Improving activity and barrier properties of epoxy modified polyurethane coating with in-situ polymerized polypyrrole functionalized graphene oxide

The limited dispersibility and lack of active anti-corrosion properties of graphene oxide (GO) have hindered its application in corrosion resistance. In this study, we graft the corrosion inhibitor benzotriazole (BTA) on the surface of GO to achieve pH response release, providing active corrosion protection for epoxy modified polyurethane (EP-PU) coating. An environmentally friendly composite material with a sandwich-like structure was synthesized by in-situ polymerization of pyrrole to improve GO dispersion and prevent rapid release of BTA. The electrochemical impedance spectroscopy (EIS) shows that adding 0.15% GBP enhances the coating's anti-corrosion performance. The dual functions of passive and active corrosion enable the GBP/EP-PU coating to provide long-term corrosion protection for metal substrates.


Introduction
Carbon steel is a highly valued structural material in various industrial fields because of its exceptional performance [1][2][3][4].However, its susceptibility to extensive corrosion poses potential safety hazards and can lead to significant economic losses.The use of physical barriers to prevent direct contact between metal devices and corrosive substances can reduce corrosion [5,6].Polymer coatings are effective in mitigating or controlling corrosion, but exposure to harsh environmental conditions such as UV radiation, salt spray, water penetration, and temperature fluctuations can cause the coating properties to deteriorate.Coating defects such as pinholes, cracks, or detachment can allow the corrosive medium to penetrate the metal substrate [7][8][9].Incorporating fillers, anti-corrosion additives/pigments, into paint formulations is a recommended approach for creating durable protective systems with long-lasting corrosion resistance.Corrosion inhibitors can enhance coating performance by plugging corrosion areas with insoluble products and promoting self-healing effects [10][11][12].Various nanocontainers, including carbon-based nanostructures [13,14], mesoporous nanoparticles [15,16], layered double hydroxides [17,18], and polymer capsules [19,20], have been successfully incorporated into self-healing and active protective coatings.
Graphene's high specific surface area, excellent mechanical properties, high thermal stability, and good electrical conductivity have made it a focal point of scientific research and industrial applications [21].Graphene's high Young's modulus (1100 GPa) and tensile strength (130 GPa) make it a suitable material for constructing polymer composites with a wide range of industrial applications [22,23].Graphene oxide (GO), an important derivative of graphene, contains abundant oxygen functional groups, including hydroxyl, epoxy, and carboxyl groups [24].Common methods for surface modification of GO include organic molecular grafting, surfactant adsorption, inorganic nanoparticle decoration, and polymer encapsulation, such as polydopamine [25], polyaniline [26], and polypyrrole [27], etc.However, GO nanoparticles face challenges of low compatibility and poor dispersion within polymer substrates.Modifying GO sheets with nano pigments and polymer chains can enhance their dispersion quality and self-healing ability in epoxy coatings.Samiee et al. [28] developed a nontoxic nanocarrier, graphene oxide@polyaniline-praseodymium (III), for producing active/passive anticorrosive coatings.Controlled release of Pr 3+ under corrosive conditions and passivation of polyaniline form a stable oxide film that provides exceptional active protection for the silane coating.Lin et al. [29] used polyvinyl alcohol-functionalized graphene oxide as a nanocontainer for cerium oxide to enhance the anti-corrosion properties of polyvinyl butyral coating on carbon steel surfaces.Results demonstrate that incorporating graphene/polyvinyl alcohol/cerium hydroxide improves the long-term anticorrosion performance of the PVA coating and significantly enhances its self-healing capability against defects.Ye et al. [11] synthesized porous polyhedral oligomeric silsesquioxane (POSS)-grafted graphene nanocontainers for benzotriazole (BTA) and demonstrated that the compactness of 8-PG-BTA and BTA release provide excellent long-term and active corrosion resistance for the epoxy coating.
Conducting polymers (CPs) have emerged as a promising surface modifier for coatings in recent years [30,31].Polyaniline [26], polypyrrole [27], polyindole [32], and polythiophene [33] are increasingly used as surface modifiers due to their electrical conductivity, simple synthesis, and environmental stability.CPs have been widely studied in anti-corrosion research for their ability to reduce accumulated electrons on the substrate, stabilize the dissolution of passive film, and maintain the surface potential of the substrate.These properties enable CPs to function as a physical barrier, inhibiting the penetration of corrosive substances.Lu et al. [34] developed hydrophobic polypyrrole-hexagonal boron nitride nanocomposites (f-BNNs/PPy) and mixed them with epoxy resin to protect Q235 carbon steel in a 3.5 wt.% NaCl solution.The coating exhibited excellent hydrophobicity and impermeability, and the corrosion rate of the composite coating containing 0.5% f-BNNs/PPy was found to be 15 nm/year.
In this study, in-situ polymerization was used to prepare a new type of sandwich structure material, GBP, as a smart nanofiller to improve the passive and active properties of EP-PU coating.First, BTA was introduced onto the GO surface by chemical grafting to form a GO/ BTA composite (GB).Then, pyrrole was deposited on the surface of GB by in-situ polymerization to obtain GBP where the polypyrrole chain (PPy) maintains structural integrity for a long time.Finally, the sandwich structure enables corrosion inhibitor (BTA) to be stably stored in GBP.The obtained nanoparticles (NPs) were analyzed by XRD, FT-IR, X-ray photoelectron spectroscopy (XPS), FE-SEM, transmission electron microscope (TEM), and thermal gravimetric analyzer (TGA) techniques.Polarization (Tafel) and EIS were used to research the corrosion inhibition mechanism and performance of NPs in 3.5% NaCl solution.

