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Corrosion behavior of Cu during graphene growth by CVD
Corrosion Science 89 (2014) 214–219
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Corrosion behavior of Cu during graphene growth by CVD Yuhua Dong ⇑, Qingqing Liu, Qiong Zhou College of Science, China University of Petroleum (Beijing), Beijing 102249, China
a r t i c l e
i n f o
Article history: Received 20 June 2014 Accepted 20 August 2014 Available online 27 August 2014 Keywords: A. Copper B. EIS B. SEM
a b s t r a c t The corrosion performance of Cu samples may be affected by annealing at high temperatures during graphene growth via the chemical vapor deposition method. In this study, multiple graphene films were deposited on Cu and characterized by Raman spectroscopy and transmission electron microscopy. The corrosion behavior of Cu immersed in 3.5 wt.% NaCl solution was investigated using electrochemical impedance spectroscopy. The Cu morphology was observed by optical microscopy and scanning electron microscopy. Results indicated that annealing affects the corrosion process of Cu. The presence of graphene films on the Cu substrate improved the corrosion performance of the material for a short period of time. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental
Numerous studies have demonstrated that graphene film, which feature chemical inertness and thermal stability, are excellent anticorrosion barriers for Cu [1–6]. Graphene film is generally prepared through chemical vapor deposition (CVD), during which Cu sheets are annealed at high temperatures. Raman et al. [1] believed that heat treatment of Cu specimens during graphene growth does not significantly influence the Cu corrosion rate. However, several researchers have found that the Cu microstructure, grain size, and mechanical properties are affected by annealing [7,8]. A previous study has suggested the significant influence of grain size on the corrosion behavior of Cu and demonstrated the influence of microstructure on the Cu corrosion kinetics [9]. Lin et al. investigated the corrosion behavior of red Cu in 3.5 wt.% NaCl solution [10] and suggested that varying grain sizes and second-phase inclusions mainly induce different corrosion properties. To determine whether or not annealing heat treatment affects the corrosion performance of Cu specimens during graphene growth, three Cu specimens, namely, as-received, annealed, and graphene film-coated, were immersed in 3.5 wt.% NaCl solution, and their electrochemical properties were evaluated by electrochemical impedance spectroscopy (EIS). The morphologies of the samples were observed by scanning electron microscopy (SEM).
2.1. Graphene synthesis A Cu sheet was ground using emery paper (500 grit), mechanically polished, and washed with deionized water. A 100 lm-thick Cu foil (99.96%) and polished Cu sheets were used for graphene growth by CVD. The samples were cleaned by ultrasonic oscillation in alcohol and acetone for 10 min. The temperature of the chamber was set to 940 °C with Ar and H2 flowing at 200 and 15 sccm, respectively. The samples were annealed for 20 min under a pressure of 11 Torr to remove contaminants and oxides from the Cu surface. Afterward, 35 sccm of CH4 gas, along with 4 sccm of H2 gas, was introduced to the samples for 20 min. After growth, the samples were cooled to room temperature at a cooling rate of 10–12 °C/min. Annealed Cu sheets (without graphene film) were prepared by subjecting the Cu sheets to the same conditions described above but without introduction of CH4 gas. The samples were named according to their heat treatment and graphene film. U-Cu and A-Cu respectively represent the exposed Cu sheet with and without annealing heat treatment. G-Cu denotes the Cu sheet on which graphene film had been deposited. The morphology and structure of graphene deposited onto the Cu foil were determined. 2.2. EIS measurements
⇑ Corresponding author. Tel./fax: +86 010 89733973. E-mail address: [email protected] (Y. Dong). http://dx.doi.org/10.1016/j.corsci.2014.08.026 0010-938X/Ó 2014 Elsevier Ltd. All rights reserved.
