resistance at the interface between the coating and the substrate, reflecting the corrosion status of the metal under the coating precisely. Furthermore, its broad measurement frequency range allows for obtaining more kinetic and electrode interface structural information than other conventional electrochemical methods. Therefore, EIS is a primary electrochemical method for evaluating the corrosion resistance of coatings.

5. The texts in Fig. 5 are not clear and also there are too much. Please check the other papers in the journal and fix it.

RESPONSE: Thank you for your insightful comment. We have made revisions to the content and annotations of Figure 5 in this manuscript, taking into reference other journal articles. The revised content is as follows:

Figure 5. (a) Microhardness comparison of eta phase in coatings at 440 °C; (b) Tafel plots comparing coatings at 440 °C; (b) Tafel plots of different coatings made at 440 °C for comparison (Reproduced with permission from Ref. [79]); (c) Corroded Zn-55Al-1.6Si-1.5Mg alloy surface morphology after dissolving corrosion products (d) EIS-Nyquist plots of zinc alloy coated steels in 3.5 wt.% NaCl; (Reproduced with permission from Ref. [84]); Low-mag SEM images of Zn–Al–Mg alloy coatings with (e) 1 & (f) 5 wt.% Mg, cleaned for 30 s. (Reproduced with permission from Ref. [96]).

6. The conclusion section is very long and messy. Please make it shorter and write only the central findings of the manuscript.

RESPONSE: We appreciate the constructive comments from the reviewer. In response to the reviewer's recommendations, we have refined the conclusion section of the manuscript. The revised conclusion and prospect is as follows:

Hot dip galvanizing and its alloys are currently one of the most widely used metal anti-corrosion technologies. This article briefly summarizes its development history and introduces its main processes, including alkaline washing, acid washing, water washing, auxiliary plating, galvanizing, cooling, passivation, etc. At the same time, some problems in each process are pointed out and corresponding solutions are discussed. Finally, the effect of adding Al, Mg, Sn, Pb, Bi and rare earth metal elements in hot-dip galvanizing is introduced.

The development of hot-dip galvanizing technology has become relatively mature as a whole. In recent years, research on hot-dip galvanizing technology has mainly focused on reducing pollution caused by the production process, reducing production costs, and improving the comprehensive performance of coatings, such as the following directions:

(1) Alloying control: By adjusting the alloy composition, ratio, and preparation process, the microstructure and composition distribution of the alloy are controlled to improve its comprehensive properties such as mechanical properties and corrosion resistance.

(2) Surface modification: By forming chemical reaction products such as oxidation and vulcanization on the surface of zinc and its alloys, the physical and chemical properties of the coating surface are changed to improve the corrosion resistance and adhesion performance of the coating.

(3) Multi-layer composite coating: By combining zinc with other metal or organic coatings, a multi-layer composite coating structure is formed to improve the mechanical properties, wear resistance, and corrosion resistance of the coating.

(4) Environmental friendly materials: acid mist inhibitor, chromium free passivator, hydrogel-based waste water treatment materials, etc. are used to reduce the environmental pollution caused by the hot-dip galvanizing process in an economical and convenient way without changing the hot-dip galvanizing process.

In summary, this article offers both theoretical and technical insights to those involved in the hot-dip galvanizing industry, as well as potential research directions for those in the field. The latest research efforts aim to enhance the performance and cost-effectiveness of coatings, aiming to better meet the diverse requirements for coating materials across various sectors. Although the industry still faces challenges, such as uneven distribution and thickness of coating alloys, zinc-aluminum alloy coatings' tendency to leak when aluminum content is too high, and the tendency for coatings to crack when bismuth content is too high, with the collaborative and unwavering efforts of industry and academia, the hot-dip galvanizing and its alloy industry is on a journey towards environmental protection, efficiency, and sustainable development.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The requested modification has been implemented as requested. Congratulations to the authors!

Reviewer 2 Report

The manuscript is acceptable for publication.