Covalently-grafted graphene oxide nanosheets to improve barrier and corrosion protection properties of polyurethane coatings

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Covalently-grafted graphene oxide nanosheets to improve barrier and corrosion protection properties of polyurethane coatings B. Ramezanzadeh a,*, E. Ghasemi b, M. Mahdavian a, E. Changizi c, M.H. Mohamadzadeh Moghadam d a

Department of Surface Coatings and Corrosion, Institute for Color Science and Technology (ICST), PO 16765-654, Tehran, Iran Department of Nanomaterials and Nanocoatings, Institute for Color Science and Technology (ICST), PO 16765-654, Tehran, Iran c Institute for Color Science and Technology (ICST), PO 16765-654, Tehran, Iran d Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, Iran b

A R T I C L E I N F O

A B S T R A C T

Article history:

Surface modification of graphene oxide (GO) has been performed by grafting of polyiso-

Received 20 February 2015

cyanate (PI) resin. Results obtained from X-ray photo electron spectroscopy, thermal gravi-

Accepted 25 May 2015

metric analysis and X-ray diffraction analysis revealed that the PI resin chains were

Available online 3 June 2015

successfully attached onto the surface of GO nanosheets through covalent bonding with hydroxyl and carboxylic groups leading to amides and carbamate esters bonds formation. Subsequently, the PI functionalized GO sheets were incorporated into the polyurethane (PU) matrix. The X-ray diffraction analysis showed that surface modification of GO nanosheets with PI enhanced the level of exfoliation of PI–GO in the PU matrix. Scanning electron microscope analysis showed the enhanced interaction between GO and PU matrix after functionalization by PI resin. The electrochemical impedance spectroscopy and salt spray tests were performed to reveal the effects of addition of 0.1 wt.% GO and PI–GO nanosheets on the corrosion protection properties of the PU coating. Also, the adhesion loss of the coatings was obtained by pull-off adhesion test after 30 days immersion in 3.5 wt.% NaCl solution. It was found that incorporation of 0.1 wt.% surface modified GO nanosheets into the PU matrix resulted in significant improvement of the coating corrosion protection properties and ionic resistance.  2015 Elsevier Ltd. All rights reserved.

1.

Introduction

Organic coatings, i.e. epoxy and polyurethane, have been widely applied on metal substrates to protect them against corrosion. They provide a physical barrier between corrosive environment and metal surface. However, all of the organic coatings are more or less permeable to corrosive agents i.e.

oxygen, water and ions. They cannot provide long term corrosion protection properties due to two main reasons. First, the coating suffers a degradation process after exposure to corrosive electrolyte. The electrolyte penetration into the coating is responsible for the hydrolytic degradation causing a path to the underlying surface. Therefore, the water molecules can gradually reach the metal/coating interface leading to the

* Corresponding author: Fax: +98 2122947537. E-mail addresses: [email protected], [email protected] (B. Ramezanzadeh). http://dx.doi.org/10.1016/j.carbon.2015.05.094 0008-6223/ 2015 Elsevier Ltd. All rights reserved.

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decrease of coating adhesion, and thus accelerating corrosion of the metal beneath the coating [1–5]. Therefore, attempts have been extensively done to improve the corrosion protection properties of the organic coatings by different ways. Among different methods the incorporation of additives, fillers and pigments into the organic paints formulations have been introduced as the most effective way of approaching the mentioned target. A wide range of fillers and pigments have been incorporated into the paint coatings to improve theirs corrosion protection properties. These are barrier pigments, e.g. lamellar aluminum pigment [6,7], micaceous iron oxide [8], zinc oxide and glass flake, inhibitive pigments, e.g. zinc phosphates [9–13], and sacrificial pigment, e.g. zinc powder. Recently, nanoparticles have attracted much more attention due to their effectiveness. In addition, it has been reported that nanoparticles provide much better barrier properties than conventional micron size pigments due to their higher surface area. The role of nanoparticles including SiO2 [14], Al2O3 [15], Fe2O3 [16], ZnO [17], Zn, Si [18,19] and halloysite clay [20] on the corrosion resistance of the epoxy coatings over the steel substrates in NaCl solution has been extensively studied. The results obtained from these studies revealed that the epoxy coatings containing nanoparticles offer acceptable barrier properties for corrosion protection. It seems that graphene oxide based nanosheets engineering has enabled the possibility of designing the corrosion resistant coatings which can last much longer compared to traditional ones. However, the great potential of graphene to be served as an effective barrier to diffusion of corrosive species, i.e. H2O, O2 and Cl, into the organic coating has not been widely pursued. The barrier properties of the graphene oxide nanosheets come from its impermeability to oxygen and water and its very high surface area and nanometric thickness. Singh et al. [21] studied the corrosion resistance of a hydrophobic graphene oxide-polymer composite coating on the copper surface. They found that the GO filled coating behaved as an effective protective shield for oxidation and corrosion of metal. Kirkland et al. [22] investigated the potential for graphene based coatings to provide barrier performance for aqueous corrosion. They revealed that the deposited graphene layers on the metal surface are capable of enhancing the corrosion protection of pure metals effectively. There have been thoroughly studies of the general electrochemical properties of graphene, such as that by Ratinac et al. [23]. What is however lacking in the literature is the study of the effects that graphene oxide and functionalized GO (FGO) have on the corrosion protection properties of the organic coatings. A little research has been performed on the impact of GO and FGO on the corrosion resistance of the organic paints on the metal substrates in exposure to corrosive mediums. Chang et al. [24] studied the corrosion protection properties of the epoxy/graphene composites on coldrolled steel. They found that incorporation of graphene nanosheets into the epoxy coating matrix enhanced its barrier properties and corrosion resistance. Polyurethane is one of the most important polymers which has been widely used in the coating industry for different applications due to its superior properties, such as flexibility at low temperature, high abrasion resistance, high impact and tensile strength, high transparency, excellent

