Effect of Oxidation Time on the Structure and Corrosion Resistance of Micro-Arc Oxidation Coating of AZ91D Magnesium Alloy in (NH₄)₂ZrF₆ Electrolyte System

Round 1

Reviewer 1 Report

The manuscript entitled “Effect of Oxidation Time on the Structure and Corrosion of Micro-arc Oxidation Coating of AZ91D Magnesium Alloy in (NH4)2ZrF6 Electrolyte System” is well written. However, some modifications need to be done to improve the quality of this manuscript:

 Major:

1.       Since this manuscript emphasizes the importance of the oxidation time, authors should include 0 min as the control Kindly include 0 min in all experiments.

      Minor:

2.       The resolution of Figure 3 needs to be enhanced.

3.       Put some indicators or captions in Figure 4.

4.       There are some grammatical errors. Kindly send to proofreading service.

Author Response

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Reviewer 2 Report

This manuscript focuses on the characterisation of micro-arc oxidation (MAO) coatings produced on the surface of AZ91D alloy with different growth times. The authors put nice efforts in the characterisation, namely structural and chemical. In general, the manuscript is fine by me, however I have some questions and comments, as follows.

1.     In line 67, the authors mention that literature is shorthanded relatively to the use of (NH₄)₂ZrF₆-based electrolytes. How does this electrolyte compare to other electrolytes for MAO, why is it relevant or worth of research? I suggest the authors to answer this question as a mean to highlight in the Introduction the innovative and relevance of their work.

2.     Figure 3 show be presented after its mention in the text.

3.     In Figure 5, the compositional profiles should match the colour of the EDS spectrum for improved readability, which I believe is not the case for P.

4.     In line 290 it is not clear what the authors mean with ‘valve metals’?

The values of R_p seem to be too high for this alloy/coating (up to 47 MΩ); the same occurs with I_corr which seems to be too low. Please, have this values confirmed.

Author Response

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Reviewer 3 Report

The reviewed paper is very interesting and certainly worthy of publication. It deals with the problem of oxidation of alloys in terms of their corrosion. This is a very important topic due to the high costs of this process, both economically and socially. Much time and work is currently being devoted to the study of the corrosion process as well as ways to prevent it. 

Despite such an important topic, the reviewed paper has some shortcomings as outlined below:

1. In line 23 in the abstract it should be stated what is the reference material

2, In real systems, structural materials including alloys are protected against the corrosion process by corrosion inhibitors. In recent years, DFT methods have been very popular in the design of suitable inhibitors. Please refer to the following publications for an introduction: https://doi.org/10.1007/s00894-017-3528-0, https://doi.org/10.3390/ma14206197.

3. SEM analysis, 3D analysis, cross-sectional analysis, EDS, XRD, is incomplete. The authors should also consider the non-oxidised reference material

4. For the determination of the standard deviation of the porosity, were multiple oxidations performed, or was an SEM analysis performed from multiple areas of the same sample?

After an in-depth analysis of the submitted work, I reccomend paper publication after major revisions. 

Author Response

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Reviewer 4 Report

The manuscript presents an interesting study about the effect of different oxidation times on the MAO coatings deposited on the AZ91D magnesium alloy. The paper needs major revisions before it is processed further, some comments follow:

Abstract:

The abstract must be improved. The abstract must contain information about:

-       Background: Please highlight the novelty of the study;

-       Methods: Describe briefly the main methods used to obtain and characterize the coating.

-       Results and conclusions: Indicate the main conclusions or interpretations.

Also, please try to use the long form when appears for the first time in text, e.g. MAO

Materials and methods

Insert the chemical composition of AZ91D into a table. Also, how was determined or is given by the manufacturer? Please introduce the information.

Lines 104-112. Please introduce the software used.

Results

Figures 1 and 4. Introduce figure labels in order to highlight the interest zone for the reader.

Subsection 3.4. Calculate the corrosion rate and discussed it, see an example: DOI: 10.3390/ma13153410

The discussion section is missing. Add discussion by comparing the obtained results with other types of coatings.

Conclusions

The conclusion must be improved. Add limitations and suggestions.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

This manuscript has incorporated the suggested comments, therefore can be considered for publication.

Reviewer 3 Report

The authors made corrections according to my suggestions. The work can be published in its present form

Reviewer 4 Report

The manuscript can be published in the present form.