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Liquid-phase exfoliated fluorographene as a two dimensional coating filler for enhanced corrosion protection performance
Accepted Manuscript Title: Liquid-phase exfoliated fluorographene as a two dimensional coating filler for enhanced corrosion protection performance Author: Zhengqing Yang Wen Sun Lida Wang Sijia Li Tianzhen Zhu Guichang Liu PII: DOI: Reference:
S0010-938X(15)30139-6 http://dx.doi.org/doi:10.1016/j.corsci.2015.10.039 CS 6535
To appear in: Received date: Revised date: Accepted date:
6-10-2015 27-10-2015 28-10-2015
Please cite this article as: Zhengqing Yang, Wen Sun, Lida Wang, Sijia Li, Tianzhen Zhu, Guichang Liu, Liquid-phase exfoliated fluorographene as a two dimensional coating filler for enhanced corrosion protection performance, Corrosion Science http://dx.doi.org/10.1016/j.corsci.2015.10.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Liquid-phase exfoliated fluorographene as a two dimensional coating filler for enhanced corrosion protection performance
Zhengqing Yang, Wen Sun, Lida Wang, Sijia Li, Tianzhen Zhu, Guichang Liu*
Department of Materials Science and Chemical Engineering, Dalian University of Technology, No.2 Linggong Road, Dalian, 116024, China
*Correspondence. E-mail: [email protected] Tel. / Fax: 86-411-84986047
1
Abstract: Though a promising corrosion protection material, graphene can accelerate metal corrosion due to its thermodynamic stability and high conductivity. In this work, few-layer fluorographene (FG) is prepared by a liquid-phase exfoliation method and incorporated into polyvinyl butyral (PVB) coating to enhance its corrosion protection performances. The results show that the introduced FG confers enhanced barrier property on the PVB coating by preventing the penetration of aggressive species. Besides, unlike graphene, FG cannot promote metal corrosion because the insulating nature of FG impedes the formation of metal-filler galvanic corrosion cell.
Keywords: A. Organic coatings; B. EIS; B. SEM; C. Oxidation; C. Oxygen reduction
2
1. Introduction
Graphene, a single atom-thick sheet of carbon, has many outstanding properties, such as excellent electric and thermal conductivity, impermeable to molecules and so on [1-4]. Since its discovery, graphene has attracted a great deal of attention within the scientific community due to its unique physical and chemical properties. At present, graphene not only displays application potentials in electronics, catalysis, magnetics, but also provides new chance for applications in corrosion protection. Graphene has been named as “the thinnest corrosion resistant coating” [5, 6].
Up to now, numerous papers and patents on graphene-based corrosion resistant coating have sprung up mainly because graphene itself as a coating material displays outstanding protection performance. To prevent chemical corrosion, Kang et al. [7] successfully prepared few-layer reduced graphene oxide on the surface of iron and copper foils. They demonstrated that the graphene coating can prevent the metallic substrate from oxidation in air at 200 oC (oxidation time is 2 hours). Nayak et al. [8] stated that graphene grown on Ni substrate by CVD was significantly resistant to both thermal oxidation in air at 500 oC and chemical oxidation in H2O2 electrolyte. For inhibition of electrochemical corrosion, Prasai et al. [9] demonstrated that graphene coating can effectively inhibit the oxidation reaction process of metal, thus providing good corrosion protection for metal substrates. Subsequently, Kirkland et al. [2] systemically studied the corrosion behaviors of graphene coated Ni and Cu in aqueous medium. They found that graphene can enhance the corrosion resistance of Ni and Cu because it slows down corresponding anodic dissolution and cathodic reduction reactions, respectively. Kousalya et al. [10] suggested that few-layer graphene can serve as a protective coating even under flow boiling conditions.
Besides, researchers also have applied graphene nanosheet as filler to improve the corrosion resistance of protective coatings. Chang et al. [11], for the first time, prepared polymer/graphene composite coating for the corrosion protection of steel. 3
The coating presents enhanced corrosion protection due to its excellent impermeability to O2 and H2O. Chang et al. [12] prepared epoxy/graphene coating by adding graphene into epoxy coating. They found the epoxy/graphene coating can provide good corrosion protection for cold-rolled steel because well-dispersed graphene with high aspect ratio can effectively enhance its oxygen barrier performance.
