Corrosion Resistance of Epoxy Coatings Modified by Bis-Silane Prepolymer on Aluminum Alloy

Round 1

Reviewer 1 Report

Coating-1288628

Corrosion resistance of epoxy coatings modified by bis-silane 2 prepolymer on aluminium alloy

In this paper a modification of epoxy resin with silane prepolymer is used to enhance the corrosion resistance of epoxy coatings of aluminium alloy substrates. The corrosion behaviour is intended by electrochemical impedance in saline concentrated water. A comparative study of monomer and bis-silane modified epoxy coating is carried out. From this study Authors found that the bi-silane modification of the epoxy coating has higher resistance to water permeation and stronger protection of the substrate corrosion.

The paper has good quality and deserves to be published, but several details need to be previously improved:

The paper needs to take care of the nomenclature and organization.

I understand that the area referred in the experimental section is the geometric one. What is the real surface area? Have Authors tried to estimate this data in some way? It would be interesting to know whether there is some correlation with the existence of pores.

In lines 87-88 it can be read that potentiodynamic polarization and EIS measurements were carried out after a stable OCP was reached in the electrolyte. However, details of experimental conditions are missing, as well as the OCP value and the time to reach it.

In table 1, indeed entitled “corrosion dynamics parameters of samples in different solutions”, Tafel slopes of anodic and cathodic branches are given but no reference to them is made in the text, neither to their value of potential corrosion and their comparison between different epoxy coatings.

In figure 2, labels (a), (b), (c) and (d) indicated in the figure legend have not explicit indication in the proper chart.

In lines 152-154, what does EECS mean? Please define acronyms whenever they first appear in the text.

Authors should clarify what is the rationale behind each of the electrical circuits. I believe that a short comment explaining the characteristics and benefits that justify the fitting strategy is needed to evaluate the equivalence and the physical meaning in real interfaces. Perhaps a scheme may help in this sense.

It is necessary to revise nomenclature of the different circuit elements in the text. For instance, in line 156, it appears Cc, Rc, Cdl, Rdl, whereas in lines 163 and 164, it appears Rct, Cdl and Rb and some of these symbols do not appear in any of the circuits proposed for fitting.

Similarly, to Figure 2, in Figure 4 name the plots with the (a), (b), (c) and (d) labels as reported in the figure title, “Evolution of the electrical parameters Cc (a), Rc (b), Cdl (c), and Rdl (d) derived from EIS data”.

There is a clear decrease on BISE-PE with respect to other PE or silane-epoxy coatings. What is the cause of the decreasing after an initial period? Some comments should be provided.

The conclusions need clarification. Authors start by referring to a compound, and then continue arguing about other studied compounds. This may mislead readers in the assignation of properties to the different systems studied.

 

Author Response

Dear Editors and Reviewer:

Thank you for your comments concerning our manuscript entitled “Corrosion resistance of epoxy coatings modified by bis-silane prepolymer on aluminum alloy” (ID: coatings-1288628). Those comments are all valuable and very helpful for revising and improving our paper. We have studied comments carefully and have made correction which we hope meet with approval. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the reviewer’s comments are as flowing:

 

1. In lines 87-88 it can be read that potentiodynamic polarization and EIS measurements were carried out after a stable OCP was reached in the electrolyte. However, details of experimental conditions are missing, as well as the OCP value and the time to reach it.

Reply: Thanks for reviewer’s suggestion. A stable open circuit potential was reached in the electrolyte needs about 10 min, and different coatings have different OCP values. EIS measurements were carried out directly after immersion test.

2. In table 1, indeed entitled “corrosion dynamics parameters of samples in different solutions”, Tafel slopes of anodic and cathodic branches are given but no reference to them is made in the text, neither to their value of potential corrosion and their comparison between different epoxy coatings.

Reply: Thanks for reviewer’s suggestion. It is indeed not reference to Tafel slopes of anodic and cathodic branches in the text, we have deleted in the revised version.

3. In figure 2, labels (a), (b), (c) and (d) indicated in the figure legend have not explicit indication in the proper chart. In lines 152-154, what does EECS mean? Please define acronyms whenever they first appear in the text. Similarly, to Figure 2, in Figure 4 name the plots with the (a), (b), (c) and (d) labels as reported in the figure title, “Evolution of the electrical parameters Cc (a), Rc (b), Cdl (c), and Rdl (d) derived from EIS data”.

