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Jul 20, 2016 - such coatings may promote metal corrosion, particularly when the defects appear. ... (C2H5OH, 99.5%) and ammonia−water were purchased from. Guangdong .... diffraction peak also appeared at 2θ = 11.5°, which represented the diffraction .... weight (56 g/mol), and ρm is the density (7.85 g/cm3). The PE.
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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*,† †

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China School of Chemical Engineering, Kunming University of Science and Technology, Chenggong Campus, Kunming 650504, China



S Supporting Information *

ABSTRACT: Functionalized graphene (PGO)/polyaniline (PANI) nanocomposites with effective synergistic reinforcement on anticorrosion have been prepared via an in situ redox polymerization−dedoping technique. PGO nanosheets are obtained through the modification of graphene oxide with pphenylenediamine to improve the dispersion stability in acidic conditions and compatibility with the polymer. Also, PGO/PANI composites are synthesized via the in situ redox polymerization of aniline. The results show that PGO is highly exfoliated and intercalated among the PANI matrix. In potentiodynamic polarization tests, the anticorrosion efficiency of the films with reinforcement of PGO/PANI composites increases from 85.16% to 99.98%. Moreover, the lowest corrosion rate is 1.68 × 10−4 mm/year, which is much better than that with individual PGO or PANI. Electrochemical impedance spectroscopy, where the Warburg impedance component emerges, further reveals that well-dispersed PGO in the film can retard or defend permeation of the corrosive material from the environment.

1. INTRODUCTION Since its first preparation via micromechanical exfoliation,1,2 graphene, as one of the most compelling materials, has attracted remarkable research attention. Possessing considerable performance such as excellent mechanical strength,3 outstanding barrier property,4 and high thermal conductivity,5 graphene has been promoted for application as a catalyst carrier, an energy storage material, and an electronic component. One of the most promising applications is the preparation of functional polymer nanocomposites.6 Generally, the three kinds of techniques to fabricate graphene-based polymer nanocomposites are solution mixing,7,8 melt blending,7,9 and in situ polymerization.7,10 Associated with lower density and higher aspect ratio compared with the traditional filler, such as mica or glass flakes, graphene was first used as a novel barrier filler in the anticorrosion polymer composite coating system. Chang et al.11 fabricated poly(methyl methacrylate)/ graphene composites via in situ polymerization. The welldispersed graphene in the polymer matrix acts as a 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 technique.12 Polystyrene (PS)/graphene was first prepared successfully via in situ miniemulsion polymerization and applied in corrosion protection.13 The corrosion protection efficiency of PS/graphene increases from 37.90% to 99.53% with respect to pure PS. However, Schriver et al.14 found that severe rusty textures appear on a graphene-coated copper surface because of oxygen diffusion through the graphene © 2016 American Chemical Society

defects for a long time (e.g., more than 1 month). They mentioned that conductive graphene provides a pathway from the environment to an internal copper surface, inducing a driving force for anodic polarization. Furthermore, it has been reported that graphene could effectively enhance the conductivity of some polymers,8 such as polycarbonate15 and polyurethane,6 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 et al. presented silicon oxide/graphene composites16- and (3-aminopropyl)triethoxysilane (APTES)/ graphene composites17-modified poly(vinyl butyral) coatings, which avoid the environment−graphene−metal connection and inhibit the corrosion promotion of graphene. Polyaniline (PANI)-functionalized polymer anticorrosion coatings have a better performance than others whose mechanism is due to the increased corrosion potential18 and redox ability in the formation of a passive metal oxide layer.19 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 for exertion of its specific anticorrosion property. However, two main problems exist in the fabrication of PANI/graphene composite coatings. First, graphene or Received: Revised: Accepted: Published: 8576

May 22, 2016 July 15, 2016 July 20, 2016 July 20, 2016 DOI: 10.1021/acs.iecr.6b01975 Ind. Eng. Chem. Res. 2016, 55, 8576−8585

