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

: In order to broaden the selectivity of graphite nanosheet additives on epoxy resin-based coatings and verify the size effect


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 − , H 2 O, and O 2 ) 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 selfcleaning 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 − , H 2 O, and O 2 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 Coatings 2024, 14, 769 3 of 11 (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.

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, Wottonunder-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.

Synthesis of the Different Sizes of Graphite Sheets
First, 4 g of graphite powder (500 mesh), 2 mm, and 0.2 mm ZrO 2 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 EG 1000 , 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 EG 3000 ; 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 EG 5000 and EG 8000 , respectively.

Preparation of EGx/C 1 P 0.2 EP 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% EG 5000 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 EG 5000 /C 1 P 0.2 EP.The EG 5000 /C 1 P 0.2 EP 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 EG 1000 /C 1 P 0.2 EP, EG 3000 /C 1 P 0.2 EP, and EG 8000 /C 1 P 0.2 EP were also fabricated using the same procedures by varying the additional amount of dimensional graphite sheets.In addition, hydrophobic epoxy coatings with 0 wt% C 1 P 0.2 EP, as well as 0.025, 0.04, and 0.075 wt% EG 5000 , were prepared to assess the effect of varying the EG 5000 content.

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, incorpo-Coatings 2024, 14, 769 4 of 11 rating 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 EG 1000 , EG 3000 , EG 5000 , and EG 8000 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 EG 1000 , EG 3000 , EG 5000 , and EG 8000 , 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 EG 5000 /C 1 P 0.2 EP contains few micropores, which hampers the diffusion of O 2 , H 2 O, and Cl − through these microchannels to the substrate and thereby considerably enhances its anti-corrosion properties.

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 ballmilling 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/C1P0.2EPcontains 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.

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, EG 1000 and EG 3000 exhibit a sharp (002) diffraction peak, indicating a highly regular spatial arrangement between lamellae [25].However, the (002) diffraction peaks for EG 5000 and EG 8000 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 EG 1000 , 0.11 for EG 3000 , and 0.17 for EG 5000 to 0.23 for EG 8000 , 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.

Structural Characterization of Samples and Coatings
The molecular structure of the EGx samples was comprehensively analyzed via Xray 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 (ID/IG) 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 ID/IG 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.

Structural Characterization of Samples and Coatings
The molecular structure of the EGx samples was comprehensively analyzed via Xray 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 (ID/IG) 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 ID/IG 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.

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/C 1 P 0.2 EP coatings.As shown inFigure 4a and Figure S5, the water contact angles of all four types of EGx/C 1 P 0.2 EP coatings are greater than 140 • within 60 days, indicating that these coatings have low surface energy and excellent hydrophobicity.The EGx/C 1 P 0.2 EP 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/C 1 P 0.2 EP decreased, but the water contact angle of the EG 5000 /C 1 P 0.2 EP coating remained the highest, indicating that EG 5000 /C 1 P 0.2 EP had the highest hydrophobicity and the lowest porosity.The significant decrease in the water contact angle for EG 8000 /C 1 P 0.2 EP coatings appears to be related to the size of the nanofiller (graphite nanosheets) in the resin; in fact, the smaller size of Coatings 2024, 14, 769 6 of 11 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/C 1 P 0.2 EP coatings with different EG 5000 contents as the soaking time increases.At the initial stage of immersion, the contact angles of the four coatings with different EG 5000 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 EG 5000 content, the contact angle also gradually increases, with the maximum angle of the EG 5000 /C 1 P 0.2 EP coating with 0.075 wt% EG 5000 exceeding 151 • , indicating that the 0.075 wt% addition of EG 5000 better filled the pores on the surface of the epoxy resin, reducing its porosity.
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/C1P0.2EPdecreased, but the water contact angle of the EG5000/C1P0.2EPcoating remained the highest, indicating that EG5000/C1P0.2EPhad the highest hydrophobicity and the lowest porosity.The significant decrease in the water contact angle for EG8000/C1P0.2EPcoatings 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/C1P0.2EPcoatings 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/C1P0.2EPcoating 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.

