Corrosion Resistance of Li-Al LDHs Film Modified by Methionine for 6063 Al Alloy in 3.5 wt.% NaCl Solution

Round 1

Reviewer 1 Report

It is an interesting work. However, the following comments must be considered prior to acceptance in order to satisfy the quality of this journal. 

1. Please discuss the physical meaning of CPEs parameters and correlate that to the surface morphologies of the samples.
2. The CPE  has two parameters (n and Y) but the results in Table 3 only presented the values of Y. Both parameters should be discussed.
3.  The introduction and electrochemical results can be strengthened by citing the relevant literature, for example: (a) https://doi.org/10.1016/j.ultsonch.2018.08.023 (b) https://doi.org/10.1016/j.jallcom.2017.04.016
4. Please balance all equations 

Author Response

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Reviewer 2 Report

Reviewer comments on the article “Corrosion resistance of Li-Al LDHs Film Modified by Methionine for 6063 Al Alloy in 3.5wt.% NaCl Solution” by Ji Ma et al. (Coatings-1609610)

The article deals with an interesting topic and shows that methionine can be incorporated inside Li-Al-NO3 LDH coatings. The effect of methionine improved the corrosion resistance of the coatings and provided self-healing ability. The article is rather well written and the overall study is well conducted. However, the article cannot be accepted. Various points of major concern have to be addressed and the conclusions are drawn from various erroneous interpretations of the experimental data:

1. Section 3.1., XRD analysis of LDH films: The authors observe that d003 changes from NLDH to MLDH and attribute it to the incorporation of methionine in the interlayers of the LDH. However, looking at the XRD patterns of Fig. 2d, it is seen that d006 does not change. This is strange (to say the least) and must be commented and explained.
2. Section 3.1., XRD analysis of LDH films, about crystallinity: The authors wrote “Meanwhile, the half-width of the (003) and (006) refection in MLDHs is smaller than that in NLDHs, proving that the grain size composed of LDHs nanosheets in MLDHs is smaller”. Actually, this is wrong: the thinner the diffraction lines, the larger the crystal size. Moreover, “refection” must be changed to “reflection”.
3. Section 3.2., polarization curves: The authors state they carried out a “fitting” of the experimental data (page 6, line 229). How they did it? Did they use mathematical expressions for electrochemical kinetic laws? This must be fully explained. Moreover, if a fitting procedure is used, the computed curve must be compared with the experimental one.
4. Tables: In general, the number of significant digits is too high. For instance, I do not think that a corrosion potential can be given with four digits, which would imply that the accuracy is lesser than ±0.1 mV. The same applies for jcorr, given with an accuracy that should be better than ±1 nA/cm2.
5. Reproducibility: Following the previous comment, the authors did not indicate whether each experiment was carried out just one time or two, three, or more times. Usually, corrosion/electrochemical experiments are achieved in triplicate to assess reproducibility. The error for each measured parameter (Ecorr, jcorr and so on) is then easier to observe because is corresponds to the dispersion of obtained values. Actually, it is very likely that most of the slight variations over time observed for jcorr are not significant.
6. Interpretation of polarization curves, page 7, lines 237-247: This interpretation is wrong. First, there is no “bimetal” as Li and Al are present in the LDH structure as oxidized species (Li+ and Al3+). Secondly, the decrease of Ecorr with the LDH layers is clearly due, as revealed by Figure 3a, to a decrease of the cathodic reaction rate. The LDH layer also induces a drastic change of the anodic Tafel slope, which is not even commented.
7. Figure 4: The Cl 2p peak cannot be seen because the figure is too small. The presence of Cl- in the LDH is important, and a detailed view of the Cl 2p spectral region is then necessary.
8. Figure 5, EIS Nyquist plots: The axes of the Nyquist plots have not the same scale and the capacitive loops seem more flattened than they actually are. The Nyquist plots must be drawn using an orthonormal basis. If not (as it is currently the case) "perfect" semi-circles may for instance seem depressed.
9. EIS and CPE: a CPE is not a capacitance and cannot be discussed as such. The value of the capacitance associated with a CPE can be computed using various models (see related literature). Another consequence is that the unit for CPE is not F cm-2, but F s^(a-1) cm^-2. The value of the coefficient a of the CPE is then of utmost importance and must be given and commented (which is currently not the case).
10. Section 3.4, about the anti-corrosion mechanism: First, the term “corrosive medium” used (lines 385, 401 and 406) for Cl- is not appropriate. Cl- is a corrosive species. Secondly, the authors wrote that “Al forms a galvanic cell in the solution” (line 404). A galvanic cell is formed by two electrodes, e.g. two metals, not by only one. This wording is incorrect.
11. Finally, the English must be improved, and numerous sentences must be rewritten.

