Size Effect of Graphite Nanosheet-Induced Anti-Corrosion of Hydrophobic Epoxy Coatings

Abstract

In order to broaden the selectivity of graphite nanosheet additives on epoxy resin-based coatings and verify the size effect, this work aims to dope graphite nanosheets of different sizes into the three-dimensional structure produced by cross-linking and curing epoxy resin and polyamide resin. In addition, a micro-nano level secondary structure and a surface with special roughness are constructed to obtain the composite epoxy hydrophobic coating. The influence of the size effect of graphite nanosheets on the hydrophobic performance and corrosion resistance of the coating is summarized as well. Among them, the optimized doping size (2.2 μm) of graphite nanosheets in the epoxy coating showed the largest impedance arc of 2.58 × 10⁸ Ω cm², which could form an excellent nano-network covering the micropores to impede the diffusion of corrosive medium. Through simulation calculation analysis, we also found that the edge site of graphene is more effective in capturing H₂O and O₂; therefore, a smaller size of graphene with a large edge can be more favorable. This work will be used as a reference for the industrial application of graphite anti-corrosive coating.

1. Introduction

Metal corrosion, induced by an electrochemical or chemical reaction at the surface of a metal, will significantly destroy its inherent properties (such as toughness, electrical, plasticity, and optical), thus posing a serious threat to construction, marine environments, transportation, and daily life [1,2,3]. The mechanism of corrosion phenomena is involved in a series of electrochemical reactions at the interface between corrosive media (such as Cl⁻, H₂O, and O₂) and substrates [4,5]. Thus, it is necessary to design a surface coating to prevent the metal substrate from contacting these corrosive media [6,7,8]. Engineering a hydrophobic nanostructured coating is a significant method for anti-corrosive and self-cleaning applications for marine environments or transportation [9,10]. The hydrophobic surface of the substrate results in a reduced rolling angle, elevated water contact angle, and self-cleaning and anti-corrosion properties [11,12,13,14,15]. Hydrophobic nanostructured coatings act as a corrosion barrier by (1) preventing blister formation and establishing difficult pathways for Cl⁻, H₂O, and O₂ and (2) prohibiting adhesive failure and enhancing corrosion resistance. However, most hydrophobic anti-corrosive coatings suffer from inadequate surface durability [16].

An organic–inorganic nanostructured coating has the features required to develop hydrophobic and anti-corrosive materials with long-term surface stability [17,18]. Developing carbonaceous nanomaterials, particularly graphene/graphite nanosheets, is favorable to employing durable and outstanding coatings for anti-corrosive protection because of their excellent electrical conductivity, physical properties, and honeycomb lattice structure, which can be used to obtain coatings with excellent barrier properties [19,20]. For example, Monetta et al. reported that 0.5 wt% and 1 wt% of graphene nanosheets were added to an additive-free waterborne epoxy resin applied to aluminum alloy. Electrochemical analysis revealed an improvement in the protective properties of the coating and a lesser amount of absorbed water than the unloaded film [21]. Kopsidas et al. also reported that the low content of graphene nanosheets (≤0.5 wt%) for EP coatings demonstrated 0% damage [22]. The doping of graphene nanosheets is only effective at low loadings, as higher contents result in accelerated corrosion that facilitates the conduction of corrosion currents. Li et al. fabricated sulfonated graphene to fill in a coating by introducing sulfonic acid groups on graphene sheets, and the most remarkable improvement was achieved by adding 1.0 wt% of graphene, which was effective for blocking the penetration of corrosive molecules [23]. In our previous work, the anti-corrosion and mechanical properties of Gr_0.04-ZnO_0.4 coatings were improved after adding the appropriate 0.04 wt% content of graphite nanosheets. Moreover, the parallel assembly of graphene occurs spontaneously, which leads to a remarkable improvement in corrosion protection [24,25]. Generally speaking, these graphene-based polymer coatings with thinner thickness and lighter weight of doping graphite nanosheets exhibit a more superior anti-corrosion effect than noumenal coating [26]. However, do the different sizes of graphite nanosheets as additives also have an impact on corrosion resistance? There are almost no relevant reports on the influence of the size effect of graphite nanosheets on the corrosion protection of coatings, and further exploration of the reaction mechanism is still needed.

