Review Reports - Anticorrosion Properties of Epoxy Composite Coating Reinforced by Molybdate-Intercalated Functionalized Layered Double Hydroxide

Round 1

Reviewer 1 Report

In the present manuscript the Authors propose the use of a composite coating constituted by molybdate intercalated layered double hydroxide embedded in an epoxy matrix as corrosion barrier for steel.

Even if the use of molybdate as corrosion inhibitor is well known, the use of layered double hydroxide as Mo-reservour is interesting and constitutes a novelty in this field. However, before I can recommend the publication of the present manuscript there are some aspects that have to be fixed or better described. below the list of my concerns:

No data about the coatings thickness were provided. It is important to compare the thickness of the different coatings since the introduction of the functionalized hydroxide layer could result in thickening of the epoxy coating. In such a way the improved corrosion resistance could be related to the more thick organic alone.

I am not an exper of the TGA analysis , however, the data presented in figure 3 apparently do not fit the description and interpretation provided in the text (rows 215-233)

Please, check the accuracy of the electrochemical measurements and reduce the number of digits of the value collected in table 1 and table 2 accordingly (Es. 6 digits for the Ecorr value in table 2).

The type of steel used for the test has not specifyed

Some typo errors and grammatical mistakes are present trought the text (es. row 129 "..was washed"; row 131 "o" is missing; rows 290-291 "...being diffused" and so on) .

Author Response

Manuscript ID: jcs-415042

Title: “Anti-corrosion properties of the epoxy composite coating reinforced by molybdate intercalated functionalized layered double hydroxide"

Dear Editor,

Thank you very much for kindly evaluating our manuscript (Manuscript ID jcs-415042) entitled “Anti-corrosion properties of the epoxy composite coating reinforced by molybdate intercalated functionalized layered double hydroxide”. We are also thankful to the Reviewers for their valuable comments and suggestions which would certainly made our manuscript more informative. The answer to the reviewers is given below for your kind reference and the revision made in the manuscript are in red color.

Reviewer #1:

Answer: We are thankful to you for kindly evaluating our manuscript. We have carefully checked the manuscript and corrected accordingly.

Q1: No data about the coatings thickness were provided. It is important to compare the thickness of the different coatings since the introduction of the functionalized hydroxide layer could result in thickening of the epoxy coating. In such a way the improved corrosion resistance could be related to the more thick organic alone.

A2: Thank you very much for meticulously evaluating our manuscript. The valuable suggestion has made our article more informative. We have calculated the thickness of the coating using Atomic Force Microscopy image analysis. The coating thickness was found to be ~ 255 nm.

Q2: I am not an expert of the TGA analysis; however, the data presented in figure 3 apparently do not fit the description and interpretation provided in the text (rows 215-233).

A2: Thank you for evaluating our manuscript thoroughly, we regret our mistake. We have revised the TGA analysis part as pointed out by the respected reviewer and the revision is included in the revised manuscript.

Q3: Please, check the accuracy of the electrochemical measurements and reduce the number of digits of the value collected in table 1 and table 2 accordingly (Es. 6 digits for the E_corrvalue in table 2).

A3: We are thankful for your sympathetic evaluation of our manuscript. The accuracy of our machine (PARSTAT 4000, Princeton Applied Research) is in the range of picoampere. We provided maximum no of digits after decimal in our findings, so that the reported value would be more accurate.

Q4: The type of steel used for the test has not specified.

A4: We regret our mistake and the careful evaluation of our manuscript by the reviewer are highly appreciated. We have included the type and grade of the mild steel used in our study in the revised manuscript.

Q5: Some typo errors and grammatical mistakes are present trought the text (es. row 129 "..was washed"; row 131 "o" is missing; rows 290-291 "...being diffused" and so on).

A5: We regret for our mistakes and acknowledge your valuable suggestions. Following your suggestion, the manuscript has been thoroughly revised and necessary correction has been done.

Author Response File: Author Response.docx

Reviewer 2 Report

In this work, molybdate anions intercalated functionlized Mg-Al LDH was used to reinforce epoxy polymeric coating to improve the corrosion resistance. The ant-corrosion performance of the composite coating was evaluated electrochemical experiments. The work has presented some interesting results for publication. However, revision is required to further improve the quality of the manuscript by considering the following comments.

1. English used should be improved and proof reading is needed to remove errors and typos.

2. Abstract should be concise and include more main results.

3. Figure 1 should be in Section 2 rather than in Introduction. In addition, the quality of the figure should be improved because it is hard to read.

4. Figure 2: all the peaks in the figure should be identified. The authors have claimed that “After intercalation of molybdate anion, a slight displacement of diffraction peaks towards low 2θ angle was observed. However, the basal spacing were not affected significantly, which might be due to the small amount of molybdate anion or there might also be a probability that molybdate ion assumed some unusual structure within the LDH interlayer.” However, it seems that there is reaction from the figure.