Article

Industrial & Engineering Chemistry Research

Figure 1. FTIR spectra of (a) GO and PGO and (b) PANI and PGO/PANI nanocomposites. 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 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). A total of 1.0 g of PPD and 0.8 mL of a NH3·H2O aqueous solution were added. The GO solution was refluxed with mechanical stirring at 90 °C for 3 h. Upon filtration with a membrane with a pore size of 0.22 μm, the filter cake was washed three times each with water and ethanol. The PGO product was collected after freeze-drying for 24 h. Preparation of PGO/PANI Nanocomposites. The as-prepared PGO was weighed and dissolved in 100 mL of a HCl aqueous solution (1 mol/L) through bath ultrasonication for 15 min, followed by probe ultrasonication for another 30 min. A total of 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. A total of 2.28 g of APS was well dispersed in 20 mL of a HCl aqueous solution (1 mol/L) via stirring, which was noted as solution B. Finally, solution B was slowly added to solution A with continuous stirring, and the mixture was continuously stirred for another 6 h at room temperature. After the reaction, the mixture was filtered with qualitative filter paper and washed with deionized water and ethanol three times each. Subsequently, the filter cake was dispersed in 120 mL of a NH3·H2O aqueous solution (1 mol/L) and stirred continuously for 4 h. The precipitate was collected via filtration and washed three times with water. Finally, the resultant was dried in an oven for 3 days. The products using 3.0, 5.0, and 10.0 wt % PGO were noted as PGO/ PANI03, PGO/PANI05, and PGO/PANI10, respectively. Preparation of PGO/PANI-Based Coatings. A total of 0.2 g of PGO/PANI nanocomposites was dispersed in 30 mL of N-methyl-2pyrrolidone (NMP) with continuous stirring for 2 h. Afterward, 10 g of PS was added to the solution under stirring for 2 h at a 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 an 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

graphene oxide (GO, a precursor with rich oxygen-containing functional groups) is likely to agglomerate in an acidic environment. GO is negatively charged in a neutral or an alkaline aqueous solution, which means that GO should be modified in a positively charged derivative to make it stable in an acidic aqueous solution.20 Thus, most of the past research adopted multistep modification under rigorous conditions.21 Second, the compatibility between the 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 an in situ polymerization−dedoping technique, and its application in anticorrosion coatings was introduced. First, GO was covalently functionalized and reduced with p-phenylenediamine, 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 an anticorrosion coating. The exfoliated PGO encapsulated with PANI nanocomposites was homogeneously 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), and pphenylenediamine (PPD) were of analytical grade and were 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 were obtained from Tianjin Fuchen Reagent Co., Ltd. (China). Polystyrene (PS; GPPS, PG33) was obtained from Chimei Chemical Co., Ltd. (China). Preparation of GO. GO was synthesized via a modified Hummers’ method.22,23 Typically, 5.0 g of graphite powder and 5.0 g of NaNO3 in a 3000 mL round-bottomed flask were mixed with 275 mL of concentrated H2SO4 in an ice bath for about 20 min. A total of 30 g of KMnO4 was tardily added into the flask for 30 min in the ice bath. After continuous stirring for 48 h at ambient temperature, the mixture 8577

DOI: 10.1021/acs.iecr.6b01975 Ind. Eng. Chem. Res. 2016, 55, 8576−8585

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Industrial & Engineering Chemistry Research Scheme 1. Strategy for the Synthesis of PGO

Figure 2. C 1s XPS spectra of (a) GO and (b) PGO. 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. Fourier transform infrared (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) in the wavelength region from 200 to 1000 nm, where the PGO/PANI nanocomposite samples were in the form of a NMP solution. Raman spectroscopy was carried out by a 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), conducted at a scanning rate of 0.01°/s. Thermogravimetric analysis (TGA) was employed with a Q600SDT thermoanalyzer instrument (TA, USA), with heating from room temperature to 600 °C at a rate of 10 °C/min under a N2 atmosphere. Transmission electron microscopy (TEM) images were studied using a Hitachi H-7650 instrument at an accelerating voltage of 80 kV. The morphologies of GO and PGO/PANI nanocomposites were observed by scanning electron microscopy (SEM; ZEISS Merlin) 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 a CO carboxyl at 1730 cm−1, CC bonds in the remaining sp2 character of graphite at 1628 cm−1,24 and peaks at 1395, 1243, and 1044 cm−1 caused by the vibration of C−OH, −COO−, and C−O−C, respectively.25 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 vibrations of the benzene ring framework. Moreover, the new peaks at 1562 and 829 cm−1 inferred 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 a nucleophilic addition reaction forming C−NH−C,26 as displayed in Scheme 1. 8578