Through graphene has already displayed apparent advantages over traditional corrosion protection materials, graphene, like a double-edged sword, also exhibits non-negligible disadvantages in corrosion protection. Recently, Dong et al. [13, 14] stated that graphene coating can only maintain its protection property in a short period of time. However, during the long-term immersion process, graphene can promote the metal corrosion at coating defects. Zhou et al. [15] also found the graphene induced corrosion promotion phenomenon of copper. And they believed that the corrosion promotion effect was caused by the defects and high electrical conductivity of graphene. Besides, graphene as coating filler also displays potential corrosion promotion effect to some extent. Our group found that when graphene was introduced into PVB coating as filler, the copper substrate apparently exhibited accelerated corrosion once the coating is mechanically damaged [16].
Currently, avoiding the corrosion promotion activity of graphene and developing other graphene-based materials are tough challenges for the further development of graphene for corrosion protection. However, to the best of our knowledge, there are few researches concerning this field. At present, it is believed that high potential (thermodynamic stability) and high conductivity are the two significant factors that contribute to the corrosion promotion activity of graphene. Hence, the corrosion promotion activity is essentially the galvanic corrosion of graphene-metal couple [15, 17]. Based on the high conductivity of graphene, our group introduced graphene encapsulation strategy to inhibit this effect of graphene, such as insulating pernigraniline modification [16], silane coupling agent decoration [18] and nanosized 4
silicon oxides isolation [19].
Besides of the encapsulation strategy, another method to inhibit the corrosion promotion activity of graphene is seeking substitute materials. FG is also a promising carbon material with unique physical and chemical properties, such as thermal and chemical stability, excellent insulation property [20-24]. In this paper, FG is prepared from fluorographite (FGi) by a liquid-phase exfoliation method and introduced into the organic coating. The FG enhanced corrosion resistant mechanism is discussed together with their corrosion promotion activity.
2. Experimental
2.1. Fabrication of FG FG was prepared from FGi (Grade I, Hubei Zhuoxi Fluorochemical Co., Ltd) through a sonochemical exfoliation process [23]. The F/C atomic ratio is about 0.925, and the specific resistance is about 109 Ω cm. In a typical experiment, 1 g of FGi was added to 200 mL N-methyl-2-pyrrolidone (NMP) in order to prepare FG/NMP suspension. Then, under continuously magnetic stirring, the mixture was placed in 60 oC water bath for 2 h. Subsequently, the obtained black and homogeneous dispersion was sonicated in a low power (40 W) sonication bath (Kunshan KQ-100DB) for 15 h after the solution was cooled down to room temperature. Finally, the exfoliated FG dispersions were centrifuged, washed by methanol and dried.
2.2. Preparation of coating Copper substrate was embedded in epoxy resin, leaving an exposed area of 0.785 cm2. Before coating, the substrates were respectively ground with 800 , 1000 and 1200 SiC grit papers, then ultrasonically cleaned in acetone for 15 minutes, washed by deionized water for 3 times and blow-dried. Meanwhile, 8 mg FG were dispersed in 20 mL methanol by ultrasonic dispersion for 1 h, producing an evenly dispersed brown FG dispersion. Next, 2 g PVB powders (Sinopharm Chemical Reagent Co., Ltd) 5
were added to the dispersion. Finally, the FG/PVB solution was obtained by stirring the sticky mixture and standing for 24 h. A dip-coating method was employed to prepare FG-modified PVB coating on the substrates, and then dried at 30 oC for 1 day, which was marked as FG0.4. Analogously, other FG contents (0, 10, 14 and 28 mg) were marked as FG0, FG0.5, FG0.7 and FG1.4, respectively. The controlled coating thickness was fundamentally the same and the range was 43~50 μm.
