Microbiological Deterioration of Epoxy Coating on Carbon Steel by Pseudomonas aeruginosa

Round 1

Reviewer 1 Report

The manuscript submitted for review deals with the microbiological damage of epoxy coatings. Manuscript is very well written and interesting. However, in my opinion some major corrections should be apply before publication.

Major remarks

1.     Introduction: there is no information on how common microbial corrosion is, especially with the presence of P.aeruginosa.

2.     Materials and methods: epoxy resins are basically resistant to alcohol, however, I have the impression that this substance has been in contact with the samples too often. Too long/frequent contact with ethanol may damage the surface and distort test results.

3.     There is no information on the exchange of medium during sample incubation.

4.     I am wondering about the sense of performing comparative corrosion tests. Since damage to the coatings is observed, it is obvious that the corrosion resistance will deteriorate, so performing a corrosion test is pointless.

5.     Subchapter 2.5: The composition of the electrochemical tests solution is not specified. If these were analogous solutions as for incubation, this is a serious methodological error - they had different concentrations.

6.     No control sample was used in the corrosion tests.

7.     There is no assessment of coating damage depth and no prediction of damage rate.

Minor remarks

1.     By nature, polymers are organic materials. So why do authors notoriously use the term "organic polymers"?

2.     Page 2, line 1-3: sentence suggests that the polymer chain breaks down at certain points, which is not the case.

3.     P2, L11: “destroy the mechanical properties” – rather reduce than destroy

4.     Term “morphology”: according to Cambridge Dictionary, morphology is “the scientific study of the structure and form”. In fact, the authors did not assess the surface morphology, but the condition of the surface. However, the distribution of bacteria on the surface can be described as their morphology. Please, correct whole manuscript.

5.     Subchapter 2.1 “All specimens” or “steel substrate” instead of “All carbon steels”

6.     Next page: rod applicator – can you explain ? Was it a brush, since "the obtained mixture was evenly brushed"?

7.     Marine medium 2216E – it needs to be explained/cited

8.     “pH was adjusted to 7.6” – what was added to change the pH?

9.     SEM observations: in high or low vacuum mode ?

10.  “The structure of coupon surfaces…” was it really “structure” ? Please, use proper nomenclature.

Author Response

Comments and Suggestions for Authors

The manuscript submitted for review deals with the microbiological damage of epoxy coatings. Manuscript is very well written and interesting. However, in my opinion some major corrections should be apply before publication.

- Thank you for your comments.

Major remarks

1. Introduction: there is no information on how common microbial corrosion is, especially with the presence of P.aeruginosa.

- The information about the significance of microbial corrosion study was added to the first paragraph of Introduction part. MIC refers to the material degradation caused by microbial activities or corrosive microbial metabolites, which accounts for 20% of global corrosion cost. The relevant literatures were cited. The influence of Pseudomonas aeruginosa in MIC was described in the third paragraph of Introduction part. Pseudomonas aeruginosa is a typical gram-negative bacterium which is widely inhabited in the marine environment. The severe corrosion of carbon steels and stainless steels caused by marine Pseudomonas aeruginosa in simulated marine environment has been frequently reported.

2. Materials and methods: epoxy resins are basically resistant to alcohol, however, I have the impression that this substance has been in contact with the samples too often. Too long/frequent contact with ethanol may damage the surface and distort test results.

- Indeed, too long/frequent contact between epoxy and alcohol can damage the coating surface. In our experiment, the epoxy coating was wiped with 75% alcohol solution for about 10 seconds. Therefore, the contact time was very short and could not damage the coating surface. We added relevant information to the second paragraph of Subchapter 2.1 to explain it.

3. There is no information on the exchange of medium during sample incubation.

- During sample incubation, the culture medium was not changed.

4. I am wondering about the sense of performing comparative corrosion tests. Since damage to the coatings is observed, it is obvious that the corrosion resistance will deteriorate, so performing a corrosion test is pointless.

- Indeed, it is obvious that the corrosion resistance of epoxy coatings will deteriorate. But the presence of Pseudomonas aeruginosa may accelerate the failure process of epoxy coatings. Therefore, the comparative corrosion tests are meaningful.

