Electrochemical and surface evaluation of the anti-corrosion properties of reduced graphene oxide

Abstract

Herein, reduced graphene oxide nanosheets (RGON) were electrochemically grown onto a carbon steel alloy from graphene oxide (GO). The produced RGON was characterized using FT-IR, Raman, XRD, SEM and TEM techniques. Then, the anti-corrosion performance of the RGON-deposited layers was evaluated by weight loss and electrochemical methods such as open circuit potential (OCP), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in NaCl 3.5%. Covering the surface with RGON led to a negative shift in the corrosion potential (E_corr) and a decrease in the slope of the cathodic (β_c) and anodic (β_a) reactions, in which both demonstrate that the RGON can hinder the corrosion reaction. Parameters affecting film formation and consequently anti-corrosion performance, such as applied voltage and deposition time, were optimized. After applying 12 V during 180 s, the formed RGON layer on carbon steel showed the lowest corrosion current density (J_corr) and corrosion rate (CR), and the highest charge transfer resistance (R_ct). Finally, the morphology of the uncovered surface and that covered by RGON after 72 h immersion in NaCl 3.5%, was studied using SEM and EDXA techniques. The observed results are presented and discussed here.

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1. Introduction

Iron and iron-based alloys are extensively used in a wide variety of industrial and engineering applications, including piping and construction. Between them, carbon steels are the most famous and most popular. Due to their high mechanical strength, formability, weldability and low cost, carbon steels have been utilized in a myriad of fields such as nuclear power plants, marine utilities, chemical processing, petroleum, and construction.¹ However, the major drawback of carbon steel is its limited resistance to corrosion under harsh environments. Generally, in seawater and marine solutions, corrosion of any exposed metal surfaces is inevitable and more intense.² Certainly, presence of chloride ions in seawater induces corrosion of metals used in facility around the seacoasts that lead to economic costs.³ Therefore reducing or preventing the corrosion is very significant.⁴

The metal surfaces can protect by painting and varnishing⁵ and anodizing,⁶ or coating with other metals or alloys,⁷ oxide layers,⁸ organic layers⁹ and polymers.¹⁰ Recent investigations are focused on application of novel protective nanomaterial films, with thermal and chemical stability and minimum changes in surface physical characteristics.

Graphene as a flat monolayer of carbon atoms packed into a two-dimensional (2D) honeycomb shears sliding at contact interfaces, provides corrosion protection, and reduces friction and wear.¹¹ The high ability of graphene as a protective layer can be understood through its unique chemical and physical features.¹² Recently, due to ultrathin nanostructures and chemical stability of graphene, it is applied as anticorrosive coating layers on various metal substrates to protect them from oxidation.^13–21 Prasai et al. reported that formation of thin layers of graphene via mechanical transfer or electrochemical method can inhibit the metal substrate from corrosion.¹³ Chen et al. deposited graphene films by using chemical vapor deposition method, to improve the corrosion resistance of Cu and Cu/Ni alloys from oxidation in air.¹⁴ Sahu et al. reported a highly corrosion protective coating on Cu substrate by using graphene nanosheets.¹⁵ Raman et al. reported on ability of chemically vapor deposited graphene in protecting cooper against electrochemical degradation.¹⁶ Also, Kang et al. deposited reduced GO sheets on Fe and Cu foils to improve their oxidation resistance.¹⁷ Recently, Krishnamurthy et al. reported on employing graphene as a passivating coating material in retarding microbially-induced galvanic corrosion of metals.¹⁸ Furthermore, Huh et al. used acetone-derived graphene coating for enhancement of seawater corrosion resistance in copper.¹⁹ Mayavan et al. reported that graphene ink can play a corrosion inhibition role for iron in an aggressive chloride environment.²⁰ And also, Singh et al. improved corrosion resistance of copper by fabrication of graphene reinforced composite coating.²¹

In the present work, electrochemical deposition technique was applied to deposit reduced graphene oxide nanosheets (RGON) onto a carbon steel substrate to create a protective layer. Produced RGON was characterized by using FT-IR, Raman, XRD, SEM and TEM techniques. The anti-corrosion performance of the RGON-deposited layers was evaluated by weight loss and electrochemical methods such as open circuit potential (OCP), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in NaCl 3.5%. Observed results will be presented and discussed.