2.3. Characterization The morphology of FG was determined by transmission electron microscopy (TEM, Tecnai G220). Energy dispersive analysis (EDS) was used to characterize the exfoliated FG. Atomic force microscopy (AFM, Veeco Multimode) was used to determine the thickness of FG nanosheet. The coating thickness was measured by a portable coating thickness gauge (TT260, Beijing Time High Technology Co., Ltd). The cross-section of FG-modified coating was determined by field emission scanning electronic microscope (FE-SEM, ZEISS Ultra 55, Germany). Metallographic microscope (BX51M, Olympus, Japan) and Raman spectra (DXR, Therom Fisher Scientific, America) were used to characterize the corrosion products on the copper substrates.
2.4. Electrochemical tests Electrochemical impedance spectroscopy (EIS) measurement was performed on the FG-modified PVB coatings during 3 months of immersion in 3.5 wt. % NaCl aqueous solution. The tests were implemented by a conventional three-electrode system, which consists of a platinum sheet as counter electrode, a saturated calomel electrode (SCE) as reference electrode and tested sample as working electrode. All the EIS tests were conducted on a CHI 750e electrochemical workstation (Shanghai Chenhua Device Company, China) in the frequency of 105~10-2 Hz and the single amplitude was 50 mV. 6
3. Results and discussion
3.1. Morphology and structure of FG TEM image in Fig.1a shows that the exfoliated FG materials consist of large electron transparent flat nanosheets. Fig. 1b reveals that the chemical composition of FG is basically the same with that of pristine FGi. AFM image in Fig. 1c shows that FG nanosheet is about 3.5 nm in thickness. Usually, the thickness of a single layer FG is ca 0.8~1.0 nm [21, 25], it is believed that the layer number of as-prepared FG is about 3~5.
3.2. Properties of FG suspension and coating Fig. 2a displays FGi and FG dispersions after standing for 30 days. It shows that FGi is slowly setting to the bottom of glass bottle, while FG can be uniformly dispersed in NMP and exhibits no sediments. SEM image in Fig. 2b clearly shows that there are numerous micropores formed in the matrix of blank coating, which are marked by red circle. Fig. 2c shows that there are fewer micropores in FG0.4 coating matrix when compared with blank coating. However, for FG0.5, FG0.7 and FG1.4 samples (Fig. 2d-f), no microdefects are observed in the coatings. The SEM images reveal that the addition of FG into coating matrix can reduce microdefects. Especially, the microdefects disappear when embedding 0.5~1.4 wt. % FG in the coating matrix.
3.3. Corrosion behavior of Cu coated by FG-modified coatings EIS measurements were employed to evaluate the corrosion protection performance of the coatings according to the electrochemical processes in the interface of metal/medium [26]. The Bode plots of the FG-modified and blank FG0 coatings during immersion are presented in Fig. 3. At the initial stage of immersion of all coatings, as is shown in Fig. 3a-j, the slope for log|Z| against logf is about 1 in the whole frequency range, and the phase angle is close to 90o at a broad frequency range, indicating that those coatings correspond to a isolating layer with no solution permeating at this time [27, 28]. With prolonging the immersion time to the 8th day, 7
two time constants appear in the impedance spectra of the blank coating (Fig. 3b). It is well known that the time constant at high frequency region can be assigned to the response of coating, while that at low frequency region corresponds to the property of the electrical double layer of metal/medium [27, 29-31]. Hence, it is concluded that there are numerous defects in the blank coating and the copper substrate suffered from corrosion attack. However, for FG-modified coatings (Fig. 3d, f, h and j), there is no corrosion response appearing in the Bode-phase plots, and only the response of coating can be discovered at the medium/high frequencies range. The Bode plots prove that FG-modified coatings display a good corrosion protection performance.
The EIS results were then fitted by ZSimpWin in order to quantitatively assess the performance of these coatings. The equivalent circuits are shown in Fig. 4a. For perfect coating, Qc represents constant phase element, Rc is the coating resistance and Rs is the solution resistance [32]. For failed coating, Rpo denotes pore resistance, Rt and Qdl are metal corrosion reaction resistance and capacitance of double-layer, respectively [28]. The fitting results are shown in Fig. 4b. It can be seen that the Rc of FG0 decreases from 6.6×1010 to 7.9×105 Ω cm2 during 3 days of immersion, then the value keeps almost stable at about 3.3×105 Ω cm2, which manifests that FG0 rapidly lose its corrosion resistant performance. On the contrary, the Rc of FG0.4 decreases from 9.1×1010 to 2.3×108 Ω cm2 on the 44th day. Though the Rc of FG0.4 decreases, it is still 3 orders of magnitude higher than that of FG0, which manifests that the corrosion protection performance of FG0.4 is more superior to that of FG0. For FG0.5, FG0.7 and FG1.4, the Rc remain almost unchanged and are kept at the range from 1010 to 1011 Ω cm2 during the 3 months of immersion, manifesting that they can exhibit an outstanding corrosion resistance for a long time.