Synthesis of GO
The modified Hummer's method [35] was used to fabricate GO nanosheets.In brief, a mixture of 120 mL concentrated H 2 SO 4 (98%) and 10 mL concentrated HNO 3 was prepared in a three-necked flask.Then, 5 g of graphite powder was slowly added to the solution and stirred for 1 h, followed by the slow addition of 30 g of KMnO 4 while controlling the system temperature below 3 °C.The oxidation reaction was allowed to continue for 6 h, after which the system temperature was raised to 95 °C.Next, 400 mL of deionized water was added to the mixture for dilution, and then 70 mL of H 2 O 2 (35%) solution was gradually added to complete the oxidation reaction.Finally, the resulting GO was washed with deionized water until pH = 7, and then freeze-dried.

Synthesis of GB
Initially, 0.01 g of GO was ultrasonically dispersed in 20 mL of deionized water.Then, 50 mL of an ethanol solution containing 1 g of BTA was added to the GO solution.The resulting solution was continuously stirred for 24 h at room temperature to ensure even dispersion of BTA on the GO nanosheets.

Synthesis of GBP
The GBP sandwich structure was prepared by in-situ polymerization of pyrrole monomer on the surface of GB nanosheets in the presence of APS.First, 0.58 mL of pyrrole monomer was added to 20 mL of 1 M HCl to obtain a homogeneous, bright yellow solution.The pyrrole monomer solution was gradually added to the solution containing GO/BTA nanosheets, followed by sonication for 10 min.Next, 0.36 g of APS was added to the product solution to initiate the polymerization process, which quickly turns the solution black.The final solution was stirred at room temperature for 24 h to complete the polymerization reaction.To remove impurities and unreacted monomers, the final solution was centrifuged and washed three times with deionized water at a speed of 8000 r min −1 for 5 min each time.The collected precipitate was dried at 60 °C for 24 h to obtain the GBP powder.The synthesis process of the GBP sandwich structure is illustrated in Fig. 1.

Synthesis of GBP/EP-PU coatings
First, GO, GB, and GBP (0.05%, 0.10%, 0.15%, 0.20%) were ultrasonically dispersed in 2.27 g of epoxy modified polyurethane resin and 1 g of curing agent, and then stirred thoroughly to ensure complete dispersion of the nanoparticles in the resin.Second, DMF was added as a viscosity modifier.The coating solution is dip-coated onto Q235 carbon steel, which had been pretreated by grinding with 600, 800, and 1200 emery paper, followed by degreasing with acetone.After drying at room temperature for 3 days, the coated specimen was pre-cured by heating at 90 °C for 12 h to ensure the completion of the curing process.The average dry film thickness of all coatings was controlled at 50 ± 5 µm.

