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Synthesis of Functionalized Graphene/Polyaniline Nanocomposites with Effective Synergistic Reinforcement on Anticorrosion Xinxin Sheng, Wenxi Cai, Li Zhong, Delong Xie, and Xinya Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01975 • Publication Date (Web): 20 Jul 2016 Downloaded from http://pubs.acs.org on July 22, 2016

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Industrial & Engineering Chemistry Research

Synthesis of Functionalized Graphene/Polyaniline Nanocomposites with Effective Synergistic Reinforcement on Anticorrosion Xinxin Sheng1, Wenxi Cai1, Li Zhong1, Delong Xie*1,2, Xinya Zhang*1 1. School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China 2. School of Chemical Engineering, Kunming University of Science and Technology, Chenggong Campus, Kunming, 650504, China ABSTRACT: Functionalized graphene (PGO)/polyaniline (PANI) nanocomposites with effective synergistic reinforcement on anticorrosion have been prepared via in situ redox polymerization–dedoping technique. PGO nanosheets are obtained through modification of graphene oxide (GO), with p-phenylenediamine to improve the dispersion stability in acidic condition and compatibility with polymer. And PGO/PANI composites are synthesized via in situ redox polymerization of aniline. The results show that the PGO are highly exfoliated and intercalated among the PANI matrix. In potentiodynamic polarization tests, the anticorrosion efficiency of the films with the reinforcement of PGO/PANI composites increases from 85.16% to 99.9%. Moreover, the lowest corrosion rate is 1.68×10-4 mm/year, which is much better than the one with individual PGO or PANI. Electrochemical impedance spectroscopy, where the Warburg impedance component emerges, further reveal that the well-dispersed PGO in the film can retard or defend the permeation of corrosive material from the environment. KEY WORDS: Graphene, Polyaniline, Nanocomposites, Anticorrosion Coatings

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1. INTRODUCTION Since its first preparation via micro-mechanical exfoliation1, 2, graphene, as one of the most compelling materials has attracted remarkable research attention. Possessing considerable performance such as excellent mechanical strength3, outstanding barrier property4, and high thermal conductivity5, graphene has been promoted for application as catalyst carrier, energy storage material, and electronic component. One of the most promising applications is the preparation of functional polymer nanocomposites6. Generally, the three kinds of techniques to fabricate graphene-based polymer nanocomposites are solution mixing7, 8, melt blending7, 9, and in situ polymerization7,

10

. Associated with lower density and higher aspect ratio

compared with the traditional filler, such as mica or glass flake, graphene has first been used as a novel barrier filler in the anticorrosion polymer composite coatings system. Chang et al.11 fabricated polymethyl methacrylate (PMMA)/graphene composites via in situ polymerization. The well-dispersed graphene in the polymer matrix acts as the barrier and the superhydrophobic surface can repel moisture, which results in the enhancement of corrosion protection. The enhanced epoxy/graphene composite coating was fabricated through a similar technique12. Polystyrene (PS)/graphene was first prepared successfully via in situ miniemulsion polymerization and applied in corrosion protection13. The corrosion protection efficiency of PS/graphene increases from 37.90% to 99.53% with respect to pure PS. However, Schriver14 found that severe rusty textures appear on graphene-coated copper surface, because of oxygen diffusion through the graphene defects for a long time (e.g., more than one month). They mentioned that conductive graphene provides a pathway from the environment to the internal copper surface inducing a driving force for the anodic polarization. Furthermore, it has been reported that graphene could effectively enhance the conductivity of 2


