Preparation and Performance Study of Carboxy-Functionalized Graphene Oxide Composite Polyaniline Modified Water-Based Epoxy Zinc-Rich Coatings

Abstract

Graphene oxide is obtained by oxidation of graphite followed by ultrasonic exfoliation. It is a two-dimensional layered material with a large number of oxygen-containing functional groups on its surface. Polyaniline is a conductive polymer and has a unique corrosion protection mechanism. In this study, carboxy-functionalized graphene oxide/polyaniline (CGO/PANI) composites with a lamellar structure were prepared by in situ polymerization. The lamellar layer was used to form a labyrinthine structure in the coating to effectively retard the penetration of corrosive media. The electrical conductivity of polyaniline can promote the formation of conductive pathways between zinc particles and improve the utilization of zinc powder. Polyaniline is also able to passivate the substrate, further improving the coating’s ability to protect steel substrates against corrosion. In this paper, the in situ polymerization of aniline on carboxy-functionalized graphene oxide flakes was confirmed by scanning electron microscopy (SEM), Fourier infrared spectroscopy (FT-IR), and X-ray diffraction (XRD), and the improvement of the corrosion resistance of the prepared composites on the epoxy zinc-rich coatings was evaluated by SEM, electrochemical impedance spectroscopy (EIS), and salt spray resistance tests. The results showed that aniline was successfully polymerized in situ on carboxy-functionalized graphene oxide, and the modified coating had significantly improved anticorrosive properties, where the best anticorrosive improvement was achieved when CGO: PANI = 0.03.

1. Introduction

With the increasing awareness of environmental protection in various countries and regions of the world, it is cause for concern that the traditional oil-based zinc-rich coating contains high volatile organic compounds (VOC), which can hurt human health and the environment. To ensure its anti-corrosion effect, a large amount of zinc powder needs to be added as a sacrificial anode, which can bring a series of defects such as porous coating and poor denseness [1]. In addition, because the corrosion products (zinc oxides) are not electrically conductive, the metal substrate and zinc particles cannot form a conductive pathway at a later stage, which affects the utilization of the remaining zinc powder [2,3,4,5]. Therefore, the modification of traditional epoxy zinc-rich primers has important research significance.

Among many modification schemes, the addition of conductive carbon materials (graphite powder [6], carbon nanotubes [7,8,9], graphene derivatives [4,10,11,12], nanocrystalline diamond [13], etc.) to epoxy zinc-rich paints has received extensive and continuous attention. Graphene is a two-dimensional layered material with high specific surface area, high electrical conductivity, high strength, and other excellent properties [14,15,16]. If a small amount of graphene is evenly dispersed in the coating, it can form a labyrinth-like structure, which effectively slows down the invasion path of corrosive media, and its excellent electrical conductivity can also promote the formation of conductive pathways between zinc particles in the epoxy zinc-rich primer and improve the utilization of zinc powder [17]. However, the large surface energy of graphene, which is not easily dispersed in water and organic solvents, has limited the application of graphene in the field of corrosion protection. Graphene oxide (GO) is a derivative of graphene, and the surface of the flake layer has hydrophilic functional groups such as carboxyl, hydroxyl, and epoxy groups, which makes the dispersion of graphene oxide in water excellent [18]. However, graphene oxide no longer has a conjugated structure, and the strong hydrophilicity also makes it easier for corrosive media to invade the coating; thus, graphene oxide should not be used directly. Graphene oxide sheet layers are rich in functional groups, so graphene oxide can be modified. Shen et al. [19] grafted highly conductive MXene (Ti₃C₂) on graphene oxide nanosheets, and open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and salt spray tests confirmed the effective cathodic protection of epoxy zinc-rich coatings (EZRC) containing 0.5 wt% Ti₃C₂ or GO–Ti₃C₂. Polyaniline is a conductive polymer with rigid molecular chains formed by the polymerization of aniline, which has many applications in chemical sensors, photovoltaic cells, and corrosion devices because of its excellent electrical properties, chemical environmental stability, and simple preparation process; however, the rigid molecular chains of polyaniline have poor solubility and need further modification [20]. Akbarinezhad et al. [21] used a supercritical CO₂ process to layer exfoliate Cloisite 30B nano clay. The fast-mixing polymerization of aniline monomers in supercritical CO₂ prepared exfoliated polyaniline/clay nanocomposites with high barrier properties, which improved the anticorrosive properties of the coatings. Lei et al. [22] synthesized polyaniline by using graphene as an internal template and applied it to an epoxy-rich zinc coating to monitor the variation of free corrosion potential of the coatings with time and the local electrochemical impedance spectrum. The results showed that the coating mixed with 0.6% graphene compounded with polyaniline exhibited the best anti-corrosion performance.