EIS measurements were performed in a conventional threeelectrode cell in situ with a carbon rod as the counter electrode and a saturated Hg/HgCl2 electrode as the reference electrode. Cu
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Y. Dong et al. / Corrosion Science 89 (2014) 214–219
circuit potential using a CHI 660 E electrochemical workstation for 14 d. 2.3. Characterization The morphologies and structures of the graphene films were characterized by Raman spectroscopy (RM2000, Renishaw) and high-resolution transmission electron microscopy (HRTEM, FEI F20 system). Sample morphologies were analyzed using an optical microscopy (OLYMPUS CK40M) and an SEM system (Quanta 200F, FEI Holland). 3. Results and discussion 3.1. Characterization of graphene films Raman spectroscopy was used to determine the structure of graphene in the samples. Fig. 1 shows that the graphene film displays sharp G (1580 cm 1) and 2D (2650–2700 cm 1) bands with a high G/2D ratio (1.15), a typical feature of graphene with multiple layers. The graphene film was clearly multilayered, as evidenced by the high-resolution TEM images shown in Fig. 2.
Fig. 1. Raman spectroscopy of graphene film.
3.2. Sample morphology
5nm
Fig. 2. TEM image of graphene film.
Samples obtained before the corrosion experiment were observed using an optical microscopy and relevant results are shown in Fig. 3. The morphologies of U-Cu and A-Cu clearly differ. Compared with those of U-Cu, the grain boundaries of A-Cu were clearer and more distinguishable, and the grain sizes increased after annealing (Fig. 3a and b). After graphene deposition on Cu, grain boundaries could still be observed but were not as clear as those of A-Cu (Fig. 3c). Fig. 4 shows the electron backscattered diffraction images of the samples. The grains exhibited various shapes and sizes after annealing (Fig. 4b). Moreover, twin crystals were observed (Fig. 4b and c). This phenomenon is mainly caused by the recovery and recrystallization of the Cu sheet when annealed at 940 °C [7], which results in the heterogeneous microstructure of the A-Cu (Fig. 4a). 3.3. Electrochemical measurements
sheets with or without graphene film with an area of 19.625 cm2 were used as working electrodes. Measurements were recorded within the frequency range of 0.01–100 kHz using a 20 mV amplitude sinusoidal voltage. The test solution was prepared from analytical-grade NaCl (3.5 wt.%) dissolved in distilled water. EIS measurements were performed at room temperature with an open
Three samples were immersed in 3.5 wt.% NaCl solution to investigate their electrochemical performances. The Nyquist and Bode plots of U-Cu showed similar shapes for all immersion times (Fig. 5a and b). The Nyquist plots mainly exhibited a capacitive loop. Initially, the radius of the capacitive loop rapidly decreased after immersion for 2 d and then slowly decreased in the
(c)
(b)
(a)
70μm
70μm
Fig. 3. Optical morphology of the samples (a) U-Cu, (b) A-Cu and (c) G-Cu.
70μm
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(a)
(c)
(b)
400μm
400μm
400μm
Fig. 4. EBSD images of the samples (a) U-Cu, (b) A-Cu and (c) G-Cu.
Fig. 5. EIS of the samples immersed in 3.5 wt.% solutions for different time. (a–b) U-Cu, (c–d) A-Cu and (e–f) G-Cu.
