Hybrid silane coating reinforced with silanized graphene oxide nanosheets with improved corrosion protective performance

Abstract

Silane coating has been used as a steel surface treatment for the promotion of adhesion between a substrate and an organic coating. However, the corrosion protection properties of this coating are not sufficient. Incorporation of various additives and nano-fillers into the coating is a promising way to improve the coating barrier action. In this paper a new nano-filler based on graphene oxide is recommended for this type of coating. The graphene oxide is silanized in the first step through a sol–gel route by using aminopropyl triethoxysilane (ATPES). Then, it is employed in a hybrid silane coating based on the mixture of aminopropyl triethoxysilane and tetraethyl orthosilicate (TEOS). The graphene oxide functionalization was characterized by Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The silane coatings filled with silanized and untreated graphene oxide were also characterized by FT-IR, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results obtained from different characterizations showed successful silanization and dispersion of graphene oxide in the silane hybrid coating. The silane coating containing silanized graphene oxide showed superior corrosion protective performance compared to the unfilled silane coating in electrochemical impedance spectroscopy (EIS) and polarization measurements, indicating the considerable corrosion barrier effect of this nano-filler.

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1. Introduction

The importance of the surface treatment of metallic surfaces and its vital role in promoting adhesion between a substrate and the organic coating and in preventing corrosion is hardly lost on anyone. Yet, the search for innovative, environmentally friendly and cost efficient conversion coatings is still an ongoing process. The use of chromate conversion coating, which has long dominated the industry owing to its highly efficient corrosion protection, has been severely hindered in the past decades due to its carcinogenic and toxic nature.¹ But in its absence many other surface treatment techniques have thrived: electrodeposition, polymer plating, anodic oxidation, physical vapor deposition, chemical vapor deposition as well as several different conversion coatings, are but a few examples of these.^2–5 In recent years, silane based coatings, employed using the sol–gel technique, have proven to be a promising substitute for Cr⁶⁺ based surface treatments of metallic substrates.⁶

Silane coatings, much akin to other hybrid inorganic–organic materials, possess the combined advantages of both inorganic and organic moieties. Advantages such as high thermal stability and good chemical resistance are mainly associated with former component, while high flexibility and good impact resistance are among the known benefits of the latter.^7,8 These coatings form strong covalent bonds with the substrate through the hydrolysable siloxane groups of silane molecules. The thickness of the resulted self-assembled coating can range from tens to hundred nanometers. These coatings protect the metallic substrate from aggressive environment through establishing a physical barrier.⁹

However, despite being quite hydrophobic, silane films often fall short in offering adequate long term corrosion protection. The reason for this is their tendency to form cracks, microforms and also their possession of low cross-linked areas. Therefore, they will eventually facilitate the diffusion of electrolytes to the interface of coating/substrate and in doing so provide ideal sites for corrosion reactions to take place.^10,11 In recent years many efforts have gone to the enhancement of corrosion protection of silane coatings. A promising rout to achieve this aim, is through utilizing a mixture of different silane molecules instead of one.^12,13 It was depicted by the investigation of Lei et al.¹³ that the composite silane coating made of a mixture of γ-aminopropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane offers better corrosion protection than a coating based on either one of the silanes alone. Another way to improve the protective properties of silane coatings is by employing them together with another corrosion protection system such as anti-corrosion pigments. This can be accomplished through the addition of a controlled amount of anti-corrosion dopant to alter the bulk qualities.⁹ Depending on their nature, and the way these pigments protect the system against corrosion, they can be categorized into three classes: barrier, for instance lamellar aluminum pigment,¹⁴ micaceous iron oxide,¹⁵ zinc oxide and glass flake; inhibitive such as zinc phosphate pigments;^15–20 and sacrificial, like zinc powder. Substituting microsize particles by nanosize ones is another effective change that can promote corrosion protection. Through lengthening the electrolyte pathway due to their high surface area and small particle size, nanofillers contribute to the coating's corrosion resistance.²¹ That is why application of nanofillers especially those in the form of nanosheet in silane coating is of great interest. Asadi et al.²² studied the effect of nanoclay on the corrosion protective properties of a silane coating. They found considerable increase in charge transfer resistance and coating film resistance in the presence of nanoclay. Such an increase was attributed to the flake morphology of nanoclay which improved barrier protection of the silane coating.

