3.1 FT-IR analysis
The rGO particles were modified with SiO2 nanoparticles through methods I and II. The FT-IR analysis was performed on the rGO nanosheets dispersed in silica matrix through method I) and II at different reaction times (24h, 48h and 72 h). The FT-IR spectra of various samples are presented in Fig. 2.
According to Fig. 2 there are various peaks in the FT-IR spectrum of rGO including -OH (3700 cm− 1), C = C (1636 cm− 1), C-O (918 cm− 1), and C-O-C (670 cm− 1), indicating the presence of hydroxyl, carboxyl, and epoxide groups on the surface of rGO, respectively. Three new intensive peaks appeared in the FT-IR spectra of rGO nanosheets dispersed in silica matrix at all silanization times. These are Si-O-Si- (asymmetric vibration at 1090 cm− 1 and bending vibration at 465 cm− 1), Si-O-C (asymmetric vibration at 1124 and bending vibration at 694 cm− 1) and -NH2 (3260 cm− 1). These all confirm the presence of silane moieties on the rGO surface [19–20].
Observation of an intensive peak corresponded to -Si-O-Si- bond at 1090 cm− 1 in all samples indicates the significant self-condensation of silane precursors forming silica clusters on the rGO surface. In addition, Si-O-C bond at 1124 and 694 cm− 1 confirms chemical grafting of silanes on the rGO surface through reaction with carboxylic groups. Results show disappearance of the carboxyl and epoxide groups of the rGO located at 1708 cm− 1 indicating the reaction of silanes with the rGO surface [21]. It can be seen from Fig. 2 (a) that the carboxyl and epoxy groups on rGO-SiO2-II/24h completely disappear, while there are still some unreacted carboxyl and epoxy groups on rGO-SiO2-I/24h. These confirm the more silane groups reaction with the rGO surface in the second method when the mixture of silanes were hydrolyzed for 24 h and then the rGO nanosheets were added to the hydrolyzed solution [22–23].
The above results indicated that the silylation method can significantly affect the way SiO2 generated by TEOS hydrolysis was grafted onto the surface of rGO. In method I, SiO2 was mainly grafted with functional groups on the surface of rGO while SiO2 particles were covered on the surface of rGO in method II.
3.2 XRD analysis
It can be seen from Fig. 3 that rGO is a lamellar structure, and the sharp and strong diffraction peak appeared at near 2θ = 26°, which corresponds to the diffraction peak of graphene (002) crystal plane [24], indicating that the spatial arrangement of graphene microcrystals is very neat.
According to the Prague formula [25]:
2dsinθ = nλ (2)
where d is the interlayer spacing between crystal planes, θ is the diffraction angle, n is the diffraction order (n = 1), λ is the wavelength of the X-ray (λ = 0.15406), the interlayer spacing of rGO can be calculated to 0.3424nm. The weak diffraction peak near 2θ = 54.6° corresponds to the graphene (004) crystal plane [24]. In addition, although the diffraction intensity of 43.3° and 44.4° is very weak, the diffraction peak also appears, which corresponds to graphene (100) and (101) crystal planes [26]. This may be caused by graphene destroying its lamellar structure or breaking some layered blocks during the processing.
On the diffraction pattern of rGO-SiO2, an additional diffraction peak appears at 2θ = 22.26°, which corresponds to the diffraction of SiO2 [27]. However, in the rGO-SiO2 hybrid material, the diffraction peak of graphene becomes wider and the strength is obviously weakened, the diffraction peak position of rGO-SiO2 prepared by method I is more left compared with that of method II, the interlayer spacing can be calculated to 0.4298nm (method I) and 0.3935nm (method II), which is because after coating SiO2, the size of graphene lamella shrinks, the integrity of crystal structure decreases, and the degree of disorder increases. According to the peak position of rGO-SiO2 hybrid material and the PDF standard card, it can be concluded that SiO2 can be successfully coated on the surface of graphene [28] due to the presence of hydroxyl, carboxyl and other oxygen containing groups on the graphene sheet.
