Enhancing the Corrosion Resistance of Passivation Films via the Synergistic Effects of Graphene Oxide and Epoxy Resin

Abstract

Silane-based passivation films have been widely utilized for corrosion protection in metal materials. In order to further improve the anticorrosion performance of the silane passivation film, this paper adopts the hydrolysis method to add graphene oxide (GO) to the silane coupling agent (3-(2,3-glycidoxy)propyltrimethoxysilane) (KH560). The synthesized KH560-GO passivation solution was then mixed with epoxy resin (EP) to prepare a silane composite passivation film layer (KH560-GO/EP) containing GO and epoxy resin. For comparison, EP and KH560-GO films were also prepared, and the corrosion performance of the composite film was compared with that of the single film. The structure of the KH560-GO film was characterized by X-ray diffraction analyzer (XRD) and infrared spectroscopy (FTIR). The microstructure of the composite film was analyzed by scanning electron microscopy (SEM), while its corrosion resistance was tested through polarization curves and electrochemical impedance spectroscopy (EIS). Additionally, neutral salt spray tests were conducted to evaluate the corrosion resistance of the samples, and rubber wiping tests were performed to assess the adhesion of the film. The results demonstrated that the KH560-GO/EP film exhibited a higher corrosion potential (E_corr) of −0.239 V compared to the EP and KH560-GO films, along with the lowest self-corrosion current density (I_corr) of 6.157 × 10⁻⁷ A/cm². These findings indicate that the KH560-GO/EP film possesses excellent corrosion resistance. The results showed that the corrosion potential (E_corr) of the KH560-GO/EP film was higher than that of EP and KH560-GO film layer is −0.239 V, and the self-corrosion current density (I_corr) is the smallest, which is 6.157 × 10⁻⁷ A/cm². The KH560-GO/EP film layer shows excellent corrosion resistance. Experiments show that the KH560-GO/EP passivated film has excellent bonding properties and corrosion resistance.

1. Introduction

Metal corrosion is a common form of material failure in industry [1]. Corrosion will not only lead to the damage to the metal facilities, but also affects the safety of engineering equipment [2]. Therefore, metal corrosion is an important problem to be solved urgently [3]. In order to extend the service life of metals, anti-corrosion treatment technology has become one of the key means. Traditional anti-corrosion methods include cathode/anode protection, anti-corrosion filler, corrosion inhibitor, and coating [4]. As a surface treatment method, passivation treatment has attracted much attention because of its simple, convenient, economical, and practical characteristics [5]. Many scholars have shown great interest in it, mainly due to its significant advantages in improving the corrosion resistance of materials [6]. Traditional passivation solutions mainly include strong oxidizing agents such as concentrated sulfuric acid, concentrated hydrochloric acid, and other strong oxidants [7]. However, the composition and process applicability of the passivation solutions are limited due to the different metal substrates [8]. Currently, acidic chromate passivation solutions are widely used because of their excellent film forming quality and simple operation process [9]. However, due to the hazards of hexavalent chromium ions to human health and the environment, its use has been strictly limited [10]. The current focus of research is organic passivation and organic-inorganic composite passivation. In terms of organic passivation, ionic liquids have received extensive attention due to their unique physical and chemical properties. Ionic liquid not only has the characteristics of non-volatilization and strong designability, but also can be effectively modified and passivated in a variety of environments [11]. For example, recent studies have shown that ionic liquids have shown great potential in polymer modification and electrochemical applications, where they can be used as catalysts or mediators to effectively improve reaction efficiency and selectivity while reducing environmental impact. Nevertheless, due to its high cost, complex production process, and lack of performance, the combination of organic and inorganic passivation is considered to be a more promising direction [12]. The passivation film’s quality is significantly improved by combining the performance of inorganic and organic materials, leading to improved corrosion resistance and bonding ability with the substrate of the film layer, thus achieving the objectives of green environmental protection, good corrosion resistance, and good bonding ability [13].

