Anodic ZnO Microsheet Coating on Zn with Sub-Surface Microtrenched Zn Layer Reduces Risk of Localized Corrosion and Improves Bioactivity of Pure Zn

Round 1

Reviewer 1 Report

In this manuscript,  the authors analyzed zinc based alloys are emerging as an alternative to magnesium and iron based alloys for biodegradable implant applications, due to their appropriate corrosion performance and biocompatibility. However one is a problem  the zinc surface localized corrosion occurring. Authors to improve corrosion behaviour studied the effect of a dual layer nanostructured Zn/ZnO layer fabricated on pure Zn via anodic oxidization. Newly formed  layer showed favorable Ca-phosphate precipitation as well as bovine serum albumin adsorption properties which increased corrosion resistance.

The presented a review manuscript of the authors are interesting. The some issue are not clearly presented in manuscript. Therefore before publishing should be consider the following comments:

1. In electrochemical measurements description there is no information about the temperature at which the potentiodynamic tests were carried out.
2. The reactions during anodization of Zn presented in page 4 are widely known.
3. The discussion of the results is very short. Please combine points Results and Discussion into one whole.
4. Based on so many studies conducted, the conclusions are too short. Please provide the most important information resulting from the research. Please introduce them in the subparagraphs.

Author Response

Reviewer 1:

In this manuscript, the authors analyzed zinc based alloys are emerging as an alternative to magnesium and iron based alloys for biodegradable implant applications, due to their appropriate corrosion performance and biocompatibility. However one is a problem  the zinc surface localized corrosion occurring. Authors to improve corrosion behaviour studied the effect of a dual layer nanostructured Zn/ZnO layer fabricated on pure Zn via anodic oxidization. Newly formed layer showed favorable Ca-phosphate precipitation as well as bovine serum albumin adsorption properties which increased corrosion resistance.

The presented a review manuscript of the authors are interesting. The some issue are not clearly presented in manuscript. Therefore before publishing should be consider the following comments:

1. In electrochemical measurements description there is no information about the temperature at which the potentiodynamic tests were carried out.

A: Thank you for the careful reading of this manuscript. In this work, potentiodynamic tests were carried out at room temperature. This information is added in revised manuscript.

2. The reactions during anodization of Zn presented in page 4 are widely known.

A: Thank you for this comment. Yes, anodization is a facile and well-established method to fabricate a coating on the surface of metals. We nevertheless would like to keep equations (1) and (2) in the manuscript, as they help to explain for instance observations on the influence of anodization time on the surface morphology.

3. The discussion of the results is very short. Please combine points Results and Discussion into one whole.

A: Thank you for this suggestion. We edited our manuscript according to your suggestion.

4. Based on so many studies conducted, the conclusions are too short. Please provide the most important information resulting from the research. Please introduce them in the subparagraphs.

A: Thank you for this comment. the most important information in this work is that anodization treatment can significantly enhance the bioactivity of Zn (accelerated precipitation of Ca-phosphate during immersion in SBF), enhance protein (BSA) adsorption, and optimize the corrosion behaviour – most noteworthy, reduce the risk of localized corrosion. We have revised the conclusions according to the reviewers recommendation.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors study the microstructured ZnO coatings on Zn and its corrosion behavior in pseudo-physiological solution. The authors tried to stall the primary release of Zn ions via a micristructured coating. Also, the authors claimed an increase in the bioactivity of pure Zn by microstructuring process. The paper has potential -; however, there are certain points that need serious attention. Hence, I would like to decide after the major revision of the following comments.

1- It is unclear as to how the authors are calling ZnO coating as a dual layer nanostructured coating on Zn? What is the analogy of using the phrase "dual layer"? Explain for readers' understanding.

2- Provide the CAS numbers of all the chemicals used in the experimental section.

3- Provide the reason for choosing low concentration of (NH4)2SO4 as an electrolyte? What other options do we have in this backdrop?

4- With the scalebar available in the figure 1 and the apparent average size  the deposited sheet/film of 3 microns, how could the authors brand the deposited structures as "nanostructures". I believe the authors should replace the word "nanostructures" with "microstructures". Also, there would be a gigantic shift in the device character while changing the dimenssions from nano to micro scale. Did the authors only named then nanostructures or also considered the dimensional effects of the films on the devices?

5- Figure 4: The subsection labels (a,b,c,d) are too big in size. Figure 4(b) is partially cut at the y-axis. Replace the X-axis units in Figure 2(a) from symbol to full name "degree". Figure 4(d) is not completely labeled. The blue, red and black graphs are label-free.

