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Synthesis of Graphene Oxide-based Sulphonated Oligoanilines Coatings for Synergistically Enhanced Corrosion Protection in 3.5% NaCl Solution Hao Lu, Shengtao Zhang, Weihua Li, Yanan Cui, and Tao Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13722 • Publication Date (Web): 10 Jan 2017 Downloaded from http://pubs.acs.org on January 11, 2017

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ACS Applied Materials & Interfaces

Synthesis of Graphene Oxide-based Sulphonated Oligoanilines Coatings for Synergistically Enhanced Corrosion Protection in 3.5% NaCl Solution Hao Lu,a Shengtao Zhang *,a Weihua Li*,b Yanan Cui,c Tao Yang*,c

a

School of Chemistry and Chemical Engineering, Chongqing University,Chongqing 400044, P.R.China b

Key Laboratory of Marine Environmental Corrosion and Bio-fouling, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, 266071, P.R.China

c

College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, P.R.China

* Corresponding author. E-mail: [email protected]; [email protected]

Phone: +86-532-84022665.

Fax: +86-532-84023927.

ACS Paragon Plus Environment


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ABSTRACT As a vital derivative of graphene, graphene oxide (GO) is widely applied in various fields, such as transparent electrodes, solar cells, energy storage and corrosion protection due to the large specific surface area and abundant active sites. However, compared with graphene, the application of GO has been less reported in metal corrosion protection field. Therefore, in our study, 3-aminobenzenesulfonic acid (m-ABSA) was selected to combine with oligoanilines to fabricate the GO-based sulphonated oligoanilines coatings for marine corrosion protection application. The obtained composite coatings were covered on the surface of Q235 steel, which is one of the most important structural marine materials. FT-IR spectra was utilized to prove the existence of different bonds and functional groups of aniline trimer (AT) and sulfonated aniline trimer (SAT). And scanning electron microscope (SEM) was applied to verify the combination of GO and SAT. What’s more, transmission electron microscope (TEM) was applied to observe the surface appearance of the obtained GO-SAT composite material. Besides, the results of electrochemical measurements performed in 3.5 wt% NaCl solution showed the excellent corrosion protective properties of GO/SAT-coated epoxy resin with a dosage of 10 mg of GO compared with the pure epoxy resin. Moreover, the enhancement of surface hydrophobic property, to some extent, is in favor of preventing the absorption of corrosive medium and water molecules revealed by contact angle test. The addition of GO can make the diffusion pathway of the corrosive medium longer and more circuitous, while SAT has displayed excellent solvent solubility while maintaining corrosion protective properties similar to those of PANIs so that the corrosion protective properties of the


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modified coatings improve significantly due to the synergistically enhanced corrosion protection of GO and SAT. KEY WORDS:

graphene

oxide,

sulfonated

oligoanilines,

electrochemical

measurements, hydrophobic, corrosion protection INTRODUCTION Carbon steel has been applied to nearly the whole industrial field for its outstanding properties as one of the most important structural materials. 1 However, severe corrosion of carbon steel always leads to multitudinous potential safety matters and heavy economic losses.2 Hence, corrosion protective studies are greatly concerned by a wide range of researchers.3 Metal substrate improvement, corrosion inhibitor immersing treatment, surface coating protection, and partial cathodic protection are constantly applied to delay the corrosion process of metal.4,5 Graphene, which is a representative two-dimensional (2D) layered material, has already attracted global attention due to the high thermal conductivity, light transparency, and high electrical conductivity.6,7 Moreover, the special π-π* band structure makes graphene share certain characteristics with the semi-metallic material8 so that graphene is widely applied in transparent electrodes, solar cells, energy storage and corrosion protection as well. Prasai et al9 had demonstrated a coating of graphene decreases the corrosion of copper, on which graphene was grown by chemical vapor deposition (CVD) method on the surface of copper directly. However, the graphene grown on the metal surface was proved to provide a short-period corrosion protective