After 3 months of immersion, the coatings were mechanically peeled off from the copper substrate, and the coating residues were removed by washing the copper substrate with methanol for a few times. The micrographs manifest that there are corrosion products generated on the surfaces of the copper coated by FG0 and FG0.4 8
(Fig. 5a and b), and much less corrosion product is shown on the copper substrate coated by FG0.4 compared with that coated by FG0. Meanwhile, Fig. 5c-e show that the morphologies of copper surface coated by FG0.5, FG0.7 and FG1.4 are similar with that of a ground copper. Raman spectra in Fig. 5f demonstrates that the copper surface coated by FG0 displays obvious Raman activity. Five peaks are observed at 145, 215, 413, 528 and 633 cm-1, respectively, which can be attributed to Γ15−(1), 2Γ12−, 4Γ12−, Γ25+ and Γ15−(2) vibration modes of Cu2O. In contrast, the five peaks located at the same wavenumber are not obvious on the substrate of FG0.4. However, the spectra of FG0.5, FG0.7 and FG1.4 are similar with that of ground Cu, which indicates that these coatings can exhibit long-term corrosion protection performance.
FG, a graphene derivative, can enhance corrosion protection performance of PVB coating due to not only its graphene-like structure, but also properties inherited from graphene (e.g. molecule impermeability). Similar to graphene, well-dispersed FG increases the diffusion resistance of corrosive species (H2O, O2 and Cl-) in PVB matrix, and thus prolongs service lifetime of coating. More importantly, FG shows properties that are superior to graphene in corrosion protection. For FG, the introduction of fluorine decreases the conductivity of graphene. The effect is due to interaction of the p orbitals F with π orbitals C producing sp3 bonds that modify the charge densities and introduce scattering centers for conduction [33]. Therefore, unlike graphene, micro-galvanic corrosion between FG and copper at coating defects would be suppressed by the insulating nature of FG. Hence, we believe that FG may be used as a promising substitute material for graphene in corrosion protection applications.
4. Conclusions
In this work, few-layer FG nanosheet was successfully exfoliated from FGi by an ultrasonication method. FG was used as a novel corrosion protection filler to enhance the physical barrier performance of PVB coating. The EIS measurement shows that 9
PVB coating can keep outstanding corrosion resistance when appropriate amounts of FG are added into the coating matrix. The enhanced corrosion resistant performance was due to the reason that the FG nanosheet can effectively prevent the corrosive medium from permeating through the coating matrix to the copper/coating interface. Furthermore, the obtained FG does not induce the corrosion acceleration of copper substrate. FG may be used as an attractive substitute of graphene for corrosion protection applications in the future.
Acknowledgements The authors gratefully acknowledge the natural support of the National Natural Science Foundation of China (No. 21403030), Natural Science Foundation of Liaoning Province (No. 2015020178) and Fundamental Research Funds for the Central Universities (No. 852014).
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Fig. 1. (a) TEM image; (b) EDS spectrum; (c) AFM image of FG.
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Fig. 2. (a) Dispersions standing for 30 days before and after exfoliation; (b-f) SEM images of different FG-modified coatings. (b) FG0, (c) FG0.4, (d) FG0.5, (e) FG0.7, (f) FG1.4.
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Fig. 3. Bode plots of the coating during the period of immersion in 3.5 wt.% NaCl aqueous solution for three months. (a)(b) FG0, (c)(d) FG0.4, (e)(f) FG0.5, (g)(h) FG0.7, (i)(j) FG1.4.
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Fig. 4. (a) Equivalent circuits and (b) The fitting results of the EIS of different coatings.
