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Synergistic effect of polypyrrole-intercalated graphene for enhanced corrosion protection of aqueous coating in 3.5% NaCl solution Shihui Qiu, Wei Li, Wenru Zheng, Haichao Zhao, and Liping Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08325 • Publication Date (Web): 15 Sep 2017 Downloaded from http://pubs.acs.org on September 17, 2017

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ACS Applied Materials & Interfaces

Synergistic effect of polypyrrole-intercalated graphene for enhanced corrosion protection of aqueous coating in 3.5% NaCl solution

Shihui Qiua,b, Wei Lib, Wenru Zhenga, Haichao Zhao∗,a, Liping Wang*,a

a

Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key

Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, P. R. China b

Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo

315211, P. R. China

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ABSTRACT: Dispersion of graphene in water and incorporation of it into waterborne resin have been rarely researched and hardly achieved owing to its hydrophobicity. Furthermore, it has largely been reported that graphene with impermeability contributed to the improved anticorrosion property. Here we show that highly concentrated graphene aqueous solution up to 5 mg/mL can be obtained by synthesizing hydrophilic polypyrrole nanocolloids as intercalators and ultrasonic vibration. Based on the π-π interaction between polypyrrole and graphene, stacked graphene sheets are exfoliated to the thickness of 3~5 layers without increasing defects. The corrosion performance of coatings without and with polypyrrole and graphene is collected by potential and impedance measurements, Tafel curves and fitted pore resistance immersed in 3.5 wt % NaCl solution. It turns out that composite coating with 0.5 wt % graphene additive exhibits superior anticorrosive ability. And mechanism of intercalated graphene-based coating is interpreted as the synergistic protection of impermeable graphene sheets and self-healing polypyrrole and proved by the identification of corrosion products and scanning vibrating electrode technique. KEYWORDS: graphene, polypyrrole, synergistic effect, self-healing, corrosive protection

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INTRODUCTION Graphene is the two-dimensional (2-D) nanosheet composed of the sp2-hybridized carbon atoms which has caught researchers’ eyes due to its high conductivity, stability, super transmittance and powerful mechanical properties. It is characterized by the high specific surface area and weak van der Waals interactions between adjacent layers,1 but its function and advantage depended on its exfoliated state (such as single- or few-layers) to a large extent.2 Therefore, the key issue on its application and development is to overcome or destroy the non-covalent interaction so as to obtain the graphene sheets with free basal plane defects.3-4 Aqueous-based exfoliation of graphene is one of the prospective alternatives in consideration of low-cost and environmental impacts. And dispersing graphene in water have been rarely researched and hardly achieved owing to its hydrophobicity. For years, the surfactants have been commonly used to accomplish the water-dispersion of graphene.5 Notley reported that highly concentrated exfoliation up to 1.5% w/w (15 mg/mL) could be achieved via the continuous addition of the nonionic surfactant in water.6 Recently, Bepete studied the surfactant-free stable water-dispersion of single-layer graphene (SLG) via mixing graphenide (negatively charged graphene) solutions in tetrahydrofuran with degassed water and evaporating the organic solvent.7 And films prepared from these dispersions exhibited the conductivity up to 32 kS/m. The discovery and dispersibility of graphene in various polymer matrices have established a new field of polymer nanocomposites.8-9 It has largely been reported that 3

