Preparation of polyaniline/graphene composites with excellent anti-corrosion properties and their application in waterborne polyurethane anticorrosive coatings

Abstract

Polyaniline, a novel conductive polymer, has been widely used as an anti-corrosive filler. In order to further improve the anti-corrosion performance, polyaniline/graphene (PANI/RGO) composites were prepared by in situ polymerization. And PANI/RGO composite anti-corrosion coatings were also prepared using PANI/RGO as anti-corrosive filler and waterborne polyurethane (WPU) as matrix. The anti-corrosion properties were proven by potentiodynamic polarization curves (Tafel polarization curves), electrochemical impedance spectroscopy and salt spray test analysis of the WPU anti-corrosive coatings. The results showed that the obtained composite coatings reinforced by 0.75 wt% of PANI/RGO composites possessed superior anti-corrosive performance when the graphene content of the filler was 4 wt%.

---

1. Introduction

Metal corrosion is the phenomenon whereby metal materials are destroyed by chemical or electrochemical reaction, and it is a serious threat to national economies and industry structures. Therefore, corrosion plays a very important role in the modern metalworking industry.^1,2 However, it is unfortunate that corrosion cannot be fully prevented, and corrosion control which is a way of slowing the kinetics and/or altering the mechanism becomes the only way. The non-controversial methods include cathodic protection, use of protective coatings and corrosion inhibitors, or any combination thereof, among which use of protective coatings is the most widely used method.^3,4 The present challenge as regards protective coatings is to find novel multifunctional materials as anticorrosive fillers which can meet different environmental needs due to their other good properties. In addition, in some cases fully carbon-based materials are preferred. Now, the challenge may be met on account of the advent of graphene.^5,7

Graphene is a novel two-dimensional material where carbon atoms form a honeycomb structure, and first discovered by Konstantin Novoselov and Andre Geim in 2004.^6,7 Nowadays, the preparation methods of graphene include micromechanical stripping, epitaxial growth, chemical gas phase deposition and graphite oxide (GO) reduction. In this work, reduced graphene oxide (RGO) (graphene) was prepared by GO reduction.^8–11 The possible applications of graphene have been intensively studied over the last few years. Recently, applications of graphene in the field of anti-corrosion have been investigated. Previous studies found that pure graphene as anti-corrosion packing is unable to significantly improve coating anti-corrosion performance. Therefore, improved methods that involve graphene composites with other materials have been studied, polyaniline (PANI) being one of the choices.¹²

In general, PANI, a kind of high molecular weight compound, was used for biological or chemical sensors, electrode materials, conductive fibers and so on.¹³ Its anti-corrosive performance is often overlooked. Previous literature has reported that well-dispersed PANI in a polymer coating could lead to a significant enhancement of the corrosion protection of a metallic substrate as compared with that of a neat polymer coating.¹⁴ However, PANI is easy to reunite in the polymerization process, and so forming a composite with a flake material is an effective way to reduce this reunion. There are many flake materials, of which graphene, carbon nitride and clay platelets are much-researched materials. Li et al. stated that graphene tended to have higher aspect ratio than clay platelets, which could enhance the barrier properties in polyurethane (PU) coatings.^15,16

Currently, the matrix of coatings is an organic polymer, and PU is one of the most widely used and versatile polymers with the typical urethane functional group (–NHCO–O–), which is usually obtained by a reaction between isocyanate and hydroxyl groups in the presence of some suitable additives. PU is well known for its outstanding corrosion resistance, excellent flexibility, strong adhesion to substrates, possibility of tailoring its properties, and also some other specific properties.¹⁶ Therefore, using it as a film-forming material for coatings can give the coating excellent mechanical properties and outstanding corrosion resistance performance. In addition, with the development of coatings, waterborne coatings have become the development trend. Therefore, waterborne PU (WPU) will attract more attention.