3.3 SEM analysis
The surface morphology of pristine rGO sheets before and after silanization thorough methods I and II (rGO-SiO2) microcapsules were studied by SEM analysis. The SEM micrographs of the rGO and rGO-SiO2 48h are compared in Fig. 4. In Fig. 4 (a), the lamellar structure of rGO is clearly visible and the surface is smooth, but there are large agglomerations. After 48h of silylation, silicon spheres precipitate on the surface of rGO, mostly spherical or nearly spherical. Moreover, rGO-SiO2 presents a fluffy form, indicating that silica particles as spacers can prevent Gr from agglomeration due to intermolecular van der Waals force during drying to some extent [29]. In Fig. 4(b), some spherical particles can be seen on the surface of rGO, which indicates that the powder prepared by method I is chemically grafted silane on the surface of rGO to generate SiO2, rather than the precipitation of silica clusters. However, in Fig. 4(c), the lamellar structure of rGO is almost invisible, and the silicon sphere completely encloses rGO. Based on these explanations the pristine rGO sheets successfully covered with silica nanoparticles and nanohybrids were obtained.
3.4 Bonding strength analysis
The bonding strength between the coating and the substrate, an essential index for evaluating the mechanical properties of the coating, was related to the reliability of the coatings. In this paper, the pulling method was used to test the adhesion of the coatings, and the test results were shown in Fig. 5(a). The adhesion of the coatings with the addition of the hybrid materials has been improved to different degrees depending on the test results, among which the adhesion of the coating with rGO-SiO2-I/48h hybrid materials had the best performance, reaching 6.2 MPa, which is 180% higher than the bond strength of the EP coating. rGO and SiO2 have large specific surface area, which can easily produce strong interaction with epoxy resin to form a dense lattice structure [30–31]. In addition, there are still some oxygen-containing functional groups on the surface of the lamellar rGO, which can produce van der Waals force interaction with the substrate, and epoxy resin to form an excellent compatibility and bonding surface structure, which further enhances the bonding strength between the coating and the substrate [32].
3.5 Contact angle analysis
Test the effect of adding hybrid materials on the hydrophilicity of epoxy coatings by measuring the size of the contact angle, as shown in Fig. 5(b). As shown in Fig. 5(b), the contact angle of the EP coating was 46.61°, indicating that the EP coating is hydrophilic. After adding different hybrid materials, the contact angle of the coating increased to 59.85°, 64.31°, 66.56°, 57.93°, 52.09°, and 60.78°, respectively. This indicated that the hydrophobicity of rGO-SiO2@EP was stronger than that of EP, which helps to slow down the corrosion of steel substrate by corrosive media in the environment [33–34].
3.6 EIS analysis
The effect of rGO-SiO2 hybrids produced through method I and method II on the corrosion and ionic resistances of the epoxy coating was studied by EIS technique. The polarization curve, Nyquist and Bode plots of different samples after 24h immersion were displayed in Fig. 6–7. In addition, the experimental results were fitted with suitable electrical equivalent circuits.
Figure 6 showed polarization curve of the EP coating with or without rGO-SiO2 hybrids. It can be obviously seen that the corrosion potential and current density of EP were − 0.670V and 2.01×10− 6A·cm− 2, respectively, with a corrosion impedance value of 5911.6 Ω· cm2, and a passivation plateau appeared in the range of -0.40V to -0.35V, indicating the formation of oxide films on the metal surface [35]. After adding rGO-SiO2 to the coating, there was no passivation plateau on the polarization curve, and Iccor significantly decreased, while the impedance value significantly increased.
