Protective Efficiency of ZrO₂/Chitosan “Sandwich” Coatings on Galvanized Low-Carbon Steel

Round 1

Reviewer 1 Report

To improve the manuscript, please provide additional data and perform the following changes:

1. In Abstract, in lines 14 and 17, (toplayer) and (with the toplayer) should be written as (top layer) and (with the top layer), and in line 22, the word "model" should be deleted from "in a model 5% NaCl" since it is the electrolyte solution;
2. On page 1, in the Introduction section, lines 37-38, the phrase "In that sense, their application must also be applied in the light of financial possibilities." should be reformulated to avoid the use of similar words (application, applied) in the same phrase;
3. On page 2, in the Introduction section, lines 68-70, the phrase "Recently organic-inorganic composite (hybrid) films have received great attention in the field of corrosion protection due to the combination of usefull properties of the both organic and inorganic components" should be replaced "usefull properties of the both" with "useful properties of both", and the useful properties should be clearly specified;
4. On page 2, in the Introduction section, in line 73, "fue to its low cost" should be replaced with "due to its low cost", and in line 81, "varous" should be replaced with "various";
5.  On page 3, in 2.1. Chemicals and Sample Types, in line 99, referring to medium molecular-weight chitosan should be specified the value of the molecular weight of the chitosan, and in line 101, referring to low carbon steel substrates with sizes 3 x 1 x 0.1 cm for the low carbon steel should be specified the steel grade and chemical composition, as well as the steel producer, while the sizes of the substrate should be expressed as 3 cm x 1 cm x 0.1 cm;
6. Referring to the Methodology, on page 3, lines 107-108 "Chitosan solution was prepared by dispersing chitosan (0.5 g) in 3% water solution of tartaric acid and stirring with a magnetic stirrer for 2 weeks" and in lines 111-112 "for the first the CS layer was dried at room temperature (RT) 1 week" should be clarified in the Results and Discussion section why were selected long times for stirring and drying; Should be clarified also how was monitored and determined the thickness of the multilayers of Zn, CS, and ZrO₂;
7. On page 3, lines 116-117 should be specified the amounts of the reagents instead of expressing them in a general way (i.e. small amounts);
8. On page 3, line 121, "after the withdrowing (rate of 30 mm / min)" should be replaced with "after the withdrawing (rate of 30 mm/min)", and in lines 123-124 "The depositon and treatment of ZrO2 coating were repeated 3 times" should be replaced "depositon" with "deposition" and in the Results and Discussion section should be clarified why was necessary to perform 3 times deposition and treatment of ZrO₂ coatings and how the thickness, uniformity, and adherence of the top coating of ZrO₂ was influenced after each deposition and treatment process;
9. References should be provided for the equations (1) and (2) and all equations should be written with an Equation Editor, not inserted as images;
10. On page 3, line 130, heating rate 10 ^o/min should be written as 10 ^oC/min;
11. On page 4, line 159, should be specified the type of the liquid drops even on page 8, line 246 were specified water drops;
12. In Fig. 1 - XRD, the phase identification should be visible in the XRD spectrum and the identified phases should be correlated with the ICDD pdf reference cards; the sample code of the analyzed “sandwich” multilayer coating structure should be also specified in Fig. 1 caption;
13. The corrosion rate should be specified for the coated steel samples of T25 and T100 type, and steel substrate;
14. SEM analysis of the developed coatings T25 and T100 should be performed both in longitudinal and cross-section and presented in the revised manuscript, along with marking on the SEM images from cross-section the thickness of each layer of ZrO₂/CS/Zn deposited on steel substrate;
15. The strength and adhesion resistance of the synthesized coatings should be proved by specific tests since on page 10, line 306 is mentioned a  strong adhesion of the coatings to the metal, and in line 312 is  mentioned "The final hybrid coating type possesses higher strength and increased resistance to peeling than the single coating" but there are not provided in the manuscript such results; It should be also interesting to compare the obtained results for ZrO₂/CS/Zn coatings deposited on steel with Zn coatings (1 µm) deposited on steel to highlight the improvement in properties of the multilayer coatings;
16. Referring to section 2.9.  Reproducibility (page 4, lines 174-176) it was claimed that all electrochemical tests were realized for 5 samples of each type and the average values were presented; It is hard to believe that all the results were reproducible and the standard deviation was equal to 0, especially the depositions of CS and ZrO₂ layers were performed by immersion and sol-gel method, and the thickness and uniformity of the layers of ZrO₂ and CS are not clear; Please clarify this issue;
17. In Abstract and Conclusions should be provided the main quantifiable results.

