Electrochemical study of corrosion behavior of graphene coatings on copper and aluminum in a chloride solution

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Electrochemical study of corrosion behavior of graphene coatings on copper and aluminum in a chloride solution Vesna Misˇkovic´-Stankovic´ a b

a,b

, Ivana Jevremovic´ a, Inhwa Jung b, KyongYop Rhee

b,*

University of Belgrade, Faculty of Technology and Metallurgy, Karnegijeva 4, 11000 Belgrade, Serbia Department of Mechanical Engineering, College of Engineering, Kyung Hee University, 446-701 Yongin, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Article history:

Electrochemical characteristics and corrosion behavior of graphene coatings on Cu and Al

Received 14 November 2013

in a 0.1 M NaCl solution were investigated. The graphene coatings were deposited on a Cu

Accepted 5 April 2014

surface by chemical vapor deposition. Multiple graphene layers were then mechanically

Available online 13 April 2014

transferred from the growth substrate, Cu, onto Al surface by a transfer technique. The corrosion stability of graphene coatings was determined by electrochemical impedance spectroscopy and open circuit potential, while the corrosion rate was evaluated using potentiodynamic sweep measurements. Surface morphologies of the graphene coatings were analyzed by scanning electron microscopy and energy dispersive spectroscopy. Obtained results indicate that Cu coated with graphene grown using chemical vapor deposition shows corrosion-inhibiting properties in 0.1 M NaCl. On the other hand, Al coated with a multilayer graphene film mechanically transferred from the Cu surface exhibits electrochemical characteristics similar to an Al oxide on bare Al. Better protective properties of graphene coating on Cu compared to the graphene coating on Al were observed, probably due to the breakage of Al oxide film, causing the corrosion of Al to proceed rapidly in the presence of chloride electrolyte. Ó 2014 Elsevier Ltd. All rights reserved.

1.

Introduction

The use of refined metals is widespread, but they are often chemically reactive, requiring protective coatings for many applications, which are well known to retard corrosion. Strategies to protect the surface of reactive metals have employed many different approaches, including coatings with organic layers [1–6], electroconductive polymers [7,8] silanes [9,10], phosphates [11,12], oxide layers [13], and metal or alloy coatings [14]. The structure of the passive films is considered to be a crucial factor in metal stability, and the determination of

* Corresponding author: Fax: +82 31 202 6693. E-mail address: [email protected] (K. Rhee). http://dx.doi.org/10.1016/j.carbon.2014.04.012 0008-6223/Ó 2014 Elsevier Ltd. All rights reserved.

surface properties, i.e. the formation of a stable passive layer, is critical in the design and engineering of new materials. However, most conventional methods modify the physical properties of metals being protected. The addition of a protective coating changes the dimensions of the metal due to the finite thickness of the coating. Changes also occur in the appearance and the optical properties of the metal surface; furthermore, decreases in the electrical and thermal conductivity can also be observed. One important approach to overcome these problems would be to develop an ultrathin protective coating to minimize the variation in physical