Graphene is a two dimensional, single atom thick, carbon nanostructure with sp² bonded atoms arranged in a hexagonal shape. It possesses many desirable inherent characteristics including chemical inertness, mechanical strength, chemical and thermal stability.²³ But what makes it quite a strong anti-corrosion candidate for protecting metallic substrates, is its impermeability to oxygen and water which renders it an effective barrier against the diffusion of corrosion species.²⁴ Despite all the aforementioned merits, implementing graphene in a corrosion protection system accompanies certain hardships chiefly due to its poor dispersibility in organic and inorganic solvents. Add to that the chemical inertness of graphene which hinders its interaction with polymer matrices, causing aggregation of this nano-filler in the composites.^23,25–27 Graphene oxide, on the other hand, made from oxidation and exfoliation of graphite, consists of hexagonal sp² and sp³ bonded carbon atoms with oxygen containing groups. Hydroxyl and epoxide groups that frequent the basal plane of graphene oxide (GO) and carbonyl and carboxyl groups forming at its edges render graphene oxide more hydrophilic than graphene sheets. It is also more susceptible to form covalent bonds with either polymer matrices of organic coatings or other functional groups and thus, can be tailored for a special physical or chemical trait.²³ Quite a number of approaches have been taken to modify the surface of carbon material and its derivatives, namely graphene; among those, silanization has proven to be an effective way to enhance their solubility.²⁸

Utilization of graphene and its derivatives as an anti-corrosion pigment and coating has been investigated on various substrates. Kirkland et al.²⁹ for instance, asserted that graphene coatings on Ni and Cu surfaces can act as an effective barrier. Corrosion prevention was accomplished through either slowing the anodic dissolution reactions or cathodic reduction reactions depending on the nature of metal. In the previous study³⁰ we evaluated corrosion protection performance of an epoxy coating through wet transfer of amino functionalized graphene oxide (fGO). It was found out that introduction of 0.1 wt% of fGO into the epoxy coating considerably improved the corrosion resistance of the epoxy coating through enhancing barrier properties.

Neupane et al.,⁵ implementing electrochemical measurements, investigated the corrosion behavior of Mg substrate covered with silane coating, as well as silane functionalized graphene oxide coating in commercial saline solutions. It was shown that the anti-corrosion performance of silane coating was far inferior to that of silane functionalized graphene oxide layer.

This investigation was carried out with the aim to use graphene oxide (GO) and aminopropyl triethoxy silane (APTES) functionalized GO (fGO) in a composite silane coating (SC) consisting of a mixture of APTES and tetraethyl orthosilicate (TEOS) as a corrosion resistant system. The ratio of APTES to TEOS was chosen in accordance with a study by Ramezanzadeh et al.,¹² in which the optimum mixture of APTES and TEOS for offering maximum corrosion protection on steel substrates was determined. Both GO and fGO nanosheets along with SC samples and silane composite doped with fGO (SC/fGO) were characterized by FT-IR, thermal gravimetric and X-ray diffraction analyses. Furthermore the anti-corrosion performance of these coatings on steel substrate was evaluated by open circuit potential measurement (OCP), electrochemical impedance spectroscopy (EIS) and polarization analyses in 3.5 wt% NaCl solution.

This work is novel over the previously published works as it reports the effect of functionalized graphene oxide through sol–gel route by silanization on the corrosion protection performance of a hybrid silane coating on mild steel. This work is also critical providing results and discussion on improvement of silane coatings protection reported in literature not only due to the barrier effect of nano-filler but also its positive side effect on the crosslinking reaction.

2. Experimental

2.1. Materials

The mild steel plates with dimensions of 150 mm × 100 mm × 2 mm and with elemental (wt%) composition of Fe (balance), Si (0.016), C (0.037), S (0.0086), P (0.005), Cu (0.065) and others (0.13), were purchased from Foolad Mobarakeh Co (Iran). The mild steel panels were first abraded utilizing sand papers 600, 800 and 1200 grades. Prior to painting, the mild steel substrates were wiped with acetone several times.

Silane solutions were prepared using tetraethyl orthosilicate (TEOS, C₈H₂₀O₄Si) and (3-aminopropyl) triethoxysilane (APTES, C₉H₂₃NO₃Si). TEOS and APTES were obtained from Aldrich Co. (Germany) and utilized without further purification. Acetic acid (C₂H₄O₂) was purchased from Merck Co. and ethanol (C₂H₅OH, 98%) was acquired from Razi Co (Iran). Natural graphite flake (<50 lm), KMnO₄, H₂SO₄ (98%) and H₂O₂ (30%) were purchased from Aldrich Co.