5. Subchapter 2.5: The composition of the electrochemical tests solution is not specified. If these were analogous solutions as for incubation, this is a serious methodological error - they had different concentrations.

- In previous MIC studies containing starvation tests, the electrochemical tests were all carried out in the culture media with different nutrient concentrations [1-3]. We refer to these studies to design electrochemical tests. Therefore, electrochemical tests were also conducted in the culture media of Pseudomonas aeruginosa with different nutrient concentrations in this paper.

[1] Dou, W.; Liu, J.; Cai, W.; Wang, D.; Jia, R.; Chen, S.; Gu, T. Electrochemical investigation of increased carbon steel corrosion via extracellular electron transfer by a sulfate reducing bacterium under carbon source starvation. Corros. Sci. 2019, 150, 258-267.

[2] Jia, R.; Yang, D.; Xu, J.; Xu, D.; Gu, T. Microbiologically influenced corrosion of C1018 carbon steel by nitrate reducing Pseudomonas aeruginosa biofilm under organic carbon starvation. Corros. Sci. 2017, 127, 1-9.

[3] Dou, W.; Jia, R.; Jin, P.; Liu, J.; Chen, S.; Gu, T. Investigation of the mechanism and characteristics of copper corrosion by sulfate reducing bacteria. Corros. Sci. 2018, 144, 237-248.

6. No control sample was used in the corrosion tests.

- The corrosion tests in the sterile culture medium were conducted in this paper as control group.

7. There is no assessment of coating damage depth and no prediction of damage rate.

- Indeed, the damage depth and damage rate of the coatings can directly reflect the destructive extent of the coatings. Nevertheless, electrochemical testing is more suitable method to study the failure process of coating. In particular, electrochemical impedance spectroscopy (EIS) can sensitively monitor the evolution process of coating shielding properties. In many studies about the coatings, the corrosion resistance was mainly studied by electrochemical test [1-3]. Therefore, in this paper, we also mainly use electrochemical methods to monitor the change and difference of corrosion resistance of the coatings. Combining with the results of SEM observation, FTIR tests and contact angle tests, The microbial degradation behavior of epoxy coatings was clarified.

[1] Ye, Y.; Liu, Z.; Liu, W.; Zhang, D.; Zhao, H.; Wang, L.; Li, X. Superhydrophobic oligoaniline-containing electroactive silica coating as pre-process coating for corrosion protection of carbon steel. Chem. Eng. J. 2018, 348, 940-951.

[2] Wang, H.; Xu, J.; Du, X.; Wang, H.; Cheng, X.; Du, Z. Stretchable and self-healing polyurethane coating with synergistic anticorrosion effect for the corrosion protection of stainless steels. Prog. Org. Coat. 2022, 164, 106672.

[3] Tabish, M.; Zhao, J.; Wang, J.; Anjum, M. J.; Qiang, Y.; Yang, Q.; Mushtaq, M.; Yasin, G. Improving the corrosion protection ability of epoxy coating using CaAl LDH intercalated with 2-mercaptobenzothiazole as a pigment on steel substrate. Prog. Org. Coat. 2022, 165, 106765.

Minor remarks

1. By nature, polymers are organic materials. So why do authors notoriously use the term "organic polymers"?

- We have revised all “organic polymers” to “polymers”.

2. Page 2, line 1-3: sentence suggests that the polymer chain breaks down at certain points, which is not the case.

- This information was obtained from the literatures. We cited relevant references at this location, as shown in references [20, 21].

3. P2, L11: “destroy the mechanical properties” – rather reduce than destroy

- As requested, the “destroy” was changed to “reduce”.

4. Term “morphology”: according to Cambridge Dictionary, morphology is “the scientific study of the structure and form”. In fact, the authors did not assess the surface morphology, but the condition of the surface. However, the distribution of bacteria on the surface can be described as their morphology. Please, correct whole manuscript.

- As requested, all “morphology” in the manuscript were corrected.

5. Subchapter 2.1 “All specimens” or “steel substrate” instead of “All carbon steels”

- As requested, the “All carbon steels” was revised to “All specimens”.

6. Next page: rod applicator – can you explain ? Was it a brush, since "the obtained mixture was evenly brushed"?

- The photo of rod applicator and the diagram of its instruction are showed below. The advantage of the rod applicator is that the coating thickness can be controlled.

