graphene composites as corrosion inhibitor for cold-rolled steel

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Available at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/carbon

Room-temperature cured hydrophobic epoxy/graphene composites as corrosion inhibitor for cold-rolled steel Kung-Chin Chang a, Min-Hsiang Hsu a, Hsin-I Lu b, Mei-Chun Lai a, Pei-Ju Liu a, Chien-Hua Hsu a, Wei-Fu Ji a, Tsao-Li Chuang b, Yen Wei c, Jui-Ming Yeh a,*, Wei-Ren Liu d a

Department of Chemistry, Center for Nanotechnology and Institute of Biomedical Technology at Chung at Yuan Christian University (CYCU), Chung Li 32023, Taiwan, ROC b Master Program in Nanotechnology and Center for Nanotechnology at CYCU, Chung Li 32023, Taiwan, ROC c Department of Chemistry and Key Lab of Organic Optoelectronic & Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China d Department of Chemical Engineering, CYCU, Chung Li 32023, Taiwan, ROC

A R T I C L E I N F O

A B S T R A C T

Article history:

Nanocasting was used to develop epoxy/graphene composites (EGCs) as corrosion inhibi-

Received 6 June 2013

tors with hydrophobic surfaces (HEGC). The contact angle of water droplets on a sample

Accepted 28 August 2013

surface can be increased from 82 (epoxy surface) to 127 (hydrophobic epoxy and

Available online 4 September 2013

EGC). It should be noted that EGC coating was found to provide an excellent corrosion protection effect on cold-rolled steel (CRS) electrode. Enhancement of corrosion protection using EGC coatings could be attributed to the following three reasons: (1) epoxy could act as a physical barrier coating, (2) the hydrophobicity repelled the moisture and further reduced the water/corrosive media adsorption on the epoxy surface, preventing the underlying metals from corrosion attack, and (3) the well-dispersed graphene nanosheets (GNSs) embedded in HEGC matrix could prevent corrosion owing to a relatively higher aspect ratio than clay platelets, which enhances the oxygen barrier property of HEGC.  2013 Elsevier Ltd. All rights reserved.

1.

Introduction

Corrosion poses a serious threat on economy and industry, and results in potential danger to humans. Unfortunately, corrosion cannot be fully prevented, and thus, corrosion control strategies focus on slowing the kinetics and/or altering its mechanism. These strategies include cathodic protection [1,2], use of protective coatings [3–5], use of corrosion inhibitors [6,7], or any combination thereof [8–10].

Considerable interest has now been given to the use of super-hydrophobic materials as protective coatings [11–16]. Briefly, super-hydrophobic coatings are characterized by a water contact angle of at least 150 and known to be very resistant to water absorption [17]. This anti-wetting property is relevant to its corrosion prevention. Nanocasting [18], based on soft lithography [19,20], is a method widely used in nanofabrication. By directly replicating the template to create larger uniform patterns, the cost of larger-scale fabrication is reduced [21]. In this method, a

* Corresponding author: Fax: +886 3 2653399. E-mail address: [email protected] (J.-M. Yeh). 0008-6223/$ - see front matter  2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.08.052