The silane coupling agent is an organosilicon compound [14]. Silanization treatment technology can effectively increase the corrosion resistance of metal materials [15]. Feng et al. [16] prepared silanized GO nanosheets with amino functional silanes by cathodic electrophoretic deposition. The nanosheets are then stacked in parallel on the metal surface. The optimized coating provides excellent protection. Jena et al. [17] prepared Hybrid Silanized Graphene Oxide (SGO) on 316L Stainless Steel (SS). It was found that the prepared hybrid silanized coatings exhibited excellent corrosion resistance. The corrosion current density is reduced by three orders of magnitude, suitable for marine applications [18]. However, in practical applications, silane films are often limited by their low thickness, and a single silane film cannot provide long-term or reliable protection for metals. Therefore, improving the durability and protective performance of silane films for metal substrates remains a critical challenge [19].

Graphene is a two-dimensional carbon layer structure material with excellent barrier properties. Despite its single atomic thickness, it exhibits excellent conductivity, mechanics, hydrophobicity, and oil resistance [20]. Currently, researchers have utilized graphene to develop coatings, primarily by leveraging the stacking effect between graphene layers to enhance the coating’s impermeability. Nevertheless, once the coating is damaged, the degree of corrosion of the metal will be significantly increased. In addition, the coating shows limited dispersion in solvents and is prone to problems such as re-stacking and agglomeration, which greatly limit its application range [21]. Therefore, researchers have functionalized graphene and synthesized a derivative of graphene—graphene oxide. The conductivity of graphene oxide has decreased, and the surface contains a large number of oxygen-containing groups. The presence of these oxygen-containing groups enhances the dispersion and compatibility of graphene oxide [22].

In order to achieve a green and environmentally friendly passivation film that combines the current process conditions, many researchers have begun to study organic-inorganic composite passivation, fully utilizing the unique layered structure of graphene oxide and its function of containing a large number of oxygen-containing groups. According to Tang et al. [23], graphene oxide possesses good barrier qualities and numerous active functional groups. They also comprehensively illustrated the “maze effect” as the mechanism by which composite coatings defend against metal corrosion. The main drawbacks of coating filler applications, they noted, are the poor dispersion in graphene oxide polymers and the propensity to agglomerate into nano fillers in composite coatings. Geng et al. [24] developed a graphene oxide (GO)-modified silane composite coating for corrosion protection consisting of isobutyltriethoxysilane and ethyl orthosilicate. They believe that the good corrosion resistance of the coating is due to the structure of the silane coupling agent coating formed by the covalent bond between GO and silanol, which improves the stability of the coating and thus delays the corrosion [25]. Although the researchers have done a lot of work on the preparation of green and environmentally friendly composite passivation film by GO, due to the special hierarchical structure of GO, the membrane layer also corresponds to holes while increasing the passage path of corrosives, resulting in the risk of corrosion of matrix materials [26]. In this paper, GO is added to the KH560 hydrolysate by the hydrolysis method [27]. Then, the synthetic KH560-GO passivation solution was mixed with epoxy resin to prepare a silane composite passivation film containing GO and epoxy resin [28]. The purpose of this composite film is to form a composite film by adding GO to improve the corrosion resistance of the passivation film, and, finally, add EP for secondary sealing to eliminate surface pores [29].

In this paper, we propose a novel dual-protection strategy by synergistically combining KH560-hydrolyzed silanol, GO, and epoxy resin (EP). The innovation lies in the following: (1) covalent bonding between silanol (Si–OH) and oxygen-containing groups of GO, which not only improves GO dispersion but also forms a dense Si–O–C network to block corrosive pathways; (2) secondary sealing with EP to eliminate surface defects (e.g., micropores) inherited from single-component films, thereby achieving a pore-free composite structure; (3) a green fabrication process that avoids toxic hexavalent chromium while ensuring strong interfacial adhesion through chemical bonding (Si–O–Fe) between the composite film and metal substrate. This approach addresses the limitations of existing GO/silane systems and provides a robust solution for long-term corrosion protection.

2. Experimental Procedures

2.1. Sample Preparation

The Q235 steel substrate is polished with 600 grit sandpaper to reduce surface corrosion and increase roughness. Residual oil on the substrate surface is removed using a 99.5% ethanol solution.

