A Nano-CeO2/Zn–Mn Composite Conversion Coatings on AZ91D Magnesium Alloy Surface of Corrosion Resistance Research
Round 1
Reviewer 1 Report
The authors presented an article about “Preparation and corrosion resistance of AZ91D magnesium alloy nano-ceria/Zinc-Mn composite coatings.”
In this study, the authors conducted scientific research on the preparation and corrosion resistance of AZ91D magnesium alloy nano-ceria/Zinc-Mn composite coatings. But the paper should clearly distinguish itself from the previous literature. It was seen that the tests and analyzes in Paper were carried out successfully. However, it is obvious that these experiments and tests are not adequately explained.
I think the paper is not well organized and appropriate for the “Coatings” journal, but the paper will be ready for publication after major revision.
· The abstract looks good. Please include all significant numerical results.
· Please correct the citation style according to the journal rules.
· What is the problem? Why was the manuscript written? Please explain the reason in the introduction part. In the last paragraph of the introduction, the novelty of the study and the differences from the past in detail should be expressed.
· In general, it is necessary to use different sandpapers from coarse to fine. Why did the authors polish with only 2000 grain SiC sandpaper?
· The quantities and proportions of the additive particles are not given, which is a major shortcoming. Please specify in the material method section.
· Specify the chemical and physical properties of the particles in the material method section.
· Which international standards were tested and analyzed according to? Please specify.
· Please add a figure containing the macro photograph of the test instruments and the produced materials to the material method section.
· In the SEM analysis of the materials, the etching liquid used for the materials was not specified. Specify the etching fluid.
· Give more information about the production of the materials used in the study.
· Explain the properties of the phases obtained according to the XRD results. Expand the XRD results.
· Please fix the typographical and eventual language problems in the paper.
· The paper is well-organized, yet there is a reference problem. First, your reference list contains no paper from the “Coatings” journal. If your work is convenient for this journal’s context, then there are many references from this journal. Secondly, cited sources should be primary ones. Namely, the indexed area shows the power of a paper and directly your paper’s reliability. Please make regulations in this direction.
*** Authors must consider them properly before submitting the revised manuscript. A point-by-point reply is required when the revised files are submitted.
Comments for author File: 
Comments.pdf
The typographical and eventual language problems in the paper. Minor editing of English language required.
Author Response
Dear expert：
First of all, thank you very much for taking time out of your busy schedule to put forward valuable comments and suggestions to this paper. Secondly, for every question and suggestion you put forward, I will reply and adopt it seriously. In the original text, I marked it with red for your review. The following is my answer to each question:
1. The abstract looks good. Please include all significant numerical results.
Dear expert：Thank you for your comments. I tried to further modify the summary to include important numerical results to make it more complete. The summary is marked in red for your review. (Please see lines 8-21 for detailed changes)
2. Please correct the citation style according to the journal rules.
Dear expert：Thank you for your comments. The format of the reference that is not standard has been changed in Coatings required by the periodical, specifically, such as the references 1, 2, 4, 5, 6, 7, 9, 11, 12, 21, 24, 26, 30, 41.
3. What is the problem? Why was the manuscript written? Please explain the reason forthe introduction part. In the last paragraph of the introduction, the novelty of the study and the differences from the past in detail should be expressed.
Dear expert：Thank you for your comments. In this experiment, by arranging written manuscripts to express the process and novelty of the experiment. Compared with existing literature reports, the unique feature is that nano-CeO2 is added to Zn-Mn composite coating layer. The experiment shows that it can well make up for the deficiencies caused by the Zn-Mn coating layer in the coating forming process of the alloy base, make up for the cracks not completely covered by the deposition, further refine the riverbed surface, and promote the corrosion resistance. (Please see lines 75-85 for detailed changes)
4. In general, it is necessary to use different sandpapers from coarse to fine. Why did the authors polish with only 2000 grain SiC sandpaper?
Dear expert：Thank you for your question. I apologize for not being specific in the description of my manuscript. I in the process of the experiment, from 400#, 800#, 1200#, 1500#, 2000#SiC sandpaper in turn for grinding in this process should ensure that the scratches for the same direction, until the sample surface bright, smooth. After grinding, the surface of the sample was washed with distilled water, dried by cold air, and placed in a drying dish for later use. (Please see lines 105-110 for detailed changes)
5. The quantities and proportions of the additive particles are not given, which is a major shortcoming. Please specify in the material method section.
Dear expert：Thank you for your comment. The main components of the phosphating solution consisted of: 6.08 g/L Marjiv salt, 1 g/L NaF, 4.6 g/L EDTA-4Na, 20 g/L Mn(NO3)2, 0-5 g/L ZnO, 2.4 g/L CeO2, and 0.5 g/L OP-10. The phosphate coatings were prepared at a temperature of 70 ℃ and pH=3 (pH adjusted by phosphoric acid and ammonia).The formulation and the corresponding substance content for this experiment has been added to the Materials and Methods module as required. (Please see lines 112-115 for detailed changes)