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soft and deformable material is utilized to cast and replicate the structures of the template surfaces. After this soft material solidification, the complement of the surface structures on the original template is transferred to the final material. This new template can be repeatedly used to replicate the surface structures or patterns as long as the template is not damaged by any means. Recently, nancomposites synthesized containing graphene-based materials such as graphene nanoplates (GNPs), graphene nanosheets (GNSs), and graphene oxide (GO), have displayed excellent beneficial properties when employed in diverse applications [22–28]. The research activities associated with conductive graphene with a relatively high aspect ratio of ca. 500 [29] have attracted much interest. The lower density and higher aspect ratio of conductive graphene, as compared with those of non-conductive clay platelets, triggered their potential application as advanced gas barrier polymer nanocomposites [30]. Several studies have been previously conducted on the use of polymers in various applications such as hydrophobic/ super-hydrophobic surfaces [31–37]. Therefore, development of hydrophobic/super-hydrophobic polymers has become an interesting subject in materials science. In addition, not only has the anticorrosive properties of hydrophobic/super-hydrophobic polymer surfaces rarely been reported to date [15,16,38–40]. But also, graphene-based composite corrosion inhibitors with/without hydrophobic surfaces have also rarely been studied [41–43]. Here, to directly duplicate the surface features of fresh plant leaves (super-hydrophobic Xanthosoma sagittifolium), we used nanocasting technique to develop epoxy/graphene composite (EGC) corrosion inhibitors. The imprint of the leaf was transferred onto the composite surface so that the resulting composite exhibited hydrophobicity for corrosion prevention. Traditionally, the epoxy resin utilized for anticorrosion usually involves the use of organic solvents, which significantly increase human health risks although the resin has excellent metal adhesion properties and high resistance to heat, water, and chemicals. As new regulations regarding the emission of volatile organic compounds (VOCs) are put into effect, increased demands for environmentally friendly (green) coating systems require the progressive substitution of hazardous coatings with greener compounds. Among the alternatives are water-borne coatings and solvent-free coatings. Thus, in this study, we also developed an environmentally friendly process to prepare composite corrosion inhibitors at room temperature without involving any solvent. The composite corrosion inhibitors developed in this study provided threefold protection for metals from corrosion. First, the cured epoxy coating acted as a good barrier to limit the accessibility of moisture and oxygen to the metal surface. Second, the hydrophobicity repelled the moisture and further reduced the water/corrosive media adsorption on the epoxy surface, preventing the underlying metal from corrosion. Finally, the dispersed GNSs in the epoxy matrix increased the tortuosity of oxygen diffusion pathway (reduced diffusion length). The detailed anticorrosion performance of the developed hydrophobic epoxy/graphene composite (HEGC) coatings was evaluated by a series of electrochemical

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corrosion measurements. Corrosion protection studies were performed on sample-coated cold-rolled steel (CRS) immersed in a corrosive medium (3.5 wt% sodium chloride aqueous solution).

2.

Experimental section

2.1.

Materials and measurements

4,4 0 -Diaminodiphenylamine sulfate (Aldrich), bisphenol A diglycidyl ether (DGEBA, Aldrich), and poly(propylene glycol) bis(2-aminopropyl ether) (B210, Epocone Chemical Co. Ltd.) were used as received. The liquid components (Sylgard 184) of polydimethylsiloxane (PDMS) were supplied by Dow Corning Corporation. Graphene nanosheets (SFG44-GNS) prepared from SFG44 synthetic graphite powders (TIMCAL) were used. All reagents were of reagent grade unless otherwise stated. Fourier transform infrared spectra (FTIR) were recorded using an FTIR spectrometer (JASCO FT/IR-4100) operating at room temperature. The nanostructure of composite materials was imaged with a JEOL-200FX transmission electron microscope (TEM). The samples for TEM study were cut into 60– 90 nm-thick sections with a diamond knife. Surface morphologies of the hydrophobic samples were observed by using scanning electron microscopy (SEM, Hitachi S-4200). Contact angles were measured using a First Ten Angstroms FTA 125 at ambient temperature. Water droplets (about 4 lL) were carefully dropped onto the surfaces of the samples, and the contact angle was determined from the average of five measurements at various positions on the samples surface. The corrosion potential and corrosion current of sample-coated CRS electrodes were electrochemically measured using a VoltaLab 50 potentiostat/galvanostat. Electrochemical impedance spectroscopy (EIS) measurements were recorded on an AutoLab (PGSTAT302 N) potentiostat/galvanostat electrochemical analyzer. Gas permeability (O2 permeation) experiments were performed using GTR-31 analyzer (Yangimoto Co., Kyoto, Japan).

2.2.