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some polymers8, such as polycarbonate15, and polyurethane6. over the electrical percolation threshold. Thus such coatings may promote metal corrosion particularly when the defects appear. Modifiers should be determined to weaken the conductivity of graphene. Sun presented the silicon oxide/graphene composites16 and (3-aminopropyl)-triethoxysilane (APTES)/ graphene composites17-modified

polyvinyl

butyral

coating,

which

avoids

the

environment-graphene-metal connection and inhibits the corrosion promotion of graphene. Polyaniline (PANI) functionalized polymer anticorrosion coatings have better performance than those of others, whose mechanism is on account of the increased corrosion potential18 and redox ability in the formation of a passive metal oxide layer19. To further enhance the corrosion protection performance of graphene, graphene should be coated with emeraldine base PANI not only as a current barrier, but also to exert its specific anticorrosion property. However, two main problems exist in the fabrication of PANI/graphene composite coatings. First, graphene or graphene oxide (GO, a precursor with rich oxygen containing functional groups) is likely to agglomerate in acidic environment. GO is negatively charged in neutral or alkaline aqueous solution, which means that GO should be modified in positively charged derivative to make it stable in acidic aqueous solution20. Thus, most of the past research adopted multistep modification under rigorous condition21. Second, the compatibility between functionalized graphene (PGO) and polymer dominates the performance of the coatings. In this paper, a facile route was proposed for the preparation of polymer nanocomposites with modified graphene uniformly covered with PANI via in situ polymerization–dedoping technique and its application in anticorrosion coatings was introduced. First, GO was 3



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covalently functionalized and reduced with p-phenylenediamine (PPD), thereby obtaining an amino-terminated PGO. Subsequently, PGO was covered with PANI to prepare the PGO/PANI nanocomposites. Second, the PGO/PANI nanocomposites were blended with PS, which was used as anticorrosion coatings. The exfoliated PGO encapsulated with PANI nanocomposites was homogenously dispersed in the PS matrix. Significant enhancement of anticorrosion was achieved with PGO/PANI nanocomposite incorporation at low loading. 2. EXPERIMENTAL METHODS Raw Materials Natural graphite (400 meshes, 99.5%) was purchased from Tianjin Yongda Chemical Reagent Co., Ltd. (China). Sulfuric acid (H2SO4, 98.0%), sodium nitrate (NaNO3), potassium permanganate (KMnO4), hydrochloric acid (HCl, 36.5%), hydrogen peroxide (H2O2, 30.0%), sodium chloride (NaCl), p-phenylene diamine (PPD) were of analytical grade and offered by Tianjin Kemiou Chemical Reagent Co., Ltd. (China). Ethanol (C2H5OH, 99.5%) and ammonia water were purchased from Guangdong Guanghua Chemical Reagent Co., Ltd. (China). Aniline and ammonium persulfate (APS) were of analytical grade and obtained from Tianjin Fuchen Reagent Co., Ltd. (China). Polystyrene (GPPS, PG33) was gained from Chimei Chemical Co., Ltd. (China). Preparation of GO GO was synthesized via a modified Hummers’ method22, 23. Typically, 5.0 g of graphite powder and 5.0 g of NaNO3 in a 3000 mL round flask were mixed with 275 mL of concentrated H2SO4 in an ice bath for about 20 min. 30 g of KMnO4 was tardily added into the flask in 30 min in the ice bath. After stirring continuously for 48 h in ambient temperature, the 4


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mixture was mixed with 460 mL of deionized water, which was added dropwise. Subsequently, 460 mL of warm water (50–60 °C) and 100 mL of H2O2 were added to the mixture to transform the residual KMnO4 and MnO2 into soluble manganese sulfate. For further purification, the mixture was washed using a solution of 6 wt% H2SO4/1 wt% H2O2 and centrifuged in turns. Afterward, the mixture was washed with water for several times. The GO dispersion was dialyzed for 2 weeks and the GO sample was obtained after freeze-drying for 48 h. Functionalization and reduction of GO with PPD To prepare PGO, 100 mg of GO was dissolved in 200 mL of deionized water and exfoliated via ultrasonication for 30 min (600 W, output power; 2 s, work time; 2 s, pause time). 1.0 g of PPD and 0.8 mL of NH3·H2O aqueous solution were added. The GO solution was refluxed with mechanical stirring at 90 °C for 3 h. Filtrated with a membrane with pore size of 0.22 µm, the filtration cake was washed with water and ethanol each for three times. The PGO product was collected after freeze-drying for 24 h. Preparation of PGO/PANI nanocomposites The as-prepared PGO was weighed and dissolved into 100 mL of HCl aqueous solution (1 mol/L) through bath ultrasonication for 15 min followed by probe ultrasonication for another 30 min. 1.86 g of aniline was added to the PGO solution and rapidly stirred for 30 min until it formed a uniformly dispersed solution, which was noted as solution A. 2.28 g of APS was well dispersed in 20 mL of HCl aqueous solution (1 mol/L) via stirring, which was noted as solution B. Finally, solution B was slowly added into solution A with continuous stirring, and the mixture was continuously stirred for another 6 h at room temperature. After the reaction, 5