Author Response

Comments and Suggestions for Authors

To improve the manuscript, please provide additional data and perform the following changes:

1. In Abstract, in lines 14 and 17, (toplayer) and (with the toplayer) should be written as (top layer) and (with the top layer), and in line 22, the word "model" should be deleted from "in a model 5% NaCl" since it is the electrolyte solution;

The reviewer's comment is taken into consideration. The text has been corrected.

2. On page 1, in the Introduction section, lines 37-38, the phrase "In that sense, their application must also be applied in the light of financial possibilities." should be reformulated to avoid the use of similar words (application, applied) in the same phrase;

The reviewer's comment is taken into consideration. The text has been corrected.

3. On page 2, in the Introduction section, lines 68-70, the phrase "Recently organic-inorganic composite (hybrid) films have received great attention in the field of corrosion protection due to the combination of usefull properties of the both organic and inorganic components" should be replaced "usefull properties of the both" with "useful properties of both", and the useful properties should be clearly specified;

The reviewer's comment is taken into consideration. The text has been corrected and specified.

4. On page 2, in the Introduction section, in line 73, "fue to its low cost" should be replaced with "due to its low cost", and in line 81, "varous" should be replaced with "various";

      The reviewer's comment is taken into consideration. The text has been corrected.

5. On page 3, in 2.1. Chemicals and Sample Types, in line 99, referring to medium molecular-weight chitosan should be specified the value of the molecular weight of the chitosan, and in line 101, referring to low carbon steel substrates with sizes 3 x 1 x 0.1 cm for the low carbon steel should be specified the steel grade and chemical composition, as well as the steel producer, while the sizes of the substrate should be expressed as 3 cm x 1 cm x 0.1 cm;

The reviewer's comment is taken into consideration. The text has been corrected and specified according to the requirements. The following text was added in the artilcle: The value of the molecular weight of the chitosan is 190–310 kDa for medium molecular weight chitosan (MMW)

 

 

 

 

 

6. Referring to the Methodology, on page 3, lines 107-108 "Chitosan solution was prepared by dispersing chitosan (0.5 g) in 3% water solution of tartaric acid and stirring with a magnetic stirrer for 2 weeks" and in lines 111-112 "for the first the CS layer was dried at room temperature (RT) 1 week" should be clarified in the Results and Discussion section why were selected long times for stirring and drying; Should be clarified also how was monitored and determined the thickness of the multilayers of Zn, CS, and ZrO₂;

Two approaches were chosen for drying the CS layer - for the T25 samples the layer was dried at room temperature for 1 week, and for the second group of samples the T100-CS layer was initially heat treated at 50 °C for 0.5 h, after which the temperature was raised to 100 °C for 1 hour. The dissolution of the chitosan in tartaric acid is carried out with stirring for a long time, as the aim is to obtain a completely homogeneous solution of the polymer. The good adhesion of the inorganic-organic composite is due to the formation of hydrogen bonds in the interaction of ZrO₂ with the polymer Chitosan.

The long period of drying at room T of the CS layer was applied aiming to remove completely the water molecules from the internal volume of the polymer (up to achieve constant weight value of the material). Based on this we have chosen this drying period (2 weeks).

 

7. On page 3, lines 116-117 should be specified the amounts of the reagents instead of expressing them in a general way (i.e. small amounts);

     We have enriched the text in part 2.1.3 with exact quantity of the reagents.