Synthesis and characterization of carboxyl-GNSs

GO derived from SFG44 synthetic graphite powders (TIMCAL) was synthesized by a modified Hummers’ method [44]: 4.0 g of synthetic graphite powder and 2.0 g of NaNO3 were put into 280 mL of concentrated H2SO4 solution and subsequently stirred for 2 h. Then 16 g of KMnO4 was slowly added into the flask with an ice bath for 2 h. The mixture was diluted with 400 mL of de-ionized water. After that, 5% H2O2 was added into the solution until the color of the mixture changed to brown to ensure that KMnO4 was fully reduced. The as-prepared GO slurry was re-dispersed in de-ionized water. Then, the mixture was washed with 0.1 M HCl solution to remove ions. Subsequently, the GO solution was washed with SO2 4 distilled water to remove the residual acid until the solution pH was ca. 5 and then vacuum dried at 50 C. The GO powder was put into the furnace at 1000 C for 30 s for thermal exfoliation. Finally, we could obtain few layers of carboxyl-GNSs. On the other hand, chemically converted graphene was

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obtained by hydrazine reduction under reflux at 100 C for 24 h and subsequent drying at 100 C in an oven. Fig. 1 shows the XPS spectra of C 1s of carboxyl-graphene and chemically converted graphene. The C 1s spectra were normalized by the C@C characteristic peak at 284.5 eV. The C 1s spectrum of carboxyl-graphene shows three peaks assigned to oxygen functional groups: C–O from phenol and ether (at 286.1 eV), C@O from carbonyl and quinone (at 287.5 eV), and –COO from carboxyl and ester (at 288.7 eV). The relative intensity of C–O peak is much larger than those of C@O and –COOH. The carboxylic group content of carboxyl-graphene nanosheets were calculated from the XPS spectra using the following equation [45]: Carboxylic group content ¼ ðAreaACAOH þ AreaAC@O þ AreaACOO Þ=Areatotal According to the quantitative analysis by Gaussian fitting with three peaks, the content of C–O, C@O and –COO in carboxyl-graphene nanosheets were 11%, 5%, and 4%, respectively. GNSs were also characterized using X-ray diffraction (XRD) patterns, Raman spectra, and SEM and TEM micrographs using similar conditions as those previously reported [46,47].

2.3.

Preparation of PDMS template

The PDMS prepolymer was obtained by mixing the elastomer base and a curing agent (Dow Corning 184 silicone elastomer) in a proper ratio (10:1, w/w). The PDMS pre-polymer was poured into 3 · 6 cm2 molds fixed to a piece of fresh, natural Xanthosoma sagittifolium leaf (The veins of the leaf were removed in an area of about 3 · 6 cm2.) and then cured in a 60 C oven for 4 h. After curing, the PDMS blocks were separated from the molds and used as templates for imprinting.

2.4.

Preparation of epoxy coating

The epoxy coating was prepared at room temperature by reacting DGEBA resin with curing agent (hardener) B210

without using any solvent. In a typical epoxy synthesis procedure, 0.5 g of B210 and 1.5 g of DGEBA were mixed with a three-roll mill at room temperature. After mixing, the mixture was coated on CRS and cured at room temperature.

2.5.

Preparation of HEGC coatings

In a typical HEGC coatings synthesis procedure, 0.5 g of B210, 1.5 g of DGEBA and 0.02 g of graphene (i.e., 1 wt%) were mixed at room temperature using a three-roll mill. After mixing, the mixture was coated on the CRS. The PDMS template was subsequently pressed against the coating. The sample was cured at room temperature and then the HEGC coating was obtained after peeling off the PDMS template from the EGC coating.

2.6.

Preparation of EGC membranes

The typical procedure for the preparation of EGC membranes with 1 wt% of GNSs (denoted EGC1) was given as follows: first, 0.5 g of B210, 1.5 g of DGEBA and 0.02 g of graphene were mixed with a three-roll mill at room temperature. After mixing, the mixture was casted onto a clean Teflon mold followed by curing at room temperature. The cured EGC membranes were obtained with a thickness of ca. 120 lm.

2.7.