The influence of the size effect of graphite nanosheets and the level of interface roughness on the hydrophobic performance of coatings was explored. In addition, the influence on stability, wear resistance, and corrosion resistance of the coating was summarized as well [27,28,29]. The operation mechanism of micro-nanoscale structure and size effect on the anti-corrosion process was determined by DFT simulation calculation [30,31]. Such a reaction mechanism can be used to adjust the size and morphology of two-dimensional materials doped in the coating and filtered into stable and high-efficiency anti-corrosion coatings. This method is capable of providing new design ideas for producing two-dimensional materials and epoxy matrices. We innovatively propose the design concept of regulating the size effect of two-dimensional materials in terms of preventing the corrosion of super-hydrophobics. This work will be used as a reference for industrial applications.

Herein, micro-kaolin, PTFE, and different sizes of graphite nanosheets were doped into WEP coatings (denoted as EGx/C₁P_0.2EP, X represents centrifugal rate). They were synthesized using a cost-effective stirring process in the presence of EP and alcohol. During the stirring process, different sizes of EGx were extensively introduced to EP coatings, and simultaneously, micro-kaolin and PTFE were uniformly created on the surface to form hydrophobic long chains. Among them, different sizes of EGx were prepared using a low-cost ball-milling and gradient centrifugation approach. Then, the EGx samples were characterized by XRD, Raman, SEM, and TEM to verify and successfully prepare the different sizes of EGx. The impedance arc of EG₅₀₀₀/C₁P_0.2EP in these coatings was the largest, with an impedance of 2.58 × 10⁸ Ωcm² after 7d immersing, indicating that the corrosion resistance of EG₅₀₀₀/C₁P_0.2EP is higher than that of EG₁₀₀₀/C₁P_0.2EP, EG₃₀₀₀/C₁P_0.2EP, and EG₈₀₀₀/C₁P_0.2EP. Even after 300 h NSS time, the EG₅₀₀₀/C₁P_0.2EP coating did not exhibit obvious corrosion features; it presented more corrosion resistance than the other sizes of EGx coatings. In addition, the theoretical calculation also showed that the chemisorption of H₂O will cause the natural decomposition of H-O, producing the OH and H species on the edge of s-Gra and d-Gra. The smaller Gra can attract H₂O and O₂, reducing their attack on the coating, which can prohibit the corrosion of EGx/C₁P_0.2EP by oxidation. These results suggest that doping the optimized size (2.21 μm) of EG₅₀₀₀ could form a nano-network covering the micropores to impede the diffusion of the corrosive medium.

2. Materials and Methods

2.1. Materials

Graphite powders (CAS: 7782-42-5) were supplied by Sigma-Aldrich (Shanghai, China). Kaolin (CAS:1332-58-7), Poly(tetrafluoroethylene) (PTFE, CAS:9002-84-0), NaCl (99.9%), and 1H,1H,2H,2H-Perfluorodecyltriethoxysilane were purchased from Aladdin Holdings Group Co. Ltd. (Beijing, China). Waterborne epoxy dispersion and cosolvent were purchased from Shanghai Macklin Biochemical Co. Ltd. (Shanghai, China). The Q235 steel substrates (size: 20 mm × 20 mm × 0.3 mm) for electrochemical corrosion were firstly polished with 180-, 600-, and 1500-mesh abrasive paper, then ultrasonically cleaned in alcohol, and finally dried at room temperature. All reagents were used without further treatment.

2.2. Characterizations of Samples

The surface morphologies and physical size of samples were observed under a Scanning Electron Microscope (SEM, S-4800, Hitachi, Tokyo, Japan). The microstructure and lattice structure of the samples were observed by high-resolution transmission electron microscopy (HRTEM, JEM-2100F, Hitachi, Tokyo, Japan). Raman spectra were analyzed using 532 nm laser excitation by a Raman Spectroscope (Via Reflex, Renishaw, Wotton-under-Edge, UK). The XRD patterns were scanned at a rate of 5°/min from the 2θ angle of 5° to 90° with a characteristic wavelength using X-ray powder diffraction (XRD Terra, Innov-X, Woburn, MA, USA). Fourier transform infrared spectroscopy was taken by a spectrometer with He-Ne radiation (IR Prestige-21, Shimadzu, Kyoto, Japan). Static contact angle measurements were performed via a contact angle meter (OCAH200, Dataphysics, Filderstadt, Germany) using a sessile drop technique.