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properties of the protected metal. Graphene is a single atomic planar sheet of carbon, with a near perfect two-dimensional honeycomb crystal lattice. This material has many useful electrochemical characteristics, and with its high thermal conductivity, high inherent capacity and extremely large specific surface area, graphene outperforms many other materials. Other outstanding properties of graphene include its high strength, ductility and remarkable chemical inertness unless exposed to harsh reaction conditions [15]. Graphene also provides oxidation resistance while its hydrophobicity prevents hydrogen bonding with water [16]. Due to its many promising properties, graphene is being widely investigated across a number of fields, but the potential for ultrathin graphene coatings as a barrier to aqueous corrosion has not been thoroughly investigated [17–22]. Recently, several pioneering experiments have demonstrated that graphene can effectively decouple the surface under it from the environment. First, it was shown that single-atomic graphene films are impermeable to gas molecules [23]. Second, it has been demonstrated that graphene can inhibit the oxidation of the underlying Cu and Ni [24,25]. It was observed in several recent studies that graphene is a promising anticorrosion material with the best performance under harsh corrosive conditions over a short time scale, while at the room-temperature, over a long period of time it was found that graphene coating accelerates the oxidation of Cu due to its conductive nature, which promotes the electrochemical corrosion process [26]. It was reported that nanometer-sized structural defects in the graphene are responsible for the limited passivation. Consequently, only a completely suppresed ion conduction throught microscopic channels in graphene coating can prevent the formation of a galvanic cell. Depending on how the graphene coatings were grown on Cu by the CVD method they are expected to be more or less porous and for certain applications must be sealed by appropriate means in order to ensure an enhanced corrosion protection [27]. The aim of this research was to further investigate graphene as a protective coating for Cu on which a single layer graphene was grown by chemical vapor deposition (CVD). In addition, a transfer technique has been applied to transfer the graphene film to an Al substrate using Cu etching, creating films of different thicknesses and inhibition properties, with the aim to investigate graphene as a protective coating for Al. In this study, electrochemical impedance spectroscopy (EIS), open circuit potential (Eocp) measurements as well as potentiodynamic sweep (PDS) measurements and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) were conducted in order to investigate the corrosion stability of graphene coatings on Cu and Al substrates during exposure in a 0.1 M chloride solution.

2.

Material and methods

2.1.

Growth and transfer of graphene films

Graphene films were prepared according to a procedure described by Li et al. [28]. Graphene films were primarily grown on Cu foils (0.07 mm in thickness, Wacopa) in a hot wall furnace consisting of a 50-mm ID fused silica tube heated in a tube furnace. The furnace was first evacuated and then

Cu foils were heated up 1000 °C under hydrogen flow, wherein the flow rate was 10 sccm. H2 pressure was maintained at 100 mTorr. After reaching 1000 °C, Cu foils were annealed at the same temperature for 30 min. After annealing the Cu film, CH4 was introduced at a flow rate of 20 sccm for 15 min. The total pressure was maintained at 300 mTorr. After processing, the samples were cooled rapidly to room temperature by removing the heater. The heating rate was higher than 300 °C/min. After growing the graphene film, a transfer technique was been applied to transfer the graphene film to the Al surface using Cu etching. Graphene films were removed from the Cu foils by etching in an aqueous solution of ammonium persulphate, (NH4)2S2O8.

2.2.

Sample preparation

We investigated the electrochemical properties of graphene coatings on Cu and Al specimens. For graphene coated Cu, we used graphene grown on Cu foil by the CVD method. Then, graphene coated Al samples were made by transferring CVDgrown graphene onto an Al surface. The transfer process began with growing graphene layers on Cu foils, then a layer of polymethyl methacrylate (PMMA) (average Mw 120,000, Sigma–Aldrich) in solution (50 mg/ml in chlorobenzene) was spin-deposited on the Cu foils at 4000 rpm for 30 s, followed by curing at 180 °C for 1 min. Finally, the Cu foil was etched in an ammonium persulphate solution (NH4)2S2O8 (0.1 M). After Cu etching, the PMMA/graphene sample was removed and placed into a deionised (DI) water bath. After rinsing several times with DI water, the PMMA/graphene was carefully deposited onto the target Al surface. Samples were dried at room temperature until the PMMA/graphene was flat on the metal surface. PMMA was then carefully dissolved with acetone. In order to vary the thickness of the protective coating, we investigated samples where the transfer procedure was repeated several times. Thereby, single (1trGr/Al), double (2trGr/Al), triple (3trGr/Al) or quadruple (4trGr/Al) transfer applied samples were successfully made. The graphene coated Al and Cu specimens were used for electrochemical tests without further surface preparation. The bare Cu foil specimen was used as a reference. These samples were made by annealing in the CVD furnace, and the process was exactly the same as that with the graphene except that the CH4 gas was not included during the process. Uncoated Al specimens were also used as a reference, and these samples were first rinsed with acetone and then washed with DI water and were finally air-dried.

2.3.