According to the data analysis in Table 2, the rGO-SiO2-I/48h@EP coating had the highest impedance value (154121.0 Ω· cm2), and the CPE of this coating was also the largest among the six coatings, up to 90.15%. The above data all indicate that the addition of rGO-SiO2 can greatly enhance the protective effect on the steel substrate, improving a more effective anti-corrosion barrier for the steel substrate. This is because rGO-SiO2 has good hydrophobicity and barrier performance [36]. After adding the coating, it can effectively fill the pores and cracks on the surface of the coating, effectively blocking the penetration channel of the corrosive medium, and playing a role in protecting the metal. According to the corrosion protection efficiency CPE, these coatings are sorted in the following order: rGO-SiO2-I/48h@EP > rGO-SiO2-I/72h@EP > rGO-SiO2-I/24h@EP > rGO-SiO2-II/24h@EP > rGO-SiO2-II/72h@EP > rGO-SiO2-II/48h@EP. The rGO-SiO2@EP coating prepared by Method I has better corrosion resistance than Method II, with the best corrosion resistance still being the rGO-SiO2-I/48h@EP coating..
Figure 6 Polarization curves of EP and rGO-SiO2@EP coatings
Table 2
Potentiodynamic polarization parameters
Coatings
|
Eccor/V
|
Iccor/A·cm− 2
|
Rp/Ω·cm2
|
CPE(%)
|
EP
|
-0.670
|
2.012×10− 6
|
5911.6
|
/
|
rGO-SiO2-I/24h@EP
|
-0.478
|
2.683×10− 7
|
144640.1
|
86.89
|
rGO-SiO2-I/48h@EP
|
-0.477
|
1.909×10− 7
|
154121.0
|
90.51
|
rGO-SiO2-I/72h@EP
|
-0.526
|
2.268×10− 7
|
124202.0
|
88.73
|
rGO-SiO2-II/24h@EP
|
-0.538
|
3.209×10− 7
|
110846.3
|
84.05
|
rGO-SiO2-II/48h@EP
|
-0.478
|
4.028×10− 7
|
111133.8
|
79.98
|
rGO-SiO2-II/72h@EP
|
-0.652
|
3.414×10− 7
|
104213.8
|
83.03
|
From Fig. 7, it can be seen that the EP coating exhibits a characteristic impedance spectrum controlled by charge transfer in the high-frequency region, with a semi-circular shape. The impedance spectrum appearing in the low-frequency region was the diffusion impedance of the solution, which corresponds to the Bode diagram with two time constants. This indicated that the corrosive electrolyte gradually diffuses towards the coating substrate through defects such as micro pores and cavities generated during the coating and curing process within 24 hours of immersion, leading to coating damage, loss of barrier performance [29], as shown in Table 2, the Rc and Rct of the EP coating were 7487 Ω cm2 and 1678 Ω·cm2, respectively. At this time, the corrosion process of the EP coating was mainly controlled by two factors: Rc and Rct.
The radius of the arc represents the size of the resistance, and the larger the radius of the arc, the better the protective effect of the coating. It can be clearly observed that the arc radius of the rGO-SiO2-I/48h@EP coating was much larger than that of all other coatings, indicating that the rGO-SiO2-I/48h@EP coating had the highest resistance and the best protective effect on metals. From Table 2, it can be seen that compared with EP, the Rc of rGO-SiO2@EP significantly increased, indicating that the coating had good barrier performance, with the most significant improvement being the rGO-SiO2-I/48h@EP coating, with an Rc of 2.405×104 Ω·cm2. However, further increasing the silylation time (rGO-SiO2-I/72h@EP) will actually lead to a decrease in the protective effect of the coating. Except for the Bode diagram of rGO-SiO2-II/24h@EP coating where two time constants can be observed, all other coatings had only one time constant. Usually, oxygen, water, and corrosive ions (Cl−) enter the interior of the coating through cracks or pores, causing corrosion and detachment under the coating. Among them, the time constant in the high-frequency region reflects the response between the electrolyte and coating interface, while the time constant in the low-frequency region reflects the corrosion process between the electrolyte and substrate interface [37]. Although the impedance value of the rGO-SiO2-I/48h@EP coating was at a high level, the electrolyte solution had gradually begun to penetrate. However, the layered structure of rGO-SiO2 is two-dimensional, which can extend the time for corrosive electrolyte solution to reach the middle of the coating and metal substrate, and reduce the corrosion rate.