2.3. Synthesis of the Different Sizes of Graphite Sheets

First, 4 g of graphite powder (500 mesh), 2 mm, and 0.2 mm ZrO₂ balls (weight ratio: 1:1) were added to a 250 mL jar along with 100 mL of N-vinyl pyrrolidone (NVP) and subjected to grinding for 200 h. After grinding, the obtained dispersion was collected and then centrifuged at 500 r/min for 20 min to remove any unexfoliated graphite. The resulting dispersion was then centrifuged at 1000 r/min for 20 min; the sediments, termed EG₁₀₀₀, were collected using a PTFE membrane and subsequently dried at 80 °C. The remaining supernatant, designated as A, was subjected to centrifugation at 3000 r/min for 20 min to yield sediments named EG₃₀₀₀; the leftover supernatant was designated as B. Similarly, exfoliated graphite sediments obtained via an identical centrifugation procedure conducted at 5000 and 8000 r/min were named EG₅₀₀₀ and EG₈₀₀₀, respectively.

2.4. Preparation of EGx/C₁P_0.2EP Composite Epoxy Coatings

Initially, a homogeneous solution was prepared by combining 1 g of micro-kaolin, 0.33 g of PTFE, 0.33 g of epoxy resin, and 0.05 wt% EG₅₀₀₀ in 3.3 mL of ethanol, followed by stirring for 1 h. Subsequently, 0.2 mL of 1H, 1H, 2H, and 2H-perfluorodecyltriethoxysilane was added to this solution and stirred for 2 h to form component A. Component B comprised 0.33 g of a waterborne epoxy curing agent. These two components were mixed and stirred at 1000 r/min for 30 min. The mixture was allowed to solidify at 30 °C until the surface was dry and then left to cure naturally for 48 h to form the hydrophobic coating, designated as EG₅₀₀₀/C₁P_0.2EP. The EG₅₀₀₀/C₁P_0.2EP nanocomposite coatings were applied to Q235 steel substrates using a 120# wire-wound rod and cured at 60 °C for 4 h. For comparative analysis, coatings EG₁₀₀₀/C₁P_0.2EP, EG₃₀₀₀/C₁P_0.2EP, and EG₈₀₀₀/C₁P_0.2EP were also fabricated using the same procedures by varying the additional amount of dimensional graphite sheets. In addition, hydrophobic epoxy coatings with 0 wt% C₁P_0.2EP, as well as 0.025, 0.04, and 0.075 wt% EG₅₀₀₀, were prepared to assess the effect of varying the EG₅₀₀₀ content.

3. Results

3.1. Characterization of Samples and Coatings

3.1.1. Morphology Characterization of Samples and Coatings

EGx (where X denotes the centrifugation rate) samples were designed and fabricated using a simple, cost-effective ball-milling and gradient centrifugation method, incorporating graphite nanosheets and an NVP solvent, as described in our previous study [25]. Typical surface morphologies of these samples and coatings were characterized via SEM and HRTEM. The SEM images (Figure 1) reveal that the sizes of EG₁₀₀₀, EG₃₀₀₀, EG₅₀₀₀, and EG₈₀₀₀ decrease progressively, illustrating the effectiveness of the gradient centrifugation method. Figure 2 demonstrates that the exfoliated EGx samples, processed through ball-milling and centrifugation, exhibit no fractures or agglomeration, maintaining uniform size and length. The average diameters of EG₁₀₀₀, EG₃₀₀₀, EG₅₀₀₀, and EG₈₀₀₀, measured across 100 samples, were found to be 4.91, 4.00, 2.21, and 1.04 μm, respectively. Figure S3 shows the actual samples, while Figure S1 provides a close-up view of various microholes and microcracks on the surface of the EP coating. These imperfections are likely due to poor structural density and substantial solvent evaporation during the curing process. Microscopic analysis at different sites revealed the average length of these microholes and microcracks to be between 2 and 4 μm. To address these defects, different sizes of graphite were introduced into the coating to decrease porosity and lengthen the penetration path for corrosive media, thereby enhancing corrosion resistance. The SEM images presented in Figure S2 show that the surface of EG₅₀₀₀/C₁P_0.2EP contains few micropores, which hampers the diffusion of O₂, H₂O, and Cl⁻ through these microchannels to the substrate and thereby considerably enhances its anti-corrosion properties.

