Enhancing the Corrosion Protection of AA2024-T3 Alloy by Surface Treatments Based on Piperazine-Modified Hybrid Sol–Gel Films
and Antonia Jiménez-Morales 1,*
Round 1
Reviewer 1 Report
The article investigates the effect of the incorporation of piperazine in a sol-gel matrix in terms of corrosion protection. According to literature, piperazine is acting as a corrosion inhibitor by adsorption or complexation with metal surfaces. Such inhibition effect has been proven in this article for 21PIP film using conventional electrochemical techniques (EIS: Figure 11d), e), f) and Figure 14 b) and d)) and localized electrochemical techniques (LEIM, Figure 17). Moreover, authors have shown with detailed characterization that piperazine is not crosslinked (FTIR results) and it can be available to move along a more viscous and dense 3D structure (NMR characterization).
It is important to point out author´s effort to synthesize a reference sample (212) with similar thickness to 21PIP in order to minimize the impact of the thickness in the corrosion protection. It provides more credibility to the discussion and improves the scientific quality of this innovative article.
In my opinion, the article can be published as it is from a technical point of view but in order to improve the manuscript, you can find the following comments below:
- Probably I did not get the explanation along the document about the different curing conditions (temperature and time) with/without the presence of piperazine (i.e. 20ºC for 12h) compared to unmodified hybrid sol-gel films (i.e. 120ºC for 2h). Otherwise, authors should indicate it: e.g. Did you play with different time and temperature or did you used only one condition? Which one was the evaluation criteria to select the final conditions for 21PIP sample?
- Which one was the temperature during hydrolysis and condensation for unmodified hybrid sol-gel? It was only indicated in table 1 for 212 and 21PIP samples.
- Could you provide the viscosity value for 212 sample? This value could provide new information for the discussion about the different protection properties between 212 and 21PIP samples.
- It should be indicated that was not able to obtain information for 212 film during the fitting of the time constant Rct*Cdl for the diagrams at 1, 4, 8 and 12 hours (these values are missing in the graph).
- Figure 14. In order to facilitate the comparison, the same scale could be used for a) and b) plots (e.g. 103 to 109 Ωcm2) and c) and d) plots (e.g. 10-10 to 10-4 Fcm-2), respectively.
- Section 3.8: To indicate the area that was exposed for EIS measurements.
- Line 243_Type: Table 2 instead “Tabla 2”.
- Line 252: To clarify why authors discuss about GPTMS precursor when they have indicated in the experimental section that the precursors are MAPTMS and TMOS.
- Line 319: Indicate Figure 11 d), e) and f).
- Line 324: Add some references from literature: e.g. Figure 11 “S. Chimenti et al., J. Coat. Technol. Res., 14 (4) 829–839, 2017”
- Line 456_Typo: films instead fiñms.
Author Response
Authors are grateful for the reviewer's comments, since it is important for us that our work awakens the interest of other researchers
In addition, we think that the work has improved significantly thanks to the suggestions of the reviewer.
Comments and Suggestions for Authors
The article investigates the effect of the incorporation of piperazine in a sol-gel matrix in terms of corrosion protection. According to literature, piperazine is acting as a corrosion inhibitor by adsorption or complexation with metal surfaces. Such inhibition effect has been proven in this article for 21PIP film using conventional electrochemical techniques (EIS: Figure 11d), e), f) and Figure 14 b) and d)) and localized electrochemical techniques (LEIM, Figure 17). Moreover, authors have shown with detailed characterization that piperazine is not crosslinked (FTIR results) and it can be available to move along a more viscous and dense 3D structure (NMR characterization).
It is important to point out author´s effort to synthesize a reference sample (212) with similar thickness to 21PIP in order to minimize the impact of the thickness in the corrosion protection. It provides more credibility to the discussion and improves the scientific quality of this innovative article.
In my opinion, the article can be published as it is from a technical point of view but in order to improve the manuscript, you can find the following comments below:
- Probably I did not get the explanation along the document about the different curing conditions (temperature and time) with/without the presence of piperazine (i.e. 20ºC for 12h) compared to unmodified hybrid sol-gel films (i.e. 120ºC for 2h). Otherwise, authors should indicate it: e.g. Did you play with different time and temperature or did you used only one condition? Which one was the evaluation criteria to select the final conditions for 21PIP sample?
In order to answer to these questions we have introduced several changes in text for the sections 2.2 and 2.3 and Table 1
- Which one was the temperature during hydrolysis and condensation for unmodified hybrid sol-gel? It was only indicated in table 1 for 212 and 21PIP samples.
