Anticorrosion Properties of Zn–Al Composite Coating Prepared by Cold Spraying
Round 1
Reviewer 1 Report
1. EDS spectra in Fig. 2 are not clear for the reader, suggesting to make the Figures bigger or re-draw the figures by transferring the data to the EXCEL.
2. For the potentiodynamic polarization test, the scanning rate is 0.5 mV / s is fast, why the authors used this scan rate?
3. The potentiodynamic polarization test should be carried out for the bare sample, as well, the potential curve of the bare sample in NaCl solution needs to be added to Figure 4.
4. For reproducibility, how many times the potentiodynamic polarization tests were carried out?
5. The Ecorr and Icorr for the bare sample need to be compared with the coated ones after exposure to the corrosive medium to measure the improvement for the developed coating. The Ecorr and Icorr of the bare sample need to be included in Table 2.
6. The scatter for Icorr in Table 2 is missing.
7. The manuscript needs to be checked for typo errors.
Author Response
1. EDS spectra in Fig. 2 are not clear for the reader, suggesting to make the Figures bigger or re-draw the figures by transferring the data to the EXCEL.
My original picture is large and the data is clear. Since the plate requires me to combine the three pictures together, the enlargement can be seen clearly.
2. For the potentiodynamic polarization test, the scanning rate is 0.5 mV / s is fast, why the authors used this scan rate?
Some references have scan speeds of 0.5 mv/s, 1 mV/s, 5 mV/s, and 10 mV/s, the polarization corrosion caused by the test is serious, so the scanning speed of the small point is selected. In terms of the dynamic potential polarization curve test, it is generally below 1 mv/s.
3. The potentiodynamic polarization test should be carried out for the bare sample, as well, the potential curve of the bare sample in NaCl solution needs to be added to Figure 4.
The polarization curves of the coatings for different corrosion periods are measured, and have little to do with the values of the bare samples. If it is a comparison between different materials, it is necessary to add a comparison of the bare sample data.
4. For reproducibility, how many times were the potentiodynamic polarization tests carried out?
The potentiodynamic polarization test was performed approximately 100 times, most of which was not ideal, and then the data were selected for more appropriate data.
5. The Ecorr and Icorr for the bare sample need to be compared with the coated ones after exposure to the corrosive medium to measure the improvement for the developed coating. The Ecorr and Icorr of the bare sample need to be included in Table 2.
The measurement of the coating data for different corrosion cycles has little to do with the value of the bare sample. If it is a comparison between different materials, you need to add a comparison of the bare sample data.(Reading similar references, there is no comparison of the base steel data for a single coating.
6. The scatter for Icorr in Table 2 is missing.
Data has been added in the article.
7. The manuscript needs to be checked for typo errors.
Some errors have been corrected in the article.
Acknowledgement:Thank you for reading my article, pointing out a lot of shortcomings and giving valuable suggestions I will seriously modify it. Thank you again for your help.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Dear Author:
The paper can be significantly improved if the experimental procedure and design is better explained
I strongly suggest that you review your data again, particularly the electrochemical part is weak and not supportive of your conclusions
The salt spray tests mentioned are properly used to support the conclusions either
Comments for author File: 
Comments.pdf
Author Response
1. Please specify if the salt spary or salt fog test conforms to particular standard
procedure or at least please be more clear on the test conditions not only equipment description.
The composite coating was investigated by neutral salt spray (NSS) and electrochemical accelerated corrosion tests method to evaluate the corrosion resistance of the composite coating. A sample having a surface area of 10 × 10 mm2 was prepared before the test. Neutral salt spray test in a static environment, where the ambient temperature of the salt spray is stable at 35 ± 1 ° C, and 100 ml of 3.5 ± 0.5% NaCl solution at 100% relative humidity is prepared, and its pH is between 6.5 and 7.2. The neutral salt spray test equipment used is model LRHS-108-RY and it includes an atomizing nozzle to atomize the salt solution in the chamber, the salt solution pool, the sample supports the vertical direction of 15° to 20°, the heating element and the controller, as well as the humidification tower, is connected to the reservoir. In the electrochemical accelerated corrosion experiment, the accuracy of the impact test is prevented. The non-coated surface is sealed, and the polarization curve is obtained by scanning. The sample was first immersed in NaCl solution for 30 min to test the open circuit potential, allowing the sample to reach a constant open circuit potential in NaCl solution. Taking four samples and put them into NaCl solution for 24h, 48h, 72h, 96h, and then the Tafel curve test was carried out with a scan frequency of 0.5 mV/s, The Icorr was obtained by using Butler-Volmer analysis (ThalesXT5.1.4 software), and test data is recorded each time.