Reply: Thanks for reviewer’s suggestion. These problems have been corrected in the revised version.

4. Authors should clarify what is the rationale behind each of the electrical circuits. I believe that a short comment explaining the characteristics and benefits that justify the fitting strategy is needed to evaluate the equivalence and the physical meaning in real interfaces. Perhaps a scheme may help in this sense

Reply: Thanks for reviewer’s suggestion. Section 3.2 has been reedited to solve these problems.

5. It is necessary to revise nomenclature of the different circuit elements in the text. For instance, in line 156, it appears Cc, Rc, Cdl, Rdl, whereas in lines 163 and 164, it appears Rct, Cdl and Rb and some of these symbols do not appear in any of the circuits proposed for fitting..

Reply: Thanks for reviewer’s suggestion. The problems have been corrected in the revised version.

6. There is a clear decrease on BISE-PE with respect to other PE or silane-epoxy coatings. What is the cause of the decreasing after an initial period? Some comments should be provided.

Reply: Thanks for reviewer’s suggestion. The low-frequency impedance represents the total resistance of the electrolyte in the micropores or capillary channels of the coating, and would reduce due to the permeation of electrolyte. Due to permeation of electrolyte, the low-frequency impedance value would decrease after an initial period. But due to the barrier effect of the modified epoxy coating on water diffusion was better, the rate of resistance drop was much slower than that of the PE coating.

7. The conclusions need clarification. Authors start by referring to a compound, and then continue arguing about other studied compounds. This may mislead readers in the assignation of properties to the different systems studied.

Reply: Thanks for reviewer’s suggestion. Bis-silane modified epoxy coatings were prepared on Al-Zn-Mg-Cu alloy in the present work.

Author Response File: Author Response.pdf

Reviewer 2 Report

The present manuscript deals with a scientifically interesting and technologically important topic, namely the improvement of the corrosion resistance of an aluminum alloy by employing  a modified epoxy-resin coating. Despite the significance of the topic, however, I have to say that I am rather unsatisfied with the manuscript and the presentation. First of all, many experimental details, as well as information about the data analysis  are missing. For example, the pH of the NaCl test solution is not provided, as well as the detailed composition of the Al-alloy is not includd. Details about the coating procedure are missing, i.e. how were the Al-alloys cleaned prior to the deposition, and was a protecting gas atmosphere used? How fast was the coating, i.e. the immersion speed? Drying and heating under noble-gas or in a glove box? Details about the thickness measurements? Suppliers and purities of the used chemicals? Suppliers of the used potentiostat, and software for EIS and voltammetry analysis (Zview software? Multi-Auto software?)? Many additional information are required here.

With regard to the use of references - I feel that some more general references should be given in the introduction, here it is not necessary to include self-citations, as many work has been done in the past 20 or even 30 years! Furthermore, references 22 to 29 are included in the bibliography, but are not used in the manuscript, please correct. Furthermore, please homogenize the citation style, some journals are used in an abbreviated style, some are not. Full author names are provided for some references, initials only for many others. In some case, references are cited as X. Y. et al. (only one single name), in other cases two or three authors are mentioned plus et al., in other references 8 (!) names are provided - please correct.

Most important, however, the fitting procedure of the EIS and electochemical data remains totally unclear - how are the values in Table 1 were derived? Are the fits shown in Fig. 1? If so, please indicate. What are the errors of the values provided in Table 1? How reproducible are the results? Please specify!

With regard to the fitting of the EIS data - which of the equivalent circuits presented in Fig. 3 was actually used for the fits presented in Fig. 2, and what was the selection criterion for its use? Of course, with more parameters more arbitrary wave functions can be fitted, but what is the underlying physical model? I urgently request the authors to study the availability of reference works on this topic, otherwise the list of parameters is more or less useless. In this context, what are the (physical) meaning of the parameters C_C, R_ct and R_b (the latter two by the way mentioned in the text on page 6, lines 163 and 164, but not used in Fig. 3), etc. What is the meaning of the other resistivities and capacities in Fig. 3, and why are those equivalent circuits suited or not to describe the processes on the investigated Al-alloy? Many things remain entirely unclear to me, even after repeated reading of the manuscript and referring to the literature provided / used by the authors ...

