MoOx nanocomposites: For surface protective performance of AA2024 alloys in marine environment

Surface & Coatings Technology 374 (2019) 579–590

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Promising graphitic carbon nitride/MoOx nanocomposites: For surface protective performance of AA2024 alloys in marine environment

T

⁎

A. Madhan Kumara, , Mohd Yusuf Khanb, Rami K. Suleimana, Abuzar Khanb, Hatim Dafallac a

Center of Research Excellence in Corrosion, King Fahd University of Petroleum & Minerals (KFUPM), Dhahran 31261, Saudi Arabia Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum & Minerals (KFUPM), Dhahran 31261, Saudi Arabia c Center for Engineering Research, King Fahd University of Petroleum & Minerals (KFUPM), Dhahran 31261, Saudi Arabia b

A R T I C LE I N FO

A B S T R A C T

Keywords: Graphitic carbon nitride Nanocomposite Epoxy coatings Corrosion

A wide variety of carbon nanostructured materials have been effectively inspected as nanofillers to enhance the overall performance of the polymeric coatings; however, their practical applications are still limited by the high cost. Graphitic carbon nitride (GN) is considered as the best alternative due to its low cost and unique structural and mechanical features. Herein, we synthesized the GN-MoOx nanocomposite and utilized it as nanofillers to enhance the corrosion resistant behavior of epoxy coatings on AA2024 Al alloys in chloride medium. The rodshaped MoOx nanoparticle with a diameter of 25 nm was obtained on the lamellar surface of GN in Scanning Electron Microscopic (SEM) and Transmission Electron Microscopic (TEM) images. The structural analyses results validate the monoclinic phase and formation of the GN/MoOx nanostructure. The Attenuated Total Reflectance-Infrared (ATR-IR) spectra of GN/MoOx illustrate distinctive poly-s-triazine peaks from 1200 to 1640 cm−1. The electrochemical corrosion results reveal that the nanocomposite reinforced epoxy coatings can deliver a higher corrosion resistant performance and the maximum corrosion protection can be attained via reinforcing the epoxy coatings with 3 wt% GN/MoOx. Based on the obtained outcomes of this present investigation, we envision that the GN/MoOx nanocomposite has a great potential in improving the surface protective behavior of epoxy coatings on AA2024 alloy against corrosion in chloride medium.

1. Introduction AA 2024-T3 alloy is widely employed as structural components in automotive and aerospace industries, as well as in many other industries owing to their light weight and low density with high strength/ weight ratios. Despite the aforementioned merits, it suffers from localized corrosion when exposed to hostile circumstances including hydraulic fluids and saltwater, leading to severe structural damage. In general, the existence of 4–5% of copper in AA 2024-T3 forms a microgalvanic couple with an Al matrix resulting in accelerated corrosion behavior of the alloy [1,2]. Hence, the significant mechanical strength of the alloy becomes beneficial only after the utilization of different approaches to improve the surface protection properties against corrosion. One of the keen approaches to protect the 2024 Al-alloy is by coating its surface with epoxy hybrid materials utilizing of its high cross-linking density and strong adhesion advantages, which lead to a creation of a strong barrier between the base metallic surface and the harsh environment [3–5]. Despite this, poor UV resistance and inherent

⁎

and high permeation to water are still challenges that limit their further applications in the coating industry. Uptake and permeation of water and aggressive ions into epoxy coatings is the main concern during a prolonged usage that mainly deteriorates the adhesion strength and surface protective performance of the coatings [6,7]. Among the numerous solutions proposed for this problem, the controllable incorporation of anticorrosive pigments into polymer matrix has been considered to be one of the most efficient approaches to improve the corrosion resistant behavior of epoxy coatings [8,9]. In general, this can be achieved by the incorporation of anticorrosive pigments that work through the action of the barrier, active, and sacrificial behaviors [10–12]. In recent years, inhibitive pigments are developed through reinforcing a metal oxide with inhibitive constituents, which can have both inhibitive mechanisms and strong barrier properties that can and enhance significantly the corrosion protection performance of the epoxy coatings. Owing to the small particle dimension and large specific surface area compared to other conventional pigments, anticorrosive pigments at the nanometer scale can improve the coating's integrity and

Corresponding author. E-mail address: [email protected] (A.M. Kumar).

https://doi.org/10.1016/j.surfcoat.2019.06.004 Received 26 February 2019; Received in revised form 18 May 2019; Accepted 1 June 2019 Available online 03 June 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.

Surface & Coatings Technology 374 (2019) 579–590

A.M. Kumar, et al.

added into the molybdenum salt solution following the same fashion described above. The Mo salt and GN were measured in a way that the obtained composite will contain 20% MoOx and 80% of GN. The suspension was then sonicated with a digital sonifier to ensure the thorough mixing of Mo salt over GN. Next, the mixture was kept overnight in a fume hood to evaporate the ethanol. Further, the well-mixed GNMoOx composite was scrubbed from the glass vial and was spread on an Al2O3 boat placed in a tubular furnace. The argon gas was introduced into the quartz tube and heated up to 450 °C at a heating speed of 10 °C/ min. Finally, the GN-MoOx composite powder was obtained after cooling down the furnace to RT.

durability, which probably can expand the comprehensive features of the coating such as improved corrosion resistance, and mechanical and functional characteristics [13,14]. Further, the incorporation of welldispersed nanoparticles inside the coating matrix could seal the existing micro defects created from a local shrinkage during the curing of polymer resins and perform as a conduit to join more fragments. This behavior further decreased the total free volume by increasing the cross-linking density. It has been already reported that the nanoparticles reinforcement in epoxy coatings provides a strong barrier performance for corrosion protection and decreases the coatings delamination or blistering [15]. In addition to this, the layered nanomaterial including clay, graphene oxide (GO) nanosheet can efficiently increase the permeation length of aggressive ions which can results in coatings with almost impervious to water and aggressive species [16–18]. As an analogous to GO, graphitic carbon nitride (g-C3N4, GN) has been recently consumed in many applications due to their basic surface functionalities, electron-rich characteristics, and H-bonding motifs owing to the existence of N and H atoms [19,20]. GN is an emergent 2D layered nanomaterial and has gained more attention as a fascinating new class of nanofiller to construct polymeric nanocomposites. In general, GN possesses a lamellar structure similar to graphite layers, possessing tri-s-triazine as the base structural part. This structure creates a material that owns good chemical, mechanical and thermal stabilities. Many research works reported in the literature have revealed that a combination of inhibitive nanoparticles with layered structured materials can deliver a synergetic inhibition effect that is deliberating improved corrosion protection for metallic materials [21,22]. In this regard, we aimed here to investigate the impact of embedding a GN/ MoOx nanocomposite as a nanofiller in epoxy coatings to enhance the anti-corrosion performance of an AA2024 Al substrate in 3.5% NaCl solution.

2.3. Preparation of epoxy nanocomposite coatings on AA2024 Al Substrates The epoxy coating matrix was prepared using an epoxy resin and a hardener in a proportion of 1:3. The resultant mixture was coated on Al samples by a drawdown bar coater at a constant rate. After that, the wet coated Al samples are kept to dry at RT, which leads to a homogeneous coating having an average thickness of about 60 ± 2 μm. The Al substrates samples coated with epoxy coatings that are free or embedded individually with 1, 3 and 5 wt% of GNMoOx nanocomposite were labeled as PE, PE/GNM1, PE/GNM2, and PE/GNM3, respectively. For the sake of assessment, epoxy coating with a 3% of GN (PE/GN) was also formed on Al substrates following the same conditions mentioned above. 2.4. Characterizations Surface morphologies and elemental analysis were inspected by the Field Emission Scanning Electron Microscopy (FE-SEM) coupled with Xray diffraction analyzer. Transmission Electron Microscopy (TEM) image of the synthesized samples was achieved by a JEM-2100F electron microscope at a voltage of 200 kV. X-ray Diffractometer (Rigaku, Japan) was used to find the phase and crystal structure identification of the synthesized nanocomposite in a 2θ range from 10 to 80°. Attenuated total reflectance Fourier Transform Infrared Spectroscopic measurements were accomplished to analyze the chemical structure of the synthesized nanocomposite. In order to estimate the thermal stability and content of MoOx in the synthesized nanocomposites, Thermogravimetric analyzer (TA instruments, TGA 50) was utilized with a scan rate of 10 °C/min from 30 °C up to 700 °C. X-ray Photoelectron Spectroscopic (XPS, Thermo ESCALAB 250) measurement was performed with a change neutralizer (hv = 1486.6 eV) and a monochromatic Al Ka source. Hydraulic adhesion test (Albuquerque Inc., U.S.A) was conducted according to ASTM D4541 standard. Initially, a metallic dolly with an epoxy resin was glued to the coated samples. After the completion of curing (24 h), the dolly was pulled and the maximum force by which the dolly isolated the coating from the substrate was monitored as a degree of the adhesion strength between coatings and substrate. Corrosion resistant behavior of bare and coated Al substrates in 3.5% NaCl medium was analyzed using a Gamry Reference 3000 electrochemical workstation. All the corrosion testing including electrochemical impedance spectroscopic (EIS) and electrochemical frequency modulation (EFM) was performed by a conventional threeelectrode cell using Al substrates as working electrode, a graphite rod and saturated calomel electrode (SCE) as a counter and a reference electrode, respectively. The exposure area of the working electrode had the defined circle with a surface area 1.766 cm2. EFM test was performed by a perturbing potential of 10 mV amplitude with 2 and 5 Hz sine waves. The highest intermodulation peaks were selected to determine the corrosion current density (icorr), the Tafel constants and the causality factors CF2 and CF3 [24]. The electrochemical corrosion test was done by monitoring the open circuit potential (OCP) and followed by performing the EFM and EIS tests up to 60 days of immersion. The

