Corrosion resistance of graphene/waterborne epoxy composite coatings in CO2-satarated NaCl solution

This study investigated the corrosion resistance of graphene/waterborne epoxy composite coatings in CO2-satarated NaCl solution. The coatings were prepared by dispersing graphene in waterborne epoxy with the addition of carboxymethylcellulose sodium. The structure and composition of the coatings were characterized by scanning electron microscopy, transmission electron microscopy, Fourier-transform infrared and Raman spectroscopies. The corrosion resistance of the composite coatings was investigated by potentiodynamic polarization measurements and electrochemical impedance spectroscopy. Composite coatings with more uniform surfaces and far fewer defects than blank waterborne epoxy coatings were obtained on 1020 steel. The 0.5 wt% graphene/waterborne epoxy composite coating exhibited a much lower corrosion rate and provided better water resistance properties and long-term protection than those of the blank epoxy coating in CO2-satarated NaCl solution.

Manuscript ID: RSOS-191943 Thank you for submitting the above manuscript to Royal Society Open Science. On behalf of the Editors and the Royal Society of Chemistry, I am pleased to inform you that your manuscript will be accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the reviewers' comments at the end of this email. I apologise that this has taken longer than usual.
The reviewers and handling editors have recommended publication, but also suggest some minor revisions to your manuscript. Therefore, I invite you to respond to the comments and revise your manuscript.
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Supplementary files will be published alongside the paper on the journal website and posted on the online figshare repository (https://figshare.com). The heading and legend provided for each supplementary file during the submission process will be used to create the figshare page, so please ensure these are accurate and informative so that your files can be found in searches. Files on figshare will be made available approximately one week before the accompanying article so that the supplementary material can be attributed a unique DOI. ********************************************** RSC Associate Editor: Comments to the Author: (There are no comments.) RSC Subject Editor: Comments to the Author: (There are no comments.) ********************************************** Reviewer comments to Author: Reviewer: 1 Comments to the Author(s) This paper is very interesting and the topic is of highly significance. I recommend a minor revision before its acceptance. The comments and suggestions are as follows.
1. Section 3.2: "The fitting results of polarization curves…" should be modified as"The parameters obtained from the Tafel extrapolation method…". And in Table 2, I suggest to remove the Rp since if you want to get a precise Rp, a special LPR measurement is required. 2. Please mark all of the peaks in Raman and FTIR. 3. The caption of Figure 4 should be: The cross-sectional SEM image… Reviewer: 2 Comments to the Author(s) The authors describe the development and study of an anti-corrosion, CO2 resistance coating for use in oil and gas industry. The coating consists of a waterborne epoxy with various graphene loadings. The results show that the addition of graphene enhanced the uniformity of the coating and electrochemical testing demonstrated an enhancement in corrosion resistance. The assignment of polarisation curve gradient to diffusion of Fe ions is highly speculative and such effects can be due to a number of processes. The authors should justify this further before publication.
1) The discussion of SEM results refers to micropores. Typically, this implies less than 2 nm in pore diameter but here is used to discribe larger pores Thank you for submitting the above manuscript to Royal Society Open Science. On behalf of the Editors and the Royal Society of Chemistry, I am pleased to inform you that your manuscript will be accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the reviewers' comments at the end of this email.
The reviewers and handling editors have recommended publication, but also suggest some minor revisions to your manuscript. Therefore, I invite you to respond to the comments and revise your manuscript.
Because the schedule for publication is very tight, it is a condition of publication that you submit the revised version of your manuscript before 04-Apr-2020. Please note that the revision deadline will expire at 00.00am on this date. If you do not think you will be able to meet this date please let me know immediately.
To revise your manuscript, log into https://mc.manuscriptcentral.com/rsos and enter your Author Centre, where you will find your manuscript title listed under "Manuscripts with Decisions". Under "Actions," click on "Create a Revision." You will be unable to make your revisions on the originally submitted version of the manuscript. Instead, revise your manuscript and upload a new version through your Author Centre.
When submitting your revised manuscript, you will be able to respond to the comments made by the referees and upload a file "Response to Referees" in "Section 6 -File Upload". You can use this to document any changes you make to the original manuscript. In order to expedite the processing of the revised manuscript, please be as specific as possible in your response to the referees.
When uploading your revised files please make sure that you have: 1) A text file of the manuscript (tex, txt, rtf, docx or doc), references, tables (including captions) and figure captions. Do not upload a PDF as your "Main Document". 2) A separate electronic file of each figure (EPS or print-quality PDF preferred (either format should be produced directly from original creation package), or original software format) 3) Included a 100 word media summary of your paper when requested at submission. Please ensure you have entered correct contact details (email, institution and telephone) in your user account 4) Included the raw data to support the claims made in your paper. You can either include your data as electronic supplementary material or upload to a repository and include the relevant doi within your manuscript 5) All supplementary materials accompanying an accepted article will be treated as in their final form. Note that the Royal Society will neither edit nor typeset supplementary material and it will be hosted as provided. Please ensure that the supplementary material includes the paper details where possible (authors, article title, journal name).