8. On page 3, line 121, "after the withdrowing (rate of 30 mm / min)" should be replaced with "after the withdrawing (rate of 30 mm/min)", and in lines 123-124 "The depositon and treatment of ZrO2 coating were repeated 3 times" should be replaced "depositon" with "deposition" and in the Results and Discussion section should be clarified why was necessary to perform 3 times deposition and treatment of ZrO₂ coatings and how the thickness, uniformity, and adherence of the top coating of ZrO₂ was influenced after each deposition and treatment process;

Principally, the sol-gel thin film processing differs from bulk gel processing in that the drying stage overlaps the aggregation/gelation and aging stages establishing a brief time for further condensation reactions. The deposition – drying process is repeated several times (often from two-three times to 20 times in some cases): after one deposition–drying process the layer obtained is ultrathin: the thickness is usually in the range 20-30nm (depending mainly on the precursor type and concentration; temperature of drying). For this reason, it is necessary to make several deposition-drying cycles. It is very important to obtain sufficient thickness of the protective coatings, capable to protect the metal (act as barrier for diffusion of aggressive species. The investigation of thickness, uniformity, and adherence of ZrO₂ coatings after each deposition is interesting not only from theoretical point of view. We are very thankful to the reviewer for raising this question, since it gives ideas and opens options for more detailed studies in the future with respect to the basic parameters at each stage of the multiple deposition of layers and hence elucidating the influence upon their anticorrosion properties. The present paper is the start of research work in the field of preparing some new types of corrosion resistance organic-inorganic multilayer structures. In this connection the aim of the present article is only to clarify the effect of the treatment temperature of chitosan middle layer on the physicochemical parameters and corrosion resistance of the final CS-ZrO₂-galvanized steel "sandwich" structures.

9. References should be provided for the equations (1) and (2) and all equations should be written with an Equation Editor, not inserted as images;

The equations (1) and (2) were written with Equation Editor and the corresponding reference was added in the paper: Eaton, P., West, P. (2010). Atomic  Force  Microscopy. Oxford: OXFORD University Press.

10. On page 3, line 130, heating rate 10 ^o/min should be written as 10 ^oC/min;

      The reviewer's comment is taken into consideration. The text has been corrected.

11. On page 4, line 159, should be specified the type of the liquid drops even on page 8, line 246 were specified water drops;

The reviewer's comment is taken into consideration. The text has been corrected and specified.

12. In Fig. 1 - XRD, the phase identification should be visible in the XRD spectrum and the identified phases should be correlated with the ICDD pdf reference cards; the sample code of the analyzed “sandwich” multilayer coating structure should be also specified in Fig. 1 caption;

We have introduced the XRD figure according to the reviewer remark as single figure  Fig 1-b with identification of the phases.

13. The corrosion rate should be specified for the coated steel samples of T25 and T100 type, and steel substrate;

The authors have already specified the corrosion rate (as corrosion current density, A/cm²) of the samples T25, T100 and steel, respectively in Table 4. We have converted the corrosion rate in “mm y^-1” (penetration rate) according to the calculation

1 mA.cm^-2 = 3,28 . M / n . d

where:

M = atomic mass;

n = number of electrons freed by the corrosion reaction;

d = density

An additional column has been added in Table 4.

 

14. SEM analysis of the developed coatings T25 and T100 should be performed both in longitudinal and cross-section and presented in the revised manuscript, along with marking on the SEM images from cross-section the thickness of each layer of ZrO₂/CS/Zn deposited on steel substrate;

         

According to the reviewer’s comment we have done cross-section of the structure the    latter being presented above. In our opinion the whole “sandwich type” system is visible (the white border in the left image x100). The latter seems to be the zinc layer (1 µm thickness) while the central area is the steel substrate. The CS and ZrO₂ layers cannot be detected as separate parts since they have similar structure and are very thin. Due to these reasons it is very difficult to register them and their borders.