6. Specify the chemical and physical properties of the particles in the material method section.
Dear expert：Thank you for your comment. The experimental drug used and the corresponding manufacturer has been added as required in the material method section, and the chemical and physical properties of some particles are also indicated. AZ91D magnesium alloy (density 1.82g/cm3); Marjivsalt (relative molecular weight: 248.91, white to grayish white or slightly reddish crystal, soluble in water to play the role of hydrolysis.); NaF (Relative molecular weight: 41.99, white powder, soluble in water.); EDTA-4Na (white crystalline powder, dissolved in water, acid, insoluble in alcohol); Mn(NO3)2 (relative molecular weight: 251.01, pink crystal, soluble in water.); ZnO (relative molecular weight: 81.38, white powder, becomes yellow after melting in acid heat, and becomes white after cooling.); CeO2 (relative molar mass: 172.115g·mol-1, yellowish white solid, yellow deepens when heated). (Please see lines 88-96 for detailed changes)
7. Which international standards were tested and analyzed according to? Please specify.
Dear expert：First of all, thank you for your comments and details, this is a good area I need to improve. Secondly, the following is a detailed analysis of the instruments used and the corresponding international testing and analysis standards: ①The international standards GB/T 31563-2015 and GB/T 36422-2018 were used to test and analyze the gold overcoating and its microscopic morphology in SEM. ②EDS spectrum is based on GB/T17722-1999 to test and analyze the thickness of the metal surface cover, and on GB/T25189-2010 to determine and analyze the concentration of elements on the surface of the coating.③XRD is used to determine the material composition of GB/T36655-2018 spherical silica powder for electronic packaging silica content of α-crystal test method④The impedance and polarization curves of magnesium alloy sacrificial anodes were analyzed and measured by GB/T24488-2009.⑤The GB/T33252-2016 nanometer laser confocal microscopic Raman spectrometer was used in the testing and analysis of 3D laser confocal microscopy, which can be used for the characterization of coating thickness and surface appearance.⑥The copper sulfate drop titration experiment uses GB437-1993 copper sulfate to test the corrosion resistance of the coating layer prepared on the surface of the alloy. Three parallel experiments are needed to obtain the average value. (Please see lines 128-158 for detailed changes)
8. Please add a figure containing the macro photograph of the test instruments and the produced materials to the material method section.
Dear expert：Thank you for your comment. Macro photos of SEM, XRD and LCSM have been inserted in the material method section. (Please see lines 102-103 for detailed changes)
9. In the SEM analysis of the materials, the etching liquid used for the materials was not specified. Specify the etching fluid.
Dear expert：First of all, thank you very much for your comments. Secondly, etching method is not used in SEM analysis of materials. Scanning the metal coating method, that is, on the surface of the material with poor electrical conductivity, by spraying gold treatment, so that the surface has electrical conductivity. Before SEM observation, a layer of conductive material is evaporated on the surface of the sample to eliminate the phenomenon of charge, improve the excitation energy of secondary electrons on the surface of the sample, and reduce the radiation damage of the sample.
10. Give more information about the production of the materials used in the study.
Dear expert：Thank you for your comment. I have listed the production information of the corresponding products in the material method section of the article. The following is the production information of the materials used: AZ91D magnesium alloy (density 1.82g/cm 3);Zinc oxide (Zinc oxide, molecular weight: 81.39g/mol), Sinopsin Chemical Reagent Co,LTD;Mazhiv salt (relative molecular weight: 284.94), Tianjin Damao Chemical Reagent Factory; Phosphoricacid, Molecular weight: 98g/mol), Tianjin Fuyu Fine Chemical Co,LTD. EDTA-4Na (Molecular weight: 380.17g/mol), Shanghai Aladdin Biochemical Technology Co,LTD;fluoride (Sodium fluoride, molecular weight: 41.99g/mol), Tianjin Damao Chemical Reagent Factory;Manganese nitrate (Manganese nitrate, molecular weight: 178.95g/mol), Tianjin Damao Chemical Reagent Factory;OP-10 (Molecular weight: 646g/mol), Zhongtian Fine Chemical Co, LTD; Ammonia (Ammonia-water, molecular weight: 35.045g/mol), Tianjin Beichen Founder Reagent Factory; Trisodium phosphate (molecular weight: 380.124g/mol) Tianjin Damao Chemical Reagent Factory. (Please see lines 88-96 for detailed changes)
11. Explain the properties of the phases obtained according to the XRD results. Expand the XRD results.
Dear expert：Thank you for your comment. Next, I will further expand the analysis results on the properties of the phase obtained by XRD. The XRD pattern illustrates the XRD pattern of Zn-Mn coating with different zinc oxide content treated in phosphoric acid bath for 15 min. The coating is mainly composed of Mg, Zn, Mn and their compounds, but the peaks and the intensity of different phases depend on the ZnO content. When ZnO is not present, the XRD pattern shows that the mixture is composed of MgO and Mg (OH)2. The PDF cards of XRD PDF#35-0821 and PDF#18-0787 are used for analysis. When ZnO content in phosphating solution is increased from 1g/L to 3g/L, the peak value corresponding to the main salt of coating formation is enhanced, and Zn elemental and some small crystalline particles of Mn3(PO4)2·5H2O and Zn3(PO4)2·4H2O appear in phosphating solution. Confirmed according to PDF#04-0831 and PDF# 03-0426 PDF cards. When ZnO content is 4g/L, the peak value reaches the main peak. At this time, Mn3(PO4)2·5H2O and Zn3(PO4)2·4H2O is enriched. When the ZnO content is further increased to 5g/L, the peak value of elemental Zn continues to rise, and more phosphoric acid phase will be obtained, resulting in the reduction of other mixed phases, and the corrosion resistance of the coating decreases. (Please see lines 165-174 for detailed changes)
12. Please fix the typographical and eventual language problems in the paper.
Dear expert：Thank you very much for pointing out the printing and final speech expression problems in the paper. I will humbly accept the suggestions, actively adopt them, and seriously modify them, so that the paper will look more rigorous and professional.