Graphene’s two-dimensional layer structure can form a labyrinth-like structure in the coating, which prolongs the path the corrosion medium must take to invade the surface of the substrate. The conductive property of polyaniline doping can enhance the electrical contact between zinc powder particles in the coating and improve the utilization rate of zinc powder. In addition, numerous studies [20,23,24,25,26] have shown that polyaniline also has a unique anti-corrosion mechanism, which can passivate the surface of metal substrates and effectively retard their corrosion. As mentioned earlier, graphene oxide is rich in oxygen-containing functional groups, of which epoxy and hydroxyl groups are located inside the lamellae and carboxyl groups are located at the edge of the lamellae [15]. In many studies [27,28,29], the modification of graphene oxide only utilizes the carboxyl groups at the edge of the lamellae, while a large number of oxygen-containing functional groups in the middle of the lamellae are retained; these oxygen-containing functional groups are hydrophilic and hurt the anti-corrosion performance of the coating.

In this paper, we prepared carboxy-functionalized graphene oxide (CGO), converted the oxygen-containing functional groups in the middle of the lamellae to carboxyl groups as much as possible, and then prepared carboxy-functionalized graphene oxide composite polyaniline (CGO/PANI) by in situ polymerization. The prepared CGO/PANI was used to modify the water-based epoxy zinc-rich coatings. Scheme 1 is the schematic diagram of the preparation process. In some studies [30,31,32], it has been shown that the complexation of polyaniline with graphene oxide is achieved through the carboxyl groups on the graphene oxide sheet layer. The graphene oxide functionalized with carboxyl groups was able to produce a consistent affinity for aniline to expect a more ordered composite structure [33]. The structures of the prepared CGO/PANI composites were characterized using scanning electron microscopy (SEM), Fourier infrared spectroscopy (FT-IR), and X-ray diffraction (XRD). The modified epoxy zinc-rich coatings with composite ratio CGO: PANI = 0.01~0.05 were prepared by spraying technique. Electrochemical impedance spectroscopy (EIS) and kinetic potential polarization techniques were used to study the anticorrosion properties of the coatings. In this study, the modified epoxy zinc-rich coating with the composite ratio of CGO: PANI = 0.03 had the best anti-corrosion performance.

2. Materials and Methods

2.1. Materials

Aniline, propylene glycol methyl ether, propylene glycol phenyl ether: analytical grade, Shanghai Aladdin Bio-Chem Technology Co., Ltd. Shanghai, China; graphite powder (825 mesh): purity ≥ 99%, Shanghai Macklin Biochemical Co., Ltd., Shanghai, China; concentrated sulfuric acid (H₂SO₄), concentrated hydrochloric acid (HCl), concentrated phosphoric acid (H₃PO₄), potassium permanganate (KMnO₄), 30% hydrogen peroxide (H₂O₂): analytical purity, Guangzhou Chemical Reagent Factory, Guangzhou, Guangdong Province, China; oxalic acid (C₂O₄H₂), hydrobromic acid (HBr): analytical purity, Shanghai Macklin Biochemical Co., Ltd.; ammonium persulfate (APS): analytical purity, Damao Chemical Reagent Factory, Shanghai, China; bisphenol A water-based epoxy resins, amine curing agent: industrial grade, Hexion Chemical Enterprise Management (Shanghai) Co., Ltd., Shanghai China; Anti-flash rust agent ES-150: industrial grade, Yueyang Kaimen Water-based Auxiliary Co., Ltd., Yueyang, China.