7. Marine medium 2216E – it needs to be explained/cited

- Marine medium 2216E is the most commonly used medium for Pseudomonas aeruginosa inoculation and has been used in many MIC studies about Pseudomonas aeruginosa. The chemical composition of 2216E was provided by the study of Liu et al.. These explanation and relevant literature were added in Subchapter 2.2.

8. “pH was adjusted to 7.6” – what was added to change the pH?

- The pH value was adjusted by dilute sulfuric acid solution and NaOH solution. This information was added in Subchapter 2.2.

9. SEM observations: in high or low vacuum mode?

- SEM observations were carried out in high vacuum mode. This information has been added in Subchapter 2.4.

10. “The structure of coupon surfaces…” was it really “structure” ? Please, use proper nomenclature.

- The “structure” was changed to “chemical composition”.

Reviewer 2 Report

Journal: Coatings

Manuscript ID: coatings 2239449

Manuscript Title: Microbiological deterioration of epoxy coating for the corrosion

protection of carbon steel by Pseudomonas aeruginosa

Comments:

1.     Title: After reading the entire manuscript, it is found that the title is misleading with respect to the manuscript content. Based on the current title, “… corrosion protection of carbon steel by Pseudomonas aeruginosa”, it seems to give an impression that the Pseudomonas aeruginosa microorganism would provide protection to the carbon steel but in fact the existence of this type of microorganism would deteriorate the epoxy coating. In my opinion, the title needs to be amended.

2.     Abstract: The authors did not highlight the importance and significant findings of this research.

3.     Introduction: The authors did not provide substantial background of why Pseudomonas aeruginosa and related microorganisms are important and worthwhile to be studied in or on the coating. This part is vague in this manuscript.

4.     Results and discussion: Section 3.4, Figure 6: The respective contact angles should be labelled in the figure.

5.     Results and discussion: Section 3.5, Figure 7: The FTIR observations and differences were not explained in-depth. It seems similar for all conditions except 0%.

6.     Results and discussion and conclusions: Quantitatively, how much deterioration was experienced by the epoxy coating with the presence of Pseudomonas aeruginosa?

7.     English problem: I suggest the authors to check the English language by native English speakers.  

8.     In summary, the authors are required to make substantial improvement for this manuscript. 

Author Response

Comments and Suggestions for Authors

Comments:

1. Title: After reading the entire manuscript, it is found that the title is misleading with respect to the manuscript content. Based on the current title, “… corrosion protection of carbon steel by Pseudomonas aeruginosa”, it seems to give an impression that the Pseudomonas aeruginosa microorganism would provide protection to the carbon steel but in fact the existence of this type of microorganism would deteriorate the epoxy coating. In my opinion, the title needs to be amended.

- As requested, the title was revised to “Microbiological Deterioration of Epoxy Coating on Carbon Steel by Pseudomonas aeruginosa”.

2. Abstract: The authors did not highlight the importance and significant findings of this research.

- According to your comments, the importance and significant findings of this research were added to the Abstract part. Epoxy coating is a commonly used anticorrosive coating on metal surfaces. Pseudomonas aeruginosa has been reported to be able to accelerate the corrosion of metal materials, but its effect on the corrosion resistance of epoxy coatings is rarely reported.

3. Introduction: The authors did not provide substantial background of why Pseudomonas aeruginosa and related microorganisms are important and worthwhile to be studied in or on the coating. This part is vague in this manuscript.

- We have revised the third paragraph of the Introduction part. Substantial background was added. Epoxy coating is widely used for corrosion protection of metals, which has the characteristics of high adhesion, electrical insulation, high strength, high corrosion resistance. Especially in the marine environment, the steel/epoxy composite coating system is one of the main anticorrosion strategies. Hence, the failure behavior of epoxy coatings under different marine environmental factors, including seawater, salt spray and UV light, has been clearly elucidated, and the related failure mechanism has also been revealed. In addition to above factors, microorganisms are also a major component of the marine environment. In recent years, the effect of microorganisms on the failure of epoxy coatings was concerned. Nonetheless, there are still few studies on the microbiological deterioration of epoxy coating. Pseudomonas aeruginosa is a typical gram-negative bacterium which is widely inhabited in the marine environment. The severe corrosion of carbon steels and stainless steels caused by marine P. aeruginosa in simulated marine environment has been frequently reported. However, the effect of marine P. aeruginosa on the degradation of epoxy coating has not been studied.