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performance only. Therefore, various materials were utilized to improve the corrosion protection properties, such as polyvinylbutyral (PVB)10, polyaniline (PANI)11, carboxylated oligoanilines12, etc. Sun’s group had reported a promising application of graphene/pernigraniline composites (GPCs) for the corrosion protection of copper.10 The obtained GPCs were then added into polyvinylbutyral coating (PVBc) to promote the protection properties of the pure coating. Their research revealed that the GPC-modified PVBc is a good barrier against corrosive media due to the enhanced electrical resistance of the coatings. Moreover, the graphene sheets in the GPCs were less flexible and more likely to unfold, which could prolong the diffusion pathway of corrosive media. However, the bonding and film-forming performance of the PVBc were not satisfactory in corrosion protection. Jui-Ming Yeh et al11 had demonstrated a kind of novel corrosion protective coatings prepared from composites of polyaniline and graphene. 4-aminobenzoyl group-functionalized graphene-like sheets could promote better dispersion of the graphite and lengthen the diffusion pathway that gases should effectively encounter as a conductive filler. Yu’s group had prepared a kind of water-dispersible graphene sheets stabilized by carboxylated oligoanilines for fabricating the corrosion protective coatings.12 In their research, graphene could be stably dispersed in water by using a water-soluble carboxylated AT derivative as a stabilizer. Besides, they revealed that the addition of graphene into epoxy resin remarkably improved corrosion protection by a series of electrochemical measurements performed in 3.5% NaCl solution. Obviously, higher thermal conductivity and electrical conductivity have limited the application of pure graphene


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in metal corrosion protection. To circumvent the issues, various 2D and 3D materials have been developed to modify the pure graphene for further application in various fields. It is apparent that the presence of additives mentioned above have enhanced the corrosion protective properties, but most of the chemical modification methods to fabricate the polymer additives are complicated, difficult to employ in industry, and also make the degree of polymerization fairly difficult to control. Graphene oxide (GO), as a vital derivative of graphene, has more abundant oxygen functional groups as hydroxy, epoxy group, carboxyl, etc.13 Meanwhile, a larger specific surface area and more active sites of GO make it easily to be applied in various areas, including corrosion protection field.14,15,16 Jafari et al17 had synthesized a polyaniline-graphene oxide nanocomposite film on copper electrode. The results of electrochemical measurements showed that corrosion potentials shifted to anodic regions in the presence of polyaniline-graphene oxide. Meanwhile, as a composite material, nanoparticle of polyaniline and GO increased the corrosion protective performance of the copper substrate significantly. The use of the polyaniline-graphene oxide composite contributed to the formation of a composite layer, which would shift the corrosion potential of the copper to lower values and decrease the corrosion rate.18 Yu’s team reported that the chemical bonding of styrene monomer and vinyl-grafted GO by in situ miniemulsion polymerization enhanced the dispersion of GO in the polystyrene (PS) matrix.19 And the obtained materials with 2 wt% GO showed an excellent corrosive protective property. Nevertheless, the complicated chemical modification method made the degree of polymerization fairly difficult to control.



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Inspired by their research consequences, GO was utilized in our experiment deservedly, benefiting from the abundant active sites and outstanding water dispersibility of it, while PANI was also applied in our research.11 As a kind of nanoscale conductive polymers, PANI could be obtained by electropolymerization, and applied in chemical sensors, energy storage, corrosion inhibition of metals, etc. K.C. Chang et al20 proposed the application of polyaniline/graphene composite in the field of mental corrosion protective coatings in 2014. However, most of the chemical modification methods to fabricate the PANI are complicated and difficult to be applied in industry. Therefore, simply and soluble oligoanilines would have excellent advantages in corrosion protection application, and deserve to be developed.10 Based on the above papers, m-ABSA was applied in our study as an easily decomposing and environmentally-friendly additive to enhance the corrosion protective properties of the GO material. In this paper, we demonstrated a method of fabricating sulfonic aniline trimer derivatives as a stable dispersant to GO in aqueous solution and then prepared a new kind of GO/epoxy composite coatings shown in Figure 1. Enhanced electrical and ionic conductivities were discovered with the combination of sulfonic acid-grafted GO and AT, which were attributed to the graphitic structure and the sulfonic acid groups of sulfonic acid-grafted GO. The SAT was used as GO dispersing agent. At first, the conjugate structure of SAT could facilitate the dispersion of GO in water through π−π* interactions. Then, SAT also displays excellent dispersing ability while maintaining corrosion protective properties,


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which are considered to be similar to those of PANIs. Therefore, the synergistic effect between SAT and GO is assumed to facilitate the dispersion of GO in water and obvious enhancement of the corrosion protective property of epoxy resin coatings. It is proved that the corrosion protection capability of GO-epoxy resin has been enhanced with the addition of SAT. The combination of sulfonic groups in SAT with the active sites on the surface of GO makes it easier and more homogeneous to disperse in the aqueous solution. As a result, the GO/SAT composite has provided an excellent barrier performance against corrosive electrolyte diffusion and own better corrosion protective properties than GO or SAT individually, which verified that the materials obtained the synergism of GO and SAT as a result of their specific structure. Eventually, the manufactured SAT derivative-modified GO composite possessed outstanding electrochemical activity, electrochemical stability and corrosion protective property. And the surface hydrophobic property of the GO/SAT-coated epoxy resin is in favor of preventing the absorption of corrosive medium and water molecules as well.