Electrochemical Characteristics
In this study, the impedance spectra of the EGx/C1P0.2EPcoatings 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/C1P0.2EPcoatings decreased over time, from 7 days to 60 days (Figure 5).After 7 days of immersion (Figure 5a), the impedance arc of the EG5000/C1P0.2EPcoating was the largest, with an impedance of 2.58 × 10 8 Ω•cm 2 , indicating superior corrosion resistance compared with EG1000/C1P0.2EP,EG3000/C1P0.2EP,and EG8000/C1P0.2EP.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 10 8 Ω•cm 2 to 10 7 Ω•cm 2 because of the penetration of corrosive electrolytes.However, after 60 days of immersion (Figure 5c), EG5000/C1P0.2EPstill exhibited the largest impedance arc among the coatings, with an impedance of 3.18 × 10 7 Ω•cm 2 .

Electrochemical Characteristics
In this study, the impedance spectra of the EGx/C 1 P 0.2 EP 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/C 1 P 0.2 EP coatings decreased over time, from 7 days to 60 days (Figure 5).After 7 days of immersion (Figure 5a), the impedance arc of the EG 5000 /C 1 P 0.2 EP coating was the largest, with an impedance of 2.58 × 10 8 Ω•cm 2 , indicating superior corrosion resistance compared with EG 1000 /C 1 P 0.2 EP, EG 3000 /C 1 P 0.2 EP, and EG 8000 /C 1 P 0.2 EP.This is attributed to the size of EG 5000 , 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 10 8 Ω•cm 2 to 10 7 Ω•cm 2 because of the penetration of corrosive electrolytes.However, after 60 days of immersion (Figure 5c), EG 5000 /C 1 P 0.2 EP still exhibited the largest impedance arc among the coatings, with an impedance of 3.18 × 10 7 Ω•cm 2 .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 EG 5000 /C 1 P 0.2 EP 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 EG 5000 /C 1 P 0.2 EP coatings with different doped EG 5000 contents are shown in Figure 6.After soaking for 7 days, the data in Table S6 indicate that the coating with 0.075 wt.% EG 5000 exhibited a greater electrochemical impedance (2.39 × 10 8 Ω•cm 2 ) compared with those with lower contents of EG5000-0.025wt.% (1.47 × 10 8 Ω•cm 2 ), 0.04 wt.% (1.96 × 10 8 Ω•cm 2 ), and 0.05 wt.% (2.18 × 10 8 Ω•cm 2 ).From 7 to 40 days, the permeation of electrolytes resulted in a decrease in resistance values for coatings with varying EG 5000 contents.However, after 40 days, the electrochemical impedance of the coating with 0.075 wt.% EG 5000 remained at 5.95 × 10 7 Ω•cm 2 .As EG 5000 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.
pared with those with lower contents of EG5000-0.025wt.% (1.47 × 10 8 Ω•cm 2 ), 0.04 wt.% (1.96 × 10 8 Ω•cm 2 ), and 0.05 wt.% (2.18 × 10 8 Ω•cm 2 ).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 × 10 7 Ω•cm 2 .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/C1P0.2EPcoating 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/C1P0.2EPcoating 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/C1P0.2EP.Notably, the phase angle of the EG5000/C1P0.2EPcoating approached 85°.These results demonstrate that EG5000/C1P0.2EPhydrophobic 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 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 EG 5000 /C 1 P 0.2 EP 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 EG 5000 /C 1 P 0.2 EP coating remained the highest after 60 days of immersion, maintaining the most stable anti-corrosive coating.This confirms that the appropriate size of graphite (EG 5000 ) 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 EG 1000 /C 1 P 0.2 EP.Notably, the phase angle of the EG 5000 /C 1 P 0.2 EP coating approached 85 • .These results demonstrate that EG 5000 /C 1 P 0.2 EP 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.
Coatings 2024, 14, x FOR PEER REVIEW 8 of 12 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 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 O 2 , H 2 O, and Cl − to diffuse through these microchannels to the substrate surface, thus reducing its corrosion resistance.In comparison, the corrosion potential of the EG 5000 /C 1 P 0.2 EP 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.% EG 5000 , Ecorr improved as the nanographite content increased, reaching a peak of −442.76 mV on EG 5000 /C 1 P 0.2 EP, 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.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/C1P0.2EPcoating 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/C1P0.2EP,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.

