Research on Corrosion Protection of TETA-Modified Li–Al LDHs for AZ31 Magnesium Alloy in Simulated Seawater

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This research has demonstrated that a hybrid coating of "LDH and TETA" stably improves the corrosion resistance of Mg alloys over the long term. The mechanism behind this has also been clearly elucidated. The content is sufficient for a paper, and the reviewers agree with the acceptance of this paper. However, the paper lacks sufficient explanation and there are several points that are difficult for readers to understand, so these need to be improved. In addition, in future research developments, it is recommended that observations of the cross-section of the coating be considered in addition to surface observations, as these are also effective methods for observing the coating.

• The introduction and conclusion are written clearly and I think it is fine as is.
• *Line 129: "-- At the initial immersion stage,--"

Since the experimental values ​​will be compared, please state the time t more specifically. Is t = 25 hours?

*Lines 133-134: “--and their low-frequency impedance values further increased.---”

 Please write the specific value obtained from the underlined part of the experiment. Is it 8 × 10⁵ Ω∙cm²?

* Lines 135-136: “--The highest |Z|0.01 Hz at 47.0 mM TETA was 5.33 × 10⁵ Ω∙cm²,--”

 Isn't the experimental value for "the underlined part above" 8 × 10⁵ Ω∙cm²?

(3) In Figure 3, please explain in more detail why the measurements for the TETA concentrations of 23.5 mM and 70.5 mM were limited to 3 to 7 days, rather than up to about 300 hours. Was it because corrosion had already begun?

(4) Figure 3: There are some places where the experimental values ​​in the text do not match those in Table 1. For example, in the following statement on lines 158-160, if we trust the values ​​written in Table 1, aren't the numbers in parentheses () correct? I strongly recommend that you review the entire paper.

Lines 158-160: “-- the corrosion potential Ecorr and corrosion current density icorr of the control group were -1.402 V and 1.51 × 10⁻⁶ A∙cm⁻²(1.812× 10⁻⁶？), respectively.  In contrast, with the addition of 47.0 mM TETA, the Ecorr shifted positively to -1.367 V(-1.349V？), and the icorr decreased significantly to 1.541 × 10⁻⁸ A∙cm⁻² (1.245 × 10⁻⁸?),--”

(5) Table 1:

In the TETA23.5mM sample after 24 hours of immersion, Ecorr is -1.506V. This value is less noble than the TETA0mM sample, and icorr has increased sharply to 3.321x10-4A/cm2.

This trend is significantly different from other TETA samples. The peculiar corrosion behavior of the TETA23.5mM sample needs to be considered.

In addition, the numbers in the text do not correspond to those in Table 1. The values ​​need to be corrected as appropriate.

(6) Regarding the explanation in the text for Figure 5:

Figure 5 shows FT-IR for three samples. An explanation of the experimental results is given, but it is not clear which sample the results are for. Please add an explanation so that readers can understand. Also, although it says that a peak for Mg(OH) can be observed, please consider adding an arrow or explanation in Figure 5.

(7) Lines 206-207:

　The text states, "--the Mg 2p spectrum exhibits four fitted peaks at 50.21 eV (Mg(OH)2), 49.68 eV (Mg-N), 49.27 eV (MgO), and 48.61 eV (Mg), indicating contribution--," but Figure 6(b) shows 49.68 eV (MgO) and 49.27 eV (Mg-N).

(8) In Figures 7b and 7c, please provide explanatory terms for the membrane layers. In Figures 7d and 7e, please provide explanatory terms for the corroding organisms, corrosion pits, and cracks.

(9) Increase the size of the text relative to the color bar in Figure 8.

Author Response

Dear Prof./Dr.:

We would like to thank the Reviewer for his/her thoughtful comments and efforts toward improving our manuscript.