gloss, color retention and good weathering resistance [25,26]. The polymeric coating can be obtained through the reaction of polyols, e.g. acrylic and polyester, with curing agents. Isocyanates are highly reactive chemicals with high tendency of reaction with AOH and ANH functional substances [27,28]. However, one main drawback of PU is its low corrosion resistance. Attempts have been carried out to improve the corrosion protection properties of the PU through addition of different kinds of nanoparticles [29,16,30]. The objective of this investigation is to use GO and functionalized GO as corrosion resistant materials in the polyurethane coating (PU). The GO was functionalized with polyisocyanate resin (PI–GO). The unmodified and modified GO nanosheets were characterized by X-ray photo electron spectroscopy, thermal gravimetric analysis and X-ray diffraction analysis. In addition, the GO and PI–GO nanosheets were incorporated into the polyurethane coating. The corrosion protection properties of the GO/PU and PI–GO/PU composite coatings were investigated on the steel surface in 3.5 wt.% NaCl solution at different pHs by electrochemical impedance spectroscopy (EIS) analysis. Also, these composite coatings were used as topcoats on the epoxy coated steel samples and the corrosion protection properties of the coating systems were evaluated by salt spray test. The adhesion properties of the coating systems were investigated by pull-off test before and after immersion in 3.5 wt.% NaCl solution.

2.

Experimental

2.1.

Materials

The steel panels (ST-37) were purchased from Foolad Mobarakeh Co (Iran). The chemical composition (wt.%) of the steel was as follows: Fe (balance), Si (0.016), C (0.037), S (0.0086), P (0.005), Cu (0.065) and others (0.13). Mild steels panels were cut into the size of 150 mm · 100 mm · 1 mm. Then they were abraded using sand papers 600, 800, 1200 and 2400 grades. Before painting, the mild steel substrates were washed with deionized water and acetone in turn, and then dried in air. Acrylic polyol (1780 M), with solid content and hydroxyl value of 60% and 49%, respectively, was purchased from DSM Co. Also, the polyisocyanate (Desmodur N75), with the NCO and solid contents of 16.5% and 75%, respectively, was purchased from Bayer Co. Natural graphite flake (<50 lm), KMnO4, H2SO4 (98%) and H2O2 (30%) were purchased from Aldrich Co. Industrial grade xylene and butyl acetate were used to prepare the polyurethane coatings. N,Ndimethylformamide (DMF) and methylene chloride were prepared from Merck Co. All of these materials were used without further purification. The epoxy resin based on bisphenol-A (Araldite GZ7 7071X75) was purchased from Saman Co (Iran). The solid content, epoxy value and density of the resin were 74–76%, 0.1492–0.1666 Eq/100 g, and 1.08 g cm3, respectively. A polyamide resin, (Crayamid 115, from Cray Valley) was used as the epoxy hardener.

2.2.

Synthesis and chemical functionalization of GO

A modified Hummer’s method has been followed to prepare GO nanosheets [31,32]. First, 0.5 g of graphite powder was

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added to 60 ml of H2SO4 under constant stirring. After 1 h, 1.5 g of KMnO4 was gradually added to the solution while keeping the temperature less than 20 C. The mixture was stirred at ambient temperature for 12 h and the resulting solution was diluted by adding 600 ml of distilled water under strong stirring. After completion of the oxidation reaction in the presence of KMnO4, the suspension was further treated with 30% H2O2 solution (5 mL). The mixture was centrifuged and washed twice with hydrochloric acid (1 M). Then, it was washed three times with de-ionized water. The aqueous solution containing graphene oxide sheets were obtained at last. In the next step, the GO nanosheets were functionalized with PI resin. For this purpose, the graphite oxide suspension in water was sonicated for an hour and centrifuging for 10 min at 4000 rpm to obtain homogeneous dispersion of GO. The resultant aqueous GO suspension was subjected to a solvent-exchange process to get a fine GO dispersion in DMF. The solvent-exchange process was performed by adding DMF to the aqueous GO, sonication, centrifugation, and removal of the supernatant liquid for three times. Then, the functionalization of GO sheets with aliphatic isocyanate (Desmodur N75) was done. For this purpose, the polyisocyanate (PI) was dissolved in DMF and mixed with GO suspension. In a typical procedure, the graphene oxide (50 mg) was loaded into a 250-mL round-bottom flask equipped with a magnetic stir bar at a speed of 200 rpm. The anhydrous DMF (5 ml) was then added under nitrogen to create an inhomogeneous suspension. The PI (2 mmol) was next added to the solution and the mixture was allowed to stir under nitrogen purge for 48 h. Then, the reaction mixture was poured into methylene chloride (50 ml) to coagulate the product. The product was filtered, washed with additional methylene chloride (50 ml). The modified-GO was finally dispersed in DMF by 1 h sonication. The procedure of obtaining PI–GO is schematically shown in Fig. 1.

2.3.

Sample preparation

2.3.1.

Samples for EIS experiment

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GO/PU and PI–GO/PU composite coatings were prepared and directly applied on the steel substrate for EIS analysis. In this way, the effect of addition of GO and PI–GO nanosheets to the PU coating on its barrier protection properties can be well studied. In fact, the PU composite investigated in this study is the top coat layer in a multilayer coating system including epoxy primer. The main reasons for doing this experiment on the PU coatings applied directly on the steel surface are decreasing the EIS analysis time duration and obtaining more precise data regarding PU coating corrosion resistance. Evaluating the PU composites by EIS in a multilayer coating system is time consuming due to its high thickness. Therefore, the experiment applied on the PU coatings which were directly applied on the steel substrate. 0.1 wt.% of GO and PI–GO nanosheets (2.5 mg/l of GO and PI–GO in DMF) were separately incorporated into the acrylic resin. The mixtures were then mixed for 15 min with a high shear mixer. Finally, desmodure N75 was added to the GO/acrylic and PI–GO/acrylic dispersions as

curing agent. The ratio of acrylic resin to the hardener was 4:1. The resultant composites as well as the neat PU were applied on the mild steel specimens using the Elcometer 3540 four-sided film applicator. The PU films were applied on the specimens in two ways. In the first method, the PU coatings were applied directly on the mild steel panels at wet film thickness (WFT) of 120 ± 10 lm. Then, samples were kept in an ambient temperature for 24 h for flash-off. Finally, they were cured in an oven at 120 C for 30 min. The dry thickness of the cured film (DFT) was 60 ± 5 lm.

