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Anticorrosion performance of 4-fluoro phenol functionalized graphene oxide nanocomposite coating on mild steel
Journal Pre-proof Anticorrosion performance of 4- fluoro phenol functionalized graphene oxide nanocomposite coating on mild steel Saurav Ramesh Nayak, Mahesh Bhaskar Hegde, Kikkeri N Mohana
PII:
S0022-1139(19)30176-9
DOI:
https://doi.org/10.1016/j.jfluchem.2019.109392
Reference:
FLUOR 109392
To appear in:
Journal of Fluorine Chemistry
Received Date:
28 May 2019
Revised Date:
5 October 2019
Accepted Date:
8 October 2019
Please cite this article as: Nayak SR, Hegde MB, Mohana KN, Anticorrosion performance of 4- fluoro phenol functionalized graphene oxide nanocomposite coating on mild steel, Journal of Fluorine Chemistry (2019), doi: https://doi.org/10.1016/j.jfluchem.2019.109392
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Anticorrosion performance of 4- fluoro phenol functionalized graphene oxide nanocomposite coating on mild steel
Saurav Ramesh Nayak, Mahesh Bhaskar Hegde, Kikkeri N Mohana*
Department of Studies in Chemistry, University of Mysore, Mysuru- 570006, Karnataka, India.
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*[email protected]
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Graphical abstract
Highlights
Graphene oxide (GO) is functionalized with 4-fluoro phenol by carrying out esterification. Functionalized graphene oxide (FGO) was characterized by FTIR, XRD, Raman, contact angle, TEM and SEM.
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Different wt.% of FGO+epoxy (EP) composites were coated on the surface of mild steel by spin coating. FGO+EP composites were showing more corrosion resistance than pure EP in 3.5% NaCl medium.
Abstract
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The present work is intended to enhance the anticorrosion performance of epoxy resin (EP) coating on mild steel (MS) by incorporating the functionalized graphene oxide as nanofiller material. In this context the graphene oxide (GO) was functionalized using 4- fluoro phenol and the functionalized graphene oxide (FGO) was well characterized by Fourier-transform infrared (FT-IR) spectroscopy, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectral studies. The thermal stability of FGO is ascertained by thermogravimetric analysis (TGA) and compared with that of GO. The anticorrosion behavior of epoxy (EP) and EP with different wt.% of FGO on MS samples in 3.5% NaCl solution was evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. The electrochemical data clearly showed that addition of 0.25 wt.% of FGO into EP drastically increases the corrosion resistance of the coating by 30.46% when compared to pure EP coating. Well dispersed composite coating effectively increases the barrier protection properties by blocking the pathway of the corrosive electrolyte to reach the metal-coating interface. Keywords: Corrosion resistance coating; graphene oxide; mild steel substrate; electrochemical impedance spectroscopy.
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1. Introduction
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Mild steel is widely used in different fields because of their unique properties. Unfortunately, steel undergoes corrosion very easily when it comes in contact with corrosive electrolytes. Metallic corrosion is one of the serious issues that people are facing ever since the use of metals, with huge losses in materials and the economy too. Thus, corrosion studies on steel are of significant interest due to its collective existence in many fields. Hence, the design of anticorrosion coatings with superior protection efficiency is still a hot topic of advanced research [1]. Graphene is a single-layered two-dimensional (2D) graphitic carbon material arranged in a honeycomb lattice. Graphene and graphene-based composites have created massive attention in the last few years after its discovery by Novoselov and coworkers in 2004 by mechanical exfoliation [2]. Graphene can be prepared by different methods such as thermal annealing [3] chemical vapor deposition [4], unzipping of carbon nanotubes [5], electrochemical [6],
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solvothermal [7], thermal decomposition [8], calcination [9], etc. [10]. Graphene possesses superior thermal, electrical and mechanical properties with chemical firmness due to its one atom thick planer sheet-like structure with hexagonally arranged sp2 bonded carbon atom network. Graphene has exceptional advantages such as high surface area, excellent electrical conductivity, durable mechanical properties, high thermal conductivity, large aspect ratio and outstanding transparency. Graphene and graphene-based materials have been widely studied in various fields including hydrogen storage, rechargeable batteries, supercapacitors, sensors, transparent electrodes, drug delivery and catalysis [11-15]. Recently, graphene/graphene oxide has attracted great attention to corrosion protection [16-18]. Graphene-based materials such as graphene, graphene oxide (GO) and modified graphene derivatives present excellent corrosion protection properties by their ultra-thin layered structure and very high aspect ratio. The presence of functional groups like epoxy, hydroxyl and carboxyl in GO nanosheet makes them suitable for functionalization with organic and inorganic materials. The presence of these functional groups at basal planes of GO assist the distribution of GO in the epoxy matrix and provide covalent bonding with epoxy networks [19]. In this regard, GO-based fillers have attracted considerable attention as they attain a higher degree of nanofiller dispersion in polymer matrix and prevent GO aggregation due to strong π-π interactions and Van der Waals forces between GO sheets.
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The conventional epoxy coatings are porous and allow corrosive electrolytes to infiltrate through them. So, nanofillers are mixed with epoxy and used to decrease the rate of corrosion. Nanocomposites synthesized by incorporating graphene/graphene oxide (GO) into polymers have been well established as barrier coating material over mild steel substrates [20-22]. Recent studies have clarified that the incorporation of graphene/GO in conventional protective metal coatings considerably advances their corrosion resistance performance [23]. This is due to alterations in morphology and microstructure of the coatings in addition to the chemical inertness and impermeability towards the electroactive media.
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Qian et al. [24] reported amine-terminated polyether functionalized GO and mixed with polyurea matrix via in situ polymerization. They revealed that the presence of modified GOs in the polymer coatings resulted in improved mechanical properties as well as thermal stability when compared to pure polyurea. Ding et al. [25] reported graphene-modified low zinc waterborne epoxy resin coating on steel surface. They found that graphene forms many isolation layers to obstruct the diffusion of corrosive particles. Abbas et al. [26] reported fluorine modified graphene nanofiller which were coated on copper substrate by drop-casting method. The fabricated film shows super hydrophobic property with water and good protection to the copper substrate from the corrosive electrolytes in 3.5% NaCl medium. Mi-Seon Park et al. [27] showed that fluorinated graphene oxide exhibits better thermal stability, high dispersibility and good adhesion property. Mathkar et al. [28] reported fluorinated graphene oxide and highly fluorinated graphene oxide, which were synthesized from fluorinated
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graphite, in which aliphatic C–F bonds within the aromatic area of the graphene basal plane exists in addition to epoxy, hydroxyl and carbonyl functional groups. They also reported that the prepared material has superamphiphobic properties because of the presence of fluorine on the surface of GO. Yang et al. [29] incorporated the fluorographene nanofiller into epoxy which forms a superhydrophobic epoxy coating. This coating enhances the corrosion protection performances of epoxy by repelling water molecules. Also, fluorographene nanofiller can effectively prevent the corrosive medium from permeating through the coating matrix to the copper/coating interface. However the anticorrosion behaviors of 4-fluoro phenol functionalized GO for MS has not been explored.
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In view of hydrophobic character and good anticorrosion behavior of fluorographene nanofillers, the present work, for the first time we have functionalized the GO with 4-fluoro phenol by a simple and cost effective esterification method and employed as a nanofiller into epoxy resin. The product obtained was confirmed by FT-IR spectroscopy, XRD and Raman spectroscopy. Different wt.% of FGO are dispersed with epoxy resin and coated on the MS substrate using spin coating technique. The surface morphology was studied by SEM and TEM analysis. The electrochemical and potentiodynamic polarization results indicated that the prepared FGO+EP nanocomposite (0.25 wt.% of FGO) shows excellent barrier and corrosion protection properties. 2. Experimental
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2.1 Materials and instruments
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4-fluoro phenol, hydroxy benzotriazole (HOBt) and N-(3-dimethylaminopropyl)-N1– ethylcarbodiimide hydrochloride (EDC.HCl) were purchased from Sigma Aldrich. All other chemicals used were of reagent grade. Araldite epoxy resin and hardener were purchased from Huntsman Advanced Materials. The mild steel samples of measurement 5 cm 2 cm 0.3 cm were used for corrosion studies. The chemical compositions of the mild steel specimens were given in Table 1.