2. Experimental 2.1. Materials and methods The Aluminum alloy substrate used in the present study was bare AA2024-T3 alloy, procured from Q lab, UK and its elemental composition is presented in Table 1. All used chemicals were of analytical reagent grade. Melamine (Aldrich; 99%), Ammonium molybdate (Aldrich; reagent grade, 98%), absolute ethanol (Aldrich; ≥99.8%) were used as received. Bisphenol A diglycidyl ether based-epoxy resin and cycloaliphatic polyamide curing agent (Huntsman Advanced Materials, U.S.A) were used to prepare the base epoxy coatings. 2.2. Synthesis of GN and GN-MoOx nanocomposite The GN was synthesized through the thermal condensation polymerization of melamine. In a typical procedure, 10 g of melamine was loaded in a covered porcelain crucible and placed into the muffle furnace. The muffle furnace was slowly heated with a heating speed of 10 °C/min and held at 550 °C for 4 h. After cooling down the muffle furnace to room temperature (RT), the resultant GN was crushed into a fine powder [23]. The GN-MoOx composite was produced by making a solution of Mo salt (Ammonium molybdate) in ethanol. The above-synthesized GN was Table 1 Composition of AA2024 Al alloy substrates. Alloy

AA2024

Main alloying elements (wt%) Cr

Ti

Mg

Si

Zn

Fe

Cu

Mn

Al

0.1

0.15

1.2–1.8

0.5

0.25

0.5

3.8–4.9

0.3–0.9

Balance

580

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Fig. 1. SEM images of (a) GN, (b) GN-MoOx and (c) EDS results of GN-MoOx and TEM images of (d) GN, (e) GN-MoOx and HR-TEM image of (f) GN-MoOx nanocomposite.

having a 2D structure of thin layered GN sheets and the nanorod structured MoOx particles are homogeneously dispersed onto the surface of the GN sheets. The combination of 2D functional GN sheets with 3D MoOx nanorods provides efficient synergetic features including high surface area, which is required for better performance as nanofillers in polymeric coatings. The selected area electron diffraction (SAED) pattern of the GN/MoOx nanocomposite (inset of Fig. 1d) exhibits bright spots that demonstrate the polycrystalline nature of the synthesized nanocomposite. The high-resolution TEM (HRTEM) image displays the interface of GN-MoOx and the interplanar distance of lattice fringes was calculated to be about 0.21 nm corresponding to the hkl (211) lattice plane of MoOx (Fig. 1f) and about 0.34 nm corresponding to the (002) plane of GN sheets [28].

EIS data were recorded with an amplitude of 10 mV in the frequency range of 100 kHz- 1 mHz under OCP. The attained EIS curves were further inspected using an equivalent circuit fitting and every measurement was repeated at least three times to ensure consistency in the generated data.

3. Results and discussion 3.1. Surface characterization results SEM micrograph of pure GN (Fig. 1a) exhibits a hierarchical structure of 2D layered nanosheets with < 60 nm thickness as described earlier [25]. It has been already reported that the thermal condensation of melamine during the calcination process (550 °C) delivered the construction of the typical layered structure [26]. The SEM image of GN-MoOx nanocomposite (Fig. 1b) exhibited a two-dimensional flakelike morphology completed covered with rod-like 3D hierarchical nanostructures, revealing good interfacial contact between them. EDS results (Fig. 1c) did not show any impurity peaks other than carbon (C) nitrogen (N), molybdenum (Mo), and oxygen (O) peaks which further validates the formation of the GN nanocomposite. A close inspection of the GN-MoOx SEM micrograph (Fig. 1b) shows hierarchical nanorods of MoOx with a uniform diameter of 25 nm and a length of about 300 nm embedded on the surface of 2D flakes like GN. The nanostructured MoOx particles are homogeneously dispersed onto the surface of the GN thin layers and the existence of MoOx nanorods between the layers of the GN protects efficiently the stacking of GN layers. The SEM images pointed out the probability of synergic interaction of MoOx nanoparticles with two-dimensional GN nanosheets. Liu et al. have also found that the homogenous distribution of Fe2O3 nanoparticles on GN nanosheets could effectually protect the GN layers from stacking [27]. Moreover, the incorporation of 2D functional GN layers with 3D MoOx nanostructures provides efficient synergetic features including a high surface area, which is necessary for improving the performance of polymeric coatings. In addition, TEM images were obtained to get more evidence about the composition and morphology of the GN/MoOx nanocomposite and the resultant images are displayed in Fig. 1d–f. The HRTEM images of GN obviously showed > 2 layers at the edge of the GN flakes. The TEM image depicted in Fig. 1e reveals that the GN/MoOx nanocomposite is

3.2. Structural analyses Fig. 2 shows the XRD patterns of the synthesized GN/MoOx nanocomposite. Two characteristic peaks of GN positioned at 13.10° and 27.70° are aroused from the inplane structural packing motif (100) and

Fig. 2. XRD pattern of pure GN and synthesized nanocomposite. 581

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considerably lesser than that of pure GN (812 cm−1) and the peaks in the range of 1200–1650 cm−1 were observed as a continuous wide peak. This observation was obtained due to the coordination effect of Mo and nitrogen atoms of the tr-s-triazine units of GN sheets. The unoccupied electronic orbits of Mo3d attracted the peripheral electrons of nitrogen pots of tri-s-triazine units of GN and diluted the inductive effect between C and N atoms in tri-s-triazine ring, resulting in a red shift and reduction of typical breathing vibration peak of tri-s-triazine units [39,40]. Thus, the obtained slight changes in the FTIR spectra of GN nanocomposite verifies the presence of an interfacial interaction between the GN sheets with the incorporated MoOx nanoparticles [41]. The XPS analysis was also employed to investigate the composition and binding state of the elements and the obtained data are presented in Fig. 4. The wide scan spectra of the synthesized nanocomposite indicate the presence of four major elements that are, C, N, O, and Mo in the nanocomposite (Fig. 4a). The C1s spectrum of the GN/MoOx nanocomposite (Fig. 4b) is further deconvoluted into three peaks appeared at 284.7, 286.6288.2 eV, which are attributed to the signals of sp2-hybridized carbon atom, the CeO and C]O bonds, and the sp2-hybridized carbon (N]CeN) in the N-containing aromatic ring, respectively [27]. The N1s spectrum of the GN nanocomposite (Fig. 4c) reveals two peaks at 397.9 and 401.3 eV, which are allocated to the N1s signal of the C] N and CeNH2 bonds, respectively [27,39]. The XPS survey spectrum of GN nanocomposite (Fig. 4e) also exhibited the four-characteristic signal at 233.9 (Mo 3d), 398.0 (Mo 3p3/2), 415.6 (Mo 3p1/2), and 531.9 (O1s) eV, demonstrating the formation of molybdenum oxides on GN sheets [42]. The Mo 3d spectrum of the GN composites is further deconvoluted into four peaks at 229.4, 231.8, 232.5 and 235.3 eV. The characteristic doublet of core-level, Mo 3d 5/2, Mo3d 3/2 at 229.4 and 232.5 eV with a spin energy separation of 3.1 eV designates the Mo (IV) oxidation state of MoO2. Further, the peaks at 231.8 and 235.3 eV are attributed to Mo (VI) 3d5/2 and 3d3/2 of MoO3, respectively, due to the slight surface oxidation of metastable MoOx in air. From the quantitative analysis results, the chemical compositions of pure GN were found to be 70.54, 26.53 and 2.93 at.% for C, N and O, respectively. In contrast, the proportions of GN/MoOx were estimated to be 59.43, 19.94, 8.12 and 12.51 at.% for C, N, Mo and O, indicating the formation of GN/MoOx nanocomposite with the ratio of about 20% MoOx and 80% of GN nanosheets. Furthermore, it was found that Mo exists in both MoO2 and MoO3 in significant quantities (~65:35) and hence, the representation of molybdenum oxide in the synthesized nanocomposites was denoted as MoOx throughout the current investigation. The peaks values are slightly varied from the reported values of pure MoOx, which probably attributed to the interaction between the MoOx and GN surfaces that leads to the formation of strong interface between them [43]. These obtained findings agree well with the IR, XRD and TEM results and further confirms the formation of GN/ MoOx nanocomposite.

