Hybrid Zinc Coating with CuO Nanocontainers Containing Corrosion Inhibitor for Combined Protection of Mild Steel from Corrosion and Biofouling
Round 1
Reviewer 1 Report
The manuscript is related to the anti-corrosion properties of zinc-based coatings. The authors developed a hybrid Zn coating with CuO nanocontainers containing corrosion inhibitor (Safranin) and compared it with a popular zinc coating. From the scientific and practical point of view, the article is very interesting. The manuscript is organized and well-written. Obtained coatings can be applied to improve the corrosion properties of low-carbon steel used in marine and industry. Despite the high level of presented work some comments, suggestions, and questions are presented below:
1) The article title is not fully adequate for the presented research results. Anti-biofouling properties have not been tested in the presented work.
2) Line 71. What is the mechanism of the anti-corrosion effect of Safranin?
3) Lines 143-148. Information about the measuring apparatus is not consistent
4) Line 162. What were the dimensions of the counter electrode? How the working electrode was attached? Was the entire surface of the sample (3 x 1 x 0.1 cm) immersed in the electrolyte?
5) The images presented in Figure 5 are unnecessarily stretched.
6) Results of the electrochemical tests presented in Table 2 are almost equal. What were the standard deviations for Icorr and Ecorr? I wonder if the differences in the presented results are statistically significant?
7) In my opinion, the presentation of the average Ipass values in Table 3 is not very accurate and redundant. The values of the standard deviations for Icorr and Ecorr should be completed.
I hope these comments will be helpful to improve the quality of this paper.
Author Response
Reviewer 1
The manuscript is related to the anti-corrosion properties of zinc-based coatings. The authors developed a hybrid Zn coating with CuO nanocontainers containing corrosion inhibitor (Safranin) and compared it with a popular zinc coating. From the scientific and practical point of view, the article is very interesting. The manuscript is organized and well-written. Obtained coatings can be applied to improve the corrosion properties of low-carbon steel used in marine and industry. Despite the high level of presented work some comments, suggestions, and questions are presented below:
- The article title is not fully adequate for the presented research results. Anti-biofouling properties have not been tested in the presented work.
Principally, the authors agree with the reviewer’s comment which is quite accurate. Anti-biofouling properties have not been directly tested in that work. Our presumption is that due to the well-known biocide action of CuO it could be expected that this coating will present also anti-biofouling effect. Our further investigations will include tests in artificial sea water (ASW) etc. If the reviewer insists, we could partially change the title.
- Line 71. What is the mechanism of the anti-corrosion effect of Safranin?
The reviewer’s comment is taken into consideration and a corresponding text was added in the manuscript in Part 3.3. Like many other inhibitor molecules, Safranin was found to adsorb on the steel−solution interface, thus blocking the metal surface from coming into contact with the corrosive solution [Ref. 28 in the manuscript]. The Safranin molecule contains N atoms (having high electron density) that could donate electrons to vacant d orbitals of iron resulting in a donor-acceptor bond (chemisorption process).
- Lines 143-148. Information about the measuring apparatus is not consistent.
The reviewer’s comment is taken into consideration and a corresponding text was added in the manuscript in Part 2.4.
- Line 162. What were the dimensions of the counter electrode? How the working electrode was attached? Was the entire surface of the sample (3 x 1 x 0.1 cm) immersed in the electrolyte?
Аll working samples are immersed and fixed in the same way in order to ensure reproducible conditions for comparing the results obtained. The counter electrode is a coiled platin wire – diameter of about 1 mm and length of about 5 cm. Тhe working electrode is secured using a platinum hook. The entire surface of the sample was immersed in the electrolyte.
- The images presented in Figure 5 are unnecessarily stretched.
The reviewer’s comment is taken into consideration. The images in Figure 5 are now presented in a correct way to show the real hybrid coatings thickness. The scale bar is 10 μm (the coatings thickness is about 12 μm).
- Results of the electrochemical tests presented in Table 2 are almost equal. What were the standard deviations for Icorr and Ecorr? I wonder if the differences in the presented results are statistically significant?
According to Part 2.6. the experimental results from the investigations are an average from the data of 5 samples per type, i.e. for each measurement 5 replicates of a zinc or both hybrid coatings were conditioned. The standard deviations were in the range of ±10%. Аccording to the authors, the data obtained are somewhat close given the fact that these are still samples with a zinc matrix.
- In my opinion, the presentation of the average Ipass values in Table 3 is not very accurate and redundant. The values of the standard deviations for Icorr and Ecorr should be completed.