Some minor points:

• Acronyms should be explained when used first (i.e. OCP (page 2, line 88), EEC (page 5, line 154)).
• How are the error bars in Fig. 4 are derived?
• Reference 37, 39, 40 are not cited in the text.
• There are many grammatical errors, incomplete sentences, wrong uses of adverbs and adjectives, etc. Please check thoroughly!

In conclusion, I cannot recommend the manuscript for publication in its present state, major revisions are mandatory.

Author Response

1. many experimental details, as well as information about the data analysis  are missing. For example, the pH of the NaCl test solution is not provided, as well as the detailed composition of the Al-alloy is not include.

Reply: Thanks for reviewer’s suggestion. The electrochemical experiments were carried out in a 3.5 wt% NaCl solution at room temperature . The pH value of NaCl test solution is neutral. We don’t think it’s necessary to provide. The composition of Al–Zn–Mg–Cu alloy has been listed in Table 1 in the revised version.

2. Details about the coating procedure are missing, i.e. how were the Al-alloys cleaned prior to the deposition, and was a protecting gas atmosphere used?

Reply: Thanks for reviewer’s suggestion. The surface of the samples was ground with up to 1200 grit SiC abrasive papers, then degreased in pure acetone before deposition and washed with deionized water. No gas atmosphere was used.

3. How fast was the coating, i.e. the immersion speed? Drying and heating under noble-gas or in a glove box? Details about the thickness measurements? Suppliers and purities of the used chemicals?

Reply: Thanks for reviewer’s suggestion. The Al-Zn-Mg-Cu alloy was dip-coated in the modified solution for 10 seconds, quickly take out, and dried at room temperature environment for 12h and cured in an oven at 100 ℃ for 90 min to obtain epoxy coating. The thickness of the coatings evaluated by thickness gauge TT260 is about 35±5 µm. All of the solutions were purchased from Aladdin Industrial Corporation (Shanghai, China) and were used without further modification.

4. Suppliers of the used potentiostat, and software for EIS and voltammetry analysis (Zview software? Multi-Auto software?)? Many additional information are required here.

Reply: Thanks for reviewer’s suggestion. A MULTI AUTOLAB M204 electrochemical workstation (Metrohm, Herisau, Switzerland) was used to collect the EIS spectra and voltammetry analysis.

5. References 22 to 29 are included in the bibliography, but are not used in the manuscript, please correct. Furthermore, please homogenize the citation style, some journals are used in an abbreviated style, some are not. Full author names are provided for some references, initials only for many others. In some case, references are cited as X. Y. et al. (only one single name), in other cases two or three authors are mentioned plus et al., in other references 8 (!) names are provided - please correct.

Reply: Thanks for reviewer’s suggestion. The problems in the references have been corrected in the revised version.

6. How are the values in Table 1 were derived? Are the fits shown in Fig. 1? If so, please indicate. How reproducible are the results? Please specify!

Reply: Thanks for reviewer’s suggestion.The potentiodynamic polarisation curves was analyzed by using the special analysis Software of MULTI AUTO M204 electrochemical work station. The analysis process is shown in the following diagram. Three similar samples were measured for each coating to check reproducibility of the results.

7. With regard to the fitting of the EIS data - which of the equivalent circuits presented in Fig. 3 was actually used for the fits presented in Fig. 2, and what was the selection criterion for its use? In this context, what are the (physical) meaning of the parameters C_C, R_ctand R_b(the latter two by the way mentioned in the text on page 6, lines 163 and 164, but not used in Fig. 3), etc. What is the meaning of the other resistivities and capacities in Fig. 3, ..

Reply: Thanks for reviewer’s suggestion. the selection criterion for the equivalent circuits is based on goodness of fit (the relative standard error within 10% (error %<10%), or the sum of the squares of the residuals is in the order of 10-3). The (physical) meaning of the parameters: Rs, Rc, Rdl, and Rdiff stand for solution resistance, electrolyte resistance in the coatings, charge transfer resistance and diffusion resistance, respectively. Cc, Cdl and Cdiff represent coating capacitance, double-layer capacitance of coating/substrate interface and diffusion capacitance of corrosion products, respectively. Rct and Rb are Rdl, which have been corrected in the revised version.