See Lines 126-128, page 3:
We have redrafted these sentences with the intention of making the text more understandable. You can read now the following paragraph:
“During hydrolysis and condensation, the mixture was magnetically stirred for 12 h. at room temperature (ca. 20 °C)”
- Could you provide the viscosity value for 212 sample? This value could provide new information for the discussion about the different protection properties between 212 and 21PIP samples.
A new Table has been added to the manuscript, containing this information:
Table 2, page 6
- It should be indicated that was not able to obtain information for 212 film during the fitting of the time constant Rct*Cdl for the diagrams at 1, 4, 8 and 12 hours (these values are missing in the graph).
For sample 212 at 1, 4, 8 and 12 hours by using the equivalent circuit shown in Figure 13a the fit plots obtained by using the ZView software are very good. The Chi-Squared values and the Weighted Sum of Squares are also sutable, but the physical meaning of the Rct values is lost, because they are greater than 10 ^ 20 ohm.
The equivalent circuit shown in Figure 13b is more suitable, but we can use shown in Figure 13a if we assume that te Rct values tend to infinite.
Following the reviewer's suggestions we have entered the information generated using the equivalent circuit shown in Figure 15
- Figure 14. In order to facilitate the comparison, the same scale could be used for a) and b) plots (e.g. 103 to 109Ωcm2) and c) and d) plots (e.g. 10-10 to 10-4 Fcm-2), respectively.
We have introduced these suggestions in Figure 14 (now Figure 15)
- Section 3.8: To indicate the area that was exposed for EIS measurements
R.- In the new version, we have introduced this information in the experimental section (page 5, line 179)
- Line 243_Type: Table 2 instead “Tabla 2”.
R.-ok, this mistake has been corrected
- Line 252: To clarify why authors discuss about GPTMS precursor when they have indicated in the experimental section that the precursors are MAPTMS and TMOS.
R.- We are sorry but in connection with this matter, an important paragraph was accidentally lost in the process of editing the first version of the manuscript. We have now incorporated this paragraph into the new version. Now you can read this discussion without cuts in page 9, lines 304-316.
But this is a good question because our initial idea was to use piperazine as an crosslinking agent. You can read the next paragraph related with this subject in page 2, lines 81-86:
A promising approach for improving the corrosion protection performance of the hybrid sol-gel coatings on metallic surfaces can be in the direction of incorporating amine crosslinkers into the organic-inorganic matrix [31, 32]. Following this idea, some research groups focused their studies on the addition of different crosslinking agents based on piperazine derivatives in sol-gel matrices prepared from tetramethoxysilane (TMOS), and γ-glycidoxypropyltrimethoxysilane (GPTMS), and polyamine cross-linking agents for increasing some features of the sol-gel matrix [33,34].
- Line 319: Indicate Figure 11 d), e) and f).
R.-ok, this mistake has been corrected
- Line 324: Add some references from literature: e.g. Figure 11 “S. Chimenti et al., J. Coat. Technol. Res., 14 (4) 829–839, 2017”
R.-This is a good suggestion and we have added this reference in the text (Ref. 59)
- Line 456_Typo: films instead fiñms.
R.-ok, this mistake has been corrected
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Piperazine is well known as a corrosion inhibitor. Cyclic aromatic heterocycles, i.g. pyridine, have long been used in sol-gel technology. Nevertheless, the paper undoubtedly causes scientific and practical interest.
However, the mechanism of the effect of piperazine on hydrolysis and polycondensation of sol-gel precursors remains not completely clear.
The experiment is described in detail by the authors; the paper is very well designed and can be recommended for publication.
Author Response
Comments and Suggestions for Authors
Piperazine is well known as a corrosion inhibitor. Cyclic aromatic heterocycles, i.g. pyridine, have long been used in sol-gel technology. Nevertheless, the paper undoubtedly causes scientific and practical interest.
However, the mechanism of the effect of piperazine on hydrolysis and polycondensation of sol-gel precursors remains not completely clear.
The experiment is described in detail by the authors; the paper is very well designed and can be recommended for publication.
Authors are grateful for the reviewer's comments, since it is important for us that our work is well valued and awakens the interest of other researchers
Author Response File: Author Response.pdf
Reviewer 3 Report
The article is in agreement with the studies that are being carried out in environmentally friendly research. However, I consider that some adjustments should be made, for example:
1. Improve the quality of Figures 3, 7 and 15.
2. It was not chemically clear enough, why does the addition of piperazine significantly improve the anticorrosive behavior of the films?