The paper is well-organized, yet there is a reference problem. First, your reference list contains no paper from the “Coatings” journal. If your work is convenient for this journal’s context, then there are many references from this journal. Secondly, cited sources should be primary ones. Namely, the indexed area shows the power of a paper and directly your paper’s reliability. Please make regulations in this direction.
Dear expert：Thank you very much for your valuable advice. As for the reference issue, I was careless in my consideration. I sincerely apologize for this. I have read and revised some of the literatures carefully, such as the references 1, 2, 4, 5, 6, 7, 9, 11, 12, 21, 24, 26, 30, 41, given that the latest literatures are retrieved and quoted by Coatings. It is typical and representative. Secondly, I have also read and modified the source of the quotation. Finally, thank you again for your valuable comments and suggestions on this paper.
We tried our best to improve the manuscript and made some changes marked in red in revised paper which will not influence the content and framework of the paper. We appreciate for expert warm work earnestly, and hope the correction will meet with approval. Once again, thank you very much for your comments and suggestions.
Author Response File: Author Response.pdf
Reviewer 2 Report
The manuscript entitled, "Preparation and corrosion resistance of AZ91D magnesium alloy nano-ceria/Zinc-Mn composite coatings" reports on the corrosion experiments and characterization of surface layers formed on Mg alloys from ZnO in Mn phosphating solutions in varying concentrations and CerO2 nanoparticles in the 4 mg/L Zn-Mn phosphating solution. I found the topic to be very interesting, but the article needs some improvements.
1. Please elaborate on how the "home-made" nanoparticles of Ce2O were made. How do you know they were nanoparticles?
2. Figure 1 is confusing. The lines are labeled a - f, yet there is also a legend with the colors included. I could not discern what the rotated Cin, the delta_sub_2, the 20, and the epsilon_sub_2 were meant to convey. Also, there did not appear to be that much difference in the XRD signatures. Could the authors plot the delta between the XRD patterns instead of the actual patterns?
3. Figure 2 is also difficult to follow. It is difficult to determine what the important features are in the inset images - and they blend into the larger images. I recommend including arrows pointing to the features that are described in the text. Also, the morphology shown in Figure 2f looks more like dendritic growth than what is seen in Figure 2e. Was any compositional analysis done on these features? Do they have the same compositions?
4. How many replicates were performed for the polarization curves in Figure 3? It looks like there is a decrease in the pitting potential as the ZnO concentration increased up to 3 mg/L, then dramatically increased at 4 mg/L before decreasing again at 5 mg/L. Is that behavior consistent with error bars? If so, what is the source of the change in behavior? The morphologies of the films do look different in Figure 1, but is that enough to explain the corrosion behavior?
5. Why plot the EIS data in Figure 4 using the Nyquist approach and the same EIS data (I think) in Figure 11 as Bode plots? I think the Bode plots provide more information for this system. However, there looks to be a high-frequency change in the phase that differs among the samples. Usually, we associate the high-frequency response with the solution resistance - which should have a zero degree phase lag. How were the EIS experiments performed? Was the same procedure followed for each sample? Could there be some parasitic inductance at the high frequencies?
6. It looks like the pitting/film breakdown potential increases by about 100 mV in Figure 9. I'm not familiar with the environments where AZ91D alloys are used but that could be a significant improvement and should be discussed.
7. Was the NAF drip test performed on the film formed with the Ce2O particles? Did it show any improvement over the others?
I recommend reducing the number of run-on sentences by decreasing the number of commas. For example, this should not be one sentence:
- "Figure 9 shows the electrochemical polarization curves of Zn-Mn phosphate film and CeO2/Zn-Mn composite film layer in 3.5% NaCl solution, the anodic region of the polarization curves both show obvious pitting characteristics, for the composite film layer of magnesium alloy, the formation of this characteristic corresponds to the penetration potential of the film layer [37], according to the extrapolation of the Tafel curve[38]，at the transformation time of 25 min, the corrosion current density of the film layer is the smallest, and the corrosion potential reflects the corrosion resistance of the film layer to a certain extent, but its priority is much smaller than the corrosion current density [39], which can be clearly seen that the corrosion current density at the transformation time of 25 min is 9.538× 10-7 A/cm2, which is higher than the corrosion current density of the Zn-Mn system monolayer of 1.86 × 10-6 A/cm2, and Figure 10 shows the microscopic 3D morphology of the film layer under laser confocal microscopy, Figure 10 (a) the thickness of the single layer shows 27.743 μm, and Figure 10 (b) the composite film shows a thickness of 49.736 μm, this 3D shows that the thickness of the composite film is consistent with the SEM, indicating that the corrosion resistance is significant, and the results of this analysis are also consistent with the SEM and EIS The results of this analysis are also consistent with SEM and EIS."