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The synthesized GO and FGO were characterized by FT-IR, XRD, SEM, TEM, TGA and Raman spectral studies. FT-IR spectra were recorded using the Nicolet-5700 FT-IR spectrophotometer. The powder X-ray diffraction patterns were recorded using Rigaku miniflex II desktop X-ray diffractometer (Cu-K radiation, λ= 1.54A˚) with a scan rate of 0.02 /s ranging from 0 to 60. TGA was done using SDT Q600, TA Instruments, New Castle, Delawave, USA. TEM analysis was done using Jeol/JEM 2100 having 200 kV voltage capacity. The Raman spectra were recorded with the PeakSeeker ProTM Raman system. The sample was excited with inbuilt 785 nm wavelength laser and contact angle measurements were done using the KYOWA interface measurement and analysis system of FAMAS.
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Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements were done using an electrochemical workstation (EIS, CHI608E, Austin, USA). A conventional three-electrode cell consisting of |Ag/AgCl| as a reference electrode, platinum as a counter electrode and blank and coated MS sample as a working electrode with 1 cm2 exposed area. 2.2. Chemical modification of graphene oxide
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Modified Hummer's method was used to prepare GO. Chemical modification of GO was done by sonicating 1 g of GO in 20 ml DCM for 1 hour and then mixed with 2.5 g (0.0223 mol) 4-fluoro phenol, 4.27 g (0.0223 mol) of EDC.HCl and 3.01 g of HOBt (0.0223 mol). In the end, it was refluxed with tri-ethyl amine for 24 hours under the nitrogen atmosphere in an ice bath with continuous stirring. The resultant mixture was washed with water followed by ethanol. The schematic diagram for the preparation of FGO is shown in Fig. 1a and possible mechanism during the reaction is represented in Fig. 1b.
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2.3. Preparation of nanocomposite
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3. Results and discussion
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0.1 wt. % and 0.25 wt. % of FGO was dispersed in ethanol using ultra-sonicator for 2 h. After addition of epoxy resin to the beaker it was again sonicated for 4 hours to enable the transmission of FGO from ethanol to epoxy medium. The ethanol was then removed by heating at 85 °C for 4 h, and sonication was continued for 2 h. Hardener was added to the FGO-epoxy resin (FGO+EP) mixture in a stoichiometric amount. This mixture was coated on the steel sample using the spin coating method and cured at room temperature for 24 h and then kept in hot air oven for 4 h at 100 °C.
3.1.1 FT-IR spectral studies
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FT–IR analysis was done to check the functionalization of GO and also to examine the ester bond formation. FT–IR spectra of GO and FGO are presented in Fig. 2. The distinguishing peaks obtained at different spectral range show the main functional groups present in GO and FGO. The characteristic broad peak displayed at 3414 cm−1 in GO is that of acidic O-H bond present in it. The peak of GO at 1730 cm−1 and 1389 cm−1 correspond to the stretching frequency of C=O and C-O bonds of COOH groups. The peak at 1230 cm−1 in GO corresponds to stretching of the C-O-C bond related to the epoxy group present in it. The absorption frequencies of =C-H and aromatic C=C can be observed near 2925 cm−1 and 1624 cm−1 respectively. This confirms the presence of hydroxyl, carboxylic and epoxy groups in the synthesized GO nanosheets which are in agreement with the results that are reported in the literature [30,31].
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The possible reaction between 4-fluoro phenol and hydroxy group of the carboxyl present in GO is by ester bond formation. FT–IR analysis of FGO shows a peak at 1722 cm−1 representing the presence of –C=O (ester) bond stretching frequency and the small peaks were detected in the range of 1050 cm−1 – 1210 cm−1 indicate the presence of –C-O stretching frequency of ester bond. A strong peak at 1506 cm−1 shows the stretching frequency of the newly formed C-O bond. The peaks at 1711 cm−1 and 1400 cm−1 relate to C=O and C-O bonds of carbonyls of the ester bond, respectively. The peaks at 1221 cm−1 and 1646 cm−1 correspond to the stretching frequency of epoxy (C-O-C) and C=C groups, respectively [32]. 3.1.2 Raman spectroscopy
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Raman spectra of GO and FGO were recorded to detect the structural variations and chemical functionalization of GO. The Raman spectrum of the GO contains two characteristic peaks D (~1356 cm-1) and G (~1578 cm-1) as shown in Fig. 3. The D band refers to the structural defect present in the graphitic structured material and the G band refers to doubly degenerate E2g mode and shows the presence of isolated double bonds. Besides, the value of the intensity ratio (ID/IG) is an effective method to evaluate the degree of disorder and defects within carbon materials. The higher the value of ID/IG the greater will be the degree of disorders and defects [18, 33, 34]. The D and G band of FGO are detected at 1347 cm−1 and 1606 cm−1, respectively. An intense 2D peak detected for FGO at 2660 cm-1 – 2800 cm-1 corresponds to the overtone of the D peak. The intensity ratio of D and G bands (ID/IG) quantify the structural distortion for carbon materials. ID/IG ratio of the GO is 0.9523 (less than 1) whereas ID/IG ratio of FGO is 1.0255 (more than 1). The slight increase in the ratio of ID/IG is not only due to the intercalation associated strain, but also notifies the functionalization of the GO.
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3.1.3 X-ray diffraction (XRD) studies
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Fig 4 shows the XRD patterns for GO and FGO. It is witnessed that GO shows a sharp and intense peak at 2θ=11.3°, which corresponds to a well-arranged layer structure with a dspacing of 8.09 Å based on Bragg's equation [35, 36]. During the functionalization of GO, the oxygen was intercalated between the GO layer and 4-fluoro phenol. So, the Bragg’s angle of FGO gets moved towards the higher diffracted angle 2θ (24.42°) with the analogous d-spacing of 4.154 Å with a notable decrease in peak intensity when compared to GO. The significant change in the d-spacing specifies the new bond formation between GO and 4-fluoro phenol. 3.1.4 Thermal analysis The thermal effect on GO and FGO has been studied by thermogravimetric analysis. Fig. 5 shows the thermogravimetric curves for the GO and FGO nanoparticles. The steady decrease in weight percent of GO (23 wt.%) at 100 C shows the elimination of moisture/water content present in the GO. The significant weight loss of about 28% between 180 °C - 240 °C elucidates
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the loss of oxygen-containing functional groups such as hydroxyl (-OH), carboxyl (-COOH) and epoxy (C-O-C) groups between the layers of GO nanosheets which were acquainted by oxidative treatment [37]. For temperature ranging from 240 °C to 800 °C, there is a weight loss of about 16 wt. % due to the decay of the remaining carbonaceous component of the graphene skeleton. In the TGA curve of FGO, the weight loss in the first stage narrowed to 9.8 wt.% upto 100 ºC due to the elimination of water molecules. A moderate weight loss of about 21 wt. % (88 wt.% to 67 wt.%) from 220 °C - 490 °C is because of the pyrolysis of oxygen-containing hydroxyl, carboxyl, epoxy and ester (-COOR) groups attached to the edge of GO nanosheets [38]. After 500 °C, the weight loss is considerably less because of the decomposition of carbonyls. From 500 °C -800 °C there is a weight loss from 67 wt. % to 56 wt. %. Hence the TG analysis shows that the synthesized FGO has good thermal stability compared to that of GO.