2.2. Silane solution preparation and application

The silane solution was prepared by adding 3.44 g TEOS and 1.56 g APTES to a solution containing 80 g ethanol and 15 g deionized water under constant stirring. In order to satisfy the hydrolysis conditions, pH was adjusted at 4.5 using a few drops of acetic acid. Upon mixing all of the components, the solution was left to stir for 3 h at ambient temperature. The silane solution at the end of the allocated time was clear, implying a low degree of condensation at the adjusted pH.¹²

2.3. Synthesis and chemical functionalization of GO

In order to prepare GO nanosheet, a modification of Hummer's method previously used by Ramezanzadeh et al.,²⁴ was adopted. The method is described in details elsewhere; however, a brief is provided here. Graphite powder (0.5 g) was added to sulfuric acid (60 ml) under constant stirring. After 1 h, KMnO₄ (1.5 g) was added gradually to the solution whilst the temperature was kept under 20 °C. The mixture was stirred at ambient temperature for 12 h. Then, distilled water (600 ml) was added to the mixture under strong stirring. After that, H₂O₂ solution (5 ml of 30%) was added to the suspension. The mixture was then centrifuged and washed twice with hydrochloric acid (1 M) and after that, three times with deionized water. The result was an aqueous solution containing graphene oxide nanosheets. Functionalization of graphene oxide with APTES, was accomplished through a hydrolysis–condensation sequence in sol–gel route by the organosilane. Prior to functionalization process, 50 ml of as-prepared aqueous solution of graphene oxide nanosheets with the concentration of 2.5 g l⁻¹ was sonicated for 10 min. Then, it was added to a solution containing 5 g APTES, 1.15 g deionized water and 93.84 g ethanol under constant stirring. In order to facilitate the hydrolysis reactions, pH of the mixture was adjusted at 4.5 by addition of acetic acid. The mixture was under stirring for 2 h. After that, an aqueous solution of NaOH was used to promote condensation reactions by raising the pH up to 11. The mixture was then left to stir for 1 h at room temperature. In order to eradicate silane residue and other impurities from the functionalized graphene oxide (fGO), the mixture contents were centrifuged at 4000 rpm for 2 min. After that the supernatant was removed and the fGO residue was washed with mixture of (60 : 40 w/w) deionized water and ethanol. This process was repeated five times to insure the removal of all impurities. The functionalized graphene oxide obtained, was dispersed in 50 ml deionized water.

2.4. Sample preparation

The fGO nanosheets (0.1 wt%) were employed in silane coating solution to prepare nanocomposite coating (SC/fGO). Silane coating was applied on the mild steel substrates using air spray under air pressure of 2 atm, with 20 ml min⁻¹ spraying rate at a distance about 30 cm on the mild steel surface. The spraying was repeated three times in order to ensure uniform film thickness. The samples were then dried by warm air (40 °C) and cured at 120 °C for 30 min. The procedure was repeated for the silane coating with no fGO (SC) to be used as a reference.

2.5. Characterization

2.5.1. fGO, SC and SC/fGO characterization. In order to assess the nature of modified graphene oxide and silane films, a series of analyses were carried out. X-ray photoelectron spectroscopy (XPS) measurements were carried out by Specs EA 10 Plus with Al Kα (at pressure of 10⁻⁹ mbar) as the radiation source. The binding energies (BE) were calibrated with shift of the peak of carbon to binding energy of 285 eV. Fourier transform infrared (FTIR) spectra of GO and fGO nanosheets and SC and SC/fGO films were obtained by Spectrum one spectrometer from Perkin Elmer. The X-ray diffraction (XRD) analysis was performed on SC and fGO/SC powders using Philips X-ray spectrometer, PW 1800 type (Netherlands) with Cu-Kα filament.

2.5.2. Surface analysis. The surface morphology of the steel samples coated with SC and SC/fGO was studied by a scanning electron microscope (SEM) model Philips XL30.

2.5.3. Electrochemical measurements. The open circuit potential (OCP) of the mild steel samples (1 cm²) coated with SC and SC/fGO was measured in 3.5 wt% NaCl solution with respect to Ag/AgCl (3 M KCl) reference electrode. The electrochemical techniques including electrochemical impedance spectroscopy (EIS) and polarization were employed to investigate the corrosion resistance of the samples treated with SC and SC/fGO. The electrochemical measurements were conducted in a conventional three-electrode cell including Ag/AgCl (3 M KCl), platinum and mild steel specimens as the reference electrode, counter electrode and working electrode, respectively. The EIS was measured within the frequency range of 10 kHz to 10 mHz and at 10 mV (peak to zero) amplitude of voltage by using the Ivium Compactstat at open circuit potential. Polarization was measured within ±100 mV of OCP with scan rate of +0.5 mV s⁻¹. Electrochemical measurements were conducted on an area of 1 cm² of the coated samples, while the remaining areas of the samples were sealed with hot melt mixture of beeswax and colophony (3 : 1.2 w/w). The test electrolyte was a solution of 3.5 wt% NaCl. In order to ensure the repeatability of the results, the measurements were carried out on triplicate samples.