The reviewer’s comment is taken into consideration and the required corrections are done in Table 3.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Section 3.4. Surface morphology : Line 282-303:
BSE images conveys information about the sample’s composition. SE images are very beneficial for the inspection of the topography of the sample’s surface. It is recommended to add a map of Cu element and an EDAX analysis of the coating to show the presence and distribution of the nanoparticles. SE images can not imply that the white aggregates are CuO nanoparticles.
How much is the concentration of the nanoparticle embedded in the coating (coating’s nanoparticles content)
In Fig. 5: how long is the scale bar of the images?
The layer that involves nanoparticles might be detectable in BSE images.
Section 3.5. Electrochemical Tests
The OCP value of the specimens should be reported.
In this section the findings have been reported and have not discussed enough. The authors have presented some reasons for the coatings behavior without experimental evidences or citation to proper references.
What is the importance of passive behavior for a coating that acts as a sacrificial coating?
How the Ecorr changes of the coatings (after 55 days of immersion) can affect their sacrificial roles against steel substrate
Line: 348-349: The appearance of crack and pores for the developed coatings is not a good characteristic.
Passive behavior can be seen in Fig. 7
“longer anodic curves” is not a corrosion term
Section 3.6. Surface Morphology and XPS Studies of Treated Hybrids
When the authors could not detect Cu containing products on the surface how the corrosion properties could be related in previous section to the role of these nanoparticles on the corrosion behavior.
What’s the role of the inhibitor in corrosion inhibition and corrosion product composition?
The electrochemical section of the work needs to be revised considerably.
Author Response
Section 3.4. Surface morphology : Line 282-303:
BSE images conveys information about the sample’s composition. SE images are very beneficial for the inspection of the topography of the sample’s surface. It is recommended to add a map of Cu element and an EDAX analysis of the coating to show the presence and distribution of the nanoparticles. SE images can not imply that the white aggregates are CuO nanoparticles.
The authors generally accept the reviewer's comments. In our opinion, the SE images present reliable information about size and distribution of the CuO nanoparticles and CuO-based nanocontainers, deposited through electrophoresis on the bare zinc coatings (Figure 4 A, C). SE images of the zinc coatings on steel were not presented since they seem similar to those presented in Figure 4 B and D for hybrid coatings obtained after deposition of the top zinc layers.
How much is the concentration of the nanoparticle embedded in the coating (coating’s nanoparticles content)
The working concentration in our study was 10-1 g/l in both suspensions used for electrophoretic deposition of CuO nanoparticles and CuO-based nanocontainers. It is too low concentration to be detected by EDAX measurements [14,30]. We noted in the manuscript: “Earlier studies reported that coatings obtained from 10-1 g/l and 1 g/l suspensions of similar (flake-like zinc oxide) nanoparticles could not be detected by scanning electron microscopy [14,30]”. (Lines 307-310 in the manuscript after corrections, Part 3.4.). Based on this information, the authors believe that this it is not uncommon result.
In Fig. 5: how long is the scale bar of the images?
The scale bar in Figure 5 is 10 μm (added to the Figure 5 caption).
The layer that involves nanoparticles might be detectable in BSE images.
The working concentration in our study was 10-1 g/l in both suspensions used for electrophoretic deposition of CuO nanoparticles and CuO-based nanocontainers. It is too low concentration to be detected by EDAX measurements [14,30]. We noted in the manuscript: “Earlier studies reported that coatings obtained from 10-1 g/l and 1 g/l suspensions of similar (flake-like zinc oxide) nanoparticles could not be detected by scanning electron microscopy [14,30]”. (Lines 307-310 in the manuscript after corrections, Part 3.4.). Based on this information, the authors believe that this it is not uncommon result.
Section 3.5. Electrochemical Tests
The OCP value of the specimens should be reported.
The authors have registered the OCP values of the coatings during the 55 days’ immersion in the corrosive medium. The results are not presented in a separate table due to fact that the OCP values of both hybrid coatings were very close during the whole investigation period (being however more negative than these of the ordinary zinc, i.e. remaining their better expressed sacrificial properties) as presented below:
Day OCP of Zn, mV OCP of Zn/CuO, mV OCP of Zn/NCs, mV
1 - 1050 - 1070 - 1065
10 - 1029 - 1040 - 1045
20 - 1015 - 1035 - 1038
30 - 998 - 1028 - 1035
40 - 970 - 1030 - 1038
55 - 910 - 1035 - 1035
If the reviewer insists, we could add these values in a separate table.