DOI: 10.1021/acs.iecr.6b01975 Ind. Eng. Chem. Res. 2016, 55, 8576−8585

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Industrial & Engineering Chemistry Research

Figure 3. XRD patterns of (a) graphite, GO, and PGO and (b) PGO, PANI, and PGO/PANI nanocomposites.

In Figure 1b, the peaks at 1589 and 1494 cm−1 were attributed to the CC stretching vibrations of the quinone and benzene rings.27 Similar peaks had blue shifts of 4, 6, and 8 cm−1 and 4, 5, and 7 cm−1 in PGO/PANI03, PGO/PANI05, and PGO/PANI10, respectively. The blue shifts could be attributed to π−π interactions between the benzene and quinone rings of PANI and the PGO layer to form a greater πelectron 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 slight blue shifts of 3, 4, and 8 cm−1 in 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 the PGO layer plane were activated. The chemical composition on the surface of GO and PGO and the reduction of GO by PPD were also confirmed through X-ray photoelectron microscopy (XPS) analysis (Figure 2). As shown in Figure 2a, the C 1s 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, respectively.28,29 After 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 oxygencontaining groups in GO were eliminated and GO was effectively reduced by PPD. Furthermore, a new peak corresponding to the C−N groups occurred at 285.7 eV, thus inferring that PPD was successfully grafted onto the surface of GO sheets.26 XRD was utilized to measure the distance between the graphene interlayers and 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, 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 water.30 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 the interlayer spacing as a result of the intercalation of functionalized PPD between the GO layers. In addition, there is one more broad peak around 2θ = 23.6° after modification, indicating that most of the oxygen groups have been removed. The broadened peak and reduced peak intensities indicate that graphene is exfoliated into a single layer or a few layers. 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. The PANI pattern showed a characteristic peak at 2θ = 20.1° corresponding to the (020) lattice plane.31 Similarly, in the pattern of PGO/PANI nanocomposites, this peak appeared; its intensity decreased, whereas the PGO content increased, which indicated 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, which corresponds to the crystallinity.32 As shown in Figure 4, graphite possessed a narrow sharp G band and a small D band, thereby indicating its

Figure 4. Raman spectra of graphite, GO, and PGO. 8579

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Industrial & Engineering Chemistry Research 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 the sp2-conjugated structure of oxygencontaining groups, such as carboxyl, hydroxyl, and epoxy groups. Furthermore, the G band of PGO shifted to 1594 cm−1, which inferred high exfoliation of PGO. The D/G intensity ratio (ID/IG) of PGO (1.19) was larger than that of GO (1.01), which resulted from grafting with the PPD molecule and an increase of the disorder degree after functionalization.33 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

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

Figure 5. UV−vis spectra of PANI and PGO/PANI nanocomposites.

PANI exhibits two characteristic features: a relatively narrow band at 328 nm caused by the π−π* transition of an aromatic C−C bond and a wide band at 629 nm attributed to the π−π* transition of a quinonoid C−C bond.34,35 For PGO/PANI nanocomposites, these two bands were largely intensive and showed red shifts in the incremental degree with an increase in the content of PGO. These results should be due to the formation of large electronic conjugation domain because of the grafting and interfacial adhesion between PGO and PANI, both of which were conjugated structural molecules. 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, PGO showed an obvious sheetlike structure with more crumple (Figure 6b) because the covalently grafted rigid 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 filmlike form of GO nanosheets. Figure 6d confirms that exfoliated PGO without apparent aggregation possessed a thin thickness and a wavy structure, which were the same as those for the intrinsic characteristics of GO nanosheets.36 The SEM and TEM images of the PGO/PANI composites are shown in Figure 7. The SEM image (Figure 7a) shows that the composite was a rigid flake with a rough surface, which could infer that the PGO sheet was covered with the nanotubelike PANI in the composites (see high-magnification SEM images in Figure S1). Apparently, the PANI nanotubes were