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 C 1 P 0.2 EP, EG 1000 /C 1 P 0.2 EP, EG 3000 /C 1 P 0.2 EP, EG 5000 /C 1 P 0.2 EP, and EG 8000 /C 1 P 0.2 EP were subjected to a neutral salt spray test (NSS).As shown in Figure S9, all coatings experienced damage to varying degrees.However, the pristine C 1 P 0.2 EP exhibited the worst anti-corrosion performance, showing apparent rust at the cross-section and separation from the Q235 steel.In contrast, while the EG 1000 /C 1 P 0.2 EP, EG 3000 /C 1 P 0.2 EP, and EG 8000 /C 1 P 0.2 EP coatings developed brown rusty products and blisters after 300 h, the EG 5000 /C 1 P 0.2 EP coating did not exhibit conspicuous corrosion features such as rust or blisters.Thus, the NSS tests indicated that EG 5000 /C 1 P 0.2 EP exhibited the best anti-corrosion performance among these coatings.Notably, among the various nanographite-doped EG 5000 /C 1 P 0.2 EP coatings, the one with 0.075 wt% EG 5000 maintained the best appearance after 400 h, demonstrating superior anti-corrosion effects (Figure S10).

Theoretical Calculation and Anti-Corrosion Mechanism of EGx/C 1 P 0.2 EP
To elucidate the mechanism by which graphite fragments enhance the corrosion resistance of EGx/C 1 P 0.2 EP, 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 O 2 and H 2 O are the primary oxidants in the EGx/C 1 P 0.2 EP 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 H 2 O and O 2 at body center sites, with adsorption energies close to zero.However, at edge sites, chemisorption occurred for both O 2 and H 2 O 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 H 2 O (−0.95 eV) and O 2 (−0.11 eV) adsorption on the EGx/C 1 P 0.2 EP 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 H 2 O and O 2 , reducing their potential to damage the coating and thus inhibiting the corrosion of EGx/C 1 P 0.2 EP by oxidation.This theoretical insight aligns well with our experimental findings.

Theoretical Calculation and Anti-Corrosion Mechanism of EGx/C1P0.2EP
To elucidate the mechanism by which graphite fragments enhance the corrosion resistance of EGx/C1P0.2EP,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/C1P0.2EPenvironment, their adsorption was investigated.As depicted in Figures 8 and 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/C1P0.2EPbody 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/C1P0.2EPby oxidation.This theoretical insight aligns well with our experimental findings.

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/C1P0.2EPcoating was the largest, registering an impedance of 2.58 × 10 8 Ω•cm 2 , indicating superior corrosion resistance compared with the EG1000/C1P0.2EP,EG3000/C1P0.2EP,and EG8000/C1P0.2EPcoatings.Furthermore, even after 300 h of NSS testing, the EG5000/C1P0.2EPcoating did not exhibit conspicuous corrosion features, demonstrating greater corrosion resistance than the other

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

Figure 1 .
Figure 1.The surface SEM of EG1000 (a), EG3000 (b), EG5000 (c), and EG8000 (d); the red frame represents the field of vision, and the blue frame represents the graphene nanosheet.Figure 1.The surface SEM of EG 1000 (a), EG 3000 (b), EG 5000 (c), and EG 8000 (d); the red frame represents the field of vision, and the blue frame represents the graphene nanosheet.

Figure 1 .
Figure 1.The surface SEM of EG1000 (a), EG3000 (b), EG5000 (c), and EG8000 (d); the red frame represents the field of vision, and the blue frame represents the graphene nanosheet.Figure 1.The surface SEM of EG 1000 (a), EG 3000 (b), EG 5000 (c), and EG 8000 (d); the red frame represents the field of vision, and the blue frame represents the graphene nanosheet.

Figure 4 .
Figure 4. (a) Water contact angle of EG x /C 1 P 0.2 EP coatings with different storage times.(b) Water contact angle of EG 5000 /C 1 P 0.2 EP with different EG 5000 contents.

Figure 8 .
Figure 8.The adsorption structures and energies of O2 and H2O at varying sites on single layers of graphite sheet.

Figure 8 .
Figure 8.The adsorption structures and energies of O 2 and H 2 O at varying sites on single layers of graphite sheet.Coatings 2024, 14, x FOR PEER REVIEW 10 of 12

Figure 9 .
Figure 9.The adsorption energies of O2 and H2O at varying sites on single and double layers of graphite sheet as well as EGx/C1P0.2EP.

Figure 9 .
Figure 9.The adsorption energies of O 2 and H 2 O at varying sites on single and double layers of graphite sheet as well as EGx/C 1 P 0.2 EP.