Superhydrophobic Coating on 6061 Aluminum Alloy Fabricated by Femtosecond Laser Etching and Anodic Oxidation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Authors reports a study on superhydrophobic coating on AA6061 by Femtosecond Laser Etching and Anodic Oxidation. The study is interesting although needs major revision before the publication based on the following comments:

1) Please, cite other references about superhydrophobic coatings on AA alloys: i) Robust superhydrophobic composite coating with dual-sized particles for self-cleaning and anti-corrosion of 5083 aluminum alloy, Colloids and Surfaces A: Physicochemical and Engineering Aspects Volume 708, 5 March 2025, 136020, ii) Enhancing corrosion resistance of anodized AA7075 alloys by electrodeposition of superhydrophobic coatings, Colloids and Surfaces A: Physicochemical and Engineering Aspects Volume 675, 20 October 2023, 132040

2) Microstructural characterization: nanotube and nanopores are two distinguished morphology. I would avoid the term "nanotube" since pores are not completely separated, therefore I would mention only "pores" or "nanopores" morphology

3) It is very hard to detect crystallinity of Al2O3 with XRD if it is not in GIXRD mode. Which is the thickness of Al2O3? Are you sure you are able to detect Al2O3? That peak in XRD diffractogram is not related to crystalline species, it is too wide. I expect a more in-depth discussion on XRD data

4) Please, report error bars when reporting contact angle values. Are you sure of that difference in contact angle values or it is only a normal variation in the data? 

5) You need to estimate what is the effect of FAS on superhydrophobic character of the coating. Did you perform contact angle measurements after etching and after anodic oxidation without FAS? That would be very interesting because I'm afraid that the superhydrophobic character is given by FAS and not from the morphology of the coating.

6) Corrosion resistance discussion is not totally correct. As you can see from polarization curves, current density increases for potential a bit more positive than corrosion potential, indicating the onset of pitting corrosion phenomena. You need to test your coating after immersion in seawater simulating solution to have real information on durability of the coating.

Author Response

The response to the reviewers is in the attached Word document. Please check it. Thank you!

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In this study, the authors successfully fabricated a superhydrophobic surface with hierarchical micro/nano-array structures on 6061 aluminum alloy using a combination of femtosecond laser etching and anodic oxidation. This hierarchical structure significantly increased the water contact angle to 162°. The presence of these micro/nano features enhanced the mechanical durability, superhydrophobic performance, and corrosion resistance of the surface.

While the authors have investigated the surface wettability and anti-corrosion performance of a superhydrophobic aluminum surface, the proposed approach lacks novelty. The manuscript does not offer original insights and fails to present comprehensive mechanical and chemical durability tests, such as adhesion tests (e.g., tape-peeling) and acid–alkali resistance tests, which are essential for determining the coating’s practical applicability. Therefore, I believe the manuscript requires major revisions and substantial improvement before it can be considered for publication. My main concerns are outlined below:

• In the keywords section, the authors should avoid using keywords that are already present in the title. Please review and revise carefully.
• The fabrication approach presented, combining laser etching and anodic oxidation to develop superhydrophobic aluminum surfaces, is not novel, as similar methods have been widely reported in the literature. The manuscript currently lacks a clear distinction from previous works. The authors should clearly highlight the novelty and advantages of their method, emphasizing what sets it apart from existing studies to justify its contribution to the field.
• The introduction section is currently insufficient and requires improvement. The authors should provide a more comprehensive discussion of existing methods for fabricating superhydrophobic surfaces, especially the commonly used two-step techniques for developing robust coatings. Including recent publications related to the development of robust superhydrophobic surfaces and their applications will strengthen the background and highlight the relevance of the work. For example,

 https://doi.org/10.1016/j.cplett.2022.139567, https://doi.org/10.1016/j.memsci.2020.118716,  https://doi.org/10.1016/j.surfcoat.2023.129293

• In section 2.1. (Preparation of Experimental Samples), what was the purpose of storing the FAS coating samples in self-sealing bags?
• In Section 3.1 (Microstructural Characterization), the manuscript lacks detailed information regarding the size and dimensions of the fabricated surface features, such as bumps and pits. Additionally, the description of 'nanotube pores' formed after anodic oxidation is unclear, are these structures nanotubes or nanowires? The authors should include morphological details in the revised draft.
• In Figure 2, the SEM images of the prepared surface appear unclear and do not effectively reveal the hierarchical micro/nano-array structures described in the manuscript. It is recommended to provide higher-magnification SEM images that clearly illustrate the surface morphology and confirm the presence of the reported micro/nanostructures.
• The motivation behind selecting anodic oxidation for 30 minutes at a voltage of 60 V is not clearly explained. A systematic study on how variation in time or voltage influences the surface wettability and other properties of the surface should be included.
• What is the thickness of the prepared surfaces? It is recommended to provide the side-view SEM images of the LE layer and LE/AAO coating.
• The authors should include water sliding angle measurements alongside contact angle data to more accurately evaluate the superhydrophobic behavior of the surface (Figure 4). A high water contact angle alone does not guarantee low sliding angles, and both parameters are essential to confirm the surface’s ultra-repellence to water
• In the introduction, author reported that superhydrophobic coatings prepared by traditional processes commonly suffer from poor mechanical durability, restricting their long-term service performance. However, here the mechanical durability was evaluated using an abrasion test with a 20 g load. The authors should clarify the justification behind selecting this specific load, as 20 g may be insufficient to fully assess the coating’s mechanical durability. It is recommended to conduct additional tests using higher loads to provide a more comprehensive evaluation of the coating’s mechanical durability.
• In the mechanical durability test under a 20 g load, the gradual increase in contact angle suggests a stepwise loss of superhydrophobicity and degradation of the coating. This raises concerns about the long-term durability of the surface. It would be beneficial for the authors to compare their results with previous studies on mechanical durability to help readers better evaluate the extent of improvement achieved in this work.
• In Figure 9, please include the water sliding angle data during the mechanical cycle test to provide a more comprehensive understanding of the surface wettability. Additionally, Figure 10 appears blurry; it is recommended to replace it with a higher-resolution SEM image for better clarity.

Author Response

The response to the reviewers is in the attached Word document. Please check it. Thank you!

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Authors corrected manuscript according to my comments. The manuscript is now ready for publication

Reviewer 2 Report

Comments and Suggestions for Authors

I have carefully reviewed the revised manuscript, and most of the comments from the authors are well-addressed.  In my opinion, the manuscript can be accepted for publication.

I also confirm that I have no conflicts of interest related to this manuscript.

With Regards,