Figure 7. (a) SEM and (b) TEM images of PGO/PANI nanocomposites. SEM images of PGO/PANI nanocomposites with reaction times of (c) 15 min, (d) 30 min, (e) 2 h, and (f) 3 h.

approximately 300 nm in length and about 30 nm in diameter. A similar nanostructure can also be observed in the TEM image (Figure 7b), which further confirms the thin composites, where the dark area should be the overlapping PANI nanotubes on the surface. Furthermore, the in situ polymerization process is presented in Figure 7c−f. From the beginning of the reaction, a grainlike polymer appeared on the surface of PGO. As the reaction proceeded, the grainlike PANI started to transform to a tubelike shape, which was attributed to the increasing molecular weight and periodicity parallel of the polymer chain. In addition, several clusters of nanotubes in the surroundings were observed in each image, thereby inferring that PANI was arrayed on various bulky substrates and polymerized via selfnucleation. 8580

DOI: 10.1021/acs.iecr.6b01975 Ind. Eng. Chem. Res. 2016, 55, 8576−8585

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Industrial & Engineering Chemistry Research Thermal and Dispersion Stabilities of GO and PGO. TGA was employed to examine the thermal stability of the materials, and the results are shown in Figure 8. From Figure 8,

indicating that a PGO dispersion in acidified conditions was quite stable. This observation could be attributed to the functional modification of PPD, which endowed GO with amino groups on its surface and led the possibility for further in situ polymerization of PANI. Anticorrosion Property of PGO/PANI Nanocomposites. Potentiodynamic polarization curves of different coatings in a 3.5 wt % NaCl aqueous solution are presented in Figure 10

Figure 8. TGA curves of graphite, GO, and PGO.

we can 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 a more than 50% loss was observed in the range of 200−300 °C for GO, which was mainly the result of unstable oxygen-containing groups decomposing into CO2 or other vapors. 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 functionalization of GO with PPD could reduce the quantity of oxygen-containing groups, which could improve the thermal stability. In addition, the lower quantity of oxygencontaining groups made PGO a good barrier filler in the coating to resist water or other aggressive corrosives. To detect the stability of GO and PGO in the solution state, 0.5 mg/mL GO and PGO aqueous solutions were prepared. As shown in Figure 9, GO could uniformly disperse in water

Figure 10. Tafel plots for bare Q235A steel, PANIc, PSc, PANIc, PGOc, PPCc03, PPCc05, and PPCc10.

(the setup of a coating evaluation electrolytic cell is shown in 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 of equilibrium. Figure 10 shows that the coated samples possessed a larger positive corrosion potential (Ecorr) than the pure carbon steel, which revealed the obvious protection from the polymer coating. Icorr was obtained through extrapolation of 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 by applying the following Stern−Geary equation:37,38 Rp =

kak b 2.303(ka + k b)Icorr

(1)

where Icorr is the corrosion current (μA/cm2), whose value was determined via the intersection of the anodic and cathodic lines and ka and kb are the anodic and cathodic slopes (ΔE/Δ 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:

Figure 9. GO and PGO dispersions in water and 1 mol/L HCl(aq) with a concentration of 0.5 mg/mL.

without any precipitation, and PGO could form a slightly heterogeneous dispersion with a small amount of precipitate and a floating solid powder. Furthermore, 0.5 mg/mL GO and PGO dispersions in a 1 mol/L HCl aqueous solution were prepared. On the contrary, a homogeneous dispersion of PGO was obtained, and GO was hardly decentralized in the HCl solution with apparent precipitate and dross. In addition, the ζ potentials 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 and +7.2 and +30.5 mV for PGO dispersions in pure water and acidified water, respectively,

