In Situ Growth of High-Performance ZnAl-Layered Double Hydroxides/Al₂O₃ Composite Coatings on Aluminum Alloy

Round 1

Reviewer 1 Report

The manuscript was dedicated to a study entitled "In-Situ Growth of High-Performance ZnAl-LDHs/Al2O3 Composite Coatings on Aluminum Alloy". The manuscript itself is theoretical and structurally relevant. Also, it is original, and results are generally very well supported by experimental evidence. I recommend this paper may be considered for publication after addressing the following points:

- On page 2, lines 49-51, the sentence "Although the anodization of Al alloys produces …. surface area." needs the following reference:  https://doi.org/10.1016/j.matpr.2018.04.177.

- The scan rate for potentiodynamic polarization should be mentioned in 2.2. 

- How did the author find Ecorr and Icorr values? For Tafel extrapolation, both anodic and cathodic sides of the curves were considered, or only the cathodic side? Authors should provide a brief description for calculating Icorr and Ecorr.

- Anodic layer roughness - does not seem to be mentioned anywhere else in the manuscript. Is this property important for corrosion and tribological performances?

- Table 2: provides chi-square values for EIS fitting.

- What is the mechanism of corrosion protection? 

- In the manuscript, the authors sometimes use x/y, and sometimes x.y-1 and x y-1 and xy-1; please check and choose the unified unit style for the values. For example, A.cm-2 and mol/L. 

 Minor editing of English language required!

Author Response

Responses to the comments of reviewers

Dear reviewers,

Thank you for giving us a chance to resubmit the revised manuscript (ID: coatings-2568726). Your comments are valuable and helpful. I have studied them carefully and the point-to-point responses are listed as the following:

Reviewer #1:

1) On page 2, lines 49-51, the sentence "Although the anodization of Al alloys produces …. surface area." needs the following reference:  https://doi.org/10.1016/j.matpr.2018.04.177.

Response: The corresponding reference has been added. Thanks.

[1] Rahimi, S.; Khiabani, A. B.; Yarmand, B.; Kolahi, A. Comparison of corrosion and antibacterial properties of Al alloy treated by plasma electrolytic oxidation and anodizing methods. Materials Today-Proceedings 2018, 5, 15667-15676.

2) The scan rate for potentiodynamic polarization should be mentioned in 2.2.

Response: The experiments were scanned from −0.5 V to +0.5 V with respect to the open circuit potential at a scan rate of 1 mV/s. The corresponding description has been added. Thanks.

3) How did the author find Ecorr and Icorr values? For Tafel extrapolation, both anodic and cathodic sides of the curves were considered, or only the cathodic side? Authors should provide a brief description for calculating Icorr and Ecorr.

Response: The E_corr and I_corr were obtained from the built-in slope algorithm of software after electrochemical tests. Thanks.

4) Anodic layer roughness - does not seem to be mentioned anywhere else in the manuscript. Is this property important for corrosion and tribological performances?

Response: As known, the smooth surface generally has lower average surface roughness namely arithmetic average height of the surface (R_a). Currently, a large number of researchers found that the tribological behavior of contact parts would be better with an appropriate range for R_a ^[1]. Moreover, some researchers found that the tribological properties for tribo-pair were greater with lower or larger R_a. However, surface roughness parameters include various components such as root mean square height of the surface, skewness of height distribution (S_sk), kurtosis of height distribution (S_ku), core roughness depth, reduced peak height, reduced valley depth and so on, which act as varied influence on the tribological properties for tribo-pair under different operation conditions and lubrication regimes. Therefore, solely on the basis of R_a, tribological behaviour properties of contact surfaces, either under dry or boundary lubricated conditions, cannot be anticipated. Surface roughness parameters S_ku and S_sk were found to show a good correlation to the tribological properties of contact surfaces and could be used for planning surfaces and surface topographies with desired tribological behavior in boundary lubrication regime. Under boundary lubrication, contact friction tends to get lower when parameter S_ku gets higher. However, the most dominant parameter is parameter S_sk ^[2]. The more negative it is, the lower the friction we can expect, even at higher average surface roughness. In conclusion, evaluation of only R_a for anodic layers cannot reflect their tribological behavior comprehensively, which should be compared with other surface roughness parameters experimentally. But the quantitative analysis for surface roughness parameters and tribological properties of tribo-pair is necessary and prospect. Meanwhile, the study on the effect of surface roughness on corrosion behavior is similar with that of tribological behavior. Based on our current works, we will further explore the effect of dominant surface roughness parameter of anodic layers on tribological behavior and anti-corrosion performance of composite coatings.