In this section the findings have been reported and have not discussed enough. The authors have presented some reasons for the coatings behavior without experimental evidences or citation to proper references.
Тhe remark is too general. However, the comment of the reviewer is taken into consideration and some corrections and discussions have been added in some places of the text and also below, especially in the cases when specific questions have been asked.
What is the importance of passive behavior for a coating that acts as a sacrificial coating?
As well-known zinc is generally used as a sacrificial coating to protect the iron and steel (generally, low-carbon steel) due to its more negative potential which leads to its corrosion before the iron in the case of corrosion attack. This results in the appearance of zinc-based corrosion products which characterize with definite barrier effect. However, ordinary zinc coating practically does not demonstrate passive zone of the anodic curve (at external polarization) in the case of chloride containing medium unlike, for example, some composite zinc-based coatings. When immersed in corrosive medium this metal can dissolve without or with corrosion products (in the latter case it will passivate) depending on the surrounding condition and the medium itself.
How the Ecorr changes of the coatings (after 55 days of immersion) can affect their sacrificial roles against steel substrate
The shift of the corrosion potential values in negative direction will lead to better expressed sacrificial nature of the coating (with or without the newly appeared corrosion products). Contrary to this, the potential shift in positive direction will make the sample more noble, which means in that case the coating needs to be without surface defects in order to ensure better protection. If some defects appear they could decrease the protective ability. In our case both hybrid coatings demonstrate more negative trend of the Ecorr values compared to the ordinary zinc which means that their sacrificial action is improved.
Line: 348-349: The appearance of crack and pores for the developed coatings is not a good characteristic.
The comment indicted by the reviewer refer to the corrosion products appeared on the samples after the immersion test. The presence of the abovementioned cracks and pores follow from the influence of the corrosive medium and can be expected.
Passive behavior can be seen in Fig. 7
The authors do not fully agree with this comment of the reviewer. We take the part of the anodic curves of both hybrid coatings in the potential interval from -960 mV to -670 mV rather as a retardation in the rate of the anodic process. In our opinion, there is no real observable potential zone in which the value of the current density to stay constant during the variation of the potential.
“longer anodic curves” is not a corrosion term
Тhe meaning of the text written by the authors is that under conditions of external anodic polarization the hybrid coatings dissolve completely (up to the appearance of the steel substrate) for a longer period of time, which is a sign of a slower rate of anodic dissolution under these conditions and greater corrosion resistance, respectively. Nevertheless, the required corrections have been made in the text – Page 10 and Page 11.
Section 3.6. Surface Morphology and XPS Studies of Treated Hybrids
When the authors could not detect Cu containing products on the surface how the corrosion properties could be related in previous section to the role of these nanoparticles on the corrosion behavior.
Тhe authors' idea is to achieve a coating with simultaneous action against local corrosion and bio-fouling. In this sense, the presence of CuO nanoparticles is to counteract or delay the occurrence of bio-fouling, and that of the included inhibitor is to minimize/slow down the local corrosion of the protected substrate. Having in mind the results reported by others (e.g. Ref. 7 in the manuscript) and our own results (Ref. 21) for hybrid zinc coatings containing ZnO or CuO nanoparticles, the influence of CuO nanoparticles on the corrosion behavior seems quite reliable as well.
What’s the role of the inhibitor in corrosion inhibition and corrosion product composition?
Like many other inhibitor molecules, Safranin was found to adsorb on the steel−solution interface, thus blocking the metal surface from coming into contact with the corrosive solution [Ref. 28 in the manuscript]. The Safranin molecule contains N atoms (having high electron density) that could donate electrons to vacant d orbitals of iron resulting in a donor-acceptor bond (chemisorption process).
The electrochemical section of the work needs to be revised considerably.
The comment of the reviewer is taken into consideration and the needed corrections have been added.
Author Response File: Author Response.docx
Reviewer 3 Report
Kamburova et al. reported a hybrid zinc “sandwich” coating containing CuO-based nano- containers with corrosion inhibitor Safranin for protection of steel from both corrosion and biofouling. Poly(ethylenimine) (PEI) and poly(acrylic acid) (PAA) are chosen as polyelectrolytes to encapsulate Safranin. The electrostatic interactions between the Safranin and the polyelectrolyte molecules can decrease corrosion and the subsequent release of the inhibitor is expected to slow down the corrosion rate. The coatings can improve the anticorrosion behavior of steel for a time interval of 55 days and under conditions of external polarization. It can be expected that the newly developed hybrid coatings would also demonstrate potential for marine application due to the main characteristics of their components.