8. Acronyms should be explained when used first (i.e. OCP (page 2, line 88), EEC (page 5, line 154)). Reference 37, 39, 40 are not cited in the text. There are many grammatical errors, incomplete sentences, wrong uses of adverbs and adjectives, etc.

Reply: Thanks for reviewer’s suggestion. These mistakes have been corrected in the revised version.

Author Response File: Author Response.pdf

Reviewer 3 Report

The work presented a quite new process to obtain coating corrosion resistance for Al alloys. The corrosion test are well prepared and presented. The text is well organized and written.

I suggest only some considerations to enrich the text with other contents:

1) Please add some description or image or investigation of the polymer coating: thickness, surface  and quality

2) How the authors controlled the coating obtained (chemical composition et al...). They think that a reflection investigation by Infrared spectroscopy could be useful? 

Author Response

Reply: Thanks for reviewer’s suggestion. The chemical composition of the coating is obtained by controlling the volume ratio of each reagent, The thickness of the coating is controlled by the immersion time and the pulling speed. A reflection investigation by Infrared spectroscopy could be useful for analyze the hydrolysis-polycondensation reaction of the siloxane in the coating.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Although the authors have done some slight modifications of their manuscript, I am still very unhappy and not convinced by their work.

First of all, the major and substantial criticism of the EIS data fitting procedures was not touched at all - still the justification of the shown equivalent circuits and their use in the fitting procedure is unclear and/or entirely missing, as the presentation of the results and their interpretation is more than speculative, and in part inconsistant. Simply by comparing the raw EIS data in Fig. 2 and the results of the fitting procedures (Fig. 4) one can easily recognize that something is wrong. For example, the GPTMS-PE sample shows a decreasing trend of R_C and R_dl in Fig. 4, while Fig. 2 suggests that the order of the resistance is larger after 7 days of immersion compared to 1 day. The same is true for the BISE-PE sample, in Fig. 4 R_C is almost constant after 3 days of immersion, and R_dl only reveals a slightly increasing trend, while in Fig. 2, the resistance shows the minimum resistance after 5 days of immersion, but strongly enhanced resistivity after 30 days ... This needs more explanations as already mentioned in my previous report.  Last not least, the authors have to explicitly justify, which of the equivalent circuits were employed for fitting. 

Fig. 5 (water content) only shows the same tendences as the C_c values shown in Fig. 4 - again I am wondering about the ABSOLUTE values, because e.g. TEOS - PE and GPTMS-PE are much more different in Fig. 4 than in Fig. 5 ... Please explain.

Some additional points:

- The literature still needs additional work, some references (e.g. ref 32) are simply wrong (i.e. wrong volume, wrong pages, wrong authors), some Journal names are abbreviated, some are not, etc. I did not went through all of the references, but please check all of them carefully.

- The choice of literature used in the introduction is (as already mentioned in my previous review) arbitrarily chosen - instead of two self-citations (Ref. [2, 3] to support the importance of their own work) I would prefer to see a textbook reference instead.

- Experimental part: Supplier or manufacturer of the chosen Al-alloy? Why, for a fundamental study, is not high purity Al used instead? The alloying components (in particular Zn amd Cu) may substantially affect corrosion ... Please include additional information.

- The preparation procedure is unclear: Is the washing with destilled water (which quality) done prior to the coating, or after the coating? Please clarify!

- The purity of all used chemicals is not provided. Please complete.

- The rotation speed during the preparation of the different precursors is not included. Please add.

- Please include the pH of the solution, this is crucial for Al due to its electrochemical properties ...

- Suppliers / manufacturer and purities of PE, polyamide 560, xylene, n-propanol, etc. are not included. Please specify!

- The thickness gauge TT260 (page 2, line 86) is unclear. Which physical phenomenon is used for thickness measurements with this device? Please include more information on the underlying mechanisms and principles, and the accuracy of the measurement. What about the homogeneity of the coating? I have a lot of unclear questions, the more I went into the details of the present work! When the precision of the measurement is +/- 5 micron in thickness, then all the evaluation of the electrochemistry would be influenced or even dominated by such a large variation in coating thickness. Please include additional discussion.