Electrochemical corrosion studies

The electrochemical corrosion measurement was performed using VoltaLab 50. All the electrochemical corrosion measurements were also performed in a double-wall jacketed cell, which is covered with a glass plate and the water inside was maintained at a constant operational temperature of 25 ± 0.5 C. Open-circuit potential (OCP) at the equilibrium state of the system was recorded as the corrosion potential (Ecorr in mV versus saturated calomel electrode (SCE)). The Tafel plots were obtained by scanning the potential from 1000 to 500 mV above the Ecorr at a scan rate of 10 mV/min. Corrosion current (Icorr) was determined through superimposing a straight line along the linear portion of the cathodic or anodic curve and extrapolating it through Ecorr. The corrosion rate (Rcorr, in milli-inches per year, MPY) was calculated from the following equation [48]: Rcorr ðMPYÞ ¼ ½0:13 Icorr ðE:W:Þ=½A  d

Fig. 1 – XPS C 1s spectra of carboxyl-graphene and chemically reduced graphene. (A colour version of this eigure can be viewed online.)

where E.W. is the equivalent weight (g/eq.), A is the area (cm2), and d is the density (g/cm3). AutoLab (PGSTAT302N) potentiostat/galvanostat was employed to perform the a.c. impedance spectroscopy measurements. Impedance measurements were carried out in the frequency range of 100 kHz to 100 MHz with pure iron (area, 1 · 1 cm2) as working electrode embedded in epoxy, Pt as counter electrode, and SCE as reference electrode. The working electrode was first maintained in the test environment for 30 min before the impedance study/measurement. All experiments were operated at room temperature. All raw data were repeated at least three times to ensure reproducibility and statistical significance.

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Fig. 2 – Preparation process for the hydrophobic surfaces of composite using the nanocasting technique. (A colour version of this eigure can be viewed online.)

3.

Results and discussion

The preparation process for the fabrication of HEGC coating materials is shown in Fig. 2. First, a PDMS pre-polymer is casted against a fresh Xanthosoma sagittifolium leaf surface, curing under proper conditions. The PDMS template with negative Xanthosoma sagittifolium leaf surface structures is obtained after peeling the leaf off. The substrate is then covered with the epoxy precursor mixture solution, and the template is pressed against the CRS. After curing at room temperature, the PDMS template is peeled off, and a Xanthosoma sagittifolium-leaf-like surface is formed on the CRS.

3.1.

Characterization of epoxy and EGC materials

FTIR was utilized to investigate the completion of curing of the epoxy and EGC materials. The FTIR spectra of graphene, epoxy, and EGC materials is presented in Fig. 3. In Fig. 3(a) and (b), the characteristic peaks of the different materials

Fig. 3 – FTIR spectra for (a) epoxy, (b) EGC1, and (c) carboxylgraphene. (A colour version of this eigure can be viewed online.)

are depicted, including those at 1609 cm1, 1508 cm1 (C–C skeletal stretching), 1036 cm1 (aromatic deformation), and 915 cm1 (epoxide ring). The curing process of epoxy and EGC materials can be monitored from the decrease in the intensity of characteristic bands of epoxide ring and the increase in the –OH stretching band intensity (3380 cm1) after completion of the curing process. Moreover, a comparison of the two curves of epoxy and EGC materials shows that there is no obvious different absorption peak.

3.2.

Morphology of EGC materials

The dispersion capability of graphene nanosheets in polymer matrix can be identified by TEM observations. The image at low magnification was used to determine graphene dispersed situation in the polymer, while the high-magnification image enables the description as the degree of exfoliation. Fig. 4 shows the TEM micrographs of the epoxy composites of graphene, and the content of graphene of composites is 1 wt%. Fig. 4(a) was an image of EGC1 at low magnification (·50,000), while Fig. 4(b) was the image of the same sample at higher magnification (·200,000). The gray regions of the photograph at high magnification represent the domain of epoxy matrix, and the dark lines correspond to the cross section of graphene nanosheets. The image in Fig. 4(b) shows that relatively well-dispersed graphene nanosheets existed in the epoxy matrix. This indicates that the attachment of carboxylic groups onto the graphene surface could effectively enhance the compatibility between carboxyl-graphene nanosheets and the epoxy matrix, leading to improved dispersion. In addition, the better dispersion also results from the formation of hydrogen bonds between the carboxyl groups of carboxyl-graphene nanosheets and the hydroxyl groups of epoxy [49].