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3.2 Surface Characterization of coated sample 3.2.1. Transmission electron microscopic analysis
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Fig. 6 displays the morphology and microstructure of FGO which were examined by transmission electron microscopy (TEM). TEM images in Fig.6 (a-b) show that the sheets typically looked wrinkled with a lot of folds that are indicative of the ultrathin nature of the functionalized GO, whereas Fig 6c displays darker FGO which is because of its thickness. The Selected Area Electron Diffraction (SAED) pattern obtained from TEM is shown in Fig. 6d. The diffuse ring pattern formed shows that the synthesized FGO is amorphous [39]. Thus from the TEM images, it is evident that the size of FGO lies within the nano range. 3.2.2. Scanning electron microscopic (SEM) analysis
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Fig. 7 shows the microstructure and morphologies of the FGO and FGO+EP composites by SEM analysis. Fig 7 (a-c) show low and high-resolution SEM images of powder FGO, The obtained powder consists of overlapped large sheets. Its surface is comparatively smooth and contains some slight wrinkles, indicating that the FGO nanosheets favorably tend to allow the strong interaction between each other, whereas Fig 7 (d) displays the SEM images of the FGO+EP nanocomposite coated specimen. It can be observed from this image that FGO is uniformly dispersed and with epoxy, it forms a good nanocomposite coating exhibit compact and crack-free morphology without any defects indicating it has good interfacial interaction of FGO with the epoxy [40]. 3.2.3. Contact angle measurement The wetting behavior of water on FGO+EP coated specimen and blank MS specimen were observed at room temperature. A water droplet of about 0.5L was injected on the surface of the coated specimen by a computerized micro-syringe. The shapes of water droplets are recorded by a charge coupled device (CCD) camera and transformed into an enlarged image. The
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images are shown in Fig. 8. The water droplet injected on the surface of the blank specimen formed an angle of about 75 whereas FGO+EP coated specimen gives 86. This clearly shows that the hydrophobicity of the MS specimen increases on coating with FGO+EP composite [41,42], which means it prevents moisture/ corrosive electrolytes from striking the surface of MS specimen by diminishing the adsorption of corrosive electrolyte and prevents it from undergoing corrosion. 3.3 Corrosion studies
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The influence of FGO on the anti-corrosion property and barrier performance of epoxy coating on the mild steel in 3.5 wt.% NaCl solution were studied by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. The electrochemical measurements were carried out using mild steel (blank), mild steel coated with epoxy (EP) and mild steel coated with different wt. % of FGO dispersed in epoxy (FGO+EP) in a 3.5% NaCl solution. Using Open Circuit Potential (OCP)-Time technique the constant potential values of the working electrodes were measured potentiostatically. FGO+EP nanocomposite coatings display higher (more positive) OCP values compared to that of EP and blank. The shift of OCP from lower values to higher values {blank < EP < [FGO+EP (0.1 wt. %)] < [FGO+EP (0.25 wt.%) 1st day] > [FGO+EP (0.25 wt. %) 8th day] > [FGO+EP (0.25wt.%) 15th day]} is the sign of resistance to the corrosive electrolyte diffusion in the metal-coating interface [43]. The shift of OCP to negative values after a few days of immersion is a sign of corrosive electrolyte diffusion to the metal-coating interface. The OCP Vs time graph of blank, EP, FGO+EP (0.1 wt.%), FGO+EP (0.25 wt.%) 1st day, FGO+EP (0.25wt. %) 8th day, FGO+EP (0.25wt.%) 15th are plotted in Fig. 9. This specifies that FGO nanocomposite in epoxy coating has better barrier property by decreasing the diffusion of corrosive electrolyte to the surface of the metal.
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3.3.1. Electrochemical impedance spectroscopy
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The Nyquist and Bode plots of the samples which were dipped in 3.5% NaCl solution are presented in Fig. 10 and Fig. 12. Fig. 10 shows the EIS data in a complex plane diagram (Nyquist plot), where the plot of the real component (Z′) of the impedance is plotted against the imaginary component (Z"). From the Nyquist plot, it is evident that FGO+EP coated sample has the biggest capacitive loop compared to EP and blank samples which show that the corrosion resistance values for FGO+EP coated on the mild steel specimens were found to be much higher compared to the blank mild steel and mild steel coated with epoxy. An electrical equivalent circuit was used to get the replicated impedance data. The capacitive loops shown in the Nyquist plots are not perfect semi-circles. This phenomenon is known as the dispersing effect [44]. Because of this fact, the double-layer capacitance does not behave as an ideal capacitor, so a constant phase element (CPE) is normally used instead of a capacitor. The virtual data produced using the electrical equivalent circuit is in good agreement with the experimentally obtained data.
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The obtained equivalent electrical circuit is shown in Fig. 11, i.e. [Rs(Qc(Rc(QdlRct)))], where Rs is the resistance of the solution (the resistance between working and reference electrodes), Qc is the constant phase element of the coating, Rc is the coating resistance, Qdl is the constant phase element of double layer and Rct is the charge transfer resistance. The higher Rct and Rc values corresponding to the FGO+EP sample represent the blockage of the corrosive electrolyte pathway by the FGO nanocomposites. The values obtained by fitting the above-said circuit are shown in Table 2. From the Nyquist plot, it is clear that the size of capacitive loops decreases with an increase in an exposure time of the sample in corrosive electrolyte [45]. The Rct values of the coated sample decrease with an increase of immersion time indicating the decrease of barrier property of the coating with time. This is due to the deterioration of coating during a longer period of exposure.
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From the bode plot, modulus of impedance at low frequency (Z0.01Hz) and phase angle at high frequency ( 0.1 MHz) were notified for all the samples. Fig. 12 (a) represents the Bode plot of blank, EP, 0.1 wt.% of FGO in EP and 0.25 wt.% of FGO in EP in 3.5 wt.% NaCl solution. The impedance at lower frequency displays that both the FGO+EP samples are having higher impedance compared to blank and EP. The well-dispersed FGO+EP coating shields the diffusion of corrosive electrolytes by blocking the pores and microscopic defects [46, 47]. In comparison, 0.25 wt.% of FGO+EP coated sample shows better performance than 0.1 wt.% of FGO+EP coated sample in 3.5 wt.% NaCl solution. It is observed from the Bode plot that Z0.01Hz of 0.25 wt. % of FGO+EP is 9.7804 106 and that of 0.1 wt. % of FGO+EP is 6.4818 104 whereas epoxy and blank samples showed relatively less values of initial impedance. The Bode plots of 0.25 wt.% of FGO+EP with different immersion time is shown in Fig 12 (b). After 7 days of immersion in 3.5 wt.% NaCl solution, FGO+EP sample showed a small decrease in the Z0.01Hz value (2.2872 106), attributed to the permeation of corrosive electrolyte into the coating. But after 14 days of immersion theZ0.01Hz value of (FGO+EP) decreases significantly (3.3238 105) due to the delamination of the coating.
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The phase angle at high frequency is another factor for calculating the corrosion resistant performance of coatings. The corrosion resistances of the coatings were examined using phase angle at high frequency ( 0.1 MHz) and presented in Fig. 12 (a). Both 0.1 wt.% and 0.25 wt.% FGO+EP samples showed higher phase angle values suggesting their excellent barrier properties compared to that of EP and blank samples. The 0.1 MHz of 0.25 wt.% FGO+EP was observed at 89 while that of 0.1 wt.% is observed at 73 which clearly shows that the diffusion of corrosive electrolyte is less in 0.25 wt. % FGO dispersed in EP. The phase angle at different immersion time with 0.25 wt.% FGO+EP is presented in Fig 12 (b). After 7 days of immersion, 0.1 MHz value of FGO+EP is observed at 82 which show that the coating was still stable and the barrier property of the coating is very good. But after 15 days of immersion, the phase angle suddenly decreases to 21 because of the delamination of the coating.
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3.3.2. Polarization studies
𝑃𝑒𝑓𝑓 % =
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Potentiodynamic polarization studies of blank MS specimen, EP and FGO+EP coated samples were done in a 3.5 % NaCl solution. Potentiodynamic polarization curves (Tafel plots) obtained are shown in Fig. 13. From Tafel plot corrosion current density (Icorr) was determined by extrapolating the linear portion of the curve to corrosion potential (Ecorr) with the help of the CHI608E software. The slope of the tangent of the cathodic curve is known as cathodic Tafel slope (bc) and the slope of the tangent of the anodic curve is anodic Tafel slope (ba). The results obtained are detailed in Table 3. According to Tafel plots, the nanocomposite coatings show quite different behavior from that of neat epoxy coating. The corrosion potential of the FGO+EP coated sample shows an extensive positive shift when compared to the EP coated specimen which shifts to more positive potential compared to that of a blank sample. The icorr of the FGO+EP coated sample decreases considerably in comparison to that of blank and EP samples. Whereas FGO+EP (0.25 wt.%) show less icorr compared to that of FGO+EP (0.1 wt.%). Also, it appears a more positive shift in Ecorr of FGO+EP (0.25 wt.%) compared to that of Ecorr of FGO+EP (0.1 wt.%). Meanwhile, both anodic Tafel slope and cathodic Tafel slope have changed by incorporation of FGO nanofillers in epoxy, showing the corrosion protection properties of coatings has changed compared to that of EP and blank coatings. A similar observation occurred in the corrosion rate, where a blank sample was showing a high corrosion rate compared to that of EP samples. The poor protection performance of EP coating is due to the formation of pores in the coating. However, these pores were effectively blocked by the FGO nanofillers in FGO+EP coating and result in a good protection performance. Thus by restricting the permeation of corrosive electrolytes through the coating, it increases the pathway of the corrosive electrolyte to the metal-coating interface. This suggests that the FGO nanocomposite coating acts as a strong protective layer that resists the corrosion of mild steel effectively in 3.5 wt.% NaCl solution [48]. The protection efficiency (Peff %) of the coating can be calculated using the equation (1), 𝑖𝑐𝑜𝑟𝑟(𝑏𝑙𝑎𝑛𝑘) − 𝑖𝑐𝑜𝑟𝑟(𝑐𝑜𝑎𝑡) 𝑖𝑐𝑜𝑟𝑟(𝑏𝑙𝑎𝑛𝑘)
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(1)
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where 𝑖𝑐𝑜𝑟𝑟(𝑏𝑙𝑎𝑛𝑘) and 𝑖𝑐𝑜𝑟𝑟(𝑐𝑜𝑎𝑡) are the corrosion current density of blank MS specimen and that of the coated specimen, respectively. Polarization curves of the 0.25 wt.% of FGO samples immersed in 3.5 wt.% NaCl solution with the different intervals of time are also plotted in Fig. 13. From Table 3 it is seen that for the 0.1 wt.% of FGO+EP nanocomposite, protection efficiency was about 91.08% and that of 0.25 wt.% of FGO+EP nanocomposite was about 99.73%. Thus FGO+EP (0.25 wt.%) enhances up to 20% corrosion protection behavior in comparison with that of FGO+EP (0.1 wt.%) coating. FGO+EP (0.25 wt.%) is having higher protection efficiency (Peff) compared to that of FGO+EP (0.1 wt.%) and blank. It renders effective coating at the initial days of immersion (up to 8 days), with a small decline in the Peff by
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blocking corrosive electrolyte to the metal/coating interface. But, the Peff of the samples gradually decreased on prolonged immersion in a corrosive electrolyte owing to the decrease in the barrier performance of the coating. On the 15th day, it shows a significant decrease in the Peff. In previous work, we reported [43] functionalization of GO with 4-nitroaniline through amide bond formation, where the addition of FGO to epoxy was showing 99.58% of protection efficiency but after one week of immersion, it decreases drastically to 89.58%. While in the present work, addition 4-fluorophenol functionalized GO to the epoxy matrix shows 99.73% protection efficiency and maintains its protection efficiency by displaying about 98.53% even after one week of immersion.
