Review Reports - Local Microstructure and Hardness of the Oxide-Layer of Oxalic Acid-Anodized A356 Alloy

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Manuscript coatings-44119285 Round 1

Alexandra Musza’s and company’s manuscript is devoted to study of anodic oxide layers obtained on cast A356 aluminum alloy in oxalic acid electrolyte at different current densities. The results are based on various analytical methods by SEM, EDS nanoindentation in situ in cross-section, the morphology, chemical composition and local hardness.

Q1. Keywords should not be repeated in the title and abstract.

Q2 line 81-84. Authors should be careful when making statements regarding the homogeneity of alumina. These statements should be supported by references.

Q3 The title is uninformative and requires revision.

Q4 The introduction is generally well written. However, it requires more analysis of the mechanical characteristics and anodizing processes 10.1007/s00366-021-01566-6. I would recommend the authors cite these two additional works.

Q5 It’s written that Al have importance in applications where "adequate durability must be achieved simultaneously", but then it's written "Therefore, surface modification 44 is frequently required to improve wear resistance, surface hardness, corrosion resistance, 45 and service life". It's strange to use materials which have problems with durability to applications where that durability must be achieved.

Q6. The link [22] doesn't specify the pages correctly.

Q7 The title, purpose of the paper, and conclusion should be linked. The purpose of the paper is an expanded version of the title and should be formulated so that it was achieved in the conclusion.

Q8 Line 123. Authors should indicate the area and size of the alloy sample, as well as its origin.

Q9 Line 132. Concentration should be given in moles.

Q10 The origin of all materials and reagents must be indicated.

Q11 Was the menesc protected before anodizing?

Q12 Why was the anodizing temperature chosen at 10 degrees? How was the temperature controlled?

Q13 Why was the anodizing time chosen to be 1 hour?

Q14 Why was anodizing performed in galvanostatic mode? Why were currents of 2 and 6 A/dm2 chosen?

Q15 The authors should provide volt-current-time curves for anodization. Without data, it is difficult to evaluate the results.

Q16. Section 2.2 authors must indicate the origin of the equipment.

Q17. Figure 1a is of poor quality. It has either been removed or replaced.

Q18. Unfortunately, it's difficult to agree with the results presented in Table 1. More precisely, it's difficult to agree with the accuracy of these results. At a minimum, the authors should have correlated these results using voltage-current-time of the potentiostatic anodizing and the charge calculation. Furthermore, the authors should have produced SEM images that clearly showed the thickness of the porous aluminum oxide for each sample. The SEM images presented in Figure 2 are taken from the authors' previous paper 10.3390/COATINGS12101528, which is somewhat of a violation.

Current density correlates very precisely with oxide thickness. The authors' thicknesses differ by almost a factor of two for the same current density. This shouldn't happen. Either the authors' thickness measurement methods are very inaccurate, or the anodizing technique was flawed, with the anodizing conditions chosen incorrectly. The authors formed an irreproducible film. In that case, this work is meaningless.

Comments for author File: Comments.pdf

Author Response

See the attached Report

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript presents an investigation of the microstructure, chemical composition and local mechanical properties of oxide layer produced by oxalic and anodizing an A256 aluminum alloy. The authors applied optical microscopy, SEM/EDS analysis and in-situ nanoindentation to evaluate oxide morphology and interface-related hardness changes.

In my view, the topic is interesting and fits within the scope of surface engineering and coating characterization. The manuscript coatings valuable experimental results; however, several points should be clarified before publication.

The manuscript contains valuable experimental results and may be suitable for publication after revision.