 

15. The strength and adhesion resistance of the synthesized coatings should be proved by specific tests since on page 10, line 306 is mentioned a  strong adhesion of the coatings to the metal, and in line 312 is  mentioned "The final hybrid coating type possesses higher strength and increased resistance to peeling than the single coating" but there are not provided in the manuscript such results; It should be also interesting to compare the obtained results for ZrO₂/CS/Zn coatings deposited on steel with Zn coatings (1 µm) deposited on steel to highlight the improvement in properties of the multlayer coatings;

 We thank the reviewer for the correct question. In our article we used the term “adhesion” in accordance with our previous studies, which revealed that the oxide layers, deposited directly upon the same type of zinc-coated steel, as the one studied in the present paper, and we established the fact that there was poorer wetting of the surface and the layers /whatever their number is/ - after the thermal treatment they are almost detached from the metal surface. It is for this reason that we were looking for a way to improve the wetting ability and the adhesion of the finally formed film. The hydrophilic nature of the surface layer of chitosan /according to the present investigation has a contact angle of about 40о – Figure 5 /together with its other positive qualities: (i) good adhesion to both metallic substrate and sol-gel coatings; (ii) the presence of hydroxyl, amine and carbonyl functional groups, serving as chelation sites of metal ions made it a good choice in its quality of intermediate layer between the metal and the final sol-gel ZrO₂ film. All this gave us the reason to claim that the multi-layer coating has greater strength and adhesion to the substrate in comparison with the single component oxide coating. Probably the formation of hydrogen bonds in the inorganic-organic composite layers is responsible for achieving of good contact between the metal and deposited layers.

 

According to reviewer’s comment we have compared the obtained results for ZrO₂/CS/Zn coatings deposited on steel with Zn coatings (1 µm) deposited on steel in order to highlight the improvement in properties – please, see the figure below. However, we have some doubts about this comparison. The reason is that the Zn layer is very thin – 1 µm – and in the anodic zone after the corrosion potential it is not very clear the place when the zinc coating will dissolve totally and the dissolution process of the substrate will begin. Of course, we could add this curve, if necessary. 

 

 

16 Referring to section 2.9.  Reproducibility (page 4, lines 174-176) it was claimed that all electrochemical tests were realized for 5 samples of each type and the average values were presented; It is hard to believe that all the results were reproducible and the standard deviation was equal to 0, especially the depositions of CS and ZrO₂ layers were performed by immersion and sol-gel method, and the thickness and uniformity of the layers of ZrO₂ and CS are not clear; Please clarify this issue;

Тhe authors accept the reviewer's remark. We agree with the reviewer that it is really difficult to achieve good reproducibility of all the samples investigated. However, in order to avoid the inaccuracy, we chose a representative PD curve with average characteristics from a group of PD curves. In such a way we sought to show the most probable possible PD curve. The authors share their belief that a purely mathematical choice of an averaged variant of a curve would not give better and more realistic results.

17 In Abstract and Conclusions should be provided the main quantifiable results.

     The reviewer's comment is taken into consideration. The text has been corrected.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper is good overall. It would be good to improve the discussion section by detailing more each tests results. English language needs some minor revision.

Author Response

Comments and Suggestions for Authors

The paper is good overall. It would be good to improve the discussion section by detailing more each tests results. English language needs some minor revision.

 

The reviewer's comment is taken into consideration. The English text has been partially improved by the Editor of journal “Nanoscience and Nanotechnology” of the Bulgarian Academy of Sciences ISSN:1313-8995 following the Instructions of “American Chemical Society Style Guide – A Manual for Authors and Editors” Janet S. Dodd, Editor, Marianne C. Brogan, Advisory Editor, ACS Washington DC 1986

Author Response File: Author Response.pdf

Reviewer 3 Report

Please see the attachment.