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It is a well-known fact that the graphene oxide sheet is impermeable to gases and water molecules due to the high surface area. However, FGO composites provide better barrier protection properties against oxygen and water diffusion. The diffusion of corrosive electrolyte into the metal-coating interface is because of the delamination of coating on metals or because of the small pores/defects present in the coating. Once the delamination process starts, the rate of corrosion increases. This results in the oxidation and reduction reactions to take place at the metal-coating interface, which causes corrosion phenomenon to occur on the metal substrate. This is the reason for the significant decrease in Peff. From Peff it is evident that the inclusion of FGO in EP increases the barrier and corrosion protection properties.
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Conclusion
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The corrosion prevention ability of epoxy (EP) coating is successfully enhanced by dispersing the functionalized graphene oxide (FGO) into EP. The graphene oxide (GO) was functionalized with 4- fluoro phenol and characterized by FT-IR, XRD, SEM, TEM and Raman spectral studies. The TGA study confirms the increase in thermal stability of FGO when compared to GO. From EIS and Tafel results it is evident that the incorporation of FGO significantly increases the anticorrosion property and barrier property of EP. Addition of 0.25 wt.% of FGO into EP increases the corrosion resistance of the coating by 30.46 % when compared to pure EP coating.
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Conflict of Interest and Authorship Conformation Form Please check the following as appropriate: o
All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
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o
This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.
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The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript
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The following authors have affiliations with organizations with direct or indirect financial interest in the subject matter discussed in the manuscript:
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Acknowledgment
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One of the authors, Saurav Ramesh Nayak acknowledges the financial support provided by the University Grants Commission, New Delhi under the scheme UGC-JRF.
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Fig. 1: Schematic representation of a) functionalization of graphene oxide, b) possible reaction mechanism of the ester bond formation. Fig. 2: FT-IR spectra of graphene oxide and functionalized graphene oxide with labelled functional groups. Fig. 3: Representative Raman spectra of graphene oxide and functionalized graphene oxide with labelled D band and G band. Fig. 4: XRD patterns of GO and FGO.
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Fig. 5: TGA thermograms of graphene oxide and functionalized graphene oxide.
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Fig. 6: TEM micrographs of (a-c) functionalized graphene oxide, (d) selected area electron diffraction pattern of functionalized graphene oxide.
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Fig. 7: SEM micrographs of (a-c) FGO powder with different magnification, (d) FGO+EP coated on MS specimen.
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Fig. 8: Contact angle images of blank and FGO+EP coated specimens by placing 0.5 L water droplet on the surface of the specimen.
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Fig. 9: OCP c of blank, EP, FGO+EP (0.1 wt.%) and FGO+EP (0.25 wt.%) at different intervals of time in 3.5% NaCl solution. Fig. 10: Nyquist plots of blank, EP and FGO+EP coated specimens at different immersion time in 3.5% NaCl solution.
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Fig. 11: Representation of fitted equivalent circuit for EIS experimental data. Fig. 12a: Bode plots of blank MS, EP, FGO+EP (0.1 wt.%) and FGO+EP(0.25 wt.%) coated specimens in 3.5% NaCl solution. Fig. 12b: Bode plots of FGO+EP (0.25 wt.%) coated specimens for different time intervals in 3.5% NaCl solution.
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Fig. 13: Tafel plot of blank MS, epoxy and FGO + EP samples in 3.5% NaCl solution.
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OH
O
OH O OH
O
Hummers method Graphite
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Ultra sonication
O
4-fluoro phenol 80-90 °C EDC(1eq.), HOBt(1eq) DCM
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OH
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F
OH
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O
F
OH
FGO
Fig. 1b
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EDC.H+ C
N
N+
N
H
N+
N
H
H
C
N
NH
O
O
O
R
H+
O
R
H
HN
O
C
O O
R
OH
N
O
R R O
R1O
O
N
O N
N
N OR1
N
N N
HOBt
N N HO
R1
where
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R-COOH = graphene oxide HO-R1 = F
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HO
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Fig. 2
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Fig. 8
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Fig. 10
Fig. 11
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Fig. 12 b
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Fig. 13
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Table 1: Chemical composition of the mild steel specimen. Chemical
S
Mn
C
Si
P
Wt. %
0.012%
0.130% 0.050% 0.050% 0.010%
Al
Fe
0.100%
99.600%
n
(-1 cm-2 sn)
Yo (. cm2)
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Yo
Qdl
Rct
n
(-1 cm-2 sn)
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Sample
Rc
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Table 2: The electrochemical parameters obtained from fitting EIS equivalent circuit for blank, EP, FGO+EP and FGO+EP (8th day) which are immersed in 3.5% NaCl solution.
(. cm2)
6.24710-6
0.391
6.605102
4.39610-5
0.781
9.417102
EP
2.35410-7
0.641
2.683104
5.22110-7
0.744
1.087104
FGO+EP
1.01110-5
0.956
1.304104
5.02010-5
0.465
1.650105
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(0.1 wt. %)
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Blank
FGO+EP
1.09610-9
0.955
1.062106
2.81210-7
0.500
2.468107
3.97010-8
0.715
7.750105
1.89710-9
0.814
1.500106
5.49410-7
0.502
2.058103
1.08510-5
0.688
1.573104
(0.25 wt. %) FGO+EP
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(0.25 wt. %) (8th day)
FGO+EP
(0.25 wt. %) (14th day)
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Ecorr
icorr
bc
ba
Corr. Rate
(V)
(A/cm2)
(V/dec)
(V/dec)
(Angs/min)
-0.594
2.32810-6
10.227
11.406
5.12010-1
EP
-0.429
7.15510-7
3.544
5.888
1.57310-1
FGO+EP
-0.153
7.71410-9
5.123
4.848
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-0.110
6.13910-9
7.365
4.050
1.35010-3
99.73
-0.440
1.93910-7
2.885
7.612
4.26510-2
98.53
-0.453
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Table 3: Polarization parameters for blank, EP and FGO+EP which are immersed in 3.5% NaCl solution.