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Fig. 5. (a-e) Micrographs of the copper substrates coated by different coatings. (a) FG0, (b) FG0.4, (c) FG0.5, (d) FG0.7, (e) FG1.4, Inset: Ground copper; (f) Raman spectra of the copper coated by different coatings.
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S0010-938X(15)30139-6 http://dx.doi.org/doi:10.1016/j.corsci.2015.10.039 CS 6535
To appear in: Received date: Revised date: Accepted date:
6-10-2015 27-10-2015 28-10-2015
Please cite this article as: Zhengqing Yang, Wen Sun, Lida Wang, Sijia Li, Tianzhen Zhu, Guichang Liu, Liquid-phase exfoliated fluorographene as a two dimensional coating filler for enhanced corrosion protection performance, Corrosion Science http://dx.doi.org/10.1016/j.corsci.2015.10.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Liquid-phase exfoliated fluorographene as a two dimensional coating filler for enhanced corrosion protection performance
Zhengqing Yang, Wen Sun, Lida Wang, Sijia Li, Tianzhen Zhu, Guichang Liu*
Department of Materials Science and Chemical Engineering, Dalian University of Technology, No.2 Linggong Road, Dalian, 116024, China
*Correspondence. E-mail: [email protected] Tel. / Fax: 86-411-84986047
1
Abstract: Though a promising corrosion protection material, graphene can accelerate metal corrosion due to its thermodynamic stability and high conductivity. In this work, few-layer fluorographene (FG) is prepared by a liquid-phase exfoliation method and incorporated into polyvinyl butyral (PVB) coating to enhance its corrosion protection performances. The results show that the introduced FG confers enhanced barrier property on the PVB coating by preventing the penetration of aggressive species. Besides, unlike graphene, FG cannot promote metal corrosion because the insulating nature of FG impedes the formation of metal-filler galvanic corrosion cell.
Keywords: A. Organic coatings; B. EIS; B. SEM; C. Oxidation; C. Oxygen reduction
2
1. Introduction
Graphene, a single atom-thick sheet of carbon, has many outstanding properties, such as excellent electric and thermal conductivity, impermeable to molecules and so on [1-4]. Since its discovery, graphene has attracted a great deal of attention within the scientific community due to its unique physical and chemical properties. At present, graphene not only displays application potentials in electronics, catalysis, magnetics, but also provides new chance for applications in corrosion protection. Graphene has been named as “the thinnest corrosion resistant coating” [5, 6].
Up to now, numerous papers and patents on graphene-based corrosion resistant coating have sprung up mainly because graphene itself as a coating material displays outstanding protection performance. To prevent chemical corrosion, Kang et al. [7] successfully prepared few-layer reduced graphene oxide on the surface of iron and copper foils. They demonstrated that the graphene coating can prevent the metallic substrate from oxidation in air at 200 oC (oxidation time is 2 hours). Nayak et al. [8] stated that graphene grown on Ni substrate by CVD was significantly resistant to both thermal oxidation in air at 500 oC and chemical oxidation in H2O2 electrolyte. For inhibition of electrochemical corrosion, Prasai et al. [9] demonstrated that graphene coating can effectively inhibit the oxidation reaction process of metal, thus providing good corrosion protection for metal substrates. Subsequently, Kirkland et al. [2] systemically studied the corrosion behaviors of graphene coated Ni and Cu in aqueous medium. They found that graphene can enhance the corrosion resistance of Ni and Cu because it slows down corresponding anodic dissolution and cathodic reduction reactions, respectively. Kousalya et al. [10] suggested that few-layer graphene can serve as a protective coating even under flow boiling conditions.
Besides, researchers also have applied graphene nanosheet as filler to improve the corrosion resistance of protective coatings. Chang et al. [11], for the first time, prepared polymer/graphene composite coating for the corrosion protection of steel. 3
The coating presents enhanced corrosion protection due to its excellent impermeability to O2 and H2O. Chang et al. [12] prepared epoxy/graphene coating by adding graphene into epoxy coating. They found the epoxy/graphene coating can provide good corrosion protection for cold-rolled steel because well-dispersed graphene with high aspect ratio can effectively enhance its oxygen barrier performance.