6- Figure 5: It looks like the data are cured and drawn at different instances and levels. It would be better if the authors follow the same trend in drawing the plots for all the three subsections of the figure for readers' understanding.

7- The authors have presented the corresponding elemental percentage data taken from the SEM-EDS. I want the authors to provide the SEM-EDS data plots (elemental peak maps) for clarity.

Author Response

Reviewer 2.

The authors study the microstructured ZnO coatings on Zn and its corrosion behavior in pseudo-physiological solution. The authors tried to stall the primary release of Zn ions via a micristructured coating. Also, the authors claimed an increase in the bioactivity of pure Zn by microstructuring process. The paper has potential; however, there are certain points that need serious attention. Hence, I would like to decide after the major revision of the following comments.

1. It is unclear as to how the authors are calling ZnO coating as a dual layer nanostructured coating on Zn? What is the analogy of using the phrase "dual layer"? Explain for readers' understanding.

A: Sorry for the unclear statement in the manuscript. This anodic layer consists of a top layer of ZnO/Zn₃O(SO₄)₂ sheets layer and beneath it a region of trenched Zn (formed due to etching of the substrate during anodization). We revised the title and the description. The new title is:

Anodic ZnO microsheet coating on Zn with sub-surface microstructured Zn layer reduces risk of localized corrosion and improves bioactivity of pure Zn.

2. Provide the CAS numbers of all the chemicals used in the experimental section.

A: We added CAS numbers for all the chemicals in the revised manuscript.

3. Provide the reason for choosing low concentration of (NH4)2SO4 as an electrolyte? What other options do we have in this backdrop?

A: Thank you for this comment. High concentration of (NH4)₂SO₄ results in stronger acid etching effect as compared with low concentration, Zn dissolves into solution with a fast rate, thus Zn trenches are not well formed under this condition. Low concentration of (NH4)₂SO₄ is therefore beneficial for the formation of Zn trenches. Moreover, it has been reported in the literature that a high concentration leads to large size (around 30 µm) sheets. (Zhao, J., Wang, X., Liu, J., Meng, Y., Xu, X., & Tang, C. (2011). Controllable growth of zinc oxide nanosheets and sunflower structures by anodization method. Materials Chemistry and Physics, 126(3), 555-559.), In our work, the low concentration was used targeting small size microsheets (around 3 µm). This size-scale is expected to be more beneficial for bioactivity.

4. With the scalebar available in the figure 1 and the apparent average size the deposited sheet/film of 3 microns, how could the authors brand the deposited structures as "nanostructures". I believe the authors should replace the word "nanostructures" with "microstructures". Also, there would be a gigantic shift in the device character while changing the dimenssions from nano to micro scale. Did the authors only named then nanostructures or also considered the dimensional effects of the films on the devices?

A: Thank you for point this out. Indeed, the length of the sheet is around 3 microns, but the thickness of each ZnO/Zn₃O(SO₄)₂ sheet is in nanometric scale range, thus we labeled it as "nanostructures". Considering the morphology of this structure, we believe microstructure is in fact suitable, especially as the surface morphology is more relevant for the properties we are studying (e.g., bioactivity, protein adsorption) than the thickness of these structures. We therefore now use the term "microstructure" in the revised manuscript.

5. Figure 4: The subsection labels (a,b,c,d) are too big in size. Figure 4(b) is partially cut at the y-axis. Replace the X-axis units in Figure 4(a) from symbol to full name "degree". Figure 4(d) is not completely labeled. The blue, red and black graphs are label-free.

A: Thank you for the careful reading. We edited Fig. 4 in the revised manuscript according to your suggestion.

6. Figure 5: It looks like the data are cured and drawn at different instances and levels. It would be better if the authors follow the same trend in drawing the plots for all the three subsections of the figure for readers' understanding.

A: Thank you for this suggestion. We edited Fig. 5 according to your suggestion.

7. The authors have presented the corresponding elemental percentage data taken from the SEM-EDS. I want the authors to provide the SEM-EDS data plots (elemental peak maps) for clarity.