I went through the manuscript and realized that this is a simple and clever story. I suggest a slight revision of the English before publication.
Author Response
Kamburova et al. reported a hybrid zinc “sandwich” coating containing CuO-based nano- containers with corrosion inhibitor Safranin for protection of steel from both corrosion and biofouling. Poly(ethylenimine) (PEI) and poly(acrylic acid) (PAA) are chosen as polyelectrolytes to encapsulate Safranin. The electrostatic interactions between the Safranin and the polyelectrolyte molecules can decrease corrosion and the subsequent release of the inhibitor is expected to slow down the corrosion rate. The coatings can improve the anticorrosion behavior of steel for a time interval of 55 days and under conditions of external polarization. It can be expected that the newly developed hybrid coatings would also demonstrate potential for marine application due to the main characteristics of their components.
Тhe authors express their gratitude to the reviewer for the positive evaluation of their work
I went through the manuscript and realized that this is a simple and clever story. I suggest a slight revision of the English before publication.
The comment of the reviewer is taken into consideration and some places in the text have been revised.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Dear Author
New comments added to the previous comments.
Comments for author File: Comments.docx
Author Response
Reviewer 2 Second revision
Section 3.4. Surface morphology : Line 282-303:
BSE images conveys information about the sample’s composition. SE images are very beneficial for the inspection of the topography of the sample’s surface. It is recommended to add a map of Cu element and an EDAX analysis of the coating to show the presence and distribution of the nanoparticles. SE images can not imply that the white aggregates are CuO nanoparticles.
The authors generally accept the reviewer's comments. In our opinion, the SE images present reliable information about size and distribution of the CuO nanoparticles and CuO-based nanocontainers, deposited through electrophoresis on the bare zinc coatings (Figure 4 A, C). SE images of the zinc coatings on steel were not presented since they seem similar to those presented in Figure 4 B and D for hybrid coatings obtained after deposition of the top zinc layers.
I am reluctant to accept the author’s opinion. The authors should use proper method to show the presence of CuO nanoparticles. The authors claimed that the SE images of the zinc coatings on steel have not been presented since they seem similar to those presented in Figure 4 B and D for hybrid coatings. So if they are similar how have you concluded that the white aggregates are CuO.
Answer B: The comment of the reviewer is taken into consideration and additional SEM image of ordinary zinc coating is added to Figure 4 (Figure 4-I). Тhe authors have realized sufficient investigations on the issue commented on by the reviewer. We have no direct visual evidence for the presence of CuO nanoparticles on the surface most probably due to the low concentration of the latter in the electrolyte for electrodeposition as well as since they are covered by additional zinc layer with a thickness of 4 μm, as described in Part 2.3. Our conclusion is based on careful examination of the experimental results which shows that the coatings obtained from the electrolyte containing CuO nanoparticles demonstrate improved anti-corrosion performance compared to the zinc in the test medium used. In our opinion this clearly demonstrates that the CuO nanoparticles are included in the coating (as intermediate layer). For the authors there is no other reasonable explanation for this matter. In addition, similar results for thin layers as presented in Figure 4 A,C and Figure 5 appear and are also discussed in the literature, as already demonstrated in the previous correction – articles No. 14 and No. 30 from the Reference list of the manuscript.
How much is the concentration of the nanoparticle embedded in the coating (coating’s nanoparticles content)
The working concentration in our study was 10-1 g/l in both suspensions used for electrophoretic deposition of CuO nanoparticles and CuO-based nanocontainers. It is too low concentration to be detected by EDAX measurements [14,30]. We noted in the manuscript: “Earlier studies reported that coatings obtained from 10-1 g/l and 1 g/l suspensions of similar (flake-like zinc oxide) nanoparticles could not be detected by scanning electron microscopy [14,30]”. (Lines 307-310 in the manuscript after corrections, Part 3.4.). Based on this information, the authors believe that this it is not uncommon result.
So what are your evidence for the presence of CuO in the coatings (apart from SE images). I suggest to add SEM images with higher magnifications (FESEM images) of the nano particles and the coatings involve nanoparticles.
Answer B: The evidence for the presence of CuO nanoparticles in the coatings is indirect and is based on the careful analysis of the obtained experimental results. The latter undoubtedly confirm that the coatings obtained from the electrolyte containing CuO nanoparticles demonstrate improved anti-corrosion performance compared to the ordinary zinc in the model medium. In our opinion this means that the CuO nanoparticles are incorporated in the coating acting as additional barrier. For the authors there is no other reasonable explanation for this matter. In our opinion, any additional images would not yield substantial information, since similar to our results already exist in the literature – articles No. 14 and No. 30 from the Reference list of the manuscript. Based on these literature sources and the similar results obtained by us, the authors believe that the presented data are credible.
