Green-High-Performance PMMA–Silica–Li Barrier Coatings

Round 1

Reviewer 1 Report

The manuscript by Trentin et al focuses on the development of PMMA-Si-Li barrier coatings on AA7075 with eco-friendly solvent and its corrosion protection performance. The authors have taken clear objectives and executed the same in a systemic way and the results are well discussed.

Author Response

The manuscript by Trentin et al focuses on the development of PMMA-Si-Li barrier coatings on AA7075 with eco-friendly solvent and its corrosion protection performance. The authors have taken clear objectives and executed the same in a systemic way and the results are well discussed.

Response: We appreciate your very positive evaluation.

Reviewer 2 Report

The manuscript presents an interesting study about using green agents to prepare PMMA-silica coatings deposited on the aluminium alloy to improve corrosion resistance. The coating was characterized by FTIR, SAXS, TGA, AFM, EIS etc. The paper needs major revisions before it is processed further, some comments follow:

Materials and methods

Subsection 2.2. Please introduce the chemical composition of the substrate into a table.

Subsection 2.3. Please divide this subsection into more subsections depending on the type of characterization (structural, chemical etc.).

What soft was used to obtain the Nyquist and Bode diagrams, also equivalent circuits? Please write this information.

Why did the authors choose 800°C temperature?

Also, in order to be easier to follow it is important to introduce a table with the acronyms of the studied samples and the difference between them.

Results and discussion

TGA analysis indicates the thermal behaviour of the coating, not the structural characteristics, please move it. Also, if the authors want to study the thermal behaviour it is also necessary for DSC analysis.

Subsection 3.1 must be reorganized. The AFM and XPS result must be presented first and after that the others.

Subsection 3.2. Please introduce also the Nyquist diagrams and equivalent circuits for all and of course, see the examples: DOI: 10.3390/app11177802

Conclusion

I recommend the conclusion be written with points.

References

There are too many self-citations (over 6) please remove the unnecessary ones.

 

Future recommendation. It will be interesting to compare this type of coating with another coating like phosphate coatings.

Author Response

The manuscript presents an interesting study about using green agents to prepare PMMA-silica coatings deposited on the aluminium alloy to improve corrosion resistance. The coating was characterized by FTIR, SAXS, TGA, AFM, EIS etc.

Response: We appreciate your positive and constructive review.

 

Point 1: The paper needs major revisions before it is processed further, some comments follow:

Materials and methods

Subsection 2.2. Please introduce the chemical composition of the substrate into a table.

Response 1: The chemical composition of the substrate is presented in the section “2.2 Deposition of the coatings” on page 3:

“The substrates (25 mm x 25 mm x 2 mm) were obtained by cutting a bar of the aluminum alloy AA7075-T3 (GGD Metals) with nominal composition (wt%) of 0.3 Mn, 0.09 Si, 0.24 Cr, 0.05 Ni, 1.48 Cu, 2.6 Mg, 0.37 Fe, 5.64 Zn, and 0.2 Ti and Al balance.”

 

Point 2: Subsection 2.3. Please divide this subsection into more subsections depending on the type of characterization (structural, chemical etc.).

Response 2: Thank you for your comment. Section 2.3. Characterization on page 4 has been reorganized into four different subsections: 2.3.1 Chemical and structural analysis, 2.3.2 Adhesion, thickness, and coating morphology, 2.3.3 Thermal analysis, and 2.3.4 Electrochemical measurements.

 

Point 3: What soft was used to obtain the Nyquist and Bode diagrams, also equivalent circuits? Please write this information.

Response 3: Thank you for pointing this out. A description of the software used to fit the impedance data has been added to the subsection “2.3 Characterization”, on page 5:

“The impedance plots were fitted with Zview® (Scribner Associates), using electrical equivalent circuits discussed below”.

 

Point 4: Why did the authors choose 800°C temperature?