1. Line 129: "-- At the initial immersion stage,--" Since the experimental values will be compared, please state the time t more specifically. Is t = 25 hours?
Thanks for your kind suggestion. The text now explicitly states: “immersing from 0 hours to 12 hours”.
2. Lines 133-134: “--and their low-frequency impedance values further increased.--” Please write the specific value.
Thanks for your kind suggestion. The meaning to be expressed here is that after soaking for 24 hours, the low-frequency impedance values of the Li-Al LDHs coating with different concentrations of corrosion inhibitors added are all increasing. Therefore, no specific values are written here. However, after this sentence, it is clearly stated that the maximum |Z|_0.01Hz at 47.0 mM is 7.56×10⁵Ω∙cm².
3. In Figure 3, please explain in more detail why the measurements for the TETA concentrations of 23.5 mM and 70.5 mM were limited to 3 to 7 days, rather than up to about 300 hours. Was it because corrosion had already begun.
Thanks for your kind suggestion. The measurement of the concentrations of 23.5mM and 70.5mM TETA was limited to 3 to 7 days because the corrosion had already begun. The reasons are described in lines 166 to 168 of the text: “whereas those with 23.5 mM and 70.5 mM TETA showed abrupt impedance drops after ~60 hours and ~132 hours, respectively, indicating corrosion onset and termination of measurements”.
4. Figure 3: There are some places where the experimental values ​​in the text do not match those in Table 1. For example, in the following statement on lines 158-160, if we trust the values ​​written in Table 1, aren't the numbers in parentheses () correct? I strongly recommend that you review the entire paper.

Thanks for your kind suggestions. The numbers in the parentheses () are correct. We have corrected numerical values in the text to match Table 1.

5. Table 1: In the TETA23.5mM sample after 24 hours of immersion, Ecorr is -1.506V. This value is less noble than the TETA0mM sample, and icorr has increased sharply to 3.321x10-⁴A/cm².

This trend is significantly different from other TETA samples. The peculiar corrosion behavior of the TETA 23.5 mM sample needs to be considered.

In addition, the numbers in the text do not correspond to those in Table 1. The values ​​need to be corrected as appropriate..

Thanks for your kind suggestions. The abnormal corrosion at 23.5mM was due to the insufficient concentration of TETA (23.5mM), which failed to form a continuous adsorption film, resulting in local corrosion and accelerated failure. In addition, we have corrected numerical values in the text to match Table 1.

6. Regarding the explanation in the text for Figure 5: Figure 5 shows FT-IR for three samples. An explanation of the experimental results is given, but it is not clear which sample the results are for. Please add an explanation so that readers can understand. Also, although it says that a peak for Mg(OH)₂ can be observed, please consider adding an arrow or explanation in Figure 5.

Thanks for your kind suggestions. The explanations of the samples corresponding to FT-IR have been added in the text. In addition, arrows have been added in Figure 5 to indicate the peaks of Mg(OH)₂.

7. Lines 206-207: The text states, "--the Mg 2p spectrum exhibits four fitted peaks at 50.21 eV (Mg(OH)₂), 49.68 eV (Mg-N), 49.27 eV (MgO), and 48.61 eV (Mg), indicating contribution--," but Figure 6(b) shows 49.68 eV (MgO) and 49.27 eV (Mg-N).

Thanks for your kind suggestions. The incorrect annotations have been corrected in the text, and now the content in the text is consistent with the picture.

8. In Figures 7b and 7c, please provide explanatory terms for the membrane layers. In Figures 7d and 7e, please provide explanatory terms for the corroding organisms, corrosion pits, and cracks.

Thanks for your kind suggestions. Explanatory term annotations have been provided in the figure.

9. Increase the size of the text relative to the color bar in Figure 8.

Thanks for your kind suggestions. Modifications have been made to Figure 8.

 

Finally, again, thank the reviewer for his/her valuable comments. Your comments are very important for improving the quality of our article.

Reviewer 2 Report

Comments and Suggestions for Authors

Se attached.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Needs a lot of improvement.

Author Response

Dear Prof./Dr.:

We would like to thank the Reviewer for his/her thoughtful comments and efforts toward improving our manuscript.