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Figure 1 Schematic illustration of the preparation process for GO/SAT-coated epoxy resin composite coatings.


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EXPERIMENT SECTIONS Apparatus and Reagents. Aniline, p-phenylenediamine sulfate, tetrahydrofuran (THF) were purchased from Aladdin Industrial Corporation. Ammonium persulfate, petroleum ether, ammonium hydroxide, sodium chloride and concentrated hydrochloric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. m-ABSA was purchased from Adamas Reagent Co., Ltd. GO was fabricated by the modified Hummers’ method.21,22,23 Epoxy resin and homologous curing agent were purchased from Feichengdeyuan Chemicals Co., Ltd. All the reagents were used as received. The substance used for corrosion protection measurement was the Q235 steel electrode. The Q235 steel electrode surface was polished with 800, 1200 SiC sandpaper, and 2000 metallographic sandpaper separately. Alumina polishing powder (0.05μm) purchased from Gaoss-Union Co., Ltd. was prepared to burnish the surface next. Then the electrode surface was rinsed ultrasonically with ethanol, acetone and plentiful distilled water, and finally dried at room temperature. Fourier transform infrared spectroscopy (FT-IR) spectrum was obtained from Tensor 27 FT-IR spectrophotometer (Bruker Company, German). And Scanning electron microscopy (SEM) measurements were carried out via a JSM-6700F scanning electron microscope (Japan Electron Company). Meanwhile, transmission electron microscopy (TEM) were carried out by a JEM 2100 transmission electron microscope. All the corrosion measurements of electrochemical test were operated on CHI-660D electrochemical workstation. Moreover, contact angle tests were carrid out by a JC2000C goniometer.



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Preperation of GO. The GO powder used in our experiment was fabricated by the modified Hummers’ method21,22. 2.5 g graphite powder was mixed with 115 mL 98% H2SO4 and 1.25 g NaNO3, followed by continuous stirring in an ice-salt bath for 30 min. Successively, 7.5 g KmnO4 was added into the mixture slowly under stirring for 2 h at 20 °C. Then, the whole reaction system was transferred to a flask in a water bath and kept there for 30 min at 35±3 °C. Afer that, the water bath was heated up to 98 °C, and the flask was retained for 15 min with 115 mL DI water added in immediately. Then, 350 mL DI water was added into the reaction to dilute the obtained mixture. Next, 30% H2O2 was added dropwise until the color of the mixture was turned to bright yellow. After that, the suspension was filtered then washed by 800 mL 5% HCl and 300 mL DI water successively to remove the metal ions, H + and SO42−. At last, the product was dried in a vacuum drying oven overnight at 50 °C then grinded to obtain the GO powder. Preparation of AT. The method formulated by Weng et al24 to fabricate the AT was adopted in this paper. 1.853 g aniline and 2.956 g p-phenylenediamine sulfate were mixed and then dissolved in 150 mL 1.0 M HCl solution at -5 °C. Besides, 50 mL 1.0 M HCl solution within 4.541 g ammonium persulfate was dropwise added into the above solution. Collected the solid product by suction filtration after a continuous stirring (1 h) at room temperature. Cyan solid product was obtained after washing with 1.0 M HCl solution pre-cooled to 0.100mL 0.5 M NH3(aq) was added to the solid product every 30 minutes. The total volume of NH3H2O was 400 mL. Then a large amount of distilled water was added to rinse the product for several times. The


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product was dried at 60 °C in the DZ-1BCII vacuum oven with a 2XZ-2 vacuum pump overnight. The AT was acquired as a red or amaranthine solid. Preparation of SAT. 1.24 g AT and 1.73 g m-ABSA were mixed then dissolved in 40 mL tetrahydrofuran (THF) with a mighty and continuous stirring under 40 °C atmosphere for 5 h. 200 mL petroleum ether was dropwise added into the mixture above afterwards. Simultaneously, suction filtration was used for collecting the solid product. And then, washing the solid with 40 mL THF was performed. Finally, the sulfonated aniline trimer was acquired as a deep purple solid after vacuum drying at 60 °C. Preparation of (GO-SAT) Composites of GO and SAT. GO-SAT composites were fabricated on base of GO dispersion liquid. 0.6 g SAT and 0.08 g NaOH were mixed then dissolved in 95 mL distilled water with an ultraphonic dispersing for 1 h to form a SAT aqueous solution. Then 10 mg GO was added into the as-prapared SAT solution and sonicated for 2 h to acquire a homogeneous GO-SAT composite dispersion. Preparation of GO-SAT Corrosion Protective Coatings. 10 g epoxy resin curing agent was selected to mix with 5 mL of GO-SAT composites dispersion. Then 5 g epoxy resin was added into the mixture after a stirring for 10 min. Another 10 min was arranged for stirring with the composites above. After these operations, the mixture was carefully coated on the Q235 steel electrode materials with a bar coater, and dried at room temperature for 72 h.