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graphene/polymer composite can be used as the anti-corrosion coating by forming a physical barrier and increasing the diffused path of corroding media (e.g., water, O2 molecules and Cl‾ ions).10-12 Migkovic-Stankovic investigated the corrosion inhibition property of single layer graphene as a protective coating for Cu grown by chemical vapor deposition (CVD).11 Luo and his co-workers prepared reduced graphene oxide (RGO)/ZnAl layered double hydroxide (LDH) composite film on 6N01 Al alloy surface via hydrothermal continuous flow method and demonstrated its superior corrosion protection performance compared with ZnAl-LDH film immersed in 3.5 and 5.0 wt % NaCl solution.13 Sun tried to choose (3-aminopropyl)-triethoxysilane (APTES) as an insulating encapsulation material to attach to the surface of graphene oxide (GO) by covalent interaction and studied its corrosion-promotion activity.14 The aims of this research were to investigate the aqueous dispersion of graphene on the basis of π-π interaction and graphene-based aqueous composite coating as a protective layer for mild steel. Polypyrrole-based nanocolloids were synthesized as the intercalator to achieve the water-dispersion of graphene and graphene-based composite coatings were prepared. The conventional electrochemical measurements were conducted to evaluate the corrosion stability of coatings with and without additives on Q235 steel during exposure in 3.5 wt % NaCl solution. Furthermore, the synergistic protective effect of intercalated graphene-based coating was attributed to the impermeable graphene sheets and self-healing polypyrrole and proved by the identification of corrosion products beneath the coatings. The self-healing effect of polypyrrole was also verified by the scanning vibrating electrode technique (SVET). 4

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EXPERIMENT SECTION Materials. Pyrrole and ferric chloride (FeCl3) were purchased from Aladdin Industrial Corporation without further purification and poly (vinyl alcohol) (PVA, Mw = 1700~1800) was purchased from obtained Sinopharm Chemical Reagent Co. Ltd. Graphene was provided from Ningbo Morsh Tech Co. Ltd. Epoxy resin (E-51) and waterborne curing agent were provided by Hangzhou Hanma Paint & Coatings Co., Ltd. The carbon steels with the area of 20 × 20 mm was polished by 400 and 800 C sand papers and cleaned in ethanol and acetone via ultrasonic vibration, and its elemental composition was shown in Table S1. Synthesis of Polypyrrole Nanocolloids. In a single-neck round bottomed flask, 2.7 g of FeCl3 was dissolved into the 100 mL 5 wt % PVA aqueous solution until homogeneous solution was obtained. The reactor was kept at 5 °C with continuous stirring for next procedure. Then 1.82 g of pyrrole monomer was added dropwise to the above mixture. The reaction was in progress with mechanical stirring for 4 hours. Polypyrrole (PPy) nanospheres were collected by the centrifugation (TG16-WS, 15000 rpm) and washed with deionized water several times to removed unreacted oxidant. Finally, the PPy powder was obtained by drying at 40 °C in vacuum. Fabrication of G-PPy Hybrids and Composite Coating. The PPy powder was completely dissolved in 20 mL deionized water. Then graphene was added and dispersed in the above solution under ultrasonic vibration (1500 W) for 1 hour to obtain PPy-intercalated graphene dispersion. Based on our previous reports,15 the 5

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mass ratio between graphene and polypyrrole remained 2:1. Aqueous epoxy resin (10 g) was mixed fully with the PPy-intercalated graphene dispersion and excess water was removed via rotary evaporation. The curing agent was weighed 14.6 g and stirred with the above epoxy resin component. The mixture was painted to the pre-treated carbon steels via automatic coater (AFA - II) for the controllable coating thickness (20 µm). The coated carbon steels were stood and cured under the room temperature for 48 hours. The additive amounts of graphene were 123 mg and 246 mg, thus the samples were named as PPy-G0.5% and PPy-G1%. Besides, the neat epoxy coating and composite coating with 0.5 wt % PPy were fabricated for comparison and named as blank and PPy0.5%, respectively. Instruments. The physical properties of G, PPy and PPy-G hybrids were characterized by Raman spectroscopy (Renishaw inVia Reflex) and X-ray powder diffractometer (D8 ADVANCE, BRUKER). XRD patterns were obtained by using monochromatic Cu Kα radiation at a speed of 5°/min in a range of 10-60° and the interplanar distance (d) was calculated by the Bragg equation: 2dsinθ = nλ

(1)

where λ is the wavelength of the X-ray (0.1541 nm, Cu-Kα) and n is the order of diffraction. The particle size distribution of polypyrrole was conducted by the dynamic light scattering (DLS, Zetasizer Nano ZS). The scanning probe microscope (SPM, Dimension 3100), scanning electron microscope (SEM, HITACHI S4800) and transmission electron microscope (TEM, Tecnai F20) were used to exhibit the 6