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the mixture was filtrated with qualitative filter paper and washed with deionized water and ethanol each for three times. Subsequently, the filter cake was dispersed in 120 mL of NH3·H2O aqueous solution (1 mol/L) and stirred continuously for 4 h. The precipitate was collected via filtration and washed thrice with water. Finally, the resultant was dried in the oven for three days. The products using 3.0 wt%, 5.0 wt%, and 10.0 wt% PGO were noted as PGO/PANI03, PGO/PANI05, and PGO/PANI10, respectively. Preparation of PGO/PANI-based coatings 0.2 g of PGO/PANI nanocomposites was dispersed in 30 mL of NMP with continuous stirring for 2 h. Afterward, 10 g of PS was added to the solution under stirring for 2 h at the speed of 300 r/min, thus yielding a blue viscous solution with uniformly dispersed PGO/PANI nanocomposites and without settling. A Q235 carbon steel sheet (80 mm×60 mm×1 mm) was polished with SiC-400 paper, washed with acetone, and blow-dried. The above coating solution was cast onto the steel sheet. The coatings were dried in the oven at 60 °C for 24 h to obtain the final coating film. The coatings incorporated with PGO/PANI03, PGO/PANI05 and PGO/PANI10 were noted as PPCc03, PPCc05, and PPCc10, respectively. For comparison, pure PANI and PGO substituted for PGO/PANI nanocomposites as fillers in the coatings, were noted as PANIc, and PGOc, respectively. Characterization and Instruments FTIR analysis was performed using a Spectnlm2000 spectrometer (PerkinElmer Co. USA). The samples were prepared in potassium bromide pellets. UV-Vis analysis was conducted by UV2450 (Shimadzu, Japan) from the wavelength from 200 to 1000 nm, where the PGO/PANI nanocomposites samples were in the form of NMP solution. Raman spectroscopy was carried 6


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out by LabRAM Aramis instrument (Horiba Jobin Yvon, France). X-Ray Diffraction (XRD) was carried out using a Bruker D8 ADVANCE diffractometer with Cu Kα radiation (λ = 0.15418), conducting a scanning rate of 0.01°/s. Thermogravimetric analysis (TGA) was employed with a thermo-analyzer instrument Q600SDT (TA, USA), heating from room temperature to 600 °C at a rate of 10 °C /min under N2 atmosphere. TEM images were studied using a HITACHI H-7650 instrument at an accelerating voltage of 80 kV. The morphology of GO and PGO/PANI nanocomposites were observed using a ZEISS Merlin SEM at an acceleration voltage of 5.0 kV. 3. RESULTS AND DISCUSSION Structures and morphologies of GO, PGO, and PGO/PANI nanocomposites The functionalization and reduction of GO using PPD were detected by the FTIR spectrum. Figure 1a, shows the following characteristic peaks of GO: stretching vibration of –OH at 3419 cm−1, stretching vibration of C=O carboxyl at 1730 cm−1, C=C bonds in the remaining sp2 character of graphite at 1628 cm−1,24 and the peaks at 1395, 1243, 1044 cm−1 caused by the vibration of C–OH, –COO–, and C–O–C, respectively25. For PGO, the characteristic peaks of C–OH, –COO–, and C–O–C were remarkably weakened, hence indicating the magnitude of GO reduction. The new peaks at 1610 and 1497 cm−1 represented the vibration of the benzene ring framework. Moreover, the new peaks at 1562 and 829 cm−1 inferred with the internal and external surface bending of –N–H. A new characteristic peak at 1268 cm−1 appeared, which indicated the C–N stretching vibration in the C–NH–C groups. Therefore, PPD was grafted onto the GO layer via nucleophilic addition reaction forming C–NH–C26, as displayed in Scheme 1. 7



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Figure 1. FTIR spectrum of (a) GO and PGO, (b) PANI and PGO/PANI nanocomposites.