2.2. Preparation of CGO

The CGO was prepared by the method concerning the literature [33]. Specifically, the modified hummers method was used to prepare graphene oxide, 375 mg of GO obtained from the preparation was dissolved in 150 mL of deionized water, ultrasonically dispersed for 2 h, 25 mL of HBr was added and stirred for 12 h, and then 7.5 g of oxalic acid was added and stirred for 4 h. Next, it was centrifuged, washed, and dried in a vacuum drying oven at 50 °C for 24 h.

2.3. Preparation of CGO/PANI

The compound products were prepared by taking 20 mg, 40 mg, 60 mg, 80 mg, and 100 mg of the prepared carboxy-functionalized graphene oxide and 2 g of aniline, respectively, and the compounding ratio was from 1% to 5% (noted as CGO/PANI-1; CGO/PANI-2; CGO/PANI-3; CGO/PANI-4; CGO/PANI-5). Specifically, the corresponding amount of CGO was configured in a suspension of 2.5 mg/mL and ultrasonically dispersed for 30 min. 100 mL of deionized water and 2 g of aniline were added to a three-mouth flask and stirred for 10 min at 0°C. The above ultrasonically dispersed CGO suspension was added to the three-mouth flask after adding deionized water to 100 mL, and stirred for 1 h at 0 °C. Next, 4.9 g of ammonium persulfate was dissolved in 25 mL of 1 mol/L HCl solution and slowly added dropwise, whilst stirring, into the above three-necked flask at 0 °C using a dropping funnel. After 24 h of full reaction, it was washed and filtered with deionized water and anhydrous ethanol, dried in a vacuum drying oven at 50 °C for 24 h, and ground for use. GO/PANI-3 and pure PANI were prepared using the same method and served as control groups.

2.4. Preparation of CGO/PANI/W-B EZRC

This modified water-based epoxy zinc-rich coating consists of a mixture of two components, A and B. Component A is prepared as follows: add 7.00%~8.00% of propylene glycol methyl ether, 1.60%~2.14% of propylene glycol phenyl ether, and 0.42%~0.58% of thickener in the mixing vessel, then disperse at medium speed for 30 min at 1000 r/min. When the thickener is fully dissolved, add 6.10%~7.18% curing agent, 0.30%~0.45% dispersant, 0.54%~0.70% anti-settling agent, and disperse at low speed for 10 min. After stirring well, add 40.07%~80.05% zinc powder and disperse at high speed for 25~30 min at 1400~1800 r/min. Component B is made as follows: take a stirring vessel, add 26.00%~30.00% epoxy resin, 0.30%~0.50% anti flash rust agent, and 0.3% CGO/PANI, and disperse at medium speed for 10 min. Mix A and B components, disperse at low speed for 10 min. After mixing evenly, filter (200 mesh), add an appropriate amount of deionized water to adjust the viscosity, and spray the coating on a sandblasted steel plate: a cold-rolled steel plate (Q195) with sanding treatment, cleaned with alcohol, with the dimensions of 120.00 mm × 50.00 mm × 0.28 mm, and a tinplate with the dimensions of 120.00 mm × 25.00 mm × 0.28 mm. The prepared coatings are noted as CGO/PANI-1/W-B EZRC; CGO/PANI-2/W-B EZRC; CGO/PANI-3/W-B EZRC; CGO/PANI-4/W-B EZRC; CGO/PANI-5/W-B EZRC; PANI/W-B EZRC; pure PANI coatings, GO/PANI-3 coatings, and no PANI or CGO/ PANI coatings as blank controls, noted as PANI/W-B EZRC, GO/PANI-3/W-B EZRC, and W-B EZRC, respectively.

2.5. Characterization

2.5.1. Characterization of CGO/PANI

The GO, CGO, PANI, GO/PANI-3, and CGO/PANI-3 morphologies were analyzed by a scanning electron microscope (SU8220, Carl Zeiss, Jena, Germany). The crystal structures of GO, CGO, PANI, GO/PANI-3, and CGO/PANI-3 were characterized by X-ray diffractometer from PANalytical, Almelo, Netherlands (radiation source was Cu/kα (λ = 0.15418 nm), operating voltage 40 kV, current 100 mA, diffraction angle 2θ of (5~60°). Analysis of GO, CGO, PANI, GO/PANI-3, and CGO/PANI moieties were conducted by Fourier Transform Infrared Spectroscopy (VERTEX 70, Billerica, MA, USA) using KBr Compression.

