Study of Polarization Characteristics of Corrosion Films on Magnesium in Sulfate-Containing Electrolytes

Round 1

Reviewer 1 Report

In general, the motivation of this study as well as the novelty should be better presented. The SEM characterization of the metals is poor. I strongly recommend to obtain cross-section images and EDX analysis of the elements distribution through the formed coatings. The analysis of the SEM images should be improved. The English should be highly improved. Maybe the reference list might be much wider, but the paper is not a review, so it's up to authors to show whether they consulted the extended literature on the subject. At least 25% of citations must be from the last 3 years. I would still like to see some more modern references from this Journal in your reference list.  The reason is that this Journal has the highest standards of reviewing, as well as of rigor and of correctness: papers publish there tend to be relatively reliable. References should be given in exactly the style of this Journal. All graphs must show four axes: left, right, top and bottom. The analysis of the polarization graphs should be improved; especially the potential-logarithm graphs. It is not stated if the electrochemical data were recorded in triplicate. Please not, that if your have the reproducibility data, it is always best to show that reproducibility (or scatter) because it makes the paper very much stronger. The presented study should be also performed in a pH range from acidic to basic solutions. Additional electrochemical characterizations should be conducted (such as EIS) in order to support confirm the presented conclusions.

Considering the aforementioned reasons, this manuscript is suggested for publication only after mandatory major revision.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

What is the pH and conductivity of the MgSO4 and Na2SO4 solution? The pH and conductivity are important factors affecting the film growing and polarization curves.  In fig.1-3, the SEM images were taken at different magnifications, which make the comparison between these images unreliable.  What are the open circuit potentials of Mg electrode in MgSO4 and Na2SO4?  In part 3.2, since the authors discuss the polarization curve in Na2SO4 first, the order of fig.5 and fig.4 should be exchange to avoid confusion. The conclusion part should be more concise. Discussion should not be included in the conclusion part.  

Author Response

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Reviewer 3 Report

This article reports the polarization properties of magnesium with corrosion film in several electrolytes.
The represented data show the composition dependence of the electrochemical properties.
The interpretation of the polarization curves seems to be appropriate and significant.
However, authors should address considerations as following.

At first, authors should confirm and blush up the Figures.
I think that ref 15 can be a possible reference for constructing Figures.
Point-by-point description is written below.

1) At all figures, I recommend to embed the notations, such as “a)”, in the figure’s appropriate region.
The notation place of the figures in the presented manuscript is dependent on figure now.

2) The scale of the SEM images is not clear, especially Figure 1 and 3.
I think that authors use the exported images from the SEM machines directly.
Then, the font size of the scale values looks small to see.
Please add the more visible scales.

3) At figure 4-7, font sizes of each axis is very small, and larger font size is better.
Please confirm the “printed” manuscript carefully.
Especially at the inset figures, it is difficult to correspond to main text. Then, I cannot confirm the description in the main text easily.

4) Why did authors show both linear and semi-log polarization curves at figure 4 and 5?
I feel that semi-log plot can be enough to describe the results.

5) At figure 7, the correspondence between Figures and captions is opposite.

At the modeling section:
6) At equations 6 and 7, “lg” can be changed to “ln”.
Authors used “ln” at equation 2. At equation 6 and 7, I think that authors intended to take “log-natural”.

At experimental section:
7) At line 93-94, authors described “7-aqueous magnesium sulfate”.
I think that authors used “Magnesium sulfate heptahydrate”.
Please confirm the label of this substance.

At results and discussion section;
8) At line 126, “(Figure 2)” can be “(Figure 3)”

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have made a remarkable work to improve the manuscript. All the reviewer's comments have been defended. Therefore, the manuscript is recommended for publication in its present form.

Author Response

Thank you very much for your work.

Reviewer 2 Report

1. More recent references discussing the effect of solution composition and pH could be added, for example:

Lin, J., Battocchi, D., & Bierwagen, G. P. (2017). Degradation of Magnesium-Rich Primers over AA2024-T3 During Constant Immersion in Different Solutions. Corrosion, 73(4), 408-416. Cao, F., Zhao, C., Song, G. L., & Zheng, D. (2019). The corrosion of pure Mg accelerated by haze pollutant ammonium sulphate. Corrosion Science, 150, 161-174.   2. Subsections could be used in the experimental section to present the methods more clearly   3. Grammar and English expressing should be improved.

Author Response

We thank the reviewer for analyzing the article to improve it.

We used the article by Cao et al. to strengthen the provisions (points) and conclusions in the review and discussion (noted in the text). The use of the first two articles proposed by the reviewer in the discussion does not seem relevant to us, since they are talking about highly doped magnesium and specifically for cathodic protection.  Added two subsections:

3.3. Analysis of the laws of charge transfer during polarization

3.4. Determination of charge transfer parameters using EIS

English Grammar and especially terminology have been improved.