In Fig. 5: how long is the scale bar of the images?
The scale bar in Figure 5 is 10 μm (added to the Figure 5 caption).
The layer that involves nanoparticles might be detectable in BSE images.
The working concentration in our study was 10-1 g/l in both suspensions used for electrophoretic deposition of CuO nanoparticles and CuO-based nanocontainers. It is too low concentration to be detected by EDAX measurements [14,30]. We noted in the manuscript: “Earlier studies reported that coatings obtained from 10-1 g/l and 1 g/l suspensions of similar (flake-like zinc oxide) nanoparticles could not be detected by scanning electron microscopy [14,30]”. (Lines 307-310 in the manuscript after corrections, Part 3.4.). Based on this information, the authors believe that this it is not uncommon result.
Section 3.5. Electrochemical Tests
The OCP value of the specimens should be reported.
The authors have registered the OCP values of the coatings during the 55 days’ immersion in the corrosive medium. The results are not presented in a separate table due to fact that the OCP values of both hybrid coatings were very close during the whole investigation period (being however more negative than these of the ordinary zinc, i.e. remaining their better expressed sacrificial properties) as presented below:
Day OCP of Zn, mV OCP of Zn/CuO, mV OCP of Zn/NCs, mV
1 - 1050 - 1070 - 1065
10 - 1029 - 1040 - 1045
20 - 1015 - 1035 - 1038
30 - 998 - 1028 - 1035
40 - 970 - 1030 - 1038
55 - 910 - 1035 - 1035
If the reviewer insists, we could add these values in a separate table.
It is suggested to report the trend of potential changes within the coatings. This observation needs to be discussed. Why more negative values for the developed coatings.
Answer B: The comment of the reviewer is taken into consideration and additional Figure 6-II with the OCP measurements has been added in Page 10. The potential changes are a result from the corrosion processes occurred on the investigated samples. Since during these processes additional corrosion products appear the latter can affect the potential and the current density values. Actually, the surface on which the processes occur, is “mixed”, i.e. parts of the coating, corrosion products, Safranin, Polyethyleneimine (PEI) and probably bare steel substrate simultaneously present from a certain time onwards. In the case of the ordinary zinc, the demonstrated potential trend is most likely due to some exposure of the steel substrate, whose potential is more positive than that of the zinc. The potential values of both hybrid coatings at the end of the test are more negative compared to the ordinary zinc. This is a sign that their sacrificial nature concerning the protected substrate is better expressed compared to the ordinary zinc, i.e. the hybrid coatings will protect the steel substrate in a greater degree. Similar comment has been added to the text under the figure.
In this section the findings have been reported and have not discussed enough. The authors have presented some reasons for the coatings behavior without experimental evidences or citation to proper references.
Тhe remark is too general. However, the comment of the reviewer is taken into consideration and some corrections and discussions have been added in some places of the text and also below, especially in the cases when specific questions have been asked.
In lines 320-328 the author reported the results without analyzing the achievements. Why this behavior was seen? Comparing the data obtained for the Rp values and the polarization curves that is shown in fig. 6. The differences between the coatings’ polarization resistance is not seen in the polarization curves in fig. 7. How the polarization resistance were measured? Furthermore, each finding and behavior and difference between the comings’ behavior should be discussed and justified.
Answer B: The most possible reason for this behavior seems to be the additionally incorporated in the zinc matrix polymer modified CuO nanoparticles or nanocontainers (NCs), respectively. Both latter act as additional physical barrier and affect positively the anti-corrosion properties of the coating in that medium especially in the case of the NCs. The latter contain corrosion inhibitor Safranin which is released (in the surrounding area of the NCs) during the corrosion process impeding in such a way the penetration of the aggressive corrosion agents deeply inside.
The authors do not understand the meaning of this comment “Comparing the data obtained for the Rp values and the polarization curves that is shown in fig. 6.”. The polarization curves are presented in Figure 7 and the Rp measurements in Figure 6. In our opinion both figures show very interesting and important information with some well visible differences in the corrosion characteristics of the investigated samples. For example, at conditions of external anodic polarization the hybrid coatings dissolve completely (up to the appearance of the steel substrate) for a longer period of time compared to the ordinary zinc, i.e. their corrosion resistance and protective ability are greater. However, the results obtained by both methods cannot be directly compared due to the fact that they are different in their essence and principles – polarization curves are realized at external polarization (cathodic and anodic) for a relatively short period of time (accelerated method - 20-30 minutes) while the Rp measurements of the samples are carried out for a prolonged time interval staying during the immersion in the test medium at open-circuit potential. In our case the Rp measurements were carried out with a special device (“Corrovit”, France) – the samples were immersed in the test solution and at selected periods of time the Rp values were measured according to the device requirements. The comment of the reviewer is taken into consideration and similar text has been added to the manuscript.
