Robust Superhydrophobic Coatings for Enhanced Corrosion Resistance and Dielectric Properties

Round 1

Reviewer 1 Report

In the manuscript, the authors describe an approach to creating superhydrophobic coatings based on PVDF and SiO2 particles. I want to note that the draft of the article does not contain fundamentally new approaches to the creation of superhydrophobic surfaces. The scopus database contains publications on this topic (for example, the article https://doi.org/10.1016/j.apsusc.2015.05.068 uses the same materials, only the substrate and the set of research methods differ). This casts doubt on the novelty of this study.

Also, as a result of reading, the following remarks appeared:

1) There is no description of materials and reagents.

2) Clause 2.1. This experiment is not reproducible, there are no reagent concentrations.

3) Fig. 1. Photos need to be enlarged. The photo of the drop moved down to the caption for the drawing.

4) Line 178. What do you mean by "polymer-like" and "nano ceramic"? These terms are not related to polymer chemistry.

The authors write that as a result of testing for abrasion resistance, the samples demonstrate high resistance. But the authors conducted only 40 test cycles and it is completely incomprehensible whether this is good or bad. Where did you get this technique and how reproducible is it? Give a comparison with the literature data.

Line 181-182. "The coating remains superhydrophobic after exposure to salt spray (Fig. 3c), showing a good chemical stability." Give specific results in the description. It can be seen from the figure that the contact angles are significantly reduced.

Temperature cycling resistance also needs to be compared with literature data.

Line 185-187. In the figure, the contact angle is more than 160°. At the same time, this experiment is not indicative of the resistance of the coating to aggressive media. The authors needed to measure the contact angle of the surface after immersion in solutions.

5) Clause 3.2. No description of the experiment; Samples must be kept in salt solution for a certain time before being measured.

The PTFE + 10% SiO2 sample does not show SH properties, what is the reason for the improvement in protective properties? Comparison with pure PVDF coating should be made. I think the result will be close to PTFE+10% SiO2.

Author Response

Reviewers' comments:

Reviewer #1: In the manuscript, the authors describe an approach to creating superhydrophobic coatings based on PVDF and SiO2 particles. I want to note that the draft of the article does not contain fundamentally new approaches to the creation of superhydrophobic surfaces. The scopus database contains publications on this topic (for example, the article https://doi.org/10.1016/j.apsusc.2015.05.068 uses the same materials, only the substrate and the set of research methods differ). This casts doubt on the novelty of this study.

 

Also, as a result of reading, the following remarks appeared:

1) There is no description of materials and reagents.

 

Answer: Thank you for your valuable comment. We have added the description of materials and reagents.

 

Revision: The AZ31 magnesium alloys (2.5–3.0 wt.% Al, 0.70–1.3 wt.% Zn, 0.2 wt.% Mn, and balanced Mg) used as matrix materials were polished with silicon carbide paper fol-lowed by ultrasonic cleaning in deionized water and ethanol. To construct superhy-drophobic coating, the PTFE layer modified with nanometer silicon dioxide (≈40 nm) (Dongguan XinWeiJin Industry Co., Ltd.) (purity=99.9%) decorated by KH-570 silane coupling agent (Hangzhou Jiexika Industry Co., Ltd.) at different concentrations (0, 10 wt%, 30 wt%), was covered on the magnesium alloy substrate with subsequent room temperature curing for 5 h. While the modified PTFE suspension used in this work was synthesized by the following process: Mix the lipophilic modified nano-silica particles (0, 1 and 3 g) with butyl acetate solution (Henan Weiyuan Biotechnology Co., Ltd.) (reagent-grade) and ultrasonically disperse for 1 h to prepare a dispersion. Then slowly add the PTFE resin (Daikin Industries) (10g) to the dispersion, with magnetically stir-ring at 1000 r/min for 60 min to ensure that the resin and the solvent dispersion are thoroughly mixed; Finally, the modified PTFE suspension is defoamed in a vacuum dryer for 5 minutes to obtain a slurry. The slurry is brushed onto the magnesium alloy substrate to prepare different samples.

 

2) Clause 2.1. This experiment is not reproducible, there are no reagent concentrations.