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Electrochemical corrosion measurement. The electrochemical impedance spectroscopy (EIS) and polarization curves tests were carried out by an original three-electrode cell in imitated seawater (3.5% wt. NaCl) at room temperature. The three-electrode cell used in this experiment was consist of a platinum sheet auxiliary electrode with a work area of 2.25 cm2, a Ag/AgCl reference electrode, and a Q235 steel with coatings working electrode. The Q235 steel with coatings working electrode was immersed in the NaCl corrosion medium at first and performed an open circle potential (OCP) test for 2 h to obtain a stable electrochemical system. The alternating-current (A.C.) impedance measurements were carried out at the OCP with amplitude of 5 mV in the frequency range between 1×105 Hz and 0.01 Hz. Polarization curves were measured in a range of -300 mV and +300 mV versus the OCP value with a scan rate of 0.5 mV/s. RESULTS AND DISCUSSION FT-IR Spectroscopy Test. FT-IR spectrum was employed to characterize the process of chemical polymerization of aniline and fabrication of SAT. Figure 2 (A) shows that the 1599 cm-1 peak embodies the vibrational absorption of quinoid structure of Q=N,25 and 1506 cm-1 peak is the characteristic vibrational absorption of benzenoid structure of N-B-N.26 The strength ratio of the two characteristic peaks can evaluate the extent of PANI oxidation.27 The 1383 cm-1 and 1285 cm-1 peaks are caused by the C-N absorption in aromatic amine Ar-N.27,28 The 1166 cm-1 and 803 cm-1 peaks represent the in-plane and out-plane flexural vibration characteristic absorbing bands of benzene, respectively.29 Besides, the appearance of 512 cm-1 peak


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indicates the flexural vibration characteristic absorbing bands of aromatic rings. Furthermore, the peaks around 3213, 3373, 3429 cm-1 can be explained by N-H streching vibration characteristic absorption.30

Figure 2 FT-IR spectra of Aniline Trimer (A) and Sulfonated Aniline Trimer (B).

The FT-IR spectrum of SAT shown in Figure 2 (B) shares the similar results with the one of AT in the same figure in part (A), furthermore, the strong absorption peaks at 1185 cm-1, 1104 cm-1, 619 cm-1 and 557 cm-1 verify the existence of sulfonic acid groups in the composite,31 in which the strong peaks at 1185 cm-1 and 1163 cm-1 express the asymmetric and symmetric streching vibration absorption, respectively.32 Meanwhile, the O=S=O streching vibration and S-O stretching vibration are also embodied at the bands above.33 Obviously, the production of polymerization reaction



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contains the specific functionalized sulfonic acid groups, which can be applied in modified corrosion protective coatings as an excellent auxiliary. Preparation and Morphology of GO and GO/SAT-coated Epoxy Resin Composites. The modified Hummers’ method was adopted for obtaining the rufous GO powder and the morphology of GO was shown in Figure 3 (A). The obtained GO exhibited rough surface and slight wrinkles according to the results of SEM.34 Meanwhile, the TEM image of GO (Figure 3 (B)) displays a thin layered-structure, which reveals that the GO could be well utilized as an excellent support material with a large specific surface area.35

Figure 3 SEM (A), TEM (B) images of GO and SEM (C), TEM (D) images of GO/SAT composite.


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GO/SAT-coated epoxy resin composites were prepared by a curing reaction with the GO/SAT-coated epoxy resin and homologous curing agent. The homogeneous composites with strong adhesion were obtained to coat on the surface of Q235 steel electrode. We also studied the structure of the GO/SAT composites by SEM and TEM, for further studying the integrating state of the two functional materials. After chemical polymerization of the aniline monomers and m-ABSA, an irregular and protruding polymer film could be obtained. Compared with separated GO, the SEM image of GO/SAT composite showed the slight folds with an increased amount and broader scale, while the SAT sheets linked together to form a homogeneous and analogous network structure in Figure 3 (C). Meanwhile, the TEM image in Figure 3 (D) of GO/SAT composite shows the slight folds with large and broad scale, while the SAT sheets linked together to form a homogeneous structure. Figure 3 (D) shows that the light lamellae represented individual GO, whereas the black zone represented the SAT materials. Therefore, it is obvious that the GO/SAT composite is homogeneous and stable. Impedance Spectra and Polarization Curves Test. The impedance spectra for Q235 steel with different types of additives dispersing in the epoxy resin in 3.5% wt. NaCl solution are presented as Nyquist plots in Figure 4. It is not hard to notice that the impedance response of Q235 steel had significantly changed after the addition of different additives in the corrosive solutions. As can be seen from Figure 4 (A), The Nyquist plots shows that a single depressed semicircle and the diameter of semicircle increased with different additives.36 For pure epoxy resin coating, the radius of two