1.25810-6
8.271
2.76510-1
45.90
Sample
Blank
FGO+EP
FGO+EP
-p
(0.25 wt. %)
(0.25 wt. %)
FGO+EP (0.25 wt. %)
91.08
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(15th day)
4.409
69.27
lP
(8th day)
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(0.1 wt. %)
1.70310-3
(Peff%)
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PII:
S0022-1139(19)30176-9
DOI:
https://doi.org/10.1016/j.jfluchem.2019.109392
Reference:
FLUOR 109392
To appear in:
Journal of Fluorine Chemistry
Received Date:
28 May 2019
Revised Date:
5 October 2019
Accepted Date:
8 October 2019
Please cite this article as: Nayak SR, Hegde MB, Mohana KN, Anticorrosion performance of 4- fluoro phenol functionalized graphene oxide nanocomposite coating on mild steel, Journal of Fluorine Chemistry (2019), doi: https://doi.org/10.1016/j.jfluchem.2019.109392
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Anticorrosion performance of 4- fluoro phenol functionalized graphene oxide nanocomposite coating on mild steel
Saurav Ramesh Nayak, Mahesh Bhaskar Hegde, Kikkeri N Mohana*
Department of Studies in Chemistry, University of Mysore, Mysuru- 570006, Karnataka, India.
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*[email protected]
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Graphical abstract
Highlights
Graphene oxide (GO) is functionalized with 4-fluoro phenol by carrying out esterification. Functionalized graphene oxide (FGO) was characterized by FTIR, XRD, Raman, contact angle, TEM and SEM.
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Different wt.% of FGO+epoxy (EP) composites were coated on the surface of mild steel by spin coating. FGO+EP composites were showing more corrosion resistance than pure EP in 3.5% NaCl medium.
Abstract
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The present work is intended to enhance the anticorrosion performance of epoxy resin (EP) coating on mild steel (MS) by incorporating the functionalized graphene oxide as nanofiller material. In this context the graphene oxide (GO) was functionalized using 4- fluoro phenol and the functionalized graphene oxide (FGO) was well characterized by Fourier-transform infrared (FT-IR) spectroscopy, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectral studies. The thermal stability of FGO is ascertained by thermogravimetric analysis (TGA) and compared with that of GO. The anticorrosion behavior of epoxy (EP) and EP with different wt.% of FGO on MS samples in 3.5% NaCl solution was evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. The electrochemical data clearly showed that addition of 0.25 wt.% of FGO into EP drastically increases the corrosion resistance of the coating by 30.46% when compared to pure EP coating. Well dispersed composite coating effectively increases the barrier protection properties by blocking the pathway of the corrosive electrolyte to reach the metal-coating interface. Keywords: Corrosion resistance coating; graphene oxide; mild steel substrate; electrochemical impedance spectroscopy.
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1. Introduction
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Mild steel is widely used in different fields because of their unique properties. Unfortunately, steel undergoes corrosion very easily when it comes in contact with corrosive electrolytes. Metallic corrosion is one of the serious issues that people are facing ever since the use of metals, with huge losses in materials and the economy too. Thus, corrosion studies on steel are of significant interest due to its collective existence in many fields. Hence, the design of anticorrosion coatings with superior protection efficiency is still a hot topic of advanced research [1]. Graphene is a single-layered two-dimensional (2D) graphitic carbon material arranged in a honeycomb lattice. Graphene and graphene-based composites have created massive attention in the last few years after its discovery by Novoselov and coworkers in 2004 by mechanical exfoliation [2]. Graphene can be prepared by different methods such as thermal annealing [3] chemical vapor deposition [4], unzipping of carbon nanotubes [5], electrochemical [6],
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solvothermal [7], thermal decomposition [8], calcination [9], etc. [10]. Graphene possesses superior thermal, electrical and mechanical properties with chemical firmness due to its one atom thick planer sheet-like structure with hexagonally arranged sp2 bonded carbon atom network. Graphene has exceptional advantages such as high surface area, excellent electrical conductivity, durable mechanical properties, high thermal conductivity, large aspect ratio and outstanding transparency. Graphene and graphene-based materials have been widely studied in various fields including hydrogen storage, rechargeable batteries, supercapacitors, sensors, transparent electrodes, drug delivery and catalysis [11-15]. Recently, graphene/graphene oxide has attracted great attention to corrosion protection [16-18]. Graphene-based materials such as graphene, graphene oxide (GO) and modified graphene derivatives present excellent corrosion protection properties by their ultra-thin layered structure and very high aspect ratio. The presence of functional groups like epoxy, hydroxyl and carboxyl in GO nanosheet makes them suitable for functionalization with organic and inorganic materials. The presence of these functional groups at basal planes of GO assist the distribution of GO in the epoxy matrix and provide covalent bonding with epoxy networks [19]. In this regard, GO-based fillers have attracted considerable attention as they attain a higher degree of nanofiller dispersion in polymer matrix and prevent GO aggregation due to strong π-π interactions and Van der Waals forces between GO sheets.
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The conventional epoxy coatings are porous and allow corrosive electrolytes to infiltrate through them. So, nanofillers are mixed with epoxy and used to decrease the rate of corrosion. Nanocomposites synthesized by incorporating graphene/graphene oxide (GO) into polymers have been well established as barrier coating material over mild steel substrates [20-22]. Recent studies have clarified that the incorporation of graphene/GO in conventional protective metal coatings considerably advances their corrosion resistance performance [23]. This is due to alterations in morphology and microstructure of the coatings in addition to the chemical inertness and impermeability towards the electroactive media.
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Qian et al. [24] reported amine-terminated polyether functionalized GO and mixed with polyurea matrix via in situ polymerization. They revealed that the presence of modified GOs in the polymer coatings resulted in improved mechanical properties as well as thermal stability when compared to pure polyurea. Ding et al. [25] reported graphene-modified low zinc waterborne epoxy resin coating on steel surface. They found that graphene forms many isolation layers to obstruct the diffusion of corrosive particles. Abbas et al. [26] reported fluorine modified graphene nanofiller which were coated on copper substrate by drop-casting method. The fabricated film shows super hydrophobic property with water and good protection to the copper substrate from the corrosive electrolytes in 3.5% NaCl medium. Mi-Seon Park et al. [27] showed that fluorinated graphene oxide exhibits better thermal stability, high dispersibility and good adhesion property. Mathkar et al. [28] reported fluorinated graphene oxide and highly fluorinated graphene oxide, which were synthesized from fluorinated
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graphite, in which aliphatic C–F bonds within the aromatic area of the graphene basal plane exists in addition to epoxy, hydroxyl and carbonyl functional groups. They also reported that the prepared material has superamphiphobic properties because of the presence of fluorine on the surface of GO. Yang et al. [29] incorporated the fluorographene nanofiller into epoxy which forms a superhydrophobic epoxy coating. This coating enhances the corrosion protection performances of epoxy by repelling water molecules. Also, fluorographene nanofiller can effectively prevent the corrosive medium from permeating through the coating matrix to the copper/coating interface. However the anticorrosion behaviors of 4-fluoro phenol functionalized GO for MS has not been explored.
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In view of hydrophobic character and good anticorrosion behavior of fluorographene nanofillers, the present work, for the first time we have functionalized the GO with 4-fluoro phenol by a simple and cost effective esterification method and employed as a nanofiller into epoxy resin. The product obtained was confirmed by FT-IR spectroscopy, XRD and Raman spectroscopy. Different wt.% of FGO are dispersed with epoxy resin and coated on the MS substrate using spin coating technique. The surface morphology was studied by SEM and TEM analysis. The electrochemical and potentiodynamic polarization results indicated that the prepared FGO+EP nanocomposite (0.25 wt.% of FGO) shows excellent barrier and corrosion protection properties. 2. Experimental
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2.1 Materials and instruments
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4-fluoro phenol, hydroxy benzotriazole (HOBt) and N-(3-dimethylaminopropyl)-N1– ethylcarbodiimide hydrochloride (EDC.HCl) were purchased from Sigma Aldrich. All other chemicals used were of reagent grade. Araldite epoxy resin and hardener were purchased from Huntsman Advanced Materials. The mild steel samples of measurement 5 cm 2 cm 0.3 cm were used for corrosion studies. The chemical compositions of the mild steel specimens were given in Table 1.
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The synthesized GO and FGO were characterized by FT-IR, XRD, SEM, TEM, TGA and Raman spectral studies. FT-IR spectra were recorded using the Nicolet-5700 FT-IR spectrophotometer. The powder X-ray diffraction patterns were recorded using Rigaku miniflex II desktop X-ray diffractometer (Cu-K radiation, λ= 1.54A˚) with a scan rate of 0.02 /s ranging from 0 to 60. TGA was done using SDT Q600, TA Instruments, New Castle, Delawave, USA. TEM analysis was done using Jeol/JEM 2100 having 200 kV voltage capacity. The Raman spectra were recorded with the PeakSeeker ProTM Raman system. The sample was excited with inbuilt 785 nm wavelength laser and contact angle measurements were done using the KYOWA interface measurement and analysis system of FAMAS.
