Two-Step Plasma Electrolytic Oxidation of Advanced High-Strength Steel in Aluminate and Silicate Solutions
Round 1
Reviewer 1 Report
Comments and Suggestions for Authors
The authors investigated whether the individual advantages of aluminate-phosphate and silicate-phosphate electrolytes in the plasma electrolytic oxidation (PEO) of steel can be combined in a sequential two-step process to improve coating homogeneity. The first step involves pulsed voltages of 150–200 V in an aluminate-phosphate electrolyte, followed by a second step at 400 V in a silicate-phosphate electrolyte. A single-step silicate-based PEO process up to 400 V serves as the reference. The resulting oxide layers were analyzed using SEM, EDX, XRD, and Raman spectroscopy. The findings indicate that silicate-based coatings remain heterogeneous even at high voltages, whereas the two-step process results in a more homogeneous, nanocrystalline layer after replacing the initial structure. Although the results are interesting, I believe the manuscript requires revision before it can be considered for publication.
- Although the authors suggest in the Introduction that the two-step PEO process may enhance adhesion to the substrate, this claim is not experimentally supported. In fact, layer delamination and cohesive failure are reported in some samples, indicating potential adhesion issues.
- While the two-step process appears to improve morphological homogeneity, the manuscript does not provide evidence that this leads to improved functional performance (e.g., adhesion, mechanical strength, corrosion resistance). Without such data, the practical advantages of the approach remain uncertain
Author Response
Comment:
Although the authors suggest in the Introduction that the two-step PEO process may enhance adhesion to the substrate, this claim is not experimentally supported. In fact, layer delamination and cohesive failure are reported in some samples, indicating potential adhesion issues.
While the two-step process appears to improve morphological homogeneity, the manuscript does not provide evidence that this leads to improved functional performance (e.g., adhesion, mechanical strength, corrosion resistance). Without such data, the practical advantages of the approach remain uncertain
Response:
In the introduction, we claim that “the potential can be deduced to first produce an electrically insulating precipitation layer and PEO layer in aluminate solution and then to convert the layer during PEO in silicate solution in order to increase the adhesion to the substrate” as a general motivation. And we define that the “focus of this study is particularly on the layer formation during the PEO steps in both electrolytes and at different voltage amplitudes”.
Although the focus of the study does not include testing functional properties (e. g., adhesion, mechanical strength, corrosion resistance), we understand the aforementioned potentials should at least be addressed in the conclusions section, again. Therefore, we added the statement:
“Based on the SEM investigations, the layers resulting from the combined processes show an improved layer adhesion compared to the PEO layers formed in the aluminate electrolyte, but they are still prone to internal cracking. Future research should tackle that structural weakness and investigate application-related layer properties, e. g., adhesion strength and corrosion protection.”
We are aware that the potential of highly adherent layers has not been achieved yet. However, we believe that presenting not entirely successful work and identifying objects of further research still adds a significant value to the state of the art.
Reviewer 2 Report
Comments and Suggestions for Authors
The article entitled "Two-step plasma electrolytic oxidation of advanced high-strength steel in aluminate and silicate solutions" follows on from the ongoing research and publication activities at the University of Chemnitz. The article is written in a clear and readable manner according to all academic principles imposed on impact journals. I have no comments on the content as such, the research and results are properly described and well evaluated at the end. The only criticism I have is about the grammar, when the term micorarc appears very often in the text, which I guess should be the term for microarc. I also have a comment on the tables in the tables, there is a lack of discipline in writing lowercase and uppercase letters, I recommend unifying it to one form. On page eleven, the references to images in the text are different than on the other pages, I again recommend unifying them. And my biggest reservations are about figure 9, where I recommend that the authors choose to color-code the lines because the image is unreadable in this format (at least for me) and the differentiation of the help of the type time and the same color is insufficient and poses a problem when reading the text.
Otherwise, I have no comments on the article. After removing these minor errors, I recommend it for publication in this journal.