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capacitive impedance arcs are in the lowest position compared with other coatings within various additives. The addition of AT, SAT, GO and GO-SAT increased the diameter of semicircle signally. The results indicated that the addition of AT, SAT, GO and GO-SAT improved the corrosion protective properties of pure epoxy resin coating, in which GO-SAT showed the most excellent performance.37 Moreover, Bode plots shown in Figure 4 (B) (C) also supplementally explain the same results as well.

Figure 4 Nyquist plots (A) Bode plots (B) (C) of Q235 electrodes taken after EIS measurement and Tafel plots (D) of epoxy resin (ER) without additions and with addition of AT, SAT, GO and GO-SAT.

Potentiodynamic polarization measurements were carried out immediately after the EIS experiments. Figure 4 (D) shows the polarization curves of Q235 steel in 3.5 wt% NaCl solution and in the presence of various additives at room temperature. All the electrochemical parameters like the corrosion current density (icorr), corrosion


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potential (Ecorr), anodic Tafel slopes (βa) , cathodic Tafel slopes (βc) and ƞ% are summarized in Table 1. The icorr, βa and βc are obtained from the extrapolation of anodic and cathodic lines to the corrosion potential using CHI660d electrochemical analyzer software. In the case, the inhibition efficiency (ƞ%) on the corrosion of Q235 steel was calculated as following:

where

and icorr are the corrosion current density values without and with the

coating, respectively. Table 1 Parameters of polarization curves for Q235 steel in 3.5 wt% NaCl solution with ER and different additives of AT, SAT, GO and GO/SAT at room temperature. Ecorr (mV)

icorr (μA/cm2)

βc (mV/dec)

βa (mV/dec)

ƞ (%)

-824.7

74.51

7.389

4.722

/

Epoxy

-590.1

40.14

5.834

4.746

46.13

AT+ Epoxy

-540.3

4.65

5.056

4.969

93.76

SAT+ Epoxy

-511.7

3.07

5.336

5.168

95.88

GO+Epoxy

-507.5

3.36

3.975

6.285

95.49

GO/SAT+ Epoxy

-447.8

2.67

5.054

4.926

96.42

Additives Blank

As shown in Figure 4 (D), the polarization curves show an obvious linear Tafel region in both the anodic and cathodic areas. Overall, a lower icorr and a higher Ecorr mean better corrosion protective properties. The icorr of the GO-SAT/epoxy resin-coated Q235 steel was 2.67 μA/cm2, which was much lower than bare Q235 steel (74.51 μA/cm2) and pure epoxy resin-coated Q235 steel (40.14 μA/cm2). The icorr of the AT/epoxy resin-coated, SAT/epoxy resin-coated and GO/epoxy resin-coated



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Q235 steel were 4.65 μA/cm2, 3.07 μA/cm2, and 3.36 μA/cm2 respectively, which fell roughly between pure epoxy resin-coated and GO-SAT/epoxy resin-coated Q235 steel. What’s more, the Ecorr of the GO-SAT/epoxy resin-coated Q235 steel was -447.8 mV, which was more positive than bare Q235 steel (-824.7 mV) and pure epoxy resin-coated Q235 steel (-590.1 mV). The Ecorr of the AT/epoxy resin-coated, SAT/epoxy resin-coated and GO/epoxy resin-coated Q235 steel were -540.3 mV, -511.7 mV, and -507.5 mV respectively, which fell roughly between pure epoxy resin-coated and GO-SAT/epoxy resin-coated Q235 steel. The ƞ (%) of the GO-SAT/epoxy resin-coated Q235 steel (96.42%) was visibly higher than that of pure epoxy resin-coated (46.13%), AT/epoxy resin-coated (93.76%),

SAT/epoxy

resin-coated (95.88%), and GO/epoxy resin-coated Q235 steel (95.49%).