Mix 3-(2,3-glycidoxy)propyltrimethoxysilane, methanol (99.5%), and distilled water in a ratio of 1:1:3. The pH of the mixed solution was modified to 5 with acetic acid at a temperature of 35 °C [30]. The hydrolysis reaction was carried out for 12 h with magnetic stirring at a steady temperature to produce a silane hydrolyzed solution. In order to prepare the green passivation solution, silane hydrolysate (KH560), graphene oxide (GO, multi-layer, 3–5 layer, Nanjing XFNANO Material Technology Co., Ltd., Nanjing, China), and volatile organic solvent (methanol, 99.5%) were mixed at a mass to volume ratio of 10–50 mg:5–25 mL and functionalized at 35 °C for 4 h. After the reaction, the functionalized graphene oxide (KH560-GO) was obtained by vacuum filtration and drying at 60 °C for 12 h. KH560-GO was mixed with epoxy resin (E-51, Bisphenol A type, Nantong Star Synthetic Materials Co., Ltd., Nantong, China) at a mass-to-volume ratio of 1~5 mg:10~50 mL, and a stoichiometric polyamide curing agent (PA-651, 1:1 ratio to epoxy resin) was added to ensure proper crosslinking [31]. To prepare the KH560-GO/EP composite passivation film, the treated Q235 steel substrate was submerged in the composite passivation solution for 90 s before being removed and dried at 120 °C for 1 h [32]. The synthesis principle of the KH560-GO/EP composite passivation film is shown in Figure 1.

2.2. Surface Analysis

The surface and cross-sectional morphology of the specimen coatings were observed using a scanning electron microscope (SEM, Hitachi SU-8010, Japan). The spectra were recorded in the range of 4000–400 cm⁻¹ with a resolution of 4 cm⁻¹ using a FTIR spectrometer (KBr beamsplitter, PerkinElmer Spectrum 100, USA). Each sample was scanned 32 times at room temperature (25 °C) to optimize the signal-to-noise ratio. The crystal structure of the specimens was analyzed by X-ray diffraction (XRD, Shimadzu R-6000, Shimadzu Corporation, Japan) with an operating voltage of 40 kV, a current of 40 mA, a scanning speed of 5°/min, and a scanning angle range of 5° to 75°.

2.3. Anticorrosion Performance Test

2.3.1. Electrochemical Testing

This experiment was performed using an electrochemical potentiostat (Autolab84362, Metrohm Autolab, Switzerland) for electrochemical testing to verify the corrosion performance. All electrochemical impedance spectroscopy (EIS) measurements were conducted at open circuit potential (OCP) after 30 min of stabilization, with a frequency range of 1 × 10⁵ to 1 × 10⁻² Hz and 10 points per decade. A sinusoidal AC perturbation of 10 mV amplitude was applied. Each sample group (Q235 substrate, EP, KH560-GO, and KH560-GO/EP) was tested in triplicate to ensure reproducibility of the polarization curves and impedance data. A conventional three-electrode system was used, with a platinum electrode as the auxiliary electrode, a Saturated Calomel Electrode (SCE) as the reference electrode, and a thin film layer/substrate specimen as the working electrode [33]. The effective working area of the working electrode was about 1 cm², and the corrosion medium was a 3.5% NaCl (mass fraction) solution (200 mL) [34]. All tests were conducted at room temperature (25 ± 1 °C).

When evaluating the corrosion resistance of a material, the polarization curve reveals such a law: when the corresponding corrosion potential value increases, the corresponding corrosion current density will decrease, which usually means that the material or its surface film has a better corrosion resistance [35]. The corrosion potential and current density data of the coating system on the polarization curve were obtained by linear regression analysis, and the polarization resistance value of the coating was accurately solved by the Stern–Geary equation, so that the anti-corrosion efficiency of the coating system could be quantitatively characterized and evaluated [36].