3. Results and discussion

3.1. Characterization of functionalized graphene oxide nanosheets

Depending on variables such as temperature, pH and ratio of the two reactants, functionalization of graphene oxide by APTES can be accomplished through two major paths. One, where silanization reaction of GO and silanol group takes place due to the formation of Si–O–C bonding and the 3-aminopropylethoxysilanes are incorporated between the GO layers and the other where APTES acts, essentially, as an amine in epoxy ring opening reaction leading to the formation of C–N–C bonding. A schematic of both processes is illustrated in ESI (Fig. S1†). For the former process to occur, first, the ethoxy groups in APTES are hydrolyzed, resulting in the formation of silanols. These groups facilitate the adsorption of hydrolyzed APTES on the surface of graphene oxide by forming hydrogen bonds particularly with the hydroxyl groups at the edges of GO. The subsequent condensation reaction results in the formation of self-assembled siloxane network. The epoxy ring opening reaction on the other hand, occurs when the NH₂ group of APTES partakes in a nucleophilic substitution reaction with epoxy groups of GO. Here as well, the formation of Si–O–Si bridges is the result of condensation reaction between silanols.^5,31

In order to distinguish the prevailing mechanism in this study, FTIR analysis for both GO and fGO was carried out. The corresponding spectra are depicted in Fig. 1. The presence of characteristic peaks in GO spectra pertaining to oxygen bearing groups such as epoxy, hydroxyl and carboxyl at circa 1205, 3435 and 1714 cm⁻¹ respectively,^28,32,33 marks the successful oxidation of graphene sheets. The appearance of the peak around 2931 cm⁻¹ attributed to C–H asym/sym stretching³⁴ and at 1400 cm⁻¹ due to bending vibration of the hydrocarbon segment of APTES, as well as N–H stretching peak at approximately 3810 cm⁻¹ and C–N stretching band at 1430 cm⁻¹ on fGO spectra alludes to the presence of APTES on graphene oxide nanosheets.^33,35

Fig. 1 FTIR spectra for GO and fGO nanosheets.

The peak at 1622 cm⁻¹ on both GO and fGO spectra is attributed to the C=C sp² carbon skeleton of graphene.³² Higher intensity of the peak at this wavelength for fGO sample is connected to the appearance of N–H bending vibration. Looking at Fig. 1, there is an apparent decrease in the intensity of C–O epoxy. This, together with the presence of C–N stretching peak³⁵ which could be associated with the reaction of amine groups of APTES with epoxy group of GO confirms grafting of APTES with GO through epoxy ring opening reaction.

The absorption bands at around 800 and 460 cm⁻¹ are assigned to symmetric stretching, and bending vibration of Si–O–Si network, respectively.³⁶ In addition, asymmetric stretchings of Si–O–Si and Si–O–C were observed at around 1040 and 1080 cm⁻¹.³⁷ Appearance of these peaks and the decrease in the intensity of O–H peak for the fGO confirm the successful functionalization of GO with APTES through silanization reaction.

To further analyze the surface chemical composition of fGO nanosheets, X-ray photoelectron spectroscopy was employed. The overall XPS spectra of fGO nanosheets as well as the O 1s spectrum, deconvoluted through utilizing different combinations of Gaussian and Lorentzian functions can be viewed in Fig. 2. The signals at 105.4 eV, 289.3 eV, 404.4 eV and 537.5 eV were attributed to Si 2p, C 1s, N 1s and O 1s, respectively.^33,38 As displayed by other studies, the XPS spectrum of graphene oxide nanosheets consists of two signals i.e. C 1s and O 1s.^24,33,38 Thus, the appearance of N 1s and Si 2p regions on overall XPS survey of fGO affirms the successful functionalization of graphene oxide with APTES. The various components of O 1s signals, fitted into five peaks of O–C=O (530.2 eV), C=O (531.2 eV), C–O/C–O–Si (531.8 eV), Si–O–Si (532.7 eV) and C–O–C/OH (533.1 eV) are also depicted in Fig. 2, further confirming the result of FTIR analysis.

Fig. 2 Overall XPS spectra (a) and O 1s spectrum (b) for the GO nanosheets modified with APTES.