Electrochemical measurements

All electrochemical measurements were performed in a 0.1 M NaCl solution in a three-electrode cell made of Teflon. The working electrode was a bare or graphene-coated Cu or Al sample (tested area was 1 cm2), the counter electrode was a platinum mesh with a surface area considerably larger than that of the working electrode, and the reference electrode was a saturated calomel electrode (SCE). All the electrochemical measurements were carried out using a reference 600TM Potentiostat/Galvanostat/ZRA (Gamry Instruments, Inc., Warminster, PA, USA), while the impedance spectra were

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analyzed using the Gamry Instruments Echem Analyst fitting procedure. EIS and PDS were performed after 30 min of Eocp measurements. Eocp was monitored in order to get a stable potential with time before carrying out the corrosion tests. The EIS measurements were carried out over a frequency range of 1 MHz to 1.6 mHz using a 10 mV amplitude of sinusoidal voltage variation around the Eocp. The PDS measurements were carried out from a cathodic potential of 150 mV to an anodic potential of +150 mV with respect to Eocp with a scan rate of 0.5 mV/s.

2.4.

Surface morphology

A scanning electron microscope (JEOL JSM-6390) and an energy dispersive spectrometer (Oxford Link ISIS 300) were used to analyze the morphology and chemical composition of the as-deposited coatings. Images of bare and coated Cu and Al specimens were recorded before and after exposure to 0.1 M NaCl.

3.

Results and discussion

3.1.

Eocp measurements

Open circuit potential was recorded daily for 30 min for bare Cu, graphene coated Cu, bare Al and graphene coated Al in order to get a stable potential with time before carrying out the corrosion tests. Eocp measurements showed slight fluctuations over the narrow potential range up to 4 mV during a typical measurement, for each particular day, indicating the stability and linearity of the all investigated systems. The time dependences of Eocp for bare Cu and graphene coated Cu after different exposure times in 0.1 M NaCl are presented in Fig. 1. As it can be seen in Fig. 1, the Eocp values for the graphene coated Cu increased in the first 10 days of exposure and then remained stable at values close to 150 mV vs. SCE. Moreover, the Eocp of the graphene coated Cu was around 20 mV more positive than the bare Cu specimen. Since Eocp is the measure of corrosion susceptibility, the shift in Eocp to more positive values indicates that graphene coating should increase the corrosion stability of the Cu substrate.

Fig. 1 – Open-circuit potential, Eocp, as a function of exposure time in 0.1 M NaCl for bare Cu and graphene coated Cu. (A colour version of this figure can be viewed online.)

Fig. 2 – Open-circuit potential, Eocp, as a function of exposure time in 0.1 M NaCl for bare Al and graphene coated Al. (A colour version of this figure can be viewed online.)

Fig. 2 depicts the time dependencies of Eocp, recorded for 30 min for bare Al and graphene coated Al after different exposure times in a 0.1 M NaCl solution. As can be seen in Fig. 2, the values of Eocp for graphene-coated Al were similar to values obtained for bare Al during exposure time, meaning that graphene coating acts like an Al oxide layer on bare Al, which is considered to be a well known protective layer [29]. Graphene coated Al samples were made by transferring CVD-grown graphene onto an Al surface. It is already known that the graphene films grown on the metal directly by CVD give a better anticorrosion performance than transferred graphene coatings [25]. Nevertheless, here we can notice that, the Eocp of the graphene coated Al was around 20 mV more positive than the bare Al specimen.

3.2.