Author Response
Comments and Suggestions for Authors
The authors appreciate the reviewers' comments, since it is important for us that our work is well valued and arouses the interest of other researchers.
The article is in agreement with the studies that are being carried out in environmentally friendly research. However, I consider that some adjustments should be made, for example:
- Improve the quality of Figures 3, 7 and 15.
R.- The quality of most figures (including Figures 3, 7, 15) has been improved as much as possible during this special period of confinement
- It was not chemically clear enough, why does the addition of piperazine significantly improve the anticorrosive behavior of the films?
R.-Initially the authors expected piperazine to act as a crosslinking agent as has been shown when added to other formulations such as those containing GPTMS.
However, the study has shown that with MAPTMS it does not act as a crosslinking agent, but rather as a catalyst for the condensation reaction during the formation of the MAPTMS / TMOS organopolysiloxane network and produces an increase of the crosslinking degree of the sol-gel films.
The most significant results of the study showed that piperazine improves the barrier properties against corrosion (passive protection) and at the same time is an effective corrosion inhibitor, which can be used to enhance the active corrosion protection performance of sol-gel films.
Author Response File: Author Response.pdf
Reviewer 4 Report
Some comments that authors may consider:
The bibliographic revision should be improved with special emphasis on the articles of coatings based on MAPTMS for corrosion protection of AA2024 aluminium alloys.
Page 3: aluminium alloy samples are 150 mm x 500 mm?.
Page 5, line 180: there is a mistake: homogeneous films and films.
Why an acid catalyst is not used in the synthesis? Usually it facilitates the inorganic polycondensation allowing to obtain denser structures.
A study of the adhesion of the coatings to the substrate would be very interesting. Can the authors include an assessment of this property?
The presence of the C=C bond (around 1650 cm-1) is not assessed in FTIR analysis, whose breakage allows the organic polymerization of the structure, which is of great importance to obtain dense coatings. According to the figures, it seems that the double bond persists after the synthesis and thermal treatment, which is an inconvenience to obtain dense layers.
29Si NMR tests indicate structures with a low degree of polymerization, which does not facilitate obtaining dense layers. Perhaps the use of an acid catalyst would have changed this situation.
It is necessary to include some cross-sectional image (SEM) of doped samples.
The values of viscosity and thickness of piperazine doped coatings are not in Table 2.
Page 8, line 252: these sentences are wrong.
The inclusion of the inhibitor significantly improves the corrosion protection but it would have been more appropriate to carry out longer tests to better verify the efficiency of the doped layers.
It is necessary to comment on the possible inhibition mechanism provided by piperazine based on the literature.
The text should be edited in detail because there are numerous errors such as the text of other articles, repetition of sentences, grammatical errors, confusion with the concentration of NaCl in the different tests, etc.
Author Response
The authors appreciate the reviewers' comments, since it is important for us that our work is well valued and arouses the interest of other researchers.
In addition, we think that the work has improved significantly thanks to the suggestions of the reviewer.
Comments and Suggestions for Authors
The bibliographic revision should be improved with special emphasis on the articles of coatings based on MAPTMS for corrosion protection of AA2024 aluminium alloys.
R.- In this new version of the manuscript, we have tried to improve the Introduction with special emphasis on the articles of coatings based on MAPTMS for corrosion protection of AA2024 aluminium alloys
Page 3: aluminium alloy samples are 150 mm x 500 mm?.
R.- Added and corrected in the text (page 3, line 118):
“2024-T3 aluminium alloy sheets (3 mm thick) were cut in small 150 mm x 500 mm panels”
Page 5, line 180: there is a mistake: homogeneous films and films.
R.-Mistake corrected
Why an acid catalyst is not used in the synthesis? Usually it facilitates the inorganic polycondensation allowing to obtain denser structures.
R.- Usually we use HNO3 to reach a pH between 2 and 3, but on this occasion we desired to obtain coatings not too dense so that they had an adequate porosity for allowing that piperazine could be released to the damaged areas in the coating. In this way, due to the piperazine physically entrapped or nanoencapsulated into the pores of the organ-inorganic network, the active protective properties of the coating against corrosion were expected to improve.
In this way, for obtained the sol-gel coatings we have applied the same methodology hat we described in reference 51, without using acid catalyst in the synthesis:
El Hadad, A.A.; Carbonell, D.; Barranco, V.; Jiménez-Morales, A.; Casal, B.; Galván, J.C. Preparation of sol–gel hybrid materials from γ-methacryloxypropyltrimethoxysilane and tetramethyl orthosilicate: study of the hydrolysis and condensation reactions. Colloid Polym. Sci. 2011, 289 (17-18), 1875-1883; DOI: 10.1007/s00396-011-2504-y.