Author Response
Dear expert：
First of all, thank you very much for taking time out of your busy schedule to put forward valuable comments and suggestions to this paper. For every question and suggestion you put forward, I will reply and adopt it seriously. In the original text, I marked it with red for your review. The following is my answer to each question:
1. Please elaborate on how the "home-made" nanoparticles of Ce2O were made. How do you know they were nanoparticles?
Dear expert：Thank you very much for your valuable advice. I will explain in detail below. Nano- CeO2 was prepared by our experimental group through coprecipitate method, which is divided into three steps:①The solution component A was obtained by adding the first silane coupling agent into cerium nitrate.②The second silane coupling agent was added to the cerous nitrate solution, and the solution B with precipitation was obtained.③A and B components were fully mixed and reacted under closed conditions to obtain nano CeO2 particles. The main reason is that the method is simple, one-step synthesis, no annealing treatment, and the particle size can be seen by SEM, which is nanoparticle. (Please see lines 95-96 and 75-80 for detailed changes)
2. Figure 1 is confusing. The lines are labeled a - f, yet there is also a legend with the colors included. I could not discern what the rotated Cin, the delta_sub_2, the 20, and the epsilon_sub_2 were meant to convey. Also, there did not appear to be that much difference in the XRD signatures. Could the authors plot the delta between the XRD patterns instead of the actual patterns?
Dear expert：Thank you very much for your valuable advice. Figure 1 illustrates the Zn-MnXRD pattern of Zn-Mn with different zinc oxide contents treated in phosphoric acid bath for 15 min. These patterns confirm that the coating consists of a mixture of Mg, Zn, Mn and their compounds. However, the composition and peak strength of these substances depend on the amount of zinc oxide in the phosphate bath. In the absence of zinc oxide in the phosphate bath, the XRD pattern of the coating shows a mixture of Mg and Mg (OH) 2, which is analyzed according to the PDF cards of XRD PDF#35-0821 and PDF#18-0787, respectively..The presence of Zn and some small crystalline particles of Mn3(PO4)2·5H2O and Zn3(PO4)2·4H2O in phosphide solution has been confirmed according to the PDF cards of XRD PDF#04-0831 and PDF# 03-0426. The higher the zinc oxide content, the higher the peak value of Zn.When the zinc oxide content was further increased beyond 4g/L, it was observed that more phosphate phases were obtained at higher zinc oxide content, and the zinc oxide content in the phosphate bath significantly affected the coating composition. Therefore, when the zinc oxide content is greater than 4g/L, there are Mg and Mg (OH)2. When the ZnO content is further increased to 5g/L, the peak value of elemental Zn continues to rise, and more phosphoric acid phase will be obtained, resulting in the reduction of other mixed phases, and the corrosion resistance of the coating decreases. Meanwhile, the corresponding pattern has been adjusted by difference analysis. (Please see lines 163-165、168-172 and 176-178 for detailed changes)
3. Figure 2 is also difficult to follow. It is difficult to determine what the important features are in the inset images - and they blend into the larger images. I recommend including arrows pointing to the features that are described in the text. Also, the morphology shown in Figure 2f looks more like dendritic growth than what is seen in Figure 2e. Was any compositional analysis done on these features? Do they have the same compositions?
Dear expert：Thank you for your comment. Now I will give you a detailed reply based on your questions. Figure (a) shows the basic conversion solution without ZnO added, and Figure (b) shows the morphology of ZnO 1g/L added. It can be seen that the diameter of particles in Figure (a) to Figure(b) is about 15μm, and the particles are always in the shape of evacuated particles, and there is no large area of aggregation. The cracks in the riverbed are still clear, and it is difficult to cover the alloy substrate completely.When magnified to 5000 times, the cracks in the riverbed in Figure (b) slightly decrease compared with those in Figure (a), but they can still be clearly seen, and there are scratches on the surface of the crystalline particles, indicating poor corrosion resistance. In Figure (c) and Figure (d) at 50μm, the conversionation coating has a cluster structure covering almost the entire substrate. This phenomenon is attributed to the enrichment of Zn phase in the phosphating solution. At a ratio of 10μm, the cluster particles and tiny cracks in Figure (c) can be seen. In Figure (d), the cluster particles have become villosu and cover the entire fractures of the riverbed, indicating that the corrosion resistance has been further improved.When ZnO content changes from Figure (e) to figure (f), flower shape can be clearly seen. In figure (e)4g/L, flower structure is obvious, small petals are closely connected and interwoven, showing a "blooming" shape, completely blocking the cracks in the riverbed. At 5g/L in figure (f), the morphology does seem to have a dendritic structure, but at this time, the membrane structure is still flower-like, and the flower has been enriched, and the gaps between small petals become larger.At 10μm, it is difficult to cover the bottom completely, and the corrosion resistance becomes smaller. The test shows that it has the same composition.In addition, in the process of ZnO content increasing successively, the Zn-Mn conversion coating grows continuously and almost completely covers the substrate at 4g/L.This indicates that the composition of the conversion coating is mainly zinc phase mixture, which is attributed to the soft quality and high adhesion of zinc, so that the gaps between the crystal particles on the surface of the substrate can be filled. (Please see lines 181-182 and 196-200 for detailed changes,Fig3)
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4. How many replicates were performed for the polarization curves in Figure 3? It looks like there is a decrease in the pitting potential as the ZnO concentration increased up to 3 mg/L, then dramatically increased at 4 mg/L before decreasing again at 5 mg/L. Is that behavior consistent with error bars? If so, what is the source of the change in behavior? The morphologies of the coatings do look different in Figure 1, but is that enough to explain the corrosion behavior?