CR =

kMIcorr ρm

(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 PE was calculated through the following formula: Icorr,o − Icorr,i PE = × 100% Icorr,o (3) 8581

DOI: 10.1021/acs.iecr.6b01975 Ind. Eng. Chem. Res. 2016, 55, 8576−8585

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Industrial & Engineering Chemistry Research Table 1. Electrochemical Corrosion Measurements of Anticorrosion Coatings electrochemical corrosion measurements sample carbon steel PSc PANIc PGOc PPCc03 PPCc05 PPCc10

Ecorr (mV) −821.16 −580.21 −561.88 −531.79 −405.78 −362.46 −277.07

Icorr (A/cm2) 4.26 6.32 3.95 5.61 2.67 2.56 7.32

× × × × × × ×

Rp (kΩ·cm2)

−5

0.26 × 10 2.63 44.21 348.89 792.17 1048.20 2366.34

10 10−6 10−7 10−8 10−8 10−8 10−9

−3

CR (mm/year) 0.99 0.15 9.20 1.31 6.17 5.95 1.68

× × × × ×

10−3 10−3 10−4 10−4 10−4

PE (%) 85.16 99.07 99.87 99.94 99.94 99.98

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

where Icorr,o is the corrosion current density of the pure carbon steel (A/cm2) and Icorr,i is the corrosion current density of the coated sample (A/cm2). Ecorr mainly represents the tendency of corrosion reaction, where a high Ecorr value results in a good anticorrosion property of the coatings. Compared with the bare carbon steel, the Ecorr values of all of the coating 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 an increase in the content of PGO. According to Faraday’s law, the corrosion current density and corrosion rate are in a positive proportional relationship. Therefore, a lower Icorr value will slow the corrosion rate. Tafel analysis results revealed that the use of PSc to protect the carbon steel can decrease Icorr for 1 order of magnitude. Additionally, the use of PANIc and PGOc could further decrease Icorr for 1 and 2 orders of magnitude compared with PSc, respectively. Nevertheless, Icorr of the PPCc series coatings was lower than those of all samples above. Similarly, the decreasing amplitude of the PPC series was positively correlated with an increase in the content of PGO, which

indicated that PGO played the dominant role in the nanocomposites. Particularly among all of the samples, PPCc10 possessed the most desirable anticorrosion performance and retained the lowest CR (i.e., 1.68 × 10−4 mm/year). PSc is the basic physical isolation to keep corrosive material, such as water and oxygen, from contacting the metal surface. PANIc can also deactivate metal to form an oxidation film on the metal surface.39 Moreover, with its outstanding stability and high aspect ratio, PGO was the novel lightweight and higheffect anticorrosion filler. PGO/PANI nanocomposites possessed better anticorrosion properties than PGO or PANI individually, which verified that the composites obtained the synergism of PGO and PANI as a result of their specific structure. As shown in the SEM and TEM images, PGO/PANI nanocomposites had sandwich-like structure, where PANI covered the PGO surface. This structure endowed the PGO/ PANI nanocomposites 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. The corrosion behavior of the PGO/PANI nanocompositebased coatings was investigated through electrochemical 8582