$$R_p={\displaystyle \frac{K_a{K}_b}{2.303\left(K_a+K_b\right)I_{corr}}}$$

(1)

where (1): I_corr is the corrosion current density (A/cm²), the Tafel slopes K_a and K_b represent the reaction rate constants of the cathode and anode processes, respectively, and R_p is a measure of polarization resistance (Ω·cm²). CR stands for corrosion rate (mm/a), which is used to measure the corrosion rate of a material in a specific environment. CR is calculated by the following formula.

$$CR={\displaystyle \frac{{KM}_m{I}_{corr}}{n_{e⸱}{\rho }_m}}$$

(2)

where (2): K is defined as a fixed value of 3268.5, M_m represents the relative mass of a metal molecule in grams per mole (g/mol), n_e represents the number of electrons each molecule loses, ρ_m is the density of the metal (g/cm³), and CR is the corrosion rate (mm/year).

2.3.2. Neutral Salt Spray Test

Conduct a salt spray accelerated corrosion test with neutral NaCl solution by continuous spray according to GB/T 10125-1997, dissolve NaCl in deionized water or distilled water, and prepare a solution with a mass concentration within 50 ± 5 g/L. Hang the test sample in the center of the box, with a salt spray settling amount of (1–2 mL)/80 cm²·h. The spray pressure is within the range of 1.00 kgf/cm²~0.01 kgf/cm², the spray cycle is 48 h (continuous), and the junction between the box and the box cover is sealed with water [37]. Spray after the temperature and pressure of the test chamber reach 35 °C and 47 MPa, respectively [38]. After the experiment, rinse off the salt stains on the surface of the sample with distilled water. After the sample is naturally dried, the corrosion rate is calculated using the weight loss method [39]. Three independent replicates of each sample (uncoated Q235, EP, KH560-GO, and KH560-GO/EP) were subjected to 48 h salt spray exposure, and the corrosion rate was calculated as the average of triplicate measurements.

2.3.3. Adhesion Test of Passivation Film

Adhesion testing was conducted in accordance with GB/T 9791-2003 (Standard Test Method for Adhesion of Metallic Coatings). A sandless eraser was used to repeatedly rub the surface of the sample under a controlled pressure of 5 N/cm², with a friction frequency of 10 cycles (back-and-forth motion as one cycle) [40]. After rubbing, the sample surface is cleaned. A salt spray test is then performed on the sample, with the salt spray duration set to 48 h [41]. The corrosion performance of the passivation film was assessed through the weight loss analysis technique. This method involves measuring the mass reduction in the specimen after exposure to a corrosive environment, thereby providing quantitative data on the efficacy of the passivation layer in resisting corrosion [42]. The wiping test and subsequent salt spray evaluation were repeated three times on separate KH560-GO/EP-coated samples to confirm the consistency of adhesion performance.

3. Results and Discussions

3.1. Micromorphology

The surface and cross-sectional morphologies of the EP, KH560-GO, and KH560-GO/EP films are shown in Figure 2. As can be seen from Figure 2a–d, the EP coating exhibits a rough and porous surface, indicating the presence of numerous micropores. These micropores are formed due to heat release during the curing process of the epoxy resin, which results from solvent evaporation. In Figure 2, the KH560-GO membrane shows a relatively homogeneous layered structure with a small amount of aggregation, and the layered coatings are stacked parallel and closely arranged. The coating showed a relatively uniform layered structure with a small amount of aggregation. The close arrangement of the layered structure can effectively cut off the path of the corrosive medium into the matrix and improve the corrosion resistance of the Q235 steel matrix, from the surface morphology of the KH560-GO/EP coating shown in Figure 2c–f, the surface of the KH560-GO/EP coating is uniform and dense, with no visible pores or defects, demonstrating excellent surface properties. It is evident that GO and EP fill cracks, pores, and other surface defects. The high specific surface area and thinness of the nanofiller enable it to bridge gaps within the coating effectively, forming a dense interlayer structure.