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Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements were done using an electrochemical workstation (EIS, CHI608E, Austin, USA). A conventional three-electrode cell consisting of |Ag/AgCl| as a reference electrode, platinum as a counter electrode and blank and coated MS sample as a working electrode with 1 cm2 exposed area. 2.2. Chemical modification of graphene oxide
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Modified Hummer's method was used to prepare GO. Chemical modification of GO was done by sonicating 1 g of GO in 20 ml DCM for 1 hour and then mixed with 2.5 g (0.0223 mol) 4-fluoro phenol, 4.27 g (0.0223 mol) of EDC.HCl and 3.01 g of HOBt (0.0223 mol). In the end, it was refluxed with tri-ethyl amine for 24 hours under the nitrogen atmosphere in an ice bath with continuous stirring. The resultant mixture was washed with water followed by ethanol. The schematic diagram for the preparation of FGO is shown in Fig. 1a and possible mechanism during the reaction is represented in Fig. 1b.
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2.3. Preparation of nanocomposite
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3. Results and discussion
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0.1 wt. % and 0.25 wt. % of FGO was dispersed in ethanol using ultra-sonicator for 2 h. After addition of epoxy resin to the beaker it was again sonicated for 4 hours to enable the transmission of FGO from ethanol to epoxy medium. The ethanol was then removed by heating at 85 °C for 4 h, and sonication was continued for 2 h. Hardener was added to the FGO-epoxy resin (FGO+EP) mixture in a stoichiometric amount. This mixture was coated on the steel sample using the spin coating method and cured at room temperature for 24 h and then kept in hot air oven for 4 h at 100 °C.
3.1.1 FT-IR spectral studies
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FT–IR analysis was done to check the functionalization of GO and also to examine the ester bond formation. FT–IR spectra of GO and FGO are presented in Fig. 2. The distinguishing peaks obtained at different spectral range show the main functional groups present in GO and FGO. The characteristic broad peak displayed at 3414 cm−1 in GO is that of acidic O-H bond present in it. The peak of GO at 1730 cm−1 and 1389 cm−1 correspond to the stretching frequency of C=O and C-O bonds of COOH groups. The peak at 1230 cm−1 in GO corresponds to stretching of the C-O-C bond related to the epoxy group present in it. The absorption frequencies of =C-H and aromatic C=C can be observed near 2925 cm−1 and 1624 cm−1 respectively. This confirms the presence of hydroxyl, carboxylic and epoxy groups in the synthesized GO nanosheets which are in agreement with the results that are reported in the literature [30,31].
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The possible reaction between 4-fluoro phenol and hydroxy group of the carboxyl present in GO is by ester bond formation. FT–IR analysis of FGO shows a peak at 1722 cm−1 representing the presence of –C=O (ester) bond stretching frequency and the small peaks were detected in the range of 1050 cm−1 – 1210 cm−1 indicate the presence of –C-O stretching frequency of ester bond. A strong peak at 1506 cm−1 shows the stretching frequency of the newly formed C-O bond. The peaks at 1711 cm−1 and 1400 cm−1 relate to C=O and C-O bonds of carbonyls of the ester bond, respectively. The peaks at 1221 cm−1 and 1646 cm−1 correspond to the stretching frequency of epoxy (C-O-C) and C=C groups, respectively [32]. 3.1.2 Raman spectroscopy
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Raman spectra of GO and FGO were recorded to detect the structural variations and chemical functionalization of GO. The Raman spectrum of the GO contains two characteristic peaks D (~1356 cm-1) and G (~1578 cm-1) as shown in Fig. 3. The D band refers to the structural defect present in the graphitic structured material and the G band refers to doubly degenerate E2g mode and shows the presence of isolated double bonds. Besides, the value of the intensity ratio (ID/IG) is an effective method to evaluate the degree of disorder and defects within carbon materials. The higher the value of ID/IG the greater will be the degree of disorders and defects [18, 33, 34]. The D and G band of FGO are detected at 1347 cm−1 and 1606 cm−1, respectively. An intense 2D peak detected for FGO at 2660 cm-1 – 2800 cm-1 corresponds to the overtone of the D peak. The intensity ratio of D and G bands (ID/IG) quantify the structural distortion for carbon materials. ID/IG ratio of the GO is 0.9523 (less than 1) whereas ID/IG ratio of FGO is 1.0255 (more than 1). The slight increase in the ratio of ID/IG is not only due to the intercalation associated strain, but also notifies the functionalization of the GO.
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3.1.3 X-ray diffraction (XRD) studies
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Fig 4 shows the XRD patterns for GO and FGO. It is witnessed that GO shows a sharp and intense peak at 2θ=11.3°, which corresponds to a well-arranged layer structure with a dspacing of 8.09 Å based on Bragg's equation [35, 36]. During the functionalization of GO, the oxygen was intercalated between the GO layer and 4-fluoro phenol. So, the Bragg’s angle of FGO gets moved towards the higher diffracted angle 2θ (24.42°) with the analogous d-spacing of 4.154 Å with a notable decrease in peak intensity when compared to GO. The significant change in the d-spacing specifies the new bond formation between GO and 4-fluoro phenol. 3.1.4 Thermal analysis The thermal effect on GO and FGO has been studied by thermogravimetric analysis. Fig. 5 shows the thermogravimetric curves for the GO and FGO nanoparticles. The steady decrease in weight percent of GO (23 wt.%) at 100 C shows the elimination of moisture/water content present in the GO. The significant weight loss of about 28% between 180 °C - 240 °C elucidates
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the loss of oxygen-containing functional groups such as hydroxyl (-OH), carboxyl (-COOH) and epoxy (C-O-C) groups between the layers of GO nanosheets which were acquainted by oxidative treatment [37]. For temperature ranging from 240 °C to 800 °C, there is a weight loss of about 16 wt. % due to the decay of the remaining carbonaceous component of the graphene skeleton. In the TGA curve of FGO, the weight loss in the first stage narrowed to 9.8 wt.% upto 100 ºC due to the elimination of water molecules. A moderate weight loss of about 21 wt. % (88 wt.% to 67 wt.%) from 220 °C - 490 °C is because of the pyrolysis of oxygen-containing hydroxyl, carboxyl, epoxy and ester (-COOR) groups attached to the edge of GO nanosheets [38]. After 500 °C, the weight loss is considerably less because of the decomposition of carbonyls. From 500 °C -800 °C there is a weight loss from 67 wt. % to 56 wt. %. Hence the TG analysis shows that the synthesized FGO has good thermal stability compared to that of GO.
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3.2 Surface Characterization of coated sample 3.2.1. Transmission electron microscopic analysis
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Fig. 6 displays the morphology and microstructure of FGO which were examined by transmission electron microscopy (TEM). TEM images in Fig.6 (a-b) show that the sheets typically looked wrinkled with a lot of folds that are indicative of the ultrathin nature of the functionalized GO, whereas Fig 6c displays darker FGO which is because of its thickness. The Selected Area Electron Diffraction (SAED) pattern obtained from TEM is shown in Fig. 6d. The diffuse ring pattern formed shows that the synthesized FGO is amorphous [39]. Thus from the TEM images, it is evident that the size of FGO lies within the nano range. 3.2.2. Scanning electron microscopic (SEM) analysis
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Fig. 7 shows the microstructure and morphologies of the FGO and FGO+EP composites by SEM analysis. Fig 7 (a-c) show low and high-resolution SEM images of powder FGO, The obtained powder consists of overlapped large sheets. Its surface is comparatively smooth and contains some slight wrinkles, indicating that the FGO nanosheets favorably tend to allow the strong interaction between each other, whereas Fig 7 (d) displays the SEM images of the FGO+EP nanocomposite coated specimen. It can be observed from this image that FGO is uniformly dispersed and with epoxy, it forms a good nanocomposite coating exhibit compact and crack-free morphology without any defects indicating it has good interfacial interaction of FGO with the epoxy [40]. 3.2.3. Contact angle measurement The wetting behavior of water on FGO+EP coated specimen and blank MS specimen were observed at room temperature. A water droplet of about 0.5L was injected on the surface of the coated specimen by a computerized micro-syringe. The shapes of water droplets are recorded by a charge coupled device (CCD) camera and transformed into an enlarged image. The
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images are shown in Fig. 8. The water droplet injected on the surface of the blank specimen formed an angle of about 75 whereas FGO+EP coated specimen gives 86. This clearly shows that the hydrophobicity of the MS specimen increases on coating with FGO+EP composite [41,42], which means it prevents moisture/ corrosive electrolytes from striking the surface of MS specimen by diminishing the adsorption of corrosive electrolyte and prevents it from undergoing corrosion. 3.3 Corrosion studies
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The influence of FGO on the anti-corrosion property and barrier performance of epoxy coating on the mild steel in 3.5 wt.% NaCl solution were studied by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. The electrochemical measurements were carried out using mild steel (blank), mild steel coated with epoxy (EP) and mild steel coated with different wt. % of FGO dispersed in epoxy (FGO+EP) in a 3.5% NaCl solution. Using Open Circuit Potential (OCP)-Time technique the constant potential values of the working electrodes were measured potentiostatically. FGO+EP nanocomposite coatings display higher (more positive) OCP values compared to that of EP and blank. The shift of OCP from lower values to higher values {blank < EP < [FGO+EP (0.1 wt. %)] < [FGO+EP (0.25 wt.%) 1st day] > [FGO+EP (0.25 wt. %) 8th day] > [FGO+EP (0.25wt.%) 15th day]} is the sign of resistance to the corrosive electrolyte diffusion in the metal-coating interface [43]. The shift of OCP to negative values after a few days of immersion is a sign of corrosive electrolyte diffusion to the metal-coating interface. The OCP Vs time graph of blank, EP, FGO+EP (0.1 wt.%), FGO+EP (0.25 wt.%) 1st day, FGO+EP (0.25wt. %) 8th day, FGO+EP (0.25wt.%) 15th are plotted in Fig. 9. This specifies that FGO nanocomposite in epoxy coating has better barrier property by decreasing the diffusion of corrosive electrolyte to the surface of the metal.