the interplane n periodic stacking of the graphite-like layers along caxis (002), respectively (JCPDS 87-1526) [29]. Safaei et al. have synthesized GN nanosheets using the urea as a precursor and found less crystalline (100) peak, representing that this is due to the less hydrogen bindings due to the higher amounts of gas released during urea pyrolysis [30]. The XRD spectra of the nanocomposite contained peaks of MoOx along with that of graphitic nitride. The prominent diffraction peaks at 26.03°, 37.02° and 53.51° relate respectively to the [011], [−212], and [−311] lattice planes of the monoclinic MoOx crystal in the space group P21/c with a lattice constant of a = 5.607 Å, b = 4.859 Å, c = 5.537 Å and β = 119.37° (JCPDS 05-0428) [31]. Further, the two characteristic diffractions of GN are not changed in the nanocomposite, however, the two peaks at 13.10° and 27.54° are relatively low or absent, representing the intercalation of MoOx on the GN layers with alteration of the intraplanar packing of heptazine units [32]. Moreover, the slight shift in the lower angle for the diffraction peak (002) indicates a slight expansion of the interlayers due to the strong interaction of MoOx nanoparticles with g-C3N4 {002} facets [33]. The interfacial interactions between GN layers and MoOx nanorods are still unclear, however, the electrostatic face-to-face van der Waals type interaction between MoOx and GN sheets maybe the driving force in forming polymeric heterostructure with synergistic aspects [34]. XRD pattern of the as-synthesized nanocomposite is not observed with any additional peaks, signifying the high purity of the samples. In addition, the average crystallite size of GN/MoOx estimated using the Scherrer's equation was calculated to be 12.57 nm, which is smaller in comparison with the pure GN (13.24 nm). The decrease in the crystallite size is ascribed to the outcomes of embedded MoOx nanorods on the GN nanosheets [35]. Further, FTIR spectra (Fig. 3) were employed to recognize the functional groups present in the pure GN and its nanocomposite. A sharp peak at 812 cm−1 is related to the typical breathing mode of tri-striazine units whereas, a series of peaks from 1200 to 1650 cm−1 is assigned to the stretching vibration mode of the dramatic C]N and CeN heterocyclic bonds in GN, which confirms the graphitic structure [36]. The relatively wide peak around 3200 cm−1 relates to the stretching vibrations mode of NeH and OeH bond. The band at 510 cm−1 arises from the MoeO stretching mode of the MoOx nanoparticles [37]. It is observed that the FTIR spectrum of the GN/MoOx nanocomposite showed representative peaks of both components (GN and MoOx), indicating the characteristic graphitic structure of GN was well retained after a homogeneous hybridization with MoOx. However, few representative bands of GN red-shifted with the weakened band strengths, which reveals that the conjugated structures of GN sheets are stretched and more widely conjugated systems containing GN sheets and MoOx have been produced [38]. In particular, the peak associated with the tris-s-triazine units obtained around ∼785 cm−1, is

3.3. Thermalgravimetric analysis results

Transmittance (a.u.)

Pure GN

The thermal degradation of synthesized GN nanocomposites was estimated using the TGA analysis and the obtained TGA curves of the pure GN and GN nanocomposite are presented in Fig. 5. It is already well known that the MoOx nanoparticles are thermally stable up to 800 °C. TGA curve of GN showed only one weight loss region observed around 550 °C which is ascribed to the degradation of tri-s-triazine moieties, demonstrating the total degradation of GN. However, for GN nanocomposites, three distinguish weight loss stages were obtained which is in good agreement with the previous report [35]. First weight loss occurs around 100 °C is attributed to the expulsion of moisture and impurities from the synthesized GN. The second weight loss observed around 350 °C which shows the decomposition of nitrile groups which are not part of the stable tri-s-triazine ring structure. The third stage of weight loss obtained around ∼400 °C due to the degradation of the carbon nitride structure, resulting in the total loss of carbon and leaving

GN-MoOx

Pure MoO2

4000

3500

3000 2500 2000 1500 -1 Wavenumber, cm

1000

500

Fig. 3. ATR-IR spectra of pure GN and synthesized nanocomposite. 582

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N1s

(a)

Intensity (A.U.)

C1s O1s

Pure GN

GN-MoOx

200

400

600 800 Binding enegy, eV

1000

1200

40000

C1s

(b)

(d)

O1s

12000

Counts/s

Counts/s

30000

20000

10000

10000 8000

0 280

60000

282

284

286

Binding enegy, eV

288

290

528

530

532

534

536

538

540

Binding enegy, eV 12000

N1s

(c)

526

(e)

Mo 3d

10000 4+

Counts/s

Counts/s

50000 40000 30000

Mo (3d3/2)

8000

6+

Mo (3d5/2 )

6+

6000

Mo (3d3/2)

4+

Mo (3d5/2) 4000

20000

2000

10000

0

0

405

400 395 Binding enegy, eV

390

245

240

235

230

225

220

Binding energy, eV

Fig. 4. XPS results of pure GCN and of synthesized nanocomposite.

the adhesion strength of the coated substrates and the obtained results are displayed in Fig. 6. It can be understood that the epoxy coatings reinforced with synthesized nanocomposites significantly improved the adhesion strength between coating and substrate. The obtained result is attributed to the homogeneous distribution of synthesized GN nanocomposites in comparison to pure GN in epoxy matrix. This observation is validating the role of MoOX nanoparticles on the homogeneous distribution of GN nanosheets. The MoOX can improve the intercalation and exfoliation properties of GN nanosheets, which indicates that MoOx probably act as a dispersing agent for GN sheets, promoting the uniform distribution in epoxy matrix. The adhesion strength of coatings increased up to the incorporation of 3 wt%, and then decreased with the inclusion of 5 wt% of GN/MoOx. The improved adhesion performance through the inclusion of nanoparticles is generally obtained due to the pore-free packing of the nanofillers/pigments in the epoxy coatings.

the metallic molybdenum oxide residue of 36 wt% [44]. The weight loss percentage of pure GN at 700 °C is about 20% while that of the GN nanocomposite is about 36%. The difference in the residue's percentage between pure GN and GN/MoOx is assigned to be the content of MoOx nanoparticles. Associating the TGA data, the weight content of the MoOx in the GN/MoOx nanocomposite was calculated to be about 18.5%. It is almost close to the feeding ratio of the MoOx and GN, thus most of MoOx was incorporated into the GN nanosheets during the insitu synthesis of nanocomposites. 3.4. Adhesion test results The interfacial adhesion strength is a vital feature for anti-corrosion coatings and thus essential to be well inspected prior to corrosion assessments. Hydraulic pull-off adhesion tests were performed to evaluate 583

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Pourhashem et al. have already found that the homogeneous distribution of GO nanosheets increases adhesion strength by the formation of hydrogen bonding with metallic substrate via the hydroxyl groups of functional epoxy groups [46]. However, the agglomeration due to high loading of nanomaterials in a coating can lead to destroying the coating's integrity and adhesion due to the agglomeration inside polymer matrix and interface between polymer and metal. Hence, to get further clear illustration, the surface morphologies of epoxy coatings with the different contents of nanocomposite were analyzed and the obtained SEM images are presented in Fig. 7. The SEM observations of epoxy coatings with 1 and 3 wt% of GN nanocomposites indicate a good dispersibility of GN/MoOx in the epoxy resin. At this content level of the reinforcements, the epoxy coating exhibits a homogeneous microstructure without any micro defects. However, it can be obviously observed that the agglomeration/aggregation of GN nanocomposite occurs by increasing GN to 5 wt% and some micro cracks and shrinkage of polymer films are observed due to heterogeneous distribution of GN sheets, indicating relatively weak interfacial bonding between GN and polymer coating. It is also believed that when the content of GN nanocomposite increases, good dispersion and exfoliation of GN sheets is not obtained in polymer matrix due to the high specific area of GN sheets [45]. The obtained results revealed that the optimum incorporation of synthesized nanocomposite is 3 wt% and the above which, causes the negative impact on adhesion strength which possibly due to the agglomeration/heterogeneous distribution of nanomaterials into the coating matrix.