The article appears to be a continuation of the authors previous work (Ref. [21]). The new contribution should be emphasized more clearly. The authors should explicitly describe which measurement and conclusions are new compared with the previously published article.
Figures 2b and 2c appear to have already been presented in the previously published article: Musza, D. Ugi, Á. Vida, and N. Q. Chinh, “Study of Anodic Film’s Surface and Hardness on A356 Aluminum Alloys, Using Scanning Electron Microscope and In-Situ Nanoindentation,” Coatings 2022, Vol. 12, Page 1528, vol. 12, no. 10, p. 1528, Oct., 2022, doi: 10.3390/COATINGS12101528. However, the caption of Figure 2 in the presented manuscript does not contain any reference or information indicating that these images were reproduced or adapted from the previous publication. The authors should clarify whether these SEM images represent new measurements or previously published data. If the images are reused, the original publication must be properly cited int he figure caption and appropriate permission/reuse information should be provided according to the requirements. Moreover, the authors do not refer to figures 2b and 2c in the text of the manuscript.
Experimental details. The description of nanoindentation measurement requires additional information. Please provide: maximum indentation load, loading/unloading rate, holding time, calibration procedure, number of repetition for each region.
The reported hardness value above 20 GPa and especially values reaching approximately 70-100 GPa are unusually high for anodic aluminum oxides layer although the autors classify them as interface-related outliers, additional discussion or verification is necessary.
Parameters such as the estimated width of the anodixing-affected substrate zone (x₀), the distance from the substrate/oxide interface (d) and the hardness of the unaffected substrate (H_bulk) should be formatted in italic style (x₀, d and H_bulk) according to standard scientific notation. Please check and unify the formatting in lines 212, 214, 277, 288, 293, 324, 325, 327, 328, 330, 366, 390, 391, 393, Table 1, Table 2, Figure 3.

Author Response

Please see the attached report.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Manuscript coatings-44119285 Round 2

A11. The specimens were fully immersed below the electrolyte level throughout the anodizing process, as now clarified in Section 2.1. Consequently, the electrolyte–air meniscus did not intersect the specimen surface, and no separate meniscus protection was required.

R2Q11. In this case, it's unclear how the contact was organized and isolated from the electrolyte. Also, how were the electrical parameters (current, voltage, aria) of the anodized object separated from the contact? I recommend the authors explain this with a schematic diagram.

A12-14: Why was the anodizing temperature chosen at 10 degrees? How was the temperature controlled? Q13 Why was the anodizing time chosen to be 1 hour? Q14 Why was anodizing performed in galvanostatic mode? Why were currents of 2 and 6 A/dm2 chosen?

R2Q12: I expected the authors to somehow tie the choice of anodizing parameters to the objective of the study. Instead, I encountered general statements like a compromise, based on previous research. It's recommended to tie the choice of anodizing modes to the objective of the study.

Furthermore, 20-60 mA/cm² is a fairly high current density for high-quality oxide; 6 mA/cm² is recommended.

A15. Thank you for this comment. We have clarified that the specimens investigated in the present work belong to the same anodizing series for which the anodic potential–time curves were previously reported in Figure 6 of Ref. [23]. Because the experiments were performed under galvanostatic conditions, the current density remained constant at the prescribed value throughout anodizing; therefore, the relevant electrochemical records are anodic potential–time curves. An explicit cross-reference to the previously published curves and a brief description of their main features have now been added to Section 2.1. The exposed specimen area, nominal applied currents, anodizing time, and calculated charge densities have also been reported. To avoid duplicating previously published experimental data, the same curves have not been reproduced as a new figure in the present manuscript.

R2Q15: There is one problem: if you indicate such a spread in thicknesses under the same anodizing conditions, then you are obliged to provide voltage-current-time curves for all M1-M6 samples.

A18. Thank you for this detailed comment. We apologize that the electrochemical conditions and the origin of the previously presented SEM images were not described with sufficient clarity. We respectfully clarify that anodizing was performed under galvanostatic, rather than potentiostatic, conditions. The corresponding text and figure are now corrected in the revised version at line 134-146.

R2Q18: Unfortunately, the authors ignored my question, which is very serious. This is the reason I am forced to reject the manuscript.

Unfortunately, it's difficult to agree with the results presented in Table 1. More precisely, it's difficult to agree with the accuracy of these results. At a minimum, the authors should have correlated these results using voltage-current-time of the potentiostatic anodizing and the charge calculation.

R2Q19: So, you anodize the alloy under the same conditions, at the same current density, but you get a 100% thickness variation, from 63 µm to 124 µm (Table 1). This is concerning. Where did the charge go in this case? You don't provide volt-current-time curves for all samples, you don't calculate the charge, and the SEM images don't show the plate thickness for all samples. Why is there such a thickness variation?

I can't let this pass, sorry. You must provide evidence.