Through graphene has already displayed apparent advantages over traditional corrosion protection materials, graphene, like a double-edged sword, also exhibits non-negligible disadvantages in corrosion protection. Recently, Dong et al. [13, 14] stated that graphene coating can only maintain its protection property in a short period of time. However, during the long-term immersion process, graphene can promote the metal corrosion at coating defects. Zhou et al. [15] also found the graphene induced corrosion promotion phenomenon of copper. And they believed that the corrosion promotion effect was caused by the defects and high electrical conductivity of graphene. Besides, graphene as coating filler also displays potential corrosion promotion effect to some extent. Our group found that when graphene was introduced into PVB coating as filler, the copper substrate apparently exhibited accelerated corrosion once the coating is mechanically damaged [16].
Currently, avoiding the corrosion promotion activity of graphene and developing other graphene-based materials are tough challenges for the further development of graphene for corrosion protection. However, to the best of our knowledge, there are few researches concerning this field. At present, it is believed that high potential (thermodynamic stability) and high conductivity are the two significant factors that contribute to the corrosion promotion activity of graphene. Hence, the corrosion promotion activity is essentially the galvanic corrosion of graphene-metal couple [15, 17]. Based on the high conductivity of graphene, our group introduced graphene encapsulation strategy to inhibit this effect of graphene, such as insulating pernigraniline modification [16], silane coupling agent decoration [18] and nanosized 4
silicon oxides isolation [19].
Besides of the encapsulation strategy, another method to inhibit the corrosion promotion activity of graphene is seeking substitute materials. FG is also a promising carbon material with unique physical and chemical properties, such as thermal and chemical stability, excellent insulation property [20-24]. In this paper, FG is prepared from fluorographite (FGi) by a liquid-phase exfoliation method and introduced into the organic coating. The FG enhanced corrosion resistant mechanism is discussed together with their corrosion promotion activity.
2. Experimental
2.1. Fabrication of FG FG was prepared from FGi (Grade I, Hubei Zhuoxi Fluorochemical Co., Ltd) through a sonochemical exfoliation process [23]. The F/C atomic ratio is about 0.925, and the specific resistance is about 109 Ω cm. In a typical experiment, 1 g of FGi was added to 200 mL N-methyl-2-pyrrolidone (NMP) in order to prepare FG/NMP suspension. Then, under continuously magnetic stirring, the mixture was placed in 60 oC water bath for 2 h. Subsequently, the obtained black and homogeneous dispersion was sonicated in a low power (40 W) sonication bath (Kunshan KQ-100DB) for 15 h after the solution was cooled down to room temperature. Finally, the exfoliated FG dispersions were centrifuged, washed by methanol and dried.
2.2. Preparation of coating Copper substrate was embedded in epoxy resin, leaving an exposed area of 0.785 cm2. Before coating, the substrates were respectively ground with 800 , 1000 and 1200 SiC grit papers, then ultrasonically cleaned in acetone for 15 minutes, washed by deionized water for 3 times and blow-dried. Meanwhile, 8 mg FG were dispersed in 20 mL methanol by ultrasonic dispersion for 1 h, producing an evenly dispersed brown FG dispersion. Next, 2 g PVB powders (Sinopharm Chemical Reagent Co., Ltd) 5
were added to the dispersion. Finally, the FG/PVB solution was obtained by stirring the sticky mixture and standing for 24 h. A dip-coating method was employed to prepare FG-modified PVB coating on the substrates, and then dried at 30 oC for 1 day, which was marked as FG0.4. Analogously, other FG contents (0, 10, 14 and 28 mg) were marked as FG0, FG0.5, FG0.7 and FG1.4, respectively. The controlled coating thickness was fundamentally the same and the range was 43~50 μm.
2.3. Characterization The morphology of FG was determined by transmission electron microscopy (TEM, Tecnai G220). Energy dispersive analysis (EDS) was used to characterize the exfoliated FG. Atomic force microscopy (AFM, Veeco Multimode) was used to determine the thickness of FG nanosheet. The coating thickness was measured by a portable coating thickness gauge (TT260, Beijing Time High Technology Co., Ltd). The cross-section of FG-modified coating was determined by field emission scanning electronic microscope (FE-SEM, ZEISS Ultra 55, Germany). Metallographic microscope (BX51M, Olympus, Japan) and Raman spectra (DXR, Therom Fisher Scientific, America) were used to characterize the corrosion products on the copper substrates.