Response 4: At this temperature, the organic phase, except of coke, is completely volatilized and the pure silica phase by dehydration of remaining silanol groups is formed.

 

Point 5: Also, in order to be easier to follow it is important to introduce a table with the acronyms of the studied samples and the difference between them.

Response 5: Thank you for your suggestion. The PMMA-silica-Li samples were named according to the amount of lithium carbonate (Li₂CO₃) content. To clarify this, a table containing acronyms, Li concentrations (ppm), and added Li₂CO₃ masses has been added in Figure 1 of the Experimental procedure on page 4:

 

Figure 1. Experimental procedure used to prepare PMMA-silica-Li hybrids.

Point 6: Results and discussion

TGA analysis indicates the thermal behaviour of the coating, not the structural characteristics, please move it. Also, if the authors want to study the thermal behaviour it is also necessary for DSC analysis.

Response 6: Thank you for the suggestion. A new subsection has been added in Materials and Methods “3.2 Thermal stability” containing the TGA results (page 9).

 

Point 7: Subsection 3.1 must be reorganized. The AFM and XPS result must be presented first and after that the others.

Response 7: Thank you for your comment. Section 3. Results and Discussion has been reorganized in three different subsections (3.1 Structural properties, 3.2 Thermal stability, and 3.3 Electrochemical Barrier Properties). According reviewer's suggestion, subsection 3.1 present the results in this sequence: XPS, AFM, FTIR, and SAXS (pages 5-8).

 

Point 8: Subsection 3.2. Please introduce also the Nyquist diagrams and equivalent circuits for all and of course, see the examples: DOI: 10.3390/app11177802

Response 8: Thank you for this suggestion. The fitted Nyquist plots were included in Fig. 7 on page 11 and the fitting parameters can be found in the Supplementary material (Table S1), see Figure and Table below.

Figure 7. Time dependence of Nyquist and Bode plots for PMMA-silica coatings with increasing lithium addition; a) Li0 (0 ppm), b) Li05 (500 ppm), c) Li1 (1000 ppm), d) Li2 (2000 ppm), and e) Li4 (4000 ppm); f) electrical equivalent circuits used to fit the EIS data (black lines).

Table S1. Electrochemical parameters were obtained by fitting EIS data using the EEC of Figure 7 for the intact coatings in the first and last days of exposure to NaCl 3.5% solution. The values in brackets correspond to the error (%) of each parameter.

Coating	Li0		Li05		Li1		Li2		Li4
	2h	430d	2h	446d	2h	768d	2h	872d	2h	411d
χ2	3.9 x 10-4	4.7 x 10-4	2.5 x 10-3	4.0 x 10-4	3.1 x 10-4	3.2 x 10-4	1.7 x 10-3	8.8 x 10-4	4.9 x 10-3	3.9 x 10-3
Rs (Ω cm2)	31	28	29	25	19	19	25	24	72	67
R1 (MΩ cm2)	45 (11)	64 (8)	27 (13)	4 (18)	13 (10)	2 (15)	29 (17)	2.4 (0.2)	0.02 (14)	2.2 x 10-3
Q1 (nΩ-1 cm-2 sn)	0.25 (0.5)	0.34 (0.5)	0.83 (2)	1.0 (0.9)	0.86 (0.5)	0.98 (1.2)	0.29 (7)	0.41 (3)	0.53 (9)	270 (13)
n1	0.97 (0.04)	0.95 (0.04)	0.96 (0.15)	0.96 (0.07)	0.97 (0.04)	0.97 (0.09)	0.96 (0.51)	0.95 (0.24)	0.97 (0.62)	0.75 (1.22)
R2 (GΩ cm2)	1290 (6.8)	95 (0.7)	191 (12.2)	10.7 (0.7)	71.2 (1.3)	0.3 (0.8)	122 (11.7)	0.6 (14.1)	1.4 (1.1)	4.3 x 10-3
Q2 (nΩ-1 cm-2 sn)	0.08 (1.6)	0.07 (2.5)	0.12 (9.2)	0.45 (2.1)	0.29 (1.4)	0.92 (1.6)	0.09 (4.5)	0.69 (10.4)	1.05 (4.4)	366 (1.7)
n2	0.75 (0.35)	0.85 (0.33)	0.53 (1.53)	0.66 (0.83)	0.74 (0.33)	0.60 (1.4)	0.44 (2.01)	0.55 (1.66)	0.74 (1.97)	0.77 (0.34)
R3 (GΩ cm2)						150 (14)		7.3 (4)		2.0 x 10-3
Q3 (nΩ-1 cm-2 sn)						21.8 (0.8)		1.4 (6.4)		71300
n3						0.89 (0.39)		0.68 (1.36)		0.40 (2.62)