2. the acoustic signal has two mutations.”Mutations or changes ??
The acoustic signal has two mutated regions.(Corresponding to Figure 3)
3. Polarization curve problem
As the corrosion potential (Ecorr) gradually increases from -1.396 to -1.325V, the current density gradually decreases from 1.154 × 10−5 to 1.737 × 10−5A cm-2, and the corrosion rate can be gradually slowed down. The lower ipass illustrated that passivation behavior could occur more easily, and the passivation film was formed rapidly. Besides, the passivation films formed on the coating surface prevented the inner layer from being exposed to corrosive liquid, which could slow down the corrosion reaction [24]. Compared with the tested samples, The 96 hour corrosion-resistant coating had a lower pass rate of 1.89 x 10-4 A·cm-2 than the other samples, showing good passivation. Guessing the reason is that the coating began to experience a vigorous electrochemical reaction in the NaCl solution, which generates an oxidation product that hindered the rate of corrosion. In order to probe deeper the corrosion resistance of the coating, the NSS test was conducted.(Supplementary explanation)
4.Note: Badanodic Tafel slope, Bcdcathodic Tafel slope, icorrdcorrosion current density, Ecorrdcorrosion potential.”
They were deleted, because thay were explained in the article.
5. You mean exposure time?
The hardness measured after corrosion at different times, with Table 3 attached.
6. Please rewritted the conclusion.
Rewritten the conclusion.
Author Response File: Author Response.docx
Reviewer 3 Report
Comments provided by reviewer are provided in attached Reviewers report and comments
within the manuscript itself
Comments for author File: Comments.pdf
Author Response
1. Style, Structure and Grammar – The structure and grammar throughout in many places is very poor and doesn’t show a logical progression. Further, sections of the manuscript lack the technical professionalism expected from an engineering manuscript for publication in the open literature It is essential that the document is proof read and amended accordingly by professional proof readers who are specialised in editing scientific and technical documents, prior to publication. There are major grammatical inconsistencies, typographical errors throughout, combined with the style of writing, which makes it hard to read and understand what is being said without going back over it more than once. The reviewer started to amend some of the errors within the manuscript but gave up early in the manuscript as the purpose of the reviewer is to review the technical content and not proof reading for English grammatical construction. At this point, the manuscript is not acceptable for publication. However, the manuscript could be considered for publication further down the track after the grammatical construction is improved considerably and the technical comments are addressed (as below):
First of all, thank you for pointing out a lot of my mistakes. This is my first time writing an English article. Even if there are many mistakes, I am willing to make many revisions. Please also give me more opportunities to make me progress. Thank you again for your help.
2. A detailed Introduction is provided in terms of corrosion and biofouling in marine environments and use of cold spray technology to overcome many of the disadvantages of other techniques. However, what is lacking here is a focus on some key aspects, forming the basis for this paper. It is not clear what the novelty of this research is i.e. what is this research contribution to the existing body of knowledge.
Many parts of the article have been modified and added some content. I have read most of the relevant literature you mentioned. The reason why references were added in article is that they were Chinese references.
3. Experimental – Generally, good details provided on the methodologies adopted for this study, however the studies are carried out on the coating only deposited under one set of conditions. To improve the academic quality of this manuscript, studies should be conducted on coatings under a range of conditions (variation in the spraying parameters, thickness, etc) to determine optimum conditions. In addition, corrosion studies should be conducted on the (uncoated) base material as a reference point. A systematic, comparative study would enhance the quality of the manuscript and it is essential that more coating conditions, other systems are studied
On the one hand, the measurement of the coating data for different corrosion cycles has little to do with the value of the bare sample. If it is a comparison between different materials, you need to add a comparison of the bare sample data. (Reading similar references, there is no comparison of the base steel data for a single coating.)On the other hand, the article mainly writes a fixed data coating without orthogonal test of material composition parameters.If it needs to compare the base data, I will add it to the lower part.