2. Experimental

2.1. Chemicals and materials

All reagents were of analytical reagent-grade and were used as supplied. Natural graphite powder, sulfuric acid, potassium peroxodisulfate, phosphorus pentoxide, phosphoric acid, potassium permanganate, hydrogen peroxide, hydrochloric acid and sodium chloride were purchased from Merck®. All solutions were prepared with purified water (18 MΩ cm, Millipore-MilliQ, Millipore Inc.). All other chemicals used were either analytical or reagent grade.

2.2. Apparatuses and methods

Fourier-transform infrared (FTIR) spectroscopy analysis was performed with Nicolet 400D model. Raman spectroscopy was also performed using Almega Thermo Nicolet Dispersive Raman Spectrometer with laser: second harmonic at 532 nm of a Nd:YLF laser, and a power of 30 mW. The structure of the prepared samples was studied by X-ray diffractometer (XRD, Philips PW 1800 model). The surface morphology of the samples was observed by scanning electron microscopy, SEM equipped with EDXA, (Philips XL30 model). TEM images were obtained using transmission electron microscopy (EM 208 S 100 kW model).

Weight loss measurements concerns with measuring the mass of sample before and after they exposed to aggressive medium followed by calculation the corrosion rate.²² A piece of carbon steel, its composition is summarized in Table S1 (ESI†), was cut to 0.2 × 0.5 × 1 cm³ dimensions for weight loss measurement. Prior to measurement, treatment of specimen was carried out according to Section 2.3. The specimen was weighed on a weighing balance, 0.1 mg precision, and the weight recorded. Then it is incubated in a sodium chloride corrosive solution (NaCl 3.5%). After 3 days interval (72 hours), the specimen was removed, washed with water, dried and weighed on a weighing balance to determine the weight loss during exposure. Corrosion rate in mm per year was calculated by using following equation, in ASTM practice standard G-31;²³

corrosion rate (CR) = 8.75 × 10⁶W/ATD

where W; weight loss in g, D; density of the specimen in g cm⁻³, A; area in cm², T; exposure time in h.

All electrochemical measurements including open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization were carried out using an Autolab PGSTAT300N Potentiostat/Galvanostat equipped with a NOVA software (Eco Chemie, Utrecht, The Netherlands). A three-electrode cell including an Ag/AgCl reference electrode, a Pt electrode as the counter electrode, and a carbon steel, 0.2 × 0.5 cm (0.1 cm²), as a working electrode was used for electrochemical measurements. Anticorrosion performance was evaluated by using different electrochemical testing techniques during immersion in NaCl 3.5% corrosive solution.

For the EIS measurements, a sine wave potential with 10 mV amplitude superimposed on OCP was applied, a wide frequency range from 10 kHz to 100 MHz was scanned, and the impedances were recorded. The EIS data analysis was performed using Zview 2® software and the complex nonlinear least square (CNLS) approximation method. A modified Randles' model in which double layer capacitance (C_dl) was replaced by a constant phase element (CPE), was enough to explain the impedances obtained in the whole frequency range, from which kinetic information were extracted.^24–30 The polarization curves were measured by scanning rate 10 mV s⁻¹ in range ±200 mV around OCP. All experiments were carried out at atmospheric pressure and room temperature and performed at least thrice to ensure repeatability.

2.3. Electrode pretreatment

The working electrode with a surface area of 0.1 cm² was prepared from carbon steel alloy. A copper wire was attached to one side of a 0.1 cm² piece of alloy. Then, the assembly was embedded in epoxy resin, leaving the smooth side exposed. This electrode was gradually ground with different grade of emery paper, and placed in an ultrasonic bath to the water–ethanol–water for 5 minutes each, then washed with distilled water and dried with nitrogen gas. All experiments were performed using the carbon steel electrode with freshly prepared surface.