 

Point 9: Conclusion

I recommend the conclusion be written with points.

Response 9:

Thank you for this suggestion, however, we prefer a compact and concise version of Conclusions, allowing an easier correlation between the obtained results.

 

Point 10: References

There are too many self-citations (over 6) please remove the unnecessary ones.

Response 10:

Thank you for this suggestion 4 self-citations were removed from the references.

 

Point 11: Future recommendation. It will be interesting to compare this type of coating with another coating like phosphate coatings.

Response 11: Thank you, a new table (Table 2) was included comparing electrochemical and other reported properties of PMMA-silica coatings. In future studies, we will compare our results of barrier coatings with those based on different materials reported in the literature. Tables of recently reported high-performance barrier coatings, based on different organic and inorganic materials, can be found in our recent works: https://doi.org/10.1016/j.cej.2019.123219 and references: [11, 24].

Table 2. Reported properties of PMMA-silica coatings for anti-corrosion applications.

Coating	Solvent	Substrate	Electrolyte	Thickness (µm)	ǀZlfǀ (Ωcm-2)	Lifetime (days)	Thermal stability* (°C)	Adhesion (MPa)	Ref.
PMMA-silica-Li	2-propanol	AA7075	0.6 M NaCl	10	1010	> 872	245	15.5	This work
PMMA-silica-Ce	THF	Mild steel	0.6 M NaCl	26	109	354	-	-	[29]
PMMA-silica	THF	AA7075	0.1 M NaCl	5	108	> 60	-	-	[21]
PMMA-silica	THF	AA7075	0.1 M NaCl	1.4	109	> 120	90	-	[30]
PMMA-silica	THF	AA2024	0.1 M NaCl	4	109	> 180	-	-	[31]
PMMA-silica	THF	Steel	0.6 M NaCl	4	109	> 180	-	-	[32]
PMMA-silica	THF	AA7075	0.85 M NaCl	5	109	> 216	-	-	[33]
Acrylic polyol -silica	Butyl acetate	Mild steel	0.6 M NaCl	75	109	> 90	-	-	[34]
Acrylic polyol -silica-ZnO	Xylene	Mild steel	0.6 M NaCl	75	109	> 30	-	-	[35]
GMA-EHA-silica	THF	AA1050	0.1 M NaCl	1	109	21	~250	-	[36]

THF: tetrahydrofuran; GMA: glycidyl methacrylate (GMA); EHA: 2-ethylhexylacrylate

*5% mass loss

Author Response File: Author Response.pdf

Reviewer 3 Report

This work report that organic-inorganic based on polymethyl enthacrylate (PMMA)-silica-lithium are strong layer to protect the metal against corrosion. However, hybrid materials are important to protect light metal such as Mg, Ti, and Al. Several issues should be resolved. 

-Author should be focused hybrid organic-inorganic materials on materials in the introduction such as like doi.org/10.1016/j.jcis.2020.03.117 and doi.org/10.1039/d1cc06340e

-Novelty should be more clear to clarify the new thing compared with other reports.

-Why author not use other polymer such as PVA,....

-How authors confirmed the distance 3 nm between chains?