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structure and morphology of polypyrrole, PPy-G hybrids and fracture surfaces for coatings. The electrochemical data of the coated carbon steels immersed in 3.5 wt % NaCl solution was collected by Modulab electrochemical workstation (Solartron) equipped with a typical three-electrode device including the reference electrode (saturated calomel electrode (SCE)), the counter electrode (platinum plate with 2.5 cm2 area) and the working electrode. It’s noted that every content and blank were tested with three parallel samples. Bare carbon steels or carbon steels with coatings were clasped in the PTFE cell with a 1 cm2 area opening to the electrolytic solution. Open circuit potential (OCP) was collected for the coatings up to 55 days of immersion time. The impedance measurements were recorded in the frequency range from 100 kHz to 0.01 Hz using an alternating current (AC) signal with the amplitude of 10 mV. The potentiodynamic polarization curves were carried out from cathodic direction to the anodic direction (Eocp ± 250 mV) with the sweep rate of 1 mV·s-1. Based on the test error and corrosion susceptibility of insulated epoxy resin specimens, coatings on the Q235 electrodes should be drilled a 2 mm diameter hole to expose the substrate.16 The scanning vibrating electrode technique (SVET) instrumentation used in these experiments was from AMETEK. The microelectrode with the diameter of 50 µm vibrated at a frequency of 80 Hz in perpendicular direction to the surface, with amplitude of 30 µm (peak to peak). Samples prepared for SVET were of 1 cm × 1 cm squares with scan area of 3000 µm × 3000 µm and 31 × 31 points X and Y-axis. The coated sample was scribed to introduce an artificial defect of size ranging from 0.1 to 0.3 mm2. The sample was mounted in a teflon holder and tested in 3.5 wt % NaCl 7

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solution. The crystalline forms and morphology of rust layers were identified through XRD patterns by monochromatic Cu Kα radiation in the range of 5-85° and FEI-SEM (Quanta FEG 250), respectively.

RESULT AND DISCUSSION

Figure 1. Schematic representation of the preparation of PPy-G hybrids. The schematic representation of the preparation of PPy-G hybrids was showed in Figure 1. We have attempted to synthesize the polypyrrole (PPy) nanospheres in 5 wt % poly (vinyl alcohol) (PVA) aqueous solution via micro-emulsion polymerization. As a result, PPy nanospheres stabilized with PVA were obtained and endowed with well-dispersion ability in water. Based on the above and π-π interaction between the heteroaromatic PPy and graphene, we have successfully prepared PPy-intercalated graphene in water assisted with ultrasonic vibration. An optical photograph showing 10 mL green PPy aqueous solution (CPPy = 0.25 mg/mL) and 10 mL dark-green PPy-G aqueous dispersion (CG = 5 mg/mL, mG / mPPy = 2:1) were exhibited in Figure 1.

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Figure 2. (a) XRD patterns and (b) Raman spectra for graphene, PPy and PPy-G. Identification for PPy-G Hybrids. The crystalline structures of graphene, as-prepared polypyrrole (PPy) and PPy-G hybrids were characterized via WAXD analysis in Figure 2a. Graphene possessed a sharp 002 diffraction peak at 2θ = 26.5°, indicating its interplanar distance was 0.335 nm. And polypyrrole showed a broad band of amorphous structure centered at 2θ =19.4°. After intercalated by PPy, the diffraction peaks of PPy-G hybrids were shifted to some extent and its interplanar distance (0.337 nm) is slightly larger than blank graphene. Raman spectra were collected to the structural changes among graphene, PPy and PPy-G hybrids as shown in Figure 2b. Graphene exhibited three distinct bands centered at 1341 (D band), 1580 (G-band) and 2713 cm-1 (2D-band),17-18 which reflected defects, in-plane vibration of sp2 carbon atoms and , respectively.19 And the Raman spectrum of polypyrrole included the characteristic peaks at 936/975, 1043, 1225, 1337/1370 and 1571 cm-1 attributed to the ring deformation vibrations in dication units and radical cation, symmetrical C– H in-plane bending in dication units and radical cation, antisymmetrical C–H deformation

vibrations,

C–N

stretching

and

C=C

stretching

vibrations,

respectively.20-21 In terms of the PPy-G hybrids, its prominent peaks were consistent 9