XRD was also employed to evaluate the effect of silane functionalization on interlayer distance of graphene oxide. As depicted in ESI (Fig. S2†), upon functionalization with APTES the interlayer spacing increased from d = 10.22 Å to 13.3 Å, presumably due to accommodation of the APTES chains on the basal plane of graphene oxide.^39–41

Further characterization of GO and fGO was performed by thermal gravimetric analysis (TGA). As shown in ESI (Fig. S3†), GO displays higher thermal stability than fGO due to degradation of sp³ carbon in alkane chain of APTES.^42–44 This result again confirmed successful grafting of APTES to GO nanosheets observed in the FTIR and XPS results.

3.2. Characterization of silane coating films

The FTIR spectra of both SC and SC/fGO coatings are presented in Fig. 3. The presence of N–H and C–H bands at approximately 3816 cm⁻¹ and 2925 cm⁻¹, respectively, alludes to the existence of APTES in the film resulted by the co-condensation of silanes.³⁴ The peaks at 754, 953 and 2414 cm⁻¹ were associated with, respectively, Si–O bending, Si–OH stretching of the three dimensional silicate network produced due the incomplete condensation reaction, and Si–C stretching bond.^34,45

Fig. 3 FT-IR spectra for the SC and SC/fGO silane composites.

The peak at 1629 cm⁻¹ corresponds to the presence of adsorbed water, while the broad peak at circa 3425 cm⁻¹ is due to the O–H of both adsorbed water and silanol groups.³³ As stated in Section 3.1., the absorption bands at around 1040, 800, and 460 cm⁻¹ are assigned to asymmetric stretching, symmetric stretching, and bending vibration of Si–O–Si network, respectively.^30,36 Intensification of these absorption bands upon inclusion of fGO in the silane film indicates better condensation of silanol groups of the film. Based on the bonds identified through FTIR analysis it is safe to assume that the formation of silane film on the surface of steel substrate follows a mechanism roughly sketched in ESI (Fig. S4†).

Meanwhile, due to intensification of N–H bond peak, it seems logical to assume that the major body of APTES on fGO is still in possession of its NH₂ functionality. And it is the silanol groups of the APTES molecule that have reacted with steel substrate.

The XRD pattern of both SC and SC/fGO samples can be seen in ESI (Fig. S5†). The broad peak at around 23° for SC powder can be attributed to the characteristic diffraction peak of amorphous siloxane matrix.⁴⁶ Adding fGO nanosheets to SC resulted in the appearance of a second peak attributed to the fGO at 7.62° with an interlayer spacing of 12.45 Å, which displays a relative decline in 2 theta angle and an increase in d-spacing compared with that of the original graphene oxide (8.64° and 10.22 Å, respectively). These observations declare that fGO nanosheets were successfully intercalated in the silane film.

The thermal stability of SC/fGO and SC samples was evaluated by TGA. The TGA curves of both SC/fGO and SC samples are provided in ESI (Fig. S6†). While the initial weight loss of approximately 7.33% and 7.11% for SC and SC/fGO samples, respectively, in the temperature range of 0–100 °C is due to the loss of surface absorbed water, the weight loss observed for both samples within 100–350 °C (approximately 11% for both) ascribes to the removal of residual water and ethanol molecules yielded by condensation reaction of unreacted silanol groups. Despite the lower weight loss of GO compared to fGO (see Section 3.1.), in the temperatures above 350 °C, SC/fGO sample displays more heat resistance than SC sample revealing that the incorporation of fGO into the SC enhanced its thermal stability. This can be possibly related to the strong chemical interaction between fGO and SC matrix which reduces the frequency of deformation vibrations, internal rotations and torsional vibrations of the siloxane network. A schematic cross-linked structure in the presence of fGO nanosheets is provided in Fig. 4.

Fig. 4 Film formation of silane coating on steel substrate in the presence of 0.1 wt% fGO.

The surface morphology of coated samples (SC and SC/fGO) along with the uncoated (blank) sample as a reference is shown in Fig. 5. The none-smooth topography of the surface of blank steel is the result of surface preparation using sand papers, which has led to the creation of multiple grooves and ripples. After being coated by SC, many grooves disappear. However, the smoothest surface is created when steel substrate is treated with SC/fGO. The coatings appear to be uniform, with no notable defects or cracks, owing to the layered structure of fGO and the homogenous dispersion of functionalized graphene oxide nanosheets through the coating thickness.

Fig. 5 SEM micrographs from the surface of blank (uncoated), SC and SC/fGO samples.