A study of the adhesion of the coatings to the substrate would be very interesting. Can the authors include an assessment of this property?
R.- We are sorry but we do not have now a systematic study on this asssemnet. We have applied cross-cut adhesion tests, which are described in the references 20 (Hegde et al and 29 (Andreatta et al) of this manuscript for similar systems for sol-gel coatings based on methacryloxypropyltrimethoxysilane on aluminium and AA-2024 aluminium alloy.
The result are very satisfactory because this kind of coatings develop a chemical adhesion on the metallic substrates.
We also have suitable results for similar coating on magnesium alloys in the following reference:
El-Hadad, A.A., Barranco, V., Samaniego, A., Llorente, I., García-Galván, F.R., Jiménez-Morales, A., Galván, J.C., Feliu, S.Influence of substrate composition on corrosion protection of sol-gel thin films on magnesium alloys in 0.6 M NaCl aqueous solution (2014) Progress in Organic Coatings, 77 (11), pp. 1642-1652. DOI: 10.1016/j.porgcoat.2014.05.026
The presence of the C=C bond (around 1650 cm-1) is not assessed in FTIR analysis, whose breakage allows the organic polymerization of the structure, which is of great importance to obtain dense coatings. According to the figures, it seems that the double bond persists after the synthesis and thermal treatment, which is an inconvenience to obtain dense layers.
R. The presence of the C=C bond was not highlighted in the analysis because we thought it was not considered an object of evaluation. The C=C bond existence after synthesis and heat treatment does not represent a problem for the purpose of our research. The obtained film must have an intermediate density property, to achieve both, protection as a physical barrier, as well as the introduction and subsequent leaching of piperazine. On the one hand, the obtained film has enough condensation to act as a physical barrier, considering that it is only the pre-treatment of a full paint system. On the other hand, this film must be not very dense to allow piperazine leaching in the presence of an electrolytic medium to achieve corrosion protection of the AA2024-T3 alloy. Remember that to reach this last aim, the film must form a reticulated network capable of physically trapping piperazine, together with good adhesion to the substrate. This crosslinking is based on the formation of polysiloxane bonds (Si-O-Si).
29Si NMR tests indicate structures with a low degree of polymerization, which does not facilitate obtaining dense layers. Perhaps the use of an acid catalyst would have changed this situation
R.- The need to obtain a pretreatment coating with low densification is justified in the previous-question answer.
It is necessary to include some cross-sectional image (SEM) of doped samples.
R.- We do not have in this special period of confinement very good cross-sectional image (SEM) of doped samples, but we have include some images. Please see Figure 6 in the new version of the manuscript
The values of viscosity and thickness of piperazine doped coatings are not in Table 2.
R.- These results are now included in Table 3
Page 8, line 252: these sentences are wrong.
R.- We are sorry but in connection with this matter, an important paragraph was accidentally lost in the process of editing the first version of the manuscript. We have now incorporated this paragraph into the new version. Now you can read this discussion without cuts in page 9, lines 304-316.
The inclusion of the inhibitor significantly improves the corrosion protection but it would have been more appropriate to carry out longer tests to better verify the efficiency of the doped layers.
R.- Figure 15 shows now results for longer tests (180 hours)
It is necessary to comment on the possible inhibition mechanism provided by piperazine based on the literature.
Piperazine has been used as inhibitor just for aqueous media, but this is the first time that it was used entrapped into a sol-gel coating (see for example Ref 46.
In preliminary studies by using complementary electrochemical techniques we found that:
Piperazine is regarded to be a very promising compound as it promotes both a less porous silane layer and an efficient inhibition of charge transfer reactions at the metal/oxide interface. This surface layer maintains its protective characteristics even when the intermediate metal oxide layer is electrochemically reduced:
Carbonell, D.J., García-Casas, A., Izquierdo, J., Souto, R.M., Galván, J.C., Jiménez-Morales, A. Scanning electrochemical microscopy characterization of sol-gel coatings applied on AA2024-T3 substrate for corrosion protection (2016) .Corrosion Science, 111, pp. 625-636. DOI: 10.1016/j.corsci.2016.06.002
The text should be edited in detail because there are numerous errors such as the text of other articles, repetition of sentences, grammatical errors, confusion with the concentration of NaCl in the different tests, etc.