Comparing the OCP values of the coatings after 55 days of immersion and the Ecorr values of the coatings that can be estimated from fig 8. The results are opposite.
Answer B: As already commented above, the results obtained by both methods cannot be directly compared due to the fact that they are different in their essence and principles. The experimental data presented in Figure 8 is mostly generalizing in nature, since in a certain sense it represents an attempt to combine the two methods, i.e. to check the influence of external polarization on the corrosion resistance of samples that have already been immersed in the test medium. Apart from the fundamental difference between the two methods, the presence of a “mixed” surface on which successively cathodic and anodic external polarization are applied must be taken into account here. In this sense, the values of the potentials have a somewhat probabilistic character. In our opinion, the main result here is that the hybrid coatings after 55 days stay in the corrosive medium last longer under anodic polarization conditions compared to the ordinary zinc.
What is the importance of passive behavior for a coating that acts as a sacrificial coating?
As well-known zinc is generally used as a sacrificial coating to protect the iron and steel (generally, low-carbon steel) due to its more negative potential which leads to its corrosion before the iron in the case of corrosion attack. This results in the appearance of zinc-based corrosion products which characterize with definite barrier effect. However, ordinary zinc coating practically does not demonstrate passive zone of the anodic curve (at external polarization) in the case of chloride containing medium unlike, for example, some composite zinc-based coatings. When immersed in corrosive medium this metal can dissolve without or with corrosion products (in the latter case it will passivate) depending on the surrounding condition and the medium itself.
The current work reports passivation behavior for the developed coatings compared to the ordinary Zn coating. You should discuss why this behavior was seen and emphasize its role on the sacrificial behavior by citing proper references.
Answer B: The authors have not used the term “passivation behavior” in the manuscript. We have used the term “passive” 2 times as presented below: Line 379 of the corrected version - “None of the coatings has a passive zone in the anodic branch” and Lines 419 - 420 of the same version – “Both hybrid coatings demonstrate “pseudo-passive” (areas with lower current density) zones with close lengths in the anodic zone but with different current values”. The authors think that practically no passive behavior can be observed and discussed from the data in Figure 7. In Figure 8 a “pseudo-passive” unstable zones can be observed for both hybrid coatings in the potential interval -0.1 V and +0.5 V. These zones exist as a result of the occurrence of some newly appeared corrosion products in the presence of chloride ions. The latter can relative easily penetrate deeply inside leading to some oscillations (in this case significantly) of the current as a result of consecutive dissolution and passivation of the coating (and the corrosive products) in that medium at these conditions. In our opinion, it makes no sense to comment on a phenomenon that is practically not observed in our investigation and has no practical connection with the obtained experimental data. From other side, the authors have already commented (essentially and in principle) the importance of the passive behavior according to the previous requirement of the reviewer.
How the Ecorr changes of the coatings (after 55 days of immersion) can affect their sacrificial roles against steel substrate
The shift of the corrosion potential values in negative direction will lead to better expressed sacrificial nature of the coating (with or without the newly appeared corrosion products). Contrary to this, the potential shift in positive direction will make the sample more noble, which means in that case the coating needs to be without surface defects in order to ensure better protection. If some defects appear they could decrease the protective ability. In our case both hybrid coatings demonstrate more negative trend of the Ecorr values compared to the ordinary zinc which means that their sacrificial action is improved.
According to Fig 8, the Ecorr of the developed coatings are more positive than the ordinary Zn coating. The letter I (capital) is used instead of i (lower case) in the polarization curves and tables to show corrosion current density.
Answer B: As already discussed, the results obtained by both methods cannot be directly compared due to the fact that they are different in their essence and principles. The experimental data presented in Figure 8 is mostly generalizing in nature, since in a certain sense it represents an attempt to combine the two methods, i.e. to check the influence of external polarization on the corrosion resistance of samples that have already been immersed in the test medium. Apart from the fundamental difference between the two methods, the presence of a “mixed” surface on which successively cathodic and anodic external polarization are applied must be taken into account here. In this sense, the values of the potentials have a somewhat probabilistic character. In our opinion, the main result here is that the hybrid coatings after 55 days stay in the corrosive medium last longer under anodic polarization conditions compared to the ordinary zinc.