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with these of graphene and polyporrole in addition to some peaks with a few shifting. Besides, the intensity ratio of D and G band (ID / IG) was used to evaluate the disordered degree of graphene in the different systems.22 PPy-G hybrids showed the lower intensity ratio (ID / IG = 0.41) compared to the neat graphene (ID / IG = 0.63), indicating the lower defect sites and the effect of polypyrrole to compensate for the vacancy of graphene.

Figure 3. Morphology of the polypyrrole and PPy-intercalated graphene. (a/b) SEM images; (c) TEM image; (d) SPM images. Morphology of PPy and PPy-G Hybrids. As-made polypyrrole (PPy) nanocolloids are regular particles and 50~90 nm in diameter confirmed by its morphology and particle size distribution (Figure 3a). The microstructure of PPy-G hybrids was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning probe microscopy (SPM), as shown in Figure 3. Obviously, a large amount of polypyrrole nanospheres were “adsorbed” on the surface of graphene 10

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sheets and PPy-intercalated graphene possessed the thickness of 1~2 nm, which was typical for 3~5 layers graphene. After successful preparation and clear characterization for PPy-G hybrids, aqueous polypyrrole-intercalated graphene composite coatings (PPy-G0.5% and PPy-G1%) on the steel substrate were then made and its corrosive behavior was tested in 3.5 wt % NaCl aqueous solution compared with pure epoxy coating (blank) and 0.5 wt % PPy/epoxy coating (PPy0.5%). Finally, the synergistic protective effect between polypyrrole and graphene was interpreted and proved via the analysis of corrosive products.

Figure 4. SEM images of fracture surfaces for (a) blank, (b) PPy0.5%, (c) PPy-G0.5% and (d) PPy-G1%. Morphology of PPy-G Composite Coatings. All the samples were pre-immersed into the liquid nitrogen for few seconds and then broken to provide the fracture surfaces for SEM observation as shown in Figure 4. For neat epoxy coating (Figure 4a), its fracture surface exhibited obvious holes (yellow dashed circle) left from volatilization of water during the epoxy matrix curing and long crack initiation. In Figure 4b, the 11

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addition of polypyrrole into epoxy resin resulted in rougher fracture surface but shorter crack length than blank. The composite coating with 0.5 wt % graphene (Figure 4c) showed the fracture surface with sparse and smooth cracks. With the loading of graphene up to 1 wt %, the cross section of composite coating formed more holes and messy cracks owing to the aggregated graphene sheets.

Figure 5. Evolution of OCP value for the specimens with the continuous immersion. Open Circuit Potential Test. The evolution of open circuit potential (OCP) for samples was recorded in 3.5 % NaCl solution as shown in Figure 5. To some extent, the OCP value could be regarded as a reference for the inclination of corrosion. In the initial immersion, the mild steels with blank epoxy, PPy0.5% and PPy-G0.5% coatings exhibited notable potential values (-0.05 V~-0.1 V) because corrosive medium has not yet induced the corrosion of substrate. OCP value tended to come down with the permeation of water and electrolyte through the coating and the slowest declined rate has been appeared in PPy-G0.5%. In terms of PPy0.5%, its OCP value has dropped by 0.4 V from 15 to 20 days’ immersion and gone up slightly during the following immersion. The former results from the water absorption of coating with PPy (as proven in Figure S1) and porous coating structure, the latter can be attributed to the 12

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accumulation for corrosive products. The potential of PPy-G1% sample has reduced sharply by 0.5 V in the initial 10 days’ immersion and kept steady at -0.63 V. It’s indicated that the hydrophilic polypyrrole and excess graphene induce the defect of coating and accelerate the corrosion beneath the coating.

