Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Synthesis of L‑Histidine-Attached Graphene Nanomaterials and Their Application for Steel Protection Chengbao Liu,†,‡ Peng Du,†,§ Haichao Zhao,*,† and Liping Wang*,† †
Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China § Institute of Marine Science and Technology, Shandong University, Qingdao 266237, P. R. China S Supporting Information *
ABSTRACT: Graphene-based carbonaceous materials have aroused great attention among the material protection fields due to their excellent impermeability. However, graphene nanosheets tend to aggregate in a polymer matrix and trigger microgalvanic corrosion, which potentially aggravates metal deterioration. Herein, we present a facile strategy to improve the protective property of graphene by tailoring graphene oxide with L-histidine molecules. The resultant functionalized graphene nanomaterials, without direct connections, can be well-dispersed in a polymer matrix. Electrochemical impedance measurements demonstrated that embedding a small percentage of well-dispersed functionalized graphene in epoxy coatings significantly enhanced the impermeable properties of the as-prepared composite coatings. The probability of galvanic corrosion of graphene with metal was largely inhibited and verified through the scanning vibrating electrode technique. The protection mechanism of composite coatings is interpreted as the barrier property of graphene nanosheets by suppressing the diffusion of corrosive species and the isolate function of L-histidine at the edge of graphene lamellae via increasing the electrical resistance. KEYWORDS: graphene, functionalization, anticorrosion, epoxy coating, galvanic corrosion
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INTRODUCTION Organic coatings have been considered as one of the most effective ways to protect a metal substrate via providing a dense barrier to prevent corrosive species penetration.1−4 Although a traditional solvent-borne organic coating can provide good anticorrosion protection for metals, a certain amount of volatile organic compounds (VOCs) will be directly discharged into the air during the drying process, which are seriously harmful to the environment and human health. Thus, waterborne epoxy resins become an appropriate candidate for the preparation of environmentally friendly coatings.5 However, the long-term anticorrosion performance of conventional waterborne epoxy systems is unsatisfactory when exposed to extreme corrosive environments. For instance, the existence of hydrophilic groups and the reaction of flash rust are prone to weaken the protective ability of the waterborne coatings.6,7 Numerous studies have been done to overcome these problems and to design coating © XXXX American Chemical Society
systems with enhanced anticorrosion performance through incorporating various additives and pigments.8−11 Among these strategies, using nanosized materials has attracted the extensive attention of researchers.12−15 Graphene, a two-dimensional lamellar-structured carbonaceous material, has been reported as an excellent material for metal protection for its superior characteristics, including excellent stability, impermeability, powerful mechanical property, and high specific surface area.16−23 Previous studies revealed that a graphene coating cannot provide long-term anticorrosion protection for metal because of the formation of microgalvanic corrosion.24−27 Preparing a graphene/polymer coating is a reasonable method to harness the characteristics of Received: January 26, 2018 Accepted: March 6, 2018 Published: March 6, 2018 A
DOI: 10.1021/acsanm.8b00149 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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graphene for metal protection.28,29 The obtained composite materials not only possessed enhanced anticorrosion performance but also exhibited superior mechanical properties.30−33 However, the high surface energy and the weak van der Waals forces between adjacent nanosheets lead to graphene aggregation, resulting in an inferior protective ability of coating systems. On one hand, the aggregated graphene nanosheets severely impair the barrier performance of composite coatings. On the other hand, the graphene−graphene and graphene− metal connections in the polymer matrix will induce microgalvanic corrosion once the corrosive electrolyte penetrates. For an enhancement of the anticorrosion property of the graphene-based composite coating, two necessary conditions should be fulfilled, which include (1) a homogeneous dispersion of graphene in the coating matrix and (2) the inhibition of the connections of graphene−graphene and graphene−metal. To date, noncovalent and covalent strategies have been used to improve the compatibility between graphene and polymer matrix.34,35 Compared with noncovalent methods, covalent modification of graphene exhibited its advantages for durability. It has been reported that covalent functionalized GO via diamine,36 (3-aminopropyl) triethoxysilane (APTES),30,37 and 3-aminoproplyphosphoic acid (APSA)38 possessed good compatibility with polymers, which endowed coating systems with enhanced barrier performance against