Figure 5 Electrical equivalent circuit diagrams of Q235 electrodes taken after electrochemical measurements without additives and with addition of AT, SAT (A) and GO-SAT (B)

The electrical equivalent circuit employed to model the investigated system is


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shown in Figure 5. This equivalent circuit consisted of three different resistors and two capacitors and the fitted corrosion parameters were listed in Table 2. The Rs is defined as solution resistance, while Rc and Rq representing the coating resistance and extra resistance brought by dispersion effect, respectively. Q is known as constant phase-angle element (CPE), which is an equivalence element formed by dispersion effect. The impedance function of the CPE is as follows: ZCPE = Y -1 ( ϳω ) -n Where, ZCPE is impedance of CPE, Y is proportional factor, ω is angular frequency, and n is the surface irregularity. The CPE, which is defined as the surface irregularity of the Q235 steel electrode, causes more obvious depression in the Nyquist semicircle, where the metal solution interface acts as a capacitor with an irregular surface. The exponential value (n) will become equal to 1 when the electrode surface is homogeneous and planar. Meanwhile, the metal solution interface will behave as a capacitor with a regular surface. Rs keeps in a nearly fixed value in the changeless corrosion medium. Rc value expresses the numbers of O2 and H2O molecules penetrated into the coatings. Rq is a parameter to describe the resistance to electron transfer across the metal surface which varies inversely to the corrosion rate. It is obvious that the Rc and Rct values of the GO-SAT/epoxy resin-coated Q235 steel were higher than others after EIS measurement. The phenomenon may ascribe to the barrier effect of GO against O2 and H2O molecules. The value of Qc would increase after the corrosion medium



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solution starting absorbing in the coatings. As shown in Table 2, the values of Qc of AT, SAT, and GO-SAT/epoxy resin-coating were lower than that of pure epoxy resin after electrochemical mearsurements. Besides, The Qc of GO-SAT/epoxy resin-coated Q235 steel electrode was much lower than others with additives of AT and SAT. It indicated that GO-SAT/epoxy resin-coating has the great utility value in corrosion protection engineering as a electrolyte solution barrier. Table 2 Electrochemical impedance parameters for Q235 steel in 3.5% wt. NaCl solution without additives and with pure epoxy resin, AT/epoxy resin, SAT/epoxy resin, GO/epoxy resin and GO/SAT-coated epoxy resin at room temperature. Rs Additive

Qc

Rc

CPE n

Rq

Qdl

Rq

(Ω cm2)

(μF cm−2)

(Ω cm2)

(Ωcm2)

(μF cm−2)

(Ω cm2)

(S sec^n cm-2)

Blank

7.625

4.675×10-2

1422

1.894×10-3

0.8401

10.91

—

—

Epoxy

9.54

1.791×10-8

2882

1.967×10-3

0.4074

611.8

—

—

6.774

8.675×10-10

1.195×10

-5

0.4827

1759

—

—

8.919

2.245×10-10

1.611×104

5.372×10-5

0.3520

2932

—

—

7.079

4.152×10-10

1.522×104

2.199×10-5

0.4515

4375

—

—

8.161

1.242×10-7

1.419×104

0.2321

0.01

3.938×10-10

9962

AT+ Epoxy

SAT+ Epoxy

GO+ Epoxy

GO/SAT+ Epoxy

4

6.605×10

9.190×10-5

However, the addition of GO/SAT composite had brought a considerable error to the electrochemical impedance parameters fitted from the equivalent circuit. Therefore, we chose another equivalent circuit for fitting in the following electrochemical impedance measurements. The Rs is solution resistance, while Rc and


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Rq representing the coating resistance and extra resistance brought by dispersion effect, respectively. Qc and Qdl represent the coating and double-layer capacitance, respectively. As we can see from the Table 2, the addition of GO/SAT-coated epoxy resin made a signally resistance between the corrosion solution and metal substrate. It might result from the barrier properties of GO against H2O and O2 in the solution. Obviously, compared with pure epoxy resin-coated Q235 steel, the addition of AT, SAT, and GO-SAT was confirmed to improve the corrosion protective properties. Then, the inhibition efficiency of the GO-SAT/epoxy resin-coated Q235 steel reached the highest. Therefore, GO/SAT-coated epoxy resin was selected to be applied in the next electrochemical measurements. Subsequently, different dosages of GO were added into the epoxy resin to evaluate the corrosion protective properties of coatings. Figure 6 (A) shows that the diameter of semicircle of the depressed semicircle increased with increasing dosage of GO while the maximum came to 10 mg. The cause of this behaviour might be the barrier effect offered by GO/SAT composite.12 Meanwhile, dispersed GO in the epoxy resin could increase the tortuosity of corrosive medium diffusion pathway due to the barrier effect.38 However, the resistance values of GO/SAT-coated epoxy resin were weaken, as the content of GO was further added in excess of optimized additive amount. This might be attributed to the growing number of cracks across the epoxy resin coatings, which would lead to an easily electron transfer between the substrate and etching solution due to the increased electric conductivity.39



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Figure 6 Nyquist plots (A) Bode plots (B) (C) of Q235 electrodes taken after EIS measurement and Tafel plots (D) with dosages of 6, 8, 10, 12 and 14 mg GO in GO/SAT coatings.