The thickness of both SC and fGO/SC films obtained at the cross-section is displayed in Fig. 6 (350 nm and 280 nm, respectively). In the presence of fGO, the film is thinner presumably due to the better condensation of the silanol groups. This result confirms better condensation of silanol groups due to intensification of the Si–O–Si absorption bands observed in Section 3.2.

Fig. 6 Film thickness of the SC and SC/fGO samples obtained from SEM cross section analysis.

As the dispersion state of fGO in the silane coating can affect the corrosion protective properties, distribution of the fGO nanosheets in the silane solution just before application of the coating was assessed by a particle size analyzer (Mater Sizer 2000, Malvern, England). The result provided in ESI (Fig. S7†) shows a good dispersion of the fGO nanosheets in the silane solution.

3.3. Corrosion studies

Corrosion protection performance of the silane coating in the presence and absence of fGO was studied by electrochemical impedance spectroscopy (EIS) in 3.5% NaCl at OCP. The variation of OCP for SC and SC/fGO samples is depicted in Fig. 7. The OCP shows two typical behaviors: at the initial stages of immersion (up to around 200 min), OCP shows a decrease; after that OCP remains almost constant. The initial decrease in OCP is related to the diffusion of water/electrolyte through the silane coating to reach the coating-metal interface and develop electrical double layer. The OCP in the next stage, where it remains almost constant is related to the extent of water present in the coating. In the presence of fGO, the initial variation of OCP is slower, indicating the slower penetration of water in the coating. Also, in the presence of fGO the final OCP is more positive revealing lower saturation level of the water in the coating and more specifically in coating metal interface. This results show the superior barrier protection effect of the silane coating in the presence of fGO not only because of its barrier effect,⁵ but also due to the better condensation of the silanol groups in the presence of fGO which was observed in Section 3.2. and 3.3.

Fig. 7 OCP values for the blank, SC and SC/fGO samples immersed in 3.5 wt% NaCl solution for various times.

The EIS results obtained from the silane coatings during 2 h immersion, in the course of which diffusion of water takes place are shown in Fig. 8–10. In this figures the results obtained from blank sample (uncoated sample) are also shown as a reference for comparison. All the Nyquist and Bode plots show one-time constant behavior revealing that only charge transfer resistance is present in the electrochemical cell. It seems that the time constant related to the silane coating is too close to that of corrosion leading to screening of the coating process by the corrosion. The Nyquist plots show an increase in the semicircle diameter related to the surface blocking in the presence of silane coating. For the fGO incorporated coating, the increase in the semicircle diameter is more pronounced. In addition, in the presence of fGO, the decrease of semicircle diameter during immersion related to silane coating degradation is more noticeable compared to that of SC/fGO, again owing to the better condensation of the silanol groups in the presence of fGO, and the barrier effect of fGO.

Fig. 8 Nyquist (a) and Bode (b) diagrams for the blank, SC and SC/fGO samples immersed in 3.5 wt% NaCl solution for 0.5 h.

Fig. 9 Nyquist (a) and Bode (b) diagrams for the blank, SC and SC/fGO samples immersed in 3.5 wt% NaCl solution for 1 h.

Fig. 10 Nyquist (a) and Bode (b) diagrams for the blank, SC and SC/fGO samples immersed in 3.5 wt% NaCl solution for 2 h.

The magnitude of impedance at low frequencies, reflecting total resistance of the sample, shows an increase in the presence of SC. The impedance magnitude at 10 mHz is presented in Fig. 11. The highest impedance magnitude is obtained for the SC/fGO sample indicating the effectiveness of fGO on the coating resistance.

Fig. 11 Values of impedance at low frequency limit (10 mHz) and phase angle (at 10 kHz) extracted from Bode diagrams for the blank, SC and SC/fGO samples immersed in 3.5 wt% NaCl solution for 0.5, 1 and 2 h.

The Bode plots show broadening and shift of relaxation frequency toward higher frequencies in the case of SC sample compared to the blank sample. This broadening and shift is also pronounced in the presence of fGO indicating the shift of electrochemical behavior from resistive toward a more capacitive behavior. This change in the electrochemical behavior is more evident by looking at the phase angle at higher frequencies, i.e. 10 kHz. The extent of phase angle at 10 kHz is reported in Fig. 11. The most negative phase angle is obtained for SC/fGO indicating the most capacitive behavior for this sample; again, confirming the improved protective performance of silane coating by fGO.

According to FTIR results, inclusion of fGO in the silane film led to better cross-linking of silanol groups of the film. This, together with the increase in the length of diffusion pathways caused by the presence of fGO nanosheets is responsible for the increase in protective performance of the silane coating. Therefore, the main protective mechanism of fGO is the barrier effect.