2.5.2. Characterization of CGO/PANI/W-B EZRC

The surface morphology of the coatings before and after corrosion was observed by scanning electron microscope of FEI type EVO18; the electrochemical test was carried out on the CS310 electrochemical workstation of Wuhan COST Instruments Co., Ltd. (Wuhan, Hubei Province, China) The working electrode was Q195 steel with a coating, the exposed area of the specimen was 1 cm², the film thickness was (90 ± 10) μm, the auxiliary electrode was platinum electrode, and the reference electrode was a saturated calomel electrode (SCE). Electrochemical Impedance Spectroscopy (EIS) was tested in the frequency range of (1 × 10⁵~1 × 10⁻²) Hz, and the electrolyte was 3.5 wt% NaCl, measured at open circuit potential with an AC sine wave signal amplitude of 20 mV. Finally, the prepared coatings were tested for salt spray resistance according to GB/T 1771-2007.

3. Results and Discussion

3.1. Morphological Study of CGO/PANI by SEM

Figure 1 shows the microscopic morphology of the products of GO, CGO, PANI, GO/PANI-3, and CGO-PANI-1~5 after vacuum drying. The dried GO (Figure 1a) and CGO (Figure 1b) flakes stacked to form a large lamellar structure with many folds on the flakes. These folds are formed along the path of moisture loss between the lamellae during the drying process [34]. The SEM images of PANI (Figure 1c) show short rods stacked together irregularly, while the SEM images of GO/PANI-3 and CGO/PANI-3 (Figure 1d,e) show a clear lamellar structure with closely arranged, uniformly sized granular protrusions on the lamellae, which visually confirms that aniline undergoes in situ polymerization in an orderly manner on the lamellae. Compared with GO/PANI-3, the loading of PANI on the lamellae of CGO/PANI-3 is denser and more uniform. The degree of separation between the lamellae is better. It has been shown that the electrical conductivity of polyaniline strongly depends on its structural ordering [4,35]. To illustrate the effect of the compound ratio on the structure, we provide microscopic morphology images of the products with different compound ratios at 50,000× magnification. From Figure 1f to Figure 1j, the size of the lamellae gradually becomes smaller and the distance between the lamellae shrinks with the increase in the composite ratio. In CGO/PANI-1 (Figure 1f), we can observe the PANI protrusion on the lamellae. However, in CGO/PANI-5 (Figure 1j), the PANI protrusions on the lamellae are more difficult to observe. This is because as the number of CGO increases, some of the CGO lamellae do not have in situ polymerized PANI on them, resulting in the smaller size of the lamellae. The CGO lamellae without in situ polymerization of PANI tend to agglomerate during drying due to their higher surface energy, and therefore the distance between the lamellae is smaller in the composite product. Subsequently, we will confirm from other experiments that this tendency to agglomerate makes the composite product less effective in corrosion protection.