R2Q20: Lines 261-262 indicate that Figure 2 shows optical micrographs of samples M1-M6, but there's only one photo of sample M2. Yes, these photos would probably clarify the situation somewhat, but for some reason you only showed one. Why? Where are the others?

R2Q21: In lines 261-263, you state that the values in the table 1 should not be interpreted as thickness. However, in the table 1, you indicate that this is film thickness in the column heading.

So is it the thickness or something else?

R2Q22: In lines 272-274, the author writes that there is no precise relationship between film thickness and current density. This is unfortunate. According to the author, this contradicts the law of conservation of energy. In electrochemistry, there is a direct relationship between film thickness and current density, since all the charge transferred is used to form the film. And under the same conditions, the film thickness will always be the same.

R2Q23:In lines 274-275 he writes that the observations are consistent with something, but does not refer to any work.

A19: The Reviewer is right. The current density correlates precisely with oxide thickness. Unfortunately, there was a mix-up in sample identifiers, which caused an ambiguous correlation. We have carefully checked the data and corrected the content of Table 1 and the corresponding texts in the revised version.

R2Q24: Sorry, but I didn't notice any significant changes in Table 1. I don't understand how the same current density can result in a thickening of 63 µm and 124 µm. This requires serious study and explanation. Current density is very much a measure of the amount of charge that passes through to form the film. According to your data, it appears that half of the charge went somewhere. We need to look at the volt-current-time curves for samples M2 and M6 and figure out where the current went. We also need to look at the microscopy. There's a very large variation in thickness.

Comments for author File: Comments.pdf

Author Response

Please see the attached file.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

I would like to thank the Authors for their thorough and detailed responses to my comments. The revised manuscript has been significantly improved, and the Authors have adequately addressed all of the issues raised during the review process.

Overall, I am satisfied with the revision made by the Authors. In my opinion, the manuscript is now suitable for publication in Coatings and can be accepted in the present form.

Author Response

Thank you for accepting our revised version.

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors

Manuscript coatings-44119285 Round 3

AR2Q11: We apologize for having misunderstood the Reviewer’s question in our previous

response. We addressed only the electrolyte–air meniscus and did not adequately explain

the electrical contact arrangement and its isolation from the electrolyte. To make the

situation clear, a schematic representation (Fig. 1) and the corresponding description have

now been added to Section 2.1 of the revised manuscript.

Q1R3: Thank you for the clarification. However, questions remain. The electrolyte/wire insulation interface is certainly a viable option. But what about the portion of the wire submerged in the electrolyte? It also undergoes anodization. Furthermore, there must be some kind of fastener that will press the aluminum wire to the component, such as a bolt or screw. So, the portion of the wire submerged in the electrolyte and the aluminum screw will also undergo anodization. How can you rule this out using the kinetics? The kinetics curve should only show the curve for the anodization of the alloy. Furthermore, with each new anodization, the fastener will be at least slightly different, which will introduce error and differences in the kinetics, thickness, and microscopy. After all, part of the current will be spent anodizing the wire and the fastener. That's why I asked you about Menesc. It's the only option in your case. You could have made Menesc in citric or boric acid at up to 150 V and then easily anodized porously. Additionally, there is a special fluoroplastic fixture to eliminate the Menesque effect; it is presented with a photo in this paper: https://ieeexplore.ieee.org/document/8782150.

Was the anodizing area recalculated to account for the fixture?

AR2Q12–14: Thank you for the clarification. We agree that in our previous response, we

did not sufficiently relate the selected parameters to the purpose of the study. We selected

the conditions from our previous screening experiments [23] with the aim of producing

oxide layers with clearly distinct thicknesses and porous morphologies for cross-sectional

SEM, EDS, and nanoindentation analyses. We outlined the specific objectives in the

Introduction section.

Q2R3: I would recommend formulating the purpose of the work even more clearly - the last paragraph of the introduction.

AR2Q15: Thank you for this comment. We agree that referring only to previously

published representative curves did not provide sufficient sample-specific evidence.

Therefore, the individual cell-voltage–time curves recorded for all samples M1–M6 have

now been added to the revised manuscript as Figure 2.

We're sorry, there was a mix-up. Re-examination of the experimental records has shown

that M6 was anodized for only 30 for comparison purposes. This has been corrected in

Section 2.1 and Table 1.