corrosive medium. In regard to the second condition, Liu and co-workers have fabricated functionalized graphene anticorrosion coatings, which can inhibit the corrosion-promotion activity of graphene. By encapsulating graphene nanosheets in low-conductivity pernigraniline39 or insulated SiO240 and APTES,41 the resultant composite coatings showed an improved anticorrosion performance. Generally, the most extensively studied amine compounds applied in graphene functionalization are synthetic and have a certain toxicity, which impede its potential applications. Environmentally friendly modifiers derived from nature, which are readily accessible, rich in content, and biodegradable, are suitable candidates for graphene functionalization. LHistidine contains nitrogen and oxygen electronegative atoms, which can interact with the metal substrate and form an adsorption film via coordination interaction.42,43 In view of its biodegradable and environmentally friendly properties, Lhistidine has been widely employed to the metal corrosion protection field. Compared with other amine reactions (requiring a high temperature), the 1-ethyl-3-(3(dimethylamino)propyl)-carbodiimide-assisted (EDC-assisted) amidation reaction exhibited its advantages, such as mild conditions, high yields, and high chemoselectivity. In this paper, L-histidine was attached to the surface of GO with the assistance of EDC. The obtained functionalized graphene nanosheets displayed good dispersibility in water and waterborne coatings. The anticorrosion performance of prepared composite coatings was characterized by electrochemical impedance spectroscopy and scanning vibrating electrode technique (SVET) measurements in 3.5 wt % NaCl solution. The enhanced anticorrosion performance of composite coatings for steel was attributed to (1) the impermeable property of graphene nanosheets and (2) the elimination of graphene− graphene and graphene−metal connections by grafting Lhistidine molecules.
Article
MATERIALS AND METHODS
Materials. L-Histidine, 1-ethyl-3-(3-(dimethylamino)propyl) carbodiimide (EDC), hydrazine hydrate, and N,N-dimethylformamide (DMF) were purchased from Aladdin Industrial Corporation without further purification. Graphene oxide was provided from Shanghai Ashine Technology Development Co., Ltd. Waterborne epoxy resin (E-20) and curing agent were provided by Shanghai Run Carbon New Material Technology Co., Ltd. The carbon steel electrodes with the area of 1 cm2 were polished by 400 and 800 SiC sand papers and cleaned in acetone and ethanol via ultrasonic vibration. Preparation of Functionalized Graphene. Covalent functionalization of graphene nanosheets (fG) was accomplished via amidation reaction between carboxylic acid groups of GO and the amine group of L -histidine, as illustrated in Scheme 1. For this purpose, a
Scheme 1. Schematic Representation of the Preparation Process of fG
homogeneous suspension of GO was prepared by dissolving 100 mg of GO in 40 mL of DMF and ultrasonication for 1 h, followed by the addition of 25 mg of EDC. After stirring for 15 min, a given amount of DMF solution containing 20 mg of L-histidine was mixed with the above suspension with magnetic stirring for 24 h at room temperature. The covalently attached GO nanosheets were subsequently reduced through adding hydrazine hydrate. The unreacted L-histidine was removed after centrifugation and washed with ethanol and deionized water several times. Finally, the functionalized graphene (fG) powder was obtained by drying the slurry under 50 °C in vacuum oven. For comparison, the chemically reduced graphene oxide (rGO) was also prepared using hydrazine solution. Fabrication of Composite Coatings. The general procedure for preparation of fG/epoxy composite coatings is as follows. A 50 mg portion of fG powder was dissolved in 20 mL of deionized water and ultrasonicated to obtain a homogeneous solution. Then, 3 g of waterborne curing agent was mixed fully, and the excess solvent was removed via rotary evaporation. After that, 2 g of epoxy resin was added and stirred for 30 min. In addition, the trapped air bubbles were removed by degassing the mixture in a vacuum oven at room temperature. Finally, the mixtures were painted to the pretreated steel electrodes via a bar coater with a thickness of 30 ± 3 μm. The coated electrodes were cured under room temperature for 72 h. For comparison, the rGO/epoxy composite coating and pure epoxy coating were also prepared in a similar way. It is noted that every test was carried out three times with three parallel samples to ensure repeatability. Characterization. The structure properties of GO and fG were characterized through Fourier-transform infrared (FT-IR) spectra and Raman spectroscopy (Renishaw in Via Reflex). FT-IR spectra were obtained from 32 scans at a resolution of 1 cm−1 between 400 and 4000 cm−1. In addition, the X-ray photoelectron spectroscopy (XPS) measurement (AXIS ULTRA DLD) was used to determine the chemical composition of GO and fG nanosheets. Transmission electron microscopy (TEM, JEOL2100) and scanning probe B
DOI: 10.1021/acsanm.8b00149 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Figure 1. (a) FT-IR and (b) Raman spectra for GO and fG.