4. Results and discussion: Section 3.4, Figure 6: The respective contact angles should be labelled in the figure.

- As requested, the respective contact angles were labelled in the figure.

5. Results and discussion: Section 3.5, Figure 7: The FTIR observations and differences were not explained in-depth. It seems similar for all conditions except 0%.

- In order to show the differences in the FTIR spectrum more clearly, several major peaks were enlarged and showed in Fig. 7.

6. Results and discussion and conclusions: Quantitatively, how much deterioration was experienced by the epoxy coating with the presence of Pseudomonas aeruginosa?

- Indeed, quantitative analysis is a better method to study coating deterioration. Some studies try to quantify the analysis by fitting electrochemical impedance spectroscopy by equivalent electrical circuits. But the fitting results are easy to be affected by the selected equivalent electrical circuits, which may produce large errors. In addition, the coating resistance from the fitting results can only be used for comparison, but cannot be used as the actual corrosion resistance of coatings. Considering these problems, we did not carry out quantitative analysis in this paper. We mainly used low frequency impedance modulus form EIS curves and corrosion current density from polarization curves to analyze the coating deterioration.

7. English problem: I suggest the authors to check the English language by native English speakers.

- As requested, we have checked the English language of this paper and revised some errors.

8. In summary, the authors are required to make substantial improvement for this manuscript.

- Thank you very much for your comments. We have carefully revised this manuscript according to the reviewer's comments.

Reviewer 3 Report

Review of the Article: “Microbiological Deterioration of Epoxy Coating for the Corrosion Protection of Carbon Steel by Pseudomonas aeruginosa” by Shuyuan Zhang et al. submitted to Coatings.

The title is kind of misleading and needs rewording or even changed. “Corrosion protection”??

Major:

1.     Authors claim that they have studied the deterioration mechanism. The manuscript does not show that.

2.     “The epoxy coating in 10% nutrient medium had the highest damage and suffered the most severe deterioration”. How come authors chose only one value of 10% nutrient medium? I say one value because of that 0% and 100% are considered as control. There is a methodological issue here. Usually, a scientists chooses more variables for an experiment to produce reliable results and conclusions. If there are more nutrient values, then authors need to include them. In the present study, this is not acceptable to draw solid conclusions.

3.     Electrochemical analysis results should be summarized in a table to show differences clearly.

4.     Explain why optical density value at 600 nm is a measure of growth?

5.     “The decrease of nutrient concentration leaded to the further decrease of corrosion resistance of the coating, and the coating in the 10% nutrient medium exhibited the lowest |Z|0.01Hz value and maximum Icorr value after 14 days”. According to this then the 0% should give the highest corrosion. Have you tried the 5% or 15% or 20%??

6.     “The results of contact angle tests showed that the adhesion of P. aeruginosa increased the hydrophilicity of the coating surface”. Explain this conclusion in terms of the aim of the research. Is hydrophilicity of the coating surface related to the destruction of surface molecular structure?

7.     FTIR results cannot easily show an acceleration in the degradation of the epoxy. This method is mostly qualitative and has much less accuracy than electrochemical methods. Bonds exist in the epoxy (and on the surfaces) all the time. Experimental error is beyond the claim of authors.

8.     We suggest a study period of longer than 14 days for more applicable results.

9.     TEM is better than SEM for such studies. Please discuss.

Author Response

Comments and Suggestions for Authors

The title is kind of misleading and needs rewording or even changed. “Corrosion protection”??

- Thank you for your comments. The title has been revised to “Microbiological Deterioration of Epoxy Coating on Carbon Steel by Pseudomonas aeruginosa”.

Major:

1. Authors claim that they have studied the deterioration mechanism. The manuscript does not show that.

- Indeed, this paper mainly studied the failure behavior of epoxy coatings. The “mechanism” was inappropriate description. According to your suggestion, we changed the “mechanism” to “behavior”.

