Graphene Coatings for Durable and Robust Resistance to Caustic Corrosion of Nickel

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. The potentiodynamic polarization (PDP) curves presented in Figure 3a cover only a relatively narrow anodic potential window (±250 mV vs. OCP). Given that the primary corrosion mechanism for Ni in strong alkalis involves the formation and transformation of Ni(OH)₂/NiOOH passive films — processes that typically occur at more positive potentials — do the authors believe this limited scan range might underestimate the true anodic behavior and breakdown tendency of both the bare Ni and graphene-coated samples? Could extending the anodic scan to at least +500 mV vs. OCP provide clearer insight into the stability of the passive or protective layers?
2. While the Bode and Nyquist plots in Figures 4 and 5 provide valuable qualitative insights into the corrosion resistance and interfacial capacitance of the samples, the manuscript lacks quantitative fitting using appropriate equivalent electrical circuits (EECs). Without circuit modeling, it is difficult to extract meaningful physical parameters such as pore resistance (Rp), charge transfer resistance (Rct), coating capacitance (Cc), and constant phase elements (CPEs). Could the authors perform EIS data fitting using suitable EEC models (e.g., R(QR)(QR) for bilayer systems), and report the fitted parameter values with confidence intervals? This would significantly strengthen the mechanistic interpretation of the barrier properties conferred by the graphene coatings.

3.The authors are encouraged to include a more detailed discussion on the interpretation of low-frequency impedance modulus  and phase angle plateaus in the context of long-term coating durability. For instance, the use of |Z| as a proxy for corrosion resistance is well-established, but its correlation with actual degradation modes (e.g., pitting, delamination) benefits from comparison with prior studies on passive film evolution under alkaline conditions.

In particular, the authors may consider citing recent work on Ti-based alloys in alkaline media, which provides excellent frameworks for analyzing time-dependent EIS responses and interfacial stability. Relevant reference : Corrosion Science, 2023, 212, 111013. doi：10.1016/j.corsci.2023.111013 can be considered.

4.Figure 5 clearly shows the time-dependent degradation of Gr_Ni_DF, with a monotonic decrease in |Z| over 80 days, likely due to progressive electrolyte penetration through defects. However, the underlying kinetic process — whether Fickian diffusion, percolation, or reaction-limited transport — remains unexplored. Have the authors considered applying a time-constant model or power-law fitting (e.g., |Z| ∝ t⁻ⁿ) to quantify the rate of coating deterioration? Such an analysis could help distinguish between initial defect-dominated leakage and later-stage bulk degradation, offering deeper insight into failure mechanisms.

 

5.The SEM results (Figs. 6–7) show excellent morphological stability of Gr_Ni even after 80 days of immersion, with no visible cracks or pits. Yet, the EIS data (Fig. 5a) show a slight initial drop in |Z| during the first 10–15 days. Could the authors elaborate on the possible causes of this transient impedance loss despite apparent film continuity? Is there evidence of sub-surface oxidation, ion intercalation, or micro-defect wetting that may not be visible in top-down SEM imaging?

 

Author Response

Please see attached

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This paper systematically investigates the long-term corrosion protection performance of low-pressure CVD-grown multilayer graphene (MLG) coatings on nickel substrates in a highly alkaline environment (0.5 M NaOH). By comparing the electrochemical behavior and morphological evolution of a robust graphene coating (Gr_Ni) with a less robust, defect-rich coating (Gr_Ni_DF) during immersion for up to 80 days, the study clarifies the critical influence of coating quality on protection durability. The work makes a substantive contribution to the understanding of the protective mechanisms of graphene in alkaline environments and is suitable for publication after revisions. The specific suggestions for revision are as follows:

1. In section "2.2 Graphene Growth via LPCVD," key CVD parameters such as the system pressure and the partial pressure of n-hexane during the growth process should be clearly specified to enhance the reproducibility of the experiments.
2. The description for Figure 3b mentions "occasionally seen in the local surface morphology." It is recommended to clarify whether this SEM image corresponds to typical localized corrosion morphology of the Gr_Ni coating after PDP testing and to annotate the corroded areas in the figure.
3. The PDP curves in Figure 3a exhibit excessive data fluctuation, which is generally unacceptable. Please provide an explanation for this or consider retesting.
4. The EIS data for the samples lack Nyquist plots, only Bode plots are presented. Please supplement the Nyquist plots and provide the equivalent circuit model used for fitting, along with the values of the circuit elements. For reference and citation, some recent electrochemical articles are recommended, such as Corros. Commun. 1 (2021) 47–57 and Corros. Sci., 260 (2026) 113560.

Author Response

Please see attached

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript presents a systematic and well-executed study on the long-term corrosion resistance of LPCVD-grown multilayer graphene (MLG) coatings on nickel in highly alkaline media (0.5 M NaOH). The combination of Raman mapping, PDP, EIS, and long-duration (80 days) immersion testing provides strong mechanistic and durability insights. The work addresses a clear industrial and scientific gap, particularly for caustic service environments, and is largely suitable for publication after minor revisions.

The novelty of the study lies primarily in the long-term (>80 days) durability assessment in strong alkali.

This aspect should be more explicitly highlighted in the Abstract and Introduction, distinguishing the work from prior short-term or neutral/acidic studies.

Bode plots are well interpreted, equivalent circuit fitting is not presented.

Including fitted EIS parameters (e.g., coating resistance, charge-transfer resistance, CPE values) would quantitatively strengthen the mechanistic claims, especially regarding defect-mediated transport.

Ensure consistent notation for graphene samples (e.g., Gr_Ni vs Gr_Ni_DF) throughout text and figures.

Minor grammatical corrections are required (e.g., spacing in units, occasional typographical errors such as “attcaks”).

The SEM discussion is strong; however, indicating magnification or scale consistency across comparative images would further improve clarity.

 

 

Author Response

Please see attached

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

No

Author Response

The Nyquist plots, as suggested by the reviewer, have been provided in a Suppl Doc to the manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

The explanation given by the author for the excessive data fluctuations in the PDP curve mentioned last time is not accepted. If it is due to the substrate, a substrate with low roughness needs to be tested.
In addition, the EIS data of all samples still lack Nyquist plots.

Author Response

Please find attached.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript is improved.

Author Response

Reviewer 3 did not provide any new comments.