Q3R3: The anodizing voltage–time curves presented for four of the samples exhibit an unusually high degree of similarity. Not only do they show nearly identical overall trends, but the positions of all characteristic features (initial rise, transition regions, inflection points, and subsequent evolution) coincide almost exactly throughout the entire anodizing process. The curves appear to differ primarily by an approximately constant voltage offset.

Since anodizing experiments performed on different samples, even under nominally identical conditions, normally exhibit at least some variation due to differences in alloy microstructure, surface preparation, local temperature, electrolyte conditions, and measurement noise, such an exceptionally close correspondence is unexpected.

The authors are requested to explain the electrochemical mechanism that could give rise to such remarkably similar voltage–time behavior in independent anodizing experiments. Specifically, what physical or electrochemical processes would be expected to produce nearly identical curve shapes and point-by-point evolution while maintaining only a nearly constant difference in voltage between different samples?

In view of the inherent variability associated with anodizing experiments, including differences in alloy microstructure, surface preparation, oxide nucleation, local temperature, and other experimental factors, such an unusually close correspondence appears unexpected. The authors are therefore requested to provide a mechanistic explanation for this observation.

As an example, I would like to give the anodization curves for different current densities of the same structure from the work https://www.mdpi.com/1996-1944/14/17/5118

AR2Q20: Thank you for pointing out this inconsistency. The previous text incorrectly

stated that optical micrographs of M1–M6 were presented, although only the

representative SEM image of M2 was included. This figure reference has been corrected.

A new figure containing calibrated cross-sectional optical micrographs of all six specimens

(M1–M6) has now been added.

Q4R3: We conducted our own analysis of the thicknesses shown in the optical images. These results are inconsistent with the results in the table. How do the authors explain this?

М1

Measurements

1)45,57 μm

2)34,81 μm

3)38,61 μm

4)36,08 μm

5)30,38 μm

6)34,81 μm

7)41,14 μm

8)40,51 μm

9)40,51 μm

10)42,41 μm

Result

38,483 μm (average) = 38,5 μm (rounded)

М2

Measurements

1)124,31 μm

2)123,61 μm

3)125 μm

4)114,38 μm

5)93,75 μm

6)97,22 μm

7)100,61 μm

8)92,36 μm

9)78,74 μm

10)88,19 μm

Result

103,817 μm (average) = 103,8 μm (rounded)

М3

Measurements

1)105,25 μm

2)108,02 μm

3)109,26 μm

4)112,96 μm

5)109,26 μm

6)100 μm

7)102,78 μm

8)105,56 μm

9)100 μm

10)93,52 μm

11)100,93 μm

Result

104,322 μm (average) = 104,3 μm (rounded)

М4

Measurements

1)76,6 μm

2)86,17 μm

3)86,17 μm

4)80,85 μm

5)90,43 μm

6)88,3 μm

7)84,04 μm

8)91,49 μm

9)91,49 μm

10)100 μm

Result

87,554 μm (average) = 87,6 μm (rounded)

Measurements

1)92,31 μm

2)89,23 μm

3)93,85 μm

4)92,31 μm

5)90,77 μm

6)93,85 μm

7)96,92 μm

8)90,77 μm

9)90,77 μm

10)93,85 μm

Result

92,463 μm (average) = 92,5 μm (rounded)

Measurements

1)48,83 μm

2)52,58 μm

3)56,96 μm

4)48,83 μm

5)52,58 μm

6)53,83 μm

7)52,58 μm

8)60,09 μm

9)50,08 μm

10)45,07 μm

Result

52,143 μm (average) = 52,1 μm (rounded)

Conclusion. Overall, the work contains experimental results and could have been published. The volt-current-time curves are concerning; the authors should present the raw data. During anodizing, there will always be some peculiarities from sample to sample. The curves cannot be so precise. Furthermore, the anodizing technique is of great concern. The results must be accurate and reproducible. The authors do not indicate which wire and screw were anodized along with the alloy, its grade, or composition. How, if at all, did this affect the results? These are serious questions, and I cannot recommend this work for publication.

Comments for author File: Comments.pdf

Author Response

Please see the attached file.

Author Response File: Author Response.pdf