3.1.2. Structural Characterization of Samples and Coatings

The molecular structure of the EGx samples was comprehensively analyzed via X-ray diffraction (XRD), Raman spectroscopy, and Fourier transform infrared (FTIR) spectroscopy. The XRD patterns of the EGx nanosheets (Figure S4) show diffraction peaks at 25.7° and 52.1°, which correspond to the (002) and (004) diffraction reflections of the graphite structure (PDF#75-1621). Notably, EG₁₀₀₀ and EG₃₀₀₀ exhibit a sharp (002) diffraction peak, indicating a highly regular spatial arrangement between lamellae [25]. However, the (002) diffraction peaks for EG₅₀₀₀ and EG₈₀₀₀ are broader, suggesting fewer layers and a reduced size of graphite nanosheets. The Raman spectroscopy analysis (Figure 3a) reveals that bulk graphite powders and EGx, excited by a 532 nm laser, display approximate resonance peaks at 1383 cm⁻¹ and 1588 cm⁻¹, representing the D and G bands, respectively. The intensity ratio of the D band to the G band (I_D/I_G) slightly increases from 0.03 for bulk graphite, 0.06 for EG₁₀₀₀, 0.11 for EG₃₀₀₀, and 0.17 for EG₅₀₀₀ to 0.23 for EG₈₀₀₀, indicating the introduction of a few defects during ball-milling [25]. Furthermore, Figure 3b demonstrates that the I_D/I_G ratio increases with increasing centrifugation speed, indicating that the defects are related to the edge structure and are influenced by the centrifugation process. Moreover, a blue shift is observed in the 2D peak of exfoliated EGx, signifying a reduction in the size of graphite nanosheets. In addition, the molecular structure of bulk graphite and EGx was examined using FTIR, as depicted in Figure 3c. The characteristic absorption peaks at 3410 cm⁻¹, 2928 cm⁻¹, 1630 cm⁻¹, and 1080 cm⁻¹ correspond to –OH, C–O, C–H, and C–O–C functional groups, respectively. The infrared spectra reveal that the functional groups in the EGx samples align closely with those of the original graphite, indicating that the chemical integrity of the graphite is maintained after ball milling.

3.1.3. Wettability Characterization of Samples and Coatings

The influence of different sizes of EGx on the wettability of coatings was studied by measuring the contact angles of four types of EGx/C₁P_0.2EP coatings. As shown inFigure 4a and Figure S5, the water contact angles of all four types of EGx/C₁P_0.2EP coatings are greater than 140° within 60 days, indicating that these coatings have low surface energy and excellent hydrophobicity. The EGx/C₁P_0.2EP coating effectively prevents the penetration of corrosive ions, making it difficult for corrosive media to adhere to the coating surface, thereby slowing down the corrosion process of the substrate (Table S1). After immersion in water for 60 days, the contact angles of all four types of EGx/C₁P_0.2EP decreased, but the water contact angle of the EG₅₀₀₀/C₁P_0.2EP coating remained the highest, indicating that EG₅₀₀₀/C₁P_0.2EP had the highest hydrophobicity and the lowest porosity. The significant decrease in the water contact angle for EG₈₀₀₀/C₁P_0.2EP coatings appears to be related to the size of the nanofiller (graphite nanosheets) in the resin; in fact, the smaller size of the nanofiller is more likely to move with increasing immersion time, slightly reducing the surface’s hydrophobicity. Figure 4b shows the contact angles of the EGx/C₁P_0.2EP coatings with different EG₅₀₀₀ contents as the soaking time increases. At the initial stage of immersion, the contact angles of the four coatings with different EG₅₀₀₀ contents were all greater than 145°, indicating that these coatings had excellent hydrophobic properties. From Figure 4b and Table S2, it can be observed that with increasing EG₅₀₀₀ content, the contact angle also gradually increases, with the maximum angle of the EG₅₀₀₀/C₁P_0.2EP coating with 0.075 wt% EG₅₀₀₀ exceeding 151°, indicating that the 0.075 wt% addition of EG₅₀₀₀ better filled the pores on the surface of the epoxy resin, reducing its porosity.