The interface between the coating and the substrate is not obvious, and there is a convex anchor point protruding, indicating that the bonding strength between the coating and the substrate is high. There is a van der Waals force between the graphene oxide sheets, and in the composite coating, large-scale lamellar accumulation can be avoided by controlling the dispersion conditions, and an orderly multi-layer distribution structure can be formed. This layered structure helps to improve the mechanical strength of the coating. From the above analysis results, it can be seen that the significant increase in the interlayer spacing of GO after silane modification significantly enhances the dispersion of GO, and the epoxy resin can effectively fill the surface gap. Therefore, the SEM surface morphology of the coating presents a smooth and flat surface.

3.2. FTIR Analysis

Figure 3 shows the presence of organic matter in the graphene oxide and KH560-GO film layers, respectively. The presence of organic components in the graphene oxide silane epoxy films was detected by infrared spectroscopy.

The FTIR spectra confirmed covalent bonding between GO’s oxygen groups (e.g., C=O at 1723 cm⁻¹) and silanol (Si–OH), forming Si–O–C linkages (1022 cm⁻¹). The GO, characterized by a multilayer structure (3–5 layers) with an interlayer spacing of 1.17 nm (XRD), exhibited high oxygen content. These properties enhanced dispersion stability and interfacial bonding within the silane matrix. In Figure 3b, the absorption peaks at 1104 cm⁻¹ and 1022 cm⁻¹ are attributed to Si-O-Si and Si-O-C bonds, respectively. Indicating the presence of hydroxyl, carbonyl, and epoxide in GO. The peak at 1050 cm⁻¹ is characteristic of the epoxy group in graphene oxide, while the peak at 1723 cm⁻¹ corresponds to C=C bonds, representing the unoxidized graphene domain structure. Figure 3b shows that the absorption peaks of 1104 cm⁻¹ and 1022 cm⁻¹ are the characteristic peaks of Si-O-Si and Si-O-C, respectively. The peak of Si-O-Si illustrates the dehydration condensation reaction of KH560 after hydrolysis, and the characteristic peak of Si-O-C indicates the covalent bonding of KH560 with the epoxy group of GO. Compared to curve (a), curve (b) shows weaker peaks at 1723 cm⁻¹ and 1410 cm⁻¹, demonstrating the successful modification of GO by KH560. Additionally, new peaks appear at 2933 cm⁻¹ and 2888 cm⁻¹, corresponding to the -CH groups of KH560. Infrared spectroscopy results show that a large amount of silanols (Si-OH) are generated by the hydrolysis of silane, and part of the Si-OH itself undergoes dehydration condensation reaction to generate Si-O-Si bonds with a network or cyclic polymer structure. A large amount of the Si-O-Si bond forms a dense silane film that wraps the Q235 steel substrate; part of the Si-OH is bonded to the oxygen-containing groups of GO to form Si-O-C bonds. A large number of Si-O-C bond layers cover the Q235 steel substrate and also fill the micropores on the surface of the silane film. Some Si-OH groups interact with iron atoms on the surface of Q235G steel and form Si-O-Fe chemical bonds, which strengthen the bonding force between the composite film and the surface of the matrix.

3.3. XRD Analysis

Figure 4 shows the XRD results of GO and KH560-GO. Both samples exhibit relatively strong diffraction peaks at 2θ = 11.5° and 7.44°. Using the Bragg equation (2dsinθ = nλ), the crystal plane spacing (d) was calculated, where θ is the angle of incidence, n is the diffraction order, and λ is the wavelength of the incident wave. The calculated crystal plane spacing for the (001) plane of GO is d_GO = 0.76 nm. After modification with KH560, the crystal plane spacing of the (001) plane increases to d_KH560-GO = 1.17 nm. This result indicates that silane molecules have been successfully inserted between the graphene oxide layers, effectively increasing the interlayer distance. As a result, the diffraction peak corresponding to the (001) plane shifts to a lower angle (leftward) in the XRD pattern.

3.4. Electrochemical Analysis

3.4.1. Electrochemical Polarization Curve Analysis

The Tafel curves for the Q235 steel substrate, EP, KH560-GO, and KH560-GO/EP coatings are shown in Figure 5, and the corresponding corrosion rates are presented in

Table 1. Tafel test results of different samples.