2.4. Electrochemical tests Electrochemical impedance spectroscopy (EIS) measurement was performed on the FG-modified PVB coatings during 3 months of immersion in 3.5 wt. % NaCl aqueous solution. The tests were implemented by a conventional three-electrode system, which consists of a platinum sheet as counter electrode, a saturated calomel electrode (SCE) as reference electrode and tested sample as working electrode. All the EIS tests were conducted on a CHI 750e electrochemical workstation (Shanghai Chenhua Device Company, China) in the frequency of 105~10-2 Hz and the single amplitude was 50 mV. 6
3. Results and discussion
3.1. Morphology and structure of FG TEM image in Fig.1a shows that the exfoliated FG materials consist of large electron transparent flat nanosheets. Fig. 1b reveals that the chemical composition of FG is basically the same with that of pristine FGi. AFM image in Fig. 1c shows that FG nanosheet is about 3.5 nm in thickness. Usually, the thickness of a single layer FG is ca 0.8~1.0 nm [21, 25], it is believed that the layer number of as-prepared FG is about 3~5.
3.2. Properties of FG suspension and coating Fig. 2a displays FGi and FG dispersions after standing for 30 days. It shows that FGi is slowly setting to the bottom of glass bottle, while FG can be uniformly dispersed in NMP and exhibits no sediments. SEM image in Fig. 2b clearly shows that there are numerous micropores formed in the matrix of blank coating, which are marked by red circle. Fig. 2c shows that there are fewer micropores in FG0.4 coating matrix when compared with blank coating. However, for FG0.5, FG0.7 and FG1.4 samples (Fig. 2d-f), no microdefects are observed in the coatings. The SEM images reveal that the addition of FG into coating matrix can reduce microdefects. Especially, the microdefects disappear when embedding 0.5~1.4 wt. % FG in the coating matrix.
3.3. Corrosion behavior of Cu coated by FG-modified coatings EIS measurements were employed to evaluate the corrosion protection performance of the coatings according to the electrochemical processes in the interface of metal/medium [26]. The Bode plots of the FG-modified and blank FG0 coatings during immersion are presented in Fig. 3. At the initial stage of immersion of all coatings, as is shown in Fig. 3a-j, the slope for log|Z| against logf is about 1 in the whole frequency range, and the phase angle is close to 90o at a broad frequency range, indicating that those coatings correspond to a isolating layer with no solution permeating at this time [27, 28]. With prolonging the immersion time to the 8th day, 7
two time constants appear in the impedance spectra of the blank coating (Fig. 3b). It is well known that the time constant at high frequency region can be assigned to the response of coating, while that at low frequency region corresponds to the property of the electrical double layer of metal/medium [27, 29-31]. Hence, it is concluded that there are numerous defects in the blank coating and the copper substrate suffered from corrosion attack. However, for FG-modified coatings (Fig. 3d, f, h and j), there is no corrosion response appearing in the Bode-phase plots, and only the response of coating can be discovered at the medium/high frequencies range. The Bode plots prove that FG-modified coatings display a good corrosion protection performance.
The EIS results were then fitted by ZSimpWin in order to quantitatively assess the performance of these coatings. The equivalent circuits are shown in Fig. 4a. For perfect coating, Qc represents constant phase element, Rc is the coating resistance and Rs is the solution resistance [32]. For failed coating, Rpo denotes pore resistance, Rt and Qdl are metal corrosion reaction resistance and capacitance of double-layer, respectively [28]. The fitting results are shown in Fig. 4b. It can be seen that the Rc of FG0 decreases from 6.6×1010 to 7.9×105 Ω cm2 during 3 days of immersion, then the value keeps almost stable at about 3.3×105 Ω cm2, which manifests that FG0 rapidly lose its corrosion resistant performance. On the contrary, the Rc of FG0.4 decreases from 9.1×1010 to 2.3×108 Ω cm2 on the 44th day. Though the Rc of FG0.4 decreases, it is still 3 orders of magnitude higher than that of FG0, which manifests that the corrosion protection performance of FG0.4 is more superior to that of FG0. For FG0.5, FG0.7 and FG1.4, the Rc remain almost unchanged and are kept at the range from 1010 to 1011 Ω cm2 during the 3 months of immersion, manifesting that they can exhibit an outstanding corrosion resistance for a long time.