As shown in Figure 6 (D), the polarization curves also show an obvious linear Tafel region in both the anodic and cathodic areas. It was observed that the icorr of the GO-SAT/epoxy resin-coated Q235 steel with a dosage of 10 mg of GO reached the minimum value of 2.67 μA/cm2 according to the Table 3. Then, the icorr of the GO-SAT/epoxy resin-coated Q235 steel with a dosage of 6, 8, 12, 14 were 29.57 μA/cm2, 3.62 μA/cm2, 21.68 μA/cm2, and 32.33 μA/cm2, respectively, which were obviously higher than that of 10 mg of GO. Moreover, the Ecorr of the GO-SAT/epoxy resin-coated Q235 steel with a dosage of 10 mg of GO was -447.8 mV, which was more positive than the dosage of 6 mg GO (-627.2 mV), 8 mg GO (-473.7 mV), 12 mg GO (-500.1 mV), and 14 mg GO (-603.9 mV). The ƞ (%) of the GO-SAT/epoxy


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resin-coated Q235 steel with a dosage of 10 mg of GO (96.41%) was visibly higher than others. And the ƞ (%) of the GO-SAT/epoxy resin-coated Q235 steel with a dosage of 6, 8, 12, 14 were 60.31%, 95.14%, 70.90%, and 56.61%, respectively. Meanwhile, Bode plots shown in Figure 6 (B) (C) also supplementally explain the same results as well. Eventually, the consequences of polarization curves were consistent with the results of EIS measurements above. Table 3 Parameters of polarization curves for Q235 steel in 3.5 wt% NaCl solution with different dosage of GO in GO/SAT-coated epoxy resin at room temperature. Dosage of GO (mg)

Ecorr (mV)

icorr (μA/cm2)

βc (mV/dec)

βa (mV/dec)

ƞ (%)

-824.7

74.51

7.389

4.722

—

6

-627.2

29.57

5.462

4.718

60.31

8

-473.7

3.62

4.660

5.020

95.14

10

-447.8

2.67

5.054

4.926

96.41

12

-500.1

21.68

4.785

5.404

70.90

14

-603.9

32.33

5.554

4.654

56.61

Blank

Contact Angle Test. One of the most intuitionistic tests of the surface modification is the contact angle (CA) test by using the sessile drop method. And the contact angles of obtained surfaces were measured, as shown in the Figure 7. The measurements were performed at room temperature with three repetitions for each specimen. In this picture, it was observed that an addition of SAT-GO resulted in an obvious increase at CA values. As for the bare Q235 steel substrate shown in Figure 7 (A), the CA was only 77.9°± 2°. After the pure epoxy resin and GO-SAT/epoxy resin coating on the Q235 steel substrate, the corresponding contact angle value increased to 95.6°and 106.2°in Figure 7 (B) (C), respectively. The surface hydrophobicity had



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substantially distinctly improved in comparison with the CA of Q235 steel surface.

Figure 7 Contact angles of bare Q235 electrode (A), with pure epoxy resin (B) and with GO/SAT-coated epoxy resin (C).

In addition, GO/SAT-coated epoxy resin with surface hydrophobic property is in favor of preventing the absorption of corrosive medium and water molecules shown in Figure 8. Generally, the enhancement of hydrophobic property will be useful to protect the metal substrate avoiding the corrosion attack. Therefore, it will take much more time for corrosive medium and water molecules to diffuse from the solution


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interface to coatings surface. Besides, after permeating in the coatings, corrosive medium and water molecules usually walk in a straight line, which will cause the corrosion in a quite shorter time. However, the addition of GO can make the diffusion pathway of the corrosive medium longer and more circuitous, so that the corrosion protective properties of the modified coatings improve significantly due to the barrier effect of GO with a dosage of 10 mg.

Figure 8 Schematic of pure epoxy resin (A), GO/SAT-coated epoxy resin composite coating (B) during corrosion process.