2.4. Electrochemically reduction of graphene oxide

Reduced graphene oxide was electrochemically deposited on carbon steel electrodes. Graphene oxide was synthesized by the natural graphite powder according to a modified Hummers method,³¹ a well-known method for producing monolayered GO by liquid-phase exfoliation of graphite. An aqueous suspension of GO (1 g L⁻¹), a carbon steel electrode as anode, a Pt wire as cathode and DC power supply model 8303, were used for the electrochemically deposition of RGON. The oxygen functional groups of GO were significantly removed by the EPD process. Possible mechanism of deoxygenation that occurred during the EPD process at the anode is well documented by Ruoff and coworkers (ESI†).³² Finally, prepared samples were removed from the solution and dried at 60 °C in an oven overnight to avoid corrosion owing to water condensation in the deposited layers.

3. Results and discussion

3.1. Structural and compositional analysis of GO and RGON

3.1.1. FT-IR. Electrochemical deposition technique was applied to deposit reduced graphene oxide nanosheets (RGON) onto a carbon steel substrate to create a protective layer. In this process, the negatively charged GO nanosheets were well dispersed in distilled water and moved to the positive electrode under an applied electric field. Fig. 1 shows FT-IR spectrums of graphene oxide (spectrum a) and electrochemically reduced graphene oxide (spectrum b). Presence of different types of oxygen functionalities in graphene oxide was confirmed at 3414 cm⁻¹ (O–H stretching vibrations), 1736 cm⁻¹ (C=O stretching vibrations), 1398 cm⁻¹ (O–H bending vibration), 1220 cm⁻¹ (C–OH stretching vibrations), and 1042 cm⁻¹ (C–O stretching vibrations).³¹ As can be seen the C=C stretching vibration at 1644 cm⁻¹ is observed for both graphene oxide and electrochemically reduced graphene oxide. However, the O–H stretching vibration at 3414 cm⁻¹ is significantly decreased due to the deoxygenation, and characteristic absorption bands of other oxide groups are decreased obviously (Fig. 1 spectrum b). Observed result demonstrated that the GO was successfully electrochemically reduced to the RGON.

Fig. 1 FT-IR spectra of GO (a) and RGON (b).

3.1.2. Raman. Raman spectroscopy is one of the most widely used techniques to characterize the structural and electronic properties of graphene including disorder and defect structures, defect density, and doping levels. In many cases, the Raman spectrum of graphene is characterized by two main features, the G mode arising from the first order scattering of the E_2g phonon of sp² C atoms (usually observed at around 1600 cm⁻¹) and the D mode arising from a breathing mode of K-point photons of A_1g symmetry (around 1370 cm⁻¹).³³ Herein, as shown in Fig. 2, after reduction of GO during applied voltage, the Raman spectrum of RGON also exhibits the presence of D and G bands at 1369 and 1614 cm⁻¹, respectively. It should be noted that the frequency of the G and D bands in the RGON are very similar to that observed in the GO. A universal observation is that higher disorder in graphite leads to a broader G band as well as to a broad D band of higher relative intensity compared to that of the G band. Therefore, the intensity ratio of D band to G band (r = I_D/I_G) is usually used as a measure of the disorder.³⁴ The intensity ratio (r) for RGO (0.84) shows a lower value compared to that for GO (0.94), indicating the high reduction of Go during applied potential and thus GO was converted to a graphene network in high order. The second order Raman feature, namely the 2D band at around 2600 cm⁻¹, is very sensitive to the stacking order of the graphene sheets along the c-axis as well as to the number of layers and shows more broadened shape (often a doublet) with an increasing number of graphene layers.³⁵ However, these specific features are not seen in our sample, where only a weakly 2D band can be seen along with the D + G combination band at 2960 cm⁻¹. Thus, it is conceivable that deposited layer contains highly ordered and arranged graphene flakes. Observed results are in good agreement with the data documented in literature.^15,36

Fig. 2 Raman spectra of GO (a) and RGON (b).