2.3.2. Sample experiments

preparation

for

alt

spray

and

pull-off

Salt spray test was also performed on a two layer coating system including epoxy primer and PU topcoat layers. It has been previously mentioned that the main role of PU coating in a multi-layer coating system is as topcoat layer to provide outdoor properties as well as mechanical and corrosion protection performances. Therefore, evaluation of the corrosion protection properties of the PU composites applied on the epoxy primer is great of importance. Considering the high thickness of the coating layer in this case, the best technique to evaluate the corrosion protection properties of the system was accelerated salt spray test. The results of this experiment and EIS could show the barrier effect of the GO and PI–GO nanosheets on the corrosion protection properties of the PU systems. To prepare epoxy coating, additives, i.e. leveling agent (BYK-306: 0.5 wt.%), defoamer (Efka-2025: 0.1 wt.%) and dispersing agent (Delta-4242: 1 wt.% of total weight of pigments and fillers), pigments and fillers, i.e. iron oxide (10 wt.%), TiO2 (14 wt.%) and talc mineral (20 wt.%), solvents, i.e. toluene (10 wt.%), were added to the epoxy resin. The mixture was mixed for 3 h by a pearl-mill in order to obtain an average particle size up to 15 lm. An aliphatic polyether (Delta-4242), which is an anionic dispersing agent with acidic groups, was used to provide proper dispersion of the pigments and fillers in the epoxy resin. Then, the polyamide curing agent (Cray amide 115, solid content 50 wt.%) was added to the epoxy resin at weight ratio of 4:1 (epoxy:polyamide) and mixed for 10 min. The resulting coatings were applied on the bare steel by an air spray. Samples were then kept at room temperature for 24 h. The dry thickness of the cured epoxy coating was 70 ± 5 lm. Then, the PU composites were applied on the epoxy coating using a film applicator. Finally, samples were cured in an oven at 120 C for 30 min. The dry thickness of the cured coatings was 120 ± 5 lm.

2.4.

Characterization

2.4.1.

GO and PI–GO nanosheets characterization

Thermal gravimetric analysis (TGA) model SDTA 851 was utilized to evaluate the amount of PI resin grafted on the surface of GO sheets. The experiment was performed at temperature region of 25–700 C and heating rate of 5 C/min in nitrogen atmosphere. X-ray photoelectronic spectroscopy (XPS) was utilized to evaluate the chemical structure of the GO and PI– GO nanosheets. The measurements were carried out by

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Fig. 1 – Schematic illustration of the PI chains grafting onto the surface of GO. (A color version of this figure can be viewed online.)

Specs EA 10 Plus. In this experiment, the radiation source was Al Ka (at pressure of 109 mbar). Moreover, the shift of binding energies (BE) was calibrated with respect to the reference peak of carbon at binding energy of 285 eV. The surface charges (zeta potential) of the GO and PI–GO nanosheets were measured by ZEN 3600 (Malvern, UK) in a range of pH 5–11. The X-ray diffraction (XRD) analysis was performed on the GO and PI–GO powder, and GO/PU and PI–GO/PU composites using Philips X-ray spectrometer, PW 1800 type (Netherlands) with Cu-ka filament. The GO/PU and PI–GO/PU interfacial interactions were evaluated by scanning electron microscope (SEM) model Philips XL30.

2.4.2.

Electrochemical and corrosion tests

The effects of addition of GO and PI–GO nanosheets on the corrosion protection properties of the PU coating were evaluated by electrochemical impedance spectroscopy (EIS) and salt spray test. The EIS analysis was carried out in a conventional three-electrode cell including saturated Ag/AgCl as reference electrode, Platinum as counter electrode and steel samples coated with PU coatings as working electrode. The measurements were done in the frequency range of 10 kHz to 10 mHz (peak to zero) and at 10 mV amplitude sinusoidal voltage by using the Ivium Compactstat at open circuit potential. Measurements were performed on 1 cm2 of the PU coating immersed in the 3.5 wt.% NaCl solutions at two different pHs (7 and 11) and at different immersion times (7, 14 and 30 days). Also, the measurements were implemented 3 times to ensure the repeatability of the measurements. Salt spray exposure was performed on the steel samples coated with epoxy coating as first layer and PU coating as topcoat layer. The test was done on the scribed samples for 300 h in salt spray cabine according to ASTM B117. The pH and NaCl concentration of salt fog were 7 and 5 wt.%, respectively.

2.4.3.

Pull-off adhesion test

The adhesion loss of the samples coated with epoxy coating and PU composites was studied by a Posi testpull off adhesion tester (DEFELSKO) according to ASTM/D 4541-95 and ISO 4624. For this purpose the aluminum dollies were glued on the surface of the PU coating using a two-part Araldite 2015 (Huntsman advanced materials, Germany) adhesive. Samples were then kept at ambient temperature for 24 h to ensure that the glue was fully cured. Finally, a slot was made around dollies and they were pulled at a speed of 10 mm/min normal to the coating surface until the coating was detached from the steel substrate. The adhesion test was performed on all samples before and after 30 days immersion in 3.5 wt.% NaCl solution to calculate the adhesion loss. All tests were carried out using three replicates to ensure the measurements repeatability.

3.

