Comparative Study of the Structure, Properties, and Corrosion Behavior of Sr-Containing Biocoatings on Mg0.8Ca

Round 1

Reviewer 1 Report

Referee report of the paper

 

 Comparative Study of the Structure, Properties and 2 Corrosion Behavior of Sr-containing Biocoatings on Mg0.8Ca

By M Sedelnikova et al.

 

Manuscript number: materials-764729

 

The authors performed the complex study in which microstructure, phase composition, corrosion behaviour of two coatings on a magnesium alloy were compared. These investigations were then completed with biological studies of the toxicity with respect to cell test cultures. The study was made profoundly, and results can be useful for other authors.

 

The paper is overall very interesting.

 

I have only small remarks:

 

Line 55: probably better … very good mechanical properties…

Explain abbreviation SAD patterns

 

 

 

In conclusion, I recommend publication of the paper with minor revisions.

 

Author Response

Response to Reviewer 1 Comments

Point 1:

Line 55: probably better … very good mechanical properties…

Response 1:

Authors agree with the Reviewers' comment and the text of manuscript was corrected.

Point 2:

Explain abbreviation SAD patterns

Response 2:

SAD is “selected area diffraction”. This specification has been added to the manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper reports on the corrosion behavior in biological environment of Mg alloys as a function of the coating composition and microstructure. The paper is interesting and well organized with a large number of experimental results. However, there are a few questions to be addressed before accepting the manuscript for publication according to the following comments.

The authors discuss about the composition of the coatings focusing on phosphates (eqs. 3-7). However, they do not mention the role of anodic polarization during MAO apart for the production of Mg2+ ions. Is only the high temperature reached during MAO the key for understanding the formation mechanism of the coatings? Did they reveal MgO? What about the fluoride ions? The authors add NaF to the electrolytic bath but they do not mention the role of F- in the coating formation and the map of F is missing. Did they check if F is incorporate at the Mg/coating interface? Its presence can strongly affect the corrosion resistance of the produced coatings (Coatings 2019, 9, 688).

The authors assessed the corrosion rate/resistance of the coated Mg using mass loss measurements, polarization curves and EIS spectra. First of all the authors should state hoe they explain a shift toward negative values of Ecorr estimated by the polarization curves. The explanation of soluble salts formation is not convincing since the chemical (not electrochemical) dissolution is not expected to affect the corrosion behavior of Mg. is it possible that uncoated areas are still present? This also can account for the presence of an inductive contribution in the EIS spectra which is associated with the cathodic half cell process, i.e. H2 evolution, and nit necessarily with reactions (12-13). The presence of an inductive behavior of partially coated Mg has been physically and mathematically rationalized in ref. Curioni et al. Electrochim. Acta 2018, 274, 343, where the problem of negative differential effect is also discussed.

Finally, it is interesting to mention if you have in some way quantified the evolved H2 directly (with a balance), to compare the results with the accepted limits for biological applications.

Author Response

Response to Reviewer 2 Comments

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 3 Report

Dear Author,

This manuscript focused on improving Mg alloy for biomedical applications with a coating. The structure of the manuscript is well-designed. However, there are some questions to be addressed.

• Firstly, the manuscript needs Professional English editing.

Introduction:

• Clarify and highlight what improvements were made so far in this type of coating, but needs further improvement. So, this study is proposing a solution for the need in this field.

Materials and Methods:

• What is the significant difference to form HA or TCP for sample preparation and coating processes?
• Refer a study for chemical precipitation method.

Results:

• 1. section mentioned “morphology” in the section title, but there are no morphology results in this section.
• Low (Ca+Sr)/P ratio is clarified as Mg replacing Ca atoms and amorphous phase might contribute to the low ratio. However, how the author explains the low Ca/P ratio of precipitated particles on the top of the coating, which no possibility for Mg to interact with the particles?
• Cross-section of the coating raised the question about its adhesion quality. Please refer to this issue. As reported, ionic incorporation might promote better adhesion or otherwise and it is important for the long-term use of the implant materials. Check the ref. study “In vitro performance of Ag-incorporated hydroxyapatite and its adhesive porous coatings deposited by electrostatic spraying”.
• About Fig. 5; how did you determine the ratio of amorphous and crystalline phases? Please include the explanation in the manuscript.
• About Fig. 8; the values of Sr content seem to be similar quantity with a Ca/P ratio, but it is not. So, it is recommended to express differently on the figure, for example, such as a symbol with error bars not with a column same as the Ca/P ratio.
• 4. section used twice. Corrosion behavior…. section must be numbered as 3.3.!
• In corrosion section (3.3.), Fig. 10 should be the first to explain and then the morphology should follow.
• Dissolution is conducted in a Cl-containing solution. So, refer to a study that evaluates the CaP dissolution and incorporated ion dissolution in Cl-containing solution, such as “Antibacterial activity of Ag nanoparticle-containing hydroxyapatite powders in simulated body fluids with Cl ions”.
• There are obvious punctuation mistakes, check it carefully.
• 14(a) needs X-axis information (Days?).

Discussion:

• To begin with, the discussion section is just the summary of results, needs more depth discussion for the phenomena occurred in your material an coating. A comparison with literature studies is necessary.
• Additionally, the author claimed to obtain not Sr-substituted CaP, but Sr, Mg co-substitution. Therefore, this needs to be addressed in the title or not at least in the abstract.

Regards.

Author Response

Response to Reviewer 3 Comments

Please see the attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

You cannot find the evidence of MgF2 by XRD and TEM analysis, since the latter is present as thin layer at the Mg/oxide interface. What I tried to stress is that an inner layer of MgF2 can play a role in the corrosion resistance of the coatings. I was expecting a comment about this point according to the suggested literature or any work you think it is relevant to comment this aspect.

Author Response

Dear Reviewer, we are very grateful to you for the constructive comment. We have modified the text of the manuscript in the lines 328–330 in accordance with your recommendation. In addition, we added a link to another source in the list of references in the lines 737–739.

Reviewer 3 Report

Dear Author,

 

The manuscript has been improved with your revisions. In this form, it will be informative and easy to follow. Of course there are some points to further clarify related to this study, however, those might be a follow-up study. 

 

Regards.

Author Response

Dear Reviewer, we are very grateful to you for the constructive comments. We have improved our manuscript due to your recommendations.