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succeeding 3–6 d. After immersion for 8 d, the radius of the capacitive loop fluctuated with time. This fluctuation occurs because U-Cu is immersed in the 3.5 wt.% NaCl solution. U-Cu rapidly corrodes and the corrosion product adheres to the metal surface. With prolonged immersion time, a compact Cu2O film is formed and this film inhibits the corrosion process, thereby increasing the impedance modulus. The chloride ions break the passive Cu2O film on the Cu surface because of the formation of soluble species, such as CuCl and CuCl2 . The CuCl2 produced is further oxidized into Cu2 (OH)3Cl. Cu2 (OH)3Cl is then subsequently transformed into the more stable Cu2 (OH)2CO3 because of the increasing pH caused by CO2 dissolution on the electrolyte surface. Local corrosion progresses with prolonged immersion time and decreases the impedance modulus [11]. The Nyquist plot of A-Cu displayed an obvious capacitive loop at high frequencies and a straight line at low frequencies for the first 3 d of immersion (Fig. 5c). The straight line, which is called the Warburg impedance, could be attributed to mass transport during corrosion. This impedance indicates that anodic Cu dissolution in the solution and cathodic reduction of dissolved oxygen are diffusion-controlled processes. The Bode plots of A-Cu for the first 3 d showed a small platform at low-to-moderate frequencies (Fig. 5d). The Warburg impedances disappeared after immersion for 4 d, which implies that the corrosion product film serves as a good inhibitor and inhibits diffusion to a certain extent [12]. The radius of the capacitive loop significantly increased over 8 d of immersion and then subsequently decreased thereafter. The variation of the Nyquist plots of G-Cu differs from those of U-Cu and A-Cu. The Nyquist plots mainly exhibited a capacitive loop at all immersion times, similar to U-Cu. However, the radius of the capacitive loop was large for the first 5 d of immersion, rapidly decreased after 6 d, and subsequently fluctuated with time (Fig. 5e). The Bode plots showed slightly changes over all immersion times except on the first day (Fig. 5f). The equivalent electrical circuit is often used to analyze the impedance spectra of samples. The equivalent circuit shown in Fig. 6a was used for fitting the EIS data displaying the Warburg impedance. Fig. 6b was also used for fitting other EIS data. In the two equivalent circuits, Rs is the solution resistance, Qf and Qdl are constant phase elements representing the capacitance of the corrosion product film and the double-charged layer, respectively, Rt is the charge-transfer resistance, Rf is the resistance of the corrosion product film, and W is the Warburg impedance. Relevant EIS parameters can be derived from the models presented in Fig. 6. The reciprocal of Rt, shown in Fig. 7, was used to characterize the corrosion rate [11]. The corrosion rate of A-Cu was always higher than those of U-Cu and G-Cu at all immersion times. G-Cu showed the lowest corrosion rate over 5 d of immersion. Afterward, G-Cu corroded more severely than U-Cu. This phenomenon may be explained by the fact that graphene films can obstruct electrolyte penetration into the Cu surface at initial immersion stages. However, with prolonged immersion time, besides the grain boundary, the edges of the graphene layers also become points through which electrolytes gain access to the Cu surface. The graphene deposited on the Cu surface through CVD is physically absorbed and has a binding energy of DE < 0.04 eV per carbon atom [13]. However, graphene cannot strongly adhere to the metal surface. Therefore, the graphene film detaches from the Cu surface after immersion for several days, causing local corrosion. EIS results indicate that the graphene film improves the corrosion properties of Cu. However, after Cu annealing, the anticorrosion properties of A-Cu deteriorated compared with those of U-Cu, which indicates that annealing during graphene deposition on the Cu surface may affect the Cu corrosion performance. Annealing causes Cu to recover and recrystallize into different
Qf Qdl
Rs Qdl
Rs Rf Rt
W
(a)
Rt
(b)
Fig. 6. Equivalent circuit models used to fit the experimental impedance data (a): for fitting the data displaying the Warburg impedance, (b): for fitting the data without displaying the Warburg impedance.
Fig. 7. Plot of the corrosion rate (1/Rct) as a function of the immersion time.