3.2. Anti-Corrosion Properties of EGx/C₁P_0.2EP Coatings

3.2.1. Electrochemical Characteristics

In this study, the impedance spectra of the EGx/C₁P_0.2EP coatings were measured in a 3.5 wt.% NaCl solution at room temperature. The corrosion protection properties of the materials correlate with the radius of the capacitive arc [14], and it was observed that the corrosion resistance of the EGx/C₁P_0.2EP coatings decreased over time, from 7 days to 60 days (Figure 5). After 7 days of immersion (Figure 5a), the impedance arc of the EG₅₀₀₀/C₁P_0.2EP coating was the largest, with an impedance of 2.58 × 10⁸ Ω·cm², indicating superior corrosion resistance compared with EG₁₀₀₀/C₁P_0.2EP, EG₃₀₀₀/C₁P_0.2EP, and EG₈₀₀₀/C₁P_0.2EP. This is attributed to the size of EG₅₀₀₀, which closely matches the pore size of the epoxy resin, effectively filling the holes and crevices to enhance corrosion resistance. After 30 days of immersion (Figure 5b), the impedance values of all coatings decreased by an order of magnitude from 10⁸ Ω·cm² to 10⁷ Ω·cm² because of the penetration of corrosive electrolytes. However, after 60 days of immersion (Figure 5c), EG₅₀₀₀/C₁P_0.2EP still exhibited the largest impedance arc among the coatings, with an impedance of 3.18 × 10⁷ Ω·cm². The EIS data were analyzed using ZSimpWin software version 3.30. The equivalent circuits in Figure S7 fit the EIS data with one time constant, where Rc represents coating resistance and C represents coating capacitance. As depicted in Table S3 and Figure 5, the Rc value of EG₅₀₀₀/C₁P_0.2EP is the largest, suggesting that this coating can more effectively prevent the penetration of the corrosion medium into the metal matrix when the size of the graphite nanosheet matches the pore size of the epoxy resin. Comparatively, the impedance spectra of EG₅₀₀₀/C₁P_0.2EP coatings with different doped EG₅₀₀₀ contents are shown in Figure 6. After soaking for 7 days, the data in Table S6 indicate that the coating with 0.075 wt.% EG₅₀₀₀ exhibited a greater electrochemical impedance (2.39 × 10⁸ Ω·cm²) compared with those with lower contents of EG5000—0.025 wt.% (1.47 × 10⁸ Ω·cm²), 0.04 wt.% (1.96 × 10⁸ Ω·cm²), and 0.05 wt.% (2.18 × 10⁸ Ω·cm²). From 7 to 40 days, the permeation of electrolytes resulted in a decrease in resistance values for coatings with varying EG₅₀₀₀ contents. However, after 40 days, the electrochemical impedance of the coating with 0.075 wt.% EG₅₀₀₀ remained at 5.95 × 10⁷ Ω·cm². As EG₅₀₀₀ content increases, the pores on the surface of the epoxy are fully filled, which reduces the porosity of the coating, increases the electrical impedance, and effectively compensates for the defects, thus protecting the steel matrix. Thus, the anti-corrosion properties of the hydrophobic epoxy resin coating were enhanced by the appropriate addition of graphite.

