Effect of Mo and Cr on S-Induced Intergranular Fracture in γ-Fe

Round 1

Reviewer 1 Report

The manuscript concerns problem of grain boundary corrosion in sulfur-containing environment at the atomic level. Super-austenitic stainless steels are of the scientific focus now and fundamental studies which could explain and predict corrosion mechanisms are of great importance.

 

Despite facts that authors mentioned super-austenitic stainless steels but there is no specific alloy or composition was proposed. Wherein modeling was based on “optimized lattice parameter of ϒ-Fe, which agree well with the experimental value…”, but pure ϒ-Fe – is not an alloy.

Thus, it is not clear based on what material modeling was performed. Are results for ϒ-Fe applicable for austenitic stainless steels with lots of alloying elements?

In present form of the manuscript Introduction section containing information about super-austenitic stainless steels looks unfounded. Some more discussion and suggestions are needed to clarify that point.

Moreover, authors concluded that “Mo or Cr in the GB has a certain inhibitory effect…”, but there is no information about which concentrations of these elements were considered. Is there any concentration dependence of observed effects? Could you give suggestions about optimal alloy composition based on provided research?

How anther alloying elements such nickel, silicon, copper, etc. which are usually used in super austenitic stainless steels will affect on the obtained results?

 

Summarizing, provided study seems fundamental research with lack of correlation with real needs and practice. Thus, I would recommend to reconsider the manuscript after major revision.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Ø  Was the temperature variable considered in this study?

 

Ø  Is the mobility of the analyzed species (atoms) independent of temperature?

 

Ø  For what range of temperatures is the study valid?

 

Ø  In “The negative value of the segregation energy indicates that solute atoms tend to segregate at this plane, and a negative value indicates strong segregation tendency.”

 

The first sentence and the second are the same, redundant phrase.

 

Ø  Regarding the following sentences:

 

“The binding energies of S at different positions in the three GBs are all negative, indicating that S can exist in the three GBs and form stable structures.”

 

“This result indicates that Cr is basically uniformly distributed in fcc-Fe. Segregation tendency of Mo and Cr is weaker than that of S. The above calculation results show that Mo easily to exists at the GB in the fcc-Fe.”

 

“From the above, Mo is easily segregated at grain boundaries, Cr segregation tendency is not obvious, but can be located at grain boundaries.”

 

Authors must show reported experimental evidence that strengthens their findings.

 

Ø  In “The above calculation results show that S is easy to permeate at the grain boundary, indicating that S is prone to corrode the grain boundary.”

 

This reviewer is confused by this phrase. Does the permeation of atomic S into grain boundary cause corrosion of the grain boundary? Where is the electrolyte for the corrosion process to be possible?

 

Ø  In “As one of the most corrosive media in stainless steel, S often produces intergranular corrosion [21].”…. “Therefore, S is easy to cause corrosion on grain boundaries and their adjacent areas.” 

 

This reviewer is still confused. Does the study consider the permeation (diffusion) of atomic species or of anions? In a corrosive medium, the species present are found as anions or cations.

 

Ø  In “From the above, Mo is easily segregated at grain boundaries, Cr segregation tendency is not obvious, but can be located at grain boundaries.”

 

However, there is much experimental evidence showing that Cr is segregated at grain boundaries as chromium carbide.

 

Ø  In “Results show that Mo and Cr can inhibit the erosion of GBs by sulfur, and this finding is consistent with experimental results.”

 

The reference is lost.

Sulfur erosion? I do not find the connection between a study of mobility of atomic species within a solid phase with surface processes such as corrosion and erosion.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Please clarify and improve the text on page 2, lines 84 - 88.

Enhance figure 1. The numbers are poorly visible, maybe changing the color of the dots can better

The paragraphs are too large in all document. I recommend separating it into shorter sections.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Thank you for detailed answers and attention to given comments. The revised version of manuscript becomes more logical due to the corrections made. However, some fundamental points are still unclear to me.

1. Since calculations were provided based on lattice parameter of pure γ-Fe and "... it does not show the actual situation of stainless steel containing many alloying elements..." what's the point of mentioning steels and even more so super-austenitic stainless steels? Maybe the title like "Effect of Mo and Cr on S induced intergranular fracture in γ-Fe" would be more appropriate? And then some discussion about applicability of the calculations and results to steels will be appropriate.

If you insist on the current version, a more informed discussion of the connection and applicability of the results for SASS needs to be given.

2. I am not satisfied with the response to point 4. The response does not answer the given question about concentrations that were considered. Specific values should be given, especiialy since concentration of S was mentioned in conclusions but without any values. Moreover, the response raises a number of issues that demonstrate the limitations of the model and disagreement with prectice, which calls into question the significance of the results and the relevancy of the conclusions. Additional comments, explanation and discussion connecting this point should be given in the manuscript or new calculations based on corrected model should be provided.

3. Additional comment regarding the given keywords: some of them such "SEM analysis" and "X-ray diffraction" are inappropriate, in my opinion. Corrections are needed.

Author Response

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Reviewer 2 Report

Dear authors, I appreciate your responses to the questions raised. Although I still have some doubts, I consider that your answers have been adequate and the modifications that you have made to the manuscript have improved it. Thank you

Author Response

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Author Response File: Author Response.pdf