Reviewer #1: The effect of TETA presence on magnesium corrosion in the electrolyte medium has been evaluated. Although a variety of experimental work has been done, but the article has several weaknesses, some of which are mentioned below, and if Molecular dynamics (MD) simulations, Surface Adsorption Energy a were not done, the article would have been rejected. In any case, the major flaws must be fixed so that the article can be re-evaluated.
1) The title is not appealing. It is missing the important fact that the medium is simulated seawater (3.5 wt% NaCl solution.). Improve.

Thanks for your kind suggestion. The title has been revised to: " Research on Corrosion Protection of TETA-Modified Li-Al LDHs for AZ31 Magnesium Alloy in Simulated Seawater "
2) The abstract needs rewriting. Very few figures/numbers/values are presented. The results are not clear especially on the effect of adding TETA.  What other concentrations were tested (other than 47 mM)? Same applies to immersion times.

Thanks for your kind suggestion. Other relevant data such as concentrations and immersion times have been added to the abstract.

3) Use “After 24 h of immersion” or “After 24 h immersion time” in the whole manuscript. Also with other times and similar description.

Thanks for your kind suggestion. The phrasing "After 24 h immersion" (or equivalent for other times) has been standardized throughout the manuscript.

4) Line 16-18: “After 300 h immersion, the low-frequency impedance remained above 105 Ω·cm² , demonstrating superior long-term protection.” There is no experimental or results or even discussion mentioning the 300 h (or even any time longer that 24 h) in the whole manuscript? Remove from conclusions as well! This is 12.5 days.

Thanks for your kind suggestion. Our Figure 3, which is the summary graph of the low-frequency impedance modulus values (|Z|_0.01Hz) summarized from the Bode graph of EIS, shows the coating conditions of different concentrations of TETA after 300 hours of immersion. It has now been replaced with a clearer description at the line 166.

5) Lines 26: Add a proper reference. Apply to similar cases elsewhere.

Thanks for your kind suggestion. References have been added in appropriate places.

6) Lines 62-66: Add a proper reference. Apply to similar cases elsewhere. Lines 62-66. Authors need to be careful with reporting information. Avoid using strong/absolute description like “significantly lower” and similar.

Thanks for your kind suggestion. This sentence has been modified now.

7) Line 85: Add Mg composition. It is not necessarily the balance.

Thanks for your kind suggestion. The composition has been clarified in the original text.

8) TETA should be explained more in the experimental and materials section, source, method of preparation, why such concentrations were chosen, solubility, etc.

Thanks for your kind suggestion. The Experimental section has been expanded. Those concentration of TETA inhibitor was confirmed in our previous study.( Insight into the Corrosion Inhibition Performance of Triethylenetetramine (TETA) for AZ31 Mg Alloy. Colloids Surf A Phyicochem Eng Asp 2025, 710.)

9) Line 93: Improve the language “The working electrode exposed area: 1 cm² , saturated”. Change to “The working electrode with an exposed area of 1 cm² , saturated …”

Thanks for your kind suggestion. The sentence has been corrected.

10) The surface area of the used sample is over 40 cm². Why only 1 cm² was exposed and tested? The larger the area, the smaller the error in measured current densities.

Thanks for your kind suggestion. We acknowledge the reviewer's point regarding error minimization. The sample size (50x40x3 mm) was chosen for practical handling and compatibility with our electrochemical setup and holder, which is designed to expose a standardized 1 cm² area. This is a common practice in corrosion electrochemistry to ensure consistent exposed area across samples and studies, facilitate comparison with literature, and manage edge effects. While a larger exposed area can reduce error, the 1 cm² area provides sufficient signal-to-noise ratio for accurate EIS and polarization measurements. All samples were prepared and tested identically to ensure comparability.

11) Lines 108-113: Do not start the section with a figure or table. They should be placed “after” their first mention.

Thanks for your kind suggestion. The figures or tables in the text have been modified and placed.

12) Figure captions (and tables) should be clearer and contain full description of the content. For example, in figure 1 caption, it is confusing to say “EIS Bode and Nyquist plots for different concentrations of TETA…”. Please explain what each figure is representing clearly. (a) …, (b) …. If a condition if common, there is a better way to describe. Also improve the quality of images/scales.

Thanks for your kind suggestion. Figures have been thoroughly revised for clarity and completeness. Image quality and scale bars have been checked and improved where necessary.