 

Answer: Thank you for the elaborative advice. We have supplemented reagent concentrations.

 

Revision: To construct superhy-drophobic coating, the PTFE layer modified with nanometer silicon dioxide (≈40 nm) (Dongguan XinWeiJin Industry Co., Ltd.) (purity=99.9%) decorated by KH-570 silane coupling agent (Hangzhou Jiexika Industry Co., Ltd.) at different concentrations (0, 10 wt%, 30 wt%), was covered on the magnesium alloy substrate with subsequent room temperature curing for 5 h. While the modified PTFE suspension used in this work was synthesized by the following process: Mix the lipophilic modified nano-silica particles (0, 1 and 3 g) with butyl acetate solution (Henan Weiyuan Biotechnology Co., Ltd.) (reagent-grade) and ultrasonically disperse for 1 h to prepare a dispersion. Then slowly add the PTFE resin (Daikin Industries) (10g) to the dispersion, with magnetically stir-ring at 1000 r/min for 60 min to ensure that the resin and the solvent dispersion are thoroughly mixed; Finally, the modified PTFE suspension is defoamed in a vacuum dryer for 5 minutes to obtain a slurry. The slurry is brushed onto the magnesium alloy substrate to prepare different samples.

 

 

 

3) Fig. 1. Photos need to be enlarged. The photo of the drop moved down to the caption for the drawing.

 

Answer: Thanks to the referee for the helpful suggestion. I have revised this Figure.

 

 

 

 

 

 

 

 

 

 

Figure 1. Surface micro-topography and contact angle change of (a) PTFE, (b) PTFE+10%SiO₂ and (c) PTFE+30%SiO₂. (d) Variation of contact angle with particle addition.

4) Line 178. What do you mean by "polymer-like" and "nano ceramic"? These terms are not related to polymer chemistry.

The authors write that as a result of testing for abrasion resistance, the samples demonstrate high resistance. But the authors conducted only 40 test cycles and it is completely incomprehensible whether this is good or bad. Where did you get this technique and how reproducible is it? Give a comparison with the literature data.

Line 181-182. "The coating remains superhydrophobic after exposure to salt spray (Fig. 3c), showing a good chemical stability." Give specific results in the description. It can be seen from the figure that the contact angles are significantly reduced.

Temperature cycling resistance also needs to be compared with literature data.

Line 185-187. In the figure, the contact angle is more than 160°. At the same time, this experiment is not indicative of the resistance of the coating to aggressive media. The authors needed to measure the contact angle of the surface after immersion in solutions.

 

Answer: To better answer the reviewer’s questions, we separated the comments into four questions. In addition, we tried to give the point by point response to each question as follows:

 

Question (1): Line 178. What do you mean by "polymer-like" and "nano ceramic"? These terms are not related to polymer chemistry.

 

Answer: "polymer-like" refers to PTFE, "nano ceramic-particles" refers to nanoparticles. I agree with you that These terms are not related to polymer chemistry, so I have revised the words.

 

Revision: Due to the PTFE coating with wear-resistant nanoparticles and anti-wear hierarchical morphology, the contact angle is still greater than 150° after 40 abrasion cycles (Fig. 3b), which demonstrates the durable superhydrophobicity of the coating.

 

Question (2): The authors write that as a result of testing for abrasion resistance, the samples demonstrate high resistance. But the authors conducted only 40 test cycles and it is completely incomprehensible whether this is good or bad. Where did you get this technique and how reproducible is it? Give a comparison with the literature data.

 

Answer: We have re-supplemented this part of the experiment and found high reproducibility. Besides, we further re-described this part and compared it with the existing Refs.