- Which analysis software was used for the analysis of the polarization curves? If the NOVA-software from Autolab was used, then please say so. I was however wondering about the lines in Fig. 1 - are those fit lines or exprimental data? With respect to the experimental capabilities of the usd Autolab potentiostat - as far as I know (we have the same model in our lab), the model 204 can handle currents of down to 1 nA, so I wonder how corrosion currents of 8x10^-11 A were measured ... Or are those values just estimates? If so, I would expect an error estimated, as well as if the numbers in Table 2 are fit values.

- Figure caption of Fig. 1: I would propose to write "Polarization curves of an Al-alloy coated with different PE-coatings.

- Fig. 2 - please add labels to the sub-figures.

- Please include reference for the Z-view software.

- Please provide some more insights into the "goodness of the fit". The information included is not appropriate for a scientific communication.

- Page 6, end of second paragraph: "... Meanwhile, the self-healing behavior could be observed in the silane modified epoxy coatings, especially in the bis-silane modified epoxy coating, in which, C_c,
C_dl decreased and R_c , R_dl increases after 7 days immersion." This is not true - there is not a single curve in which the RC increases, only for the BISE-PE sample, R_c remains constant within the error of the experiments and data evaluation provided. All other R_C are decreasing with time. Please provide a less simplified description.

- The Figure caption of Fig. 4: Lower cases for CC, RC, Cdl and Rdl!

- The discussion of Fig. 5 is a bit simplified and speculative, there is no experimental proof given for either leaching and/or cross-linking of the polymers. In particular, referencing to the literature might be dangerous if not one and the same polymers are used in the coating. So additional data is required here, alone from EIS data, such a discussion is highly speculative.

-English languare usage, in particular grammar etc, must be improved. Attached is a pdf in which some of the errors are marked. Please correct thoroughly.

In conclusion, I cannot accept the manuscript for publication, major improvements are required.

Comments for author File: Comments.pdf

Author Response

Dear Editors and Reviewer:

Thank you for your comments concerning our manuscript entitled “Corrosion resistance of epoxy coatings modified by bis-silane prepolymer on aluminum alloy” (ID: coatings-1288628). Those comments are all valuable and very helpful for revising and improving our paper. We have studied comments carefully and have made correction which we hope meet with approval. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the reviewer’s comments are as flowing:

1. First of all, the major and substantial criticism of the EIS data fitting procedures was not touched at all Simply by comparing the raw EIS data in Fig. 2 and the results of the fitting procedures (Fig. 4) one can easily recognize that something is wrong. For example, the GPTMS-PE sample shows a decreasing trend of R_Cand R_dl in Fig. 4, while Fig. 2 suggests that the order of the resistance is larger after 7 days of immersion compared to 1 day. The same is true for the BISE-PE sample, in Fig. 4 R_C is almost constant after 3 days of immersion, and R_dl only reveals a slightly increasing trend, while in Fig. 2, the resistance shows the minimum resistance after 5 days of immersion, but strongly enhanced resistivity after 30 days. 

Reply: Thanks for reviewer’s suggestion. Thanks to the self-healing properties related to the hydrolysis-polycondensation reaction of the siloxane in the coating, the resistance is larger after 7 days of immersion compared to 1 day. In BISE-PE coating system, RC and Rdl gradually increases after 5 days immersion.

2. The authors have to explicitly justify, which of the equivalent circuits were employed for fitting.

Reply: Thanks for reviewer’s suggestion. Section 3.2 has been reedited to solve these problems.

3. 5 (water content) only shows the same tendences as the C_c values shown in Fig. 4 - again I am wondering about the ABSOLUTE values, because e.g. TEOS - PE and GPTMS-PE are much more different in Fig. 4 than in Fig. 5 ... Please explain.

Reply: Thanks for reviewer’s suggestion. Water content is a description of water transmission during the coating. From the formula , water volume fraction is linearly proportional to the coating capacitance - time square root (logCC-t^0.5) curve.

4. The literature still needs additional work, some references (e.g. ref 32) are simply wrong (i.e. wrong volume, wrong pages, wrong authors), some Journal names are abbreviated, some are not, etc. I did not went through all of the references, but please check all of them carefully. The choice of literature used in the introduction is (as already mentioned in my previous review) arbitrarily chosen - instead of two self-citations (Ref. [2, 3] to support the importance of their own work) I would prefer to see a textbook reference instead.