	Ecorr (V)	Icorr (A/cm2)	Rp (Ω·cm2)	CR (mm/year)	Ba (mV/dec)	Bc (mV/dec)
Q235 steel substrate	−0.499	5.030 × 10−5	1.765 × 10−4	7.372 × 10−3	120	−85
Epoxy resin coating	−0.391	3.609 × 10−6	4.191 × 10−4	1.12 8 × 10−3	115	−78
KH560-GO	−0.315	5.855 × 10−6	2.844 × 10−4	1.918 × 10−3	105	−70
KH560-GO/EP	−0.239	6.157 × 10−7	6.876 × 10−3	7.370 × 10−4	95	−65

. It can be observed that the corrosion potentials of the EP, KH560-GO, and KH560-GO/EP film layers are higher and shifted positively compared to the Q235 steel substrate. This indicates that these film layers inhibit the anodic reaction and enhance anodic polarization, acting as anodic corrosion inhibitors and providing anodic protection for the Q235 steel. Among the film layers, the KH560-GO/EP composite film exhibits the highest corrosion potential (−0.239 V), the lowest self-corrosion current density (6.157 × 10⁻⁷ A/cm²), and the largest polarization resistance (6.876 × 10⁻³ Ω·cm²). The corrosion current density of KH560-GO/EP composite film in this study (6.157 × 10⁻⁶ A/cm²) was significantly lower than that of Dun et al. [25]’s silane/GO composite coating (8.2 × 10⁻⁶ A/cm²), indicating that the secondary sealing effect of epoxy resin effectively reduces the coating defects’ improved densification.

Its corrosion rate is an order of magnitude lower than that of the other films, demonstrating superior corrosion resistance. This enhanced performance is attributed to the denser and more uniform structure of the GO-modified silane coating, which has an increased thickness. The FTIR results reveal that the increased interlayer spacing and the interlacing of multiple GO layers extend the diffusion path and time for corrosive media to reach the metal substrate, thereby inhibiting base metal corrosion. Furthermore, the combined XRD and FTIR results indicate that the Si-O-Si bonds in the silane interact with GO after the disappearance of the OH groups, reducing the hydrophilic energy of KH560-GO. This reduction minimizes surface redox reactions and decreases the corrosion rate on the metal surface. The incorporation of epoxy resin provides a dual protection mechanism for the film layer. It not only fills and seals the micropores within the film but also significantly reduces the number of defects. This process enhances the film’s resistance to the penetration of external corrosive media, effectively preventing erosion damage to the metal surface.

3.4.2. Electrochemical Impedance of Film Layer (EIS)

Electrochemical impedance (EIS) testing is an indispensable means of evaluating the corrosion resistance of coated films. The electrochemical information of the coating surface and protected substrate can be reflected by the Nyquist diagram and Bode diagram. In the Bode diagram, the high frequency region reflects the impedance of the coating itself, while the low frequency region represents the impedance value (Z_f) of the solution/metal interface. The total impedance value in the low frequency range is often used as an indicator of the corrosion resistance of a metal surface. In this study, the low-frequency impedance value is employed as a key parameter to assess the corrosion resistance of the composite film. Figure 6 shows the electrochemical AC impedance curves of EP, KH560-GO, and KH560-GO/EP coatings under specific conditions. Figure 6a shows the Nyquist plot, and Figure 6b shows the Bode plot. In the Nyquist plot, each coating exhibits a semicircular capacitive arc. The radius of the capacitor arc is inversely proportional to the corrosion rate, and a larger radius means a lower corrosion rate and better corrosion resistance. The epoxy resin (EP) coating has the smallest capacitive arc radius. Combined with the SEM results, the rough and porous surface of the EP coating allows corrosive media to penetrate through the pores and reach the coating/metal interface, leading to corrosion. In contrast, the Nyquist plot shows an increase in the capacitive arc radius for the KH560-GO and KH560-GO/EP coatings. This improvement is attributed to the shielding effect of the layered structure of graphene oxide (GO), which hinders the penetration of corrosive media.

As shown in

Table 2. EIS fitting parameters.