grain crystallographic orientations (Fig. 2b and c). Lapeire et al. [14–16] confirmed that, besides the grain orientation, the orientation of neighboring grains may also substantially contribute to the dissolution rate. 3.4. SEM morphologies of the samples Corroded samples obtained from the electrolyte were analyzed by SEM after removal of the corrosion products with 5 wt.% sulfuric acid. Before immersion, no difference between U-Cu and A-Cu, except for variations in grain size (Fig. 8c and f), were observed. Some white particles were also observed on the Cu surface. After graphene deposition, the white particles decreased and the Cu grain size increased (Fig. 8i). After immersion in the 3.5 wt.% NaCl solution, numerous granulometric corrosion products were observed on the sample surface. The corrosion products of A-Cu were looser than those of U-Cu and G-Cu (Fig. 8a, d and g). After removal of the corrosion products, evident differences were observed. Some pit corrosions were distributed on the U-Cu surface (Fig. 8b). A-Cu was severely corroded and its surface became porous (Fig. 8e). The G-Cu sample also showed significant differences compared with the other samples and was generally dissolved (Fig. 8h). These SEM images further verify the results presented in Fig. 7, which shows that A-Cu is the most severely corroded sample. 4. Conclusions The corrosion behavior of as-received, annealed, and graphene film-deposited Cu samples were investigated by EIS. The corrosion rate of annealed Cu was higher than that of Cu without annealing
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(a)
(b)
(f)
(e)
100μm
20μm
(g)
100μm
100μm
20μm
(d)
(c)
(i)
(h)
20μm
100μm
100μm
100μm
Fig. 8. SEM images of the Cu surface before (left), after (middle) corrosion product removed and before corrosion (right) (a–c) U-Cu; (d–f) A-Cu; (g–i) G-Cu.
treatment. The deposited graphene film improved Cu corrosion properties for a short period of time. After prolonged immersion, the graphene film detached from the Cu substrate and no longer provided barrier protection, leading to more severe corrosion in this sample than in U-Cu. SEM images showed that the three samples exhibit different morphologies after immersion. U-Cu was locally corroded, whereas A-Cu and G-Cu were generally corroded. Moreover, the morphology of A-Cu indicated that this sample has the poorest corrosion property among all of the samples studied.
Acknowledgements This work was supported by Science Foundation of China University of Petroleum Beijing (No. KYJJ2012-06-19). The authors would like to thank Prof. Yongfeng Li and Master Zhiqiang Tu for their supply of graphene.
References [1] R.K.S. Raman, P.C. Banerjee, D.E. Lobo, H. Gullapalli, M. Sumandasa, A. Kumar, L. Choudhary, R. Tkacz, P.M. Ajayan, M. Majumder, Protecting Cu from electrochemical degradation by graphene coating, Carbon 50 (2012) 4040–4045.
[2] G. Kalita, M.E. Ayhan, S. Sharma, S.M. Shinde, D. Ghimire, K. Wakita, M. Umeno, M. Tanemura, Low temperature deposited graphene by surface wave plasma CVD as effective oxidation resistive barrier, Corros. Sci. 78 (2014) 183–187. [3] A.S. Kousalya, A. Kumar, R. Paul, D. Zemlyanov, T.S. Fisher, Graphene: an effective oxidation barrier coating for liquid and two-phase cooling systems, Corros. Sci. 69 (2013) 5–10. [4] N.T. Kirkland, T. Schiller, N. Medhekar, N. Birbilis, Exploring graphene as a corrosion protection barrier, Corros. Sci. 56 (2012) 1–4. [5] S.S. Chen, L. Brown, M. Levendorf, W.W. Cai, S.Y. Ju, J. Edgeworth, X.S. Li, C.W. Magnuson, A. Velamakanni, R.D. Piner, J.Y. Kang, J. Park, R.S. Ruoff, Oxidation resistance of graphene-coated Cu and Cu/Ni alloy, ACS Nano 5 (2011) 1321– 1327. [6] D. Prasai, J.C. Tuberquia, R.R. Harl, G.K. Jennings, K.I. Bolotin, Graphene: corrosion-inhibiting coating, ACS Nano 6 (2012) 1102–1108. [7] J.E. Lee, D.H. Bae, W.S. Chung, K.H. Kim, J.H. Lee, Y.R. Cho, Effects of annealing on the mechanical and interface properties of stainless steel/aluminum/Cu clad-metal sheets, J. Mater. Process. Technol. 187–188 (2007) 546–549. [8] W.Z. Han, S.D. Wu, S.X. Li, Y.D. Wang, Intermediate annealing of pure Cu during cyclic equal channel angular pressing, Mater. Sci. Eng. A 483–484 (2008) 430– 432. [9] K.D. Ralston, N. Birbilis, Effect of grain size on corrosion: a review, Corrosion 66 (2010) 075005. [10] Y.F. Lin, X.D. Kong, Z.Q. Tian, G. Wu, Comparison on corrosion behavior of two kinds of red Cu in 3.5% NaCl solution, Equip. Environ. Eng. 8 (2011) 29–34. [11] X.N. Liao, F.H. Cao, L.Y. Zheng, W.J. Liu, A.N. Chen, J.Q. Zhang, C.N. Cao, Corrosion behavior of Cu under chloride-containing thin electrolyte layer, Corros. Sci. 53 (2011) 3289–3298. [12] T.T. Qin, J. Li, H.Q. Luo, M. Li, N.B. Li, Corrosion inhibition of Cu by 2,5dimercapto-1,3,4-thiadiazole monolayer in acidic solution, Corros. Sci. 53 (2011) 1072–1078.