5. Figure 4: please provide one image for the three cases at the same magnification. The image should show the typical microstructural features from the three conditions. In addition, professional scale bars are required and the phase should be indicated on the figures.

6. Figure 5 and Table 1, the meanings of the values of Rs and Rct should be described, by referring to and citing Corrosion Science 102 (2016) 484-489 and ACS Biomaterials Science & Engineering 4 (2018) 2633-2642.

7. The authors have mentioned OCP but there are no results presented in the manuscript.

8. Figure 6: please indicate the method to define the parameters in Table 2, by referring to and citing Electrochimica Acta 232 (2017) 89-97 and Corrosion Science 123 (2017) 289-296.

9. It was mentioned that the anti-corrosion is related to the interfacial interaction with epoxy matrix and to firmly adhere the coating on steel substrate. However, there is no direct evidence for that. Please provide microstructure images after corrosion.

10. Conclusions: more results should be included.

Author Response

Manuscript ID: jcs-415042

Title: “Anti-corrosion properties of the epoxy composite coating reinforced by molybdate intercalated functionalized layered double hydroxide"

Dear Editor,

Reviewer #2:

Answer: We are thankful to you for kindly evaluating our manuscript. We have carefully checked the manuscript and corrected accordingly.

Q1: English used should be improved and proof reading is needed to remove errors and typos.

A6: We highly acknowledge the thorough evaluation of our manuscript; the suggestion has been heartily addressed in the revised manuscript. We have thoroughly revised the manuscript to remove minimize the error typos.

Q2: Abstract should be concise and include more main results.

A2: We are thankful for your constructive suggestion. The abstract has been made concise and results have been included in the revised manuscript.

Q3: Figure 1 should be in Section 2 rather than in Introduction. In addition, the quality of the figure should be improved because it is hard to read.

A3: We are thankful for your valuable suggestions. The Figure 1 has been kept in section 2 and accordingly quality of the Figure has been improved.

Q4: Figure 2: all the peaks in the figure should be identified. The authors have claimed that “After intercalation of molybdate anion, a slight displacement of diffraction peaks towards low 2θ angle was observed. However, the basal spacing were not affected significantly, which might be due to the small amount of molybdate anion or there might also be a probability that molybdate ion assumed some unusual structure within the LDH interlayer.” However, it seems that there is reaction from the figure.

A4: We are thankful for your valuable suggestions. We calculated the spacing between the two basal plane (003) for LDH and Mo-LDH, but the difference was only of ~0.6 A°. Therefore, on the basis of the calculated spacing value, we wrote that the basal spacing were not affected significantly.

Q5: Figure 4: please provide one image for the three cases at the same magnification. The image should show the typical microstructural features from the three conditions. In addition, professional scale bars are required and the phase should be indicated on the figures.

A5: We are thankful for your valuable suggestions. We had provided FE-SEM image for all the cases at the same magnification in the manuscript. We acknowledge the suggestion, the inclusion of microstructural features of composite coating would have polish our manuscript but due to some technical error in electron gun, our FE-SEM machine is not providing images beyond 20 micrometer.

Q6: Figure 5 and Table 1, the meanings of the values of R_s and R_ct should be described, by referring to and citing Corrosion Science 102 (2016) 484-489 and ACS Biomaterials Science & Engineering 4 (2018) 2633-2642.

A6: We are thankful for your valuable suggestions. The meanings of the values of Rs and R_cthave been described by including the given references in the revised manuscript as suggested by the reviewer.

Q7: The authors have mentioned OCP but there are no results presented in the manuscript.

A7: We are sorry for the mistake, we intended to write that OCP is achieved before electrochemical test and all the measurements are carried out at and around OCP.

Q8: Figure 6: please indicate the method to define the parameters in Table 2, by referring to and citing Electrochimica Acta 232 (2017) 89-97 and Corrosion Science 123 (2017) 289-296.

A8: We are thankful for your valuable suggestions. The parameters have been defined by citing the reference in the revised manuscript as suggested by the reviewer.

Q9: It was mentioned that the anti-corrosion is related to the interfacial interaction with epoxy matrix and to firmly adhere the coating on steel substrate. However, there is no direct evidence for that. Please provide microstructure images after corrosion.

A9: We are thankful for your valuable suggestions. The FE-SEM provided in the manuscript was done after the electrochemical test. Due to the some error in electron gun, we could not take the image beyond 20 μm. We acknowledge the suggestion put forward by the respected reviewer as this if done would have make our work more informative, but within this short period of time we are unable to perform the FE-SEM.

Q10: Conclusions: more results should be included.

A10: We are thankful for your valuable suggestions. As per the suggestion of the reviewer, the important results have been included in the conclusion.