Figure 6. Nyquist and Bode plots of the specimens coated with different coatings. 13

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(a/b) blank; (c/d) PPy0.5%; (e/f) PPy-G0.5%; (g/h) PPy-G1% Electrochemical Impedance Spectra Test. Figure 6 showed the Nyquist and Bode-impedance plots of the specimens during the immersion of 55 days. For neat epoxy coating, its Nyquist plots exhibited the shrinking capacitive loop during the immersion, implying the declined corrosion protective performance for steel (Figure 6a). PPy0.5% (Figure 6b) showed the same shrinking tendency for its radii of capacitive impedance arcs in high frequency region. However, its radius of capacitive impedance arc at low frequency region at 10 days was suddenly expanded beyond that of 1 day. As can be seen from Figure 4g, the Nyquist plots for PPy-G1% showed a single depressed semicircle and the diameter of the semicircle decreased after 10 days’ immersion. In the Bode plots, the impedance modulus at the lowest measured frequency (most commonly |Z|0.01Hz) has been used as a semi-quantitative indicator of coating’s barrier performance.23-24 In the initial immersion, the values of |Z|0.01Hz for PPy0.5% and PPy-G0.5% were up to ~ 108 Ω cm2, higher than those of blank and PPy-G1% (closed to 3×106 Ω cm2) by two orders of magnitude. The Bode plot of PPy0.5% (Figure 6d) exhibited a sharp degradation for |Z|0.01Hz value in 20 days (1.35×106 Ω cm2) relative to 15 days (7.59×107 Ω cm2). It can be interpreted that hydrophilic polypyrrole accelerated the water absorption of coatings and weakened their barrier capability to some extent during this immersion period. Surprisingly, a subtle growth of its |Z|0.01Hz occurred at 35 days from 1.02×106 Ω cm2 to 3.31×106 Ω cm2. It turned out that electro-active polypyrrole probably reacted to passivate the metal substrate and it will be discussed in the following. For PPy-G0.5%, its Bode plot (Figure 6f) 14

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displayed a higher |Z|0.01Hz value up to ~ 108 Ω cm2 in the early immersion and kept higher than 107 Ω cm2 through 55 days’ immersion.25

Figure 7. (a) (b) Electrical equivalent circuit models and evolution of (c) Rpore, (d) Rct values with immersion in 3.5 wt % NaCl solution. For further investigations, the EIS measurements are fitted using Zsimpwin software via electrical equivalent circuits (Figure 7a and 7b). The electrical equivalent circuits consist of Rs, Rpore, Cc, Rct and Qdl which are defined by the solution resistance, pore resistance, coating capacitance, charge transfer resistance, double-layer constant phase, respectively. Constant phase element (Q) is used to compensate for the deviation from ideal capacitive behavior and its exponent (n) represents the deviation degree from an ideal dielectric behavior.26-27 For ideal capacitance (Q = C), n = 1 and if n = 0, it acts as an ideal resistor.28 Warburg element 15

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(Zw) represents the diffusion in the coating, resulting increase in cathodic reaction. Figure 7c and 7d displayed the time-dependent behavior of Rpore and Rct immersed in 3.5 wt % NaCl solution. The fitting goodness (χ2) for electrical equivalent circuits and other parameters were showed in Table S2, S3, S4 and S5. Generally, the value of Rpore and Rct for all the specimens decreased with the immersion. Rpore models ionically conducting paths across the coating and has been used to evaluate the barrier performance of coatings.26 During the immersion process, the sample PPy-G0.5% mostly exhibited the highest value of Rpore, implying its remarkable protective capability against corrosion. Furthermore, Rct is the parameter to describe the resistance to charge transfer on the metal surface which varies inversely to the corrosion rate.29 The samples named PPy0.5% and PPy-G0.5% both exhibited the value of Rct higher than 108 Ω cm2 in the initial immersion. However, the Rct for neat epoxy and PPy-G1% closed to 106 Ω cm2 during the whole immersion. The clear decline of Rct occurred at the immersion of 5 days (blank), 15 days (PPy0.5%), 20 days (PPy-G0.5%) and 5 days (PPy-G1%), indicating the onset of corrosion reaction beneath the coatings. Deserved to be mentioned, the evolution of Rct with immersion for PPy0.5% corresponded with its OCP and |Z|0.01Hz value dropped down suddenly after 15 days’ immersion and gone up slightly after 30 days. For specimens with PPy additive, the phenomena on increased Rct value in the immersion period can also be attributed to its self-healing effect. Polarization Curves Test. The Tafel curves and electrochemical parameters were shown in Figure 8 and Table 1, respectively. The corrosion current density (icorr), 16