 

Revision: Due to the PTFE coating with wear-resistant nano particles and anti-wear hierarchical morphology, the contact angle is still greater than 150° after 40 abrasion cycles (Fig. 3b), which demonstrates the durable superhydrophobicity of the coating. Li et al.[40] reported that the superhydrophobic coating prepared by electrodeposition of cobalt on the AZ31 magnesium alloy surface maintained good mechanical stability and the contact angle decreased by 6 ° (It decreased from 156.2°±0.60° to nearly 150° after 9 m wear of 800# sandpaper). Shi et al. [41] found that the WCA of polyphenylene sulfide-polytetrafluoroethylene/silicon dioxide (4g/l) coating remained 142.5 ° after wear of 10 m, and had good wear resistance. Liang et al., [42] reported that the 7.3 wt.% SiO₂@PTFE coating surface placed facedown to 400 grits SiC sandpaper under load of 5.4 kPa could maintain superhydrophobility after being rubbed 30 times. The comparison shows that that the PTFE+30%SiO₂ coating in this work has better wear resistance.

 

Papers reference list involving the proposed application areas:

40. W. Li, Z. Kang. Fabrication of corrosion resistant superhydrophobic surface with self-cleaning property on magnesium alloy and its mechanical stability. Surf Coat Tech, 2014, 253: 205-213.
41. L. Shi, J Hu, X.D. Lin, L. Fang, F. Wu, J. Xie, F.M. Meng. A robust superhydrophobic PPS-PTFE/SiO2 composite coating on AZ31 Mg alloy with excellent wear and corrosion resistance properties. J. Alloys Compd 2017, 721: 157-163.
42. Y. Liang, J. Ju, N. Deng. Super-hydrophobic self-cleaning bead-like SiO2@ PTFE nanofiber membranes for water-proof-breathable applications. Appl. Surf. Sci, 2018, 442: 54-64.

 

Question (3): Line 181-182. "The coating remains superhydrophobic after exposure to salt spray (Fig. 3c), showing a good chemical stability." Give specific results in the description. It can be seen from the figure that the contact angles are significantly reduced.

 

Answer: We've recharacterized this part of the result.

 

Revision: The coating remains superhydrophobic after prolonged exposure to salt spray for 150 h (Fig. 3c), showing a relatively good chemical stability.

 

Question (4): Temperature cycling resistance also needs to be compared with literature data.

Line 185-187. In the figure, the contact angle is more than 160°. At the same time, this experiment is not indicative of the resistance of the coating to aggressive media. The authors needed to measure the contact angle of the surface after immersion in solutions.

 

Answer: We have supplemented the literature data for the Temperature cycling resistance. The details are as follows. Moreover, after immersing the coating in the solution, there are no liquid droplets on the surface of the coating, showing stable hydrophobicity.

 

Revision: The water resistance of super water-repellent coating remains unchanged after thermal shock cycle (＞ 400 h) (Fig. 3d). Zang et al.[43] found that the superhydrophobic coating remains superhydrophobic through 18 cycles at high temperature (350 °C). Xie et al. [44] reposted that the PDMS/SiO₂ superhydrophobic coatings can withstand atmospheric temperatures up to 350 °C and has good thermal stability between room temperature (25 °C) and 350 °C. To sum up, the superhydrophobic coating in this work has temperature cycling resistance.

 

Papers reference list involving the proposed application areas:

43. D. Zang, R. Zhu, W. Zhang. Corrosion‐resistant superhydrophobic coatings on Mg alloy surfaces inspired by lotus seedpod. Adv. Funct, 2017, 27(8): 1605446.
44. J. Xie, J. Hu, X. Lin. Robust and anti-corrosive PDMS/SiO2 superhydrophobic coatings fabricated on magnesium alloys with different-sized SiO2 nanoparticles. Appl. Surf. Sci, 2018, 457: 870-880.

 

5) Clause 3.2. No description of the experiment; Samples must be kept in salt solution for a certain time before being measured.

The PTFE + 10% SiO2 sample does not show SH properties, what is the reason for the improvement in protective properties? Comparison with pure PVDF coating should be made. I think the result will be close to PTFE+10% SiO2.

 

Answer: Thanks to the referee for the suggestion. We have described the experimental details.

The reason for the improvement in corrosion protective properties is that the uniform and dense composite coating with chemical stability and density acts as a barrier layer, which prevents the corrosion of the corrosion medium and effectively reduces the corrosion rate. At the same time, the air trapped in the hydrophobic structure of the PTFE+10%SiO₂ composite coating can effectively prevent the penetration of corrosion ions and solution. Further, we compared the PTFE+10%SiO₂ composite coating with pure PTFE coating. It is found that the corrosion resistance is close to that of PTFE+10%SiO₂ composite coating.