Characterization techniques
To investigate the chemical properties of NPs, Fourier transform infrared spectroscopy (FT-IR) was performed using PerkinElmer's Spectrum 100, with a wavelength range of 4000-400 cm −1 .The crystal phase and structural information of the samples were studied using X-ray powder diffraction (XRD) with a diffractometer Fig. 1 Synthesis process of sandwich-like GBP structures (Rigaku TTR-III).The morphology of GB and GBP was examined using a field emission scanning electron microscope (FE-SEM, Thermo Fisher, USA, Helios NanoLab G3).The internal structure and element composition of GB and GBP were analyzed using transmission electron microscope (TEM, PHILIPS CM 200 FEG, 160 V).The thermal stability of the samples was evaluated using TGA from Mettler-Toledo instrument, heating NPs to 600 °C (starting from 25 °C) at a temperature increase rate of 10 °C min −1 under the continuous flow of N 2 gas.The XPS studies were conducted on the p5700 ESCA spectrometer with Al Kα radiation.Additionally, the adhesion strength of the coatings was determined via the pull-off test conducted on a PosiTest AT-M Manual Adhesion Tester.

Electrochemistry characterization
Before conducting the test, an artificial defect with a size of approximately 0.1 mm × 3 mm was created on the coating surface using a sharp knife.The electrochemical behavior of different coatings in a 3.5 wt.% NaCl solution was evaluated using an AUTOLAB (Metrohm PGSTAT302N) electrochemical workstation and a traditional three-electrode system comprising a Ag/AgCl electrode as the reference electrode, a platinum foil electrode as the counter electrode, and carbon steel with different coatings as the working electrode.The electrochemical impedance spectroscopy (EIS) test was performed under open circuit potential (OCP) control using NOVA 2.0 software.A sinusoidal disturbance with a frequency range of 10 5 ~ 10 -2 Hz and an amplitude of 20 mV was applied.The microscopic morphology of the rust layer was analyzed using a field emission scanning electron microscope (FE-SEM).

Analysis of GBP composite
The sandwich-like GBP structure is confirmed by XRD as depicted in Fig. 2(a), where the XRD pattern of GO shows a strong peak (001) at 2θ = 10.53° with an inter-layer spacing (d sp ) of 0.84 nm [36].This peak is attributed to the presence of oxygen-containing functional groups formed during the oxidation of graphite to produce GO.The covalent grafting of BTA molecules to GO flakes results in an increase in the interlayer spacing of GO.The diffraction peaks observed in GB at 8.4°, 15.06°, 16.86°, and 25.3° are characteristic peaks of BTA, indicating that the BTA molecules are inserted into the GO sheets.The XRD pattern of GBP exhibits amorphous diffraction characteristics due to the amorphous structure of PPy.This result indicates that PPy successfully covers the GB surface, forming the sandwich-like GBP structure.
Figure 2(b) shows the FT-IR analysis conducted to investigate the synthesis mechanism of GBP.The FT-IR spectrum of GO nanosheets exhibits five major absorption peaks that correspond to O-H, symmetric and asymmetric CH 2 , C = O, O = C-OH, and C-O-C bond stretching vibrations at 3415 cm −1 , 2968 cm −1 and 2883 cm −1 , 1649 cm −1 , 1388 cm −1 and 1056 cm −1 , respectively [37].The FT-IR spectrum of GB nanosheets shows a new absorption peak at 1211 cm −1 , which corresponds to the N = N double bond vibration in BTA [38].The FT-IR spectrum of GBP reveals the appearance of new absorption peaks at 3200-3600 cm −1 , 1515 cm −1 , 1253 cm −1 , 1090 cm −1 , which correspond to N-H and O-H stretching vibration, pyrrole ring symmetric vibration, C-N bond stretching vibration, and N-H in-plane deformation, respectively [30].These findings are consistent with XRD data.
As shown in Fig. 2(c), the survey XPS spectra of GB and GBP clearly show peaks corresponding to C 1 s, O 1 s, and N 1 s.The atomic content shown in Table S1 indicates a significant increase in the N/C and N/O atomic ratios (0.19 and 0.87, respectively) of GBP compared to GB (0.05 and 0.12, respectively).These results provide clear evidence that PPy is loaded onto GBP nanosheets, which is consistent with the XRD and FT-IR results.
Figure 2(d) displays the TGA curves of the prepared samples.BTA showed a comparatively steady state before 197 °C.At about 198-318 °C, BTA undergoes thermal decomposition and rapidly loses about 97% of its initial weight.GO NPs lose about 11% of their weight due to the evaporation of adsorbed water from 25-130 °C.Between 130-318 °C, the second weight loss of GO is owing to the decomposition of oxygen-containing functional groups.Upon loading BTA, the initial decomposition temperature of GB increases, and the weight loss at 194-318 °C is 31.68wt.%.The weight loss values of GB and GO suggest that the loading of BTA onto GO is 12.07 wt.%.The thermal properties of the GBP show a similar trend with GO NPs, indicating the growth of PPy NPs on the GO nanosheets and the enhancement of the thermal stability of the GO nanosheets.FE-SEM and TEM technique are utilized to investigate the morphology of GO, GB and GBP NPs (Fig. 3(a-f)), while EDS confirms the distribution of functional groups (Fig. 3(g-i)).Figure 3(a) and (d) illustrates that GO has a rough surface and a dark lamellar fold structure, indicating its potential as a high-quality supporting material.Figure 3(e) clearly shows the presence of BTA nanoparticles, providing evidence that BTA was successfully loaded onto the GO nanosheets.Figure 3(b) demonstrates that the surface morphology of GO remains unchanged after loading BTA nanoparticles, providing a basis for the preparation of sandwich-structured composites.FE-SEM and TEM images of GBP, Fig. 3(c) and (f), reveal that PPy is uniformly grafted onto the surface of GO, as shown in.The distribution of nitrogen in GBP nanomaterials (Fig. 3(h)) and the size of PPy and BTA nanoparticles also support the successful loading of PPy into the GBP nanomaterial.