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anodic Tafel slope (ba) and cathodic Tafel slope (bc) are calculated via extrapolation of anodic and cathodic lines to the corrosion potential (Ecorr) using electrochemical analyzer software. Rp is the polarization resistance determined from the slope of Tafel curve over the narrow potential range of ± 20 mV relative to the corrosion potential.30 The corrosion rate vcorr (mm/year) was obtained by the following equation:31

vcorr =

Aicorr × 87600 (mm/year) nρ F

(2)

where icorr is the corrosion current density for Q235 electrodes. The formula weight, A, is 55.85 g/mol for Q235. And the density, ρ, is 7.85 g/cm3 for Q235. The chemical valence, n, is 2 for Fe. F is the Faraday’s constant (F = 96485 C/mol = 26.8 A hr). And the inhibition efficiency (IE, %) was calculated via the following equation: IE =

bare icorr − icorr ×100% bare icorr

(3)

bare where icorr and icorr signify the corrosion current density in the absence and presence

of coatings for Q235 electrodes, respectively.

Figure 8. Tafel curves of the mild steel with coatings. Table 1. Electrochemical Parameters of Q235 Electrodes without and with 17

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Coatings Immersed in 3.5 wt % NaCl Solution Sample

Ecorr (mV)

icorr (A/cm2)

ba (mV/dec)

bc (mV/dec)

Rp (Ω cm2)

IE (%)

vcorr (mm/year)

Bare Q235

-745.1

7.455×10-5

120.98

-186.88

428.3



0.867

Blank

-624.8

8.048×10-8

222.37

-171.38

5.228×105

99.89

9.36×10-4

PPy0.5%

-505.9

1.78×10-8

196.04

-215.52

2.508×106

99.97

2.07×10-4

PPy-G0.5%

-596.3

7.728×10-9

225.33

-149.84

5.063×106

99.99

8.99×10-5

PPy-G1%

-628.8

6.825×10-8

212.36

-187.55

6.344×105

99.91

7.94×10-4

A shift in the Ecorr towards more noble values for the blank (-624.8 mV) was apparent compared to bare steel (-745.1 mV), indicating the certain barrier property for epoxy coating. The higher Ecorr value (-505.9 mV for PPy0.5% and -596.3 mV for PPy-G0.5%) for composite coating with PPy revealed that abundant imine groups on the polypyrrole backbone transformed to the Fe-NH- chelated functional groups to stabilize the potential of metal in the passive region via adsorbing the Fe2+ and Fe3+ ions. In addition, higher ratio between ba/bc revealed the application of external current strongly polarizes the anode. The addition of polypyrrole and graphene made a strong difference to the ba value, indicating the anodic dissolution has been decelerated. From icorr, PPy-G0.5% exhibited the lowest icorr value (7.728×10-9 A/cm2) with one order of magnitude lower than that of pure epoxy coating (8.084×10-8 A/cm2) and acquired the optimal condition among the tested systems. What’s more, the corrosion rate vcorr was consistent with the corrosion current density, which was 0.867 (bare Q235), 9.36×10-4 (blank), 2.07×10-4 (PPy0.5%), 8.99×10-5 (PPy-G0.5%), 7.94×10-4 mm/year (PPy-G1%), respectively. In terms of inhibition efficiency, the highest IE 18

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value from PPy-G0.5% (99.99%) revealed that only 0.5 wt % intercalated graphene loading in the coating can provide the superior barrier performance. And the lower IE for PPy-G1% (99.91%) than PPy0.5% (99.97%) indicated that agglomerate graphene increased the defect of coating and accelerated the metal corrosion under the coating.