Scheme 1. Strategy for the synthesis of PGO. In Figure 1b, the peaks at 1589 and 1494 cm−1 were attributed to the C=C stretching vibrations of quinone and benzene rings27. Similar peaks had blue shifts of 4, 6, and 8 cm−1 and 4, 5, and 7 cm−1 in the PGO/PANI03, PGO/PANI05, and PGO/PANI10, respectively. The blue shifts could be attributed to the π–π interactions between the benzene ring and quinone ring of PANI and the PGO layer to form the greater π electron domain, which hindered the movements of the functional groups of nanocomposites. Furthermore, the Q=N (Q means quinone) stretching vibrations can be observed at 1109 cm−1 in PANI, and the same peak appeared at 1112, 1113, and 1117 cm−1 with a slight blue shift of 3, 4, and 8 cm−1 in 8


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PGO/PANI03, PGO/PANI05, and PGO/PANI10, respectively. The above phenomena revealed that intercalation polymerization occurred and the new π–π interactions between PANI and PGO layer plane were activated. The chemical composition on the surface of GO and PGO, the reduction of GO by PPD were also confirmed through XPS analysis (Figure 2). As shown in Figure 2a, the C1s XPS spectrum of GO presented three different peaks at 284.7, 286.4, and 288.7 eV, corresponding to the C–C, C–OH, and C=O groups, respectively28, 29. After the functionalization by PPD (Figure 2b), the peaks corresponding to the oxygen-containing groups were significantly decreased, particularly the peak of C=O (286.4 eV). This result demonstrated that most of the oxygen-containing groups in GO were eliminated, and GO was effectively reduced by PPD. Furthermore, a new peak corresponding to C–N groups occurred at 285.7 eV, thus inferring that PPD was successfully grafted onto the surface of GO sheets26.

Figure 2. C1s XPS spectra of (a) GO and (b) PGO XRD was utilized to measure the distance between the graphene interlayers and the exfoliation of the graphene sheets in the nanocomposites. Figure 3 presents the XRD patterns of graphite, GO, PGO, and PGO/PANI nanocomposites. As shown in Figure 3a, for graphite, 9



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a sharp peak appeared at 2θ = 26.53°, corresponding to a d-spacing of 0.34 nm. A new diffraction peak also appeared at 2θ = 11.5°, which represented the diffraction peak of GO. Nevertheless, while the sharp peak of graphite disappeared after oxidation. The d-spacing increased to 0.77 nm because of the oxygen-containing groups and absorbed H2O30. However, after functionalization and reduction, the diffraction peak of PGO at 2θ = 5.3° suggested a d-spacing of 1.66 nm which presented a significant enlargement in interlayer spacing as a result of the intercalation of functionalized PPD between the GO layers. In addition, there’s one more broad peak around 2θ = 23.6° after modification, indicating most of the oxygen groups have been removed. The broadened peak and reduced peak intensity indicate that graphene is exfoliated into single-layer or few-layers.