To confirm the EIS results, the polarization measurements were conducted on the same samples after 2 h immersion in 3.5% NaCl solution. The results are shown in Fig. 12.

Fig. 12 Polarization curves for the blank, SC and SC/fGO samples immersed in 3.5 wt% NaCl solution for 2 h.

The silane coating with no fGO revealed decrease in current densities on cathodic and anodic branches compared to the blank sample with no coating; however, such decrease was more pronounced in the presence of fGO. Corrosion current densities of the samples obtained from the Tafel extrapolation method were 14.1 ± 2.3, 7.6 ± 1.2 and 1.4 ± 0.3 μA cm⁻² for the blank, SC and SC/fGO sample, respectively. The polarization results showed a good trend correlation with the EIS results confirming superior performance of SC/fGO sample.

4. Conclusion

This paper reported functionalization of graphene oxide by APTES and the effect of functionalized graphene oxide on the protective performance of a hybrid silane coating. The results obtained from different characterization techniques led us to conclude that:

(1) Silanization of graphene oxide was proved by FTIR, XRD, XPS and TGA.

(2) FIIR and SEM cross-section showed and proved higher extent of Si–O–Si condensation reaction in the presence of functionalized graphene oxide, which is a positive side effect for this nano-filler.

(3) Superior corrosion protective performance compared to the unfilled silane coating in OCP, EIS and polarization measurements, indicated considerable corrosion barrier effect of this nano-filler. This result can also be related to higher extent of curing reaction in the presence of functionalized graphene oxide.

(4) The results of this paper show that silanized graphene oxide can be used in silane coating to improve corrosion protection of mild steel with no negative side effect on the curing reaction.