After 3 months of immersion, the coatings were mechanically peeled off from the copper substrate, and the coating residues were removed by washing the copper substrate with methanol for a few times. The micrographs manifest that there are corrosion products generated on the surfaces of the copper coated by FG0 and FG0.4 8
(Fig. 5a and b), and much less corrosion product is shown on the copper substrate coated by FG0.4 compared with that coated by FG0. Meanwhile, Fig. 5c-e show that the morphologies of copper surface coated by FG0.5, FG0.7 and FG1.4 are similar with that of a ground copper. Raman spectra in Fig. 5f demonstrates that the copper surface coated by FG0 displays obvious Raman activity. Five peaks are observed at 145, 215, 413, 528 and 633 cm-1, respectively, which can be attributed to Γ15−(1), 2Γ12−, 4Γ12−, Γ25+ and Γ15−(2) vibration modes of Cu2O. In contrast, the five peaks located at the same wavenumber are not obvious on the substrate of FG0.4. However, the spectra of FG0.5, FG0.7 and FG1.4 are similar with that of ground Cu, which indicates that these coatings can exhibit long-term corrosion protection performance.
FG, a graphene derivative, can enhance corrosion protection performance of PVB coating due to not only its graphene-like structure, but also properties inherited from graphene (e.g. molecule impermeability). Similar to graphene, well-dispersed FG increases the diffusion resistance of corrosive species (H2O, O2 and Cl-) in PVB matrix, and thus prolongs service lifetime of coating. More importantly, FG shows properties that are superior to graphene in corrosion protection. For FG, the introduction of fluorine decreases the conductivity of graphene. The effect is due to interaction of the p orbitals F with π orbitals C producing sp3 bonds that modify the charge densities and introduce scattering centers for conduction [33]. Therefore, unlike graphene, micro-galvanic corrosion between FG and copper at coating defects would be suppressed by the insulating nature of FG. Hence, we believe that FG may be used as a promising substitute material for graphene in corrosion protection applications.
4. Conclusions
In this work, few-layer FG nanosheet was successfully exfoliated from FGi by an ultrasonication method. FG was used as a novel corrosion protection filler to enhance the physical barrier performance of PVB coating. The EIS measurement shows that 9
PVB coating can keep outstanding corrosion resistance when appropriate amounts of FG are added into the coating matrix. The enhanced corrosion resistant performance was due to the reason that the FG nanosheet can effectively prevent the corrosive medium from permeating through the coating matrix to the copper/coating interface. Furthermore, the obtained FG does not induce the corrosion acceleration of copper substrate. FG may be used as an attractive substitute of graphene for corrosion protection applications in the future.
Acknowledgements The authors gratefully acknowledge the natural support of the National Natural Science Foundation of China (No. 21403030), Natural Science Foundation of Liaoning Province (No. 2015020178) and Fundamental Research Funds for the Central Universities (No. 852014).
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Fig. 1. (a) TEM image; (b) EDS spectrum; (c) AFM image of FG.
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Fig. 2. (a) Dispersions standing for 30 days before and after exfoliation; (b-f) SEM images of different FG-modified coatings. (b) FG0, (c) FG0.4, (d) FG0.5, (e) FG0.7, (f) FG1.4.
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Fig. 3. Bode plots of the coating during the period of immersion in 3.5 wt.% NaCl aqueous solution for three months. (a)(b) FG0, (c)(d) FG0.4, (e)(f) FG0.5, (g)(h) FG0.7, (i)(j) FG1.4.
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Fig. 4. (a) Equivalent circuits and (b) The fitting results of the EIS of different coatings.
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Fig. 5. (a-e) Micrographs of the copper substrates coated by different coatings. (a) FG0, (b) FG0.4, (c) FG0.5, (d) FG0.7, (e) FG1.4, Inset: Ground copper; (f) Raman spectra of the copper coated by different coatings.
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