Visual results of corrosion protection tests. Digital images of the Q235 steel electrodes under different treatment were prepared below shown in Figure 9. The bran-new Q235 steel electrode was polished with 800, 1200 SiC sandpaper, and 2000 metallographic sandpaper separately. The bright, smooth and clean surface of Q235 electrode without further measurements was shown as Figure 9 (A). Besides, as we could see from Figure 9 (B), the obtained GO/SAT-coated epoxy resin was coated on the working electrode evenly. After a continuous drying process of 72 h, the coated Q235 electrode was utilized to get the electrochemical measurements in 3.5 wt% NaCl solution. In summary, the results of electrochemical measurements exhibited the



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excellent corrosion protective properties of GO/SAT-coated epoxy resin. Moreover, the comparison of electrodes with and without coatings were showed in Figure 9 (C) (D). It is obvious that severe rusting appeared on the surface of Q235 steel electrode without coatings (Figure 9 (D)) while the Q235 working electrode with GO/SAT-coated epoxy resin still keep in a bright, smooth and clean condition.

Figure 9 Digital images of the pre-processed Q235 steel electrode (A), Q235 steel electrode with GO/SAT-coated epoxy resin (B), Q235 steel electrode of removing the coatings after electrochemical measurements (C), pre-processed Q235 steel electrode after electrochemical measurements (D).

CONCLUSION We demonstrated a simple process to combine the GO and SAT to obtain the functional epoxy resin coatings to provide excellent corrosion protection. The GO-SAT composite was in a stable state due to the strong π-π* interactions between GO and SAT. Obviously, the addition of AT, SAT, GO and GO/SAT remarkably improved corrosion protection compared with the pure epoxy resin coating. Among them, GO/SAT composite achieved the optimal effect, which was ascribed to the synergy of the active sites on the surface of GO sheets and functional sulphonic


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groups. As the basic physical isolation, GO is valuable to keep corrosive materials, such as water, oxygen and chloridion, from contacting the Q235 steel surface. SAT can also deactivate metal to help forming a homogeneous film with epoxy resin on the steel surface. Obviously, the synergistic effect between SAT and GO is assumed to facilitate the dispersion of GO in solvent and significant enhancement of the corrosion protective property of epoxy resin coatings. And the combination of sulfonic groups in SAT with the active sites on the surface of GO makes it easier and more homogeneous to disperse in the aqueous solution. Moreover, the GO/SAT-coated epoxy resin composite showed the most outstanding electrochemical and corrosion protective properties with a dosage of 10 mg of GO. Besides, the coating material also exhibited an enhancement of hydrophobic property which was helpful to protect the Q235 steel electrode avoiding the corrosion attack. ACKNOWLEDGEMENTS This work was supported by the Natural Science Foundation of China (No.21376282, 51525903, 21275084, 41476083, 21675092), Sail plan of Guangdong, China (No.2015YT02D025), National Science Fund for Distinguished Young Scholars (51525903), 863 program (No. 2015AA034404), Marine science and technology projects of Huangdao district (2014-4-1), and National Basic Research Program of China (No. 2014CB643304).



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TOC graphic


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Figure 1 Schematic illustration of the preparation process for GO/SAT-coated epoxy resin composite coatings.



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Figure 2 FT-IR spectra of Aniline Trimer (A) and Sulfonated Aniline Trimer (B).


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Figure 3 SEM (A), TEM (B) images of GO and SEM (C), TEM (D) images of GO/SAT composite.



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Figure 4 Nyquist plots (A) Bode plots (B) (C) of Q235 electrodes taken after EIS measurement and Tafel plots (D) of epoxy resin (ER) without additions and with addition of AT, SAT, GO and GO-SAT.


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Figure 5 Electrical equivalent circuit diagrams of Q235 electrodes taken after electrochemical measurements without additives and with addition of AT, SAT (A) and GO-SAT (B).



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Figure 6 Nyquist plots (A) Bode plots (B) (C) of Q235 electrodes taken after EIS measurement and Tafel plots (D) with dosages of 6, 8, 10, 12 and 14 mg GO in GO/SAT coatings.


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Figure 7 Contact angles of bare Q235 electrode (A), with pure epoxy resin (B) and with GO/SAT-coated epoxy resin (C).



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Figure 8 Schematic of pure epoxy resin (A), GO/SAT-coated epoxy resin composite coating (B) during corrosion process.


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Figure 9 Digital images of the pre-processed Q235 steel electrode (A), Q235 steel electrode with GO/SAT-coated epoxy resin (B), Q235 steel electrode of removing the coatings after electrochemical measurements (C), pre-processed Q235 steel electrode after electrochemical measurements (D).


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