Dear expert：First of all, thank you very much for your comments. Secondly, I will give detailed responses to the above questions. The polarization curve in Figure 3 was used for parallel experiments for three times, excluding accidental error factors, and 6 groups of experiments were carried out each time. Pitting potential decreases with the increase of ZnO concentration to 3g/L, then increases sharply at 4g/L, and then decreases again at 5g/L, indicating that there is an inflection point during the increase of ZnO content. The results show that the formation of these cracks in the coating with low ZnO content is due to the predominant manganese in the coating, which leads to the shrinkage of the coating. When the zinc oxide content is increased to 4g/L and 5g/L, the phosphate crystals change into flower-like structures due to the zinc phosphate in the coating. Small petals with flower-like structure grow on the gap of crack boundary obtained with ZnO content of 4g/L. When the content of zinc oxide is 5g/L, the membrane structure is similar to the flower structure of zinc oxide content is 4g/L, but the petals of this structure are larger, the spacing is larger, and the cracks are difficult to cover. Due to the higher crystal hardness and more compact manganese phosphate, resulting in tensile stress, the compressive stress of Mn coating is higher than that of Zn coating, and Mn's superiority leads to many basement surface cracks.In contrast, when ZnO content is higher, the weight percentage of zinc is greater than that of Mn, indicating that the coating is mainly composed of zinc compounds.Thus, because of zinc's high adhesion and low hardness, the advantage of zinc compounds in coatings can lead to a good filling of cracks between crystal boundaries on the sample surface, so that combined with electrochemical impedance and polarization, can be fully explained. (Please see lines 209-211 and 214-230 for detailed changes)
5. It looks like the pitting/coatingbreakdown potential increases by about 100 mV in Figure 9. I'm not familiar with the environments where AZ91D alloys are used but that could be a significant improvement and should be discussed.
Dear expert：First of all, thank you very much for your comments. Secondly, I will give detailed responses to the above questions. The breakdown potential of CeO2/Zn-Mn composite coating increased by about 100mv compared with that of single Zn-Mn coating. It shows that when the electrochemical workstation is used for Tafel test analysis, a small amplitude sinusoidal potential signal is used to perturb the system. Anode and cathode processes appear alternately on the electrode, corresponding to anode arc and cathode arc, and the potential increase is due to the addition of nano CeO2. The riverbed cracks on the surface of the Zn-Mn coating layer are further refined to make it look smaller under SEM, and the composite coating deposited on the surface look denser. The corrosion potential will be increased. The more positive the corrosion potential is, the better the corrosion resistance will be. (Please see lines 298-308 for detailed changes)
6. Was the NAF drip test performed on the coatingformed with the Ce2O particles? Did it show any improvement over the others?
Dear expert：Thank you for your comment. In the course of this experiment, I did not use the NAF drop test. In my experiment, NaF was used as an inhibitor in the additive. Phosphating coating can be formed on the surface of magnesium alloy substrate only when the corrosion rate of magnesium alloy substrate is much lower than the formation rate of phosphating salt conversion layer. The corrosion rate of AZ91D magnesium alloy surface can be reduced by adding NaF into the phosphating solution.
Comments on the Quality of English Language
I recommend reducing the number of run-on sentences by decreasing the number of commas. For example, this should not be one sentence:"Figure 9 shows the electrochemical polarization curves of Zn-Mn phosphate coating and CeO2/Zn-Mn composite coating layer in 3.5% NaCl solution, the anodic region of the polarization curves both show obvious pitting characteristics, for the composite coating layer of magnesium alloy, the formation of this characteristic corresponds to the penetration potential of the coating layer [37], according to the extrapolation of the Tafel curve[38]，at the conversionation time of 25 min, the corrosion current density of the coating layer is the smallest, and the corrosion potential reflects the corrosion resistance of the coating layer to a certain extent, but its priority is much smaller than the corrosion current density [39], which can be clearly seen that the corrosion current density at the conversionation time of 25 min is 9.538× 10-7 A/cm2, which is higher than the corrosion current density of the Zn-Mn system monolayer of 1.86 × 10-6 A/cm2, and Figure 10 shows the microscopic 3D morphology of the coating layer under laser confocal microscopy, Figure 10 (a) the thickness of the single layer shows 27.743 μm, and Figure 10 (b) the composite coating shows a thickness of 49.736 μm, this 3D shows that the thickness of the composite coating is consistent with the SEM, indicating that the corrosion resistance is significant, and the results of this analysis are also consistent with the SEM and EIS The results of this analysis are also consistent with SEM and EIS."
Dear expert：First of all, thank you very much for your comments. Secondly, as for the problems you mentioned above, I will take them seriously and revise them in detail.Scan the entire manuscript for the comma problem you mentioned, and thank you again for taking the time to review this paper.
We tried our best to improve the manuscript and made some changes marked in red in revised paper which will not influence the content and framework of the paper. We appreciate for expert warm work earnestly, and hope the correction will meet with approval. Once again, thank you very much for your comments and suggestions.
Author Response File: Author Response.pdf
Reviewer 3 Report
The title of the manuscript should be change to a proper title that shows the work. (The authors have investigated the preparation of phosphate composite coatings)
Reference [8] does not involve the coatings mentioned in lines 42-45. It is better to use conversion coating instead of transformation film.