Results and discussion

3.1.

Characterization of functionalized GO nanosheets

TGA, XPS and XRD analyses were performed on the GO and PI–GO nanosheets to investigate the extent of functionalization of PI chains on the GO surface. GO surface includes hydroxyls and carboxyls [33] groups, which provides the reactive sites for covalent functionalization with isocyanate groups of PI resin. The schematic illustration of the chemical functionalization of the functional groups on the GO surface with PI chains is displayed in Fig. 1. The amount of PI chains grafted onto the GO sheets was studied by TGA analysis. The results obtained from TGA measurement is presented in Fig. 2a. The extent of mass loss was about 3.7% below 100 C and 23% between 150 and 200 C for the GO. The former is related to the removal of physically adsorbed water

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Fig. 2 – Characterization of GO and PI–GO by (a) TGA analysis, (b) XRD analysis, (c) XPS spectrum of GO and (d) XPS spectrum of PI–GO. (A color version of this figure can be viewed online.)

on the GO surface and the latter can be ascribed to pyrolysis of the oxygen-containing functional groups. In addition, mass loss of about 25.79% was observed at temperature above 300 C, which can be assigned to the removal of more stable oxygen functionalities [34]. According to Fig. 2a it can be seen that PI–GO showed lower weight loss at 100–350 C indicating that the PI chains grafted on the GO surface decreased the rate of thermal decomposition of the oxygen-containing functional groups. However, the weight loss of the PI–GO was more significant than GO at temperatures above 350 C. This is mainly due to thermal decomposition of the PI resin grafted on the GO surface. The residue at 650 C for the PI–GO and GO are 47.46% and 55.73%, respectively. This means that the weight loss of the PI–GO

is almost 8% greater than GO which can be related to the amount of PI grafted on the GO surface. XRD patterns for the GO and PI–GO samples are depicted in Fig. 2b. A broad band over low diffraction angles (2h = 10.81), corresponding to the existence of oxygen-rich groups on both sides of the sheets and water molecules trapped between the sheets, was seen for the GO. Results show that surface functionalization of GO with PI caused downshift of the XRD band of GO to 2h = 7.6. Widening of ˚ for GO to 11.56 A ˚ for PI–GO) can be attribud-spacing (8.17 A ted to the PI chains presence between the intergalaries of the GO sheets. The chemical structure of the GO and PI–GO samples were evaluated by XPS analysis. Fig. 2c and d show the XPS

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Fig. 3 – XRD spectra of polyurethane composites containing 1 wt.% of GO and PI–GO nanosheets. (A color version of this figure can be viewed online.)

surveys of GO and PI–GO samples. Also, the single scan spectra of the samples with high resolution in the N1s, O1s and C1s regions were investigated. Fig. 2c1 exhibits C1s of GO with many functional groups [35], including (a) CAC: 284.5 eV, (b)

CAO: 286 eV, (c) C@O: 288.2 eV, and (d) OAC@O: 289.5 eV. The XPS survey of PI–GO (Fig. 2d) showed the existence of a new peak at 399 eV for N compared to GO. Two peaks were detected in the N1s XPS at 398.6 and 399.6 eV related to @NA and ANHA, respectively. Similarly, Fig. 2d1 shows 6 peaks in the C1s of PI–GO: CAC (284.6 eV), CAN (285.7 eV), CAO (286 eV), O@CAN (287.4 eV), C@O (288.2 eV) and CO(O) (289.5) eV. It can be understood from the results of XPS analysis that the atomic ratio of C/O for PI–GO is less than that of GO which is due to the PI chains grafted onto the GO surface. The intensity of CAC, CO(O) and C@O on the GO surface decreased after modification with PI, indicating the GO surface reduction. Generally, the results obtained from TGA, XPS and XRD analyses confirm that the PI chains grafting onto the surface of GO was occurred successfully.

3.2.

Characterization of PI–GO/PU and GO/PU composites

The exfoliation degree of GO and PI–GO sheets in the polyurethane matrix was studied by XRD analysis. Two diffraction peaks can be seen in the XRD spectra of the samples at 2h ranges of 4–10 and 15–30. The wide diffraction from 15 to 30 is connected to scattering of the cured polyurethane molecules, revealing its amorphous nature. The d-spacing

Fig. 4 – SEM micrographs obtained from the cross-section of the (a) neat PU, (b) GO/PU and (c) PI–GO/PU samples after tensile test.

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(a)

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(b)

(c)

Fig. 5 – Visual performance of (a) PU, (b) GO/PU and (c) PI–GO/PU coatings applied on steel substrate after 30 days exposure to 3.5 wt.% NaCl solution at pH = 7. (A color version of this figure can be viewed online.)

˚ were obtained for the GO and PI–GO values of 8.63 and 12.2 A after insertion into the PU matrix at 2hs 10.2 and 7.1, respectively. These show that the d-spacing of both GO and PI–GO sheets increased after incorporation into the PU matrix. This increment is related to diffusion of polymer chains between the interlayer distance of GO and PI–GO sheets. Furthermore, the increase of d-spacing for the PI–GO sheets was higher than GO. This may show higher degree of exfoliation of the PI–GO sheets in the PU matrix than GO (see Fig. 3). The improved dispersion and interface of GO nanosheets can affect the morphological properties of PU coating. This was studied through investigation of the morphology of fracture of these samples by SEM analysis. Fig. 4a shows that the cracks had spread freely and randomly at the fractured surface of the neat PU indicating brittleness and poor impact strength of this sample. Less cracks and ruptures were seen at the fractured surface of GO/PU sample than the neat PU (Fig. 4b). This indicates that GO sheets reduced cracks through crack deflection mechanism. Results showed that the surface fracture of the PI–GO/PU composite is almost smooth. The smoother surface of PI–GO/PU (Fig. 4c) suggests that the fracture occurred in the matrix phase rather than at

the interface; also, the pull-out mechanism in this sample rarely took place, which indicates strong bonding of modified PI–GO nanosheets to the matrix. In fact, the PI chains grafted on the GO surface produced proper cites for chemical interaction with PU matrix. The OH groups of acrylic resin could react with NCO groups on the PI–GO surface. Thus, the presence of flexible chains on PI–GO sheets plays a critical role in fracture behavior of composite.

