Corrosion Resistance of Mg(OH)2/Mn(OH)2 Hydroxide Film on ZK60 Mg Alloy
Round 1
Reviewer 1 Report
In this paper, the authors studied the corrosion behavior of an ZK60 magnesium alloy in simulated body fluid. The alloy was coated with Mg(OH)2/Mn(OH)2 to increase the corrosion resistance. The coating was prepared using a hydrothermal method. The work is interesting, novel, and sufficiently researched. The paper is publishable subject to revision.
1.It would help to list the chemical composition of the ZK60 alloy (Mg-Zn-Zr alloy). Not everyone is familiar with designation of Mg alloys.
2.The location of magnified areas in Fig. 3 (b, d, f) should be clearly marked on the left images.
3.Have you measured an open circuit potential of the alloy before the potentiodynamic polarization?
4.It is necessary to calculate the corrosion rate (CR) from the jcorr values using the Faraday law and compare it with the CR from hydrogen evolution experiments. Any differences should be discussed.
5.The results of the present work should be compared with previous studies, e.g., https://doi.org/10.1016/j.surfcoat.2016.11.111 or https://doi.org/10.3390/ma13173833. The effect of Mn(OH)2 on the corrosion resistance of the ZK60 alloy should be highlighted.
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Reviewer 2 Report
This paper provides incremental contribution to the understanding and design of corrosion resistant surface films on magnesium alloys. The authors provide an extended experimental set of data, each of which are well described and documented. On the other hand, I doubt the validity of the last sentence of the abstract, which states that their results provide new insights. This is not documented in the discussion nor in the conclusions: no clear explanation is given about in what extent their results go beyond and offer more about the mechanism or the corrosion resistance than those published e.g. in ref. 17. This why I wrote “incremental contribution”.
The paper is very badly written: it contains many awkward formulations, which sometimes even make the understanding of the content difficult.
Thus I propose a substantial correction of the English (perhaps by a native English person).
I have the following formal comments:
- in row 219 it is mentioned that “and Cl disappeared”, but in the inserted tables in Fig. the Cl-content is still not zero (of course if it is close or below the detection limit, then you can write that it decreased to close this limit)
- the inserted tables in Fig.7 cover the values of the bars indicating the typical lengths.
- in Fig. 8 the Mn content has a peak at about 5 micron below the surface (after 24h) and this peak gradually shifted deeper and flattened with increasing time. What is the explanation for this?
- “Grant N. 51701093” should be deleted from the end of (4) Conclusions.
In summary: I can propose to accept the paper only after successful major revisions.
Author Response
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Reviewer 3 Report
The authors investigated the corrosion resistance of a magnesium alloy covered with a layer of magnesium and manganese hydroxides under the conditions of a body fluid simulating solution.
My comments are as follows:
1. The title is not precise. The authors did not investigate the corrosion resistance of the composite layer as an independent object, but the corrosion rate of the magnesium alloy in the presence of a layer of hydroxides on the surface. Please correct the title to reflect the content of the manuscript.
2. To assess the influence of manganese hydroxide on the properties of the layer, it would be advisable to also make a layer without the addition of manganese hydroxide, only with magnesium hydroxide. We do not know how the presence of manganese hydroxide affected the properties of the layer. Maybe negative?
3. Sentence (lines 173-174) "The applied film samples have weak corrosion tendency and a low corrosion rate with high thermodynamic and low kinetic." should be improved in terms of the English language.
4. In "2. Materials and Methods" there is no description of how the hydrogen evolution studies were performed. Please complete this.
5. The sentence (line 284) "the main reaction in the curve was solution resistance." It makes no sense.
6. Sentence (lines 289 and 290) "inductive reactance emerges in the Nyquist diagram, indicating that pitting corrosion occurs during the corrosion process", Is any literature reference to support this thesis?
7. In the Nyquist plots (Figure 10a) the units on the Zre and Zim axes should be equal, in the manuscript, the Zim axis is stretched, in fact, the obtained semicircles are strongly flattened.
8. The analysis of the EIS measurement results is very superficial. Figure 10 shows that we are dealing with one semicircle, from which the analysis describes three semicircles? In the measured frequency range, down to 0.1 Hz, it is difficult to obtain an inductive semicircle (it occurs at lower frequencies). From the Bode diagram (not bode) it can be seen that the composite layer gives practically no impedance response, it is very, very porous, so why the film resistance? The use of the electrical equivalent circuit from Figure 11, taking into account also the large scatter of the results, does not make sense, we can observe in principle only one semicircle, characterizing the charge transfer reaction (corrosion reaction).
The obtained impedance results show that the protective effect of layer is small, it is somewhat inconsistent with the results of the polarization method (Figure 4). Any discussion, or comparison?
There is too little literature quoted in the manuscript, and there is a lot of literature in this area of ​​research, especially electrochemical studies of magnesium alloys.
Author Response
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Round 2
Reviewer 1 Report
Authors responded to most of my previous comments. The paper is publishable subject to minor revision.
1.Please, include the OCP data in the manuscript. The open circuit potential should be compared to Ecorr from polarization experiments. Therefore, it is recommended to include the OCPs in Table 2.
2.The corrosion rate should be calculated from hydrogen evolution volume (Fig. 5). It should be labelled as PH and included in Table 2 next to Pi for the sake of comparison.
3.The dimension of the corrosion rate is mm/y, not mm.y. Please, correct the misprint (Table 2).
Author Response
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Reviewer 2 Report
After the corrections made I propose to accept the paper.
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Reviewer 3 Report
The authors partly took into account my previous comments. However, some of them were misinterpreted and not corrected or omitted. This is especially true for the interpretation of the results of electrochemical tests. In response, the authors refer to the article (Wang, S .; Wang, Y .; Cao, G .; Chen, J .; Zou, Y .; Yang, B .; Ouyang, J .; Jia, D .; Zhou, Y. Highly reliable double-layer coatings on magnesium alloy surfaces for robust superhydrophobicity, chemical durability and electrical property. Ceram. Int. 2021, 47, 35037-35047) but it concerns other, very barrier coatings on Mg alloy. The impedance picture in this case is different. The authors investigate systems with a very porous layer, without barrier properties, and repeat the interpretation scheme characteristic of barrier coatings, taken from this article. One gets the impression that the authors do not understand this. The used equivalent electrical circuit does not correspond to their situation.
The authors did not transform the Nyquist plots so that the units on the Zre and Zim axes were the same (Figure 10), and then it would be seen that the points at the lowest frequencies do not form an inductive semicircle even for a pure Mg alloy. All this is done correctly in the article quoted above, so please analyze it with understanding, paying attention to the differences concerning the different type of the tested system.
The authors argue that the hydroxide layer has significant protective properties, and it can be concluded from the results obtained that this is not the case. For example, impedance values ​​are of the same order (Figure 10). Studies with polarization curves show more favorable properties. Previously, I asked how can this be explained?
Author Response
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Round 3
Reviewer 3 Report
I see progress in improving the quality of this manuscript. Most of my comments were taken into account. I propose to delete the word "excellent" (line 371), because it is not true if we look at the test results.