The published articles related to the composite and nanocomposite phosphate conversion coatings should be reviewed and reported in the introduction and then the differences between the current work and them emphasize.</p>
ZnO2 is added to the phosphating bath so why Zn-Mn film should be formed?
The manuscript is written in a way that it is hard to understand its results and discussion. The phosphating bath composition, the aim of using the ZnO particles and many of the materials that has been used have not been discussed or cited.
I am not qualified to assess the quality of English in this paper.
Author Response
Dear expert：
First of all, thank you very much for your comments on this manuscript. Secondly, as for the questions you mentioned above, I will take them seriously and revise them in detail. In addition, I have extensively revised the previous manuscript, and all the parts involved in revision and revision have been marked in red for your review. The following is my detailed reply:
①As for the title of the manuscript, I made some modifications after careful analysis. Modified to "A nano-CeO2/Zn-Mn composite conversion coatings on AZ91D magnesium alloy surface of corrosion resistance research ". (Please see lines 2-3 for detailed changes) Since the corrosion resistance study has included the preparation process, your suggestion is very useful and has been adopted. Thank you very much.
②The references and introduction you mentioned should be reviewed and reported, and should be compared with the preparation of nanocomposite membranes to show what is new about this paper. I have carefully revised the introduction and analyzed and compared the differences between them. (Please see lines 29-36、49-56、75-85 for detailed changes)
③As for the experimental process and typesetting of the manuscript, it is indeed not reasonable enough. I will try to make further modifications. The purpose of the discussion is to come to a convincing conclusion based on facts. The results show that compared with Zn-Mn coating without nano-CeO2, the addition of nano-CeO2 can fine the surface particles of the Zn-Mn coating, fill in the bed cracks on the surface of the magnesium alloy, and make the deposited coating look dense and thick, thus providing better corrosion protection.
④In the materials method, the formulation used, after modification, has been explained in detail, and the reaction corresponds to a phosphate conversion membrane. At the same time, the purpose of using zinc oxide is that the main salt of coating forming itself is Mazhiv salt, forming Mn phosphating coating.When you start trying to add ZnO, you will initially react with phosphoric acid to produce zinc phosphate. It's equivalent to adding one more coating forming agent. At this time, there are Mn and Zn series coating forming agents in the phosphating solution, and the Zn-Mn series phosphating coating is finally formed. And that's the first thing that's different from other papers.The second is the addition of nano CeO2, which further changes the composition of the liquid. (Please see lines 112-114、211-228 for detailed changes)
Finally, dear reviewer, regarding the grammar of the whole manuscript you raised, I accept your suggestions very seriously, carefully and humbly. We have done our best to polish and revise the manuscript for language issues, and have indeed invited a native English speaker from the United States to help us polish our article, We sincerely hope that the revised manuscript can be accepted by you. Once again, we would like to send our sincere blessing. Thank you for your hard work.
Author Response File: 
Author Response.doc
Reviewer 4 Report
The paper presents an interesting study on enhancing the corrosion resistance of AZ91D magnesium alloy by depositing a composite film layer containing nano-CeO2 and Zn-Mn phosphate on its surface. The growth mechanism and corrosion behavior of the composite film are well investigated and explained, with the help of XRD, EDS, SEM, EIS, LCSM, kinetic potential polarization, and spot-drop test. However, there are a few areas where the paper could be improved.
1. Abstract - provide more numerical values as properties. It looks general.
2. The introduction part includes a short story until it comes to the main objective of this paper. I think it should be written with more details to make it clear and exact.
3. What are the advantages of your composite compared to existing ones? What are the fundamental differences in improving the properties of the starting (initial) materials by you and other scientists.
4. The Reviewer recommends the Authors to rewrite the last paragraph of the introduction section in a way to stress the novelty.
5. The criteria of investigated materials selection are not explained.
6. The authors should provide more context and background information on the use of magnesium alloys in structural applications and the challenges associated with their corrosion resistance.
7. The authors should provide more detailed explanations of some of the technical terms and concepts used in the paper, particularly for readers who may not be familiar with the field.
8. The authors are advised to draw comparisons with previous literature to justify the results.
9. The authors could provide more discussion and analysis of the implications of their findings and how they could be applied in practice.
10. No need to discuss your results in the conclusions. The conclusion of the study needs to be rewritten. The conclusion should be quantified.
11. A few references need to be updated with some recent papers published in the last years.
12. The manuscript is overall well-written. However, there are many typesetting and grammatical errors in the text that should be corrected.
Overall, this paper provides valuable insights into the preparation and corrosion resistance of a composite film layer deposited on AZ91D magnesium alloy. With some revisions to provide additional context and explanations, this paper could be an important contribution to the field.
The manuscript is overall well-written. However, there are many typesetting and grammatical errors in the text that should be corrected.
Author Response
Dear expert：
First of all, thank you very much for taking time out of your busy schedule to put forward valuable comments and suggestions to this paper. For every question and suggestion you put forward, I will reply and adopt it seriously. In the original text, I marked it with red for your review. The following is my answer to each question:
1. Abstract - provide more numerical values as properties. It looks general.
Dear expert：Thank you for your comment. I have carefully modified the experimental data and experimental result parameters used in the abstract, and the important numerical results have been included. Thanks again for your correction. (Please see lines 11-21 for detailed changes)
2. The introduction part includes a short story until it comes to the main objective of this paper. I think it should be written with more details to make it clear and exact.