Figure 3. XRD patterns of a) graphite, GO and PGO and b) XRD patterns of PGO, PANI and PGO/PANI nanocomposites. As shown in Figure 3b almost no apparent peaks of PGO were observed in the XRD patterns of PGO/PANI nanocomposites. Therefore, PGO was well exfoliated in the PANI matrix without restacking together. The PANI pattern showed a characteristic peak at 2θ=20.1° corresponding to the (020) lattice plane31. Similarly, in the pattern of PGO/PANI 10


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nanocomposites, this peak appeared; its intensity decreased, whereas the PGO content increased, which represented that PGO was homogeneously distributed as the nucleating carrier of PANI inhibiting the close-packing of PANI chains. The structure of carbon-based materials could be characterized by Raman spectroscopy. Generally, Raman spectra feature a D band at 1350 cm−1 which represents the defect and disorder and a G band at 1568 cm−1 that corresponds to crystallinity32. As shown in Figure 4, graphite possessed a narrow sharp G band and small D band, thereby indicating its intensive crystallinity and well aligned structure. Different from graphite, GO showed a wide distinct D band. In addition, the G band became wider and shifted to 1582 cm−1, which could be attributed to the surface defects and disorder caused by the destruction of sp2-conjugated structure of the oxygen-containing groups, such as carboxyl, hydroxyl, and epoxy groups. Furthermore, the G band of PGO shifted to 1594 cm−1, which inferred the high exfoliation of PGO. The D/G intensity ratio (ID/IG) of PGO (ID/IG=1.19) was larger than that of GO (ID/IG =1.01), which resulted from the graft with the PPD molecule and the increase of the disorder degree after functionalization33.

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Figure 4. Raman spectra of graphite, GO and PGO. The interaction of PANI and PGO in PGO/PANI nanocomposites could be characterized by UV-Vis spectroscopy analysis. As shown in Figure 5, the UV-Vis spectrum of PANI exhibited two characteristic features: a relatively narrow band at 328 nm caused by the π–π* transition of aromatic C–C bond and a wide band at 629 nm attributed to the π–π* transition of quinonoid C–C bond34, 35. For PGO/PANI nanocomposites, these two bands were largely intensive and showed red shifts in incremental degree with the increasing content of PGO. These results should be on account of the formation of large electronic conjugation domain because of the graft and interfacial adhesion between PGO and PANI, both of which were the conjugated structural molecule.

Figure 5. UV-Vis spectra of PANI and PGO/PANI nanocomposites. SEM and TEM were utilized to observe the morphology of GO, PGO, and PANI/PGO nanocomposites. Figure 6a illustrates that the surface of GO was relatively smooth, and the film was easily formed with a slight crumple. By contrast, the PGO showed obvious sheet-like structure with more crumple (Figure 6b), because the covalently grafted rigid 12


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structure of PPD acted as a nanospacer to hinder the restacking of GO nanosheets. To further observe the morphology, the TEM image (Figure 6c) of GO also revealed the film-like form of GO nanosheets. Figure 6d confirms that exfoliated PGO without apparent aggregation possessed thin thickness and wavy structure, which was the same as the intrinsic characteristics of GO nanosheets36.

Figure 6. High-magnification SEM images of (a) GO and (b) PGO; TEM images of (c) GO and (d) PGO.

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Figure 7. (a) SEM and (b) TEM images of PGO/PANI nanocomposites; SEM images of PGO/PANI nanocomposites with reaction time of (c) 15 min, (d) 30 min, (e) 2 h, and (f) 3 h. The SEM and TEM images of the PGO/PANI composite are shown in Figure 7. SEM image (Figure 7a) exhibited that the composite was rigid flake with rough surface, which could be inferred that PGO sheet was covered with the nanotube–like polyaniline in the composites (see high-magnification of FESEM images in Supporting Information, Figure S1). Apparently, the PANI nanotubes were approximately 300 nm in length and about 30 nm in diameter. Similar nanostructure could also be observed in the TEM image (Figure 7b), which further confirmed the thin composites, where the dark area should be the overlapping PANI 14