References

1. E. V. Skorb, D. Fix, D. V. Andreeva, H. Möhwald and D. G. Shchukin, Adv. Funct. Mater., 2009, 19, 2373–2379.
2. S. Naghibi, K. Raeissi and M. Fathi, Mater. Chem. Phys., 2014, 148, 614–623.
3. W. Su and J. O. Iroh, Synth. Met., 2000, 114, 225–234.
4. Z. Kang, J. Sang, M. Shao and Y. Li, J. Mater. Process. Technol., 2009, 209, 4590–4594.
5. M. P. Neupane, S. Lee, J. Kang, I. S. Park, T. S. Bae and M. H. Lee, Mater. Chem. Phys., 2015, 163, 229–235.
6. R. B. Figueira, C. J. Silva and E. V. Pereira, Surf. Coat. Technol., 2015, 265, 191–204.
7. F. Mammeri, E. Le Bourhis, L. Rozes and C. Sanchez, J. Mater. Chem., 2005, 15, 3787–3811.
8. S. M. Hanetho, I. Kaus, A. Bouzga, C. Simon, T. Grande and M.-A. Einarsrud, Surf. Coat. Technol., 2014, 238, 1–8.
9. M. Montemor, M. Ferreira, W. Trabelsi, E. Triki, L. Dhoubi and M. Zheludkevich, Corrosion studies and analytical characterisation of galvanised steel substrates pre-treated with doped silane solutions, ed. L. Fedrizzi, H. Terryn and A. Simoes, Woodhead Publishing, Cambridge, 2007, vol. 7, pp. 83–95.
10. W. Van Ooij, D. Zhu, M. Stacy, A. Seth, T. Mugada, J. Gandhi and P. Puomi, Tsinghua Sci. Technol., 2005, 10, 639–664.
11. A. Phanasgaonkar and V. Raja, Surf. Coat. Technol., 2009, 203, 2260–2271.
12. B. Ramezanzadeh, E. Raeisi and M. Mahdavian, Int. J. Adhes. Adhes., 2015, 63, 166–176.
13. W. Lei, C.-s. LIU, H.-y. YU and C.-q. AN, J. Iron Steel Res. Int., 2012, 19, 46–51.
14. S. González, F. Cáceres, V. Fox and R. Souto, Prog. Org. Coat., 2003, 46, 317–323.
15. B. Nikravesh, B. Ramezanzadeh, A. Sarabi and S. Kasiriha, Corros. Sci., 2011, 53, 1592–1603.
16. M. Mahdavian and M. Attar, Prog. Org. Coat., 2005, 53, 191–194.
17. A. Bastos, M. Ferreira and A. Simões, Corros. Sci., 2006, 48, 1500–1512.
18. R. Naderi and M. Attar, Prog. Org. Coat., 2009, 66, 314–320.
19. S. Mousavifard, P. M. Nouri, M. Attar and B. Ramezanzadeh, Ind. Eng. Chem., 2013, 19, 1031–1039.
20. M. Heydarpour, A. Zarrabi, M. Attar and B. Ramezanzadeh, Prog. Org. Coat., 2014, 77, 160–167.
21. M. Behzadnasab, S. Mirabedini, K. Kabiri and S. Jamali, Corros. Sci., 2011, 53, 89–98.
22. N. Asadi, R. Naderi and M. Saremi, Appl. Clay Sci., 2014, 95, 243–251.
23. B. P. Singh, B. K. Jena, S. Bhattacharjee and L. Besra, Surf. Coat. Technol., 2013, 232, 475–481.
24. B. Ramezanzadeh, E. Ghasemi, M. Mahdavian, E. Changizi and M. M. Moghadam, Carbon, 2015, 93, 555–573.
25. Z. Xu and C. Gao, Macromolecules, 2010, 43, 6716–6723.
26. C. M. Santos, M. C. R. Tria, R. A. M. V. Vergara, F. Ahmed, R. C. Advincula and D. F. Rodrigues, Chem. Commun., 2011, 47, 8892–8894.
27. H. Fujimoto, Carbon, 2003, 41, 1585–1592.
28. S. Hou, S. Su, M. L. Kasner, P. Shah, K. Patel and C. J. Madarang, Chem. Phys. Lett., 2010, 501, 68–74.
29. N. Kirkland, T. Schiller, N. Medhekar and N. Birbilis, Corros. Sci., 2012, 56, 1–4.
30. B. Ramezanzadeh, S. Niroumandrad, A. Ahmadi, M. Mahdavian and M. M. Moghadam, Corros. Sci., 2016, 103, 283–304.
31. Y. Matsuo, Y. Nishino, T. Fukutsuka and Y. Sugie, Carbon, 2007, 45, 1384–1390.
32. B. Scheibe, E. Borowiak-Palen and R. Kalenczuk, J. Alloys Compd., 2010, 500, 117–124.
33. P. F. Li, Y. Xu and X.-H. Cheng, Surf. Coat. Technol., 2013, 232, 331–339.
34. X. Wang, W. Xing, P. Zhang, L. Song, H. Yang and Y. Hu, Compos. Sci. Technol., 2012, 72, 737–743.
35. M. Q. Zhang, M. Z. Rong, S. L. Yu, B. Wetzel and K. Friedrich, Wear, 2002, 253, 1086–1093.
36. L. Song, J. Chen, Y. Bian, L. Zhu, Y. Zhou, Y. Xiang and D. Xia, J. Porous Mater., 2015, 22, 379–385.
37. D. E. Leyden and J. B. Atwater, J. Adhes. Sci. Technol., 1991, 5, 815–829.
38. H. Yang, F. Li, C. Shan, D. Han, Q. Zhang, L. Niu and A. Ivaska, J. Mater. Chem., 2009, 19, 4632–4638.
39. S. Rani, M. Kumar, R. Kumar, D. Kumar, S. Sharma and G. Singh, Mater. Res. Bull., 2014, 60, 143–149.
40. S. Stankovich, D. A. Dikin, O. C. Compton, G. H. Dommett, R. S. Ruoff and S. T. Nguyen, Chem. Mater., 2010, 22, 4153–4157.
41. Z. Yu, H. Di, Y. Ma, Y. He, L. Liang, L. Lv, X. Ran, Y. Pan and Z. Luo, Surf. Coat. Technol., 2015, 276, 471–478.
42. C.-C. Teng, C.-C. M. Ma, C.-H. Lu, S.-Y. Yang, S.-H. Lee, M.-C. Hsiao, M.-Y. Yen, K.-C. Chiou and T.-M. Lee, Carbon, 2011, 49, 5107–5116.
43. Y. Talukdar, J. T. Rashkow, G. Lalwani, S. Kanakia and B. Sitharaman, Biomaterials, 2014, 35, 4863–4877.
44. M. Palimi, M. Rostami, M. Mahdavian and B. Ramezanzadeh, J. Coat. Technol. Res., 2015, 12, 277–292.
45. F. Khelifa, M.-E. Druart, Y. Habibi, F. Bénard, P. Leclère, M. Olivier and P. Dubois, Prog. Org. Coat., 2013, 76, 900–911.
46. M. Gharagozlou, R. Naderi and Z. Baradaran, Prog. Org. Coat., 2016, 90, 407–413.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04843a

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