3.8 Salt spray analysis
Salt spray corrosion refers to an accelerated corrosion method that simulates the seawater environment, and its resistance time determines the quality of corrosion resistance. Fig. S1 shows the optical photos of the coatings with or without hybrid materials at placed in a salt spray box with 5 wt.% NaCl solution for 180 hours. The surface topography and surface elements of the coatings are analyzed by SEM and EDS, which is shown in Fig. S2.
From Fig. S1, it can be seen that there were varying degrees of corrosion at the scratches on the coating surface, and some plate surfaces had obvious rust spots. The width of the expansion marks at the scratch on the surface of EP coated board were about 3mm, and there were large corrosion products and rust spots in the lower left corner of the board, with a large corrosion area and a small amount of bubbles. In contrast, after adding the hybrid materials to the EP coating, the corrosion propagation area at the scratch of the coatings decreased, and the propagation width was generally around 1.5 mm-2 mm. The number of rust spots was also significantly reduced compared to EP. According to the analysis of the number of rust spots, it can be observed that rGO-SiO2-I@EP coatings had better corrosion resistance than rGO-SiO2-II@EP coatings. Among them, the rGO-SiO2-I/48h@EP had the best surface condition and almost no rust spots. And the surface of rGO-SiO2@EP coatings did not show any blistering phenomenon, and the adhesion of the coatings above the scratch were still good, indicating that the physical barrier performance of the coating surface and the bonding strength between the coating and the steel substrate were effectively enhanced by the addition of rGO-SiO2 hybrid materials, greatly improving the protective performance of the coating on the steel substrate [38].
Additionally, it can be shown in Fig. S2 that Fe, Cl, C and Si elements were present within EP coating after salt spray test, which means the corrosive medium had reached the surface of the substrate. Compared with it, the surface of the rGO-SiO2-I/48h@EP coating and rGO-SiO2-II/48h@EP coating had only Fe, O, C and Si element but not Cl element, which indicated that the coating formed a good shielding effect on the corrosive medium and protected the metal substrate perfectly [39–40].
The corrosion mechanism of the coating is shown in Fig. 8. Due to its large structural gaps and small adhesion between particles, epoxy coatings can only provide a certain degree of protection [41], so the corrosion resistance performance of EP coating was the weakest. In conjunction with the other experiments described above, the better dispersion of rGO-SiO2 hybrids prepared by method I than the one obtained in method II is responsible for the higher corrosion resistance of the former. The silane molecules grafted on the rGO surface and SiO2 nanoparticles increased the interlayer distance of rGO sheets preventing them from agglomerations in the epoxy matrix. The rGO dispersion improvement in this case would cause significant improvement of the coating barrier properties [29]. The rGO nanosheets are impermeable against the electrolyte diffusion and could block the electrolyte pathways and increase the diffusion length of oxygen and water diffusion. In addition the rGO-SiO2 hybrids could resist against Cl− ions diffusion. This is due to the negative surface charge of rGO-SiO2 in the EP [38]. These mean that rGO-SiO2 nanosheets can enhance the barrier properties of EP against oxygen, water and corrosive Cl− ions. However, these are not the only favorites of using SiO2-GO nanosheets in the epoxy coating. There are NH2 groups on the rGO-SiO2 surface which are reactive sites that can react with epoxide groups of EP resin through a SN2 nucleophilic substitution ring opening reaction [42]. This can result in the increase of EP cross-linking density around rGO-SiO2 hybrids. In addition, a portion of the fillers in the coating shows a tendency of lateral arrangement, constructing a labyrinth shielding structure, extending the penetration and diffusion path of the road corrosion medium, further improving the anti-permeability and service life of the coatings.