PDS measurements

Corrosion rates of graphene coated Cu and Al in 0.1 M NaCl were determined using Tafel analysis. PDS measurements were performed after 30 min of Eocp measurements. The Eocp was also measured during the initial delay of the experiment in order to get a stable potential with time before carrying out the corrosion tests. By using PDS measurements it was also investigated whether graphene is an effective anticorrosion coating for the long-term protection of Cu when exposed to ambient air over 10 months. The investigated systems exhibited Eocp with a slight fluctuation over the narrow potential range less than 4 mV. The anodic and cathodic polarization curves for the corrosion of bare and graphene coated Cu after different times of exposure to 0.1 M NaCl are shown in Fig. 3. We have performed IR correction taking into account the resistance drop across the solution, in order to avoid nonlinear Tafel behavior at high currents. The corrosion rate can be determined by Tafel extrapolation of either the cathodic or anodic polarization curve alone [30,31]. In case of polarization curves for bare and graphene coated Cu anodic curve produces a longer and better defined Tafel region (anodic parts of a potentiodynamic polarization plots are constant over a decade of current density), while the cathodic polarization curves show some deviations from Tafel behavior. The anodic Tafel region is extrapolated back to zero overvoltage to give the net rate of the anodic reaction at the corrosion

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Fig. 3 – Potentiodynamic plots showing the bare and graphene coated Cu after different times of exposure to 0.1 M NaCl. (A colour version of this figure can be viewed online.)

potential, i.e. corrosion current density, jcorr. Rp is the polarization resistance determined from the slope of the potential vs. current density curve over the narrow potential range of 20 mV to +20 mV relative to the corrosion potential. The corrosion rate, vcorr, was then calculated using jcorr values according to Eq. (1): mcorr ¼

Jcorr EW qF

ð1Þ

where Ew and q are the equivalent weight and density, respectively, for Cu and Al samples. The equivalent weight, Ew, is 31.7 g for Cu (9 g for Al), the density, q, is 8.94 g/cm3 for Cu (2.70 g/cm3 for Al), F is the Faraday’s constant, (96.485 C/mol) and the sample area, A, is 1 cm2. The values of polarization resistance, Rp, corrosion current density, jcorr, and corrosion rate, vcorr, for bare and graphene coated Cu in 0.1 M NaCl are given in Table 1. From the polarization curves in Fig. 3 and data in Table 1, a shift in the Ecorr towards more noble values for the graphene coated Cu (160 mV) is apparent after 2 h of exposure compared to bare Cu (170 mV), which remains stable after 40 days (167 mV). This indicates that the graphene coating acts as a corrosion barrier for the Cu substrate. The calculated corrosion rate for graphene coated Cu after 2 h of exposure to 0.1 M NaCl solution (6.30Æ103 mm/y) was lower than the corrosion rate for bare Cu (7.53Æ103 mm/y), which decreased by a factor of two (3.24Æ103 mm/y) after 40 days of exposure. In addition, the decrease in current density from 6.50Æ107 A cm2 for bare Cu to 2.80Æ107 A cm2 for graphene coated Cu after 40 days of exposure, as well as the increase in polarization resistance (53.90 kX cm2 for graphene coated Cu after 40 days relative to 14.53 kX cm2 of bare Cu) proved that the graphene

coating acts as a barrier to the underlying Cu surface and therefore inhibits the Cu corrosion. To investigate whether graphene can also serve as an anti corrosion coating in air over a much longer time scale, a graphene coated Cu sample and a bare Cu sample were stored in a plastic Petri dish at room temperature (21 °C) for up to 10 months in the dark. Previous studies suggested that the graphen coated Cu was oxidized faster at room temperature than bare Cu [26]. Our result did not confirm that the graphene coated Cu sample underwent more severe oxidation than the uncoated one, as follows. Corrosion rates of bare and graphene coated Cu in 0.1 M NaCl after 10 months exposure to air were determined using Tafel analysis. The anodic and cathodic polarization curves for the corrosion of bare and graphene coated Cu after10 months exposure to air are shown in Fig. 4. As it can be seen the anodic and cathodic branches indicate lower anodic and cathodic corrosion current density for graphene coated Cu sample after 10 months. The calculated corrosion rate for graphene coated Cu after 10 months of exposure to ambient atmosphere (3.30Æ102 mm/y) was lower than the corrosion rate for bare Cu (4.80Æ102 mm/y). Higher polarization resistance determined from PDS measurements for graphene coated Cu (15.87 kX cm2) relative to bare Cu (2.9 kX cm2 for) also shown that the graphene coating acts as a barrier to the underlying Cu surface even after 10 months and therefore indicates that the graphene coating does not necessarily promote the corrosion of Cu over longer periods of time. In addition, after 10 months of storage coated and uncoated Cu samples were not very different in appearance. We have similarly extracted the corrosion rates for bare Al and graphene coated Al. Tafel curves of the bare Al and graphene coatings of different thicknesses (obtained by different numbers of transfers) after 35 days of exposure to NaCl solution are presented in Fig. 5. In case of polarization curves for bare and graphene coated Al cathodic curve produces a longer and better defined Tafel region (cathodic parts of a potentiodynamic polarisation plots are constant over a decade of current density), while the anodic polarization curves show some deviations from Tafel behavior. The cathodic Tafel region is extrapolated back to zero overvoltage to give the net rate of the cathodic reaction at the corrosion potential i.e. corrosion current density, jcorr. The values of polarization resistance, Rp, corrosion current density, jcorr and corrosion rate, vcorr for bare and graphene coated Al in 0.1 M NaCl are given in Table 2. The results in Table 2 indicate that the corrosion rate of bare Al (2.51Æ103 mm/y), was lower than that of graphene coated Al. The specimen where three graphene layers were transferred onto Al sequentially has the lowest corrosion rate (9.81Æ103 mm/y), among the coated specimens. It appears