The capital letter “I” in Figure 8 has been corrected according to the reviewer’s comment.
Line: 348-349: The appearance of crack and pores for the developed coatings is not a good characteristic.
The comment indicted by the reviewer refer to the corrosion products appeared on the samples after the immersion test. The presence of the abovementioned cracks and pores follow from the influence of the corrosive medium and can be expected.
The SEM images of the coatings after 55days of test should be added for all the three coatings.
Answer B: The requirement is taken into consideration and the SEM image of corrosive treated zinc coating has been added to Figure 9.
Passive behavior can be seen in Fig. 7
The authors do not fully agree with this comment of the reviewer. We take the part of the anodic curves of both hybrid coatings in the potential interval from -960 mV to -670 mV rather as a retardation in the rate of the anodic process. In our opinion, there is no real observable potential zone in which the value of the current density to stay constant during the variation of the potential.
An unstable passive behavior is seen. Why this happens? It needs more discussion.
Answer B: The authors have not used the term “passive behavior” in the manuscript. We have used the term “passive” 2 times as presented below: Line 379 of the corrected version - “None of the coatings has a passive zone in the anodic branch” and Lines 419 - 420 of the same version – “Both hybrid coatings demonstrate “pseudo-passive” (areas with lower current density) zones with close lengths in the anodic zone but with different current values”. The authors think that practically no passive behavior can be observed and discussed from the data in Figure 7. In Figure 8 a “pseudo-passive” unstable zones can be observed for both hybrid coatings in the potential interval -0.1 V and +0.5 V. These zones occur as a result of the presence of some newly appeared corrosion products from the availability of chloride ions. The latter can relative easily penetrate deeply inside leading to some oscillations (in this case significantly) of the current as a result of consecutive dissolution and passivation of the coating (and the corrosive products) in that medium at these conditions. In our opinion, it makes no sense to comment on a phenomenon that is practically not observed in our investigation and has no practical connection with the obtained experimental data. From other side, the authors have already commented (essentially and in principle) the importance of the passive behavior according to the previous requirement of the reviewer. The corresponding text has been added in the manuscript.
“longer anodic curves” is not a corrosion term
Тhe meaning of the text written by the authors is that under conditions of external anodic polarization the hybrid coatings dissolve completely (up to the appearance of the steel substrate) for a longer period of time, which is a sign of a slower rate of anodic dissolution under these conditions and greater corrosion resistance, respectively. Nevertheless, the required corrections have been made in the text – Page 10 and Page 11.
Section 3.6. Surface Morphology and XPS Studies of Treated Hybrids
When the authors could not detect Cu containing products on the surface how the corrosion properties could be related in previous section to the role of these nanoparticles on the corrosion behavior.
Тhe authors' idea is to achieve a coating with simultaneous action against local corrosion and bio-fouling. In this sense, the presence of CuO nanoparticles is to counteract or delay the occurrence of bio-fouling, and that of the included inhibitor is to minimize/slow down the local corrosion of the protected substrate. Having in mind the results reported by others (e.g. Ref. 7 in the manuscript) and our own results (Ref. 21) for hybrid zinc coatings containing ZnO or CuO nanoparticles, the influence of CuO nanoparticles on the corrosion behavior seems quite reliable as well.
XPS results show no trace of Cu in the corrosion product composition. So how you can emphasize its role in improving the corrosion resistance. The electrochemical section needs to be discussed more profoundly.
Answer B: Тhe authors agree partially with this comment. Most probably due to the low concentration of the CuO nanoparticles in the electrolyte and its incorporation as intermediate layer in the “sandwich-type” structure we have no direct evidence. However, our conclusion is based on careful analysis of the experimental results which shows that the coatings obtained from the electrolyte containing CuO nanoparticles demonstrate improved anti-corrosion performance compared to ordinary zinc in that medium. In our opinion this means that the CuO nanoparticles present in the coating. For the authors there is no other reasonable explanation for this matter. In addition, similar results for thin layers appear and are also discussed in the literature presented by the authors in the previous version – articles No. 14 and No. 30 from the Reference list of the manuscript. According to the reviewer comments we have added additional discussion in some parts of the manuscript (marked in yellow).
The electrochemical section of the work needs to be revised considerably.
The comment of the reviewer is taken into consideration and the needed corrections have been added.
