Influence of a Plasma Nitriding Treatment on the Corrosion Behavior of API 5L X70 Steel in Simulated Soil Solution

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear author,

I have some comments to your intersting paper.

• In your experimental section it is written that teh specimens have been stored for later use. How had they be stored (under air, in an desiccator, ...)?
• - In 3.1.2 You'd written the same text for 4 h as for 6 h this must be changed..
• In table 3 no carbon is listed? Why?
• In table 5 no oxygen is given. Did you found none?
• The amount of 3.3 wt.% of C in the blank steel is quite high. Is this correct?
• The EDS spectra do not differ much. The tables provide enough informations in my opinion.
• Indication of the pattern in fig. 6 would be helpful. Otherwise the reader has to read the text plus checkicg the figure 6.
• Reactions 4 and 5 indicate an acidic environment (H+) while eq. 8 is related to a basic environmnet (OH-). Can you comment on that? 
• Is there any effect of the released hydrogen (eq. 5, 6, 7)?
• Please add wt.% in table 9 for the N content.
• There is no figure caption for fig. 11 (cross section).
• What are the thicknesses of the nitride layers after 4, 6 or 8 h, respectively. This would be intersting to compare with the value after 10 h since the N content does not change much

Regards

Reviewer

Comments on the Quality of English Language

Dear author,

please check your English, e.g., six must be sixth reaction or reaction six.... as described by the following equations (9) but only one equation is given, e.g. singular. 

Author Response

Review Report Form

Open Review

( ) I would not like to sign my review report
(x) I would like to sign my review report

Quality of English Language

(x) The English could be improved to more clearly express the research.
( ) The English is fine and does not require any improvement.

 

	Yes	Can be improved	Must be improved	Not applicable
Does the introduction provide sufficient background and include all relevant references?	( )	(x)	( )	( )
Is the research design appropriate?	( )	(x)	( )	( )
Are the methods adequately described?	( )	(x)	( )	( )
Are the results clearly presented?	( )	(x)	( )	( )
Are the conclusions supported by the results?	( )	(x)	( )	( )
Are all figures and tables clear and well-presented?	( )	(x)	( )	( )

Comments and Suggestions for Authors

Dear author,

I have some comments to your intersting paper.

• In your experimental section it is written that teh specimens have been stored for later use. How had they be stored (under air, in an desiccator, ...)?
• Section 2.1 specifies the storage of the samples after roughing, indicating that they were stored in a desiccator for later use.
• In 3.1.2 You'd written the same text for 4 h as for 6 h this must be changed..

In the results section, sections 3.1.2 and 3.1.3, the texts describing the EDS of the samples with 4 and 6 h of treatment were corrected.

• In table 3 no carbon is listed? Why?

The results of the sample analysis are requested again and it appears that there was an error in the data extraction, since the characteristic carbon peak is clearly present in the spectrum.

• In table 5 no oxygen is given. Did you found none?

The presence of oxygen in the samples may be due to their tendency to corrode during storage and handling. Furthermore, when they are taken to the institute where the characterization is performed, the samples are placed in a waiting line, as many other researchers also send their samples to this facility.

• The amount of 3.3 wt.% of C in the blank steel is quite high. Is this correct?

These were the results obtained from the characterization, however, the carbon content of the sample is not the same as what would be expected from an X70.

• The EDS spectra do not differ much. The tables provide enough informations in my opinion.

I also agree with that, but on some occasions I have been asked for more information or graphics and that is why we include them.

• Indication of the pattern in fig. 6 would be helpful. Otherwise the reader has to read the text plus checkicg the figure 6.

XRD patterns were added in Figure 6

• Reactions 4 and 5 indicate an acidic environment (H+) while eq. 8 is related to a basic environmnet (OH-). Can you comment on that? 

With respect to electrochemical reactions, the solution used for the tests is of neutral pH. In reactions 4 and 5, the basic cathodic reactions that occur in any corrosion process are discussed, which would be the dissolution of hydrogen and oxygen.

• Is there any effect of the released hydrogen (eq. 5, 6, 7)?