To study the state of the coating at different stages in detail, Bode modulus experiments were conducted. Similar to the impedance arc in the Nyquist diagram, the Bode modulus (f = 0.01 Hz) is an important electrochemical parameter for evaluating the corrosion resistance of coatings; organically coated samples with higher Bode moduli tend to exhibit better anti-corrosive properties. The impedance modulus of the EG₅₀₀₀/C₁P_0.2EP coating decreased with the extension of the corrosion time (Figure 7a), indicating a transition from capacitance to resistance in electrochemical behavior. Despite this, the coating resistance value of the EG₅₀₀₀/C₁P_0.2EP coating remained the highest after 60 days of immersion, maintaining the most stable anti-corrosive coating. This confirms that the appropriate size of graphite (EG₅₀₀₀) effectively suppresses the corrosive medium. Furthermore, it is straightforward to observe the number of time constants in Bode plots (phase angle), and the phase angle at high frequency can also be considered a useful parameter for evaluating a coating’s protective performance. Because of high resistance, this results in higher phase angles between the current and the voltage. At the initial stage of immersion (Figure S10), the phase angle curves show that the phase angles of all coated samples were greater than 60°, except for those coated with EG₁₀₀₀/C₁P_0.2EP. Notably, the phase angle of the EG₅₀₀₀/C₁P_0.2EP coating approached 85°. These results demonstrate that EG₅₀₀₀/C₁P_0.2EP hydrophobic films exhibit distinct corrosion behaviors with higher phase angles at high frequencies and significantly improved corrosion resistance, aligning with the conclusions drawn from Nyquist plots. Additionally, considering the literature discussed in the Introduction, the performance and yield were compared with the other materials listed in Table S9 [17,18,28,29,30]. Although different graphene derivatives were used with varying filler contents, our materials demonstrated the highest anti-corrosion properties, the longest immersion duration, and the smallest filler content.

Tafel polarization curves for four types of coatings are presented in Figure 7b. Specific data from a series of electrochemical measurements, including corrosion current (Icorr), corrosion potential (Ecorr), and corrosion rate (η), are listed in Table S4. Information about Icorr and Ecorr was derived from the intersection points of anodic and cathodic polarization curves. Generally, lower Icorr values or higher Ecorr values indicate better corrosion resistance [18]. For the pristine Q235, the corrosive current and corrosive potential were recorded at −631.36 mV and 9.40 × 10⁻⁶ A·cm⁻², respectively. These results suggest the presence of numerous micropores or microcracks on the surface or interior of Q235, allowing O₂, H₂O, and Cl⁻ to diffuse through these microchannels to the substrate surface, thus reducing its corrosion resistance. In comparison, the corrosion potential of the EG₅₀₀₀/C₁P_0.2EP coating shifted in a positive direction, indicating that its anti-corrosion effect primarily reflected the inhibition of the anodic corrosion reaction. The optimized size of nanographite in the coating could form a nano-network that covers the micropores or microcracks of the Q235 substrate, impeding the diffusion of the corrosive medium. As the nanographite addition increased, Ecorr increased and Icorr decreased (Figure S6c and Table S7). With the addition of 0.075 wt.% EG₅₀₀₀, Ecorr improved as the nanographite content increased, reaching a peak of −442.76 mV on EG₅₀₀₀/C₁P_0.2EP, thereby exhibiting exceptional anti-corrosion properties. This effect is attributed to the moderate nanographite being adsorbed into the gaps between nano-kaolin and the epoxy resin, which reduces the likelihood of the corrosive medium penetrating the coating interior and prolongs the corrosion pathway.

3.2.2. Barrier Properties

To assess the corrosion resistance of the EP substrates doped with different sizes of EGx for over 300 h in an accelerated corrosion environment, coatings including C₁P_0.2EP, EG₁₀₀₀/C₁P_0.2EP, EG₃₀₀₀/C₁P_0.2EP, EG₅₀₀₀/C₁P_0.2EP, and EG₈₀₀₀/C₁P_0.2EP were subjected to a neutral salt spray test (NSS). As shown in Figure S9, all coatings experienced damage to varying degrees. However, the pristine C₁P_0.2EP exhibited the worst anti-corrosion performance, showing apparent rust at the cross-section and separation from the Q235 steel. In contrast, while the EG₁₀₀₀/C₁P_0.2EP, EG₃₀₀₀/C₁P_0.2EP, and EG₈₀₀₀/C₁P_0.2EP coatings developed brown rusty products and blisters after 300 h, the EG₅₀₀₀/C₁P_0.2EP coating did not exhibit conspicuous corrosion features such as rust or blisters. Thus, the NSS tests indicated that EG₅₀₀₀/C₁P_0.2EP exhibited the best anti-corrosion performance among these coatings. Notably, among the various nanographite-doped EG₅₀₀₀/C₁P_0.2EP coatings, the one with 0.075 wt% EG₅₀₀₀ maintained the best appearance after 400 h, demonstrating superior anti-corrosion effects (Figure S10).