4,5. Results / Discussion – More detailed explanation is required for the electrochemical tests. What is happening in the cathodic / anodic regions of the curves, particularly effects of cathodic reactions influencing the system. The corrosion behaviour of the coated system should be compared to the uncoated (base steel) material as a reference point to ascertain the corrosion characteristics of the coating relative to the base material, as this is one purpose of this coating, to provide protection to the substrate if the substrate becomes exposed to the environment through defects / breaks in the coating at any point. Further, this section is just a description of the results. It is not enough to describe the results but to provide a critical discussion around the results. What is lacking is a detailed, critical discussion on what these results mean, how the results can be explained and interpreted based upon previous studies. Overall it lacks depth as it stands. Conclusions – Conclusions are quite general. They should be more detailed and focussed on the specific findings from this study. In particular, the second one is an expected outcome from studying sacrificial coatings and adds no value to the paper, except confirm that it is indeed a sacrificial coating.
I rewrote the conclusions.
In this experimental study, A Zn-Al composite coating was prepared on the Q345 substrate by cold spray technique. The corrosion resistance and other properties of the composite coating were studied by salt spray test, electrochemical test and hardness test. The following conclusions can be drawn:
· The Zn-Al composite coating prepared by cold spray technology can be firmly attached to the Q345 substrate. The Energy Dispersive Spectrometer (EDS) analyzed the coating without significant oxidation during the preparation process. The SEM microstructure shows that the coating material Zn, Al powder particles are uniformly mixed and good plastic deformation occurs.
· After the salt spray test and electrochemical test of the sample, the microstructure of the sample at different corrosion time periods showed that the sample had a severe anodization reaction at the beginning of the accelerated corrosion, and a large amount of agglomerated oxidation products were formed. With the accelerated corrosion test, the oxidation products on the microscopic surface of the sample are gradually fine and dense, and the corrosion rate is gradually slowed down. The oxidation product can effectively slow the corrosion of the sample surface. The corrosion surface hardness data measured by the hardness tester shows that the longer the corrosion time in a certain range along the greater the hardness of the surface, so the better the wear resistance.
· On the one hand, the composite coating isolates the substrate from contact with the marine environment, therefore, corrosion of the substrate in direct contact with seawater is avoided. On the other hand, even if the surface of the composite coating is scratched, the coating and the substrate are anodized in a closed loop in seawater, thereby protecting the substrate from corrosion. In summary, the coating has good anti-corrosion properties.
Acknowledgement:Thank you for reading my article and pointing out a lot of my mistakes.I know that my article has many errors and shortcomings. Please give me the opportunity to correct your valuable suggestions again. Thank you again for your help.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
1. The bare sample needs to tested (after stabilization or after 24 h of immersion in NaCl) in order to compare with coated samples and measure the improvement in corrosion resistance due to the developed coating.
2. The authors claimed that the corrosion tests were carried out 100 times and the most were not ideal. Therefore the Authors need to explain why and the scatters for the icorr values need to be added in Table 2.
Author Response
1. The bare sample needs to tested (after stabilization or after 24 h of immersion in NaCl) in order to compare with coated samples and measure the improvement in corrosion resistance due to the developed coating.
The non-coated base steel Q345 sample was first immersed in NaCl solution for 24h to test Tafel curve. The polarization potential of the base steel Q345 is -1.992 V, and the corrosion current density is 6.589 × 10−5 Acm-2. By comparing the relevant corrosion parameters of the base steel Q345 and the coating, it is found that the polarization potential of Q345 is -1.192V is much smaller than the corrosion potential of the coating -1.396V, so the coating and the base steel Q345 together corrode the coating first. It has the function of protecting the base steel. The corrosion current density of the base steel Q345 is also much smaller than the corrosion current density of the composite coating, so the Q345 corrosion rate is slower than the corrosion rate of the base steel.
2. The authors claimed that the corrosion tests were carried out 100 times and the most were not ideal. Therefore the Authors need to explain why and the scatters for the icorr values need to be added in Table 2.