Y. Dong et al. / Corrosion Science 89 (2014) 214–219 [13] G. Giovannetti, P.A. Khomyakov, G. Brocks, V.M. Karpan, J.V. den Brink, P.J. Kelly, Doping graphene with metal contacts, Phys. Rev. Lett. 101 (2008) 026803–026804. [14] L. Lapeire, E. Martinez Lombardia, K. Verbeken, I. De Graeve, L.A.I. Kestens, H. Terryn, Effect of neighboring grains on the microscopic corrosion behavior of a grain in polycrystalline Cu, Corros. Sci. 67 (2013) 179–183.
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[15] A. Schreiber, J.W. Schultze, M.M. Lohrengel, F. Karman, E. Kalman, Grain dependent electrochemical investigations on pure iron in acetate buffer pH 6.0, Electrochim. Acta 51 (2006) 2625–2630. [16] K.A. Lill, A.W. Hassel, G. Frommeyer, M. Stratmann, Scanning droplet cell investigations on single grains of a FeAlCr lightweight ferritic steel, Electrochim. Acta 51 (2005) 978–983.
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Corrosion behavior of Cu during graphene growth by CVD Yuhua Dong ⇑, Qingqing Liu, Qiong Zhou College of Science, China University of Petroleum (Beijing), Beijing 102249, China
a r t i c l e
i n f o
Article history: Received 20 June 2014 Accepted 20 August 2014 Available online 27 August 2014 Keywords: A. Copper B. EIS B. SEM
a b s t r a c t The corrosion performance of Cu samples may be affected by annealing at high temperatures during graphene growth via the chemical vapor deposition method. In this study, multiple graphene films were deposited on Cu and characterized by Raman spectroscopy and transmission electron microscopy. The corrosion behavior of Cu immersed in 3.5 wt.% NaCl solution was investigated using electrochemical impedance spectroscopy. The Cu morphology was observed by optical microscopy and scanning electron microscopy. Results indicated that annealing affects the corrosion process of Cu. The presence of graphene films on the Cu substrate improved the corrosion performance of the material for a short period of time. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental
Numerous studies have demonstrated that graphene film, which feature chemical inertness and thermal stability, are excellent anticorrosion barriers for Cu [1–6]. Graphene film is generally prepared through chemical vapor deposition (CVD), during which Cu sheets are annealed at high temperatures. Raman et al. [1] believed that heat treatment of Cu specimens during graphene growth does not significantly influence the Cu corrosion rate. However, several researchers have found that the Cu microstructure, grain size, and mechanical properties are affected by annealing [7,8]. A previous study has suggested the significant influence of grain size on the corrosion behavior of Cu and demonstrated the influence of microstructure on the Cu corrosion kinetics [9]. Lin et al. investigated the corrosion behavior of red Cu in 3.5 wt.% NaCl solution [10] and suggested that varying grain sizes and second-phase inclusions mainly induce different corrosion properties. To determine whether or not annealing heat treatment affects the corrosion performance of Cu specimens during graphene growth, three Cu specimens, namely, as-received, annealed, and graphene film-coated, were immersed in 3.5 wt.% NaCl solution, and their electrochemical properties were evaluated by electrochemical impedance spectroscopy (EIS). The morphologies of the samples were observed by scanning electron microscopy (SEM).