 

Revision: The corrosion resistance test at room temperature. A stable open-circuit potential (OCP) within 600 s was allowed before potentiodynamic polarization test.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Reviewing Manuscript coatings-1953595

Robust Superhydrophobic Coatings for Enhanced Corrosion 2 Resistance and Dielectric Properties

 

Referee Comments:

 

1. Perhaps, the authors were not aware of the figures in the constructed PDF file.

Figure 1 has the image for the contact angle out of position.

The figure is small and aligned to the left. Please, correct these features.

 

 

2. When using numbers, their units should be with the correct symbol.

Pag. 3, line 106: 5 hours (twice) should be 5 h.

 

3. Pag. 4, line 154: “Fig.2..” there are two dots.

4. Pag. 4, line 161: “η arrives 99.77%”

“arrives” may not reflect what the authors mean.

 

5. Pag. 4, line 165: “substances dissolved” there is a double space.

 

6. In Figure 1c, the surface looks like individual particles, what about SEM image after the Figure 3b test?

 

7. Pag. 6, line 213: “derutilizedHence”

 

8. One final thought for the magnesium alloy covered by such a robust coating, eventually, a scratch can damage a coated piece. This alloy going to be highly corroded under the cross-cut test in a salt spray chamber. Some uses could require a test to evaluate undercutting corrosion of a scribed surface having a coating.

Author Response

Reviewer #2:

1. Perhaps, the authors were not aware of the figures in the constructed PDF file.

Figure 1 has the image for the contact angle out of position.

The figure is small and aligned to the left. Please, correct these features.

 

Answer: Thanks to the referee for the suggestion. We have corrected these features.

 

Revision:

 

 

 

 

 

 

 

 

 

 

Figure 1. Surface micro-topography and contact angle change of (a) PTFE, (b) PTFE+10%SiO₂ and (c) PTFE+30%SiO₂. (d) Variation of contact angle with particle addition.

2. When using numbers, their units should be with the correct symbol.

Pag. 3, line 106: 5 hours (twice) should be 5 h.

 

Answer: Thanks to the referee for the helpful suggestion. We have corrected this description.

 

Revision: The thermal shock test was conducted by muffle furnace, and a high temperature of 80 °C for 5 h and a room temperature of 25 °C for 5 h are set as a cycle.

 

3. Pag. 4, line 154: “Fig.2..” there are two dots.

 

Answer: Thank you for the elaborative advice.

 

Revision: The potentiodynamic polarization curves of various samples are shown in Fig.2.

 

4. Pag. 4, line 161: “η arrives 99.77%”

“arrives” may not reflect what the authors mean.

 

Answer: Thank you for your enlightening comments.

 

Revision: The I_corr of the PTFE+10%SiO₂ coating is three orders of magnitude lower than that of bare magnesium (3.47 x 10^-5 A cm^-2), reaching 7.66 x 10^-8 A cm^-2 and  is up to 99.77%.

 

5. Pag. 4, line 165: “substances dissolved” there is a double space.

 

Answer: Thanks to the referee for pointing out the problem. We have corrected the problem.

 

Revision: which is due to the generated superhydrophobic nanocomposite layer that hinders the diffusion of water and other corrosive substances dissolved in water, such as Cl^-.

 

6. In Figure 1c, the surface looks like individual particles, what about SEM image after the Figure 3b test?

 

Answer: Thank you for the elaborative advice. We supplemented the SEM after abrasion test.

 

Revision: From Fig. 4, although the abrasive test results in serious physical damage, the coating can still maintain the dual scale structure with non-wetting performance.

Fig. 4. The coating surface morphology after abrasion cycle.

 

7. Pag. 6, line 213: “derutilizedHence”

 

Answer: Thanks to the referee for pointing out the mistakes. We have corrected the problem.