Dear expert：Thank you for your comment. As for the question in the introduction you mentioned, I have made corresponding modifications to make it more complete. In the introduction part, a comparative analysis between this experiment and other literature experiments is carried out, which indicates the novelty of this experiment. At the same time, in the last paragraph of the introduction, I have analyzed and explained the purpose of this experimental research and the exploration process to a certain extent, so that it looks more detailed and can be read more clearly and accurately. (Please see lines 29-36、41-45、49-54、57-62、75-85 for detailed changes)
3. What are the advantages of your composite compared to existing ones? What are the fundamental differences in improving the properties of the starting (initial) materials by you and other scientists.
Dear expert：Thank you for your comment. As for your questions, I will give you detailed replies：①Compared with the original and existing literature, the composite membrane prepared by our experiment has two significant differences. The first point is that when nano-CeO2 is not added, the Zn-Mn coating is inside, which is the composite of Zn series phosphating coating and Mn series phosphating coating. In other words, there are two kinds of coating forming agents in the conversion liquid, Zn type coating forming agent is mainly zinc phosphate, Mn type coating forming agent is mainly manganese dihydropgen phosphate. Compared with the single Zn series or Mn series, the comparative analysis with the original literature shows that the corrosion resistance of the Zn-Mn coating layer is better than that of the single Zn series or Mn series, so the Zn-Mn mesocoating layer is selected for preparation. The second point is that on this basis, nano-CeO2 is deposited, and then nano-CeO2 /Zn-Mn composite coating is prepared. A series of testing and characterization methods showed that the surface of the composite coating deposited crystal particles were dense, and the cracks of the riverbed could not be seen completely, indicating excellent corrosion resistance. Trying to add nanoparticles, not that many others have tried so far, is a really good place to start.②Compared with other experimenters, the fundamental difference in improving the initial material lies in the fact that a layer of nano-CeO2 is deposited on the basis of the Zn-Mn composite coating layer, rather than on the surface of a single Zn or Mn series. (Please see lines 359-363、366-369 for detailed changes)
4. The Reviewer recommends the Authors to rewrite the last paragraph of the introduction section in a way to stress the novelty.
Dear expert：Thank you for your comment. I have revised the introduction of the paper. The purpose of the experimental study and the corresponding conclusions. The novelty of what I do has been explained in detail and has been marked red for processing. Finally, thank you again for your guidance and suggestions. (Please see lines 75-85 for detailed changes)
5. The criteria of investigated materials selection are not explained.
Dear expert：Thank you for your comment. This is a good place that I need to improve. The following is a detailed analysis of the instruments used and the corresponding international testing and analysis standards:①The international standards GB/T 31563-2015 and GB/T 36422-2018 were used to test and analyze the gold overcoating and its microscopic morphology in SEM. ②EDS spectrum is based on GB/T17722-1999 to test and analyze the thickness of the metal surface cover, and on GB/T25189-2010 to determine and analyze the concentration of elements on the surface of the coating.③XRD is used to determine the material composition of GB/T36655-2018 spherical silica powder for electronic packaging silica content of α-crystal test method④The impedance and polarization curves of magnesium alloy sacrificial anodes were analyzed and measured by GB/T24488-2009.⑤The GB/T33252-2016 nanometer laser confocal microscopic Raman spectrometer was used in the testing and analysis of 3D laser confocal microscopy, which can be used for the characterization of coating thickness and surface appearance.⑥The copper sulfate drop titration experiment uses GB437-1993 copper sulfate to test the corrosion resistance of the coating layer prepared on the surface of the alloy. Three parallel experiments are needed to obtain the average value. (Please see lines 127-158 for detailed changes)
6. The authors should provide more context and background information on the use of magnesium alloys in structural applications and the challenges associated with their corrosion resistance.
Dear expert：Thank you for your comment. As for your questions, I would like to make the following explanations:Magnesium and its alloys are a promising structural and biomaterial for industries such as the automotive and aircraft industries, as well as the electronics, energy and biomedical sectors. Due to its excellent properties, including light weight, high thermal conductivity, high specific strength and stiffness, low specific density, good castability and machinability, and biocompatibility with the human body. The most common magnesium alloys that use aluminum as their primary additive includes AM60, AZ91 or AZ91D, which accounts for the majority of the magnesium market in lightweight structural applications, such as replacing plastics, aluminum alloys, and even steel parts. In addition, AZ91D alloy is the most widely used magnesium die-casting alloy with excellent mechanical properties and castability. It provides a lightweight alternative to traditional metal alloys and is becoming an extremely important structural application in the automotive and light truck industries. However, due to some adverse properties such as poor creep resistance, poor plastic deformation ability, high chemical reactivity and poor wear resistance, the development and application of magnesium alloys in industry are limited to a certain extent.In order to solve these problems, surface treatment and coating technologies are two major methods that have been widely explored to improve the corrosion resistance and mechanical stability of magnesium and its alloys and to meet the requirements of practical applications. Many effective treatment processes can protect magnesium alloy surface, such as sol-gel process, electrochemical electroplating, anodic oxidation, vapor deposition, plasma electrolytic oxidation, micro-arc oxidation coating, chemical conversion coating, electrodeposition coating, biomimetics deposition coating. Chemical conversion coating technology is one of the cheapest and most effective methods, which has the advantages of rapid coating formation, simple operation, and the ability to treat uneven surfaces. In recent years, phosphate conversion coating is an environmentally friendly surface treatment method, cost saving, energy saving, has been used for wear corrosion, lubrication, to protect the metal surface.Corresponding challenges related to the use of magnesium alloys in structural applications and their corrosion resistance has been added to the text. (Please see lines 28-35、41-45、47-54、70-74、76-85 for detailed changes)
7. The authors should provide more detailed explanations of some of the technical terms and concepts used in the paper, particularly for readers who may not be familiar with the field.