Corrosion inhibition influence of GBP in solution
GO contains a large number of oxygen-containing functional groups and delocalized π electrons, and these unique structures make GO have strong adsorption capacity for BTA via hydrogen bond, ionic bond, covalent bond.When GB and GBP were added in 3.5% NaCl solution, BTA was released due to the differential concentration.To investigate the release process of BTA in GBP, UV-Vis was used to measure the absorption intensity of the dispersion at different pH conditions, details were shown in supporting information.The release amount of BTA is deduced based on these measurements (Fig. 4).After an immersion time of 14 h, the release concentration of BTA in GBP at pH values of 4, 7, and 10 is 76%, 72%, and 80%, respectively, suggesting that the release of BTA in GBP is pH-responsive [39].Moreover, the released ratio of BTA in GB is significantly higher than that in GBP under identical pH conditions, indicating that the release of BTA from GBP is controlled, and the capping of PPy on the GBP surface can act as pH-responsive gatekeepers.
To assess the solid-phase corrosion inhibition performance of GBP, 0.2 g of GB and GBP were dispersed in a 40 mL solution of 3.5 wt.% NaCl.Then, a bare carbon steel sample was immersed in the resulting solution (5 g L −1 ) and placed in a constant temperature oscillator at 25 °C for 24 h. Figure 5(a) shows optical and SEM images of carbon steel immersed in various dispersions for 4 h and 24 h.The corrosion inhibitory effect of the GBP dispersant on carbon steel is evident from the observed surface morphology.The potentiodynamic polarization test of carbon steel confirms this result after 24 h of immersion in various dispersants.The E corr value of GBP (-857.3mV) has significantly increased compared to bare steel (-988.0mV), and the I corr value has decreased from 790.76 to 233.92 μA⋅cm −2 , indicating that GBP exhibits corrosion inhibition properties.Figure 5(c) clearly shows that the cathode and anode branches in the GBP sample have changed compared to the blank sample, and the current density has decreased, indicating the inhibitor's inhibitory effect on corrosion.In addition, the conductive polymer PPy NPs increase the corrosion potential of steel and extend the diffusion length of the corrosive medium to the steel surface due to PPy's strong oxidation properties [30].Consequently, by transferring the potential of carbon steel from the active to the passive region, the anodic corrosion rate of carbon steel can be reduced.In addition, the galvanic corrosion occurs when GB is in contact with the substrate, while GBP plays a passivation and shielding role on the metal.Therefore, the concentration of corrosion inhibitor released by GB is higher than that of GBP, but the corrosion inhibition performance is worse.