Author Response

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Reviewer 3 Report

In the reviewed work devoted to the investigation of the effect of modified Li-Al LDHs protective films on the corrosion resistance of the aluminum, a significant enough experimental result is reported. In the reviewer’s opinion, the paper can be published in Coatings journal after some improving and making the following revisions:

1. The authors should expand the abbreviation LDHs” at the first mention in the text of the article. Now, the abbreviation “LDHs” is explained in the Abstract only.

2. In Introduction, the goal of the work should be formulated.

3. In Subsection 2.1, please specify the geometric sizes of the specimens, which the coatings were deposited on.

4. In Subsection 2.2, the authors should specify the temperature, which the treatment (etching) of the aluminum alloy were performed at.

5. Please clarify why the optimal treatment time was selected to be 15 min at 60 ^оC (Subsection 2.2).

6. What was the purity of the components used in making the solutions?

7. How many times the potentiometric polarization curves were recorded for each specimen and for each solution when carrying out the electrochemical tests?

8. Please specify the exact time of holding in the NaCl solution to ensure the stationary value of the potential Е.

9. In the methodical part of the paper, please provide a graphical illustration demonstrating the procedure of calculating the corrosion current density for any sample. It is very important because the slopes, which are determined by the software in the automated mode, are incorrect often.

10. Please place the figures just after the first mentioning the ones in the text. It helps the analysis of the results essentially. Please note that Figure 2 is placed two pages below its first mention in the paper.

11. The authors should introduce some quantitative characteristics of the coatings and answer the following question:

• How uniform was the LDHs film on the sample surface? Are there any problems with the edges of the samples?
• Whether the thicknesses of the modified LDHs films on the surfaces and of the non-modified ones were estimated? How uniform were the film thicknesses on different parts of the samples?
• How did the authors calculate the film surface area (S)? The quantity S enters in the formula for calculating the corrosion current density and a correct accounting for this one is very important. If the surface areas of the samples vary, how correctly one can compare the results of the electrochemical testing for several samples?

12. Did the authors account for the contribution of the instrumental function into the broadening of the XRD peaks, based on which the conclusions on the grain sizes were drawn (the first paragraph on Page 5)? If yes – how?

13. One can see in Fig. 2d that the XRD peak (003) for the Li-Al Met LDHs coating was shifted strongly (approximately in 1 deg.) relative to the (003) one for the non-modified Li-Al LDHs coating. I would stress that no shifts of the (006) XRD peaks were observed. The authors should clarify the origin of the shift of the (003) XRD peak in the modified coating.

14. Please specify the slopes of the cathode and anode parts of the Tafel curves.

15. Please describe in details the procedure of the experiment, the results of which are presented in Fig. 3b. Was the sample stored continuously in the cell connected to the measuring potentiostat system all the time? Or were different samples held during different times?

16. It follows from Table 2 that the potential varied nonmonotonously in the course of the experiment. Was it a normal scatter of the potential (from 0.82 to 0.97 V)? This range of parameters appeared to be greater than the differences in the potentials between the modified coating and the non-modified one (Table 1).

17. The results of investigating the effect of the holding time on the corrosion rate presented in Table 2 are very contradictory and the explanations presented by the authors don't seem to be convincing to Reviewer since the surface areas covered by the corrosion products were very small (Fig. 4b). The reviewer's experience points the scatter of characteristics specified in Table 2 to be normal for the aluminum alloys, and one cannot speak on the reliability of the differences observed. The authors should specify the uncertainty of the characteristics presented in Table 2 based on averaging of several measurements.