Author Response
Comment 1: The only criticism I have is about the grammar, when the term micorarc appears very often in the text, which I guess should be the term for microarc.
Response: Thank you, we corrected this mistake.
Comment 2: I also have a comment on the tables in the tables, there is a lack of discipline in writing lowercase and uppercase letters, I recommend unifying it to one form.
Response: Thank you. We found and corrected some mistakes regarding lowercase numbers on pages 8 and 9 during proof-reading.
Comment 3: On page eleven, the references to images in the text are different than on the other pages, I again recommend unifying them.
Response: Thank you. We unified the references on page 11 according to the format on the other pages. During proof-reading we further unified the usage of terms and notations.
Comment 4: And my biggest reservations are about figure 9, where I recommend that the authors choose to color-code the lines because the image is unreadable in this format (at least for me) and the differentiation of the help of the type time and the same color is insufficient and poses a problem when reading the text.
Response: Thank you. We corrected that weakness by introducing an additional color coding of the reference spectra.
Reviewer 3 Report
Comments and Suggestions for Authors
The manuscript investigates the possibility of combining the silicate- and aluminate-based electrolytes for the plasma electrolytic oxidation of steel in a two-step process. Dedicated chemical and material characterizations are performed to validate that such two-step process is possible and can be achieved with the provided technical details. I recommend accepting the manuscript after revision. My comments are as follows.
- Can authors further discuss the main expected benefit of combining aluminate and silicate electrolytes in PEO processes.
- Can authors further explain the needs of increasing applied voltage at 600 and 1200 (what time unit BTW).
- Figure 3, what is the physical or chemical reason for the J peak before t-2400.
- Figure 5, it is suggested to highlight the main features of the material surface condition that are of interest in the photo.
- For the main conclusion “the phase composition of the PEO layers is essentially determined by the final process step at a voltage amplitude of 400 V”, can authors comment on the generality of such conclusion in other pertinent PEO scenarios.
Author Response
Comment 1: Can authors further discuss the main expected benefit of combining aluminate and silicate electrolytes in PEO processes.
Response: We revised the conclusion to present the main benefits more clearly:
“The main benefit of the combined PEO in the aluminate and subsequently in the silicate electrolyte is the new formation of a more homogeneous PEO layer at a voltage amplitude of 400 V.” and “Based on the SEM investigations, the layers resulting from the combined processes show an improved layer adhesion compared to the PEO layers formed in the aluminate electrolyte”
Comment 2: Can authors further explain the needs of increasing applied voltage at 600 and 1200 (what time unit BTW).
Response: We added a short explanation of the motivation for the general approach to stepwise increase the voltage amplitude in 2.2:
“Voltage-controlled anodic pulses with stepwise increased amplitude levels were applied to first facilitate the formation of insulating layers and initial PEO layers at step-wise thickening and transformation of the initial layer.”
And we thoroughly checked the manuscript to make sure that the correct time units are given.
Comment 3: Figure 3, what is the physical or chemical reason for the J peak before t-2400.
Response: Thank you. In the discussion, we explain that “(l)ayer spallation of several square millimeters in area occurs during the PEO process exposing substrate areas completely or except from thin adhering layer residues”. We now added the statement: “This is the reason for the sharp current-density increase at around 2300 s in Figure 3.”
Comment 4: Figure 5, it is suggested to highlight the main features of the material surface condition that are of interest in the photo.
Response: Thanks for that remark. We highlighted the main features in Figures 4 and 5 refer to the numbers in the text.
Comment 5: For the main conclusion “the phase composition of the PEO layers is essentially determined by the final process step at a voltage amplitude of 400 V”, can authors comment on the generality of such conclusion in other pertinent PEO scenarios.
Response: Thank you for that important remark on the generalizability of that statement. We did not intent to generalize that statement to all pertinent PEO scenarios. We are aware that there might also be ways to influence the final phase composition by previous process steps, but we decided to dispense with speculations on that. Instead, we now added “for the investigated conditions” to clearly connect that conclusion with our study.