13) Line 113: “Figure 1 presents the Bode plots and Nyquist diagrams...”. This is not clear. Which one? The Nyquist should come before the “Bode”.

Thanks for your kind suggestion. The caption describing Figure 1 has been clarified.

14) Line 119 “. L is an inductive element”. Explain correctly in the figure caption as well.

Thanks for your kind suggestion. Explanations have been added to the captions in Figure 2.

15，16) Line 158-159: “Ecorr and corrosion current density icorr of the control group were -1.402 V and 1.51 × 10⁻⁶ A·cm⁻², respectively”. Something is wrong. Same as in the point above applies to the 47 mM results. Please revise and make correct.

Thanks for your kind suggestion. The corresponding text have been carefully re-examined against the raw polarization data. The values reported (-1.402 V, 1.51×10⁻⁶ A·cm⁻² for blank 24h; -1.367 V, 1.541×10⁻⁸ A·cm⁻² for 47 mM 24h) are correct based on our Tafel extrapolation analysis of the polarization curves shown in Figure 4b. We acknowledge the corrosion potential (-1.367 V) for the 47 mM sample at 24h is relatively noble for Mg in NaCl, which reflects the protective effect of the TETA-modified coating suppressing the anodic reaction, consistent with the low i_corr and high |Z|.

17) Either there is a missing table or there is a confusion in presenting and discussing the results. Nyquist and Tafel results are different and give different parameters. Authors need to reconsider this urgently.

Thanks for your kind suggestion. EIS is a nondestructive test, but the Tafel test is not. Results may have some differences. According to the EIS data, 47mM has better protection after 0 h and 24 or immersion in NaCl medium (47mM has the highest |z|). This finding can be further confirmed by potentiodynamic test (47mM has the lowest icorr).

18) Line 200: FTIR cannot demonstrate that TETA adsorption forms a protective film on the surface, acting as a corrosion barrier! Rewrite.

Thanks for your kind suggestion. The sentence has been corrected. The modified coating exhibits prominent absorption peaks at 2845 cm−1 and 2940 cm−1, corresponding to the symmetric and asymmetric C-H stretching vibrations of the -CH2- groups in TETA molecules [19]. Those peaks can be found in both TETA and TETA modified Li-Al LDHs coatings.

19) What is the concentration at which SEM was done? It is worth showing images at different concentrations to prove the concept that a middle concentration is better inhibitor.

Thanks for your kind suggestion. Figure caption has been modified. Our SEM images compare the optimal concentration with the control group. Other concentrations did not provide adequate protective effects, so we did not test them.

20) Since section 3 contains some discussion, merge sections 3 and 4.

Thanks for your kind suggestion. As suggested, Sections 3 ("Results and Analysis") and 4 ("Discussion and Analysis") have been merged into a single section titled "3. Results and Discussion".

21) The conclusions are not well written! They are not discussion and results or an abstract.

Thanks for your kind suggestion. This part has been modified.

22) Some repeated sentences are misleading... Improve the language and grammar in the whole text.

Thanks for your kind suggestion. The phrase regarding "inorganic-organic hybrid design" was modified into a clear statement in the mechanism discussion and conclusion.

23) Improve the language and grammar in the whole text.

Our paper has been polished by an native speaker.

24) Some references are older than 10 years. Improve.

Thanks for your kind suggestion. References that are over 10 years old have all been replaced.

 

Finally, again, thank the reviewer for his/her valuable comments. Your comments are very important for improving the quality of our article.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Authors did a good job addressing the raised points.

Some modifications are still needed.

For example, "Figure 3. Add different concentrations ..." is not the correct way to describe the figure caption.

Comments on the Quality of English Language

Better.

Author Response

Dear Prof./Dr.:

We would like to thank the Reviewer for his/her thoughtful comments and efforts toward improving our manuscript.

 

1.Some modifications are still needed. For example, "Figure 3. Add different concentrations ..." is not the correct way to describe the figure caption.

Thanks for your kind suggestion. Several modifications have been made. We deeply thank the reviewer's comments.