Comments for author File: Comments.pdf

Author Response

Review report on the manuscript titled “Protective Efficiency of ZrO2/Chitosan “Sandwich”

Coatings on Galvanized Low-Carbon Steel”

The manuscript is well organized, supported by detailed results and data, and it regards an

interesting field of study. This reviewer suggests the publication after some minor revisions listed

below:

1. In the introduction, in lines 68-70 the authors mentioned the organic-inorganic hybrid

coatings. They should briefly specify which are the most common methods to produce these

films, in particular by thin-film deposition (https://doi.org/10.1016/j.tsf.2012.08.023) or by

incorporation or surface decoration of an organic matrix with inorganic fillers

(https://doi.org/10.1177%2F1847980419862075, https://doi.org/10.3390/nano10122500).

 

According to the reviewer suggestion we have introduced the following text and  new references  in the Introduction part of the  article . “The most common methods to produce these hybrid films, in particular by thin-film deposition [20] or by incorporation or surface decoration of an organic matrix with inorganic fillers [ 22, 23].”

 

2. Why was the tartaric acid specifically used to dissolve chitosan? What makes it different

from acetic acid for the preparation of chitosan-based solutions?

 

Chitosan constitutes a biocompatible, biodegradable, mucoadhesive, pH-dependent cationic polymer, which is soluble in acidic media. Chitosan’s amine groups become protonated (R – NH₃⁺) so that the polymer is positively charged, which enables to solvate it in water. Furthermore, the ionic repulsion between the charged amine groups causes an extended linear polymer configuration. As polyelectrolyte chitosan is able to form electrostatic complexes under acidic conditions [1] R. Marguerite, Chitin and chitosan: Properties and application. Prog. Polym. Sci. 31 (2006) 603-632. ELON ISHMAEL KADOGAN et al. have studied the effect of various acids on the behavior of chitosan. They found that citric acid had the greatest potential for forming hydrogen bonding (with an elongation of 56 %) from other dissolving acids and then followed by tartaric acid, with an elongation of 37%. Acetic and oxalic acid have an elongation of 20% and 19%, respectively, due to the fact that acetic acid and oxalic acid have the least available hydrogen atoms to participate in intermolecular hydrogen bonding with chitosan interaction.

                     

citric acid                              tartaric acid

 

The dissolution of the chitosan in tartaric acid is carried out with stirring for a long time, as the aim is to obtain a completely homogeneous solution of the polymer. The good adhesion of the inorganic-organic composite is due to the formation of hydrogen bonds in the interaction of ZrO₂ with the polymer chitosan. It has to be also noted for T25 samples, the CS layer follows the topography of galvanized steel, while for T100 samples, the CS layer is "fixed" faster and unevenly distributed areas of AFM are observed.

        For the information we present here the 2D and 3D images of bare galvanized steel    substrate

 

 

3. In section 2.8 how were the samples connected to the measuring systems: were they soldered to a wire or clamped with alligator clips? The authors should insert a figure/scheme

depicting the three-electrode cell used for the electrochemical tests (references:

https://doi.org/10.3390/mi10110739, https://doi.org/10.1016/j.porgcoat.2008.08.012)

 

Electrodes with soldered cables and a hook at the lower end with a hole allowing a tight attachment of the test specimen were used. The authors have added the figure of the tri-electrode cell used in the text.

 

4. The inset in Figure 1 is hardly readable: the text should be resized

 

We are agree with the reviewer’s comment. We have added XRD as Figure 1-b with identification of the phases.

 

5. In lines 182-183, the authors should briefly explain the difference between the first and

second stage of dehydration correlated to the mass losses in the TG curve.

 

We have complied with the reviewer and have explained briefly the changes in TG during the first and second stage of dehydration.  A corresponding sentence was added in Results and discussion –p.3.1.

 

6. In lines 197-198, the authors explain that the T100 samples are rougher than the T25 ones. Is this due to the thermal treatment? How can it be explained?

 

Yes, we are completely agree with the reviewer supposition about the effect of the treatment on the surface roughness. The following sentence was added in the article ”This is due to the so called “sintering effect”: after thermal treatment the particles size increases and  proceeds  some agglomeration. The closer contact between the particles, leads to decreased surface roughness”.

 

7. As an additional point to the AFM studies, this reviewer suggests to perform an SEM

observation of the surface and the cross section of the coatings produced.