As mentioned in the introduction, hydrogen tends to promote the appearance of stress corrosion cracking (SCC).

• Please add wt.% in table 9 for the N content.

%wt of nitrogen was added to table 6

• There is no figure caption for fig. 11 (cross section).

Added title to Figure 11 (cross section)

• What are the thicknesses of the nitride layers after 4, 6 or 8 h, respectively. This would be intersting to compare with the value after 10 h since the N content does not change much

Only the thickness of the nitrided layer was measured after 10 hours of treatment, since it was the one that presented the best results.

Regards

Reviewer

Comments on the Quality of English Language

Dear author,

please check your English, e.g., six must be sixth reaction or reaction six.... as described by the following equations (9) but only one equation is given, e.g. singular. 

The English in the text was improved.

Submission Date

17 May 2025

Date of this review

23 Jun 2025 16:21:49

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,
The work is interesting and important from a practical point of view. Please pay attention to the correct numbering of figures and tables in relation to the content contained in the manuscript. In order for the article to be published, it requires explanations and additions, according to the guidelines contained in the attached file.

Regards

Reviewer

 

Comments for author File: Comments.pdf

Author Response

Dear Authors,

The work is interesting and important from a practical point of view. Please pay attention to the

correct numbering of figures and tables in relation to the content contained in the manuscript. In

order for the article to be published, it requires explanations and additions, as follows:

• Line 60 – Was cooling used during cutting?

The samples were lubricated with lard during cutting, and the cutting was carried out at low speeds with a diamond disc to avoid damaging the material.

• Line 120 – Please provide the ASTM standards on which the electrochemical tests were based.

The standards used for electrochemical tests were ASTM G03, G59 and G106 which were added in section 2.5

• Line 141 – Table 2. In the authors' opinion, are the results obtained using EDS for carbon contentreliable?

These were the results obtained from the characterization, however, the carbon content of the sample is not the same as what would be expected from an X70.

Line 145 – Do you definitely refer to Figure 3 and Table 4?

The order of figures and tables was corrected.

• Lines 150, 159, 167, 175 – Why is the C content not included in Table 3? Please explain where the differences in the content of, for example, Al and Ni in the individual tables come from.

The results of the sample analysis are requested again and it appears that there was an error in the data extraction, since the characteristic carbon peak is clearly present in the spectrum.

• Line 205. Are the Authors able to demonstrate the presence of pitting corrosion?

To demonstrate pitting corrosion we would have to apply electrochemical noise or remove the corrosion products and observe the surface with SEM to visualize what type of corrosion occurred, however, these tests were not performed.

• Line 217. This probably refers to Table 7.

The order of the tables was ordered

• Line 332. It should be Figure 8, not 12.

The order of the figures is verified and corrected.

• Line 237. It should be Table 8.

The order of the tables was verified

• Line 240 and 270. Please correct the figure numbers and separate them or describe them as (a) and (b).

The suggested modifications were made based on sections (a) and (b)

• Line 273. Please correct the figure number, there is no reference in the manuscript text.

The figures are rearranged

• Line 305. Please correct the figure number.

The order of the figures was fixed
• Line 337. There is no description under the photo. It would be valuable to compare the crosssections of the samples: non-nitrided and nitrided before and after corrosion tests.

Only the thickness of the nitrided layer was measured on the sample, which had the best results after 10 hours of treatment. Furthermore, since it is a difficult-to-obtain material, it was not possible to obtain more samples for further testing.

Best regards,

Reviewer

Reviewer 3 Report

Comments and Suggestions for Authors

Dear authors of the manuscript. The work has been done at a high level, but there are some questions, comments and suggestions that will improve the level of the manuscript and the interest of readers.