Table 1 – Corrosion parameters obtained from PDS measurements for bare and graphene coated Cu samples in 0.1 M NaCl after 2 h and 40 days of immersion at open circuit potential. Sample

Rp, kX cm2

jcorrÆ107, Acm2

Ecorr, mV vs.SCE

vcorrÆ103, mm/y

Bare Cu, 2 h Gr/Cu, 2 h Gr/Cu, 40 days

14.53 15.36 53.90

6.50 5.50 2.80

170 160 167

7.53 6.30 3.24

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tears and scratches in the graphene film that may result during its transfer onto the Al substrate increase the risk of galvanic corrosion. Contact with a chloride containing solution will accelerate electrochemical degradation of the metal. Given these considerations, at this point it is not clear if graphene can serve as an effective anticorrosion coating for the protection of Al but the insight we gain from this research helps us to better understand the mechanism of metal corrosion in the presence of graphene in order to enhance corrosion inhibiting performance of graphene as protective anticorrosion coating.

3.3.

Fig. 4 – Potentiodynamic plots showing the bare and graphene coated Cu after 10 months of exposure to ambient atmosphere. (A colour version of this figure can be viewed online.)

Fig. 5 – Potentiodynamic plots showing the bare and graphene coated Al after 35 days of exposure to 0.1 M NaCl. (A colour version of this figure can be viewed online.)

that multilayer graphene coatings transferred onto Al do not reduce the corrosion rate of Al, probably due to the breakage of the Al oxide film beneath the graphene coating. Another possible explanation could be the galvanic corrosion of Al, which can occur when there is a contact with a more noble metal or other electron conductor with a higher chemical potential than Al [32]. It is well known that even a very small concentration of impurities, especially graphitic materials, on a metal surface could greatly enhance its galvanic corrosion by serving as an electrode for oxygen reduction [26]. Also,