	Rct (Ω·cm2)	Cdl (F/cm2)	n	|Z| 0.01 Hz (Ω·cm2)
Q235 steel substrate	1.2 × 103	2.5 × 10−5	0.78	8.6 × 102
Epoxy resin coating	5.8 × 103	1.3 × 10−6	0.82	9.2 × 103
KH560-GO	1.5 × 104	6.4 × 10−7	0.85	2.7 × 104
KH560-GO/EP	6.9 × 104	2.1 × 10−7	0.89	9.4 × 104

, the Rct value of KH560-GO/EP coating (6.9 × 10⁴ Ω·cm²) was significantly higher than that of other samples, indicating that its charge transfer resistance was the highest and the penetration path of corrosive medium was effectively blocked. In addition, the Cdl value (2.1 × 10⁻⁷ guess F/cm²) is the lowest, indicating that the coating/metal interface double layer capacitance decreases, and further verifies the compatibility of the composite film. The dispersion coefficient (n) is close to one, indicating that the coating is approximately ideal capacitive behavior with fewer defects. The low-frequency impedance modulus (|Z| 0.01 Hz) is as high as 9.4 × 10⁴ Ω·cm², which is consistent with the polarization curve and salt spray experiment results, which fully proves the cooperative protection mechanism of the composite film.

The oxygen-containing functional groups in the silane hydrolysate can functionalize the surface of graphene oxide (GO), increasing the interlayer spacing and improving its dispersion. The sealing effect of the epoxy resin further enhances the corrosion resistance of the passivation solution and insulates the corrosive medium (such as Cl⁻, H₂O, O₂, etc.) from the interior of the coating and the metal surface, improving the densification and corrosion resistance durability of the passivation film. In the Bode diagram, the KH560-GO/EP coating shows the largest impedance value (9.3856 × 10³ Ω·cm²), further indicating that the combination of graphene oxide and epoxy resin enhances the corrosion resistance of the coating.

3.5. Analysis of Neutral Hydrochloric Acid Experiment

The neutral salt spray corrosion time was set to 48 h. The corrosion test results are provided in

Table 3. Neutral salt spray corrosion test results.

	Quantity Before Corrosion (g)	Quality After Corrosion (g)	Area (cm2)	Time (h)	Rate (g/(m2/h))
1	12.997	12.942	0.08	48	1.432 ± 0.121
2	11.846	11.799	0.08	48	1.224 ± 0.098
3	12.769	12.755	0.08	48	0.365 ± 0.034

(1 is the oxidized graphene passivation film layer; 2 is the oxidized graphene silane passivation film layer; 3 is the oxidized graphene silane epoxy resin composite passivation film layer).

According to the data in Table 3, the corrosion rate of the GO silane epoxy composite passivation film (sample 3) after 48 h of neutral salt spray exposure (0.365 g/(m²/h)) was significantly lower than that of the single component GO coating (sample 1, 1.432 g/(m²/h)) and GO silane passivation film (sample 2, 1.224 g/(m²/h)), which is attributed to the “maze effect” of graphene oxide, the chemical bonding of silanes and the synergistic effect of epoxy resin pore filling. The salt spray resistance of the composite film is also better than similar coatings reported in the literature: compared with the silanized GO coating of Jena et al. [17] (1.8 g/(m²/h)) and the hydrophobic silane/GO composite coating of Chen et al. [27] (0.72 g/(m²/h)), the corrosion rate decreased by 79.7% and 49.3%, respectively. The effectiveness of the epoxy resin’s secondary sealing process by extending the diffusion path of corrosive medium and filling the microhole defects was verified.

3.6. Adhesion Analysis of Passivation Film

The composite passivation solution, consisting of graphene oxide, silane, and epoxy resin, was used to form a passivation layer. This layer was then wiped ten times back and forth under a controlled pressure of 5 N/cm² (GB/T 9791-2003). We conducted neutral salt spray testing on the graphene oxide silane epoxy resin passivation film sample that had not been cleaned and the sample that had been cleaned. The experimental findings are displayed in

Table 4. Passivated film adhesion wiping experiment.