100

Weight loss (%)

90 80 70 60 50 40

GN/MoOx Pure GN

30 20

100

200

300 400 500 600 Temperature ( C)

700

800

Fig. 5. TGA curves of synthesized GN and its nanocomposites.

3.5. Electrochemical corrosion test results The EFM curves of uncoated and coated AA2024 Al alloy substrates after immersion in 3.5% NaCl solution are given in Fig. 8 and the obtained values are summarized in Table 2. The values of CF-2 and CF-3 in Table 2 are almost equal to the defined values of 2.0 and 3.0, respectively, demonstrating that the experimental data are reliable and of good quality [46]. Since the corrosion current density (icorr) is directly proportional to the corrosion rate, a lower icorr value generally signifies a higher corrosion resistant behavior [47]. From Table 2, all the coated

Fig. 6. Adhesion test results for the coated AA 2024 Al alloy substrates.

Fig. 7. SEM images of epoxy coatings with different amount of synthesized GN/MoOx nanocomposites. 584

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-6

10

-7

10

-8

10

-9

-10

10

10

-6

10

-7

10

-8

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-9

10

-10

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-11

10

-12

-11

10

-12

10

0

2

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Frequency (Hz) -5

Current (A)

Current (A)

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-10

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10

0

2

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14

-5

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-6

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-7

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-10

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PE/GN

0

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2

4

6

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-6

10

-6

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-7

10

-7

10

-8

10

-8

10

-9

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PE/GNM2

10

-10

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-11

10

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0

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Frequency (Hz)

6

8

10

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14

12

14

16

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10

PE/GNM1

-6

2

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10 10

PE

Current (A)

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10

Uncoated

Current (A)

-5

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Current (A)

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-10

10

-11

10

-12

PE/GNM3

0

2

4

6

8

10

12

14

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Frequency (Hz)

Frequency (Hz)

Fig. 8. EFM curves of uncoated and coated AA2024 alloy substrates in 3.5% NaCl solution. Table 2 EFM parameter for the uncoated and coated AA2024 substrates (the range of standard deviation for the obtained values is found to be between 0.5% and 5.5%).

1 2 3 4 5 6

Substrates

Uncoated PE PE/GN PE/GNM1 PE/GNM2 PE/GNM3

Ecorr mV

μA cm

−649 −299 −274 −220 −192 −244

4.59120 0.00300 0.00120 0.00049 0.00039 0.00091

Icorr

CF2

CF3

Rp Ohm cm2

1.62 1.72 1.83 1.88 1.89 1.73

2.84 2.69 2.82 2.89 2.94 2.77

1.346 × 106 3.967 × 107 1.890 × 108 3.645 × 109 1.508 × 1010 9.842 × 108

−2

OCP vs SCE (m V)

S. no

-200

(a)

-400

-600 Uncoated PE PEGN PEGNM1 PEGNM2 PEGNM3

-800

Al substrates exhibit lower icorr than bare substrates, particularly, PE coatings containing GN/MoOx nanocomposite exhibit significantly lowest icorr values. Uncoated and PE coated substrates showed Rp value of 1.346 × 106 and 3.967 × 107 Ω cm2, while PE/GNM1, PE/GNM2 and PE/GNM3 coatings showed improved Rp of 3.645 × 109, 1.508 × 1010 and 9.842 × 108 Ω cm2, respectively. Further, the changes in the OCP and Rp values have been monitored during the immersion up to 60 days in 3.5% NaCl solution and the obtained results are plotted in Fig. 9a–b. In the case of bare Al substrate, the OCP values were observed to be about −0.650 V and remained to be constant at −0.900 V. Slight fluctuations in OCP were identified, which indicates the continuous depassivation and repassivation leading to the formation of metastable pits on the Al surface [48]. The variation in OCP and Rp values of coated Al substrates with exposure time was completely different compared to that of uncoated Al substrate. At the initial immersion time (3 days), the OCPs of coated Al substrates were observed to be about −300 mV and slowly reached to −500 mV within 10 days and then, remained nearly constant up to the end of the immersion time. It was found that the OCP is significantly shifted to a nobler direction and also relatively stable with immersion time after the incorporation of GN/Mo nanocomposite into PE coatings. Particularly, the PE coating reinforced with 5% GN/Mo nanocomposite

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Generally, in the Bode plots, phase angles appearing in the higher frequency region are assigned to the response of coatings, whereas those appearing in the region of medium/low frequencies are accompanied with the responses of corrosion phenomenon that reveal the coating failure [50]. For all coated substrates, only one evident peak is identified at high/medium frequency after 1-day immersion, owing to the responses of coatings. In particular, all of the coated substrates exhibited the phase angle of about - 90° in the high frequency region, indicating a strong barrier protection offered by them in the initial immersion periods [51]. In the case of PE coatings (Fig. 10), a phase angle representing the response of corrosion exhibits at a lower frequency region from 100 to 10 mHz, demonstrating that the PE coated substrate has lost its barrier protection after of 60-day immersion in the chloride solution. Whereas PE/GN (Fig. 10), a phase angle at the low-frequency region that is representative of the responses of Al corrosion could not be illustrated up to 30 days, revealing that the GN reinforced PE coating has a better corrosion resistant behavior compared to that of PE coating. Bode phase angle curve of PE/GN nanocomposite coatings display only one evident peak in the high/medium frequency region, indicating that the barrier performance of PE coatings is improved with the inclusion of GN nanocomposite. It is worth mentioning here that though the width of phase angle curve slowly narrows, the response for coating defect and resultant corrosion is not identified during the entire immersion time [52]. This result indicates that the PE/GN nanocomposite coated substrates are not corroded after 60-day immersion. Moreover, the Bode-resistant plot exhibits that the impedance modulus of PE/GNM2 coatings at 10 mHz is higher than those of other substrates. Thus, the addition of 3% of GN nanocomposite is the optimum reinforcement methodology to enhance the corrosion protection performance of PE coatings. The experimentally obtained EIS curves were fitted with the equivalent circuit diagrams (Fig. 11) in order to quantitatively assess the corrosion resistant behavior of PE nanocomposite coated AA2024 Al alloy substrates. Based on the impedance and capacitance response with the function of immersion time, the three equivalent circuit models were selected in order to precisely fit the obtained EIS data [53–56]. At the primary exposure periods up to 24 h, coated substrates displayed a high resistance and low capacitance response and hence the model A was utilized to describe the impedance behavior in this stage. With extended immersion time, aggressive ions from the electrolyte penetrated through the coating to metal/coating interface, the corrosion reactions initiated and propagated which results in a decrease of impedance were observed in the low frequencies. Equivalent circuit model B was utilized to describe the appearance of the second time constant in impedance response of the coating system at this stage of exposure. Finally, due to the vigorous corrosion reaction at the metal/coating