Dear expert：First of all, thank you very much for your comments. Secondly, as for the problems you mentioned above, I will take them seriously and revise them in detail. Scan the entire manuscript for technical terms and related conceptual issues you mentioned, and thank you again for taking the time to review this paper.
8. The authors are advised to draw comparisons with previous literature to justify the results.
Dear expert：Thank you for your comment. With regard to your questions, I will take them into serious consideration and accept your suggestions with an open mind. Recently, I rearranged the comparison and summarized the following two points:①It has been reported in previous literature that in the process of making coating layers, one is used as coating forming agent, which has the advantage of being easy to use and easy to operate, but the disadvantage is that there is only one coating forming agent, which is easy to deposit uneven coating forming in the process of preparing coating layers.Therefore, in this context, I tried to participate in coating formation with two coating forming agents, namely, Mn and Zn coating forming agents. The advantage of this is that Mn system is currently widely used, and coupled with Zn system, due to the soft quality and high adhesion of Zn system, the cracks between the crystal particles on the surface of the substrate can be filled, so that the bed cracks of magnesium alloy are smaller than that of a single coating layer.②Under the condition of optimal Zn/Mn ratio, the optimal Zn-Mn coating layer is found, and then nano-CeO2 is added. At the microscopic level, nanoparticles can make up for the cracks in the bed of the composite Zn-Mn coating and further refine the crystal particles, which is indeed a good attempt in the deposition of magnesium alloy coating. (Please see lines 62-74 for detailed changes)
9. The authors could provide more discussion and analysis of the implications of their findings and how they could be applied in practice.
Dear expert：Thank you for your comment. I will humbly accept your question. Based on the above questions, I will give a detailed reply:①As for the composite coating prepared in this experiment, the significance of the discovery lies in that some attempts and innovations can be made in the field of magnesium alloy corrosion and protection deposition coating, so as to provide certain theoretical and experimental support for the further development and application of magnesium alloy.②I will also pay attention to how to make use of these findings for improvement, analysis and discussion in practice. At last, thank you again for your comments. Thank you very much.(Please see lines 80-85 for detailed changes)
10. No need to discuss your results in the conclusions. The conclusion of the study needs to be rewritten. The conclusion should be quantified.
Dear expert：First of all, thank you very much for your comments. Secondly, I will carefully revise the conclusion of the conclusion for the problems you mentioned above. As you mentioned, this is where my work falls short.It would be inappropriate and inappropriate to discuss my results in a conclusion.I will make a good arrangement, rearrange the conclusion, proceed according to the article and deal with it quantitatively. I hope the revised manuscript will be satisfactory to you. Thank you again. (Please see lines 350-369 for detailed changes)
11. A few references need to be updated with some recent papers published in the last years.
Dear expert：Thank you for your comment. As for the reference, I also made a new adjustment. The reference part needs to be supported by updated references, such as the references in Article 1, 2, 4, 5, 6, 7, 9, 11, 12, 21, 24, 26, 30 and 41. Finally, thank you again for your advice.
12. The manuscript is overall well-written. However, there are many typesetting and grammatical errors in the text that should be corrected.
Dear expert：First of all, thank you for your affirmation of my manuscript. Your encouragement is the biggest motivation for me. Next, I will treat each experiment with a more scientific and rigorous attitude, and strive to continue to produce high-quality papers. Secondly, as for the typography and grammar in the manuscript you mentioned, I will take it seriously and revise it in detail. I hope that the revised paper can get your approval and satisfaction. Finally, thank you again.
Overall, this paper provides valuable insights into the preparation and corrosion resistance of a composite coating layer deposited on AZ91D magnesium alloy. With some revisions to provide additional context and explanations, this paper could be an important contribution to the field.
Dear expert：
First of all, thank you very much for taking time out of your busy work to make detailed comments and analysis on this paper. Based on the questions you mentioned above, I have revised and checked each item in detail as required. Including the experimental purpose of the paper, materials, analysis and discussion, results obtained. I have systematically revised everything from the big frame to the subtitle, and I have also explained additional background information in the article. I hope you will be satisfied with the revised paper. Finally, thank you again for your criticism and correction.
We tried our best to improve the manuscript and made some changes marked in red in revised paper which will not influence the content and framework of the paper. We appreciate for expert warm work earnestly, and hope the correction will meet with approval. Once again, thank you very much for your comments and suggestions.
Author Response File: Author Response.doc
Round 2
Reviewer 1 Report
Thank you for revised version
Minor editing of English language required
Reviewer 4 Report
The authors have done efforts to answer all the questions addressed by the reviewers and to a large extent have done it correctly. The manuscript can now be accepted for publication. However, the entire English writing of the manuscript must be revised for improvement before publication. With special emphasis on the texts included after revision 1.
Minor editing of English language required. The entire English writing of the manuscript must be revised for improvement before publication. With special emphasis on the texts included after revision 1.