3.1.3. XRD. Fig. 3 represents the XRD patterns of graphene oxide and electrochemically reduced graphene oxide. A strong and sharp peak at 11.6° assigned to the (001) plane, is corresponding to the graphene oxide (pattern a). The disappearance of (001) reflection plane made certain the complete reduction of graphene oxide and an appearance of a new reflection plane at 24.2° ensured the presence of stacked graphene layers of reduced graphene oxide^36,37 (pattern b).

Fig. 3 XRD patterns of GO (a) and RGON (b).

3.1.4. SEM, EDXA and TEM. The TEM image (A-a), EDX analysis (A-b) and SEM image (A-b inset) of GO, are presented in Fig. 4A. As can be seen in the TEM image, GOs have thin and transparent sheets, with folded and rippled structure (Fig. 4A-a). The transparent areas were likely to be flat layers of GO, whereas the darker locations indicated wrinkled area of GO layers. It can be concluded that GO is dramatically exfoliated into individual and scrolled nanosheets. There is agreement between our observation and our previous observations.³¹ According to the SEM image of Fig. 4A-b there is a wrinkled texture associating with the presence of flexible and ultrathin graphene sheets. Chemical analysis using EDX analysis shows the presence of O and C atoms in the graphene oxide (Table 1).

Fig. 4 (A) TEM image of GO sheets (a), and SEM-EDX analysis of GO sheets (b). (B) The SEM-EDX analysis of coated RGON on the carbon steel. The scale bars are 100 nm, 10 μm and 20 μm in TEM image (A-a), and SEM images (A-b) and (B), respectively.

Table 1 Chemical composition of GO and RGON from EDX analysis in Fig. 4

Sample	Composition%
	Fe	C	O	C/O
GO	—	54.95	45.05	1.21
RGON	33.78	42.81	23.41	1.82

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The EDX analysis and SEM image of carbon steel coated by RGON are preselected in Fig. 4B. As can be seen from SEM image, the RGONs were coated uniformly on the surface. Elemental analysis of the RGON by using EDXA shows a significant decrease in oxygen content in comparison GO, indicating GO was converted to graphene by removing the surface oxygen functional groups via electrochemical reduction. Observed results are summarized in Table 1. A little decrease in carbon content of the RGON in comparison to GO may be due to the removal of carboxylic groups during the reduction. Finally, observed increasing in C/O ratio from RGO to GO, proofs conversion of GO to RGON (Table 1).

3.2. Anticorrosion behavior of RGON

The corrosion and rust formation on steel involves several steps including oxidation/reduction reactions. However, Scheme S1† depicts the corrosion phenomenon of iron base alloy in a chloride containing environment (ESI†).

3.2.1. OCP measurements. Fig. 5 shows the variation of potential versus time of the bare carbon steel electrode in comparison to the RGON coated samples in different voltages (A) and times (B). The OCP values for graphene coated carbon steel samples were shifted to more negative potential than bare electrode, due to higher resistance of RGON films towards cathodic corrosion reaction. As can be seen from Fig. 5A, for the bare carbon steel, the OCP value as −550 mV was obtained after 990 s immersion in the NaCl 3.5% solution. In contrast, the OCP value of RGON-coated samples was increased gradually to more negative potentials at shorter times, during increasing the applied deposition voltage. As can be seen, the OCP value as −600 mV was obtained after 760 s, during 180 s applying 12 V deposition voltage. Our observation showed, under higher electrical field condition (16 V), the obtained layers were thickened excessively, and cracks were appeared in the deposited layers. As can be seen in Fig. 5B the OCP values for graphene coated carbon steel samples shifted to more negative potential than bare electrode, when the applied deposition time was increased, at 12 V. This phenomenon is due to growing the graphene layers on the edges or defects of primary layers such as wrinkles, during increasing the deposition time. So the aggressive agents cannot easily penetrate to the graphene sheets. However, at long deposition times more than 180 s, the thickness of formed graphene layers is high but these will be brittle and cause to penetration of aggressive agents to the metal surface.^15,38

Fig. 5 The obtained OCP values for the bare and RGON-coated carbon steel at; different applied voltages during 180 s (A), and different deposition times at 12 V (B) in 3.5% NaCl solution.