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showed the noblest shift in OCP value among the studied coatings. The highest and the lowest decrease of OCP were identified for the bare Al and pure PE coatings, respectively. Rp values were also increased with the addition of GN nanocomposite up to 3 wt% and then remained unchanged. Reduction in Rp values with immersion time is observed in the case of pure PE coatings and this change is relatively lower in the case of PE/GN nanocomposite coatings. The obtained findings reveal that the incorporation of GN nanocomposite significantly proves a better barrier performance than that of GN in the PE coatings. Bode plots of PE coatings with and without reinforcement at the initial immersion and during 60-day immersion in 3.5% NaCl solution are presented in Fig. 10, respectively. Generally, the impedance values at 0.01 Hz is utilized to get an indication of the barrier performance of the studied coating. For the PE coatings, the impedance at 0.01 Hz was found to be 3.58 × 105 Ω.cm2 at the initial immersion and the value reduced to 2.32 × 103 Ω.cm2 after of 60-day immersion, demonstrating that the permeation of aggressive species into PE coatings causing the deterioration and complete loss of barrier protection [49]. In contrast, the initial impedance values of PE/GN, PE/GNM1, PE/GN/M2 and PE/ GNM3 at 0.01 Hz were 3.37 × 106 Ω.cm2, 5.67 × 1010 Ω.cm2, 1.38 × 1011 Ω.cm2 and 2.87× 108 Ω.cm2, respectively. These values lowered to 5.44 × 105 Ω cm2, 2.53 × 108 Ω.cm2, 6.74 × 108 Ω.cm2 and 2.56 × 107 Ω.cm2 after 60 days of immersion, respectively, that are three or four-order magnitude higher compared to that of PE coatings during the entire immersion time. In particular, the PE/GNM2 coating revealed the highest impedance values comparing to those of other coatings, signifying that this coating has the highest corrosion protection performance among the investigated coatings.

Fig. 11. Equivalent circuit models utilized for fitting of the obtained EIS results with the function of immersion time (A - Early immersion periods; B - Permeation of aggressive species into the coating interface; C - accumulation of corrosion products at metal/coating interface). 586

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species into the metal/coating interface caused the delamination of coatings which is a result of the formation of corrosion products [61]. By contrast, the Rct of the PE/GN coating decreased from 3.37 × 106 Ω.cm2 to 4.71 × 105 Ω.cm2 after 7 days of immersion in the chloride medium. Beyond 30 days of immersion, the Rct fluctuated and steadied around 7.01 × 104 Ω.cm2. Remarkably, Rct of the PE/GNM2 coating slightly decreased up to 14 days and then remained constant around 6 × 108 Ω.cm2 in the entire immersion period. The presence of GN and MoOx nanoparticles can protect the coatings defect areas and prevent the coatings from the delamination phenomenon. The CPEf of PE coated Al substrates during the immersion time are presented in Fig. 13c. In general, coatings' capacitance is utilized to inspect the water absorption into the polymeric coatings. It is clear that the CPEf of PE increases with extending exposure period, suggesting that water can readily permeate the PE coating [62]. Generally, water absorption could cause the swelling of the coatings and followed by the formation of blistering at the coating defected areas. Further, the formed blister probably breaks due to a developed osmotic pressure inside the coatings, which further leads to the formation of a direct path for corrosive ions to reach the base substrate, and hence, the onset of corrosion. The CPEf of PE/GN primarily increases after 30th day, demonstrating that PE/GN gradually drop barrier performance after 30-day of exposure. It is important to mention that the change in CPEf of PE/ GNM1 and PE/GNM2 nearly retains constant during the entire immersion time, revealing that these coatings have a compact and stable metal/coating interface without any indication of corrosion. As for PE/ GN/M3, the CPEf steadily increases during the whole immersion period, however, still less than that of pure epoxy, indicating that the permeation of water into the coating is more difficult than the case of pure epoxy. The obtained results indicate that nanocomposite acts as a filler and covers the pores and micro defects inside the epoxy resin to hinder

interface, corrosion products formed beneath the coating, the equivalent circuit model C, as a suitable model for explaining the impedance response of the coating was utilized to fit the obtained EIS curves (Fig. 12). The obtained EIS parameters are displayed in Table 3 and few important parameters with the function of immersion time are plotted as graph in Fig. 13(a–c). In the equivalent circuits, Rs, Rf, Rct, represent the solution, coating and charge transfer resistance respectively. Further, CPEf and CPEdl represent constant phase element (CPE) of coating and double layer capacitance element, respectively. The CPE is utilized to describe the non-ideal behavior of the coated metallic substrates due to the non-homogeneous surface [57–59]. Further, Warburg impedance component (W) was inserted in model C, which is representing the diffusion-controlled electrochemical corrosion reactions at the metal/ coating interface. The utilized equivalent circuit models fitted properly which was validated using the low value of χ2. Fig. 13b displays the change of coatings resistance (Rf) during the immersion time, revealing that Rf slowly drops with prolonged immersion time. The Rf of PE/GN is slightly higher compared to that of pure PE after immersion of 60 days, while the Rf of PE/GNM1 is about 3 orders of magnitude compared to that of PE/GN and PE coatings. The increase in Rf is ascribed to the intercalation of nanocomposite into the epoxy matrix [60]. Also, the Rf of PE/GNM2 is 15 times greater than that of pure epoxy, as the PE/GNM2 have a compact structure which further improves the barrier performance. The Rct values of the PE coatings (Fig. 13a) are considerably reduced during the immersion time, however, the reduction in Rct values for the GN and GN nanocomposite reinforced PE coating was not significant. The Rct values of the PE coating instantly decreased from 3.58 × 105 Ω.cm2 to 1.63 × 104 Ω.cm2 after immersion for 7 days. After 30 days of immersion, the Rct fluctuated and then steadied around 5.91 × 103 Ω.cm2. For PE coating, the permeation of electrolytic 587

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Table 3 EIS circuit parameter of coated AA2024 Al alloy substrates in 3.5% NaCl solution (the range of standard deviation for the obtained values is found to be between 1% and 9.5%). Sample

PE PE/GN PE/GNM1 PE/GNM2 PE/GNM3 PE PE/GN PE/GNM1 PE/GNM2 PE/GNM3 PE PE/GN PE/GNM1 PE/GNM2 PE/GNM3 PE PE/GN PE/GNM1 PE/GNM2 PE/GNM3 PE PE/GN PE/GNM1 PE/GNM2 PE/GNM3 PE PE/GN PE/GNM1 PE/GNM2 PE/GNM3 PE PE/GN PE/GNM1 PE/GNM2 PE/GNM3

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2.02E5 1.01E6 1.62E10 1.57E11 2.47E8 1.61E4 4.38E5 4.92E9 2.29E10 3.08E7 8.20E3 4.22E5 5.00E8 2.03E9 7.39E7 1.44E4 1.28E5 2.74E8 1.02E9 1.09E8 1.16E4 2.09E5 3.76E8 1.43E9 1.47E8 5.98E3 6.72E4 2.55E8 7.38E8 5.12E7 2.40E3 6.11E4 2.84E8 6.86E8 2.17E7

CPEdl

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ndl

5.6E−09 2.6E−10 1.8E−12 8.5E−12 1.3E−11 7.7E−08 1.4E−10 8.3E−11 2.2E−12 7.7E−10 1.3E−07 9.0E−09 1.4E−11 7.3E−11 3.2E−10 7.7E−07 6.4E−08 6.3E−10 1.2E−11 1.0E−10 8.6E−06 7.3E−07 2.7E−10 8.5E−10 7.5E−09 5.6E−05 3.1E−07 8.4E−09 1.2E−09 2.1E−09 4.6E−05 6.2E−06 2.1E−09 8.5E−08 2.3E−07

0.98 0.99 0.99 0.99 0.99 0.96 0.96 0.98 0.98 0.96 0.94 0.94 0.96 0.96 0.95 0.90 0.91 0.95 0.96 0.89 0.88 0.89 0.94 0.94 0.90 0.89 0.88 0.92 0.93 0.91 0.86 0.85 0.90 0.91 0.88

1.41E5 7.93E5 6.15E8 1.92E9 1.12E8 6.95E5 2.07E5 1.52E8 1.13E9 4.67E7 4.99E5 1.23E5 1.06E8 3.49E8 7.98E6 1.76E4 5.78E4 8.55E7 1.89E8 3.03E7 5.35E3 4.83E4 6.17E7 1.41E8 2.01E7 3.75E3 3.72E4 4.31E7 9.10E7 1.75E7 1.92E3 2.45E4 4.44E7 9.01E7 9.31E6