DOI: 10.1021/acs.iecr.6b01975 Ind. Eng. Chem. Res. 2016, 55, 8576−8585

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Industrial & Engineering Chemistry Research 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 regarding the resistance, capacity of the coatings, corrosion reaction resistance, and electric double-layer capacity of the metal surface.40 Generally, in the phase curve of the Bode plot, the peak at high frequency (i.e., 103−105 Hz) responds to protection of the coatings, and the peak at middle-to-low frequency (i.e., 10−3−103 Hz) responds to the corrosion reaction between the corrosive material and metal.41 Two peaks (i.e., two time constants) were observed at both high and middle-to-low frequency in the Bode-phase plots in Figure 11a. At high frequency, the values of the phase angles were approximately −90° and decreased toward the lowfrequency direction. This observation demonstrated the increased capacitance and decreased resistance of the coatings ascribed to the diffusion of the corrosive electrolyte with low resistance and high permittivity. The time constant at low frequency responded to the electric double-layer capacity and the corrosion reaction resistance. 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 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 the Warburg impedance, was attributed to the randomly distributed flakelike PGO with a prominent shielding effect. This result reflected a tardy process for the corrosive to penetrate through the 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 that the diameters of the semicircles increased in the order of PPCc03 < PPCc05 < PPCc10, which mainly resulted from the significant improvement of the charge-transfer resistance with the enhanced content of PGO. In addition, the Warburg impedance character was observed as the 45° oblique lines that occurred at low frequency in the plots of PPCc05 and PPCc10, which also showed that the diffusion process dominated corrosion at low frequency. Therefore, a sufficient amount of PGO that is well dispersed in the coatings can retard or prevent the penetration of a corrosive electrolyte. The equivalent circuit is shown in Figure 11d, where Rs is the electrolyte resistance and Rp is the polarization or chargetransfer resistance, which indicated the protection of the coatings against the corrosive.42 Cdl represents an electric double-layer capacitor ascribed to the accumulated charge between the metal surface and coatings. Cc is the coating 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.

Figure 12. Model of the corrosive diffusion pathways through the coatings.

4. CONCLUSION In this paper, PGO/PANI nanocomposites with distinct anticorrosion performance were fabricated via a facile in situ redox polymerization−dedoping method. Grafted with PPD onto the surface, PGO homogeneously 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 high π−π delocalization. The 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 an increase in the content of PGO in the nanocomposites. A further comparison with PANIc and PGOc showed that the PPCc series possessed 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 an increase in the 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 ecofriendly technique would be a promising method to fabricate graphene-based nanocomposites and expand their application in the anticorrosion field.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b01975. High- and low-magnification SEM images of PGO/PANI nanocomposites and assembly drawing of a coating evaluation electrolytic cell (PDF)



AUTHOR INFORMATION

Corresponding Authors

*Tel./fax: +86-20-87112047. E-mail: [email protected] (D.X.). *Tel./fax: +86-20-87112047. E-mail: [email protected] (X.Z.). Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Science and Technology Planning Project of Guangdong Province, China (Grant 2015A010105008). 8583

DOI: 10.1021/acs.iecr.6b01975 Ind. Eng. Chem. Res. 2016, 55, 8576−8585

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Industrial & Engineering Chemistry Research