 

8. One final thought for the magnesium alloy covered by such a robust coating, eventually, a scratch can damage a coated piece. This alloy going to be highly corroded under the cross-cut test in a salt spray chamber. Some uses could require a test to evaluate undercutting corrosion of a scribed surface having a coating.

 

Answer: Thanks to the referee for the helpful suggestion. In the future, we will further add a test to evaluate undercutting corrosion of a scribed coating surface, according to the special application environment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Review #2:

1) The authors did not rework the introduction, the question of novelty remains open. See my first comment in a previous review.

2) Make a separate subsection: "2.1 Materials".

3) In this edition, section 2.1 "Preparation of the superhydrophobic coating" remained practically the same, the experiment is not reproducible. Should be rewritten.

4) The question under point 3.2 remains the same: there is no comparison with pure PVDF coating without the introduction of silicon dioxide particles. In the comment, the authors write "At the same time, the air trapped in the hydrophobic structure of the PTFE+10%SiO2 composite coating can effectively prevent the penetration of corrosion ions and solution. Further, we compared the PTFE+10%SiO2 composite coating with pure PTFE coating. It is found that the corrosion resistance is close to that of PTFE+10%SiO2 composite coating", but no data is given. In this case, the contact angle is 135 degrees, i.e., probably, the Wenzel regime is realized. In this case, there can be no talk of any pockets with air, on the contrary, the liquid will wet the surface.

In this paragraph, corrosion resistance data for a coating based on pure PVDF should be given. It is logical to compare the original substrate with hydrophobic (pure PVDF), highly hydrophobic (+10% silicon particles) and superhydrophobic (+10% silicon particles).

5) The conclusions are also not corrected in accordance with the comments on the text. (for example, the term nano-ceramic particles).

Author Response

• The authors did not rework the introduction, the question of novelty remains open. See my first comment in a previous review.

 

Answer: Thank you for your valuable comment. We have revised the introduction. As mentioned by the reviewer, many superhydrophobic coatings have been made, including superhydrophobic coatings based on PTFE and SiO₂ particles. With the development of modern technology, unlike the extensive development of superhydrophobic coatings, the multi-functionalization with electrical properties and heavy anti-corrosion has not been fully utilized, which may broaden the application of superhydrophobic coatings in novel electronic devices fields. Especially with the rapid development of electronic information transmission, the demand for magnesium alloys is becoming more and more changeable, and a single performance has been unable to cope with new and strict challenges. Meanwhile, for instance, some surface coating materials for radar protection need not only good protective properties in complex environments, but also good dielectric properties, including low dielectric constant and low dielectric loss tangent. However, the effective integration of multiple functions (superhydrophobicity, corrosion resistance and electrical properties, etc.) remains a major challenge. Among various functional coatings ever reported, the organic and inorganic composites have been regarded as one of the most promising candidates. Meanwhile, most performances of coatings can be adjusted by tailoring micro/nano structures. Therefore, we hope to solve the above problems by means of organic and inorganic hybrid.

Revision: Additionally, unlike the extensive development of superhydrophobic coatings, the multi-functionalization with electrical properties and heavy anti-corrosion has not been fully utilized, which may broaden the application of superhydrophobic coatings in novel electronic devices fields [35]. Especially with the rapid development of mod-ern technology, the demand for magnesium alloys is becoming more and more changeable, and a single performance has been unable to cope with new and strict challenges. At the same time, for instance, surface coating materials for radar protec-tion need not only good protective properties in complex environments, but also good dielectric properties, including low dielectric constant and low dielectric loss tangent. However, the effective integration of a variety of functions (super-hydrophobic, corro-sion resistance and electrical properties, etc.) remains a major challenge. Another ma-jor challenge with superhydrophobic materials operating harsh environment is the mismatch between ambient temperature and hydrophobicity, where most wa-ter-repellent materials are prone to failure in high-temperature applications, which is the common problem.

 

• Make a separate subsection: "2.1 Materials".

 

Answer: Thank you for your valuable comment. We have added the "2.1 Materials" section.