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Fig. 4 – TEM micrographs of EGC materials with 1 wt% graphene at (a) low magnification (·50,000) and (b) high magnification (·200,000).

Fig. 5 – (a) Photograph of the Xanthosoma sagittifolium leaves. (b) SEM image of the fresh natural leaf. Illustration is water contact angle of Xanthosoma sagittifolium leaf. (c) PDMS negative template. (A colour version of this eigure can be viewed online.)

3.3.

Microscopic observations

A photograph of natural, fresh Xanthosoma sagittifolium leaves is shown in Fig. 5(a). Fig. 5(b) is the high magnification SEM image of the same leaf. The average contact angle on the fresh leaves is ca. 146, as shown in Fig. 5(b). In Fig. 5(b), many small papillary hills are clearly visible on the natural leaf. The diameters of the small papillary hills are between 7 and 9 lm. Fig. 5(c) is the SEM image of the PDMS template prepared by casting the liquid PDMS directly onto a natural, fresh Xanthosoma sagittifolium leaf. Many holes whose diameters range from 7 to 9 lm are shown on the surface of the PDMS template. Fig. 5(c) shows the topographic structure of the hole on the surface that complements the papillary hills on the natural, fresh leaf. This result demonstrates that the template effectively replicated the topologically inverse structures of the leaf surfaces. Fig. 6 shows the structures on the surfaces of the nanocast layers on the CRS slides observed using SEM. Numerous papillary microstructures (ca. 7–9 lm average diameter) are formed

on the surfaces. The papillary microstructures are replicas of the surface patterns on the Xanthosoma sagittifolium leaves.

3.4.

Contact angle (wettability) measurements

The coating material replicated from the fresh leaves shows hydrophobic characteristics and a larger water contact angle. Fig. 7 shows the change in the water contact angle with various times for HEGC. The durability of the water repellent on the HEGC coating is a very important parameter. The HEGC was kept at room temperature in the ambient atmosphere for 1 month, and water contact angles were measured under each set of conditions. Tested once every 5 days, total of 30 days. As shown in Fig. 7, almost no decrease in the water contact angle was observed, indicating that the hydrophobic property of the as-prepared HEGC coating is stable enough. The Xanthosoma sagittifolium-leaf-like-structured epoxy and EGC coatings obviously have a larger water contact angle (ca. 127) than the smooth-surface epoxy coating (ca. 82, Table 1).

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Fig. 6 – SEM images for (a) the Xanthosoma sagittifolium leaf-like hydrophobic surface of composite. The inset shows the contact angle of the polymeric surface. (b) Top-view of the surface of composite. (c) Section view of the composite surface.

Fig. 7 – Change in water contact angle for various times for HEGC.

As a significant amount of air was trapped between the papillary hills of the epoxy and EGC surface, a water drop on such a coating could only make contact with the top of the papillary hills. Thus, the water placed on the surface of the epoxy and EGC coatings was likely resting on a thin air cushion. Moreover, good adhesion between the coating and CRS are essential if the coating is to exert effective anticorrosive behaviour on the substrate. The adhesion of the HE and HEGC coating on the CRS substrate was synthesized according to the cross-cut method [50]. In the wake of adhesion tests, there was no significant peeling of either HE or HEGC after crosscutting through the coating, as shown in Fig. 8. In other words, adhesion between the CRS and both coatings is strong.

It should be noted that the topographic structure on the surface of the CRS complements the papillary hills of the natural, fresh Xanthosoma sagittifolium leaf. When various smooth substrates such as glass slides or CRS substrates are used, the resulting surface features are almost identical because they are replicated on a thin layer completely covered onto the substrate surfaces. The PDMS template played a key role in this approach. Due to its low surface energy and excellent solidification property, the PDMS template could be used to replicate the nanostructures on the surface of the leaf with high fidelity and could be readily peeled off without significantly damaging the surfaces. During the process, the template remained in close contact with the substrate under pressure, generating the solid nanostructures after replication. The ability of a coating to protect metal substrates against corrosion depends on three aspects: (1) sorption of water onto the coating, (2) transport of water throughout the coating, and (3) accessibility of water to the coating/substrate interface. Consequently, it is reasonable to accept that the low-wetting HEGC effectively prevented from adsorption of water onto the substrate surface; and thus exhibited an excellent corrosion resistance in wet environments.