A: Thank you for this comment. All the elemental peak maps are showed below:

Author Response File: Author Response.pdf

Reviewer 3 Report

The article deals with interesting topics related to the search for new materials for generally understood medicine. In this case, they are biodegradable zinc-based implants. The part concerning the synthesis of layers and the study of their morphology is very well documented. How were polarization tests performed? That is, whether the test was started at the lowest cathode potential and the sample potential was increased, or whether two polarizations were performed starting from the corrosion potential. Polarization from such a low potential may lead to the evolution of hydrogen and changes in the produced nano-layer. Especially that the authors of the work are primarily concerned with showing the corrosive behavior of zinc in various solutions. According to what criteria were the results of the EIS method assessed and compared? The results of polarization and EIS tests are not fully consistent. Perhaps you should not simply compare the diameter of the spectrum obtained, but try to model it. The description of the surface and the processes occurring on it, especially in the presence of anode layers, is certainly more complicated than describing the processes using a simple Randles diagram. The indication of the electrical substitution circuit is absolutely desirable! It would be useful to show the surface after corrosion testing.

Author Response

Reviewer 3

The article deals with interesting topics related to the search for new materials for generally understood medicine. In this case, they are biodegradable zinc-based implants. The part concerning the synthesis of layers and the study of their morphology is very well documented. How were polarization tests performed? That is, whether the test was started at the lowest cathode potential and the sample potential was increased, or whether two polarizations were performed starting from the corrosion potential. Polarization from such a low potential may lead to the evolution of hydrogen and changes in the produced nano-layer. Especially that the authors of the work are primarily concerned with showing the corrosive behavior of zinc in various solutions.

A: Thank you for this comment. The test was started at the lowest cathode potential (-2 V) and then the potential was scanned in the anodic region. We agree that in principle strong cathodic polarization can modify surface, for instance due to pH change in the electrolyte. However, here polarization was carried out in SBF buffered with Tris-HCl, this system can maintain a pH range from 7.0-9.0, bonded H⁺ in Tris-HCl depletes OH^- and retards the increase of pH. We therefore expect less strong change of pH during cathodic polarization as would be the case in non-buffered aqueous solutions. Therefore, less effect of alkalization on the surface layer. In principle, evolution of hydrogen is possible due to the low starting potential (-2 V), but no visible gas bubbles were observed during polarization test in cathodic branches, and we therefore expect no destructive effects on the layer as could occur by strong H₂ bubbling. Finally, the long-term immersion test in both SBF and SBF+BSA, indicate generally a good stability of the ZnO/ Zn₃O(SO₄)₂ layer.

According to what criteria were the results of the EIS method assessed and compared? The results of polarization and EIS tests are not fully consistent. Perhaps you should not simply compare the diameter of the spectrum obtained, but try to model it. The description of the surface and the processes occurring on it, especially in the presence of anode layers, is certainly more complicated than describing the processes using a simple Randles diagram. The indication of the electrical substitution circuit is absolutely desirable! It would be useful to show the surface after corrosion testing.

A: Thank you for this comment.

We carried out these electrochemical measurements in both SBF and SBF+BSA. In pure SBF, polarization curves and EIS results are fully consistent, bare Zn showed stronger corrosion resistance as compared with anodized sample. After anodization treatment, the coated sample shows a larger specific surface area, offering more attack sites for aggressive ions such as Cl^-, resulting in the worse corrosion resistance.

In SBF+BSA, adsorption of BSA improves the corrosion resistance for both bare Zn and coated Zn, this is reflected in EIS results. As EIS measurements are carried out with a small perturbation around the corrosion potential, the sample is not polarized far away from its equilibrium state. Therefore, much less reaction (cathodic and anodic) are taking place than during the polarization curve measurements. As BSA is only relatively weakly bound to the Zn surface, this weak bond between samples and BSA can be reflected in the EIS measurements: adsorbed BSA increased impedance value, more adsorbed BSA on anodic sample induce larger impedance value as compared with bare Zn. But for polarization test, rapid chemical reactions take place in both cathodic (oxygen reduction reaction) and anodic branch (Zn dissolution), unstable interface between sample and solution suppresses the adsorption of BSA. Therefore, the corrosion resistance improvement in SBF+BSA is not observed in the polarization curves. We added a short statement on this in the revised manuscript.

Due to the very complex nature of the surfaces and the high reactivity, use of equivalent circuits would be highly challenging and we therefore prefer to discuss the EIS results only in this qualitative manner.

We prefer to show the surface after long-term immersion testing instead of after the electrochemical measurements, as the immersion test is more relevant for real exposure and the surface morphology after immersion therefore can better reveal details of the corrosion behavior.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors have provided a detailed response to the review comments. I found the response complete and satisfactory. I believe the manuscript is ready for publication and would be a good contribution to Coatings. Hence, I would like to accept the paper publication in the present form.

Reviewer 3 Report

The authors escaped a bit from responding to allegations about EIS research. A "qualitative assessment" can be used, but it gives no information about the condition of the test surface.