3.3. Theoretical Calculation and Anti-Corrosion Mechanism of EGx/C₁P_0.2EP

To elucidate the mechanism by which graphite fragments enhance the corrosion resistance of EGx/C₁P_0.2EP, theoretical calculations were conducted using density functional theory with the PBE form of the generalized gradient approximation functional (GGA). The graphite (001) surface was modeled by slicing graphite along the [001] direction, choosing a single-layer and a double-layer slab for analysis. In the geometry optimizations, all atomic positions were allowed to relax. A vacuum layer of 18 Å was applied along the c and a axes to prevent periodic interactions.

Given that O₂ and H₂O are the primary oxidants in the EGx/C₁P_0.2EP environment, their adsorption was investigated. As depicted in Figure 8 and Figure S11, the adsorption sites on single-layer (s-Gra) and double-layer graphite (d-Gra) at the body center and edge locations were studied. Physisorption was observed for both H₂O and O₂ at body center sites, with adsorption energies close to zero. However, at edge sites, chemisorption occurred for both O₂ and H₂O on s-Gra and d-Gra. Figure 9 shows that the chemisorption of H2O leads to the natural decomposition of the H-O bond, producing OH and H species on the edges of s-Gra and d-Gra. Compared with the H₂O (−0.95 eV) and O₂ (−0.11 eV) adsorption on the EGx/C₁P_0.2EP body center, the edges of s-Gra/d-Gra were more favorable (Figure 8). These results suggest that smaller graphene particles, which have a large edge surface, are more effective in attracting H₂O and O₂, reducing their potential to damage the coating and thus inhibiting the corrosion of EGx/C₁P_0.2EP by oxidation. This theoretical insight aligns well with our experimental findings.

4. Conclusions

To broaden the selectivity of graphite nanosheets as additives in epoxy resin and to verify their size effect, EGx was extensively incorporated into EP coatings. Micro-kaolin and PTFE were also uniformly integrated onto the surface to form hydrophobic long chains. EGx of various sizes was prepared using a cost-effective ball-milling and gradient centrifugation approach. Notably, the impedance arc of the EG₅₀₀₀/C₁P_0.2EP coating was the largest, registering an impedance of 2.58 × 10⁸ Ω·cm², indicating superior corrosion resistance compared with the EG₁₀₀₀/C₁P_0.2EP, EG₃₀₀₀/C₁P_0.2EP, and EG₈₀₀₀/C₁P_0.2EP coatings. Furthermore, even after 300 h of NSS testing, the EG₅₀₀₀/C₁P_0.2EP coating did not exhibit conspicuous corrosion features, demonstrating greater corrosion resistance than the other sizes of EGx coatings. The optimized size (2.21 μm) of EG₅₀₀₀ in the EG₅₀₀₀/C₁P_0.2EP coating forms a nano-network that covers micropores or microcracks, effectively impeding the diffusion of corrosive media. In addition, theoretical calculations revealed that the chemisorption of H₂O leads to the natural decomposition of H–O, producing OH and H species on the edges of s-Gra and d-Gra. Compared with the adsorption energies for H₂O (−0.95 eV) and O₂ (−0.11 eV) on EGx/C₁P_0.2EP, the edges of s/d-Gra are more favorable. These findings suggest that smaller graphene particles are more effective in attracting H₂O and O₂, thereby reducing their potential to damage the coating and inhibit the corrosion of EGx/C₁P_0.2EP through oxidation.