EIS measurements

The electrochemical properties of graphene-coated Cu and Al were determined using EIS measurements. The Bode plots for the impedance of bare Cu and graphene coated Cu after different exposure times in 0.1 M NaCl are depicted in Fig. 6. The phase angle plot for bare Cu after an initial time of 30 min exposure (Fig. 6a) shows two time constants: one at low frequencies (10 mHz) corresponding to an electrochemical process (corrosion) at the Cu surface, and the second at intermediate frequencies (10 Hz) referring to a Cu oxide/ hydroxide layer. The phase angle plot for graphene coated Cu after 30 min exposure (Fig. 6a) shows two time constants: one at intermediate frequencies (2 Hz) corresponding to Cu oxide/hydroxide layer, with a broader peak compared to the peak of bare Cu at intermediate frequencies, which is often recognized as two overlapping time constants [15], and another one at high frequencies (4000 Hz) due to graphene coating. The lack of a low frequency time constant indicates the absence of corrosion processes at the metal surface and increased Cu corrosion protection by the graphene coating. After prolonged exposure, the difference is much more evident (Fig. 6b). Namely, after 30 days the graphene coated Cu still showed two time constants, one at low frequency (0.1 Hz) corresponding to the electrochemical process at the Cu/Cu oxide/hydroxide interface, and the second at high frequencies (10,000 Hz) that corresponds to the graphene coating, confirming the presence of a graphene layer even after 30 days of exposure to NaCl solution. In the case of bare Cu, the maximum at 0.1 Hz refers to the electrochemical process at the Cu/Cu oxide/hydroxide interface, while the maximum at intermediate frequencies (10 Hz) corresponds to the Cu oxide/hydroxide layer itself. The most corrosion resistant coating among the multilayer graphene coatings on Al surface, as determined by PDS, was the three layer graphene coating. This coating was further investigated by employing EIS measurements. The Bode plots

Table 2 – Corrosion parameters obtained from PDS measurements for bare and graphene coated Al in 0.1 M NaCl after 35 days of immersion at open circuit potential. Sample

Rp, kX cm2

jcorrÆ106, Acm2

Ecorr, mV vs.SCE

vcorrÆ102, mm/y

Bare Al 1trGr/Al 2trGr/Al 3trGr/Al 4trGr/Al

63.90 7.890 3.293 13.67 12.50

0.23 2.80 3.00 0.90 1.80

802 772 713 737 743

0.251 3.051 3.267 0.981 1.961

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Fig. 6 – Bode plots for bare Cu and graphene coated Cu after: (a) 30 min and (b) 30 days of exposure to 0.1 M NaCl. (A colour version of this figure can be viewed online.) for the impedance of bare and graphene coated Al after different exposure times in a 0.1 M NaCl solution are depicted in Fig. 7. As can be seen in Fig. 7a for the short time of 30 min exposure, bare Al showed only one time constant at intermediate frequencies (40 Hz), corresponding to the oxide layer on the Al surface. The graphene coated Al exhibits one maximum at intermediate frequencies, corresponding to the oxide layer on the Al surface beneath the graphene coating, and a small shoulder at high frequencies (10,000 Hz) due to the graphene film itself. After prolonged exposure time, the difference is much more evident (Fig. 7b). After 30 days of exposure, graphene coated Al still showed a maximum at high frequencies (10,000 Hz), which corresponds to a graphene coating, confirming the presence of a graphene layer even after 30 days of exposure to NaCl solution. The maximum at intermediate frequencies (10 Hz) corresponds to an Al oxide layer, while the maximum at 0.01 Hz refers to the corrosion process at the Al surface. In the case of bare Al, the maximum at 0.01 Hz also refers to the corrosion process at the Al surface, while the maximum at intermediate frequencies (10 Hz) corresponds to the Al oxide layer. To analyze the impedance data of the graphene coating, as well as of the metal/electrolyte interface, an equivalent electrical circuit (EEC) [33–40] depicted in Fig. 8 was employed.

Fig. 7 – Bode plots for bare Al and three layer graphene film on Al after: (a) 30 min and (b) 30 days of exposure to 0.1 M NaCl. (A colour version of this figure can be viewed online.)

The electrolyte resistance is represented by RO. In the case of bare Cu and bare Al, R1 is an oxide layer resistance (Rox) and CPE1 is the constant phase element (CPEox) of the oxide layer on the metal surface, while R2 is the charge-transfer resistance (Rct) in parallel with CPE2, the constant phase element related to double layer capacitance. In the case of graphene coated Cu and Al, R1 is the graphene coating resistance (Rc) and CPE1 is the constant phase element (CPEc) related to the graphene coating capacitance, while R2 is an oxide layer resistance (Rox) and CPE2 is the constant phase element (CPEox) of the oxide layer on the metal surface. CPE which includes all the frequency dependent electrochemical phenomena,

Fig. 8 – Equivalent electrical circuit used for the impedance plots fitting of the graphene film on metallic substrate for different immersion times in 0.1 M NaCl.