3.3. Corrosion protection performance of PI–GO/PU and GO/PU composites 3.3.1. Visual performance after immersion in 3.5 wt. NaCl solution Corrosion protection properties of PU, and GO/PU and PI– GO/PU composites were studied on the steel substrates. The visual performances of the samples immersed in 3.5 wt.% NaCl solution (pH = 7) for 30 days immersion are studied in Fig. 5. From Fig. 5a it can be seen that blisters appeared on the PU sample after immersion in NaCl solution. Also, the corrosion products accumulations can be seen at the coating/metal interface. These indicate that water containing chloride ions

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Fig. 6 – Bode plots of blank PU, GO/PU and PI–GO/PU coatings applied on steel substrate after (a) 7 days, (b) 14 days and (c) 30 days exposure to 3.5 wt.% NaCl solution at pH = 7.

passed through underlying metal and activated primary corrosion sites at the interface. While corrosion proceeds at anodic areas at the coating-metal interface, hydroxyl ions build up at cathodic sites. The alkaline environment at the cathodic sites seriously affects the adhesion of the coating to the substrate and produce osmotically active substances at the coating-metal interface. These substances at the interface promote either osmotic or endosmotic passage of water through the coating from its environment. All of these are responsible for the PU coating de-bonding from the steel

substrate. Also, corrosion products accumulation at the coating/metal interface could develop the disbonded area of the coating. Different results were observed for the PU coating containing 0.1 wt.% of GO nanosheets. It can be seen from Fig. 5b that a large number of black spots appeared in the PU coating matrix after 30 days immersion while no blister and corrosion product were observed on this sample. The GO/PU film was detached from the steel surface to see whether the black spots created on the steel surface or in the bulk of PU coating. It was found that there was no

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Fig. 7 – Nyquist plots of blank PU, GO/PU and PI–GO/PU coatings (DFF = 60 ± 5 lm) applied on steel substrate after (a) 7 days, (b) 14 days and (c) 30 days exposure to 3.5 wt.% NaCl solution at pH = 7.

Fig. 8 – One time constant equivalent circuit used to model experimental impedance data. (A color version of this figure can be viewed online.)

corrosion products on the metal surface and the black spots corresponds to the GO accumulation in the PU film after exposure to 3.5 wt.% NaCl solution. It is known that GO has great tendency to water due to its great hydrophilic surface

nature. The presence of many oxygen containing groups on the GO sheets is responsible for the hydrophilic nature of the GO. This means that the GO sheets may produce osmotically active substances in the coating matrix leading to the water diffusion through the coating. The higher GO tendency to water than PU matrix causes the creation of water containing regions in the coating matrix around GO sheets. In this way, the GO sheets act as physical trap for water in the coating and limits water molecules diffusion to the coating/metal interface. It seems that the NaCl salt solution diffusion into the GO/PU coating resulted in the GO nanosheets aggregation at some parts resulting in black spots creation in the coating matrix. This means that the GO could reduce the PU adhesion loss through decreasing corrosive electrolyte diffusion to the coating/metal interface. However, the diffused electrolyte causes coating bonds hydrolysis at long immersion times. Results obtained from Fig. 5c show that incorporation of PI– GO into the PU coating improved its corrosion protection performance significantly. No blister was seen on this sample and a slight corrosion product was seen at the coating/metal interface. These observations clearly demonstrate that the PI functionalized GO significantly enhanced the PU coating corrosion protection performance.

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Fig. 9 – Variations of impedance data including (a) log |Z| at 10 mHz and (b) log (fb) of PU, GO/PU and PI–GO/PU coatings as a function of immersion time in 3.5 wt.% NaCl solution at pH = 7. (A color version of this figure can be viewed online.)

3.3.2. EIS measurements of PU, GO/PU and PI–GO/PU samples 3.3.2.1. In 3.5 wt.% NaCl solution at pH = 7. Corrosion protection properties of the PU, GO/PU and PI–GO/PU samples were investigated in 3.5 wt.% NaCl solution (pH = 7) at various immersion times. The Nyquist and Bode diagrams of the samples are given in Figs. 6 and 7. From Figs. 6 and 7 it can be clearly seen that the impedance modulus at low frequency limit (10 mHz) of PI– GO/PU sample is greater than 109 ohm cm2 after 7 days immersion, and the phase angle is almost 90 at high frequency region. There is only one time constant in the impedance spectra of all samples indicating good barrier characteristics of the coatings. With prolonged immersion time the corrosive electrolyte gradually diffused into the PU matrix. This can be understood from the narrowing of the frequency range displaying capacitive behavior in the Bode phase diagrams.