Figure 2. XPS spectra of (a) GO and (b) fG. (c) C 1s and (d) N 1s spectra of fG.
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microscopy (SPM, Dimension 3100) were employed to analyze the morphology and dispersibility of rGO and fG. The electrochemical measurements were carried out using a CHI660E electrochemical station to evaluate the corrosion behaviors of different coatings. A conventional three-electrode cell system was used: the counter electrode (platinum plate with 2.5 cm2 area), reference electrode [saturated calomel electrode (SCE)], and working electrode (coated mild steel with an exposed area of 1 cm2). The electrochemical data were collected at the frequency range from 10 kHz to 10 mHz by using a sinusoidal perturbation of 20 mV amplitude at an open-circuit potential (OCP) in 3.5 wt % NaCl solution. For further investigation of the failure mechanism of different coatings, the scanning vibrating electrode technique (SVET) was employed to examine the corrosion behaviors of composite coatings with defects. Prior to the test, an artificial scratch with a size of 0.2 mm was introduced to the coated samples. A VersaSCAN microscanning electrochemical workstation (AMETEK) was used to detect the local current density on the area of 2 cm2 with 21 × 21 scanning points. The vibration amplitude was selected as 30 μm at a frequency of 80 Hz for the microelectrode. Furthermore, the corrosion products formed on the carbon steel electrodes were analyzed by XRD and SEM after peeling off the coatings.
RESULTS AND DISCUSSION Characterization of fG. The covalent interaction between GO and L-histidine was confirmed by FT-IR spectra. Figure 1a shows the comparison of FT-IR spectra of GO and fG. As can be seen, the characteristic bands of GO were CO stretching of the COOH at 1722 cm−1, epoxy vibration at 1059 cm−1, OH stretching of the COH at 3420 cm−1, and CO vibration of the COH at 1260 cm−1.44,45 In addition, a prominent peak appeared at 1628 cm−1, which is attributed to aromatic CC stretching. After functionalization, it is obvious that the characteristic adsorption bands of carboxyl at 1721 cm−1 disappeared, and a new band presented at 1626 cm−1 corresponding to amide−carboxyl stretching vibration.46 This could be interpreted as the condensation reaction between the amine group of L-histidine and carboxylic acid groups of GO. Furthermore, the fG showed the CN stretching (1500, 1090 cm−1) and asymmetric stretching (3185 cm−1) with decreased absorption intensity of the epoxy group. These observations clearly demonstrated that the L-histidine successfully bonded to the surface of graphene. C
DOI: 10.1021/acsanm.8b00149 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Figure 3. Scanning probe microscopy (SPM) images of (a) rGO and (b) fG.
Figure 4. TEM images of (a) rGO, (b) fG, (c) rGO/epoxy coating, and (d) fG/epoxy coating.
D
DOI: 10.1021/acsanm.8b00149 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Figure 5. Bode plots of different coating/steel systems with immersion in 3.5 wt % NaCl solution: (a) 1 day, (b) 10 days, and (c) 35 days.