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namely the capacitance and diffusion processes, is used in this model to compensate for inhomogeneity in the system and is defined by two parameters, Y0 and n. The impedance of CPE is represented by the following equation [41]: n

ZCPE ¼ Y 1 o ðjxÞ

ð2Þ

1/2

1

where j = (1) , x = 2pf is frequency in rad s and f is the frequency in Hz. The impedance data in the complex plane were well fitted by the proposed EEC and we used three basic criteria to evaluate the general accuracy of the fit: visual fit to Bode and Nyquist plots, low goodness of fit and low relative standard errors for every circuit element [42]. We obtained suitably low goodness of fit (<104) and the error associated with each element lower than 10%. It can be concluded that the chosen fit describes investigated systems accurately. EIS parameters corresponding to the equivalent circuit (Fig. 8) for graphene coated Cu in 0.1 M NaCl after different times of immersion at open circuit potential are listed in Table 3. Nyquist plots and the fitting curves for graphene coated Cu at different times of immersion in 0.1 M NaCl solution are shown in Fig. 9. The obtained Rc, CPEc, Rox, and CPEox values are plotted as a function of prolonged exposure time in 0.1 M NaCl in Fig. 10a–d. Slight fluctuations in Rc value (Fig. 10a) during exposure time could be the result of inhomogeneous structure of graphene coating on both substrates. The Cu surface nucleates oxide islands which grow and coalesce into an oxide coating. These oxide islands, when coalesced, may exhibit a network of microcracks and pinholes and consequently may induce the breakage of graphene coating into micro-patches and result in the decrease of Rc value [43,26]. On the other hand, the greater values of graphene resistance, Rc, on Cu compared to the graphene coating on Al demonstrate the effect of the metal substrate on the corrosion properties of the graphene coating. These observations can be explained by galvanic corrosion of Al in contact with a chloride solution due to the breakage of the passive Al oxide film beneath the graphene coating. In neutral salt solution the rate of oxygen reduction to hydroxyl ions determines the thickness of corrosion product layer. Under such conditions, the overall corrosion process could be limited by the reaction between the metal and the oxidation agent. It was assumed that Al oxide film beneath the graphene coating layer was porous and therefore during longer immersion of Al coated specimens in 0.1 M NaCl the oxide grew and filled up the pores, conse-

Fig. 9 – Nyquist plots (experimental) and the fitting curves (solid lines) for graphene coated Cu at different times of immersion in 0.1 M NaCl solution. (A colour version of this figure can be viewed online.)

quently the layer became less porous and the Rox values increased (Fig. 10c). The slow decrease in CPEox (Fig. 10d), after 20 days of immersion test indicates that probably the number and the width of the pores in the Cu oxide/hydroxides film and Al oxide film decreased, and the layers became less porous. It is reported that after the coalescence of Cu oxide islands the oxide growth rate significantly decreases, due to the change of oxygen diffusion through the existing oxide layer [43]. Consequently the formation of non-uniform oxide film on the Cu substrate can also lead to altered CPEox values. The obtained results confirmed that graphene coating on Cu surface can offer corrosion protection by extending the time required for the electrolyte to reach the metallic surface beneath the graphene coating. In order to compare the surface morphology of graphene coated Cu and graphene coated Al, electrochemical measurements were supplemented by SEM/EDS analysis. Scanning electron micrographs of the bare and graphene coated Cu and Al before and after 2 h of exposure to 0.1 M NaCl are depicted in Figs. 11 and 12, respectively. While the entire surface of the bare Cu and Al samples is damaged after 2 h of exposure to 0.1 M NaCl, the surface of the Gr/Cu and 3Gr/Al samples degrades at isolated areas, leaving most of the surface undamaged. In addition, SEM micrographs clearly show the differences between the cracked surface of the graphene coating on Al (Fig. 12d) and the homogeneous surface of graphene coating on Cu (Fig. 11d). It was observed from EDS analysis that oxygen