Electrical equivalent circuit shown in Fig. 8 was utilized to model the impedance data. The constant phase element (CPE) was utilized instead of ideal capacitor (C). Rs, Rc and CPEc represent the solution resistance, coating resistance and constant phase element of the coating, respectively. The variations in impedance at low frequency limit (10 mHz) and breakpoint frequency (fb) of different samples are shown in Fig. 9. The barrier performance of the coatings can be also explained based on the intersection of Bode plots (IBP) [36]. The IBP values as a function of immersion time are shown in Fig. 10. From Figs. 9a and 10 it can be seen that the low frequency impedance (|Z|0.01Hz) and the corresponding phase angle of all samples are high at initial immersion stage (7 days) indicating good barrier performance of the coating at the beginning of immersion. However, as the immersion time elapsed, a descending trend was seen for the |Z|0.01Hz and corresponding phase angle of the samples. Increasing the immersion time resulted in the shift of IBP of all samples to higher frequencies. These can show the continuous diffusion of corrosive electrolyte into the coating, resulting in the barrier protective performance decline. It can be obviously seen from Figs. 9a and 10 that addition of 0.1 wt.% GO nanosheets to the PU coating caused the increase of impedance at low frequency and shift of IBP to lower frequencies compared to the blank coating. Results show that the |Z|0.01Hz of the PU coating was significantly increased in the presence of PI–GO nanosheets. The increase in coating resistance of the PI–GO/PU is at least one order of magnitude higher than the neat PU. In addition, the shift of IBP to lower frequencies was most pronounced in the case of PI–GO/PU sample. All of these observations indicate that surface functionalization of GO with PI resulted in significant improvement of the barrier protection properties of the PU coating. The extent of fb (frequency at 45 phase angle) was determined from Bode diagrams to evaluate the corrosion protective performance of the coatings. It has been reported that there is a close relationship between the microscopic delaminated areas of the coating and its corrosion protective properties [37,38]. As the corrosive electrolyte reaches the coating/metal interface, the electrochemical reactions initiate at active sites of the metal surface. It can be seen from Fig. 9b that fb of all coatings increased to higher frequencies with the increase of immersion time. One can be deduced from this observation is that the capacitive behavior of the PU systems decreased as a result of electrolyte diffusion into the coating. The shift of fb to higher frequencies is considerably most pronounced for the blank coating. It can be seen that addition of GO nanosheets causes less increase in fb compared to the blank sample. However, the fb of the PU coating containing PI–GO increased relatively smaller than the one containing GO. The corrosive species diffusion to the underlying metal is responsible for the primary corrosion sites activation at the interface of the coating/metal. While corrosion progresses on the anodic sites at the PU coating/metal interface, hydroxyl ions (2H2O + O2 + e ! 4OH) build up at cathodic sites. This can accelerate the coating adhesion bonds destruction leading

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Fig. 10 – IBP values of the neat PU, GO/PU and PI–GO/PU composite coatings immersed in 3.5 wt.% NaCl solution at pH = 7 for (a) 7 days, (b) 14 days and (c) 30 days. (A color version of this figure can be viewed online.)

coating matrix. In fact, the low coating capacitance may reflect the impermeability of the coatings, i.e. its barrier effect. The effective capacitance of coating (Cc) was calculated from CPE parameters according to Eq. (1) [39]. 1

Cc ¼

Fig. 11 – Variation of coating capacitance versus immersion time for the neat PU coating and GO/PU and PI–GO/PU composite coatings. (A color version of this figure can be viewed online.)

to the coating delamination from the substrate. Therefore, the fb is proper parameter to evaluate the relative increase of delaminated part of the coating. There is a close relation between the coating capacitance and the diffusion behavior of corrosive electrolyte into the

ðY 0 Rc Þn Rc

ð1Þ

where Y0 and n represent admittance and exponent of CPE. The coating capacitance at different immersion times are shown in Fig. 11. It can be clearly seen from Fig. 11 that the Cc for the neat PU and GO/PU samples quickly increased at the initial period of immersion. The initial increase of Cc of the samples can be attributed to the water uptake into the PU coating. With prolonged immersion times the Cc of these samples declined slowly to a stable value and the water uptake gradually reached a saturation state. This can be attributed to the decrease of coating homogeneity along with the water uptake process. It can be seen from the results that incorporation of both GO and PI–GO nanosheets could reduce water uptake compared to the neat PU sample. However, the Cc of the PI–GO/PU sample is significantly lower than GO/PU sample in the whole period of immersion. Also, the increase of Cc of the PI–GO/PU sample in the initial period of immersion is not as quick as that observed for the neat PU and GO/PU

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Fig. 12 – Bode plots of PU, GO/PU and PI–GO/PU coatings (DFF = 80 ± 5 lm) applied on steel substrate after (a) 7 days and (b) 14 days exposure to 3.5 wt.% NaCl solution at pH = 11.

samples. All of these observations demonstrate that PI–GO could effectively reduce the water uptake into the PU coating. Based on the above explanations it can be said that addition of PI–GO to the PU coating significantly decreased the number of microscopic delaminated areas and improved barrier protection properties of the coating.

3.3.2.2. In 3.5 wt.% NaCl solution at pH = 11. Barrier protection properties of the PU, GO/PU and PI–GO/PU samples were also studied in 3.5 wt.% NaCl solution at pH = 11. It is believed that the GO nanosheets may improve the corrosion protection properties of the PU in alkaline condition. The coating can be encountered with such an alkaline pH during cathodic protection or in harsh corrosion environments. The Bode diagrams of the samples are given in Fig. 12. Also, the impedance at low frequency limit (10 mHz), breakpoint frequency (fb) and IBP values of different samples (obtained from Fig. 11) are presented in Figs. 13 and 14.

From Fig. 12 it can be seen that the neat PU shows a broad resistive region in the whole frequency region. Increasing the immersion time up to 14 days caused significant decrease of coating corrosion resistance. The impedance modulus at low frequency limit (10 mHz) of PI–GO/PU sample is greater than 109 ohm cm2 after 7 days immersion, and the phase angle is almost 90. There is only one time constant in the impedance spectra of the GO/PU and PI–GO/PU samples. Results show that the phase angle at high frequency region, i.e. 10 kHz, for the neat PU sample significantly decreased after 14 days immersion. However, the phase angles for the GO/PU and PI–GO/PU samples are almost 90 even after 14 days immersion. The significant decrease in phase angle for the neat PU confirms that the coating was severely deteriorated as a result of alkaline solution attack. From Fig. 13a and 14 it can be obviously seen that the incorporation of GO and PI–GO nanosheets into the PU matrix caused the increase of |Z|0.01Hz compared to the neat PU sample. It can be obviously seen that PI–GO/PU coating system has