appearance of the N atom derived from L-histidine in fG (Figure 2b), indicating the successful grafting of L-histidine on the GO surface. This finding was consistent with the FT-IR and Raman results. In addition, the high-resolution C 1s and N 1s XPS spectra for fG are presented in Figure 2c,d. The C 1s spectrum of fG contains four peaks with binding energies of 284.6, 285.4, 286.9, and 288.7 eV, which are attributed to C C/CC, CN, CO, and NCO bonds,51,52 respectively. Two peaks were exhibited in the N 1s spectrum at 399.7 and 400.8 eV related to NH and CN bonds.53 The presence of the CN peak further demonstrates the introduction of L-histidine on the GO surface via covalent interaction. Morphology and Dispersion State of fG. SPM and TEM were exploited to investigate the morphology and dispersion state of rGO and fG. As can be seen in Figure 3a, the rGO exists in thick layers with thickness of 10−20 nm, indicating that it is of poor dispersibility. However, an obvious lamellar structure could be observed for fG after grafting L-histidine (Figure 3b). The TEM images of rGO and fG dispersed in water are shown in Figure 4a,b. Compared with rGO, the prepared fG nanosheets were transparent with rare agglomerations, which means that their interlayer force was weakened. In addition, the dispersion state of rGO and fG in the epoxy matrix could be examined through TEM measurement (Figure 4c,d). It should be noted that the gray areas were related to the domain of the epoxy resin, while the profile of graphene nanosheets was displayed as dark lines. For the rGO composite coating, serious aggregation was observed due to van der Waals forces (Figure 4c). Conversely, it was clearly observed that the fG nanosheets evenly dispersed in the epoxy matrix (Figure 4d). These results confirmed that graphene nanosheets were almost exfoliated with a few layers, and the compatibility between graphene and epoxy matrix has been largely improved after grafting L-histidine. Electrochemical Behaviors for Steel Electrodes Coated with Composite Coatings. Electrochemical impe-
Figure 6. Electrical equivalent circuit models for (a) pure and rGO/ epoxy coatings, and (b) fG/epoxy coating.
The structure changes of GO were investigated by Raman spectroscopy. It can be observed that two distinct peaks at 1348 and 1575 cm−1 (Figure 1b) corresponding to D and G bands, which represent the vibration of sp3 carbon atoms from the functional groups and the in-plane vibration of sp2 carbon atoms,47,48 respectively. The intensity ratio of D and G bands (ID/IG) was employed to estimate the disordered level of the graphene crystal structure.49,50 It is obvious that the intensity ratio value increased from 0.88 to 1.10 after functionalization, which signifies the higher disorder level of fG. This result can account for the grafting reaction of L-histidine with an imidazole ring on the GO surface. The composition analysis and chemical states of fG were analyzed by XPS measurements. It was noted that the E
DOI: 10.1021/acsanm.8b00149 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Figure 7. SVET maps of the current density for steel electrodes coated with (a, b) pure epoxy, (c, d) rGO, and (e, f) fG coatings immersed in 3.5 wt % NaCl solution.
Figure 9. SEM images and XRD patterns for rust regions on the steel substrate beneath pure epoxy, GO, and fG composite coatings after 35 days of immersion.
Figure 8. Water uptake for specimens during immersion time.
dance spectroscopy (EIS) was performed to investigate the protective performance of composite coatings. Figure 5 presents the Bode plots of the specimens immersed in 3.5 wt % NaCl solution for 35 days. Usually, the appearance of multiple peaks (each peak corresponds to one time constant) in a Bode-phase plot indicates the occurrence of metal corrosion. Among them, the peak appearing at high frequency is related to the capacity and resistance of the coating, while the peak located in the low or medium frequency is assigned as the response of the corrosion reaction.54 For pure and rGO epoxy coatings, two obvious peaks were observed after 1 day of immersion, which is an indication of coating failure. Such a
rapid invalidity of the coating is displayed by an EIS response at low or medium frequency, derived from the rapid evaporation of solvents and graphene aggregation, respectively. It should be noted that water is prone to penetrate into the waterborne epoxy matrix as compared to the solvent-based resin. However, the second time constant was not apparent for the fG/ composite coating even after 35 days of immersion (Figure 5c), which revealed that fG effectively reduces the coating defects and enhances its impermeability. Moreover, the impedance modulus at the lower frequency (Zf = 0.01 Hz) could be considered as a semiquantitative F
DOI: 10.1021/acsanm.8b00149 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Figure 10. Illustration of protective mechanism for (a) rGO and (b) fG composite coatings.