Author Response File: Author Response.docx
Round 3
Reviewer 2 Report
1- I suggest to add SEM images with higher magnifications (FESEM images) of the nano particles and the coatings involve nanoparticles. The presence of nanoparticles can be concluded by comparing the shape of white aggregates and the nanoparticles. It is recommended to add EDAX analysis of the white aggregates to show the presence of CuO.
2- The scale bar of inset images in Figure 4 is not visible.
3- Comparing the OCP values of the coatings after 55 days of immersion and the Ecorr values of the coatings that can be estimated from fig 8. The results are opposite. I accept the author’s response that these two methods are different in their essence and principles, but the opposite behavior is not acceptable. Please prepare some examples in the literatures for your claim that this behavior is not unusual.
4- In fig. 8 the corrosion current density of the CuO containing coatings is higher than the Zn coating. How has the icorr been calculated?
5- In Figs 7 and 8 and tables 2 and 3 the letter I (capital) is used instead of i (lower case) in the polarization curves and tables to show corrosion current density.
Author Response
I suggest to add SEM images with higher magnifications (FESEM images) of the nano particles and the coatings involve nanoparticles. The presence of nanoparticles can be concluded by comparing the shape of white aggregates and the nanoparticles. It is recommended to add EDAX analysis of the white aggregates to show the presence of CuO.
The authors really cannot understand why there is so much emphasis on this issue. As it was already explained in revision 2 the concentration of CuO in the electrolyte is too low and similar results for thin layers appear in the literature – articles No. 14 and No. 30 from the Reference list of the manuscript. From other side CuO nanoparticles are covered by a second zinc layer with a thickness of 4 μm and their effect can be expected after dissolution of this layer. Generally, two identical electrolytes are applied which only difference is the presence of CuO in low concentration and there is no other reason for the presence of the nanoparticles on the surface – compare Figures 4A and 4B. Greater concentrations of CuO nanoparticles will be very dangerous for the environment. From other side, the authors could electrodeposit a coating at similar electrodeposition conditions but for longer time – for example twice longer – in order to try to register the nanoparticles with EDAX. However, this will be another coating with different properties and behavior which will need additional investigations.
2- The scale bar of inset images in Figure 4 is not visible.
Тhe authors agree with this. Apparently, it is not possible to see the scale bars of the insets well. On the other hand, it would be professional to assume that all pictures should be at the same magnifications, including the insets in order to compare them. However, following text has been added in Figure 4-II to reflect this remark.
Magnification of the insets – x 10000.
3- Comparing the OCP values of the coatings after 55 days of immersion and the Ecorr values of the coatings that can be estimated from fig 8. The results are opposite. I accept the author’s response that these two methods are different in their essence and principles, but the opposite behavior is not acceptable. Please prepare some examples in the literatures for your claim that this behavior is not unusual.
Regardless of the reviewer denying these results, these are the real data, for which the relevant explanation was given. It is a probabilistic process in the presence of a mixed surface with a complex composition – parts of the hybrid coating, corrosion products, zinc, PEI, Safranin. Since the potential values are measured at conditions of external polarization, similar results could be expected. During the cathodic polarization hydrogen is released, which practically changes the top layer of the investigated sample. In the case of the ordinary zinc these are the zinc corrosion products, but in the case of hybrid coatings other substances also present. Therefore, it can be assumed that after the cathodic polarization, a different surface will present when the corrosion potential is registered - at the moment of transition from cathodic to anodic polarization. Apparently, both hybrid coatings have very close corrosion potentials in contrast to the ordinary zinc which supports the above reasoning.
In addition, the authors are not sure whether it makes sense to present some evidence requested by the reviewer for such phenomena in view of the fact that the articles No. 14 and No. 30 of the Reference list are for the third time ignored by him in relation to the questions of the presence of CuO nanoparticles – see point 1 in this revision as well as previous two revisions.
4- In fig. 8 the corrosion current density of the CuO containing coatings is higher than the Zn coating. How has the icorr been calculated?
Тhe authors do not agree with this comment and find this remark a little bit strange. The commented values of the corrosion current density are indicated in Table 3 and are calculated from the equipment (VersaStat 4 PAR).
5- In Figs 7 and 8 and tables 2 and 3 the letter I (capital) is used instead of i (lower case) in the polarization curves and tables to show corrosion current density.
The authors have corrected Figure 8 in the second review, which was apparently not noticed by the reviewer. The remaining corrections - Fig. 7, Tables 2 and 3 have been placed as required.
Author Response File: Author Response.docx