Figure 9. Current density distribution maps for coated steels immersed in 3.5 wt % NaCl solution for 1 hour, 6 hours, 12 hours and 24 hours. (a) Blank, (b) PPy0.5% and (c) PPy-G0.5%. Self-healing Property for Coatings with PPy. The scanning vibrating electrode technique (SVET) was usually used to evaluate the local electrochemical corrosion reaction of metals beneath the coating in the liquid electrolyte. The obtained potential signals were transferred to the local current density (J) expressed with the Ohm’s law as shown the following equation:32 J =-∆φ

k (A cm-2) d

(4)

where ∆φ is the electric potential drop, k is the electrolyte conductivity and d is the 19

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vibration amplitude. Current density distribution maps for coated steel in 3.5 wt % NaCl solution with the immersion were shown in Figure 9. For neat epoxy coating (Figure 9a), the defect remained active during all tests and the highest anodic current density in the scan region is increased from 3.37 µA cm-2 at 1 hour’s immersion to 13.3 µA cm-2 exposed to electrolyte after 24 hours. It demonstrated that anodic dissolution occurred to the carbon steel close to scratch with the invasion of NaCl solution. Furthermore, the whole region with increasing anodic current density initiated to corrosion reaction due to the water penetration from the scratch. In Figure 9b, the addition of polypyrrole induced the steel substrate anodic reaction at 1 hours’ immersion. On the contrary, the anodic current density for scan region was gradually dropped and cathodic reaction was active from 6 hours to 12 hours. It can be explained that polypyrrole took effect on the passivation of steel and possessed the auto-suppression for anodic reaction. And the same phenomenon can be also observed in the sample of PPy-G0.5% (Figure 9c). Besides, graphene played an important role in stabilizing anodic current density at test region.

Figure 10. XRD patterns for rust regions on the steel substrate beneath blank, PPy0.5% 20

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and PPy-G0.5%. Characterization of Corrosive Products. The self-healing effect for polypyrrole mentioned above and protective mechanism of PPy-G composite coating should be proved via the characterization for corrosion products. Figure 10 showed the crystalline forms for rust regions on the steel substrate beneath neat epoxy coating, PPy0.5% and PPy-G0.5% identified by XRD. The permeation of electrolyte ions through neat epoxy coating resulted in the rust layer composed of goethite (α-FeOOH), akaganeite (β-FeOOH) and lepidocrocite (γ-FeOOH). However, the rust product was mainly made up of hematite (Fe2O3) and magnetite (Fe3O4) after the addition of electro-active polypyrrole into coating. The morphology of corroded regions on the steel substrate beneath the coatings was exhibited in Figure 11. In terms of pure epoxy coating (Figure 11a), the crack caused by the corrosion spread along the traces from pre-polishing. For the composite coating, the traces left from the sand papers turned indistinct and were covered by a large amount of nanoscale granules (Figure 11b, 11c and 11d). And from the view of density of rust layers, the specimen PPy-G0.5% (Figure 10c) exhibited the densest rust layer compared to PPy0.5% and PPy-G1% (Figure 11b and 11d), revealing its superior corrosive protection.

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Figure 11. SEM images for rust regions on the steel substrate beneath (a) pure epoxy, (b) PPy0.5%, (c) PPy-G0.5% and (d) PPy-G1%. Protective Mechanism for Coatings. Corroding media (water, O2 molecules and Cl‾ ions) permeates through defects and micro-pores of coating to coating/metal interface and results in the metal corrosion.33 Figure 12 showed the mechanism of corrosive protection for the steel with pure epoxy coating and PPy-G composite coating. In terms of pure epoxy coating (Figure 12a), it can be regarded as an isolated layer and the corroding media permeated the coatings along its thickness. The following oxidation and reduction reactions take place on the interface of coating/metal:34-35 22