1. In the "Introduction" we would like to see a more detailed description of alternative methods of corrosion protection, and not just references to the works of other authors. It would be useful if the text of the work indicated the main advantages and disadvantages of alternative methods of corrosion protection. There is also plasma-electrolytic chemical-thermal treatment of alloys. The authors could familiarize themselves with the works of Mukhacheva T.L., Kusmanov S.A. of recent years, using the published works in MDPI, so that the literature review would be more interesting.
2. In the text of the manuscript, it is necessary to indicate the composition of API 5L X70 steel in brackets. It is assumed that after the publication of your work, readers from different countries will be able to read it, where the marking of API 5L X70 steel analogues may differ.
3. The question concerns the corrosion test methodology. There are doubts that the equilibrium OCP potential will be established after 20 minutes. Usually, it takes 1-3 hours, even for stainless steel or titanium materials. Were the samples kept in the corrosive environment for only 20 minutes before testing? Also, the methodology does not specify the area of ​​the shielded surface. Was the entire surface of the sample subjected to electrochemical corrosion or a local area of ​​the surface?
4. Figure 1. Percentage of each element present in the blank sample. Incorrect figure title. The Y-axis has completely different units of measurement, not percentages. The title of figures 2, 3, 4, 5 is also incorrect.
5. Tables 3 and 6 indicate the oxygen content, but the spectra obtained as a result of the experiment, shown in Figures 2 and 5, do not indicate oxygen. It is also interesting why oxygen is contained in large quantities of 2.7 % and 7.0 %, if nitriding was carried out in a nitrogen and hydrogen gas environment?
6. Figure 8 y 9. It is probably more correct to write Figure 8 and 9.
7. Tables 8 and 9 are not found sequentially in the text of the manuscript.

Comments for author File: Comments.pdf

Author Response

Review Report Form

Open Review

(x) I would not like to sign my review report
( ) I would like to sign my review report

Quality of English Language

( ) The English could be improved to more clearly express the research.
(x) The English is fine and does not require any improvement.

 

	Yes	Can be improved	Must be improved	Not applicable
Does the introduction provide sufficient background and include all relevant references?	( )	(x)	( )	( )
Is the research design appropriate?	(x)	( )	( )	( )
Are the methods adequately described?	( )	(x)	( )	( )
Are the results clearly presented?	(x)	( )	( )	( )
Are the conclusions supported by the results?	(x)	( )	( )	( )
Are all figures and tables clear and well-presented?	( )	(x)	( )	( )

Comments and Suggestions for Authors

Dear authors of the manuscript. The work has been done at a high level, but there are some questions, comments and suggestions that will improve the level of the manuscript and the interest of readers.

1. In the "Introduction" we would like to see a more detailed description of alternative methods of corrosion protection, and not just references to the works of other authors. It would be useful if the text of the work indicated the main advantages and disadvantages of alternative methods of corrosion protection. There is also plasma-electrolytic chemical-thermal treatment of alloys. The authors could familiarize themselves with the works of Mukhacheva T.L., Kusmanov S.A. of recent years, using the published works in MDPI, so that the literature review would be more interesting.

Added some references to MDPI Works and the Mukhacheva T. L.

1. In the text of the manuscript, it is necessary to indicate the composition of API 5L X70 steel in brackets. It is assumed that after the publication of your work, readers from different countries will be able to read it, where the marking of API 5L X70 steel analogues may differ.

The composition is presented in the Results section, 3.1. SEM and EDS.

1. The question concerns the corrosion test methodology. There are doubts that the equilibrium OCP potential will be established after 20 minutes. Usually, it takes 1-3 hours, even for stainless steel or titanium materials. Were the samples kept in the corrosive environment for only 20 minutes before testing? Also, the methodology does not specify the area of ​​the shielded surface. Was the entire surface of the sample subjected to electrochemical corrosion or a local area of ​​the surface?

In this particular case, the OCP stabilized after 20 minutes. Stabilization will depend on the material and medium. It's understood that stainless steel or titanium may take that long, but in what medium and concentration? Thus, first, OCP was performed for 20 minutes, and then, once the potential stabilized, potentiodynamic polarization curves were performed. The exposed area, as mentioned, is 1 cm.

1. Figure 1. Percentage of each element present in the blank sample. Incorrect figure title. The Y-axis has completely different units of measurement, not percentages. The title of figures 2, 3, 4, 5 is also incorrect.

The figure title was changed.