indicator of the coating’s barrier capability.55,56 It can be observed that the impedance modulus values for pure and rGO/epoxy coatings decrease greatly during the immersion periods. The reduced impedance values resulted from the penetration of water into the coating matrix, which revealed that the composite coatings almost lose their barrier protective function for metal after the saturation of water absorption. In the case of the fG composite coating, its impedance remained at a relatively stable value (108 Ω cm2 or so) for 35 days of immersion, which is higher than pure and rGO/epoxy coatings. This result was consistent with a previous report by B. Ramezanzadeh et al.36 In their work, P-phenylenediamine was used to prepare functionalized graphene oxide. During the immersion period, the impedance values (Zf = 0.01 Hz) for the FGO/epoxy composite were reduced by 2 orders of magnitude, indicating the penetration of electrolyte due to coating degradation. However, the fG composite coating in our study displayed a higher value with limited variations even after 35 days of immersion. By the incorporation of functionalized graphene, the probability for corrosive species penetration has been restricted to a small extent. Thus, the as-prepared fG composite coating possessed a superior anticorrosion performance for the steel substrate. Moreover, the Nyquist plots were also used to estimate the coating’s protective properties. For the pure and rGO epoxy coatings (Figure S3), the capacitive arcs continue to decrease during the immersion process, indicating the reduced charge transfer resistance and limited anticorrosion protection for steel. After 35 days of immersion, the capacitive arcs for pure and rGO epoxy coatings were much smaller than those for fG coatings, revealing the decreased barrier performance. Compared with the pure and rGO epoxy coatings, the fG coating possessed a greater capacitive arc, which reveals its enhanced protection capability. These results demonstrated that fG could significantly improve the coating’s anticorrosion performance. For a further study of the corrosion process of different coatings in 3.5 wt % NaCl solution, the EIS data were then fitted via ZSimpWin software using the electrical equivalent
circuits presented in Figure 6. In the electrical equivalent circuits, Rs represents the solution resistance, Rc relates to the coating resistance, and Rct corresponds to charge transfer resistance.57,58 The constant phase element (Q) indicates the deviation from pure capacitance, which is referred to as the dispersion effect.59 The deviation degree from an ideal capacitive behavior can be expressed by its exponent (n). As for n = 1, Q converts to C, indicating a double-layer capacitor between electrode and solutions, and it acts as a resistor while n = 0. It is necessary to point out that the value of n is close to 1, which indicates capacitive behaviors. With the immersion time, corrosive electrolytes gradually reach the metal surface resulting in the separation of metal and coating. Because of the existence of a micropore and microcracks in the coating matrix, the saturation state of composite coatings has been realized quickly for pure and rGO/epoxy composite coatings. At this moment, the diffusion process is exhibited in the vicinity of steel electrodes, demonstrating the period of severe metal corrosion. The diffusion behavior could be revealed via Warburg component (Zw). The control step of the corrosive reaction has been converted from coating resistance into charge transfer resistance, and the mass transfer process could be demonstrated through the circuit in Figure 6a. However, the diffusion behavior has not been detected for fG/epoxy coating even after 35 days of immersion, and the corresponding circuit is presented in Figure 6b. These results proved that welldispersed fG nanosheets endow the epoxy coating with improved anticorrosion performance for the steel substrate. The time-dependent behaviors of Rc and Rct for specimens are shown in Figure S1. Usually, Rc has been considered as an indicator to estimate the barrier property of a coating by which it impedes corrosive medium penetration into the coating. As can be seen, the value of Rc for all samples decreased with the immersion time. Obviously, the Rc value for pure and rGO/ epoxy coatings reduced from 6.87 × 107 and 6.05 × 107 Ω cm2 for 1 day to 3.45 × 106 and 4.55 × 106 Ω cm2 after 35 days, respectively. However, the fG/epoxy composite coating exhibited the highest Rc values during 35 days of immersion, G