I have done many corrosion tests, and many of the data obtained are not representative or the test fails, so I have selected representative data from many experiments.On the one hand, the values of icorr and Ecorr in Table 2 also increase the Q345 data of the base steel. On the other hand, the values of icorr and Ecorr can also draw corresponding conclusions.
Author Response File: Author Response.docx
Reviewer 2 Report
Dear Authors;
To better support your conclusions you need to discuss more in detail the significance of the behavior at open circuit conditions
Comments for author File: Comments.pdf
Author Response
1. To better support your conclusions you need to discuss more in detail the significance of the behavior at open circuit conditions
On the one hand, I added the icorr and Ecorr of the bare base steel Q345, the values of icorr and Ecorr can also draw corresponding conclusions.On the other hand, the real-time monitoring of the open circuit potential results in the rate of corrosion of the sample, and the corrosion current density can also replace this set of data. By comparing the relevant corrosion parameters of the base steel Q345 and the coating, it is found that the polarization potential of Q345 is -1.192V is much smaller than the corrosion potential of the coating -1.396V, so the coating and the base steel Q345 together corrode the coating first. It has the function of protecting the base steel. The corrosion current density of the base steel Q345 is also much smaller than the corrosion current density of the composite coating, so the Q345 corrosion rate is slower than the corrosion rate of the base steel.
Author Response File: Author Response.docx
Reviewer 3 Report
Further to my original comments for major revision, (see below), please see my responses as to how I believe that they have been addressed:
1. Style, Structure and Grammar – The structure and grammar throughout in many places is very poor and doesn’t show a logical progression. Further, sections of the manuscript lack the technical professionalism expected from an engineering manuscript for publication in the open literature It is essential that the document is proof read and amended accordingly by professional proof readers who are specialised in editing scientific and technical documents, prior to publication. There are major grammatical inconsistencies, typographical errors throughout, combined with the style of writing, which makes it hard to read and understand what is being said without going back over it more than once. The reviewer started to amend some of the errors within the manuscript but gave up early in the manuscript as the purpose of the reviewer is to review the technical content and not proof reading for English grammatical construction. At this point, the manuscript is not acceptable for publication. However, the manuscript could be considered for publication further down the track after the grammatical construction is improved considerably and the technical comments are addressed (as below):
Reviewers Response to Authors Response and Actions:
The reviewer feels that while sections of the manuscript have had changes done to them, mainly addressing points for minor revision, it doesn't appear that the document has been proof read by professional proof reading services to bring it to a standard in terms of technical presentation and grammatical construction expected from an engineering manuscript for publication in the open literature
Introduction – A detailed Introduction is provided in terms of corrosion and biofouling in marine environments and use of cold spray technology to overcome many of the disadvantages of other techniques. However, what is lacking here is a focus on some key aspects, forming the basis for this paper. It is not clear what the novelty of this research is i.e. what is this research contribution to the existing body of knowledge. In addition, there are several relevant articles focussed on cold spraying of Zn-Al alloys which have not been included in this review. These are detailed below:
Enhanced bonding property of cold-sprayed Zn-Al coating on interstitial-free steel substrate with a nanostructured surface layer, Liang, Y.L.; Wang, Z.B.; Zhang, J.; Zhang, J.B.; Lu, K. Applied Surface Science, v 385, p 341-8, 2016
Deposition behavior of mixed binary metallic powders in cold spraying process, Zhou, X.L.; Mou, S.J.; Wu, X.K.; Zhang, J.S. Applied Surface Science, v 257, n 24, p 10628-33, 2011
Properties of Zn-Al Alloy Cold Spraying Coatings on Magnesium Alloy, Zhao Hui; Huang Zhanghong; Li Pingcang; Wang Hunian Special Casting & Nonferrous Alloys, v 30, n 8, p 702-4, 2010
Depositing characteristic of 65%Zn-Al coatings produced by cold gas dynamic spray, Li Hai-xiang; Sun Ming-xian; Li Xiang-bo; Wang Hong-ren; Huang Guo-sheng Chinese Journal of Nonferrous Metals, v 20, n 7, p 1353-9, 2010
Corrosion resistance of cold-sprayed Zn-50Al coatings in seawater, Li Haixiang; Li Xiangbo; Sun Mingxian; Wang Hongren; Huang Guosheng Journal of Chinese Society for Corrosion and Protection, v 30, n 1, p 62-6 2010
Novel technique for laser lap welding of zinc coated sheet steels, Li, X.; Lawson, S.; Zhou, Y.; Goodwin, F. Journal of Laser Applications, v 19, n 4, p 259-64, 2007
The Introduction needs to be rewritten to incorporate some of these important references as part of the review and highlight where the gaps are forming the basis for this research