3.2. FT-IR Spectroscopic Study

Figure 2 shows the FT-IR spectra of the products of GO, CGO, PANI, GO/PANI-3, and CGO/PANI-3 from 600 cm⁻¹ to 4000 cm⁻¹. In the FT-IR curves of GO and CGO, 1725 cm⁻¹, 1420 cm⁻¹, 1220 cm⁻¹, and 1050 cm⁻¹ correspond to the stretching vibration peaks of the oxygen-containing functional group C=O, the deformation vibration peak of O-H, the stretching vibration peak of C–OH, and the stretching vibration peak of C-O on the lamellae, respectively [36]. The presence of these peaks on the infrared curve of GO indicates that the graphite has been oxidized, resulting in the hydroxyl, epoxy, and carboxyl groups appearing on the graphite flake layer. In the FT-IR curve of CGO, all four peaks are significantly enhanced, especially the carbonyl peak located at 1725 cm⁻¹, indicating the increase in carboxyl groups in graphene oxide after carboxylation [33]. In the FT-IR curves of PANI, the peaks located at 1580 cm⁻¹ and 1493 cm⁻¹ correspond to the C=C stretching vibration absorption peaks of the quinone ring and benzene ring on PANI, respectively. The absorption peaks of both quinone and benzene rings in GO/PANI-3 and CGO/PANI -3 are red-shifted to near 1559 cm⁻¹ and 1480 cm⁻¹, respectively, which is due to the hydrogen bonding of the carboxyl group on carboxy-functionalized graphene oxide and the nitrogen-containing functional group on polyaniline, as well as the π-π conjugation between benzene rings, which indicates that the carboxy-functionalized graphene oxide was successfully compounded with polyaniline [33]. The absorption peak of CGO did not appear on the infrared curve of CGO/PANI-3 because the composite ratio of CGO was too small.

3.3. X-ray Diffraction (XRD) Study

Figure 3 shows the XRD patterns of the products of GO, CGO, PANI, GO/PANI-3, CGO-PANI-3. The XRD curve of GO has a sharp peak at 2θ = 10.04°, which corresponds to the crystalline spacing of 0.88 nm in the GO lamellae, proving that the graphite has been successfully oxidized [37]. The XRD curve of CGO also has a sharper peak at 2θ = 10.5°, which corresponds to the crystalline spacing of 0.90 nm in the GO lamellae. This is because, after the carboxylation of graphene oxide, some hydroxyl and epoxy groups are converted into carboxyl groups, and the layer spacing becomes larger due to the repulsive effect between carboxyl groups [33]. The CGO curve indicates the successful carboxylation of graphene oxide. The XRD curves of PANI show broad peaks at 8.6°, 14.5°, 20.5°, 25.3°, 26.8°, and 29.2°, corresponding to the (001), (011), (020), (200), (121), and (022) diffraction planes of the emerald green imine salt form PANI [38], respectively, which indicates that the synthesized polyaniline coexists in both an amorphous and crystalline form [35]. The GO/PANI-3 and CGO/PANI-3 curve is similar to the PANI curve; the curve does not respond to the crystalline peak with CGO, which indicates that CGO did not crystallize, further confirming the in situ polymerization of aniline on graphene sheets [39,40].

3.4. Morphological Study of CGO/PANI/W-B EZRC by SEM

Figure 4 shows the coatings (W-B EZRC, PANI/W-B EZRC, GO/PANI-3/W-B EZRC, CGO/PANI-3/W-B EZRC) without soaking, and soaked in 3.5 wt% NaCl for 4 d, 10 d, and 28 d at 1000 times magnification. Figure 4a,e,i,m show the microscopic morphology of the surface of the unsoaked coatings. It can be seen that the surface micromorphology of the four coatings before immersion is similar; the epoxy covers the zinc particles and acts as a physical barrier to the corrosive medium. During the soaking time of four days (Figure 4b,f,j,n), the corrosive medium penetrated through the micro-pores of the coatings. When the corrosive medium reaches the surface of the metal substrate, the zinc powder acts as the anode to lose electrons first and is corroded instead of the substrate. Some zinc particles on the surface of W-B EZRC (Figure 4b) started to oxidize to produce zinc oxide adhering to the coating surface, indicating that the corrosive medium had reached the metal substrate surface. After PANI/W-B EZRC, GO/PANI-3/W-B EZRC, and CGO/PANI-3/W-B EZRC were dipped for four days, the microscopic morphology of the coating surfaces did not change much compared to the unsoaked coatings, indicating that the zinc powder had not yet exerted its cathodic protection effect.

