Effect of Porosity on the Corrosion Behavior of FeCoNiMnCr_x Porous High-Entropy Alloy in 3.5 Wt.% NaCl Solution

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

HEAs (High-Entropy Alloys) are recently developed alloys that exhibit a variety of properties. However, due to the diverse elements included, segregation is prone to occur, necessitating appropriate heat treatment. Although information is provided, it is difficult to comprehend. Draw a schematic diagram of the specimen preparation process and include it. This will make it easier for readers to understand.

Corrosion tests come with various standards depending on the type. While this study conducted an immersion test, information related to the standards was not included. Confirm why the immersion test was conducted under these conditions and add the reasoning. This will help justify the experiment.

Figure 1 shows the XRD pattern analysis of the HEA. From these results, only FCC peaks can be identified. However, Cr acts as a BCC-stabilizing element, and when increased up to 27%, the Schaeffler diagram indicates that ferrite growth becomes favorable. Therefore, measure the XRD again.

HEAs exhibit coarse grains of approximately 100 µm. It appears that the study observed microvoids at a magnification of around 1000x. Provide additional images at a lower magnification, such as below 200x. By examining the overall trends and then the finer details, readers will find it easier to understand.

Figure 6 presents the results of the potentiodynamic polarization test. Despite the increase in Cr, reactivity decreases while current density increases. This effect seems to result from voids. However, the current density value of Sample 3 differs from the results of the potentiodynamic polarization test. This appears to be an error. Verify and correct the current density value of Sample 3. Additionally, omit decimal places beyond the second for potential values. Decimal places for current density are unnecessary; remove them.

The EIS results show very low values compared to Cr composition. These results suggest that the passive layer did not form correctly. Does void formation weaken the passive layer? Perform and reinforce discussions on this matter.

The discussion and conclusion sections are very inadequate. Both require significant revision and supplementation. Revise and enhance the discussion and conclusion sections comprehensively.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The aim of the research has been to evaluate the influence of Cr content on the corrosion behaviour of three porous CoCrFeMnNi HEAs. The data presented evidences that the  increase of Cr content is accompanied by an increase in the volume fraction of open porosity in the alloy. According to electrochemical measurements the authors conclude that an increase in Cr concentration results in the increase of the corrosion rate. This result is opposite to that usually reported for many Cr-containing in which a minimal amount of Cr is needed to assure a good corrosion resistance of the alloy. In any case this reviewer considers that this study should be carried out in the alloys free of porosity. It is obvious that corrosion behaviour is highly dependent on the volume fraction of open porosity. As the porosity increases, the surface directly exposed to the corrosive medium increases, i.e. the corrosion rate. Consequently, the increase in the corrosion rate is not related associated with the increase of Cr content. Moreover, XRD patterns prove that all the alloys exhibit single FCC structures, so it cannot be invoked that Cr additions promote the precipitation/segregation of second phases, which could exacerbate galvanic corrosion. In addition, there are other points which make not suitable for publishing the manuscript.
1) The alloys were processed through a route involving mechanical grinding and subsequent vacuum sintering. The manuscript does not provide information if mechanical grinding of the elemental powders could result in mechanical alloying of all elements, leading to the formation of the single FCC-phase. It should be explained the reason for the mechanical grinding: Mechanical alloying, deformation of the powders or changing the morphology of the initial powders for improving powder compactness during cold-press stage. The two latter roles will promote diffusion during the subsequent sintering stage. In addition, are all the elemental powders spherical?
2) The images presented in the manuscript are not representative of the microstructure of the alloy. Micrographs at lower magnifications are required to gain insight about the porosity of the alloys.
3) The value of total porosity calculated do not show good agreement with SEM micrographs presented in Fig. 2.
4) Why Cr diffusion should prevail over the diffusion of the other elements? The Kirkendall effect proposed to occur should be clearly described in the manuscript.
5) Many experimental details are missing. For instance, are the elemental powders spherical? What is the purity of elemental powders?  What is the geometry of corrosion samples? Was the surface of corrosion samples  corrected taking into account the open porosity of the alloys?

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

All responses to the reviews have been appropriately addressed.

Comments on the Quality of English Language

 Additionally, I recommend checking the grammar.

Author Response

Comment 1: All responses to the reviews have been appropriately addressed.

Response 1:Thank you for your support, your affirmation is a great encouragement to us.

Comment 2: Additionally, I recommend checking the grammar.

Response 2:Thanks to your comments, we've thoroughly checked the manuscript for grammatical issues to make sure there are no misunderstandings on the part of our readers.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have addressed properly some of the comments marked by this reviewer. However, the title of the manuscript should be changed because it is confusing for the readers. In the present form, it seems that Cr content influences the corrosion behaviour of the alloys, and this is not true. The only effect of Cr addition is the porosity final of the vacuum sintered alloy. I will propose to change the title as follows: "Effect of Cr content on the porosity of FeCoNiMnCrx high-entropy alloys processed through vacuum sintering". This reviewer considers that such title reflects the real effect of Cr content on the microstructure. Of course, the corrosion behaviour could be presented but the results must be directly correlated with the porosity level in the alloys, not with the Cr content.

In addition, the new manuscript should include these two mandatory changes: 1) A general view of the surface is needed. As marked in the first revision, micrographs at lower magnifications are required to gain insight about the porosity of the alloys. 2) According to the images provided in the manuscript the volume fraction of porosity is below the porosity of 80 % determined through density measurements. The authors state that the alloys have a very complex structure with holes with different sizes, morphologies and distributions. This complexity seems to account in the discrepancies between calculated total porosity values and porosity observed from SEM micrographs. To overcome such discrepancy, grinding of the sample 2-3 mm could provide a more accurate idea about the porosity in the bulk. This reviewer suggest to include these new micrographs to confirm the total porosity of the alloys,

 If the authors accomplish these changes, the manuscript would be suitable for publishing.

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Author Response

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Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have addressed properly the comments made by this reviewer, so the manuscript is now ready for publishing

Author Response

Comment 1:The authors have addressed properly the comments made by this reviewer, so the manuscript is now ready for publishing

Response 1:Thank you for your support, your affirmation is a great encouragement to us.