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nanotubes on the surface. Furthermore, the in situ polymerization process is presented in Figure 7c–f. From the beginning of the reaction, grain-like polymer appeared on the surface of PGO. With the reaction proceeding, the grain-like PANI started to transform to tube-like shape, which was attributed to the increasing molecular weight and periodicity parallel of the polymer chain. In addition, several clusters of nanotube in the surrounding were observed in each image, thereby inferring that PANI was arrayed on various bulky substrates, and polymerized via self-nucleation. Thermal and disperse stability of GO and PGO TGA was employed to examine the thermal stability of materials, and the result is shown in Figure 8. From the Figure 8, we could see that graphite was nearly lossless throughout the heating procedure. A 5% weight loss was observed at around 100 °C because of the absorbed water, and more than 50% loss in the range of 200–300 °C for GO, which was mainly the result of the unstable oxygen-containing groups decomposing into CO2 or other vapor. For PGO, at 200–300 °C, only about 12% weight loss occurred, which suggested that PGO was much more thermally stable. It could be concluded that the functionalization of GO with PPD could reduce the quantity of oxygen-containing groups, which improved the thermal stability. In addition, the less quantity of oxygen-containing groups made PGO a good barrier filler in coating to resist the water or other aggressive corrosives. To detect the stability of GO and PGO in the solution state, 0.5 mg/mL of GO and PGO aqueous solution were prepared. As shown in Figure 9, GO could disperse uniformly in the water without any precipitation, and PGO formed a slightly heterogeneous dispersion with a small amount of precipitate and floating solid powder. Furthermore, 0.5 mg/mL of GO and 15



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PGO dispersions in 1 mol/L HCl aqueous solution were prepared. On the contrary, homogeneous dispersion of PGO was obtained, and GO was hardly decentralized in the HCl solution with apparent precipitate and dross. In addition, the zeta penitential of GO and PGO dispersions were also tested to further investigate the dispersion behaviors in pure water and acidified water (1 mol/L HCl aqueous solution), e.g., -46.5 and -11.5 mV for GO dispersions, +7.2 and +30.5 mV for PGO dispersions in pure water and acidified water, respectively, indicating that the PGO dispersion in acidified condition was quite stable. This observation could be attributed to the functional modification of PPD, which endowed GO with amino groups on its surface and caused the possibility for further in situ polymerization of PANI.

Figure 8. TG curve of graphite, GO and PGO.

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Figure 9. GO and PGO dispersions in water and 1 mol/L HCl (aq) with concentration of 0.5 mg/mL. Anticorrosion property of PGO/PANI nanocomposites The potentiodynamic polarization curves of different coatings in 3.5wt% NaCl aqueous solution are presented in Figure 10 (the setup of coating evaluation electrolytic cell is shown in Supporting Information, Figure S2). The Tafel analysis plot was based on log I versus log

E constructed for a potential range of −500 to +500 mV relative to the open circuit potential (Eocp) after 30 min for equilibrium. Figure 10 shows that the coated samples possessed larger positive corrosion potential (Ecorr) than the pure carbon steel, which revealed the obvious protection from the polymer coating. Icorr was obtained through extrapolating the straight line along the linear portion of the anodic and cathodic polarization curves to the Ecorr axis to obtain the intersection point. The quantitative analyses of potentiodynamic polarization are listed in Table 1. Rp was calculated through applying the following Stern–Geary equation37, 38: ∙

= . ( )

(1)

where Icorr is the corrosion current (µA/cm2), whose value was determined via the intersection of anodic and cathodic lines; and ka and kb are the anodic and cathodic slopes (△E/△log I), respectively. The corrosion rate (CR, mm/year) and protection efficiency (PE, %) were calculated to analyze the anticorrosion performance of the coatings quantitatively. The CR was calculated using the following formula:

=

(2)

where k is a constant (3268.6 mol/A), M is the molecular weight (56 g/mol), and ρm is the density (7.85 g/cm3). The protection efficiency (PE) was calculated through the following 17



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formula:

=

, , ,

× 100%

(3)

where Icorr,o is the corrosion current density of pure carbon steel (A/cm2), and Icorr,i is the corrosion current density of coated sample (A/cm2).