After 10 d of immersion, a large amount of zinc oxidation products had accumulated on the surface of W-B EZRC (Figure 4c), indicating that more corrosive media had reached the surface of the substrate. PANI/W-B EZRC (Figure 4g) showed the accumulation of zinc oxidation products on the surface after 10 days of immersion, indicating that the passivation effect of PANI on the substrate had started to fail. The microscopic morphology of the surface of GO/PANI-3/W-B EZRC and CGO/PANI-3/W-B EZRC (Figure 4k,o) remained little changed after 10 days of immersion compared with that without immersion, indicating that the passivation effect of polyaniline and the lamellar structure of the composite effectively retarded the corrosion of the substrate and the intrusion of corrosive media. After 28 d of soaking, the surface morphology of W-B EZRC (Figure 4d) was similar to that after 10 d of soaking, indicating that the penetration of corrosive media into the coating had saturated and almost all of the zinc particles on the surface were oxidized by the cathodic protection they exerted. PANI/W-B EZRC (Figure 4h) showed a more severe buildup of the surface of zinc oxidation products after 28 d of immersion, indicating that the substrate was further corroded, and more zinc powder was corroded instead of the substrate. The morphology of the zinc particles on the surface of GO/PANI-3/W-B EZRC (Figure 4l) also changed significantly after 28 d of immersion–from regular spheres to barbed balls, which are oxidation products of zinc–indicating that the corrosion protection of GO/PANI-3/W-B EZRC reached the second stage and the zinc powder started to be corroded instead of the substrate. However, the zinc powder particle integrity of the CGO/PANI-3/W-B EZRC (Figure 4p) surface was still high, which showed that the lamellar structure formed by the carboxyl-functionalized graphene oxide compounded with polyaniline exerted the barrier effect of the lamellar structure on the corrosive medium and the passivation effect of polyaniline on the substrate, and had the best improvement on the anticorrosive performance of the coatings.

3.5. Electrochemical Impedance Spectroscopy (EIS)

Figure 5 shows the Bode and Nyquist plots of pure epoxy-rich zinc coating, PANI modified epoxy-rich zinc coating, GO/PANI-3 modified epoxy-rich zinc coating, and CGO/PANI-modified epoxy-rich zinc coating immersed in 3.5 wt% NaCl for 2 h (Figure 5 a₁,a₂), 168 h (Figure 5b₁,b₂), and 528 h (Figure 5c₁,c₂). The Bode plot of |Z|_0.01Hz is an important index to evaluate the anticorrosive performance of the coating. In general, the larger the |Z|_0.01Hz, the better the corrosion protection of the coating. The Bode plot for 2 h immersion in 3.5 wt% NaCl is shown in Figure 5. The pure W-B EZRC has the lowest |Z|_0.01Hz of 7.56 × 10⁸ Ω·cm². The |Z|_0.01Hz of the coating with PANI is 2.02 × 10⁹ Ω·cm², similar to GO/PANI-3 and higher than the pure W-B EZRC, while the |Z|_0.01Hz of the coating with CGO/PANI has a greater increase. The highest |Z|_0.01Hz is the coating with CGO/PANI-3, 5.33 × 10⁹ Ω·cm², which is nearly 10 times higher than the pure W-B EZRC. As can be seen from the Nyquist plot for 2 h immersion, all curves show a single capacitance against arc, which is due to the fact that the coating is in the early stage of immersion, when the corrosive medium has not yet made contact with the substrate through the coating. As shown in the Bode and Nyquist plots of the coating immersed for 168 h, the |Z|_0.01Hz of pure W-B EZRC coating decreased to 4.83 × 10⁵ Ω·cm² after 168 h immersion–a decrease of three orders of magnitude. The |Z|_0.01Hz of PANI/W-B EZRC decreased to 4.54 × 10⁷ Ω·cm², a decrease of two orders of magnitude, and the |Z|_0.01Hz of the remaining coatings also dropped, but the decrease did not exceed one order of magnitude. This indicates that the in situ polymerization of PANI on GO or CGO lamellae changes the original rod-like structure into a laminar structure, and this structure can effectively slow down the intrusion rate of corrosive media. The same result can be obtained from the Nyquist plot soaked for 168 h. The Nyquist plot of pure W-B EZRC coating, soaked for 168 h, is no longer a single capacitance anti-arc, but a Warburg impedance image representing the diffusion effect appears at the low-frequency end. This is due to the corrosion products of zinc powder accumulating at the interface of the metal and coating, controlling the corrosion reaction by the transfer process of the medium. The Bode and Nyquist plots of the coating immersed for 528 h are shown in Figure 5. The pure W-B EZRC |Z|_0.01Hz drops to 1.39 × 10⁵ Ω·cm² and the W-B EZRC coating with the addition of PANI |Z|_0.01Hz drops to 3.30 × 10⁶ Ω·cm²; Warburg impedance also appears at the low-frequency end. In addition, a similar situation was observed for the coating with GO/PANI-3 and with CGO/PANI-1 added. It is worth noting that in all the tests of EIS, the impedance modulus of the coating with added CGO/PANI shows an increase and then a decrease, with the increase in CGO composite ratio. This could be due to the increase in CGO ratio, meaning the PANI cannot wrap the lamellae well, thus leading to the agglomeration of the lamellae during drying (as shown in Figure 1i,j), and therefore reducing the barrier effectiveness against corrosive media. In summary, it can be concluded from the EIS data that CGO/PANI-3/W-B EZRC has the best anti-corrosion performance.