2. “The epoxy coating in 10% nutrient medium had the highest damage and suffered the most severe deterioration”. How come authors chose only one value of 10% nutrient medium? I say one value because of that 0% and 100% are considered as control. There is a methodological issue here. Usually, a scientists chooses more variables for an experiment to produce reliable results and conclusions. If there are more nutrient values, then authors need to include them. In the present study, this is not acceptable to draw solid conclusions.

- Thank you for your suggestions. Indeed, more nutrient concentration values need to be studied if we want to determine the concentration at which the failure is most severe. This study mainly showed the variation trend of coating failure rate. In order to describe the failure behavior more precisely, we revised the relevant descriptions in Abstract part, Subchapter 3.2, Subchapter 3.3 and Conclusion part. In Abstract part, the descriptions were revised to “and the coatings suffered more degradation under starvation conditions” and “and the coating had lower low frequency impedance modulus and higher corrosion current density in the inoculated medium with starvation conditions”. In Subchapter 3.2, the description was revised to “The damage of epoxy coating was more severe in nutrient deficient environments with P. aeruginosa”. In Subchapter 3.3, the descriptions were revised to “From EIS results, it can be seen that P. aeruginosa promoted the failure of the epoxy coating, and with the reduction of nutrients, the failure rate of the epoxy coating increased first and then decreased” and “Compared with that in the sterile medium and inoculated medium with 100% nutrients, the epoxy coating in the inoculated media with 10% and 0% nutrients had higher I_corr value and suffered more serious deterioration”. In Conclusion part, the description was revised to “The nutrient starvation environments in the medium leaded to the further decrease of corrosion resistance of the coatings, and the coatings in the inoculated medium under starvation conditions exhibited lower |Z|_0.01Hz value and higher I_corr value than those in the medium with 100% nutrients after 14 days”.

3. Electrochemical analysis results should be summarized in a table to show differences clearly.

- As requested, the electrochemical corrosion parameters, including the corrosion potential (E_corr) and the corrosion current density (I_corr), determined from the polarization curves are summarized, as shown in Table 1.

4. Explain why optical density value at 600 nm is a measure of growth?

- In the microbiologically influenced corrosion (MIC) studies, the growth of bacteria in the culture medium is often characterized by the optical density value, which is measured by ultraviolet spectrophotometer. With the increase of bacterial cell number, the culture medium will become turbid, which increase the optical density value. For the MIC studies caused by Pseudomonas aeruginosa, optical density value at 600 nm is widely adopted to monitor the growth of Pseudomonas aeruginosa, as shown in references below [1, 2]. This is a commonly used method to characterize the growth of Pseudomonas aeruginosa. So we used this measurement method in this paper. We added corresponding explanation to Subchapter 2.2.

[1] San, N. O.; Nazır, H.; Dönmez, G. Microbially influenced corrosion and inhibition of nickel–zinc and nickel–copper coatings by Pseudomonas aeruginosa. Corros. Sci. 2014, 79, 177-183.

[2] Huang, L. Y.; Huang, Y.; Lou, Y.; Qian, H.; Xu, D.; Ma, L.; Jiang, C.; Zhang, D. Pyocyanin-modifying genes phzM and phzS regulated the extracellular electron transfer in microbiologically-influenced corrosion of X80 carbon steel by Pseudomonas aeruginosa. Corros. Sci. 2020, 164, 108355.

5. “The decrease of nutrient concentration leaded to the further decrease of corrosion resistance of the coating, and the coating in the 10% nutrient medium exhibited the lowest |Z|0.01Hz value and maximum Icorr value after 14 days”. According to this then the 0% should give the highest corrosion. Have you tried the 5% or 15% or 20%??

- In order to summarize more accurately, we have revised this description to “The nutrient starvation environments in the medium leaded to the further decrease of corrosion resistance of the coating, and the coatings in the inoculated medium under starvation conditions exhibited lower |Z|_0.01Hz value and higher I_corr value than those in the medium with 100% nutrients after 14 days”. The starvation test in this paper was designed according to several papers with starvation tests [1, 2]. The nutrient concentrations of 100%, 10% and 0% were adopted in these papers. Therefore, we just used these three concentrations to conduct starvation test.