-Could you explain how got SEM cross-section images for organic compounds?

-If possible, please provide PDP and EIS of samples in aggressive solution as well as surface after corrosion.

 

 

Author Response

This work report that organic-inorganic based on polymethyl enthacrylate (PMMA)-silica-lithium are strong layer to protect the metal against corrosion. However, hybrid materials are important to protect light metals such as Mg, Ti, and Al. Several issues should be resolved.

 

Point 1: Author should be focused on hybrid organic-inorganic materials on materials in the introduction such as like doi.org/10.1016/j.jcis.2020.03.117 and doi.org/10.1039/d1cc06340e

Response 1: Thank you for this suggestion. The phrase in the introduction on page 3 was modified including a new reference:

A number of recent studies have investigated the effects of the inclusion of different corrosion inhibitors in hybrid coatings based on cerium, lithium, and molybdenum salts and organic inhibitors, such as 2-mercaptobenzimidazole, that are capable of significantly prolonging the service life of metallic components by retarding the progression of corrosion and the self-healing ability [7,13–17].

 

Point 2: Novelty should be more clear to clarify the new thing compared with other reports.

Response 2: Thank you, the novelty of the work was emphasized in the introduction on page 3 stating:

“Despite the high performance of these coating systems, most formulations used for the preparation involve harmful solvents, making their use on the industrial scale problematic.

Focusing on the second and fifth key factors mentioned previously concerning an environmentally compliant formulation and allowing up-scalable fabrication of reproducible high-performance barrier, this work aimed to prepare high-performance poly(methyl methacrylate) (PMMA)-silica coatings using green, low-cost reagents, which provide efficient and reliable protection for the aluminum alloy AA7075.”

 

Point 3: Why author not use other polymer such as PVA,....

Response 3: In the previous articles of our group, we used already several combinations of promising hybrid materials for barrier coatings based on different organic and inorganic phases. Such as epoxy-silica, polyurethane-silica, PMMA-silica, PMMA-CeO₂, PMMA-ZrO₂, PMMA-TiO₂. Please check it out.

 

Point 4: How authors confirmed the distance 3 nm between chains?

Response 4: This was achieved by nanostructural characterization using SAXS. Actually, the distance refers to average distance of silica nanosized nodes which crosslink the linear PMMA segments. The correlation distance (d =2π/q_max) extracted from the correlation peak in the SAXS profiles (essentially corresponding to a diffraction peak of a super lattice) is a measure of the average spacing (3 nm) between scatterers with higher electronic density (in this case silica) in a polymeric matrix with smaller electronic density (page 7).

 

Point 5: Could you explain how got SEM cross-section images for organic compounds?

Response 5: The hybrid material, consisting mainly of the organic phase (80%), is highly isolating. This leads to charging effects and thus to distortion of SEM images. Therefore, the polished cross-sections of the samples were modified by evaporation depositing a nanometric conductive carbon film.

 

Point 6: If possible, please provide PDP and EIS of samples in aggressive solution as well as surface after corrosion.

Response 6: Thank you for your suggestion. Unfortunately, potential dynamic polarization (PDP) measurements were very difficult to record, this is because the low current densities values, which are close to the detection limit of the Gamry Reference 600 instrument, result in very noisy curves. In a recent study (https://doi.org/10.1016/j.msec.2020.110713) the barrier property for PMMA-silica coating on Ti6Al4V substrate after exposure to simulated body fluid (SBF) solution were studied by PDP, the noisy polarization curve showed current densities as low as 2.8 ± 0.8 pA cm^-2.

The 3.5% NaCl solution is a standard aggressive solution used in electrochemical tests to simulate the salt concentration of seawater. The use of the more aggressive NaCl 3.5 % + HCl (pH 3) solution was reported in our recent article (https://doi.org/10.1016/j.porgcoat.2020.106129) and effects of surface damage after an alkaline attack (pH 8 and pH 14) can be found in our latest publication (https://doi.org/10.1021/acsanm.1c04281). These references were included in a new sentence in Results and discussion on page 12:

“Excellent barrier properties have been reported for PMMA-silica and acrylic-silica coatings synthesized at similar conditions even when tested under harsh conditions such as acid [9] and alkaline [14] solutions.”