Ecorr mainly represents the tendency of corrosion reaction where the high Ecorr value results in good anticorrosion property of the coatings. Compared with the bare carbon steel, the Ecorr values of all the coatings samples had positive shifts. The Ecorr values of PPCc03, PPCc05, and PPCc10 had positive shifts of 415.38, 458.7, and 544.09 mV, respectively, which indicated that the anticorrosion performance enhanced with the increasing content of PGO. According to Faraday’s law, the corrosion current density and corrosion rate are in a positive proportional relationship. Therefore, the lowe Icorr value will slow the corrosion rate. Tafel analysis results revealed that the use of PSc to protect the carbon steel can decrease the Icorr for an order of magnitudes. Additionally, the use of PANIc and PGOc could further decrease the Icorr for one and two order of magnitudes compared with PSc, respectively. Nevertheless, the Icorr of PPCc series coatings was lower than all samples above. Similarly, the decreasing amplitude of PPC series was positively correlated with the increasing content of PGO, which indicated that the PGO played the dominant role in the nanocomposites. Particularly among all the samples, PPCc10 possessed the most desirable anticorrosion performance and retained the lowest CR (i.e., 1.68×10-4 mm/year).

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Figure 10. Tafel plots for bare Q235A steel, PANIc, PSc, PANIc, PGOc, PPCc03, PPCc05 and PPCc10. PSc is the basic physical isolation to inhibit corrosive material, such as water, and oxygen from contacting the metal surface. PANIc provide can also deactivate metal to form an oxidation film on the metal surface39. Moreover, with its outstanding stability and high aspect ratio, PGO was the novel light-weight and high-effect anticorrosion filler. PGO/PANI nanocomposites possessed better anticorrosion property than PGO or PANI individually, which verified that the composites obtained the synergism of PGO and PANI as a result of its specific structure. As shown in the SEM and TEM images, the PGO/PANI nanocomposites had sandwich-like structure, where PANI covered the PGO surface. This structure endowed the PGO/PANI nanocomposites the higher compatibility to disperse better in the PS matrix than PGO and combined the deactivation effect of PANI with the stability and high aspect ratio of PGO.

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Table 1. Electrochemical corrosion measurements of anticorrosion coatings. Electrochemical Corrosion Measurements Sample

2

CR

PE

Ecorr (mV)

Icorr (A/cm2)

Rp (kΩ·cm )

(mm/year)

(%)

Carbon Steel

-821.16

4.26×10-5

0.26×10-3

0.99

--

PSc

-580.21

6.32×10-6

2.63

0.15

85.16

PANIc

-561.88

3.95×10-7

44.21

9.20×10-3

99.07

PGOc

-531.79

5.61×10-8

348.89

1.31×10-3

99.87

PPCc03

-405.78

2.67×10-8

792.17

6.17×10-4

99.94

PPCc05

-362.46

2.56×10-8

1048.20

5.95×10-4

99.94

PPCc10

-277.07

7.32×10-9

2366.34

1.68×10-4

99.98

The corrosion behavior of the PGO/PANI nanocomposite-based coatings was investigated through the electrochemical impedance spectroscopy (EIS). EIS was used to determine the impedance characteristics of the coatings in a wide range. EIS was able to collect the information of regarding the resistance, capacity of the coatings, corrosion reaction resistance, and electric double-layer capacity of the metal surface40. Generally, in the phase curve of Bode plot, the peak at high frequency (i.e., 103–105 Hz) responds to the protection of the coatings, and the peak on mid-low frequency (i.e., 10-3–103 Hz) responds to the corrosion reaction between the corrosive material and metal41. Two peaks (i.e., two time constants) were observed at both high and mid-low frequency in the Bode-phase plots in Figure 11a. At high frequency the values of phase angles were approximately −90° and decreased toward the low frequency direction. This observation demonstrated the increased capacitance and decreased resistance of the coatings ascribed to the diffusion of corrosive electrolyte with low resistance and high permittivity. The time 20


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constant at low frequency responded to the electric double-layer capacity and the corrosion reaction resistance.

Figure 11. The Bode (a) phase and (b) modulus plots, (c) Nyquist plots and (d) equivalent circuit model of PPCc03, PPCc05 and PPCc10.