 

Revision:

2.1. Materials

The AZ31 magnesium alloys (2.5–3.0 wt.% Al, 0.70–1.3 wt.% Zn, 0.2 wt.% Mn, and balanced Mg) used as matrix materials were polished with silicon carbide paper fol-lowed by ultrasonic cleaning in deionized water and ethanol. Silicon dioxide (SiO₂) powder (purity=99.9%) with a diameter of around 40nm was purchased from Dongguan XinWeiJin Industry Co., Ltd. PTFE resin was purchased from Daikin Indus-tries. All other chemicals including KH-570 silane coupling agent (Hangzhou Jiexika Industry Co., Ltd.), and butyl acetate solution (Henan Weiyuan Biotechnology Co., Ltd.) were analytical grade reagents.

 

 

• In this edition, section 2.1 "Preparation of the superhydrophobic coating" remained practically the same, the experiment is not reproducible. Should be rewritten.

 

Answer: Thank you for your valuable comment. We have added the description of materials and reagents.

 

Revision: The superhydrophobic coating was prepared by the following process: First, the mixed solution of SiO₂, KH-570 silane coupling agent and ethanol precursors with a mass ratio of 1:1:5 was mechanically stirred for 1 h, then centrifuged at 8000 rpm for 3 minutes to obtain modified hydrophobic silica particles. Subsequently, mix the hydrophobic nano-silica particles (0, 1 and 3 g) with butyl acetate solution and ultrasonically disperse for 1 h to prepare a dispersion. Then slowly add the PTFE resin (10 g) to the dispersion, with magnetically stirring at 1000 r/min for 60 min to ensure that the resin and the solvent dispersion are thoroughly mixed. Finally, the modified PTFE suspension is defoamed in a vacuum dryer (30 ℃) for 5 minutes to obtain a slurry. The slurry is brushed onto the magnesium alloy substrate with subsequent room temperature curing for 5 h to prepare different samples. For convenience, the samples prepared by adding different concentrations (0, 10 wt%, 30 wt%) of silica were called as PTFE, PTFE+10%SiO₂ and PTFE+30%SiO₂, respectively.

 

 

4) The question under point 3.2 remains the same: there is no comparison with pure PVDF coating without the introduction of silicon dioxide particles. In the comment, the authors write "At the same time, the air trapped in the hydrophobic structure of the PTFE+10%SiO2 composite coating can effectively prevent the penetration of corrosion ions and solution. Further, we compared the PTFE+10%SiO2 composite coating with pure PTFE coating. It is found that the corrosion resistance is close to that of PTFE+10%SiO2 composite coating", but no data is given. In this case, the contact angle is 135 degrees, i.e., probably, the Wenzel regime is realized. In this case, there can be no talk of any pockets with air, on the contrary, the liquid will wet the surface.

In this paragraph, corrosion resistance data for a coating based on pure PVDF should be given. It is logical to compare the original substrate with hydrophobic (pure PVDF), highly hydrophobic (+10% silicon particles) and superhydrophobic (+10% silicon particles).

 

Answer: Thank you for your valuable comment. We compared the corrosion resistance of the original substrate with hydrophobic (pure PTFE), highly hydrophobic (+10% silicon particles) and superhydrophobic (+30% silicon particles).

 

Revision: As depicted in Fig. 2 and Table 1, compared with magnesium substrate (-1.483), the Ecorr of the composite coatings obviously moves to the positive corrosion potential by about 0.226 V. The Icorr values of the PTFE and PTFE+10%SiO₂ coatings are three orders of magnitude lower than that of bare magnesium (3.47 x 10^-5 A cm^-2), reaching 7.89 x 10^-8 and 7.66 x 10^-8 A cm^-2, respectively. The corrosion inhibition efficiency (η) of PTFE and PTFE+10%SiO₂ coatings on magnesium alloy substrate is 97.69% and 97.79% respectively, which further proves that the composite coating has a good corrosion inhibition effect. Compared with PTFE, the Ecorr becomes more and more positive and the Icorr becomes smaller as the SiO₂ content increases. Especially for the PTFE+30%SiO₂ coating is more effective in terms of corrosion protection, which is due to the generated superhydrophobic nanocomposite layer that hinders the diffusion of water and other corrosive substances dissolved in water, such as Cl^-.

