An Electrochemical Study of the Corrosion Behaviour of T91 Steel in Molten Nitrates

Round 1

Reviewer 1 Report

This manuscript, entitled „ An Electrochemical Study of the Corrosion Behaviour of T91 Steel in Molten Nitrates” is relevant to the scope of this journal.

It is an interesting study that can provide valuable information to specialists in the field.

Therefore, the article can be recommended for publication only after mandatory revision according to the following suggestions:

 1.     Since the manuscript can be seen as a continuation with an electrochemical study of reference 27, it must be clearly highlighted what it brings as a novelty.

2.     The displacement of the potential in the case of potentiodynamic polarization is done against the open circuit potential and not against the free corrosion potential value, E_corr. Please check and correct.

3.     "EIS experiments were recorded at the E_corr value". Please check if this is the case and if it is somehow about OCP! Otherwise, the applied potential values must be slightly different...

4.     The same observation in the case of LPR measurements.

5.     Pay attention to the way of expression. For example, "In order to know the formed corrosion products formed on top of corroded specimens".

6.     It is not very well understood how the electrochemical tests were done! Were several samples introduced, which after different periods of immersion were tested under the same conditions or was the same sample tested several times? Or were the samples extracted after the immersion period and were electrochemically tested under other conditions? Please clarify and compete!

7.     Figure 2 must be moved in the text after it is mentioned the first time.

8.     "In this figure it can be seen that the plots displayed an anodic-limit current density value similar to a passive layer". I disagree. In Figure 3, there is no establishment of the value of the corrosion current density, so we cannot speak of passivation! On the contrary, both on the anodic and cathodic branches, the corrosion current density increases with the displacement of the potential from the value of E_corr.

9.     Software used to fit EIS data must be specified. Also, the error recorded in the case of fitting must be specified for all studied samples. Depending on this, we can know if the proposed equivalent electric circuit was good or not!

10.  In Figures 8-10, clearer and possibly color SEM micrographs should be inserted. Also, the scale is not very clear.

11.  Figure 11 is extremely unclear! It must be replaced.

12.  How was the identification of the phases from the XRD spectra done? Either the cards or the bibliographic references must be specified. In the text, a broader discussion should be made based on the XRD analyses, indicating the 2q maxima....

13.  The phases identified in Figure 11 are not found in the corrosion products formed on the surface of the studied alloy! For example, NaCrO₄ appears in the figure, and Na₂CrO₄ in the text. It must be checked and corrected.

14.   The writing style of the references is not uniform and does not fully comply with the journal's requirements.

Author Response

Reviewer 1

1. Since the manuscript can be seen as a continuation with an electrochemical study of reference 27, it must be clearly highlighted what it brings as a novelty. Answer is on page 2, lines 8-14 from bottom.
2. The displacement of the potential in the case of potentiodynamic polarization is done against the open circuit potential and not against the free corrosion potential value, E_corr. Please check and correct. Answer is on page 3, lines 12-16.

 

3. "EIS experiments were recorded at the E_corrvalue". Please check if this is the case and if it is somehow about OCP! Otherwise, the applied potential values must be slightly different. Answer is on page 3, lines 17-18.
4. The same observation in the case of LPR measurements. Answer is on page 3, lines 18-22.
5. Pay attention to the way of expression. For example, "In order to know the formed corrosion products formed on top of corroded specimens". Answer: the last part of this expression “on top of corroded specimens” has been deleted.
6. It is not very well understood how the electrochemical tests were done! Were several samples introduced, which after different periods of immersion were tested under the same conditions or was the same sample tested several times? Or were the samples extracted after the immersion period and were electrochemically tested under other conditions? Please clarify and compete! Answer is on page 3, lines 5-7 from bottom

 