Also, the experimental part was modified on page 5:

“The corrosion performance of the coatings and the uncoated AA7075 alloy was evaluated in a neutral saline environment (3.5% NaCl solution) by electrochemical impedance spectroscopy (EIS) using a Gamry Reference 600 Potentiostat.”

Author Response File: Author Response.pdf

Reviewer 4 Report

In the article“Green-high-performance PMMA-silica-Li barrier coatings”，PMMA-silica coatings were synthesized 14 using 2-propanol as a solvent and deposited on aluminum alloy AA7075 . Tests and analyses results are reasonably provided. For the benefit of the reader, a number of points need clarifying and certain statements require further justification. There are given below.

1) More related references on pare poly(methyl methacrylate) (PMMA)-silica coatings or similiar coatings, especially their development on the corrosion resistance, mechanical, thermal and chemical stability, should be supplied to make a comparison.

2) What about the surface roughness of the substrates, I think the substrates polished using only 600 and 1500 grit SiC abrasive sandpaper can impose big effect on the surface roughness of deposited coating.

3) Line 248, Higher Li concentrations (Li4) seem to not have a beneficial effect on the structure, please explain the possible mechanism in detail.

4) Line 297, Figure 5f displays a representative image of the Li2 sample on AA7075, showing a transparent, colorless, and homogeneous layer. Actually, it hard to observe  these features (transparent, colorless) in Figure 5f.

Author Response

In the article“Green-high-performance PMMA-silica-Li barrier coatings”，PMMA-silica coatings were synthesized 14 using 2-propanol as a solvent and deposited on aluminum alloy AA7075 . Tests and analyses results are reasonably provided. For the benefit of the reader, a number of points need clarifying and certain statements require further justification. There are given below.

Point 1: More related references on pare poly(methyl methacrylate) (PMMA)-silica coatings or similiar coatings, especially their development on the corrosion resistance, mechanical, thermal and chemical stability, should be supplied to make a comparison.

Response 1: We appreciate the suggestion to improve our manuscript. New references have been added and displayed in a new table (Table 2), comparing on pages 12 and 13 the findings of this work with reported studies:

“Electrochemical, thermal, barrier, and mechanical properties of PMMA-silica coatings reported so far are compared to those found in the present work in Table 2. Excellent barrier properties have been reported for PMMA-silica and acrylic-silica coatings synthesized at similar conditions even when tested under harsh conditions such as acid [9] and alkaline [14] solutions. Nonetheless, despite their performance, the most of used formulations were prepared using toxic solvents such as tetrahydrofuran, butyl acetate, and xylene. This data compilation highlights the relevance of the results obtained in this study, since no comparable outcomes have been reported for green solvents, proving the potentiality of this system as a promising alternative for anti-corrosion applications in aggressive environments”.

Table 2. Reported properties of PMMA-silica coatings for anti-corrosion applications.

Coating	Solvent	Substrate	Electrolyte	Thickness (µm)	ǀZlfǀ (Ω cm-2)	Lifetime (days)	Thermal Stability* (° C)	Adhesion (MPa)	Ref.
PMMA-silica-Li	2-propanol	AA7075	3.5% NaCl	10	1010	> 872	245	15.5	This work
PMMA-silica-Ce	THF	Mild steel	3.5% NaCl	26	109	354	-	-	[29]
PMMA-silica	THF	AA7075	0.1 M NaCl	5	108	> 60	-	-	[21]
PMMA-silica	THF	AA7075	0.1 M NaCl	1.4	109	> 120	90	-	[30]
PMMA-silica	THF	AA2024	0.1 M NaCl	4	109	> 180	-	-	[31]
PMMA-silica	THF	Steel	3.5% NaCl	4	109	> 180	-	-	[32]
PMMA-silica	THF	AA7075	5% NaCl	5	109	> 216	-	-	[33]
Acrylic polyol -silica	Butyl acetate	Mild steel	3.5% NaCl	75	109	> 90	-	-	[34]
Acrylic polyol -silica-ZnO	Xylene	Mild steel	3.5% NaCl	75	109	> 30	-	-	[35]
GMA-EHA-silica	THF	AA1050	0.1 M NaCl	1	109	21	~250	-	[36]