Author Response File: Author Response.docx

Reviewer 3 Report

This paper introduces an approach to modify the epoxy using LDH, Mo-LDH, and APTES-Mo-LDH to improve the anti-corrosion property of the epoxy. The topic is interesting, the paper is well-organized and easy to follow. Only some minor comments to consider for further improvements:

1) Page 4, Line 132, "calculated amount of APTES-Mo-LDH powder was dispered in organic solvent (THF)...." how much? How to calculate? Show the detail composition is needed for others to repeat the test if needed.

2) Page 6, Results, Line 188 to 198 and Figure 2a, there are numbers showing in the texts such as 1640, 983, 500-800... not showing in Figure 2a. Please include those in the figure.

3) Page 6, Line 192, Figure b should be Figure 2a?

4) Page 6, Figure 2b, no explination on APTES-Mo-LDH?

5) Page 7, Figure 3 is problematic. It should be "weight" in the vertical axles instead of "weight loss" based on reading the contents from Line 216-224. Also, please include some lines (indicators) in Figure 3 to show the temperature ranges for each of LDH to degrade rapidly.

6), Page 7, Figure 3, so APTES-Mo-LDH degrades very fast in high temperature and not applicable in high T application?

7) Page 8, Line 249, "LDH is able to improve the adherence of coating..." it may not be able to see from the surface structure, a cross-section image will help to show the adherence with substrate which is suggested to include.

8) Page 11, Table 2, corrosion rate is suggested to calculate based on the Tafel test to compare in Table 2.

Author Response

Manuscript ID: jcs-415042

Title: “Anti-corrosion properties of the epoxy composite coating reinforced by molybdate intercalated functionalized layered double hydroxide"

Dear Editor,

Reviewer #3:

Answer: We are thankful to you for kindly evaluating our manuscript. We have carefully checked the manuscript and corrected accordingly

Q1: Page 4, Line 132, "calculated amount of APTES-Mo-LDH powder was dispersed in organic solvent (THF)...." how much? How to calculate? Show the detail composition is needed for others to repeat the test if needed.

A1: We acknowledge your valuable suggestion the calculated amount along with the epoxy hardener ratio has been added in the revised manuscript. Epoxy hardener ratio used in our current study was 2:1. To prepare epoxy composites, 10 gm of epoxy resin and 5 gm of hardener was used. With respect to epoxy hardener system, 1 wt% of APTES-Mo-LDH i.e, ~150 mg was used.

Q2: Page 6, Results, Line 188 to 198 and Figure 2a, there are numbers showing in the texts such as 1640, 983, 500-800... Not showing in Figure 2a. Please include those in the figure.

A2: We acknowledge your valuable suggestion. We have included wave numbers in the figure as per the valuable suggestion of reviewer.

Q3: Page 6, Line 192, Figure b should be Figure 2a?.

A3: We regret our error, correction has been done in the revised manuscript.

Q4: Page 6, Figure 2b, no explanation on APTES-Mo-LDH?

A4: We acknowledge your valuable suggestion; explanation has been included in the revised manuscript. We have included the explanation of APTES-Mo-LDH in revised manuscript.

Q5: Page 7, Figure 3 is problematic. It should be "weight" in the vertical axles instead of "weight loss" based on reading the contents from Line 216-224. Also, please include some lines (indicators) in Figure 3 to show the temperature ranges for each of LDH to degrade rapidly.

A5: We acknowledge your valuable suggestion. We have marked the thermograms to indicate the thermal degradation range in the revised manuscript.

Q6: Page 7, Figure 3, so APTES-Mo-LDH degrades very fast in high temperature and not applicable in high T application?

A6: We acknowledge your critical review of our manuscript. The fast degradation of APTES-Mo-LDH at high temperature does not mean that it will not perform at high temperature. The degradation is mainly due to the decomposition of alkyl chain and loss of OH and SO₂ from surface of LDH. From the TGA thermogram that the weight loss at 700 ^oC was found to be ~ 50%. Moreover, it is rare that the polymeric coating is subjected to such high temperature.

Q7: Page 8, Line 249, "LDH is able to improve the adherence of coating..." it may not be able to see from the surface structure, a cross-section image will help to show the adherence with substrate which is suggested to include.

A7: We highly acknowledge your valuable suggestion. Due to the some technical error in electron gun, we are unable to take the micrograph below 20 micrometer using our FE-SEM machine. We respect the noteworthy suggestion given by the reviewer, but within this short period of time we could not outsource the FE-SEM facility.

Q8: Page 11, Table 2, corrosion rate is suggested to calculate based on the Tafel test to compare in Table 2.

A8: We acknowledge your valuable suggestion. We have calculated the corrosion rate in the revised manuscript.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors amended the manuscript removing and fulfilling the imprecision noted. It is my opinion the manuscript can be pubbliche in the present status.

Author Response

We are thankful to the reviewers for kindly evaluation our manuscript. The English has been checked through out the manuscript. Hope that you find everything in order in the revised version of the manuscript.

Reviewer 2 Report

The revision is acceptable for publication.

Author Response