        

According to the reviewer’s comment we have done cross-section of the structure the    latter being presented above. In our opinion the whole “sandwich type” system is visible (the white border in the left image x100). The latter seems to be the zinc layer (1 µm thickness) while the central area is the steel substrate. The CS and ZrO₂ layers cannot be detected as separate parts since they have similar structure and are very thin. Due to these reasons it is very difficult to register them and their borders.

 

 

8. Lines 249-250: is the hydrophobicity of the multilayer influenced by the hydrophilicity of the chitosan layer? Or is it due to the deposition of the inorganic layers? A contact angle

measurement of the single inorganic layers should be added as comparison.

 

We have added additional figure (as Figure 5-c), which represents the contact angle of zirconia layer only, deposited on the same type of galvanized steel, the same zirconia precursor, dipping procedures and treatments the final “sandwich” structures.

 

9. In Figure 6, the authors should illustrate the parameters listed in Table 4, i.e. Ecorr, Icorr and Ipass.

The reviewer's comment is taken into consideration. The places where the parameters are measured are marked in the figure (please, see below). Тhe authors doubt whether marking these places on the figure itself will improve its appearance, but if necessary, this will be done.

 

10. In section 3.6, the EIS curves plotted in Figure 7 are called Nyquist plots and several

parameters can be extracted besides the polarization resistance. Some softwares can be used

to evaluate these parameters through an equivalent circuit for the coatings under

consideration. This reviewer suggests to make a quantitative analysis of these plots.

The authors agree with this reviewer’s comment since we recognize the importance of these parameters for the interpreting of the results obtained. However, due to some software problems appearing we are not able to give the full analysis and the fitted curves of EIS plot but only the electrical circuit. The latter has been added in the Figure 7. We expect that the respected reviewer will agree with this amendment. If this is accepted as insufficient we could remove this section from the main text.

 

Author Response File: Author Response.pdf

Reviewer 4 Report

This contribution deals with manufacturing of a multilayer coating on low carbon steel aiming at improvement of corrosion resistance in sodium chloride solution. Despite generally being well prepared, it suffers from some weaknesses, which should be revised prior to publication. More detailed comments are listed below.  

1. In terms of language, the contribution urgently needs to be revised by a native speaker.
2. In the abstract, it should be considered to just give a short introduction in subject, methods and main results of the study. It seems rather unnecessary to show details like temperatures used for application of layers at this point.
3. Page 3, line 101: please provide a little more details on the used “low carbon steel substrates” since this is a rather unspecific definition.
4. In total, the “Materials and Methods” section seems too elaborate. Perhaps, it can be abbreviated e.g. by moving some parameters presented to a table.
5. Figure 1: the inserted image is rather small. Furthermore, there is a fourth peak which is not designated.
6. Page 7, line 230: Where is the “Zn2P peak(s)” in Figure 4? It is not designated / labelled there.
7. Figure 6: please refer to remark no. 3, since “steel” is even less specific than “low carbon steel”. And the steel substrate may have a significant impact on corrosion behavior.
8. The “Discussion” section is rather short. Since the multilayer system seems rather extensive, its advantages over concurrent solutions, e.g. using conventional corrosion protection measures, or applying stainless steel, remain unclear. This holds true both for financial aspects as well as for performance like corrosion resistance. A more elaborate comparison and discussion is suggested, which would help to illustrate the impact of the results obtained in this study.

Author Response

This contribution deals with manufacturing of a multilayer coating on low carbon steel aiming at improvement of corrosion resistance in sodium chloride solution. Despite generally being well prepared, it suffers from some weaknesses, which should be revised prior to publication. More detailed comments are listed below.  

1. In terms of language, the contribution urgently needs to be revised by a native speaker.

The reviewer's comment is taken into consideration. The English text has been partially improved by the Editor of journal “Nanoscience and Nanotechnology” of the Bulgarian Academy of Sciences ISSN:1313-8995 following the Instructions of “American Chemical Society Style Guide – A Manual for Authors and Editors” Janet S. Dodd, Editor, Marianne C. Brogan, Advisory Editor, ACS Washington DC 1986

1. In the abstract, it should be considered to just give a short introduction in subject, methods and main results of the study. It seems rather unnecessary to show details like temperatures used for application of layers at this point.