2.1. Graphene synthesis A Cu sheet was ground using emery paper (500 grit), mechanically polished, and washed with deionized water. A 100 lm-thick Cu foil (99.96%) and polished Cu sheets were used for graphene growth by CVD. The samples were cleaned by ultrasonic oscillation in alcohol and acetone for 10 min. The temperature of the chamber was set to 940 °C with Ar and H2 flowing at 200 and 15 sccm, respectively. The samples were annealed for 20 min under a pressure of 11 Torr to remove contaminants and oxides from the Cu surface. Afterward, 35 sccm of CH4 gas, along with 4 sccm of H2 gas, was introduced to the samples for 20 min. After growth, the samples were cooled to room temperature at a cooling rate of 10–12 °C/min. Annealed Cu sheets (without graphene film) were prepared by subjecting the Cu sheets to the same conditions described above but without introduction of CH4 gas. The samples were named according to their heat treatment and graphene film. U-Cu and A-Cu respectively represent the exposed Cu sheet with and without annealing heat treatment. G-Cu denotes the Cu sheet on which graphene film had been deposited. The morphology and structure of graphene deposited onto the Cu foil were determined. 2.2. EIS measurements
⇑ Corresponding author. Tel./fax: +86 010 89733973. E-mail address: [email protected] (Y. Dong). http://dx.doi.org/10.1016/j.corsci.2014.08.026 0010-938X/Ó 2014 Elsevier Ltd. All rights reserved.
EIS measurements were performed in a conventional threeelectrode cell in situ with a carbon rod as the counter electrode and a saturated Hg/HgCl2 electrode as the reference electrode. Cu
215
Y. Dong et al. / Corrosion Science 89 (2014) 214–219
circuit potential using a CHI 660 E electrochemical workstation for 14 d. 2.3. Characterization The morphologies and structures of the graphene films were characterized by Raman spectroscopy (RM2000, Renishaw) and high-resolution transmission electron microscopy (HRTEM, FEI F20 system). Sample morphologies were analyzed using an optical microscopy (OLYMPUS CK40M) and an SEM system (Quanta 200F, FEI Holland). 3. Results and discussion 3.1. Characterization of graphene films Raman spectroscopy was used to determine the structure of graphene in the samples. Fig. 1 shows that the graphene film displays sharp G (1580 cm 1) and 2D (2650–2700 cm 1) bands with a high G/2D ratio (1.15), a typical feature of graphene with multiple layers. The graphene film was clearly multilayered, as evidenced by the high-resolution TEM images shown in Fig. 2.
Fig. 1. Raman spectroscopy of graphene film.
3.2. Sample morphology
5nm
Fig. 2. TEM image of graphene film.
Samples obtained before the corrosion experiment were observed using an optical microscopy and relevant results are shown in Fig. 3. The morphologies of U-Cu and A-Cu clearly differ. Compared with those of U-Cu, the grain boundaries of A-Cu were clearer and more distinguishable, and the grain sizes increased after annealing (Fig. 3a and b). After graphene deposition on Cu, grain boundaries could still be observed but were not as clear as those of A-Cu (Fig. 3c). Fig. 4 shows the electron backscattered diffraction images of the samples. The grains exhibited various shapes and sizes after annealing (Fig. 4b). Moreover, twin crystals were observed (Fig. 4b and c). This phenomenon is mainly caused by the recovery and recrystallization of the Cu sheet when annealed at 940 °C [7], which results in the heterogeneous microstructure of the A-Cu (Fig. 4a). 3.3. Electrochemical measurements
sheets with or without graphene film with an area of 19.625 cm2 were used as working electrodes. Measurements were recorded within the frequency range of 0.01–100 kHz using a 20 mV amplitude sinusoidal voltage. The test solution was prepared from analytical-grade NaCl (3.5 wt.%) dissolved in distilled water. EIS measurements were performed at room temperature with an open
Three samples were immersed in 3.5 wt.% NaCl solution to investigate their electrochemical performances. The Nyquist and Bode plots of U-Cu showed similar shapes for all immersion times (Fig. 5a and b). The Nyquist plots mainly exhibited a capacitive loop. Initially, the radius of the capacitive loop rapidly decreased after immersion for 2 d and then slowly decreased in the
(c)
(b)
(a)
70μm
70μm
Fig. 3. Optical morphology of the samples (a) U-Cu, (b) A-Cu and (c) G-Cu.