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Fig. 13 – The values of (a) log (Z) and (b) log (fb) for the neat PU, GO/PU and PI–GO/PU composite coatings obtained from the EIS measurements after 7 and 14 days immersion in 3.5 wt.% NaCl solution at pH = 11. (A color version of this figure can be viewed online.)

|Z|0.01Hz much greater than the one containing GO. As the immersion time increased, the impedance value of the neat

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PU and GO/PU coating systems decreased. The decrease of impedance was most pronounced in the case of neat PU. Also, it can be seen that the impedance value of the PI– GO/PU sample did not change significantly at this high alkaline condition. The impedance value of the neat PU significantly decreased after 14 days immersion. However, the impedance of PI–GO/PU sample did not change significantly as the immersion time increased. According to Fig. 14 the IBP of the neat PU can be seen at lower right of Bode plots indicating poor protection performance of the coating at this alkaline condition. From Fig. 14 it can be seen that the IBP was in the middle of Bode plots for the GO/PU sample indicating intermediate protection performance of this sample. The IBP shifted to upper left of Bode plots for the PI–GO/PU samples revealing good protection properties of this sample in the alkaline condition. The IBP transferred to lower right side and lower phase angles for all of the samples after 14 days immersion. This shift of IBP for PI–GO/PU samples was considerably lower than other samples indicating good corrosion protection properties of this sample even after passing immersion time. These all show that the PI–GO could improve the coating resistance against alkaline condition. It can be seen from Fig. 13b that the fb values of the blank PU are much higher than those containing GO and PI–GO nanosheets. Incorporation of PI–GO and GO nanosheets into the PU matrix caused the significant decrease of fb. In addition, the PI–GO containing coating showed the lowest fb changes after 14 days immersion. These mean that incorporation of PI–GO into the PU matrix could reduce the coating delamination from the substrate due to its considerable barrier effect. The mechanism of improvement of the corrosion protection properties of PU in the presence of PI–GO has been discussed in Section 3.5.

3.3.3.

Salt spray exposure

Salt spray test was also performed on the two layer coating systems including epoxy primer and PU topcoat. The visual performance of the samples is depicted in Fig. 15.

Fig. 14 – IBP values of the neat PU, GO/PU and PI–GO/PU composite coatings immersed in 3.5 wt.% NaCl solution at pH = 7 for (a) 7 days and (b) 14 days. (A color version of this figure can be viewed online.)

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PU on epoxy coang

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GO/PU on epoxy coang

GO/PU on epoxy coang

PI-GO/PU on epoxy coang

PI-GO/PU on epoxy coang

Fig. 15 – Visual performance of the PU, GO/PU and PI–GO/PU coatings applied on the steel samples coated with epoxy primer after 300 h salt spray test. (A color version of this figure can be viewed online.)

From Fig. 15 it can be seen that a large number of blisters appeared around scribes on the PU sample. Addition of GO to the PU coating caused significant decrease of the numbers of blisters around scribes. A few blisters were seen on the PU coating system containing 0.1 wt.% PI–GO nanosheets. The larger disbonding of PU coating than GO/PU and PI–GO/PU coating systems confirms that GO and PI–GO nanosheets could enhance the corrosion protection properties of the coating system. Also, the lowest coating disbonding around scribes was observed for the PU coating reinforced with PI–GO. These all show that incorporation of PI–GO into the PU coating enhanced its corrosion protection performance greater than GO sample.

3.4.

Pull-off test measurements

Adhesion strength of a two layer coating system consisting epoxy /PU coatings was measured on the steel substrate before and after 60 days immersion in 3.5 wt.% NaCl solution. The pull-off strength and failure mode at tested area of different samples are shown in Figs. 16 and 17, respectively. The

adhesion loss values were calculated by Eq. (2) and results are depicted in Fig. 16b. Adhesion loss ð%Þ ¼

Dry adhesion strength  wet adhesion strength Dry adhesion strength  100 ð2Þ

From Fig. 16a it can be obviously seen that the wet adhesion strength of the PU sample is much lower than GO and PI–GO containing samples. The highest wet adhesion was observed for the PI–GO/PU sample. It can be seen from Fig. 16b that incorporation of GO and PI–GO into the PU coating decreased the adhesion loss. The lowest adhesion loss was observed for the PI–GO containing coating. Results show that the coating detachment from the surface of the steel substrate coated with neat PU and the one containing GO nanosheets was in the form of adhesive failure. However, cohesive failure was seen for the sample coated with PU coating containing PI–GO nanosheets. Cohesive failure occurs when the coating adhesion to the steel surface is high enough. This means that the PI–GO nanosheets could successfully restrict the coating delamination from

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Fig. 16 – The results obtained from adhesion test including (a) dry and wet adhesion (after 60 days immersion in 3.5 wt.% NaCl solution at pH = 7) strengths and (b) adhesion loss of the PU, GO/PU and PI–GO/PU coatings applied on the steel samples coated with epoxy primer.

the steel surface in the corrosive electrolyte. These observations are completely in agreement with the results of EIS (break point frequency (fb) measurements). In fact, the PI– GO nanosheets decreased the epoxy coating adhesion loss on the steel surface through enhancing its barrier performance.

3.5.