3.2.2. Potentiodynamic polarization curves. Fig. 6 shows Tafel polarization curves of the bare electrode in comparison to the RGONs coated carbon steel electrodes at various deposition voltages (A) and times (B). The kinetic corrosion parameters values like; corrosion current density (J_corr), corrosion potential (E_corr), and corrosion rate (CR) are summarized in Table 2. As can be seen from Fig. 6A and B in the polarization curve of RGONs coated carbon steel samples, a negative shift in the E_corr, decreasing in J_corr, anodic (β_a), and cathodic (β_c) Tafel slopes, were observed in comparison to the bare carbon steel (Table 2). The RGONs coated carbon steel at 12 V and 180 s, shows the lowest corrosion potential value as 641 mV while that measured at bare carbon steel is −590 mV. As indicated by the values of E_corr, a shift is observed to more negative potential in the cathodic direction indicating more protection as the cathodic reaction is unfavorable. The J_corr value (30.5 μA cm⁻²) for the RGONs coated carbon steel (12 V and 180 s) is remarkably lower than the corresponding value recorded for bare carbon steel surface, ca. 69.2 μA cm⁻² and the other RGONs coated electrodes. It is therefore concluded that the presence of RGONs coating resulted in corrosion protection to the surface of carbon steel. It is also important to relate the corrosion protection ability of graphene coating over carbon steel to its large specific surface area, excellent mechanical properties and two dimensional geometry of the graphene sheet. While graphene has hydrophobic nature and chemical inertness contributes to the isolation of the surface from electrolytic interactions, but due to high electronic conduction of graphene, observed low loss in corrosion current is expectable.^39,40

Fig. 6 The Tafel plots for the bare and RGON-coated carbon steel at; different applied voltages during 180 s (A), and different deposition times at 12 V (B) in 3.5% NaCl solution.

Table 2 Electrochemical parameters for different working electrodes from polarization curves after immersion in NaCl 3.5% solution

		Constant time (180 s)				Constant voltage (12 V)
	Bare	4 V	8 V	12 V	16 V	60 s	120 s	180 s	240 s
Ecorr (mV)	−590	−600	−608	−641	−636	−600	−610	−641	−618
Jcorr (μA cm^−2)	69.2	47.3	39.3	30.5	52.3	46.8	39.4	30.5	60.6
βa (mV dec^−1)	248	246	127	112	160	241	160	112	151.4
βc (mV dec^−1)	260	179	147	101	134	133	106	101	103
CR (mm per year)	0.804	0.550	0.457	0.354	0.608	0.544	0.459	0.354	0.820
PE%	—	31.6	43.2	56	24.4	32.4	43	56	12.4

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The protective efficiency (PE) of the coatings was determined using the following equation:

where J_corrbare and J_corrcoated are the corrosion current densities in absence and presence of the coating, respectively. It was observed that the PE increases to 56% when use 12 V and 180 s for deposition of RGON on carbon steel.

The simplest and oldest method of measuring CR in plant and equipment is weight loss analysis.²² The CR values measured from weight losing method as 0.7941 and 0.3839 mm per year were in good agreement with calculated values as 0.804 and 0.354 mm per year from Tafel plots for the bare and RGONs coated carbon steel (12 V and 180 s), respectively.

The observed decrease in corrosion rate may be due to the presence of RGONs layer on the surface which hindered the diffusion pathways for oxygen and water molecules to the carbon steel surface. As previously mentioned the process of the carbon steel corrosion involves several steps which depend on the presence of O₂ and H₂O. When the coating prevents O₂ gas molecules from reaching the surface, the coating becomes more effective for corrosion protection.