1. Tables 3 and 6 indicate the oxygen content, but the spectra obtained as a result of the experiment, shown in Figures 2 and 5, do not indicate oxygen. It is also interesting why oxygen is contained in large quantities of 2.7 % and 7.0 %, if nitriding was carried out in a nitrogen and hydrogen gas environment?

The presence of oxygen in the samples may be due to their tendency to corrode during storage and handling. Furthermore, when they are taken to the institute where the characterization is performed, the samples are placed in a waiting line, as many other researchers also send their samples to this facility.

1. Figure 8 y 9. It is probably more correct to write Figure 8 and 9.

The order of the figures was organized.

1. Tables 8 and 9 are not found sequentially in the text of the manuscript.

The sequence of the tables was fixed.

peer-review-47657696.v2.pdf

Submission Date

17 May 2025

Date of this review

21 Jun 2025 11:24:33

 

Reviewer 4 Report

Comments and Suggestions for Authors

1- In introduction, while plasma nitriding has been shown to enhance corrosion resistance, there are alternative methods such as electroless Ni-P coatings, pvd deposition, or hybrid systems. Can you discuss why plasma nitriding was selected over other technologies and provide a brief comparison? Please support this section by citing the following reference:

https://doi.org/10.1016/j.matchemphys.2022.127170

 

2- The study shows that a 10-hour plasma nitriding treatment yielded the highest corrosion resistance. What is the scientific basis for choosing 4, 6, 8, and 10 hours as treatment durations? Mention in the text. Furthermore, from an industrial perspective, is 10 hours a technically and economically viable process time?

 

3- According to the EDS results, the nitrogen content does not significantly increase between 6 and 10 hours, yet the corrosion resistance continues to improve. Can you clarify whether this improvement is due to microstructural changes in the nitrided layer (e.g., phase transformations or grain refinement) rather than elemental content?

4- Statistical Validation and Error Analysis. Although you mention that all tests were performed in triplicate, the manuscript lacks error bars or statistical analysis.

Author Response

Review Report Form

Open Review

(x) I would not like to sign my review report
( ) I would like to sign my review report

Quality of English Language

( ) The English could be improved to more clearly express the research.
(x) The English is fine and does not require any improvement.

Comments and Suggestions for Authors

1- In introduction, while plasma nitriding has been shown to enhance corrosion resistance, there are alternative methods such as electroless Ni-P coatings, pvd deposition, or hybrid systems. Can you discuss why plasma nitriding was selected over other technologies and provide a brief comparison? Please support this section by citing the following reference:

https://doi.org/10.1016/j.matchemphys.2022.127170

There are other alternative methods to mitigate corrosion, however, the working group specializes in plasma, which is why it collaborated with the corrosion group to carry out this research work. On the other hand, the suggested literature is cited on introduction.

2- The study shows that a 10-hour plasma nitriding treatment yielded the highest corrosion resistance. What is the scientific basis for choosing 4, 6, 8, and 10 hours as treatment durations? Mention in the text. Furthermore, from an industrial perspective, is 10 hours a technically and economically viable process time?

The treatment times were proposed based on previous work by the working group and some literature. Today, plasma treatment is no longer the expensive process it was in the past. We have more powerful and less expensive equipment, and it is an environmentally friendly method that produces no waste that could harm the environment.

3- According to the EDS results, the nitrogen content does not significantly increase between 6 and 10 hours, yet the corrosion resistance continues to improve. Can you clarify whether this improvement is due to microstructural changes in the nitrided layer (e.g., phase transformations or grain refinement) rather than elemental content?

In this case study, no metallographic study was performed, only electrochemical tests and additional characterization. However, the possibility of structural changes cannot be ruled out.

4- Statistical Validation and Error Analysis. Although you mention that all tests were performed in triplicate, the manuscript lacks error bars or statistical analysis.

In the case of the corrosion area, we rely on standards that establish the conditions for reproducible experiments, so it is not necessary to carry out a large number of repetitions of the test under the same conditions; three is more than necessary.

Submission Date

17 May 2025

Date of this review

15 Jun 2025 20:16:53

 

Round 2

Reviewer 4 Report

Comments and Suggestions for Authors

The authors have addressed the comments and this version can be accepted.