Furthermore, in the Bode modulus plots (Figure 11b), the modulus of the coatings at 0.01 Hz increasing in the sequence PPCc03 < PPCc05 < PPCc10 was an evidence to prove that the withstanding corrosive competence of the coatings was strengthened as the PGO loadings increased. In addition, the modulus curve of PPCc03 emerged as a platform at medium frequency, and the oblique lines appeared in the PPCc05 and PPCc10 curves where the oblique line of PPCc10 was particularly distinct. This kind of oblique line, a feature of Warburg impedance, was attributed to the random distributed flake-like PGO with prominent shielding effect. This result reflected a tardy process for the corrosive to penetrate through the 21



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coatings to contact the metal surface. Thus PGO was the root of enhanced anticorrosion performance of the coatings. Along with the continuous penetration and corrosion, the diameter of the Nyquist plot typically decreased with the low polarization or charge transfer resistance, which inferred the improved corrosion rate. Figure 11c shows the diameters of the semicircles increased in the order of PPCc03 < PPCc05 < PPCc10, which mainly resulted from the significant improvement of charge transfer resistance with the enhancive content of PGO. In addition, the Warburg impedance character was observed as the 45° oblique lines that occurred at the low frequency in the plots of PPCc05 and PPCc10, which also presented that the diffusion process dominated corrosion at low frequency. Therefore, sufficient amount of PGO that is well dispersed in the coatings can retard or prevent the penetration of corrosive electrolyte. The equivalent circuit is shown in Figure 11d, where Rs is the electrolyte resistance, and Rp is the polarization resistance or charge transfer resistance, which indicated the protection of the coatings against the corrosive42. Cdl represents an electric double-layer capacitor ascribed to the accumulated charge between the metal surface and coatings. Cc is the coatings capacitor,

Rpo is the pore resistance of the coatings and Zw is the Warburg impedance. The model proposed in Figure 12 demonstrated that the high-aspect PGO/PANI nanocomposites randomly distributed inside the PS coatings considerably affected the diffusion pathways of oxygen, water and other corrosives through the coatings. Consequently, the corrosion reaction occurring on the metal surface was retarded or prevented. Thus, the EIS results were well consistent with the potentiodynamic polarization results.

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Figure 12. The model of corrosive diffusion pathways through the coatings. 4. CONCLUSION In this paper, PGO/PANI nanocomposites with distinct anticorrosion performance were fabricated via facile in situ redox polymerization–dedoping method. Grafted with PPD onto the surface, PGO homogenously dispersed in the acidic system and provided the copolymerization reaction sites with PANI. The blue shift of FTIR, red shift of UV-Vis, and XRD pattern verified that PGO interacted with PANI to form the high π–π delocalization. TEM and SEM images confirmed that PGO was highly exfoliated and intercalated among the PANI matrix. In potentiodynamic polarization analysis, the corrosion current decreased, whereas the corrosion potential and polarization resistance increased with the increasing content of PGO in the nanocomposites. Further comparison with PANIc and PGOc showed that PPCc series possessed the high protection efficiency, which represented the synergistic improvement caused by PGO/PANI nanocomposites. Moreover, EIS was conducted to reveal that the impedance modulus increased with the increasing content of PGO in the PGO/PANI nanocomposites. Corrosive penetration was a tardy process because of the tortuous pathways resulting from the incorporation of PGO in the PGO/PANI nanocomposites. In summary, the facile and eco-friendly technique would be a promising method to fabricate graphene-based 23



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nanocomposites and expand their application in anticorrosion field.

Supporting Information * High-magnification of FESEM images of PGO/PANI nanocomposites and assembly drawing of coating evaluation electrolytic cell are given in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *D. Xie. Tel. /fax: +86-20-87112047, E-mail: [email protected]. *X. Zhang. Tel. /fax: +86-20-87112047, E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support from the Science and Technology Planning Project of Guangdong Province, China (Grant NO. 2015A010105008).

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