DOI: 10.1021/acsanm.8b00149 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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ACS Applied Nano Materials indicating its excellent barrier property. Similarly, the Rct values for pure epoxy and rGO coatings decreased greatly with immersion. Nevertheless, the value of Rct for the fG coating was also higher than those of other coatings, which attributed to the homogeneous dispersion of graphene nanosheets in the coating matrix. Open-Circuit Potential Test. Figure S2 presents the variations of open-circuit potential (OCP) for specimens immersed in 3.5 wt % NaCl solution. Usually, the OCP value could be considered as an indicator to reflect the corrosion tendency to some extent. Compared with solvent-based resin, corrosive mediums easily interact with waterborne epoxy resin, leading to a negative shift for the OCP value. It can be observed that all of the samples obtained relatively positive OCP values in the initial period, revealing that the corrosive ions have not yet induced metal corrosion. In addition, the OCP values decreased gradually with the penetration of corrosive electrolyte. After 35 days of immersion, the OCP values for pure epoxy and rGO/epoxy coatings dropped to 0.60 and 0.58 V, respectively. However, the fG/epoxy coating exhibited more positive OCP values during the immersion period. This result indicated that the functionalized graphene nanosheets hindered the penetration of electrolyte, and reduced the possibility of steel corrosion. Corrosion Activity of Steel beneath Composite Coatings with Scratches. Graphene-based polymer coatings exhibited improved impermeability, while the mass transfer process in the metal/coating interface could be altered because of its conductivity. For a study of the corrosion activity at the coating defects, the SVET was used to evaluate the electrochemical behaviors of coatings with scratches. The potential signals at the vicinity of scratches were detected and then converted into the local current density through Ohm’s law. Figure 7 shows the current density maps of coated steel electrodes in 3.5 wt % NaCl solution for 24 h. The aggravated corrosion reaction can be observed for pure and rGO/epoxy coatings with an enlarged corrosion area and increased corrosion current density. Because of the coating defects derived from the coating formation, corrosive ions easily arrive at the metal substrate and initiate the corrosion reaction. Conversely, the fG/epoxy coating exhibited lower corrosion activity even after 24 h of immersion. Among them, the rGO composite coating showed the highest corrosion current density, which indicates that steel substrates were corroded severely. This result can be explained by two reasons: the untreated graphene nanosheets are prone to aggregate in the polymer matrix, resulting in poor density of prepared composite coatings; and the aggregated graphene cluster can serve as a cathode when exposed to electrolyte solution, which could trigger the microgalvanic corrosion of metals. Usually, an electrode system with larger cathode-to-anode area ratio often displays a higher galvanic corrosion rate, resulting in serious metal corrosion. After functionalization, the graphene nanosheets can be well-dispersed in the polymer matrix, and the obtained composite coating showed an excellent barrier property. Moreover, the direct connections of graphene nanosheets could be greatly inhibited by grafting L-histidine, which will further reduce the probability of galvanic corrosion. Water Permeability Study of Specimens. On the basis of the above analysis, water absorption and penetration were the main reasons for coating deterioration. In the current work, we have investigated the variations of water uptake for specimens during 35 days. Because of the larger difference of
dielectric constants between the organic coating and water, the coating capacitance change can be detected even with a limited amount of absorbed water. The coating capacitance was obtained through fitting EIS results using electrical equivalent circuits, and the water content in the coating matrix (Xv, %) was also calculated based on the Brasher and Kingsbury (BK) equation:60,61
( ) × 100
log Xv , % =
Cc(t ) Cc(0)
log(80)
(1)
where Cc(t) and Cc(0) represent the capacitance at time t and initial time, and Xv (%) is the volume fraction of water in the coating. For all samples, the Xv (%) values exhibited an increased trend with the immersion (Figure 8). After 35 days of immersion, the higher water-uptake values were displayed for pure (17.31%) and rGO (16.22%) epoxy coatings, indicating the poor barrier capability. However, the value of Xv for the fG coating tends to be stable after 10 days of the test, which is greatly lower than those for pure and rGO epoxy coatings. The well-dispersed graphene nanosheets enhanced the coating’s impermeability. These results were consistent with EIS results, demonstrating the excellent protective performance of fG epoxy coatings. Characterization of Corrosive Products. For a further study of the effect of composite coatings on the steel electrodes beneath them, XRD and SEM were employed to evaluate the chemical component and morphology of the steel surface after the coatings were peeled off for 35 days of immersion. Figure 9 shows the crystallinity and morphology of corrosive products formed on steel electrodes. Because of the strong peaks of the steel substrate, the diffraction peaks for metallic oxide were relatively weak. It can be observed that the corrosive products for pure epoxy and rGO coatings mainly consisted of αFeOOH, β-FeOOH, and