CPEf Y0 (Ω−1 cm−2sn)

nf

2.1E−07 1.3E−08 8.4E−10 2.3E−10 5.7E−10 5.6E−07 8.5E−08 6.2E−10 2.3E−10 7.1E−09 3.2E−06 5.1E−08 5.5E−10 2.0E−10 2.8E−09 5.0E−06 4.3E−07 4.2E−10 1.7E−10 7.6E−09 4.7E−05 4.2E−06 4.5E−10 1.7E−10 4.9E−09 1.2E−05 2.4E−06 3.4E−10 1.5E−10 1.7E−09 3.6E−05 4.2E−06 2.3E−10 7.2E−11 1.1E−08

0.99 0.99 0.99 0.99 0.99 0.97 0.97 0.98 0.98 0.96 0.96 0.96 0.97 0.97 0.95 0.94 0.96 0.95 0.95 0.95 0.89 0.89 0.93 0.93 0.92 0.86 0.87 0.90 0.90 0.88 0.84 0.83 0.90 0.91 0.89

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χ2

– – – – – 2.3E−06 – – – – 4.5E−05 – – – – 9.2E−04 – – – – 1.4E−04 – – – – 6.7E−03 5.4E−05 – – – 2.7E−03 1.9E−05 – – –

2.3E−04 5.4E−04 5.2E−04 4.7E−04 7.4E−04 2.1E−04 4.3E−04 7.6E−04 3.7E−04 8.7E−04 8.3E−04 6.5E−04 8.3E−04 2.5E−04 4.9E−04 1.5E−04 3.2E−04 9.4E−04 5.7E−04 5.4E−04 9.6E−04 4.3E−04 3.6E−04 7.5E−04 8.3E−04 9.1E−04 8.4E−04 7.4E−04 8.2E−04 4.7E−04 8.3E−04 9.4E−04 6.4E−04 5.8E−04 6.7E−04

nanosheets via the incorporation of MoOx nanorods significantly increased the barrier and corrosion protection performance of the epoxy coating than that provided by pure GN sheets. Moreover, the surface modification of GN with MoOx probably enhances the compatibility with the epoxy matrix, resulting in the homogeneous dispersion, leading to the improved barrier performance. GN nanocomposite covers the micro defects, cavities and free volumes of the epoxy matrix, resulting in a reduction in the permeability of the coatings by zigzagging the diffusion pathway and made the diffusion path of electrolyte more tortuous. The permeation of electrolyte into the coating matrix and metal/coating interface is significantly reduced when the epoxy coatings possess GN nanocomposites. As a result, less coatings degradation and delamination occur. Moreover, due to the different size and shapes of the GN and MoOx, the reinforcement of nanocomposites can better fill the coating porosities and cavities, resulting in a greater improvement in the coating's adhesion strength and barrier performance. Concluding the discussions with the obtained results, the reinforcement of GN nanocomposites significantly improved the adhesion and corrosion protection performance of epoxy coatings on AA2024 Al alloy substrates.

the permeation and movement of aggressive ions in the coating. By increasing the loading of 5 wt% of GN/MoOx nanocomposite, the barrier performance reduces due to the agglomeration of nanosheets in the polymer matrix. From the obtained results, it was concluded that the reinforcement of GN nanocomposite provided a significant enhancement in the corrosion protection behavior of epoxy coatings. Fig. 14 displays the schematic representation of the corrosion protection mechanism for epoxy/GNMoOx nanocomposite coatings. In general, pure epoxy coatings on Al alloy surface possess few micro cracks, defects including cavities and pores, formed during the application and curing process, which are paths for the permeation of aggressive species from the electrolyte. The penetrations of aggressive species entered the metal/ coating interface and initiated the corrosion reactions as exposure periods extended, leading to the coating's delamination and deterioration on coatings integrity. Reinforcement of nanomaterials in nanosheets/lamellar forms into the epoxy coating has already been established as an approach to reduce the penetration of electrolytes into metal/coating interface. However, it depends on the nature of polymeric resin and it compatibility with reinforced nanomaterials. Compatibility of pure GN sheets with epoxy resin is poor due to the high hydrophilicity of GN sheets resulted in the agglomeration in the epoxy matrix, yielding micro defects and free volumes in the coatings matrix [63,64]. The heterogeneous distribution of GN sheets forms a pathway for the penetration of electrolytes into the coating matrix. The obtained electrochemical corrosion results exhibited that the GN sheets can improve the corrosion protection performance of epoxy coating to a certain degree. However, the reinforcement of GN

4. Conclusions A nanocomposite based on the GN and MoOx nanoparticles was successfully synthesized and examined by different surface and structural analyses. SEM and TEM images corroborated the presence of a uniformly distributed of MoOx nanoparticles on the GN matrix. Structural characterization results revealed that the influence of MoOx 588

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Acknowledgment 0

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We thank King Fahd University of Petroleum and Minerals (KFUPM Saudi Arabia) for providing all support to this project. This project has been funded by King Fahd University of Petroleum and Minerals under project no. FT161002.

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References

Rf ( cm

2

)

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[1] H. Masuda, F. Hasegwa, S. Ono, Self-ordering of cell arrangement of anodic porous alumina formed in sulfuric acid solution, J. Electrochem. Soc. 144 (1997) L127–L130. [2] Kathrin Schwirn, Woo Lee, Reinald Hillebrand, Martin Steinhart, Kornelius Nielsch, Ulrich Gosele, Self-ordered anodic aluminum oxide (AAO) formed by H2SO4 hard anodization (HA), ACS Nano 2 (2008) 302–310. [3] N.W. Khun, B.R. Troconis, G.S. Frankel, Effects of carbon nanotube content on adhesion strength and wear and corrosion resistance of epoxy composite coatings on AA2024-T3, Prog. Org. Coat. 77 (2014) 72–80. [4] M.Y. Jiang, L.K. Wu, J.M. Hu, J.Q. Zhang, Silane-incorporated epoxy coatings on aluminum alloy (AA2024), part 1: improved corrosion performance, Corros. Sci. 92 (2015) 118–126. [5] M.Y. Jiang, L.K. Wu, J.M. Hu, J.Q. Zhang, Silane-incorporated epoxy coatings on aluminum alloy (AA2024); part 2: mechanistic investigations, Corros. Sci. 92 (2015) 127–135. [6] Jianhua Liu, Qing Yu, Mei Yu, Songmei Li, Kuo Zhao, Bing Xue, Hang Zu, Silane modification of titanium dioxide-decorated graphene oxide nanocomposite for enhancing anticorrosion performance of epoxy coatings on AA-2024, J. Alloys and Comp. 744 (2018) 728–739. [7] M.G. Saria, M. Shamshiria, B. Ramezanzadeh, Fabricating an epoxy composite coating with enhanced corrosion resistance through impregnation of functionalized graphene oxide-co-montmorillonite nanoplatelet, Corros. Sci. 129 (2017) 38–53. [8] S. Palraj, M. Selvaraj, K. Maruthan, G. Rajagopal, Corrosion and wear resistance behavior of nano-silica epoxy composite coatings, Prog. Org. Coat. 81 (2015) 132–139. [9] S.Z. Haeria, M. Asgharia, B. Ramezanzadeh, Enhancement of the mechanical properties of an epoxy composite through inclusion of graphene oxide nanosheets functionalized with silica nanoparticles through one and two steps sol-gel routes, Prog. Org. Coat. 111 (2017) 1–12. [10] W. Tian, L. Liu, F. Meng, Y. Liu, Y. Li, F. Wang, The failure behaviour of an epoxy glass flake coating/steel system under marine alternating hydrostatic pressure, Corros. Sci. 86 (2014) 81–92. [11] I.A. Kartsonakis, E. Athanasopoulou, D. Snihirova, B. Martins, M.A. Koklioti, M.F. Montemor, G. Kordas, C.A. Charitidis, Multifunctional epoxy coatings combining a mixture of traps and inhibitor loaded nanocontainers for corrosion protection of AA2024-T3, Corros. Sci. 85 (2014) 147–159. [12] W. Wang, L. Xu, X. Li, Y. Yang, E. An, Self-healing properties of protective coatings containing isophorone diisocyanate microcapsules on carbon steel surfaces, Corros. Sci. 80 (2014) 528–535. [13] Samira Ghiyasi, Morteza Ganjaee Sari, Meisam Shabanian, Mohsen Hajibeygi, Payam Zarrintaj, Marco Rallini, Luigi Torr, Debora Pugli, Henri Vahabi, Maryam Jouyandeh, Fouad Laoutid, Seyed Mohammad Reza Parani, Mohammad Reza Saeb, Hyper branched poly(ethyleneimine) physically attached to silica nanoparticles to facilitate curing of epoxy nanocomposite coatings, Prog. Org. Coat. 120 (2018) 100–109. [14] Ruben F.A.O. Torrico, Samarah V. Harb, Andressa Trentin, Mayara C. Uvida, Sandra H. Pulcinelli, Celso V. Santilli, Peter Hammer, Structure and properties of epoxysiloxane-silica nanocomposite coatings for corrosion protection, J. Colloid Inter. Sci. 513 (2018) 617–628. [15] B. Ramezanzadeh, M.M. Attar, Cathodic delamination and anticorrosion per-formanceof an epoxy coating containing nano/micro-sized ZnO particles on Cr(III)-Co (II)/Cr(III)-Ni(II) post treated steel samples, Corros. Sci. 69 (2013) 793–803. [16] Bahram Ramezanzadeh, Behzad Karimi, Mohammad Ramezanzadeh, Mehran Rostami, Synthesis and characterization of polyaniline tailored graphene oxide quantum dot as an advance and highly crystalline carbon-based luminescent nanomaterial for fabrication of an effective anti-corrosion epoxy system on mild