 

Figure 2. Potentiodynamic polarization curves of AZ31 Mg, PTFE, PTFE+10%SiO₂ and PTFE+30%SiO₂ coated samples.

 

Table 1. The corresponding parameters of the PDP curves of AZ31 alloy, PTFE+10%SiO₂ and PTFE+30%SiO₂ coated samples.

Sample	Ecorr (V)	Icorr (A∙cm-2)	Rp (Ω∙cm-2)	η
AZ31 Mg	-1.483	3.47 E-05	1098.52
PTFE	-1.293	7.89 E-08	5.84×105	97.69%
PTFE+10%SiO2	-1.257	7.66 E-08	5.99×105	97.79%
PTFE+30%SiO2	-1.229	3.39 E-08	1.20×106	99.02%

 

5) The conclusions are also not corrected in accordance with the comments on the text. (for example, the term nano-ceramic particles).

 

Answer: Thanks to the referee for the helpful suggestion. We have revised these words, and check the full text, made the revision.

 

Revision: Thanks to the PTFE layer with wear-resistant nanoparticles and solid hierarchical surface texture, the super-hydrophobic coating presents outstanding thermal cycling stability, environmental stability (> 60 days), mechanical durability (> 40 cycles) and self-cleaning ability.

Author Response File: Author Response.pdf

Round 3

Reviewer 1 Report

1) The introduction is better, but the last paragraph is more in line with the conclusion than the introduction. The authors write about the obtained properties, but they must indicate the idea: what they are doing and why! The purpose of the work must be clearly defined.

2) Clause 3.2

As I said, there is practically no difference between coatings based on original and modified PTFE. The PTFE-based coating is inherently hydrophobic and inert, probably the key factor being the thickness of the coating and its adhesion to the substrate. The difference between superhydrophobic and original PTFE is less than 1.5%, which lies within the margin of error.

Author Response

Reviewer Comments to Author:

1) The introduction is better, but the last paragraph is more in line with the conclusion than the introduction. The authors write about the obtained properties, but they must indicate the idea: what they are doing and why! The purpose of the work must be clearly defined.

 

Answer: Thank you for your valuable comment. We have revised this part, and added what this paper mainly does and why.

Revision: The relationship between surface morphology and wettability is discussed. The corrosion resistance of different coatings and Mg alloy substrate was characterized by potentiodynamic polarization curves, and the corrosion protection mechanism was revealed. The appropriate organic-inorganic network structures and hierarchical surface textures give the composite coating with durable water repellency. Finally, the electrical properties of different coatings including voltage resistance, resistivity, dielectric constant and dielectric loss were characterized, and the mechanism of electrical performance improvement was revealed. This multifunctional superhydrophobic coating is demonstrated to be a candidate for magnesium alloys in complex and harsh environment.

 

 

 

 

2) Clause 3.2

As I said, there is practically no difference between coatings based on original and modified PTFE. The PTFE-based coating is inherently hydrophobic and inert, probably the key factor being the thickness of the coating and its adhesion to the substrate. The difference between superhydrophobic and original PTFE is less than 1.5%, which lies within the margin of error.

 

Answer: Thank you for your valuable comment.

 

Revision: The superhydrophobic coating can improve corrosion resistance, especially in the process of long-term immersion will have certain advantages. At the same time, the superhydrophobic coating has some additional functional features, such as self-cleaning, long-term outdoor applications, anti-icing, anti-fouling and so on. As we all know, water or moisture must be isolated in the process of electrical protection. This paper aims at the application of magnesium alloy in the process of electrical protection in harsh environment. The prepared coating has excellent water repellency on the basis of excellent electrical properties, which has a certain guiding significance for practical application.

In addition, as mentioned by the reviewer, the probably key factor in the corrosion protection effect of the coating on the substrate is the thickness and its adhesion to the substrate. This enlightens us that in the future research, we can investigate the thickness of different hydrophobic and superhydrophobic coatings and test their corrosion resistance and adhesive strength, so as to find a superhydrophobic coating thickness suitable for preparation on the substrate, simultaneously to achieve the best corrosion resistance and the lowest cost.

Author Response File: Author Response.pdf