[1] Jia, R.; Yang, D.; Xu, J.; Xu, D.; Gu, T. Microbiologically influenced corrosion of C1018 carbon steel by nitrate reducing Pseudomonas aeruginosa biofilm under organic carbon starvation. Corros. Sci. 2017, 127, 1-9.

[2] Cui, L.; Liu, Z.; Hu, P.; Shao, J. Laboratory investigation of microbiologically influenced corrosion of X80 pipeline steel by sulfate-reducing bacteria. J. Mater. Eng. Perform. 2021, 30, 7584-7596.

[3] Unsal, T.; Xu, L.; Jia, R.; Kijkla, P.; Kumseranee, S.; Punpruk, S.; Mohamed, M.; Saleh, M.; Gu, T. Microbiologically influenced corrosion of titanium by Desulfovibrio vulgaris biofilm under organic carbon starvation. Bioelectrochemistry 2023, 149, 108307.

6. “The results of contact angle tests showed that the adhesion of P. aeruginosa increased the hydrophilicity of the coating surface”. Explain this conclusion in terms of the aim of the research. Is hydrophilicity of the coating surface related to the destruction of surface molecular structure?

- As requested, the reason for the increased hydrophilicity was explained in Subchapter 3.4. A prerequisite for biofilm formation is to form a conditioning film, which contains some organic chemicals such as proteins. The conditioning film is firmly combined with the epoxy coating surface and is difficult to be completely removed. This should be the reason for the increased hydrophilicity of the coating surface. The increase of hydrophilicity will aggravate the damage of water to the epoxy coating.

7. FTIR results cannot easily show an acceleration in the degradation of the epoxy. This method is mostly qualitative and has much less accuracy than electrochemical methods. Bonds exist in the epoxy (and on the surfaces) all the time. Experimental error is beyond the claim of authors.

- Indeed, FTIR is qualitative and has much less accuracy than electrochemical methods. We want to analyze the chemical composition changes on the coating surface by FTIR, which can help analyze the reason why Pseudomonas aeruginosa can destroy the epoxy coating. Some studies about the microbiological deterioration of coatings also used FTIR to analyze the change of chemical compositions. Therefore, we referred to these literatures and also performed FTIR tests.

[1] Wang, G.; Chai, K.; Wu, J.; Liu, F. Effect of Pseudomonas putida on the degradation of epoxy resin varnish coating in seawater. Int. Biodeter. Biodegr. 2016, 115, 156-163.

[2] Feng, T.; Wu, J.; Chai, K.; Liu, F. Effect of Pseudomonas sp. on the degradation of aluminum/epoxy coating in seawater. J. Mol. Liq. 2018, 263, 248-254.

[3] Deng, S.; Wu, J.; Li, Y.; Wang, G.; Chai, K.; Yu, A.; Liu, F. Effect of Bacillus flexus on the degradation of epoxy resin varnish coating in seawater. Int. J. Electrochem. Sci. 2019, 14, 315-328.

8. We suggest a study period of longer than 14 days for more applicable results.

- Thank you for your suggestions. Indeed, longer experimental time can obtain more applicable results. However, under laboratory conditions, the activity of Pseudomonas aeruginosa was poor after 14 days, which was not enough to sustain longer experimental time. In future work, we will try to change the experimental method to meet the needs of longer experimental time.

9. TEM is better than SEM for such studies. Please discuss.

- Thank you very much for providing us with a very good research idea. Indeed, TEM may obtain more critical information about the microbial/coating interface. It is helpful to analyze the mechanism of microbiological deterioration of coating. In this paper, we mainly reveal the microbiological deterioration behavior of epoxy coating. Compared with TEM, SEM can more clearly and intuitively show the conditions of the biofilms on the epoxy coating surfaces and the destruction behaviors of the epoxy coatings. In the future work about the mechanism study, we will try to use TEM to study the interaction at the microbial/coating interface.

Round 2

Reviewer 1 Report

The manuscript was corrected, however, I am amazed by the carefree approach to the methodology of corrosion testing. These results are analyzed in complete isolation from the use of different environments. As the Authors admitted in response to comment #4: “Indeed, it is obvious that the corrosion resistance of epoxy coatings will deteriorate. But the presence of Pseudomonas aeruginosa may accelerate the failure process of epoxy coatings. Therefore, the comparative corrosion tests are meaningful”. How can you compare the results obtained in different environments? Does it make sense? It is obvious that such approach does not make any sense.