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nanoparticles on GN nanosheets. Adhesion test results confirmed that the addition of the synthesized nanocomposite does not change the adhesion strength of the epoxy coatings on Al substrates. Further, electrochemical corrosion testing results corroborated that the addition of the synthesized nanocomposite has significantly enhanced the anticorrosion behavior of epoxy coatings in 3.5% NaCl medium. Based on the obtained findings in this study, it can be concluded that the synthesized GN-MoOx nanocomposite can be a potential material for nanofillers in improving the surface protective performance of epoxy coatings in NaCl medium.

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steel, J. Taiwan Inst. Chem. Eng. 95 (2019) 369–382. [17] B. Ramezanzadeh, S. Niroumandrad, A. Ahmadi, M. Mahdavian, M.H. Mohamadzadeh Moghadam, Enhancement of barrier and corrosion protection performance of an epoxy coating through wet transfer of amino functionalized graphene oxide, Corros. Sci. 103 (2016) 283–304. [18] Nafise Parhizkar, Bahram Ramezanzadeh, Taghi Shahrabi, The epoxy coating interfacial adhesion and corrosion protection properties enhancement through deposition of cerium oxide nanofilm modified by graphene oxide, J. Indus. Eng. Chem. 64 (2018) 402–419. [19] Junjiang Zhu, Ping Xiao, Hailong Li, Sonia A.C. Carabineiro, Graphitic carbon nitride: synthesis, properties, and applications in catalysis, ACS Appl. Mater. Inter. 6 (2014) 16449−16465. [20] Ligang Zhang, Huimin Qia, Guitao Li, Daoai Wang, Tingmei Wang, Qihua Wang, Ga Zhang, Significantly enhanced wear resistance of PEEK by simply filling with modified graphitic carbon nitride, Mater. Des. 129 (2017) 192–200. [21] M. Behzadnasab, S.M. Mirabedini, M. Esfandeh, Corrosion protection of steel by epoxy nanocomposite coatings containing various combinations of clay and nanoparticulate zirconia, Corros. Sci. 75 (2013) 134–141. [22] S. Pourhashem, M.R. Vaezi, A.M. Rashidi, Investigating the effect of SiO2- graphene oxide hybrid as inorganic nanofiller on corrosion protection properties of epoxy coatings, Surf. Coat. Technol. 311 (2017) 282–294. [23] S. Chu, Y. Wang, Y. Guo, J. Feng, C. Wang, W. Luo, X. Fan, Z. Zou, Band structure engineering of carbon nitride: in search of a polymer photo catalyst with high photo oxidation property, ACS Catal. 3 (2013) 912–919. [24] I.B. Obot, B. Ikenna, Electrochemical frequency modulation (EFM) technique: theory and recent practical applications in corrosion research, J. Mol. Liq. 249 (2018) 83–96. [25] Na Xu, Yiru Wang, Mingcong Rong, Zhifeng Ye, Zhuo Deng, Xi Chen, Facile preparation and applications of graphitic carbon nitride coating in solid-phase micro extraction, J. Chromatography A 1364 (2014) 53–58. [26] M. Kim, S. Hwang, J.S. Yu, Novel ordered nanoporous graphitic C3N4 as a support for Pt–Ru anode catalyst in direct methanol fuel cell, J. Mater. Chem. 17 (2007) 1656–1659. [27] Lin Liu, Jinxiang Wang, Chengyin Wang, Guoxiu Wang, Facile synthesis of graphitic carbon nitride/nanostructured α-Fe2O3 composites and their excellent electrochemical performance for supercapacitor and enzyme-free glucose detection applications, Appli. Sur. Sci. 390 (2016) 303–310. [28] Lei Ge, Changcun Han, Jing Liu, Novel visible light-induced g-C3N4/Bi2WO6 composite photocatalysts for efficient degradation of methyl orange. Appl. Cat. B: Environment. 108–109 (2011) 100– 107. [29] Xiang dong Kang, Yuyang Kang, Xingxing Hong, Zhenhua Sun, Chao Zhen, Hu Chaohao, Gang Liu, Huiming ChengImproving the photocatalytic activity of graphitic carbon nitride by thermal treatment in a high-pressure hydrogen atmosphere, Prog. Natur. Sci.: Mater. Inter. 28 (2018) 183–188. [30] Javad Safaei, Nurul Aida Mohamed, Mohamad Firdaus Mohamad Noh, Mohd Fairuz Soh, Muhammad Arif Riza, Nurul Syafiqah Mohamed Mustakim, Norasikin Ahmad Ludin, Mohd Adib Ibrahim, Wan Nor Roslam Wan Isahak, Mohd Asri Mat Teridi, Facile fabrication of graphitic carbon nitride, (g-C3N4) thin film, J. Alloys Comp. 769 (2018) 130–135. [31] Guangdong Zhou, Wenxi Zhao, Xiaoqing Ma, A.K. Zhou, Current-voltage hysteresis of the composite MoS2-MoOx nanobelts for data storage, J. Alloys Comp. 679 (2016) 47–53. [32] Javad Safaei, Nurul Aida Mohamed, Mohamad Firdaus Mohamad Noh, Mohd Fairuz Soh, Muhammad Arif Riza, Nurul Syafiqah Mohamed Mustakim, Norasikin Ahmad Ludin, Mohd Adib Ibrahim, Wan Nor Roslam Wan Isahak, Mohd Asri Mat Teridi, Facile fabrication of graphitic carbon nitride, (g-C3N4) thin film, J. Alloys Comp. 769 (2018) 130–135. [33] L. Wang, J. Ding, Y.Y. Chai, Q.Q. Liu, J. Ren, X. Liu, W.L. Dai, Ag3PO4 nanoparticles loaded on 3D flower-like spherical MoS2: a highly efficient hierarchical heterojunction photocatalyst, Dalton Trans. 44 (2015) 11223–11234. [34] Vidyasagar Devthade, Akanksha Gupta, Suresh S. Umare, Graphitic carbon nitride−γ-gallium oxide (GCN-γ-Ga2O3) nanohybrid photocatalyst for dinitrogen fixation and pollutant decomposition, ACS Appl. Nano Mater. 1 (2018) 5581−5588. [35] Lingeswarran Muniandy, Farook Adam, Abdul Rahman Mohamed, Anwar Iqbal, Cu2+ coordinated graphitic carbon nitride (Cu-g-C3N4) nanosheets from melamine for the liquid phase hydroxylation of benzene and VOCs, Appl. Sur. Sci. 398 (2017) 43–55. [36] Nur Ruzaina, Abdul Rahmana, Wu Kun, Dongdong Chen, Wu Shuxing, Jianzhang Fang, Fan Yang, Ximiao Zhu, Zhanqiang Fang, One-step synthesis of sulfur and tungstate co-doped porous g-C3 N4 microrods with remarkably enhanced visible-light photocatalytic performances, Appl. Surf. Sci. 462 (2018) 991–1001. [37] J.J. Uhlrich, J. Sainio, Y. Lei, D. Edwards, R. Davies, M. Bowker, S. Shaikhutdinov, H.J. Freund, Preparation and characterization of iron–molybdate thin films, Surf. Sci. 605 (2011) 1550–1555. [38] Shukun Le, Tingshun Jiang, Yiwen Li, Qian Zhao, Yingying Li, Weibing Fang, Ming Gong, Highly efficient visible-light-driven mesoporous graphitic carbonnitride/ZnO nanocomposite photocatalysts, Appli. Cat. B: Environ. 200 (2017) 601–610. [39] Dashui Yuan, Jing Ding, Jie Zhou, Lei Wang, Hui Wan, Wei-Lin Dai, Guofeng Guan, Graphite carbon nitride nanosheets decorated with ZIF-8 nanoparticles: effects of the preparation method and their special hybrid structures on the photocatalytic performance, J. Alloys Comp. 762 (2018) 98–108. [40] W.Y. Huang, N. Liu, X.D. Zhang, M.H. Wu, L. Tang, Metal organic framework g-