7. Figure 2 must be moved in the text after it is mentioned the first time. This has been done.
8. "In this figure it can be seen that the plots displayed an anodic-limit current density value similar to a passive layer". I disagree. In Figure 3, there is no establishment of the value of the corrosion current density, so we cannot speak of passivation! On the contrary, both on the anodic and cathodic branches, the corrosion current density increases with the displacement of the potential from the value of E_corr. Answer: the paragraph was corrected according to the reviewer comment on page 4, lines 3-4 from bottom.
9. Software used to fit EIS data must be specified. Also, the error recorded in the case of fitting must be specified for all studied samples. Depending on this, we can know if the proposed equivalent electric circuit was good or not! Answer to this is on page 7, lines 9-17.
10. In Figures 8-10, clearer and possibly color SEM micrographs should be inserted. Also, the scale is not very clear. Answer: Clearer SEM micrographs have been included.
11. Figure 11 is extremely unclear! It must be replaced. Answer: A clearer figure has been included
12. How was the identification of the phases from the XRD spectra done? Either the cards or the bibliographic references must be specified. In the text, a broader discussion should be made based on the XRD analyses, indicating the 2q maxima....Answer: Refrence 37 has been included and a more detailed discussion of XRD patterns has been included on page 10, line 1 and lines 17-22.
13. The phases identified in Figure 11 are not found in the corrosion products formed on the surface of the studied alloy! For example, NaCrO₄appears in the figure, and Na₂CrO₄ in the text. It must be checked and corrected. Answer: This has been corrected, including in the text NaCrO₄ instead of Na₂CrO₄
14. The writing style of the references is not uniform and does not fully comply with the journal's requirements. Answer: The instructions guidelines establish that references may be in any style, provided that we use consistent formatting throughout, which I have done, and once the manuscript is accepted, I will be requested to use the journal style.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper focuses on the corrosion behaviour of T91 steel in molten nitrates by means of electrochemical measurements. Even this steel has been previously studied in this environment, the research developed in this paper is considered a valuable contribution to the field as it enhances the employment of electrochemical methods, that are of high interest for the live corrosion monitoring. In my opinion, the investigation is well structured, methodology is suitable and covers all the relevant aspects researched. The paper is suggested to be accepted after following issues are addressed.

1)      The weakest point of the paper is the SEM characterization after corrosion testing. The quality of Figures 8, 9 and 10 must be improved. It is not clear the elements distribution on the surface. It is recommended to use different colours for the different elements analysed. I miss a cross-section analysis of the samples. It would allow to quantify the increase, if any, of the corrosion products thickness along with the temperature. It provides also very relevant information about the distribution of the elements along the corroded layer.

2)      Some research of Encinas-Sánchez is referenced in the text. However, this researcher has different studies focused on the use of EIS for corrosion monitoring in molten nitrates. Specifically, this author has studied the behaviour of T91 steel in the same nitrate mixture used in this research. It is recommended to review his studies so agreement or disagreement among results can provide useful information regarding this system critical for the reinforce the driving of CSP technology in the electricity market.

Author Response

Reviewer 2

1)      The weakest point of the paper is the SEM characterization after corrosion testing. The quality of Figures 8, 9 and 10 must be improved. It is not clear the elements distribution on the surface. It is recommended to use different colours for the different elements analysed. I miss a cross-section analysis of the samples. It would allow to quantify the increase, if any, of the corrosion products thickness along with the temperature. It provides also very relevant information about the distribution of the elements along the corroded layer. Answer: Figures 8-10 have been included in different format, in a much clearer pictures. Unfortunately, the student who carried out the experiments has left the university and took with him the specimens, so it is impossible to make new analysis such as cross-section.

2)      Some research of Encinas-Sánchez is referenced in the text. However, this researcher has different studies focused on the use of EIS for corrosion monitoring in molten nitrates. Specifically, this author has studied the behavior of T91 steel in the same nitrate mixture used in this research. It is recommended to review his studies so agreement or disagreement among results can provide useful information regarding this system critical for the reinforce the driving of CSP technology in the electricity market. Answer: References 32 and 33 have been included for this, and the discussion is on page 6, lines 4-10 from bottom, page 8, lines 5-9 from bottom and page 9, lines 3-4 just after table 4.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

This manuscript entitled "An electrochemical study of the corrosion behavior of T91 steel in molten nitrates" is interesting and well-organized.  It is suitable to publish in Metals after minor revision.  Comments are given as follows:

1. In the caption of Figure 10, the temperarure used in line 302 should be 500 degree centigrade.  Please correct it.

2. How can the author prove the protective corrosion product layers are Cr2O3, FeCr2O4, Na2CrO4, and K2Fe2O4 ?  Please give practical evidence and give some discussions.

Author Response

Reviewer 3

1. 
   In the caption of Figure 10, the temperarure used in line 302 should be 500 degree centigrade.  Please correct it. THIS HAS BEEN CORRECTED.
2. How can the author prove the protective corrosion product layers are Cr2O3, FeCr2O4, NaCrO4, and K2Fe2O4 ?  Please give practical evidence and give some discussions. Answer: Since there is no more evidence that the compounds are protective than all the cited references, the statement that claimed this, in the abstract, the main text and in the conclusions, has been deleted.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

For the most part, the authors made the requested corrections and additions.

I do not agree with the answer given about the references' writing style, which is not uniform and does not fully comply with the journal's requirements.

The authors say, “The guidelines state that references can be in any style, provided we use consistent formatting throughout, which I did, and once the manuscript is accepted, I will be required to use journal style."

Metals journal has clear rules for writing references.

 

   At least it could be uniform, but that doesn't happen. I don't think we have to wait for it to be accepted to follow a very clearly detailed way of writing.

Author Response

All the references have been ordered in a uniform format according to the Journal style.