70μm
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Y. Dong et al. / Corrosion Science 89 (2014) 214–219
(a)
(c)
(b)
400μm
400μm
400μm
Fig. 4. EBSD images of the samples (a) U-Cu, (b) A-Cu and (c) G-Cu.
Fig. 5. EIS of the samples immersed in 3.5 wt.% solutions for different time. (a–b) U-Cu, (c–d) A-Cu and (e–f) G-Cu.
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succeeding 3–6 d. After immersion for 8 d, the radius of the capacitive loop fluctuated with time. This fluctuation occurs because U-Cu is immersed in the 3.5 wt.% NaCl solution. U-Cu rapidly corrodes and the corrosion product adheres to the metal surface. With prolonged immersion time, a compact Cu2O film is formed and this film inhibits the corrosion process, thereby increasing the impedance modulus. The chloride ions break the passive Cu2O film on the Cu surface because of the formation of soluble species, such as CuCl and CuCl2 . The CuCl2 produced is further oxidized into Cu2 (OH)3Cl. Cu2 (OH)3Cl is then subsequently transformed into the more stable Cu2 (OH)2CO3 because of the increasing pH caused by CO2 dissolution on the electrolyte surface. Local corrosion progresses with prolonged immersion time and decreases the impedance modulus [11]. The Nyquist plot of A-Cu displayed an obvious capacitive loop at high frequencies and a straight line at low frequencies for the first 3 d of immersion (Fig. 5c). The straight line, which is called the Warburg impedance, could be attributed to mass transport during corrosion. This impedance indicates that anodic Cu dissolution in the solution and cathodic reduction of dissolved oxygen are diffusion-controlled processes. The Bode plots of A-Cu for the first 3 d showed a small platform at low-to-moderate frequencies (Fig. 5d). The Warburg impedances disappeared after immersion for 4 d, which implies that the corrosion product film serves as a good inhibitor and inhibits diffusion to a certain extent [12]. The radius of the capacitive loop significantly increased over 8 d of immersion and then subsequently decreased thereafter. The variation of the Nyquist plots of G-Cu differs from those of U-Cu and A-Cu. The Nyquist plots mainly exhibited a capacitive loop at all immersion times, similar to U-Cu. However, the radius of the capacitive loop was large for the first 5 d of immersion, rapidly decreased after 6 d, and subsequently fluctuated with time (Fig. 5e). The Bode plots showed slightly changes over all immersion times except on the first day (Fig. 5f). The equivalent electrical circuit is often used to analyze the impedance spectra of samples. The equivalent circuit shown in Fig. 6a was used for fitting the EIS data displaying the Warburg impedance. Fig. 6b was also used for fitting other EIS data. In the two equivalent circuits, Rs is the solution resistance, Qf and Qdl are constant phase elements representing the capacitance of the corrosion product film and the double-charged layer, respectively, Rt is the charge-transfer resistance, Rf is the resistance of the corrosion product film, and W is the Warburg impedance. Relevant EIS parameters can be derived from the models presented in Fig. 6. The reciprocal of Rt, shown in Fig. 7, was used to characterize the corrosion rate [11]. The corrosion rate of A-Cu was always higher than those of U-Cu and G-Cu at all immersion times. G-Cu showed the lowest corrosion rate over 5 d of immersion. Afterward, G-Cu corroded more severely than U-Cu. This phenomenon may be explained by the fact that graphene films can obstruct electrolyte penetration into the Cu surface at initial immersion stages. However, with prolonged immersion time, besides the grain boundary, the edges of the graphene layers also become points through which electrolytes gain access to the Cu surface. The graphene deposited on the Cu surface through CVD is physically absorbed and has a binding energy of DE < 0.04 eV per carbon atom [13]. However, graphene cannot strongly adhere to the metal surface. Therefore, the graphene film detaches from the Cu surface after immersion for several days, causing local corrosion. EIS results indicate that the graphene film improves the corrosion properties of Cu. However, after Cu annealing, the anticorrosion properties of A-Cu deteriorated compared with those of U-Cu, which indicates that annealing during graphene deposition on the Cu surface may affect the Cu corrosion performance. Annealing causes Cu to recover and recrystallize into different
Qf Qdl
Rs Qdl
Rs Rf Rt
W
(a)
Rt
(b)
Fig. 6. Equivalent circuit models used to fit the experimental impedance data (a): for fitting the data displaying the Warburg impedance, (b): for fitting the data without displaying the Warburg impedance.