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

[53]

[54]

[55]

[56]

[57]

[58]

[59] [60]

[61]

[62]

[63]

[64]

590

C3N4/MIL-53(Fe) heterojunctions with enhanced photocatalytic activity for Cr (VI) reduction under visible light, Appl. Surf. Sci. 425 (2017) 107–116. Deli Jiang, Qing Xu, Suci Meng, Changkun Xia, Min Chen, Construction of cobalt sulfide/graphitic carbon nitride hybrid nanosheet composites for high performance supercapacitor electrodes, J. Alloys Comp. 706 (2017) 41–47. Yongming Sun, Xianluo Hu, Wei Luo, Yunhui Huang, Self-assembled hierarchical MoO/graphene nanoarchitectures and their application as a high-performance anode material for lithium-ion batteries, ACS Nano 5 (2011) 7100–7107. Ibrahim Khan, Nadeem Baig, Ahsanulhaq Qurashi, Graphitic carbon nitride impregnated niobium oxide (g-CN/NbO) type (II) heterojunctions and its synergetic solar driven hydrogen generation, ACS Appl. Energy Mater. 2 (2019) 607–615. Chuanbo Hu, Ying Li, Ning Zhang, Yushi Ding, Synthesis and characterization of a poly(o-anisidine)–SiC composite and its application for corrosion protection of steel, RSC Adv. 7 (2017) 11732. Sepideh Pourhashem, Mohammad Reza Vaezi, Alimorad Rashidi, Mohammad Reza Bagherzadeh, Exploring corrosion protection properties of solvent based epoxy-graphene oxide nanocomposite coatings on mild steel, Corros. Sci. 115 (2017) 78–92. A. Madhan Kumar, Bharathi Suresh, Soumyadip Das, I.B. Obot, Akeem Yusuf Adesin, Suresh Ramakrishna, Promising bio-composites of polypyrrole and chitosan: surface protective and in vitro biocompatibility performance on 316L SS implants, Carbo. Polym. 173 (2017) 121–130. A. Madhan Kumar, S. Nagarajan, Suresh Ramakrishna, P. Sudhagar, Yong Soo Kang, Hyongbum Kim, Zuhair M. Gasem, N. Rajendran, Electrochemical and in vitro bioactivity of polypyrrole/ceramic nanocomposite coatings on 316L SS bio-implants, Mater. Sci. Eng. C 43 (2014) 76–85. Klodian Xhanari, Matjaz Finsgar, Masa Knez Hrnci, Uros Maver, Zeljko Knez, Bujar Seiti, Green corrosion inhibitors for aluminium and its alloys: a review, RSC Adv. 7 (2017) 27299–27330. Haihua Wang, Shudi Qin, Xiaofang Yang, Guiqiang Fei, Mi Tian, Yanming Shao, Ke Zhu, A waterborne uniform graphene-poly(urethane-acrylate) complex with enhanced anticorrosive properties enabled by ionic interaction, Chem. Eng. J. 351 (2018) 939–951. Xinding Lv, Xitao Li, Nan Li, Hechuang Zhang, Yanzhen Zheng, Jiaojiao Wu, Xia Tao, ZrO2 nanoparticle encapsulation of graphene microsheets for enhancing anticorrosion performance of epoxy coatings, Surf. Coat. Technol. 358 (2019) 443–451. A. Madhan Kumar, R. Suresh Babu, Suresh Ramakrishna, Ana L.F. de Barros, Electrochemical synthesis and surface protection of polypyrrole-CeO2 nanocomposite coatings on AA2024 alloy, Synth. Met. 234 (2017) 18–28. A. Madhan Kumar, A. Khan, R. Suleiman, M. Qamar, S. Saravanan, H. Dafall, Bifunctional CuO/TiO2 nanocomposite as nanofiller for improved corrosion resistance and antibacterial protection, Prog. Org. Coat 114 (2018) 9–18. Xinding Lv, Xitao Li, Nan Li, Hechuang Zhang, Yan-zhen Zheng, Jiaojiao Wu, Xia Tao, ZrO2 nanoparticle encapsulation of graphene microsheets for enhancing anticorrosion performance of epoxy coatings, Sur. Coat. Technol. 358 (2019) 443–451. Javad Khodabakhshi, Hossein Mahdavi, Farhood Najafi, Investigation of viscoelastic and active corrosion protection properties of inhibitor modified silica nanoparticles/epoxy nanocomposite coatings on carbon steel, Corros. Sci. 147 (2019) 128–140. Xuehui Liu, Chuanjun Gu, Zhehua Wen, Baorong Hou, Improvement of active corrosion protection of carbon steel by water-based epoxy coating with smart CeO2 nanocontainers, Prog. Org. Coat. 115 (2018) 195–204. Y.H. Yu, Y.Y. Lin, C.H. Lin, C.C. Chan, Y.C. Huang, High performance polystyrene/ graphene-based nanocomposites with excellent anti-corrosion properties, Polym. Chem. 5 (2014) 535–550. Bahram Ramezanzadeh, Ghasem Bahlakeh, Mohammad Ramezanzadeh, Polyaniline-cerium oxide (PAni-CeO2) coated graphene oxide for enhancement of epoxy coating corrosion protection performance on mild steel, Corros. Sci. 137 (2018) 111–126. A. Madhankumar, N. Rajendran, Influence of zirconia nanoparticles on the surface and electrochemical behavior of polypyrrole nanocomposite coated 316L SS in simulated body fluid, Surf. Coat. Technol. 213 (2012) 155–166. Ji-Heng Ding, Hong-Ran Zhao, Yan Zheng, Xinpeng Zhao, Hai-Bin Yu, A long-term anticorrsive coating through graphene passivation, Carbon 138 (2018) 197–206. M. Behzadnasab, S. Mirabedini, K. Kabiri, S. Jamali, Corrosion performance of epoxy coatings containing silane treated ZrO2 nanoparticles on mild steel in 3.5% NaCl solution, Corros. Sci. 53 (2011) 89–98. A. Madhan Kumar, S. Nagarajan, Suresh Ramakrishna, P. Sudhagar, Yong Soo Kang, Hyongbum Kim, Zuhair M. Gasem, N. Rajendran, Electrochemical and in vitro bioactivity of polypyrrole/ceramic nanocomposite coatings on 316L SS bio-implants, Mater. Sci. Eng. C 43 (2014) 76–85. A. Madhankumar, P. Sudhagar, Akira Fujishima, Zuhair M. Gasem, Hierarchical polymer nanocomposite coating material for 316L SS implants: surface and electrochemical aspects of PPy/f-MWCNT, Coatings. Polym. 55 (2014) 5417–5424. Hongpeng Zheng, Yawei Shao, Yanqiu Wang, Guozhe Meng, Bin Liu, Reinforcing the corrosion protection property of epoxy coating by using graphene oxide–poly (urea–formaldehyde) composites, Corros. Sci. 123 (2017) 267–277. Bahram Ramezanzadeh, Ghasem Bahlakeh, M.H.Mohamadzadeh Moghadam, Rooshanak Miraft, Impact of size-controlled p-phenylenediamine (PPDA)-functionalized graphene oxide nanosheets on the GO-PPDA/Epoxy anti-corrosion, interfacial interactions and mechanical properties enhancement: experimental and quantum mechanics investigations, Chem. Eng. J. 335 (2018) 737–755.