Thus, at optimized deposition conditions (12 V and 180 s), the diffusion pathway of O₂ and H₂O is hindered further resulting in more effective protection of carbon steel surface against corrosion. Observed results are in good agreements with OCP observations.

3.2.3. Electrochemical impedance spectroscopy measurements. Impedance measurements provide information about the resistive behavior of surface and make it possible to study the performance of a protective coating against metal corrosion. In using EIS for understanding electrochemical degradation of metal and coated sample, the Nyquist plots (Z_imaginary vs. Z_real) are the most common representations of the processes occurring at various interfaces. Fig. 7 shows the Nyquist plots of the bare electrode in comparison to the RGONs coated carbon steel electrodes at various deposition voltages and times. To determine the important factors on electrochemical behaviors of the RGON coating on corrosion of carbon steel, the electrochemical processes on the bare and coated samples were represented using equivalent circuit models in Fig. 7. The approximated data as; solution resistance (R_s), double layer capacitor (Q_dl), coat capacitor (Q_c), charge transfer resistance (R_ct), and Warburg impedance (W) are presented in Table 3.

Fig. 7 The EIS complex plane plots obtained at the bare and RGON-coated carbon steel at different applied voltages during 180 s (A), and different deposition times at 12 V (B) in 3.5% NaCl solution. Symbols show experimental data and lines the approximated data obtained by using the equivalent circuit models for the bare (A, inset), and RGON coated (B, inset) carbon steel. (A-a) and (B-b) show the short scale plots with a higher resolution.

Table 3 Values of the interfacial parameters estimated by approximation of the EIS experimental data obtained on carbon steel electrode in different steps of covering, using the CPE equivalent circuit model and CNLS method

		Rs (Ω)	Qdl (S^n Ω^−1)	n1	Rct (Ω)	Qc (S^n Ω^−1)	n2	W (S^n Ω^−1)	n3	PE%
	Bare	31.7(±0.3)	7.9(±0.2) × 10^−5	0.75	40.0(±0.5)	—	—	1.7(±0.4)	0.5	—
Applied voltages	4 V	31.8(±0.2)	5.8(±0.6) × 10^−5	0.73	45.6(±0.9)	7.7(±2) × 10^−4	0.99	1.3(±0.4) × 10^2	0.4	12.3
	8 V	27.6(±0.1)	5.5(±0.3) × 10^−5	0.76	48.2(±0.8)	7.8(±0.2) × 10^−4	0.66	1.7(±0.5) × 10^2	0.5	17.0
	12 V	26.1(±0.1)	4.2(±0.2) × 10^−5	0.75	121.9(±2.5)	8.0(±3) × 10^−4	0.68	1.9(±0.3) × 10^2	0.4	67.2
	16 V	28.8(±0.1)	8.1(±0.3) × 10^−5	0.71	60.8(±0.6)	3.3(±0.1) × 10^−4	0.67	7.2(±0.1) × 10^2	0.5	34.2
Deposition time	60 s	29.2(±0.1)	2.3(±0.1) × 10^−5	0.78	54.8(±0.3)	2.4(±0.04) × 10^−4	0.79	1.5(±0.6) × 10^2	0.6	27.0
	120 s	25.3(±0.1)	2.8(±0.2) × 10^−5	0.80	54.9(±1.0)	4.9(±1.3) × 10^−4	0.89	1.2(±0.3) × 10^2	0.4	27.1
	180 s	26.1(±0.1)	4.2(±0.2) × 10^−5	0.75	121.9(±2.5)	8.0(±3) × 10^−4	0.68	1.9(±0.3) × 10^2	0.4	67.2
	240 s	25.3(±0.1)	3.2(±0.1) × 10^−5	0.79	60.7(±0.3)	7.0(±0.6) × 10^−4	0.81	2.1(±0.1) × 10^2	0.5	34.1