3.5.

Potentiodynamic measurements

On the basis of a series of electrochemical measurements (i.e., corrosion potential, polarization resistance, and corrosion current measured in a corrosive medium (3.5 wt% aqueous NaCl electrolyte)), we concluded the HEGC coating was superior at protecting the CRS electrode against corrosion than the

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Table 1 – Contact angle and electrochemical corrosion measurements of bare CRS, epoxy, HE and HEGC coated electrodes, and gas permeability of HE and HEGC. Sample code Electrochemical corrosion measurementsa 2

PEF%

Thickness Contact angle O2 permeability (lm) (/H2O) (barrer)

– 88.45 92.94 99.33

– 110 110 115

2

Ecorr (mV vs SCE) Rp (kX cm ) Icorr (lA/cm ) Rcorr (MPY) CRS Epoxy HE HEGC a

821 678 633 411

2.94 25.46 37.87 442.00

6.97 1.07 0.35 0.10

6.49 1.00 0.33 0.09

– 82 ± 1 126 ± 1 128 ± 1

– – 0.10 0.04

Saturated calomel electrode (SCE) was employed as a reference electrode.

Fig. 8 – SEM images of (a) HE-coated on CRS, and (b) HEGC-coated on CRS after testing for adhesion. common hydrophobic epoxy (HE) and epoxy coatings. Information about corrosion current can be obtained by extrapolating Tafel plots, from both the cathodic and anodic polarization curves for the respective corrosion processes [50,51]. Extrapolating the cathodic and anodic polarization curves to their point of intersection provides both the corrosion potential and the corrosion current. Corrosion protection studies were performed on samples with about 110-lm-thick coatings and immersed in a corrosive medium for 30 min. Tafel plots for the three samples immersed in the corrosive medium are shown in Fig. 9 and the corresponding data listed in Table 1. The protection efficiency (PEF%) values were estimated using the following equation [52]: 1 1 PEF % ¼ 100½R1 p ðuncoatedÞ  Rp ðcoatedÞ=Rp ðuncoatedÞ

where Rp is the polarization resistances (kX cm2), were evaluated from the Tafel plots. The Tafel plots for the sample-coated CRS electrode gave a corrosion potential of Ecorr = 678 and 633 mV for the epoxy and HE coatings, which was more positive than that for the bare CRS electrode, where Ecorr = 821 mV. Moreover, the corrosion current (Icorr) of the epoxy and HE-coated CRS electrodes was ca. 1.07 and 0.35 lA/cm2, which was significantly lower than that of the bare CRS electrode (i.e., 6.97 lA/cm2). The corresponding Icorr decreased considerably when we used the EGC-coated bare CRS electrode with Xanthosoma sagittifolium-leaf-like structures (HEGC) to produce hydrophobic properties. Moreover, the Ecorr of the EGC-coated CRS electrode was more positive than that of the HE-coated CRS electrode, as Ecorr increased from 633 to 411 mV (vs. SCE) for the HEGCcoated CRS electrode. These electrochemical measurement results show that the HEGC coating provided better protection against corrosion of the CRS electrode than other coatings did.

Fig. 9 – Tafel plots for (a) bare, (b) epoxy-coated, (c) HEcoated, and (d) HEGC-coated CRS electrodes measured at 25 ± 0.5 C. (A colour version of this eigure can be viewed online.)

3.6.