(22) Hummers, W. S.; Offeman, R. E. Preparation of graphitic oxide. J. Am. Chem. Soc. 1958, 80, 1339. (23) Sheng, X.; Xie, D.; Cai, W.; Zhang, X.; Zhong, L.; Zhang, H. In situ thermal reduction of graphene nanosheets based poly(methyl methacrylate) nanocomposites with effective reinforcements. Ind. Eng. Chem. Res. 2015, 54, 649. (24) Kuila, T.; Bose, S.; Khanra, P.; Kim, N. H.; Rhee, K. Y.; Lee, J. H. Characterization and properties of in situ emulsion polymerized poly (methyl methacrylate)/graphene nanocomposites. Composites, Part A 2011, 42, 1856. (25) Szabó, T.; Berkesi, O.; Dékány, I. DRIFT study of deuteriumexchanged graphite oxide. Carbon 2005, 43, 3186. (26) Ma, H.-L.; Zhang, H.-B.; Hu, Q.-H.; Li, W.-J.; Jiang, Z.-G.; Yu, Z.-Z.; Dasari, A. Functionalization and reduction of graphene oxide with p-phenylene diamine for electrically conductive and thermally stable polystyrene composites. ACS Appl. Mater. Interfaces 2012, 4, 1948. (27) Chiolerio, A.; Porro, S.; Bocchini, S. Impedance Hyperbolicity in Inkjet-Printed Graphene Nanocomposites: Tunable Capacitors for Advanced Devices. Adv. Electron. Mater. 2016, 2, 1500312. (28) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007, 45, 1558. (29) Giardi, R.; Porro, S.; Chiolerio, A.; Celasco, E.; Sangermano, M. Inkjet printed acrylic formulations based on UV-reduced graphene oxide nanocomposites. J. Mater. Sci. 2013, 48, 1249. (30) Shen, J.; Hu, Y.; Shi, M.; Lu, X.; Qin, C.; Li, C.; Ye, M. Fast and facile preparation of graphene oxide and reduced graphene oxide nanoplatelets. Chem. Mater. 2009, 21, 3514. (31) Sanches, E. A.; Soares, J. C.; Iost, R. M.; Marangoni, V. S.; Trovati, G.; Batista, T.; Mafud, A. C.; Zucolotto, V.; Mascarenhas, Y. P. Structural characterization of emeraldine-salt polyaniline/gold nanoparticles complexes. J. Nanomater. 2011, 2011, 1. (32) Ferrari, A. C. Raman spectroscopy of graphene and graphite: disorder, electron phonon coupling, doping and nonadiabatic effects. Solid State Commun. 2007, 143, 47. (33) Kudin, K. N.; Ozbas, B.; Schniepp, H. C.; Prud’Homme, R. K.; Aksay, I. A.; Car, R. Raman spectra of graphite oxide and functionalized graphene sheets. Nano Lett. 2008, 8, 36. (34) Stafström, S.; Sjögren, B.; Brédas, J. L. An INDO/S-CI study of the photoinduced absorption spectrum of polyemeraldine. Synth. Met. 1989, 29, 219. (35) Chiolerio, A.; Bocchini, S.; Scaravaggi, F.; Porro, S.; Perrone, D.; Beretta, D.; Caironi, M.; Fabrizio Pirri, C. Synthesis of polyanilinebased inks for inkjet printed devices: electrical characterization highlighting the effect of primary and secondary doping. Semicond. Sci. Technol. 2015, 30, 104001. (36) Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S. The structure of suspended graphene sheets. Nature 2007, 446, 60. (37) Huang, T.-C.; Yeh, T.-C.; Huang, H.-Y.; Ji, W.-F.; Chou, Y.-C.; Hung, W.-I.; Yeh, J.-M.; Tsai, M.-H. Electrochemical studies on aniline-pentamer-based electroactive polyimide coating: Corrosion protection and electrochromic properties. Electrochim. Acta 2011, 56, 10151. (38) Lu, W.; Elsenbaumer, R. L.; Wessling, B. Corrosion protection of mild steel by coatings containing polyaniline. Synth. Met. 1995, 71, 2163. (39) Beard, B. C.; Spellane, P. XPS evidence of redox chemistry between cold rolled steel and polyaniline. Chem. Mater. 1997, 9, 1949. (40) Yin, K. M.; Wu, H. Z. Electrochemical impedance study of the degradation of organic-coated copper. Surf. Coat. Technol. 1998, 106, 167. (41) Mahdavian, M.; Attar, M. M. Electrochemical behaviour of some transition metal acetylacetonate complexes as corrosion inhibitors for mild steel. Corros. Sci. 2009, 51, 409.