[1] Arslan, E., Totik, Y., Demirci, E. (2010), “Influence of surface roughness on corrosion and tribological behavior of CP-Ti after thermal oxidation treatment,” Journal of Materials Engineering and Performance, 19, pp 428-433. doi:10.1007/s11665-009-9504-9

[2] Marko, S., Bojan, P., Jože, V. (2011), “Correlation between standard roughness parameters skewness and kurtosis and tribological behaviour of contact surfaces,” Tribology International, 48, pp 102-112. doi:10.1016/j.triboint.2011.11.008

Thanks.

5) Table 2: provides chi-square values for EIS fitting.

Response: The corresponding description has been added. Thanks.

6) What is the mechanism of corrosion protection?

Response: ZnAl-LDHs/AAO composite coatings with good crystallinity were formed at different concentrations of Zn²⁺. With the continuous increase of Zn²⁺ concentration, the distance between LDHs nanosheets gradually increased and the distribution of LDHs nanosheets became more compact. Hence, LDHs nanosheets with higher concentrations of Zn²⁺ played a better role in sealing AAO film. Additionally, the thickness of the ZnAl-LDHs/AAO composite coatings became thicker, the binding force became stronger with increased Zn²⁺ concentration. And the effect of inhibiting the penetration behavior of corrosive ions was enhanced, which significantly improved the corrosion resistance of ZnAl-LDHs/AAO composite coatings. The corresponding description about mechanism of corrosion protection of composite coatings has been added. Thanks.

7) In the manuscript, the authors sometimes use x/y, and sometimes x.y-1 and x y-1 and xy-1; please check and choose the unified unit style for the values. For example, A.cm-2 and mol/L.

Response: This mistake has been revised. Thanks.

Thanks again. Wish you and your family all the best in the future. ^_^

Best regards

Lixia Ying

Reviewer 2 Report

Comments for author File: Comments.pdf

Author Response

Responses to the comments of reviewers

Dear reviewers,

Thank you for giving us a chance to resubmit the revised manuscript (ID: coatings-2568726). Your comments are valuable and helpful. I have studied them carefully and the point-to-point responses are listed as the following:

Reviewer #2:

1) Need more details for hydrothermal method.

Response: The in-situ growth method was used to prepare the LDHs. This mistake has been revised. The corresponding description has been added. Thanks.

2) How the LDH has been formed?

Response: There are two procedures of ionization equilibrium and coordination equilibrium in the formation of LDH. The aluminum source on the anodized aluminum surface continuously consumed the OH^- in solution, causing the ammonia to undergo ionization equilibrium in a positive direction. The occurrence of ionization equilibrium caused the concentration of NH₃ to become smaller, which promoted the dissociation of [Zn(NH₃)₆]²⁺, which in turn caused Zn²⁺ to be released and the concentration of NH₃ to increase. At this time, the presence of Al³⁺, Zn²⁺ and OH^- provided conditions for the growth of ZnAl-LDHs films, which can be generated in situ on the AAO surface. The corresponding description has been added. Thanks.

3) How samples be prepared for cross-section SEM, need more explanation, are there be coated.

Response: To observe the cross-sectional profiles of ZnAl-LDHs/AAO composite coatings, specimens were completely wrapped using epoxy resin, left to set for 12h, the cross-section of the specimens were then sandpapered until smooth. The coated samples were prepared for cross-section SEM. The corresponding description has been added. Thanks.

4) How the samples be prepared for FTIR? It seems that the results are from ATR.

Fig4. Cross sectional SEM images need improvement.

Response: During the testing process, ZnAl-LDHs films were scraped from the surface of the AAO with a razor blade, and the powder was added to an appropriate amount of potassium bromide for grinding, and finally the samples were prepared by KBr presses for infrared spectroscopy. The corresponding description about the preparation of samples for FTIR has been added. The Fig. 4 has been replaced. Thanks.

5) The absorption peaks of SO₄^2- and M-O are sharper: It seems that the amount of LDH decreases with increasing the Zn. Vibrate obviously, which indicates that the ZnAl-LDH films have high crystallinity and more nucleated ZnAl-LDHs nanosheets. Need reference

Response: I am sorry that no appropriate literature has been found on this issue, however, ZnAl-LDHs films with complete functional groups and lamellae structure can be formed under different concentrations of Zn²⁺. The corresponding description has been revised in order to make the discussion more reasonable. Thanks.

Thanks again. Wish you and your family all the best in the future. ^_^

Best regards

Lixia Ying

Round 2

Reviewer 1 Report

The authors corrected the paper following the reviewer's advice and improved the quality of the manuscript. It can be accepted as it is.