Long-term anticorrosion performance of GBP/EP-PU
EIS records the anti-corrosion performance of EP-PU based composite coatings, with a blank EP-PU coating used as the comparison sample.After 50 days of immersion in the electrolyte, the results are presented as Bode diagrams of all specimens, as shown in Fig. 6.The impedance modulus at lower frequency (|Z| 10 mHz ) of the blank EP-PU coating decreases dramatically from 1.10 × 10 10 to 9.59 × 10 7 Ω cm 2 during the immersion period.It is noteworthy that after 5 days of immersion, there are two time-constants observed, which suggests that the corrosive ions penetrate the coating through inherent defects such as cracks or vacancy defects, thereby compromising the barrier ability of the coating.In the GO/EP-PU and GB/EP-PU coatings (Fig. 6(b, c)), the peak value observed in the range of 10 3 ~ 10 0 Hz represents the response of the coating Fig. 3 FE-SEM and TEM images of GO (a, d), GB (b, e), and GBP (c, f) nanosheets, (g-i) EDS map of GBP defect.After 5 days of immersion, the GO/EP-PU and GB/EP-PU coatings have higher |Z| 10 mHz values than the blank EP-PU coating.The GBP/EP-PU coating shows only one peak value (10 5 ~ 10 3 Hz) in the Bode diagram (Fig. 6(d-g)), with a low-frequency impedance value approximately 10 times that of GB/EP-PU, indicating excellent permeability and corrosion resistance.The low-frequency impedance of EP-PU coating with 0.15% GBP remains at 10 10 even after 50 days of soaking, which is much higher than that of the blank EP-PU coating.
Figure 7 shows the Nyquist plots of pure EP-PU and EP-PU coatings with fillers.The Nyquist diagram of EP-PU coating includes two typical capacitor circuits, represented by a semicircle at high frequency and a straight line at low frequency.The semicircle region reflects the charge transfer resistance and double-layer capacitance at the coating electrolyte interface, while the linear region corresponds to Warburg impedance.The diameter of the semicircle region in the Nyquist plot shrinks sharply as the immersion time extends, indicating severe destruction of the coating and reduced effectiveness for long-term anti-corrosion.The inferior corrosion protection of the GO/EP-PU coating may be attributed to galvanic corrosion between the carbon steel and GO nanosheets.When GBP is added to the resin substrate, the Nyquist diagram displays a semicircle.Generally, a larger high-frequency capacitance loop size indicates better corrosion resistance [40].Therefore, the addition of GBP provides a physical barrier for corrosion erosion, significantly improving corrosion resistance.Among the coatings tested, the one with 0.15% GBP has the largest capacitive arc and exhibits the best anti-corrosion ability.Additionally, it suggests that PPy can effectively interrupt the chemical interaction between GO sheets and the carbon steel matrix.
To further investigate the anti-corrosion properties of the different coatings, Zsimpwin software is used to fit the EIS data, with the equivalent circuit used for fitting shown in Fig. 8.The equivalent circuit includes R s , R c , Q c , R ct and Q dl , representing solution resistance, coating pore resistance, coating capacitance, charge transfer resistance, and double layer constant phase, respectively [41].The constant phase element (Q) is used to compensate for the deviation from the ideal capacitance behavior, and its exponent (n) indicates the degree of deviation from the ideal dielectric behavior [42].In addition, the Warburg element (Z w ) represents diffusion in the coating, leading to an increase in cathodic reactions.
Table S2-S8 show the fitting results and Fig. 9 illustrates the relationship between R c and R ct and immersion time.The physical barrier provided by the composite coating can be evaluated through changes in R c .A higher R c value indicates greater difficulty for corrosive media to penetrate the coating.The R c values for both EP-PU and GO/ EP-PU coatings are initially low and decrease gradually as the immersion time extends.As exhibited in Fig. 9(a), the R c value for the GBP/EP-PU coating is consistently higher than that of the other coatings throughout the soaking process, indicating its strong anti-corrosion properties.Moreover, the R ct value for both EP-PU and GO/EP-PU coatings decrease significantly with increasing immersion time.After soaking for 5 days, the R ct of the GB/ EP-PU coating starts to increase due to the release of BTA, while the R ct value of the GBP/EP-PU coating remains higher and more stable.Specifically, the R ct of EP-PU with 0.15% GBP is the highest due to the uniform distribution of polypyrrole on the graphene nanosheets, which extends the penetration path of corrosive media uniformly throughout the coating.
The water absorption and adhesion of coatings (pulloff testing, ISO 4624-2016) are critical for evaluating their anti-corrosive performance.Figure 10(a) illustrates that the water absorption process occurs in two stages.In the first stage, the corrosive medium passes through the coating quickly and fills the micropores.In the second stage, the coating becomes saturated.GBP/EP-EP has the lowest water absorption rate due to the high dispersion of GBP in the EP-PU coating.This dispersion creates an extended diffusion path for corrosive Coating adhesion to carbon steel is a critical factor in evaluating the effectiveness of organic coatings in preventing corrosion.Coatings that have excellent anticorrosion properties typically exhibit greater adhesion [43].As illustrated in Fig. 10(b), the addition of nanomaterials to the coating results in an increased adhesion trend.After immersion in 3.5 wt% NaCl for 7 days, the GBP/EP-PU coating exhibits the highest adhesion, with values of 4.17 MPa under atmospheric and 2.35 MPa under wet conditions, when compared to EP-PU and GB/EP-PU coatings.The voids in the EP-PU matrix are filled by the prepared GBP nanoparticles, which improves the compactness of the coating.These results demonstrate that the use of GBP NPs can effectively prevent the coating from peeling off the steel substrate, thereby proving the excellent protective properties of the composite coating.