Corrosion protection mechanism of PI–GO/PU system

It has been shown that the unmodified GO nanosheets have hydrophilic nature. This is mainly due to the presence of different functional groups on the surface of GO including hydroxyls, epoxides, and carboxyls. The interfacial interactions between the graphenic sheets and the PU chains can effectively influence the dispersion properties of the GO in the coating matrix. The large surface area of the GO sheets and the strong van der Waals force among them result in poor

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interaction of the GO sheets with the PU matrix. This leads to poor dispersion and intercalation of GO in the PU matrix leading to the decrease of the barrier performance of the coating. Covalent functionalization of the reactive sites on the GO sheets with PI chains improved its surface properties. In this way, a high degree of GO sheets dispersion and good bonding to the PU matrix can be obtained. The intercalated PI–GO sheets can make the path of permeating corrosive electrolyte more tortuous (Fig. 18). The PI–GO nanosheets can block the diffusion paths through the horizontal directions between local anodes and cathodes along the coating-substrate interface. Moreover, due to the impermeability of pristine graphene the PU coating act as an excellent barrier to water, oxygen and other corrosive materials. It has been shown that addition of GO and PI–GO nanosheets to the PU matrix enhanced its corrosion resistance in 3.5 wt.% NaCl solution at pH = 11. The PU coating damage can severely occur at this high pH as a result of hydrolytic degradation of the coating bonds. This causes the decrease of the coating cross-linking density and the increase of electrolyte pathways in the coating matrix. In this way, the alkaline solution can easily reach the PU coating/metal interface leading to the coating disbondment from the steel substrate. However, addition of GO and PI–GO nanosheets could enhance the coating properties against alkaline solution. This may be attributed to two main mechanisms. First, the GO and PI–GO nanosheets can enhance the barrier properties of the PU coating causing the decrease in alkaline solution diffusion into the coating matrix. In fact, the PI–GO nanosheets, due to their good dispersion in the PU matrix, could provide excellent barrier performance against alkaline solution diffusion into the coating matrix (Fig. 18). Another possible mechanism of improvement is the effect of GO and PI–GO nanosheets on enhancing the PU coating ionic resistance. Knowing the surface charges on the GO and PI–GO is important to explain their effect on the ionic resistance of the PU coating. Therefore, the zeta potential values of the GO and PI–GO sheets were measured in a wide range of pH values, from 5 to 11. As can be seen in Fig. 19, the graphene oxide suspension showed the higher negative values at all pH levels than PI–GO. The functional groups i.e. hydroxyl and carboxylic groups presented at the edges of the graphene oxide sheets can weakly develop negative charges in the 3.5 wt.% NaCl solution at high pHs due to deprotonation [40]. The increase of pH causes more functional groups deprotonation leading to more negative charges and more negative zeta potential. Surface grafting of the GO functionalities with PI resulted in less negative charge as a result of decreasing the number of available hydroxyl and carboxylic groups on the GO surface. However, the negative zeta potential at pHs greater than 7 indicates that there are still intact hydroxyl and carboxylic functional groups which produce negative charges on the PIGO surface specially at high pHs (i.e. 11). The negative charging of the GO surface is schematically shown in Fig. 20. As can be seen in Fig. 20, the surface of

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Dry adhesion test (a1)

(a2)

Cohesive failure

(a3)

Cohesive failure

(b1)

Cohesive failure

(b3)

(b2)

Adhesive failure

Adhesive failure

Cohesive failure

Wet adhesion test aer 60 days immersion in 3.5 wt% NaCl soluon Fig. 17 – Visual performances of different samples after pull-off test; (a1 and b1) PU, (a2 and b2) GO/PU composite and (a3 and b3) PI–GO/PU composite; (a1), (a2) and (a3) are samples before immersion and (b1), (b2) and (b3) are the samples immersed in 3.5 wt.% NaCl solution at pH = 7 for 60 days. (A color version of this figure can be viewed online.)

Cl -

OHCathode

O2

H 2O

OH-

Fe2+ Anode

Fe2+

Steel substrate

Cathode

Anode

Fig. 18 – Schematic illustration of the PU coating containing PI–GO nanosheets exposed to 3.5 wt.% NaCl solution at pH = 7. (A color version of this figure can be viewed online.)

GO can be negatively charged as a result of nucleophilic attack by water and its basicity. The negatively charged GO sheets influence the coating ionic resistance. In this case, the coating exhibits cation selectivity and the anions are prohibited from penetration into the PU coating. Therefore, the Cl and OH anions cannot diffuse into the coating until the coating charge become positive or neutral. As a result, less

corrosive Cl and OH ions can reach the coating/metal interface. This is responsible for the lower coating hydrolytic degradation and adhesion bonds damage. PI–GO nanosheets provide good barrier against water molecules and O2 diffusion into the PU matrix as well as proper ionic resistance against OH and Cl anions diffusion into the coating. GO nanosheets react with OH and reduce

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the pH beneath the PU coating. Also, the GO sheets, due to their high solubility in water, reach the coating/metal interface. It seems that the GO sheets may be reduced during Fe oxidation process resulting in a substantial removal of oxygen functionalities of the GO. This may be responsible for the GO nanosheets color change into black and sedimentation at anodic sites (as seen in Fig. 5). The reduced GO sheets can cover the anodic sites and reduce the steel corrosion through pore plugging and providing barrier against electrolyte access to the steel surface. All of these are responsible for the enhanced barrier protection properties of the PU coating in the presence of PI–GO nanosheets.

4. Fig. 19 – Zeta potential measurement of GO and PI–GO suspensions at different pHs. (A color version of this figure can be viewed online.)

Conclusions

Results showed that PI polymer successfully grafted on the surface of GO through making amides and carbamate esters bonds. The surface modification of GO nanosheets enhanced

Fig. 20 – Schematic illustration of the PU coating containing PI–GO nanosheets exposed to 3.5 wt.% NaCl solution at pH = 11. (A color version of this figure can be viewed online.)

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its compatibility and dispersion in the polyurethane resin. The incorporation of 0.1 wt.% GO and PI–GO nanosheets to the PU coating enhanced its corrosion protection properties. Compared to GO, the PI–GO effect on the corrosion resistance improvement of the PU coating was most pronounced. The lowest adhesion loss was seen for the PI–GO/PU sample. Results showed that PI–GO could improve the corrosion resistance of the PU coating through enhancing its barrier performance and ionic resistance.

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