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3.3.1. Electrochemical impedance spectroscopy
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The Nyquist and Bode plots of the samples which were dipped in 3.5% NaCl solution are presented in Fig. 10 and Fig. 12. Fig. 10 shows the EIS data in a complex plane diagram (Nyquist plot), where the plot of the real component (Z′) of the impedance is plotted against the imaginary component (Z"). From the Nyquist plot, it is evident that FGO+EP coated sample has the biggest capacitive loop compared to EP and blank samples which show that the corrosion resistance values for FGO+EP coated on the mild steel specimens were found to be much higher compared to the blank mild steel and mild steel coated with epoxy. An electrical equivalent circuit was used to get the replicated impedance data. The capacitive loops shown in the Nyquist plots are not perfect semi-circles. This phenomenon is known as the dispersing effect [44]. Because of this fact, the double-layer capacitance does not behave as an ideal capacitor, so a constant phase element (CPE) is normally used instead of a capacitor. The virtual data produced using the electrical equivalent circuit is in good agreement with the experimentally obtained data.
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The obtained equivalent electrical circuit is shown in Fig. 11, i.e. [Rs(Qc(Rc(QdlRct)))], where Rs is the resistance of the solution (the resistance between working and reference electrodes), Qc is the constant phase element of the coating, Rc is the coating resistance, Qdl is the constant phase element of double layer and Rct is the charge transfer resistance. The higher Rct and Rc values corresponding to the FGO+EP sample represent the blockage of the corrosive electrolyte pathway by the FGO nanocomposites. The values obtained by fitting the above-said circuit are shown in Table 2. From the Nyquist plot, it is clear that the size of capacitive loops decreases with an increase in an exposure time of the sample in corrosive electrolyte [45]. The Rct values of the coated sample decrease with an increase of immersion time indicating the decrease of barrier property of the coating with time. This is due to the deterioration of coating during a longer period of exposure.
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From the bode plot, modulus of impedance at low frequency (Z0.01Hz) and phase angle at high frequency ( 0.1 MHz) were notified for all the samples. Fig. 12 (a) represents the Bode plot of blank, EP, 0.1 wt.% of FGO in EP and 0.25 wt.% of FGO in EP in 3.5 wt.% NaCl solution. The impedance at lower frequency displays that both the FGO+EP samples are having higher impedance compared to blank and EP. The well-dispersed FGO+EP coating shields the diffusion of corrosive electrolytes by blocking the pores and microscopic defects [46, 47]. In comparison, 0.25 wt.% of FGO+EP coated sample shows better performance than 0.1 wt.% of FGO+EP coated sample in 3.5 wt.% NaCl solution. It is observed from the Bode plot that Z0.01Hz of 0.25 wt. % of FGO+EP is 9.7804 106 and that of 0.1 wt. % of FGO+EP is 6.4818 104 whereas epoxy and blank samples showed relatively less values of initial impedance. The Bode plots of 0.25 wt.% of FGO+EP with different immersion time is shown in Fig 12 (b). After 7 days of immersion in 3.5 wt.% NaCl solution, FGO+EP sample showed a small decrease in the Z0.01Hz value (2.2872 106), attributed to the permeation of corrosive electrolyte into the coating. But after 14 days of immersion theZ0.01Hz value of (FGO+EP) decreases significantly (3.3238 105) due to the delamination of the coating.
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The phase angle at high frequency is another factor for calculating the corrosion resistant performance of coatings. The corrosion resistances of the coatings were examined using phase angle at high frequency ( 0.1 MHz) and presented in Fig. 12 (a). Both 0.1 wt.% and 0.25 wt.% FGO+EP samples showed higher phase angle values suggesting their excellent barrier properties compared to that of EP and blank samples. The 0.1 MHz of 0.25 wt.% FGO+EP was observed at 89 while that of 0.1 wt.% is observed at 73 which clearly shows that the diffusion of corrosive electrolyte is less in 0.25 wt. % FGO dispersed in EP. The phase angle at different immersion time with 0.25 wt.% FGO+EP is presented in Fig 12 (b). After 7 days of immersion, 0.1 MHz value of FGO+EP is observed at 82 which show that the coating was still stable and the barrier property of the coating is very good. But after 15 days of immersion, the phase angle suddenly decreases to 21 because of the delamination of the coating.
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3.3.2. Polarization studies
𝑃𝑒𝑓𝑓 % =
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Potentiodynamic polarization studies of blank MS specimen, EP and FGO+EP coated samples were done in a 3.5 % NaCl solution. Potentiodynamic polarization curves (Tafel plots) obtained are shown in Fig. 13. From Tafel plot corrosion current density (Icorr) was determined by extrapolating the linear portion of the curve to corrosion potential (Ecorr) with the help of the CHI608E software. The slope of the tangent of the cathodic curve is known as cathodic Tafel slope (bc) and the slope of the tangent of the anodic curve is anodic Tafel slope (ba). The results obtained are detailed in Table 3. According to Tafel plots, the nanocomposite coatings show quite different behavior from that of neat epoxy coating. The corrosion potential of the FGO+EP coated sample shows an extensive positive shift when compared to the EP coated specimen which shifts to more positive potential compared to that of a blank sample. The icorr of the FGO+EP coated sample decreases considerably in comparison to that of blank and EP samples. Whereas FGO+EP (0.25 wt.%) show less icorr compared to that of FGO+EP (0.1 wt.%). Also, it appears a more positive shift in Ecorr of FGO+EP (0.25 wt.%) compared to that of Ecorr of FGO+EP (0.1 wt.%). Meanwhile, both anodic Tafel slope and cathodic Tafel slope have changed by incorporation of FGO nanofillers in epoxy, showing the corrosion protection properties of coatings has changed compared to that of EP and blank coatings. A similar observation occurred in the corrosion rate, where a blank sample was showing a high corrosion rate compared to that of EP samples. The poor protection performance of EP coating is due to the formation of pores in the coating. However, these pores were effectively blocked by the FGO nanofillers in FGO+EP coating and result in a good protection performance. Thus by restricting the permeation of corrosive electrolytes through the coating, it increases the pathway of the corrosive electrolyte to the metal-coating interface. This suggests that the FGO nanocomposite coating acts as a strong protective layer that resists the corrosion of mild steel effectively in 3.5 wt.% NaCl solution [48]. The protection efficiency (Peff %) of the coating can be calculated using the equation (1), 𝑖𝑐𝑜𝑟𝑟(𝑏𝑙𝑎𝑛𝑘) − 𝑖𝑐𝑜𝑟𝑟(𝑐𝑜𝑎𝑡) 𝑖𝑐𝑜𝑟𝑟(𝑏𝑙𝑎𝑛𝑘)
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(1)
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where 𝑖𝑐𝑜𝑟𝑟(𝑏𝑙𝑎𝑛𝑘) and 𝑖𝑐𝑜𝑟𝑟(𝑐𝑜𝑎𝑡) are the corrosion current density of blank MS specimen and that of the coated specimen, respectively. Polarization curves of the 0.25 wt.% of FGO samples immersed in 3.5 wt.% NaCl solution with the different intervals of time are also plotted in Fig. 13. From Table 3 it is seen that for the 0.1 wt.% of FGO+EP nanocomposite, protection efficiency was about 91.08% and that of 0.25 wt.% of FGO+EP nanocomposite was about 99.73%. Thus FGO+EP (0.25 wt.%) enhances up to 20% corrosion protection behavior in comparison with that of FGO+EP (0.1 wt.%) coating. FGO+EP (0.25 wt.%) is having higher protection efficiency (Peff) compared to that of FGO+EP (0.1 wt.%) and blank. It renders effective coating at the initial days of immersion (up to 8 days), with a small decline in the Peff by
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blocking corrosive electrolyte to the metal/coating interface. But, the Peff of the samples gradually decreased on prolonged immersion in a corrosive electrolyte owing to the decrease in the barrier performance of the coating. On the 15th day, it shows a significant decrease in the Peff. In previous work, we reported [43] functionalization of GO with 4-nitroaniline through amide bond formation, where the addition of FGO to epoxy was showing 99.58% of protection efficiency but after one week of immersion, it decreases drastically to 89.58%. While in the present work, addition 4-fluorophenol functionalized GO to the epoxy matrix shows 99.73% protection efficiency and maintains its protection efficiency by displaying about 98.53% even after one week of immersion.