THF: tetrahydrofuran; GMA: glycidyl methacrylate (GMA); EHA: 2-ethylhexylacrylate, *5% mass loss

Point 2: What about the surface roughness of the substrates, I think the substrates polished using only 600 and 1500 grit SiC abrasive sandpaper can impose big effect on the surface roughness of deposited coating.

Response 2: As can be seen in the results obtained from AFM maps (page 8) and Table 1 the surface finishing has no significant influence on the surface roughness of the film. The hybrid sol fills the microscopic scratches and creates a new surface with roughness on the nanometric scale. The microscopic roughness of the substrate is helpful to improve the adhesion of the film.

 

Point 3: Line 248, Higher Li concentrations (Li4) seem to not have a beneficial effect on the structure, please explain the possible mechanism in detail.

Response 3: We agree with the reviewer that the high lithium carbonate concentration impairs the structural homogeneity, as discussed in the manuscript. According to the results, the intermediate concentrations, more specifically Li1 and Li2, provide a beneficial structural effect which can be attributed to the interaction of lithium ions with MMA monomers supporting the chain growth and higher condensation kinetics of free silanols by the charge balance provided by Li⁺.

On the other hand, the increase of polymerization defects and decrease of adhesion observed for Li4 samples suggest the presence of Li₂CO₃ aggregates, possibly due to the presence of non-dispersed salt. Similar results were observed when >3000 ppm of Ce(IV) was added to PMMA-silica coatings (https://doi.org/10.1016/j.corsci.2021.109581) indicating a threshold of this system for incorporation of additives. A more detailed discussion was included in the section “3.3 Electrochemical barrier properties” on page 10 and three new references (23, 25, and 26) were added to support these arguments:

“A possible mechanism to explain the beneficial structural effect of lithium ions is supported by the network modifier activity reported for these ions. It has been shown that lithium ions form complexes with MMA monomers, thus favoring the chain growth[25], and driving higher condensation kinetics of free silanols by providing a charge balance[26]. Concerning the performance drop at higher Li⁺ concentrations (4000 ppm), a recent study reported a similar finding showing that concentrations > 3000 ppm of Ce(IV) ions impair the structural homogeneity of PMMA-silica coatings [23], indicating a threshold for the incorporation of additives in this system. This is possibly due to the presence of non-dispersed salt after reaching the limit of solubility.”

 

Point 4: Line 297, Figure 5f displays a representative image of the Li2 sample on AA7075, showing a transparent, colorless, and homogeneous layer. Actually, it is hard to observe these features (transparent, colorless) in Figure 5f.

Response 4: Thank you for this observation. To better describe the coloration of the samples the sentence on page 7 has been changed to:

“Figure 5f (now 4f) displays a representative image of the Li2 coating on AA7075, showing a homogeneous layer obtained by dip-coating. Samples containing low Li concentrations are transparent and colorless, as reported in [14], while for higher lithium concentrations, a slightly yellowish layer was formed.”

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The paper can be accepted to be further processed.

Author Response

Response to Reviewer 2 Comments R2

 

The paper can be accepted to be further processed.

Response: Thank you for the positive response.

Reviewer 3 Report

Current form is acceptable to publish

Author Response

Response to Reviewer 3 Comments R2

 

Current form is acceptable to publish.

Response: Thank you for the positive response.