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Fe → Fe 2+ +2e −

(5)

Fe2+ → Fe3+ +e−

(6)

H 2 O+ 1 O 2 ( g ) + 2e − → 2OH − 2

(7)

2Fe2+ (aq)+O2 (g )+2H 2O → 2FeOOH+2H+

(8)

The reactions induce the pitting and delamination of coating on metal substrate so that metal substrate loses the protection. However, graphene-based composite coating exhibits the longer-term protection than pure epoxy coating attributed to the superior barrier of graphene.36 Graphene prevents the macroscopic cluster formation of pitting, sheets oriented to the vertical direction of coating’s thickness offer a winding diffusion pathway for corroding media and delay the corrosion as shown in Figure 12b.37 Furthermore, polypyrrole acts as not only an intercalator for graphene in aqueous coating but also a corrosion inhibitor. As a conducting polymer, polypyrrole is able to accept the electrons released by the metal dissolution and reduce from oxidized state (doped form) to reduced state (dedoped form).38 The increasing Fe ions (Fe2+ and Fe3+) are transformed to the passive Fe2O3 and Fe3O4 as an oxide layer in the neutral environment.39

Figure 12. Schematic representation of corrosive protection for the steel with (a) pure 23

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epoxy coating and (b) PPy-G composite coating.

CONCLUSION Based on the π-π interaction between polypyrrole and graphene sheets, highly concentrated exfoliated graphene aqueous dispersion up to 5 mg/mL was obtained by synthesizing hydrophilic nanospheres as intercalators and ultrasonic vibration. Stacked graphene sheets were exfoliated to the thickness of 3~5 layers without increasing plane defects as confirmed by Raman spectroscopy, X-ray powder diffraction, scanning probe microscope and electron microscope. The corrosion performance of coatings without and with polypyrrole and graphene on the Q235 electrodes was collected by potential and impedance measurements, Tafel curves and fitted Rp and Rct immersed in 3.5 wt % NaCl solution. Composite coating with 0.5 wt % graphene (PPy-G0.5%) exhibited the highest impedance modulus at 0.01 Hz (|Z|0.01Hz) and the lowest corrosion current density during the given immersion time. It turns out that PPy-G0.5% sample possessed superior anticorrosive ability. And the mechanism of synergistic protection is based on impermeable graphene sheets and self-healing polypyrrole and proved by the identification of corrosion products beneath the coatings. The addition of polypyrrole results the formation of passive layer composed of Fe2O3 and Fe3O4. And we hope that this environmental-friendly aqueous polypyrrole-intercalated graphene composite can be applied in the field of not only corrosion protection but also other areas such as electromagnetic interference shielding, antistatic and antimicrobial coatings. 24

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ASSOCIATED CONTENT Supporting Information A table of elemental composition for the Q235 electrodes and four tables of electrochemical parameters extracted from EIS data for specimens immersed in 3.5 wt % NaCl solution for different time; a figure of water contact angle value and time-dependent water absorption for specimens, a figure of Tafel curve for bare Q235 electrode and optical photographs of the artificial defects for SVET immersed in 3.5 wt % NaCl solution after 24 hours.

AUTHOR INFORMATION Corresponding authors ∗

E-mail addresses:

[email protected], Fax: 0574-86685159, Phone: 0574-86657094; [email protected], Phone: 0574-86325713. ORCID Haichao Zhao: 0000-0002-3558-1306 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors gratefully appreciate financial support provided by the “One Hundred Talented People” of the Chinese Academy of Sciences (No. Y60707WR04); 25

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Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (No. QYZDY-SSW-JSC009) and Natural Science Foundation of Zhejiang Province (No. Y16B040008).

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Graphical Abstract 177x120mm (150 x 150 DPI)

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Synergistic Effect of Polypyrrole-Intercalated Graphene for Enhanced Corrosion Protection of Aqueous Coating in 3.5% NaCl Solution.

Dispersion of graphene in water and its incorporation into waterborne resin have been rarely researched and hardly achieved owing to its hydrophobicit...
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