Reviewers Response to Authors Response and Actions:
The Introduction has not been rewritten to incorporate these relevant references nor has it been made clear in the Introduction what the novelty of the research is and the gaps from the literature which form the basis for conducting this research. This should be done in accordance with the reviewers original request
Experimental – Generally, good details provided on the methodologies adopted for this study, however the studies are carried out on the coating only deposited under one set of conditions. To improve the academic quality of this manuscript, studies should be conducted on coatings under a range of conditions (variation in the spraying parameters, thickness, etc) to determine optimum conditions. In addition, corrosion studies should be conducted on the (uncoated) base material as a reference point. A systematic, comparative study would enhance the quality of the manuscript and it is essential that more coating conditions, other systems are studied
Reviewers Response to Authors Response and Actions:
The authors have not included any further testing as suggested by the reviewer. The reviewer acknowledges that the authors have responded to this comment and have provided their reasons for not including further testing. However, the reviewer still believes that by not conducting further tests on a wider range of conditions (essentially the whole paper is focussed on on coating under one set of conditions) the quality of the paper is diminished
4. Results / Discussion – More detailed explanation is required for the electrochemical tests. What is happening in the cathodic / anodic regions of the curves, particularly effects of cathodic reactions influencing the system. The corrosion behaviour of the coated system should be compared to the uncoated (base steel) material as a reference point to ascertain the corrosion characteristics of the coating relative to the base material, as this is one purpose of this coating, to provide protection to the substrate if the substrate becomes exposed to the environment through defects / breaks in the coating at any point.
Further, this section is just a description of the results. It is not enough to describe the results but to provide a critical discussion around the results. What is lacking is a detailed, critical discussion on what these results mean, how the results can be explained and interpreted based upon previous studies. Overall it lacks depth as it stands.
Reviewers Response to Authors Response and Actions:
The authors have not included any corrosion testing on the base material as a comparison. The reviewer acknowledges that the authors have responded to this comment and have provided their reasons for not including further testing. However, the reviewer still believes that a comparison of the base material with the coating is essential as mentioned above.
While additional information has been incorporated into the manuscript in terms of explaining further the potentiodynamic scans, this could be more detailed. The authors have talked about ipass and passivity, but the potentiodynamic scans don't show any passive behaviour in the anodic region; what they are doing is reaching a limiting anodic current density as the samples are polarized more anodically. What the graph shows is a more positive shift in the Ecorr values (a shift in the polarisation curves to the left) with increasing immersion time may indicate the formation of passive layers as immersion time progresses, the more positive Ecorr values indicating improved thermodynamic stability; however, the increase in Icorr values with increasing exposure time indicate that higher corrosion rates are observed with increased exposure time. This has not been made clear.
Further, there is no detailed discussion on the corrosion results (nor any of the other results) in terms of the literature. The reviewer believes that this is essential in producing a high quality manuscript i.e. providing an in depth, critical discussion.
Conclusions – Conclusions are quite general. They should be more detailed and focussed on the specific findings from this study. In particular, the second one is an expected outcome from studying sacrificial coatings and adds no value to the paper, except confirm that it is indeed a sacrificial coating.
Reviewers Response to Authors Response and Actions:
The authors have rewritten the conclusions, to include more details on what the findings are from this investigation
Author Response
1. It is not clear what the novelty of this research is i.e. what is this research contribution to the existing body of knowledge. In addition, there are several relevant articles focussed on cold spraying of Zn-Al alloys which have not been included in this review.
Although there are many studies on Zn-Al coatings, the working environment and experimental conditions are different, and the application direction is different. On the one hand, the cold sprayed Zn-Al composite coating described in the article is applied to offshore equipment, and the coating has the advantages of the above cold spraying. On the other hand, this study focuses on the marine working environment, and the advantages of the composite coating are also shown in the experiment. By all means, the introduction rewrites a part and reads the references you have recommended and added them to the article.