Table 3 – EIS parameters corresponding to the equivalent circuit for graphene coated Cu in 0.1 M NaCl after different times of immersion at open circuit potential. Time of immersion, days

Rc, kO cm2

CPEc, lFcm2

n c,

Rox, kO cm2

CPEox, mFcm2

nox

3 11 15 19 26 33

9.247 49.16 55.97 60.01 30.10 29.56

73.43 112.5 103.9 90.49 85.29 101.8

0.646 0.623 0.604 0.570 0.538 0.552

56.05 32.31 49.73 47.36 62.48 60.37

0.675 1.868 7.617 14.36 0.023 0.020

0.820 0.789 0.982 0.892 0.983 0.830

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Fig. 10 – Time dependences of (a) graphene coating resistance, Rc, (b) constant phase element related to graphene coating capacitance, CPEc, (c) oxide layer resistance, Rox, and (d) constant phase element related to oxide layer capacitance, CPEox, for graphene coated Cu and Al during prolonged exposure to 0.1 M NaCl. (A colour version of this figure can be viewed online.)

concentration increased to 26.1 at.% for the Gr/Cu sample and to 32.9 at.% for the 3Gr/Al sample, indicating the formation of an oxide/hydroxide layer on Cu, and an Al oxide

Fig. 11 – Scanning electron micrographs of bare Cu before (a) and after exposure (b), and graphene coated Cu before (c) and after exposure to 0.1 M NaCl (d).

layer on the Al surface, respectively. SEM/EDS results are in accordance with the results obtained from electrochemical measurements.

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during exposure time were monitored using different electrochemical techniques. The corrosion-inhibiting properties of graphene coating on Cu were demonstrated. Eocp values of the graphene coated Cu were around 20 mV more positive than bare Cu, and PDS measurements showed a decrease in current density, while EIS confirmed the presence of graphene coating after 40 days exposure. Slight fluctuations in Rc value during immersion time could be the result of inhomogeneous structure of graphene coating on Cu, while the slow decrease in CPEox, after prolonged immersion time indicates the presence of non-uniform oxide film on the Cu substrate. The Cu oxidation occurs in micrometer-sized domains surrounded by areas having minimal oxidation. The obtained results indicate that the graphene coating acts as a barrier to the underlying Cu surface and decreases the Cu dissolution in 0.1 M NaCl. The same effect was observed from the PDS measurements after 10 months of exposure in air, where calculated corrosion rate for graphene coated Cu was lower than the corrosion rate for bare Cu. On the other hand, Eocp values of graphene-coated Al were similar to values obtained for bare Al during exposure time, while PDS measurements showed higher values of current density and lower values of corrosion rate than bare Al. EIS confirmed the presence of a graphene coating even after 35 days of exposure, while the increase of Rox values can be explained by a decrease of the number of pores in Al oxide layer with immersion time. The greater values of graphene resistance, Rc, on Cu compared to the graphene coating on Al demonstrate that graphene coating on Cu is less corrosion susceptible as compared to coatings that were mechanically transferred to an Al surface, probably due to the galvanic corrosion in the presence of small concentration of graphitic materials. The efficiency of the corrosion inhibition could be further enhanced by depositing graphene coating with higher purity in order to achieve an improved homogeneous coating structure and thereby avoid the possibility of galvanic corrosion.

Acknowledgements This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (project number: 2013R1A1A2A10063466). The authors would like to express their gratitude to the Ministry of Education, Science and Technological Development, Republic of Serbia (Grant No. III 45019), for financial support.

R E F E R E N C E S

Fig. 12 – Scanning electron micrographs of bare Al before (a) and after exposure (b), and graphene coated Al before (c) and after exposure to 0.1 M NaCl (d).

4.

Conclusions

The electrochemical properties and corrosion stability of graphene coatings on Cu and Al in a 0.1 M NaCl solution

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CARBON

7 5 ( 2 0 1 4 ) 3 3 5 –3 4 4

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