It is not a problem for the authors to shorten the Introduction part. However, two of the other reviewer insist to give more details. In that case we leave the Editor to decide how to proceed.

1. Page 3, line 101: please provide a little more details on the used “low carbon steel substrates” since this is a rather unspecific definition.

The reviewer's comment is taken into consideration. The text has been corrected and specified according to the requirements (Part 2.1.1.).

1. In total, the “Materials and Methods” section seems too elaborate. Perhaps, it can be abbreviated e.g. by moving some parameters presented to a table.

It is not a problem for the authors to present some parameters in table. However, since the materials differ in their nature we try to make the text more appropriate and readable. In that case we leave the Editor to decide how to proceed.

 

1. Figure 1: the inserted image is rather small. Furthermore, there is a fourth peak which is not designated.

We have introduced the XRD figure according to the reviewer remark as single figure  Fig 1-b with identification of the phases.

1. Page 7, line 230: Where is the “Zn2P peak(s)” in Figure 4? It is not designated / labelled there.

We agree with reviewer that the peak is not labelled. However, since the XPS analysis is a surface analytical method it is not possible to register the zinc underlayer. That’s why only the peaks of Zr core levels have been labelled.

1. Figure 6: please refer to remark no. 3, since “steel” is even less specific than “low carbon steel”. And the steel substrate may have a significant impact on corrosion behavior.

      The reviewer's comment is taken into consideration. The text under the figure has been   corrected.

1. The “Discussion” section is rather short. Since the multilayer system seems rather extensive, its advantages over concurrent solutions, e.g. using conventional corrosion protection measures, or applying stainless steel, remain unclear. This holds true both for financial aspects as well as for performance like corrosion resistance. A more elaborate comparison and discussion is suggested, which would help to illustrate the impact of the results obtained in this study.

We are very thankful to the reviewer for raising this question, since it gives ideas and opens options for more detailed studies in the future with respect to the basic parameters at each stage of the multiple deposition of layers and hence elucidating the influence upon their anticorrosion properties.

The present paper is the start of research work in the field of preparing some new types of corrosion resistance organic-inorganic multilayer structures. In this connection the aim of the present article is only to clarify the effect of the treatment temperature of chitosan middle layer on the physicochemical parameters and corrosion resistance of the final CS-ZrO₂-galvanized steel "sandwich" structures.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors answered satisfactorily to almost all the raised questions. However, there are a few amendments to be considered by the authors:

1) On page 3, line 107, in the phrase "Low carbon steel substrates with composition...." should be specified "...the composition (in weight %):  ...";

2) On page 5, Fig. 1, the title "Heat Flow (µV)" should be rotated by 180° to be able to read it from left to right; Also, the measurement unit of the heat flow should be clarified since V is the symbol from Volt specific to Voltage;

3) On page 10, in Fig. 7, the title "Current (A) per cm^2" should be written as "Current density (A/cm²);

4) Figure 8. Electrochemical cell used for the experiments should be deleted since it is not necessary to provide pictures with equipment and testing devices, and on page 4, lines 180-181, the phrase "А picture of the equipment used is shown in the section for potentiodynamic investigations according to [29,30]." should be reformulated as "The equipment used for potentiodynamic investigations was according to [29,30].";

5) SEM images (cross-section) of the developed coatings T25 and T100 should be presented in the revised manuscript, along with specific comments concerning the thickness of the multilayer coatings.

Author Response

The authors answered satisfactorily to almost all the raised questions. However, there are a few amendments to be considered by the authors:

1) On page 3, line 107, in the phrase "Low carbon steel substrates with composition...." should be specified "...the composition (in weight %):  ...";

The reviewer's comment is taken into consideration. The text has been corrected.