R.- The full text has been double checked improving the English, avoiding repetition of sentences and correcting grammatical errors.
Author Response File: Author Response.pdf
Reviewer 5 Report
The manuscript describes the development of piperazine-modified sol-gel hybrid films. Although the research appears to be scientifically sound and the results would merit publication in Metals, some points still need to be clarified before this work can be published.
One of the major points that seems to be against its publication as it stands is the notion of sustainability which is not addressed in the manuscript. Indeed, piperazine is a reagent that is widely used for the absorption of CO2, what are the proofs that in the film it does not react with CO2 and ultimately loses its anti-corrosion activity. It also lacks a part on the study of the porosity of the film and a possible release of piperazine in aqueous solution. Should we consider a loss of active product over time by simple leaching?
Concerning the synthesis part, data on the pH of the hydrolysis solution would have been very informative because it is an important synthesis parameter...
A section on the adhesion of these films would have been welcome to improve the application attractiveness of this type of deposit.
The electrochemistry part is far from my field of expertise and therefore I cannot judge its relevance.
Some typographical errors:
- l.92: soils
- l.203: remarkable
-l.24 table
-l.385-391: twice the same sentence
Author Response
The authors appreciate the reviewers' comments, since it is important for us that our work is well valued and arouses the interest of other researchers.
In addition, we think that the work has improved significantly thanks to the suggestions of the reviewer
Comments and Suggestions for Authors
The manuscript describes the development of piperazine-modified sol-gel hybrid films. Although the research appears to be scientifically sound and the results would merit publication in Metals, some points still need to be clarified before this work can be published.
One of the major points that seems to be against its publication as it stands is the notion of sustainability which is not addressed in the manuscript. Indeed, piperazine is a reagent that is widely used for the absorption of CO2, what are the proofs that in the film it does not react with CO2 and ultimately loses its anti-corrosion activity.
R.- The notion of sustainability not has been addressed in the manuscript. The objective is to design and develop novel pretreatments for metal surfaces as an alternative to the chromates. This is a promising technique to develop eco-friendly and non-toxic coatings consists in applying films based on hybrid organopolysiloxane precursors as surface chemical conversion coatings.
Piperazine can certainly lose activity in the presence of CO2. But in the results obtained in this study it is verified with very powerful electrochemical techniques which show that the protective properties of piperazine are very promising.
It also lacks a part on the study of the porosity of the film and a possible release of piperazine in aqueous solution. Should we consider a loss of active product over time by simple leaching?
R.- We do not have analysed the release. This is an interesting idea that could be a valuable complement to spread the knowledge of the corrosion protection mechanisms with the help of researchers of other field of the specialitation.
Concerning the synthesis part, data on the pH of the hydrolysis solution would have been very informative because it is an important synthesis parameter...
R.-We are sorry but we do not have we do not have a systematic monitoring of pH values.
With the synthesis protocol used, we know that if no abnormalities occur, the pH is between 4 and 5
A section on the adhesion of these films would have been welcome to improve the application attractiveness of this type of deposit.
R.- We are sorry but we do not have now a systematic study on this asssemnet. We have applied cross-cut adhesion tests, which are described in the references 20 (Hegde et al and 29 (Andreatta et al) of this manuscript for similar systems for sol-gel coatings based on methacryloxypropyltrimethoxysilane on aluminium and AA-2024 aluminium alloy.
The result are very good because this kind of coatings develop a chemical adhesion on the metallic substrates.
We have suitable results for similar coating on magnesium alloys in the following reference:
El-Hadad, A.A., Barranco, V., Samaniego, A., Llorente, I., García-Galván, F.R., Jiménez-Morales, A., Galván, J.C., Feliu, S.Influence of substrate composition on corrosion protection of sol-gel thin films on magnesium alloys in 0.6 M NaCl aqueous solution (2014) Progress in Organic Coatings, 77 (11), pp. 1642-1652. DOI: 10.1016/j.porgcoat.2014.05.026
The electrochemistry part is far from my field of expertise and therefore I cannot judge its relevance.
Some typographical errors:
- l.92: soils
R.- We are sorry but this is not a mistake: the proper word should be sols and not soils.
- l.203: remarkable
We agree, the mistake has been corrected.
-l.24 table
R.-ok, this mistake has been corrected
-l.385-391: twice the same sentence
R.-ok, this mistake has been corrected
Author Response File: Author Response.pdf
Round 2
Reviewer 4 Report
I consider that the manuscript is now acceptable after corrections.