Self-healing properties of GBP/EP-PU
To evaluate the self-healing properties of the coating, the coating samples was scratched and subjected to EIS analysis.Subsequently, the samples are immersed in 3.5 wt.% NaCl solution for varying periods.As illustrated in Fig. 11, the EP-PU, GO/EP-PU, GB/EP-PU, and GBP/EP-PU (0.05) coatings can be distinguished into two peaks.The peak at the middle frequency range of 10 0 ~ 10 3 Hz corresponds to the coating defect response, while the peak at the low frequency range of 10 -2 ~ 10 0 Hz corresponds to the metal corrosion response [44,45].The |Z| 10 mHz -time curve is presented in Fig. 11(h).It is evident from the curve that, during the initial stages of immersion, the |Z| 10 mHz value of the modified coating is higher than that of the pure EP-PU coating.This increase in impedance is mainly attributed to the addition of impervious nano-fillers.Over the next 12 h, the |Z| 10 mHz value of the modified coating begins to decline due to the penetration of the electrolyte.During the middle period of immersion (24-48 h), the curve of the GBP-modified coating shows an upward trend, potentially indicating inhibitor release and the healing process.As the immersion time prolongs, the |Z| 10 mHz of the GBP/EP-PU gradually decreases to a stable level, which corresponds to the development of a protective layer at the defect.

Mechanism discussion
Figure 12 illustrates the anti-corrosion mechanism of the GBP/EP-PU coating.Coating defects, such as microcracks and micro-pores that can arise during the preparation and use of the coating, can accelerate the penetration rate of corrosive electrolyte and induce local corrosion of the metal matrix.This happens due to the increased pathway for aggressive substances to penetrate into the coating substrate through these defects [46].At the anode, the metal undergoes oxidation and loses electrons, while the hydrolysis of metal ions leads to a decrease in the pH of the anode region.In the cathode region, oxygen is reduced to hydroxide ions, which increases the local pH.The GBP in the coating responds to changes in the pH value of the corrosion microenvironment when the coating is scratched.BTA molecules can be gradually released from the container and adsorbed onto the exposed metal surface, thereby increasing the corrosion potential of the metal and playing a crucial role in corrosion inhibition [47].The even distribution of nanoparticles within the epoxy-modified polyurethane coating can increase the diffusion path length and hinder corrosion.Moreover, the surface-modified graphene

Conclusions
In this article, a GBP nanocomposite with a sandwichlike structure was synthesized, which were analyzed by XRD, FT-IR, and XPS techniques to confirm their suitable form and successful modification.The performance of GBP as a corrosion inhibitor in salt solution was evaluated using polarization, EIS, and FE-SEM technology.The polarization curve shows that the addition of GBP to 3.5 wt.% NaCl solution decreased the corrosion current from 790.76 to 233.92 μA⋅cm −2 , indicating a significant limitation of the corrosion reaction.The EIS results indicate that after 50 days of immersion, the |Z| 10 mHz of the 0.15% GBP/EP-PU coating remains at 1.11 × 10 10 Ω cm 2 , which is two orders of magnitude higher than that of the pure EP-PU coating.Additionally, when the coating is damaged, GBP nanomaterials release the corrosion inhibitor BTA, providing additional active corrosion protection for the EP-PU coating.The |Z| 10 mHz of the 0.15% GBP/EP-PU coating increased after immersion for 4 h and did not decrease significantly during the entire immersion process, indicating the best active protection performance.Overall, the GBP/EP-PU coating exhibits dual functions of passive anti-corrosion and active selfrepair, highlighting its potential for practical anti-corrosion applications.

Fig. 4
Fig. 4 Release curves of BTA from the GB and GBP at different pH environments

Fig. 12
Fig. 12 Schematic representation of the corrosion or self-healing mechanisms for EP-PU coatings (a) and GBP/EP-PU coatings (b) in the event of surface damage