2. To improve the academic quality of this manuscript, studies should be conducted on coatings under a range of conditions (variation in the spraying parameters, thickness, etc) to determine optimum conditions. In addition, corrosion studies should be conducted on the (uncoated) base material as a reference point. A systematic, comparative study would enhance the quality of the manuscript and it is essential that more coating conditions, other systems are studied.
The requirements for spray parameters and spray thickness are not high. On the one hand, the spray parameters are set by the processing workers according to the materials, and they have years of experience in spraying. On the other hand, the coatings we prepared were larger than 100 μm, much larger than the thickness used for the test. For example, the test needs to be corroded to 50 μm, so the prepared coating should be larger than 50 μm to complete the test and draw corresponding conclusions.
3.The authors have not included any corrosion testing on the base material as a comparison. The reviewer acknowledges that the authors have responded to this comment and have provided their reasons for not including further testing. However, the reviewer still believes that a comparison of the base material with the coating is essential as mentioned above.
On the one hand, I added the icorr and Ecorr of the bare base steel Q345, the values of icorr and Ecorr can also draw corresponding conclusions.On the other hand, the real-time monitoring of the open circuit potential results in the rate of corrosion of the sample, and the corrosion current density can also replace this set of data. By comparing the relevant corrosion parameters of the base steel Q345 and the coating, it is found that the polarization potential of Q345 is -1.192V is much smaller than the corrosion potential of the coating -1.396V, so the coating and the base steel Q345 together corrode the coating first. It has the function of protecting the base steel. The corrosion current density of the base steel Q345 is also much smaller than the corrosion current density of the composite coating, so the Q345 corrosion rate is slower than the corrosion rate of the base steel.4.
4. however, the increase in Icorr values with increasing exposure time indicate that higher corrosion rates are observed with increased exposure time. This has not been made clear.
Some experimental data has been modified in Table 2. Among them, corrosion current density and open circuit potential can be used to estimate the rate of corrosion. The corrosion current density has been listed in the article, so there is no data related to the open circuit potential. If necessary, I can draw the chart to add.
Acknowledgement：You have given me a lot of suggestions, I am very grateful to you. I know that only by finding out more shortcomings can we make progress and learn more. The most important thing is that I have learned a rigorous and serious attitude from you.Thank you, the reviewer.
Author Response File: Author Response.docx
Round 3
Reviewer 3 Report
1. The authors have incorporated additional referencing into the Introduction as recommended by the reviewer.
2. The authors have defended and provided their reasons for not looking at additional coating parameters.
3. The authors have provided results for the uncoated sample
Further to point 3, the authors have incorrectly stated (lines 188-189) that "so the Q345 corrosion rate is slower than the corrosion rate of the base steel." What the results show are "that the Q345 base steel corrosion rate is slower than the corrosion rate of the coated samples". As this is quite important to the overall findings from this study, the authors should add further information as follows:
The more positive Ecorr values and lower Icorr values of the base material indicate improved corrosion resistance properties of the uncoated material. This is likely to be associated with galvanic coupling between the more active coatings and thus increased corrosion behaviour of the coated samples compared to the base material alone.
This should also be added into the conclusions as this is an important finding
Author Response
1.Further to point 3, the authors have incorrectly stated (lines 188-189) that "so the Q345 corrosion rate is slower than the corrosion rate of the base steel." What the results show are "that the Q345 base steel corrosion rate is slower than the corrosion rate of the coated samples".
To begin with, I correct an error "so the Q345 corrosion rate is slower than the corrosion rate of the base steel.". It was modified to "so the Q345 corrosion rate is slower than the corrosion rate of the composite coating.".
2. This should also be added into the conclusions as this is an important finding.
The last conclusion was revised to "The more positive Ecorr values and lower Icorr values of the base material indicate improved corrosion resistance properties of the uncoated material. On the one hand, the composite coating isolates the substrate from contact with the marine environment, therefore, corrosion of the substrate in direct contact with seawater is avoided. On the other hand, even if the surface of the composite coating is scratched, the coating and the substrate are anodized in a closed loop in seawater, thereby protecting the substrate from corrosion. In summary, The composite coating increases the corrosion behavior of the sample compared to the base material alone.".
Author Response File: Author Response.pdf