γ-FeOOH, which could be attributed to the penetration of corrosive mediums through the coating matrix. However, for the coatings containing fG, the formation of α-FeOOH and γ-FeOOH was largely inhibited because of its superior barrier capability. The morphology for specimens is also shown in Figure 9 (inset). The surface was mainly covered with loose rust for pure and rGO/epoxy-coated steel. However, there are a few apparent corrosive products on the surface of the steel beneath fG composite coatings. This result was consistent with XRD analysis, indicating that the enhanced anticorrosion performance was achieved by incorporating Lhistidine-functionalized graphene. Anticorrosion Mechanism of fG/Epoxy Coating. The compatibility between graphene and the coating matrix is a considerable factor for the protective performance of composite coatings. For rGO epoxy coating, micropores and cracks will be produced accompanied by coating formation because of its hydrophobicity. Once the corrosive ions penetrate, a microgalvanic (the rGO serves as cathode, and metal acts as anode) environment will be formed, leading to the galvanic corrosion of steel. With the immersion, more galvanic couples formed on the steel/coating interface, which further accelerate the metal corrosion (Figure 10a). The oxidation and reduction reactions appeared in the coating/metal interface as follows:
H
Fe → Fe2 + + 2e−
(2)
Fe 2 + → Fe3 + + e−
(3) DOI: 10.1021/acsanm.8b00149 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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ACS Applied Nano Materials 2H 2O + O2 + 4e− → 4OH−
(4)
2Fe2 + + O2 + 2H 2O → 2FeOOH + 2H+
(5)
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully appreciate financial support provided by the “One Hundred Talented People” of the Chinese Academy of Sciences (Y60707WR04); Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (QYZDY-SSWJSC009), Natural Science Foundation of Zhejiang Province (2015C01006), and Zhejiang Province Key Technology Project (Y16B040008).
However, the functionalized graphene nanosheets can be well-dispersed in the epoxy matrix with the assistance of Lhistidine molecules. The well-dispersed graphene, which can act as a barrier, effectively reduces the generation of micropores in the coating matrix and impedes the penetration of corrosive ions. With the grafting of L-histidine on the edge of graphene, the direct connections of graphene−graphene and graphene− metal could be largely inhibited. Because the steel oxidation process only occurs on the electrode’s surface, and the mass transfer is restricted by the coating, the electrons cannot be captured by oxygen. Thus, the oxidation reaction of steel and the reduction process of oxygen were largely inhibited, leading to its excellent anticorrosion performance. The corresponding protective mechanism is illustrated in Figure 10b.
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CONCLUSIONS In summary, we have successfully synthesized L-histidinefunctionalized graphene nanomaterials with favorable dispersibility and a galvanic corrosion inhibitive effect. This is a new feasible and environmentally friendly strategy to exert the barrier function of graphene. The obtained functionalized graphene nanosheets exhibited an intact lamellar structure in aqueous solution as characterized by TEM and SPM. The anticorrosion performance of waterborne epoxy coatings was significantly enhanced by only the incorporation of 1 wt % fG nanosheets. L-Histidine was attached to the surface of graphene via covalent interaction, which effectively reduced the possibility of graphene−metal couples. The protection mechanism of fG is based on the impermeable property by extending the diffusion pathway for the corrosive medium and an insulating function through avoiding direct connections as proven via EIS and the SVET. This novel water-dispersible functionalized graphene may open a new scope to prepare waterborne organic coatings with improved anticorrosion performance. We believe that this facile and versatile strategy for graphene functionalization can be applied in not only the field of corrosion protection but also other areas such as electromagnetic interference shielding and antistatic and antimicrobial coatings.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00149. Variation of the electrical parameters Rc and Rct obtained from EIS data, evolution of OCP values, Nyquist plots of different coating/steel systems, and the values of coating capacitance for specimens (PDF)
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REFERENCES
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AUTHOR INFORMATION
Corresponding Authors
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[email protected]. Fax: 0574-86685159. Phone: 0574-86657094. *E-mail:
[email protected]. Phone: 0574-86325713. ORCID
Chengbao Liu: 0000-0002-1506-757X Peng Du: 0000-0001-9730-7487 Haichao Zhao: 0000-0002-3558-1306 I
DOI: 10.1021/acsanm.8b00149 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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