REFERENCES

(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric field effect in atomically thin carbon films. Science 2004, 306, 666. (2) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-dimensional atomic crystals. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10451. (3) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008, 321, 385. (4) Compton, O. C.; Kim, S.; Pierre, C.; Torkelson, J. M.; Nguyen, S. T. Crumpled graphene nanosheets as highly effective barrier property enhancers. Adv. Mater. 2010, 22, 4759. (5) Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Superior thermal conductivity of single-layer graphene. Nano Lett. 2008, 8, 902. (6) Kim, H.; Miura, Y.; Macosko, C. W. Graphene/polyurethane nanocomposites for improved gas barrier and electrical conductivity. Chem. Mater. 2010, 22, 3441. (7) Kim, H.; Abdala, A. A.; Macosko, C. W. Graphene/polymer nanocomposites. Macromolecules 2010, 43, 6515. (8) Kim, H.; Kobayashi, S.; Abdurrahim, M. A.; Zhang, M. J.; Khusainova, A.; Hillmyer, M. A.; Abdala, A. A.; Macosko, C. W. Graphene/polyethylene nanocomposites: Effect of polyethylene functionalization and blending methods. Polymer 2011, 52, 1837. (9) Xu, J.; Wang, K.; Zu, S.; Han, B.; Wei, Z. Hierarchical nanocomposites of polyaniline nanowire arrays on graphene oxide sheets with synergistic effect for energy storage. ACS Nano 2010, 4, 5019. (10) Zhang, K.; Zhang, L. L.; Zhao, X. S.; Wu, J. Graphene/ polyaniline nanofiber composites as supercapacitor electrodes. Chem. Mater. 2010, 22, 1392. (11) Chang, K. C.; Ji, W. F.; Lai, M. C.; Hsiao, Y. R.; Hsu, C. H.; Chuang, T. L.; Wei, Y.; Yeh, J. M.; Liu, W. R. Correction: Synergistic effects of hydrophobicity and gas barrier properties on the anticorrosion property of PMMA nanocomposite coatings embedded with graphene nanosheets. Polym. Chem. 2014, 5, 6865. (12) Chang, K. C.; Hsu, M. H.; Lu, H. I.; Lai, M. C.; Liu, P. J.; Hsu, C. H.; Ji, W. F.; Chuang, T. L.; Wei, Y.; Yeh, J. M.; Liu, W. R. Roomtemperature cured hydrophobic epoxy/graphene composites as corrosion inhibitor for cold-rolled steel. Carbon 2014, 66, 144. (13) Yu, Y. H.; Lin, Y. Y.; Lin, C. H.; Chan, C. C.; Huang, Y. C. High-performance polystyrene/graphene-based nanocomposites with excellent anti-corrosion properties. Polym. Chem. 2014, 5, 535. (14) Schriver, M.; Regan, W.; Gannett, W. J.; Zaniewski, A. M.; Crommie, M. F.; Zettl, A. Graphene as a long-term metal oxidation barrier: worse than nothing. ACS Nano 2013, 7, 5763. (15) Kim, H.; Macosko, C. W. Processing-property relationships of polycarbonate/graphene composites. Polymer 2009, 50, 3797. (16) Sun, W.; Wang, L.; Wu, T.; Pan, Y.; Liu, G. Inhibited corrosionpromotion activity of graphene encapsulated in nanosized silicon oxide. J. Mater. Chem. A 2015, 3, 16843. (17) Sun, W.; Wang, L.; Wu, T.; Wang, M.; Yang, Z.; Pan, Y.; Liu, G. Inhibiting the corrosion-promotion activity of graphene. Chem. Mater. 2015, 27, 2367. (18) Wei, Y.; Wang, J.; Jia, X.; Yeh, J.-M.; Spellane, P. Polyaniline as corrosion protection coatings on cold rolled steel. Polymer 1995, 36, 4535. (19) Wessling, B. Passivation of metals by coating with polyaniline: corrosion potential shift and morphological changes. Adv. Mater. 1994, 6, 226. (20) Stankovich, S.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Synthesis and exfoliation of isocyanate-treated graphene oxide nanoplatelets. Carbon 2006, 44, 3342. (21) Chang, C. H.; Huang, T. C.; Peng, C. W.; Yeh, T. C.; Lu, H. I.; Hung, W. I.; Weng, C. J.; Yang, T. I.; Yeh, J. M. Novel anticorrosion coatings prepared from polyaniline/graphene composites. Carbon 2012, 50, 5044. 8584

DOI: 10.1021/acs.iecr.6b01975 Ind. Eng. Chem. Res. 2016, 55, 8576−8585

Article

Industrial & Engineering Chemistry Research (42) Mansfeld, F. Use of electrochemical impedance spectroscopy for the study of corrosion protection by polymer coatings. J. Appl. Electrochem. 1995, 25, 187.

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DOI: 10.1021/acs.iecr.6b01975 Ind. Eng. Chem. Res. 2016, 55, 8576−8585