	Quantity Before Corrosion (g)	Quality After Corrosion (g)	Area (cm2)	Time (h)	Rate (g/(m2/h))
unwiped	12.77	12.755	0.08	48	0.365 ± 0.029
wiped	12.77	12.741	0.08	48	0.729 ± 0.067

.

According to the data in Table 4, after 48 h of neutral salt spray test, the corrosion rate of the composite passivated film substrate (0.729 g/(m²·h)) after wiping was still significantly lower than that of the unpassivated galvanized sheet (1.432 g/(m²·h)), and the corrosion rate decreased by about 50.7%, indicating that the composite film was not completely stripped and had excellent adhesion to the substrate. This result is consistent with the study of Pourhashem et al. [42], which pointed out that the silane coupling agent significantly improves the interface bonding strength by enhancing the chemical bonding between GO and the substrate, thus maintaining the integrity of the coating after mechanical wiping.

4. Exploration of Anti-Corrosion Mechanism

The more defects on the surface of the metal coating, the faster the corrosive medium diffuses into the coating, which will lead to increased corrosion of the substrate. This paper mainly adopts the method of increasing the curvature of the diffusion path to effectively prevent the corrosion of the corrosive medium to the metal matrix, so as to obtain good corrosion resistance.

Figure 7 illustrates the corrosion protection mechanism of the substrate, EP, and KH560-GO/EP composite film layer. The corrosive medium (H₂O, O₂, Cl⁻) can readily flow through the EP film layer through the micropores on its surface and react chemically with the metal surface, causing the substrate to corrode. The corrosion resistance of the coating material is directly impacted by the dispersion homogeneity of EP and GO. The KH560-GO thin film layer shows a dense, staggered layer of protective metal on the substrate surface called a “labyrinth” due to the excellent specific surface area, stable chemical properties, and ultra-thin thickness of graphene oxide. The protective layer effectively blocks the passage through which the corrosive medium penetrates the metal, significantly postponing the time required for the corrosive medium to reach and be used in the substrate. Meanwhile, the oxygen-containing functional groups in the silane hydrolysis solution can functionalize the surface of GO to form functionalized graphene oxide. The secondary sealing effect of the epoxy resin, when combined with graphene oxide, can protect the Si-O-M covalent bond that has formed on the metal surface and improve the corrosion resistance of the passivation solution, thereby increasing the passivation film’s durability and density.

5. Conclusions

In this paper, EP film, KH560-GO film, and KH560-GO/EP film were successfully prepared by the hydrolysis method. The morphology of the films was observed, and their electrochemical properties were analyzed and the results showed the following:

(1)

By comparing the GO analysis with the SEM analysis of the surface morphology of the composite film layers, it was evident that the surface of the KH560-GO/EP film layer is smooth, without noticeable small pores or GO aggregation. This result is significantly better than those observed for the pure EP, GO, and KH560-GO film layers;

(2)

GO contains a large number of functional groups. The characteristic absorption peaks of these functional groups confirm the successful introduction of GO into the composite material, where it bonds with Si-OH generated by silane hydrolysis to form Si-O-C. The Si-O-C structure covers the surface of the matrix and fills the micropores, thereby enhancing the corrosion resistance;

(3)

The KH560-GO/EP film layer exhibits significantly superior anti-corrosion performance compared to both the EP and KH560-GO film layers. The KH560-GO/EP film layer has the highest polarization resistance (6.8763 × 10³ Ω·cm²) and the lowest corrosion rate (7.3702 × 10⁻⁴ mm/a). This improvement is primarily attributed to the formation of the “labyrinth effect” in graphene oxide, which isolates the pathways for corrosion medium invasion into the substrate and delays the penetration of the corrosion medium;

(4)

The average corrosion rate of the KH560-GO/EP film layer is 0.365 g/(m²/h), which is significantly lower than those of the GO and KH560-GO film layers. This result demonstrates the excellent corrosion resistance of the KH560-GO/EP composite film layer. After wiping, the average corrosion rate of the KH560-GO/EP film layer increases to 0.729 g/(m²/h). These findings suggest that the wiped composite coating sample retains a certain level of corrosion resistance, indicating that the composite passivation film has not completely peeled off.