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It is a well-known fact that the graphene oxide sheet is impermeable to gases and water molecules due to the high surface area. However, FGO composites provide better barrier protection properties against oxygen and water diffusion. The diffusion of corrosive electrolyte into the metal-coating interface is because of the delamination of coating on metals or because of the small pores/defects present in the coating. Once the delamination process starts, the rate of corrosion increases. This results in the oxidation and reduction reactions to take place at the metal-coating interface, which causes corrosion phenomenon to occur on the metal substrate. This is the reason for the significant decrease in Peff. From Peff it is evident that the inclusion of FGO in EP increases the barrier and corrosion protection properties.
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Conclusion
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The corrosion prevention ability of epoxy (EP) coating is successfully enhanced by dispersing the functionalized graphene oxide (FGO) into EP. The graphene oxide (GO) was functionalized with 4- fluoro phenol and characterized by FT-IR, XRD, SEM, TEM and Raman spectral studies. The TGA study confirms the increase in thermal stability of FGO when compared to GO. From EIS and Tafel results it is evident that the incorporation of FGO significantly increases the anticorrosion property and barrier property of EP. Addition of 0.25 wt.% of FGO into EP increases the corrosion resistance of the coating by 30.46 % when compared to pure EP coating.
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Conflict of Interest and Authorship Conformation Form Please check the following as appropriate: o
All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
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o
This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.
o
The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript
o
The following authors have affiliations with organizations with direct or indirect financial interest in the subject matter discussed in the manuscript:
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Acknowledgment
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One of the authors, Saurav Ramesh Nayak acknowledges the financial support provided by the University Grants Commission, New Delhi under the scheme UGC-JRF.
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Fig. 1: Schematic representation of a) functionalization of graphene oxide, b) possible reaction mechanism of the ester bond formation. Fig. 2: FT-IR spectra of graphene oxide and functionalized graphene oxide with labelled functional groups. Fig. 3: Representative Raman spectra of graphene oxide and functionalized graphene oxide with labelled D band and G band. Fig. 4: XRD patterns of GO and FGO.
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Fig. 5: TGA thermograms of graphene oxide and functionalized graphene oxide.
ro
Fig. 6: TEM micrographs of (a-c) functionalized graphene oxide, (d) selected area electron diffraction pattern of functionalized graphene oxide.
-p
Fig. 7: SEM micrographs of (a-c) FGO powder with different magnification, (d) FGO+EP coated on MS specimen.
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Fig. 8: Contact angle images of blank and FGO+EP coated specimens by placing 0.5 L water droplet on the surface of the specimen.
lP
Fig. 9: OCP c of blank, EP, FGO+EP (0.1 wt.%) and FGO+EP (0.25 wt.%) at different intervals of time in 3.5% NaCl solution. Fig. 10: Nyquist plots of blank, EP and FGO+EP coated specimens at different immersion time in 3.5% NaCl solution.
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Fig. 11: Representation of fitted equivalent circuit for EIS experimental data. Fig. 12a: Bode plots of blank MS, EP, FGO+EP (0.1 wt.%) and FGO+EP(0.25 wt.%) coated specimens in 3.5% NaCl solution. Fig. 12b: Bode plots of FGO+EP (0.25 wt.%) coated specimens for different time intervals in 3.5% NaCl solution.
Jo
Fig. 13: Tafel plot of blank MS, epoxy and FGO + EP samples in 3.5% NaCl solution.
17
of
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-p
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lP
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Jo Fig. 1a
18
OH
O
OH O OH
O
Hummers method Graphite
O
Ultra sonication
O
4-fluoro phenol 80-90 °C EDC(1eq.), HOBt(1eq) DCM
O
OH
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O
-p
F
OH
GO
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OH
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O
lP
O
O
O
O
F
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O
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O
O
F
OH
FGO
Fig. 1b
19
EDC.H+ C
N
N+
N
H
N+
N
H
H
C
N
NH
O
O
O
R
H+
O
R
H
HN
O
C
O O
R
OH
N
O
R R O
R1O
O
N
O N
N
N OR1
N
N N
HOBt
N N HO
R1
where
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R-COOH = graphene oxide HO-R1 = F
Jo
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lP
re
-p
HO
of
R
Fig. 2
20
21
of
ro
-p
re
lP
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Jo
of
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-p
re
lP
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Jo Fig.3
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-p
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lP
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Jo Fig.4
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of
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-p
re
lP
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Jo Fig. 5
24
of
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-p
re
lP
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Jo Fig. 6
25
of
ro
-p
re
lP
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Jo Fig. 7
Fig. 8
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lP
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Jo Fig. 9
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of
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lP
re
-p
ro
Fig. 10
Fig. 11
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lP
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29
Fig. 12 b
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of
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-p
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lP
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Jo
Fig. 13
31
of
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lP
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Jo
32
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lP
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Jo
Table 1: Chemical composition of the mild steel specimen. Chemical
S
Mn
C
Si
P
Wt. %
0.012%
0.130% 0.050% 0.050% 0.010%
Al
Fe
0.100%
99.600%
n
(-1 cm-2 sn)
Yo (. cm2)
ro
Yo
Qdl
Rct
n
(-1 cm-2 sn)
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Sample
Rc
-p
Qc
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Table 2: The electrochemical parameters obtained from fitting EIS equivalent circuit for blank, EP, FGO+EP and FGO+EP (8th day) which are immersed in 3.5% NaCl solution.
(. cm2)
6.24710-6
0.391
6.605102
4.39610-5
0.781
9.417102
EP
2.35410-7
0.641
2.683104
5.22110-7
0.744
1.087104
FGO+EP
1.01110-5
0.956
1.304104
5.02010-5
0.465
1.650105
ur na
(0.1 wt. %)
lP
Blank
FGO+EP
1.09610-9
0.955
1.062106
2.81210-7
0.500
2.468107
3.97010-8
0.715
7.750105
1.89710-9
0.814
1.500106
5.49410-7
0.502
2.058103
1.08510-5
0.688
1.573104
(0.25 wt. %) FGO+EP
Jo
(0.25 wt. %) (8th day)
FGO+EP
(0.25 wt. %) (14th day)
33
Ecorr
icorr
bc
ba
Corr. Rate
(V)
(A/cm2)
(V/dec)
(V/dec)
(Angs/min)
-0.594
2.32810-6
10.227
11.406
5.12010-1
EP
-0.429
7.15510-7
3.544
5.888
1.57310-1
FGO+EP
-0.153
7.71410-9
5.123
4.848
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-0.110
6.13910-9
7.365
4.050
1.35010-3
99.73
-0.440
1.93910-7
2.885
7.612
4.26510-2
98.53
-0.453
re
Table 3: Polarization parameters for blank, EP and FGO+EP which are immersed in 3.5% NaCl solution.
1.25810-6
8.271
2.76510-1
45.90
Sample
Blank
FGO+EP
FGO+EP
-p
(0.25 wt. %)
(0.25 wt. %)
FGO+EP (0.25 wt. %)
91.08
Jo
ur na
(15th day)
4.409
69.27
lP
(8th day)
ro
(0.1 wt. %)
1.70310-3
(Peff%)
34