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As can be seen from Fig. 7A and B in the complex plane plots of RGONs coated carbon steel samples, an increasing in the R_ct and decreasing in Q_dl were observed in comparison to the bare carbon steel (Table 3). The diameter of semicircle at high frequencies represents the resistance of electrochemical reactions on the electrode. It enlarges markedly for the coating samples which means provides more charge transfer resistance due to the graphene layer. The RGONs coated carbon steel at 12 V and 180 s, shows the highest R_ct value as 121.9 Ω while that measured at bare carbon steel is 40 Ω. Obtained results are in good agreements with potentiodynamic polarization measurements.

The protective efficiency (PE) of samples was calculated from R_ct values according to the following equation:⁴¹

where R_ctbare and R_ctcoated are the charge transfer resistance in absence and presence of the coating, respectively. It was observed that the PE increases to 67.2% when use 12 V and 180 s for deposition of RGON on carbon steel.

3.3. Surface study of carbon steel

Fig. 8A shows the EDX analysis and SEM images (insets), of carbon steel surface before (a) and after (b) 72 h immersion in NaCl 3.5% corrosive solution. As can be seen from SEM images in Fig. 8A insets, the clean and uniform surface of carbon steel was damaged after exposure to NaCl 3.5% corrosive solution (Fig. 8A-b inset). The permeation of water, dissolved oxygen and attractive Cl⁻ into the outer layer could result in oxidation of the whole surface. Furthermore appearing the strong O and little Cl peaks in the EDX analysis, Fig. 8A-b and Table 4, indicates that iron chloride formed on the surface and subsequently hydrolyzed to iron oxide layer (Fe²⁺ + 2Cl⁻ → FeCl₂, FeCl₂ + H₂O → FeO + 2H⁺ + 2Cl⁻).¹⁹

Fig. 8 The SEM-EDX analysis of; (A) the bare steel before (A-a) and after (A-b) exposing to 3.5% NaCl solution, and (B) the RGON-coated carbon steel after 72 h immersion in 3.5% NaCl solution. The scale bars are 20 μm, 20 μm and 50 μm in SEM images (A-a), (A-b) and (B), respectively.

Table 4 Chemical composition of carbon steel samples before and after corrosion and RGON-coated sample after 72 h immersion in NaCl 3.5% solution from EDX analysis in Fig. 8

Sample	Composition%
	Fe	C	O	Cl
Bare carbon steel before corrosion	96.94	3.06	—	—
Bare carbon steel after corrosion	53.34	4.50	33.54	3.62
RGON-coated carbon steel after corrosion	33.82	42.12	23.50	0.56

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Fig. 8B inset, shows the surface of RGON coated carbon steel damaged at cracks in the RGON as corrosion initiation sites, while most of the sample surface leave undamaged. It shows RGON does not completely corrode in our experiment. The EDX results indicate existence of much lower O and Cl contents on the surface of the RGON coated sample in compared to uncoated sample (Table 4).

Decreasing in O and Cl contents, in RGON coated toward bare carbon steel after immersion in NaCl 3.5% corrosive solution, implies that products of corrosion such as FeCl₂ and FeO are decreased. These data confirm the ability of RGON coating to suppression of penetrating the aggressive chlorine and dissolved oxygen ions into the underlying alloy.

4. Conclusions

In this research, the ability of RGON film as a protective layer to prevent corrosion of carbon steel has been studied. All results confirmed that the RGON layer supply protection for the carbon steel, because the coating film is well formed on the substrate and act as a barrier protector. Despite the corrosion protection role of RGON is distinguished as in the case of our present experiments, we would like to study the defects on the graphene surface which may be act as the corrosion initiation sites. So in our future studies we focus on the raising the value of corrosion resistance of carbon steel by modifying the graphene surface by aromatic molecules. This part of work is in progress.

Acknowledgements

The authors gratefully acknowledge the NSTRI and University of Birjand for providing facilities for this work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26948b

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