It would make sense to evaluate the same coatings in different environments, or different coatings in the same environment.

There is still no description of corrosive environments in the methodology.

Your reply to comment #5 stated: “… the electrochemical tests were all carried out in the culture media with different nutrient concentrations [1-3]. We refer to these studies to design electrochemical tests”. The fact that the reviewers of previous works did not notice an obvious methodological error does not release you from the obligation to use correct research methods in your works. If you want to compare corrosion resistance, you cannot introduce more variables without quantitative knowledge of their effect on the corrosion process. Now you must find the solution of the situation. This is a key point in evaluating the entire manuscript.

Author Response

Comments and Suggestions for Authors

The manuscript was corrected, however, I am amazed by the carefree approach to the methodology of corrosion testing. These results are analyzed in complete isolation from the use of different environments. As the Authors admitted in response to comment #4: “Indeed, it is obvious that the corrosion resistance of epoxy coatings will deteriorate. But the presence of Pseudomonas aeruginosa may accelerate the failure process of epoxy coatings. Therefore, the comparative corrosion tests are meaningful”. How can you compare the results obtained in different environments? Does it make sense? It is obvious that such approach does not make any sense.

- Thank you for your comments. In this paper, the corrosion testing included sterile group with 100% nutrient, inoculated group with 100% nutrient, inoculated group with 10% nutrient and inoculated group with 0% nutrient. For the comparison between sterile group and inoculated group with 100% nutrient, the only variable was Pseudomonas aeruginosa. Through this comparison, we can understand the influence of Pseudomonas aeruginosa. For the comparison between inoculated group with 100% nutrient and inoculated group with 10% nutrient and inoculated group with 0% nutrient, the only variable was nutrient concentration. Through this comparison, we can understand the influence of nutrient concentration. The same epoxy coatings were used in all corrosion testing. The comparison certainly makes sense when we control a single variable. In order to illustrate this point, we have explained it in Subchapter 2.3 and Subchapter 2.6 of the article.

It would make sense to evaluate the same coatings in different environments, or different coatings in the same environment.

- In this study, we evaluated the same epoxy coatings in different environments (sterile group with 100% nutrient, inoculated group with 100% nutrient, inoculated group with 10% nutrient and inoculated group with 0% nutrient). As answered above, we strictly control the single variable, so we think it makes sense.

There is still no description of corrosive environments in the methodology.

- In the studies of microbial corrosion or microbial degradation, the corrosive environments mainly consist of microorganisms and corresponding culture medium. For most of microbial corrosion or microbial degradation studies, the corrosion testing is carried out in microorganism inoculated culture medium. In this paper, Pseudomonas aeruginosas and the culture medium are described in the introduction part and Subchapter 2.2, respectively.

Your reply to comment #5 stated: “… the electrochemical tests were all carried out in the culture media with different nutrient concentrations [1-3]. We refer to these studies to design electrochemical tests”. The fact that the reviewers of previous works did not notice an obvious methodological error does not release you from the obligation to use correct research methods in your works. If you want to compare corrosion resistance, you cannot introduce more variables without quantitative knowledge of their effect on the corrosion process. Now you must find the solution of the situation. This is a key point in evaluating the entire manuscript.

- There may be more suitable experimental solutions and experimental methods, which need to be gradually explored and improved. At least in the current MIC researches, most of the literatures adopts this experimental method. I will share your opinion and comment with my colleagues so that we can find better research methods together. Thank you very much.

Reviewer 3 Report

Table 1 is missing some results especially the corrosion rate or corrosion resistance.

Author Response

Comments and Suggestions for Authors

Table 1 is missing some results especially the corrosion rate or corrosion resistance.

- Thank you for your comments. For organic coatings, corrosion rate is generally difficult to be calculated directly, and is often analyzed and compared by electrochemical parameters such as low frequency impedance modulus or corrosion current density. In order to better show the difference in corrosion resistance, the low frequency impedance modulus is added to Table 1.

Round 3

Reviewer 1 Report

I am not completely satisfied with the answers. The authors revised the manuscript sufficiently to be accepted for publication.