Reply: Thanks for reviewer’s suggestion. Ref. [2, 3] have been insteaded, and other mistakes of references have been corrected in the revised version.

5. Experimental part: Supplier or manufacturer of the chosen Al-alloy? Why, for a fundamental study, is not high purity Al used instead? The alloying components (in particular Zn amd Cu) may substantially affect corrosion ... Please include additional information.

Reply: Thanks. The Al-Zn-Mg-Cualloy used for oil drill pipe， belongs to high-strength aluminum alloy. In order to improve the corrosion performance, protective coatings are typically applied to metal surfaces.

6. The preparation procedure is unclear: Is the washing with destilled water (which quality) done prior to the coating, or after the coating?

Reply: Thanks. The washing with destilled water done prior to the coating

7. The purity of all used chemicals is not provided. Please complete. The rotation speed during the preparation of the different precursors is not included. Please add. Please include the pH of the solution, this is crucial for Al due to its electrochemical properties. Suppliers / manufacturer and purities of PE, polyamide 560, xylene, n-propanol, etc. are not included. Please specify!

Reply: Thanks for reviewer’s suggestion. All of those problems have been corrected in the revised version.

8. The thickness gauge TT260 (page 2, line 86) is unclear. Which physical phenomenon is used for thickness measurements with this device? Please include more information on the underlying mechanisms and principles, and the accuracy of the measurement. What about the homogeneity of the coating? I have a lot of unclear questions, the more I went into the details of the present work! When the precision of the measurement is +/- 5 micron in thickness, then all the evaluation of the electrochemistry would be influenced or even dominated by such a large variation in coating thickness. Please include additional discussion.

Reply: Thanks for reviewer’s suggestion. The thickness gauge adopts the eddy current thickness measurement method to non-destructively measure the thickness of the non-conductive coating on the metal substrate. The coating thickness obtained by dip coating method cannot be precisely controlled, and the coating thickness is controlled within +/- 5 micron, which has little effect on the electrochemical performance of the organic coating.

9. Which analysis software was used for the analysis of the polarization curves? If the NOVA-software from Autolab was used, then please say so. I was however wondering about the lines in Fig. 1 - are those fit lines or exprimental data? With respect to the experimental capabilities of the usd Autolab potentiostat - as far as I know (we have the same model in our lab), the model 204 can handle currents of down to 1 nA, so I wonder how corrosion currents of 8x10^-11A were measured ... Or are those values just estimates? If so, I would expect an error estimated, as well as if the numbers in Table 2 are fit values.

Reply: Thanks for reviewer’s suggestion. Using the Metrohm Autolab Nova 2.1 analysis software of MULTI AUTO M204 electrochemical work station, the corrosion dynamics parameters were derived from the extrapolation of the approximately cathodic Tafel region (ranging from −120 mV to −60 mV versus the corrosion potential [6]) back to the corrosion potential (as shown in Fig.2). the lines in Fig. 1 are exprimental data, the numbers in Table 2 are fit values. About how corrosion currents of 8x10^-11A were measured, We did obtain such experimental data through MULTI AUTO M204 electrochemical work station.

10. Figure caption of Fig. 1: I would propose to write "Polarization curves of an Al-alloy coated with different PE-coatings.Fig. 2 - please add labels to the sub-figures. Please include reference for the Z-view software. The Figure caption of Fig. 4: Lower cases for CC, RC, Cdl and Rdl! English languare usage, in particular grammar etc, must be improved. Attached is a pdf in which some of the errors are marked. Please correct thoroughly.

Reply: Thanks for reviewer’s suggestion. All of those problems have been corrected in the revised version.

11. The discussion of Fig. 5 is a bit simplified and speculative, there is no experimental proof given for either leaching and/or cross-linking of the polymers. In particular, referencing to the literature might be dangerous if not one and the same polymers are used in the coating. So additional data is required here, alone from EIS data, such a discussion is highly speculative.

Reply: Thanks for reviewer’s suggestion. The sharp decrease in water absorption of bis-silane modified epoxy coatings may be due to the polymer leaching and/or crosslinking reaction between silanol and epoxy groups. Experimental evidence of polymer leaching and/or crosslinking reaction needs further study.

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors improved the paper and adequately reply to my answers.

The paper is now suitable for publication

Author Response

Thanks!