2) On page 5, Fig. 1, the title "Heat Flow (µV)" should be rotated by 180° to be able to read it from left to right; Also, the measurement unit of the heat flow should be clarified since V is the symbol from Volt specific to Voltage;

The reviewer's comment is taken into consideration. The figure and measurement unit have been corrected.

3) On page 10, in Fig. 7, the title "Current (A) per cm^2" should be written as "Current density (A/cm²);

The reviewer's comment is taken into consideration. The title has been corrected.

4) Figure 8. Electrochemical cell used for the experiments should be deleted since it is not necessary to provide pictures with equipment and testing devices, and on page 4, lines 180-181, the phrase "А picture of the equipment used is shown in the section for potentiodynamic investigations according to [29,30]." should be reformulated as "The equipment used for potentiodynamic investigations was according to [29,30].";

The reviewer's comment is taken into consideration. The text has been corrected. Figure 8 has been deleted.

5) SEM images (cross-section) of the developed coatings T25 and T100 should be presented in the revised manuscript, along with specific comments concerning the thickness of the multilayer coatings.

The reviewer's comment is taken into consideration. The cross-sections have been added in Figure 9 along with some specific comments concerning the thicknesss of the coatings.

Author Response File: Author Response.docx

Reviewer 4 Report

Most isses raised have been adressed satisfactorily.

However, obe issue remains.

Remark no. 2 was related to the abstract, not to introduction. Thus, it does not stand in contradiction to remarks of the other reviewers.

 

Author Response

Most isses raised have been adressed satisfactorily.

However, obe issue remains.

Remark no. 2 was related to the abstract, not to introduction. Thus, it does not stand in contradiction to remarks of the other reviewers.

The reviewer's comment is taken into consideration. The text has been corrected and presented below as well as in the manuscript.

 

Enhanced corrosion efficiency of low carbon steel was achieved by newly developed hybrid multilayers, composed of low carbon steel coated with electrodeposited zinc sublayer (1µm), chitosan (CS) middle layer and ZrO₂ coating by sol-gel method (top-layer). The middle chitosan layer was obtained by dipping on galvanized steel substrate in 3% tartatic acid water solution of  Мedium molecular-weight chitosan, composed of β-(1-4)-linked D-glucosamine and N-acetyl-D-glucosamine with a deacetylation degree of about 75 − 85% (CS). The substrates were dipped into CS solution and  withdrawn at a rate of 30 mm/min. One part of the samples with CS layer were dried  at room temperature 2 weeks, another part – at 100^oC for 1 h, respectively. After CS deposition-treatment, the substrates were dipped into isopropanol sol of zirconium butoxide with small quantity of polyethylene glycol (PEG400). The cycles dipping-drying of the ZrO2 coatings were repeated three times. After the third cycle the final structures were treated at 180⁰C. The samples were denoted - as T25 wivch consist of CS middle layer, dried at RT and T100 with CS middle layer treated at 100⁰C, respectively. The samples were characterized by means of Differential Thermal analysis (DTA-TG), XRD analyses, X-ray photoelectron spectroscopy (XPS) and Atomic Force Microscopy (AFM). Hydrophobicity properties were evaluated by measuring the Contact angle with a Ramé-Hart automated goniometer. Two electrochemical tests - Potentiodynamic polarization technique (PD) and Electrochemical Impedance Spectroscopy (EIS) - have been used to determine the corrosion resistance and protective ability of the coatings in a 5% NaCl solution. The results obtained by both methods revealed that the applied “sandwich” multilayer systems demonstrate sacrificial character and will protect hopefully the steel substrate in corrosion medium containing chloride ions as corrosion activators. The newly obtained hybrid multilayer coating systems have dense structure and hydrophobic nature. They demonstrated positive effects on the corrosion behavior at conditions of external polarization independent of their various characteristics: morphology, grain sizes, surface roughness and contact angle. They extend the service life of galvanized steel in a chloride containing corrosion medium due to their amorphous structure, hydrophobic surface, and the combination of the positive features of the both chitosan middle layer and the zirconia top layer.

Author Response File: Author Response.docx