Supplementary files will be published alongside the paper on the journal website and posted on the online figshare repository (https://figshare.com). The heading and legend provided for each supplementary file during the submission process will be used to create the figshare page, so please ensure these are accurate and informative so that your files can be found in searches. Files on figshare will be made available approximately one week before the accompanying article so that the supplementary material can be attributed a unique DOI.
Once again, thank you for submitting your manuscript to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry. I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. Dear Editors and Reviewers, Thank you very much for taking your time to review this manuscript. I really appreciate all your comments and suggestions. The manuscript (Manuscript ID: RSOS-191943) has been revised by taking all these suggestions into account.
Here are responses to the reviewer comments: Reviewer #1: Question 1. Section 3.2: "The fitting results of polarization curves…" should be modified as "The parameters obtained from the Tafel extrapolation method…". And in Table 2, I suggest to remove the R p since if you want to get a precise R p , a special LPR measurement is required.
Answer 1: Thank you for your nice comments on our article. According to your suggestions, the expression of polarization curves in section 3.2 has been modified and the R p in Table 2 has been removed.
Fixed 1: (Section 3.2, line 3) The parameters obtained from the Tafel extrapolation method are shown in Table 2, in which I corr and E corr stand for the corrosion current density and the corrosion potential respectively. R p represents the polarization resistance. B a and B c represent the slope of the anodic and cathodic polarization respectively. Question 2. Please mark all of the peaks in Raman and FTIR.
Answer 2: We have added all the functional groups corresponding to the characteristic peaks in section 3.1.
Fixed 2: (Section 3.1, paragraph 3, line 3) The peak near 2800-3000 cm -1 was due to the C-H vibration of aliphatic group. The peak in 1244 cm -1 and the broad absorption at 3382 cm −1 were assigned to C-O-C and -OH respectively. Absorption at 916 cm −1 and 827 cm −1 were assigned to epoxy groups. Question 3. The caption of Figure 4 should be: The cross-sectional SEM image… Answer 3: The caption of Figure 4 has been modified as "The cross-sectional SEM image of 0.5 wt% graphene/waterborne epoxy composite coating on the steel 1020 test surface." Fixed 3: (Figure 4, caption) Figure 4. The cross-sectional SEM image of 0.5 wt% graphene/waterborne epoxy composite coating on the steel 1020 test surface.
Reviewer #2: Question 1. The assignment of polarization curve gradient to diffusion of Fe ions is highly speculative and such effects can be due to a number of processes. The authors should justify this further before publication.
composite coating and neat waterborne epoxy were measured after immersed in high chlorine CO 2 saturated solution. And we compared the intensity of Fe element inside of the different coatings within 2 μm (figure 1). It can be observed that with the addition of graphene, the intensity of Fe element of 0.5%wt graphene/epoxy composite coating is stronger than that of the pure epoxy coating. We suppose the increase in intensity indicates the deposition of iron ions at the interface of the substrate metal and the composite coating. And The increase of slope of the anodic polarization B a indicates the transition of anodic reaction. Therefore, we came to a conclusion that the graphene dispersed inside the composite coating retarded the diffusion of iron ions and thus the anodic reaction. The work was cited in the revised manuscript. Figure 1: The intensity of Fe element inside of the different coatings within 2 μm. Fixed 1: *(Abstract) This study investigated the corrosion resistance of graphene/waterborne epoxy composite coatings in CO 2 -satarated NaCl solution. The coatings were prepared by dispersing graphene in waterborne epoxy with the addition of carboxymethylcellulose sodium. The structure and composition of the coatings were characterized by SEM, TEM, FTIR and Raman spectroscopies. The corrosion resistance of the composite coatings was investigated by potentiodynamic polarization measurements and electrochemical impedance spectroscopy. Composite coatings with more uniform surfaces and far fewer defects than blank waterborne epoxy coatings were obtained on 1020 steel. The 0.5 wt% graphene/waterborne epoxy composite coating exhibited much lower corrosion rate and provided better water resistance properties and long-term protection than those of the blank epoxy coating in CO 2 -satarated NaCl solution. This was attributed to the distribution of graphene inside the composite coating that retarded the diffusion of iron ions and thus the anodic reaction. *(Section 3.2, line 16) Table 1 shows that B a increased with the addition of graphene while B c did not change significantly. The increase of B a indicates the transition of anodic reaction. With the addition of graphene, the intensity of Fe element of graphene/epoxy composite coating at the interface of the substrate metal and the composite coating is stronger than that of the pure epoxy coating. And the increase in the intensity of Fe element indicates the deposition of iron ions at the interface [28]. Therefore, the graphene distributed inside the composite coating was thought to impede the diffusion of Fe ions which retarded the anodic reaction. Question 2. The discussion of SEM results refers to micropores. Typically, this implies less than 2 nm in pore diameter but here is used to describe larger pores.
Answer 2: Thank you for pointing out the inaccuracy of our choice of words. The expression of "micropores" has been modified as "pores".
Fixed 2: (Section 3.1, paragraph 6, line 3) The SEM image of the blank waterborne epoxy coating in Figure 5(a) showed a surface topography containing many pores and inhomogeneous defects that is due to the rapid water evaporation during the curing process of waterborne epoxy coatings. These pores and defects will serve as active channels for aggressive species and it is not surprised that the corrosion resistance of waterborne epoxy coatings are significantly lower than those of solvent-based epoxy coatings.
We have corrected them in the revised manuscript by underlining and marking with blue. Thank you again for your consideration of our work! Yours Sincerely,

Haijun Hu
Xi'an Jiaotong University, Xi'an 710049, China  Table 3. Fitting results from the EIS measurements in Figure 8.