Electrochemical impedance measurements

Dielectric spectroscopy, sometimes called impedance spectroscopy or electrochemical impedance spectroscopy (EIS), is used to measure the dielectric properties of a medium and express them as functions of frequency [53,54]. EIS is also used to examine the activity difference between the CRS surface upon HE and EGC coating materials treatment. Impedance is a complex resistance when alternate current flows through a circuit made of capacitors, resistors, or insulators, or any of their combination [55]. EIS measurement results in currents over a wide range of frequency.

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Fig. 10 – Nyquist plot for (a) bare, (b) epoxy-coated, (c) HEcoated, and (d) HEGC-coated CRS electrodes. (A colour version of this eigure can be viewed online.)

Fig. 10 shows the Nyquist plots of the four measured samples. The first sample (a) is uncoated CRS. A series of samples denoted as (b), (c), and (d) represent CRS-coated by epoxy, HE, and HEGC, respectively. The charge transfer resistances of all samples, as determined by subtracting the intersection of the high-frequency end from the low-frequency end of the semicircle arc with the real axis, are 0.0019, 1.98, 10.6, and 29.6 MX cm2, respectively. EIS Bode plots (impedance vs. frequency) of all samples are shown in Fig. 11. Zreal is a measure of corrosion resistance [56]. Low Zreal value could be brought about by very high capacitance and/or very low resistance of the coating [57,58]. Large value of the capacitance has been related to the high extent at which water has penetrated the coating [59]. In the case of Bode plots, the value of Zreal at the lowest frequency also represents the corrosion resistance. The Bode magnitude plots for uncoated CRS and CRS-coated by epoxy, HE, and HEGC shows Zreal values of 3.2, 6.3, 7.0, and 10.6 kX cm2, respectively, at low frequency end. These results clearly demonstrate that the HEGC coating protects the CRS electrode against corrosion better than the epoxy and HE coatings. The increase in impedance for the epoxy and HEcoated electrode can be attributed to the hydrophobicity and barrier effects of GNSs dispersed in composites of the HEGC coating.

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Fig. 12 – Schematic representation of hydrophobic surface and oxygen following a tortuous path through an epoxy and EGC materials. (A colour version of this eigure can be viewed online.) The hydrophobicity and gas barrier effect of HEGC compared to HE and epoxy may result from the low-wettability and dispersed graphene in the epoxy matrix to increase the tortuosity of oxygen diffusion pathway (lower diffusion length) [60,61], as shown in Fig. 12.

3.7.

Molecular barrier measurements

The membranes of epoxy and EGC materials used for the molecular barrier measurements were prepared to have film thickness of 120 lm. Compared to epoxy, EGC membranes at 1 wt% graphene loading shows about 60% reduction in O2 permeability, as shown in Table 1. The decrease in gas permeability is attributed to the barrier properties of the layers of GNSs dispersed in the composites.

4.

Conclusions

In conclusion, we have shown the fabrication of EGC corrosion inhibitor with biomimetic hydrophobic structures by using a nanocasting technique. On the surface of as-synthesized HEGC coatings lots of micro-scaled mastoids were found, each decorated with many nanoscale wrinkles. The water contact angle of the HE and HEGC coatings whose surface was imprinted with the biomimetic pattern of the surface of a natural leaf was ca. 127, significantly larger than that of the epoxy coating (82). From the morphology of TEM observations, the coating exhibited relatively well-dispersed GNSs in the epoxy matrix. The decrease in gas permeability is attributed to the barrier properties of the GNSs layers dispersed in the composites. The hydrophobicity and barrier effect of HEGC materials provides it with excellent anticorrosive properties.

Acknowledgements

Fig. 11 – Bode plot for (a) bare, (b) epoxy-coated, (c) HE-coated, and (d) HEGC-coated CRS electrodes.

We gratefully acknowledge the financial support of the Ministry of Education (MOE), Taiwan, ROC, NSC 101-2113-M-033005-MY1, 101-3113-P-002-026 and 102-2622-E-007-016-CC1,

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the Department of Chemistry at Chung Yuan Christian University (CYCU) (CYCU-01RD-RA002-11235), and the Center for Nanotechnology and Institute of Biomedical Technology at CYCU.

R E F E R E N C E S

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