3.6. Comprehensive Anticorrosion Performance

Figure 6 shows the photos of the neutral salt spray resistance test after 1400 h. The salt spray resistance of the coating can visually reflect the comprehensive anti-corrosion performance of the coating. The corrosion of pure W-B EZRC (Figure 6a) was quite serious along the scratches after 1400 h in the salt spray chamber, and the corrosion was extended from the scratches in several places. The corrosion at the scratches of the coating with PANI added (Figure 6b) was still severe. This also indicates that the corrosion improvement of unmodified PANI is not good enough. With the addition of GO/PANI-3 coating (Figure 6c), the corrosion at the scratches was again improved. The coating with CGO/PANI added (Figure 6d–h) showed a significant improvement in corrosion at the scratches, and as the percentage of CGO in the composite increased, the corrosion at the scratches of the coating showed improvement at first, and then deterioration. This is consistent with the results of the EIS test. As the amount of CGO in the composite increases, the loading capacity of PANI becomes stronger and the dispersion of PANI improves. When CGO is excessive, PANI cannot cover the surface of the lamellae, and the lamellae tend to agglomerate with each other, so the surface of the coating shown in Figure 6h is relatively rough. Therefore, the salt spray resistance of the coating with GO/PANI-5 decreased at 1400 h. In conclusion, from the salt spray resistance performance of the coating, it can be visualized that CGO/PANI-3 has the best anticorrosion improvement ability.

4. Conclusions

Carboxy-functionalized graphene oxide composite polyaniline was successfully prepared. FT-IR, XRD, and SEM analyses showed that GO was carboxy-functionalized, and aniline was successfully polymerized on CGO sheets in an ordered manner, forming an ordered sheet-like CGO/PANI composite. The lamellar structure of CGO/PANI can play the role of physical shielding, delaying the invasion path of corrosive media. Polyaniline doping conductive properties can improve the electrical contact of zinc powder, but also passivate the surface of the metal substrate so that the corrosion performance of the coating is greatly improved. The test results show that the effect of CGO/PANI on coating corrosion resistance is better than that of GO/PANI and pure PANI. Additionally, with a gradual increase in the CGO composite ratio, the corrosion improvement effect of CGO/PANI composite on waterborne epoxy zinc-rich coatings increases first and then decreases. This is because there is an optimum ratio of loading of CGO to PANI. When the ratio of CGO is small, some PANI will not be loaded onto the CGO sheet layer, and when the ratio of CGO is large, CGO is not completely loaded by PANI, and then agglomeration will easily occur. Among them, CGO/PANI-3 has the best anti-corrosion improvement effect. The initial impedance value of the coating modified with CGO/PANI-3 was 5.3 × 10⁹ Ω·cm², which was seven times higher than that of pure W-B EZRC. After immersion in 3.5 wt% NaCl for 528 h, the impedance value was reduced by only one order of magnitude, while pure W-B EZRC was reduced by four orders of magnitude. CGO/PANI-3/W-B EZRC performed best in the salt spray resistance test. This paper provides a simple and effective method to modify the anticorrosive properties of water-based epoxy-rich zinc coatings, which can expand their application in the field of anticorrosion.