18. Analyzing the data presented in Table 3, the authors concluded that the specific surface area first decreased, then increased. The authors should provide a detailed explanation of such a nonmonotonous variation.

19. Please describe the procedure of making the scratch (see Subsection 3.3) in the Materials and methods section. Please describe the procedure calculating the surface area when testing the samples with the scratches.

20. In the electrochemical testing of the scratched samples, the authors found the corrosion current density for the damaged aluminum alloys to be very large and was ~9·10^-5 A/cm². This magnitude is much greater than the one specified in Table 1 for the aluminum alloy (~7·10^-6 A/cm²). What such an essential difference related to? I should note also that the magnitude of the potential appeared to be abnormally low (-1.4 V, see Fig. 6e) and doesn’t match the data presented in Table 1.

21. The authors should perform the mass loss tests to confirm the correctness of the results of electrochemical investigations. Without these data, one cannot make a correct conclusion on high protective properties of the modified coatings. The results of the electrochemical tests presented in the paper are very contradictory, and the methodical part of the corrosion experiments is described in not enough details.

The paper text needs a serious improvement. The authors should pay the most attention to the description of the techniques as well as to the section describing the results of the corrosion tests. 

Author Response

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Round 2

Reviewer 2 Report

Reviewer comments on the article “Corrosion resistance of Li-Al LDHs Film Modified by Methionine for 6063 Al Alloy in 3.5wt.% NaCl Solution” by Ji Ma et al. (Coatings-1609610, version 2)

 

With the revised version, various points of concern were addressed but the manuscript still requires important changes to be accepted:

1. Section 3.1., XRD analysis of LDH films: The authors observe that d003 changes from NLDH to MLDH and attribute it to the incorporation of methionine in the interlayers of the LDH. However, looking at the XRD patterns of Fig. 2d, it is seen that d006 does not change. This is strange (to say the least) and must be commented and explained.

In their replies to this comment, the authors have commented and discussed this point, but the manuscript was not modified accordingly.

2. Section 3.1., XRD analysis of LDH films, about crystallinity: The authors wrote “Meanwhile, the half-width of the (003) and (006) refection in MLDHs is smaller than that in NLDHs, proving that the grain size composed of LDHs nanosheets in MLDHs is smaller”. Actually, this is wrong: the thinner the diffraction lines, the larger the crystal size. Moreover, “refection” must be changed to “reflection”.

In their replies to this comment, the authors have correctly answered, but the manuscript was not modified accordingly.

3. Tables: In general, the number of significant digits is too high. For instance, I do not think that a corrosion potential can be given with four digits, which would imply that the accuracy is lesser than ±0.1 mV. The same applies for jcorr, given with an accuracy that should be better than ±1 nA/cm2. The number of digits must then be decreased considerably, e.g. Icorr = 6.8 X 10-6 instead of 6.7611 X 10-6.
4. Interpretation of polarization curves, page 7, lines 259-262: The authors wrote that the decrease of Ecorr indicates that the corrosion tendency has become higher. This is wrong, as illustrated by the associated decrease of Icorr. This sentence must be removed.
5. Interpretation of polarization curves: the LDH layer also induces a drastic change of the anodic Tafel slope, which is not even commented.

In their replies to this comment, the authors have commented and discussed this point, but the manuscript was not modified accordingly.

5. EIS and CPE: a CPE is not a capacitance and cannot be discussed as such. The value of the equivalent capacitance Ceq associated with a CPE can be computed using various models, and in particular the Brug formula (G.J. Brug et al, J. Electroanal. Chem., 176 (1984) 275). It is the value of Ceq that can be linked to the surface S and the film thickness d as given in line 339.
6. Section 3.4, about the anti-corrosion mechanism: The authors wrote “Al easily acts as an anode in a corrosion galvanic reaction” (line 427). A galvanic reaction implies not only an anode but a cathode. What is the cathode in this galvanic system?

Author Response

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Round 3

Reviewer 2 Report

The authors carefully amended their manuscript so that it can now be accepted in its present form.