Fig. 7. Plot of the corrosion rate (1/Rct) as a function of the immersion time.
grain crystallographic orientations (Fig. 2b and c). Lapeire et al. [14–16] confirmed that, besides the grain orientation, the orientation of neighboring grains may also substantially contribute to the dissolution rate. 3.4. SEM morphologies of the samples Corroded samples obtained from the electrolyte were analyzed by SEM after removal of the corrosion products with 5 wt.% sulfuric acid. Before immersion, no difference between U-Cu and A-Cu, except for variations in grain size (Fig. 8c and f), were observed. Some white particles were also observed on the Cu surface. After graphene deposition, the white particles decreased and the Cu grain size increased (Fig. 8i). After immersion in the 3.5 wt.% NaCl solution, numerous granulometric corrosion products were observed on the sample surface. The corrosion products of A-Cu were looser than those of U-Cu and G-Cu (Fig. 8a, d and g). After removal of the corrosion products, evident differences were observed. Some pit corrosions were distributed on the U-Cu surface (Fig. 8b). A-Cu was severely corroded and its surface became porous (Fig. 8e). The G-Cu sample also showed significant differences compared with the other samples and was generally dissolved (Fig. 8h). These SEM images further verify the results presented in Fig. 7, which shows that A-Cu is the most severely corroded sample. 4. Conclusions The corrosion behavior of as-received, annealed, and graphene film-deposited Cu samples were investigated by EIS. The corrosion rate of annealed Cu was higher than that of Cu without annealing
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(a)
(b)
(f)
(e)
100μm
20μm
(g)
100μm
100μm
20μm
(d)
(c)
(i)
(h)
20μm
100μm
100μm
100μm
Fig. 8. SEM images of the Cu surface before (left), after (middle) corrosion product removed and before corrosion (right) (a–c) U-Cu; (d–f) A-Cu; (g–i) G-Cu.
treatment. The deposited graphene film improved Cu corrosion properties for a short period of time. After prolonged immersion, the graphene film detached from the Cu substrate and no longer provided barrier protection, leading to more severe corrosion in this sample than in U-Cu. SEM images showed that the three samples exhibit different morphologies after immersion. U-Cu was locally corroded, whereas A-Cu and G-Cu were generally corroded. Moreover, the morphology of A-Cu indicated that this sample has the poorest corrosion property among all of the samples studied.
Acknowledgements This work was supported by Science Foundation of China University of Petroleum Beijing (No. KYJJ2012-06-19). The authors would like to thank Prof. Yongfeng Li and Master Zhiqiang Tu for their supply of graphene.
References [1] R.K.S. Raman, P.C. Banerjee, D.E. Lobo, H. Gullapalli, M. Sumandasa, A. Kumar, L. Choudhary, R. Tkacz, P.M. Ajayan, M. Majumder, Protecting Cu from electrochemical degradation by graphene coating, Carbon 50 (2012) 4040–4045.
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