In this paper, we manufacture anti-corrosive composite coatings on carbon steel surfaces with WPU as the organic matrix and PANI/RGO with high aspect ratio as the barrier. Structural properties were characterized by Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and X-ray diffraction (XRD). Moreover, morphological properties were observed with transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Subsequently, mechanical properties were investigated by hardness pencil test, paint film scribe, flexible tester and paint film impact tester. Furthermore, anti-corrosion properties were investigated by an electrochemical workstation and salt spray test. Finally, anti-corrosion mechanisms of PANI/RGO composite WPU coatings were analysed based on the experimental results.

2. Experimental

2.1. Materials

WPU and other auxiliaries (dispersing agent and defoamer for water-based coating) were purchased from BASF, Germany. The curing agent hexamethylene diisocyanate (HMDI) was supplied by Bayer Co. Ltd, Germany. Aniline, potassium permanganate and ammonium persulfate (APS) were obtained from Aladdin Co. Ltd, China. Graphite, sulfuric acid, phosphoric acid, ethanol, ammonia, hydrazine hydrate and hydrogen peroxide were acquired from Sinopharm Chemical Reagent Co. Ltd, China. All materials were used as received.

2.2. Instrumentation

FTIR spectra were acquired using a FTIR spectrometer (FTIR-8400S, Shimadzu, Japan). Raman spectra were obtained using a Renishaw Invia Reflex Raman spectroscopy system. All spectra were obtained using a 457 nm Ar-ion laser with a 10 s acquisition time and 50 accumulations per spectrum through a 100× objective. The XRD analyses of powdered samples were performed using an X-ray diffractometer with Cu anode (D/Max 2500 PC, Rigaku Corporation, Japan), running at 60 kV and 30 mA with a scan range from 5° to 80° at 3° per min. The nanostructures of RGO and PANI/RGO composite materials were imaged with a JEOL-200FX transmission electron microscope. The morphologies of the WPU anti-corrosive coatings were recorded using a scanning electron microscope (JEOL JSM-6360LA, Japan).

The electrochemical experiments were carried out with a CHI 660D electrochemical workstation (Beijing, China). A conventional three-electrode cell was employed using a saturated calomel electrode (SCE) as the reference, platinum foil as the counter electrode, and a sample-coated glassy carbon electrode (GCE) and sample-coated carbon steel electrode (CSE) as the working electrode. Coating mechanical properties were determined with hardness pencil test, paint film scribe, flexible tester and paint film impact tester. The salt spray test was carried out on coated tin plates (40 mm × 80 mm) with a 5% NaCl solution at 100% relative humidity at 35 °C according to ASTM B117-03. The samples were checked every 12 h and images were recorded to validate the electrochemical impedance spectroscopy (EIS) results.

2.3. Preparation of RGO

GO,¹⁷ prepared by a modified Hummers' method,¹⁸ was reduced with a chemical reduction method. Typically, 50 ml of GO/DI water solution (1 mg ml⁻¹) was mixed with 0.7 ml of hydrazine hydrate and 7.5 ml of ammonia. The mixture was stirred at 95 °C for 6 h, followed by filtration, and flushed with DI water repeatedly to eliminate the unreacted hydrazine hydrate and ammonia. The product, referred to as RGO, was dried at 60 °C and ground into very small ​pieces.

2.4. Preparation of PANI/RGO composites

1 ml of aniline monomer was added into 27.8 ml of 0.5 M H₂SO₄ and various amounts of RGO (1, 2, 3, 4 and 5 wt%) were completely dispersed in the solution under magnetic stirring with brief ultrasonication to accelerate the dispersion. After addition of 2.5 g of APS in 27.8 ml of 0.5 M H₂SO₄, the resulting solution was stirred for 8 h in an ice bath, followed by filtration, and flushed with DI water repeatedly to eliminate the unreacted aniline, APS and H₂SO₄. The product was (1, 2, 3, 4 and 5 wt%) PANI/RGO composite, and was dried at 50 °C and ground into very small pieces.

2.5. Preparation of composite WPU anticorrosive coatings

A series of WPU anticorrosive coatings with 0 wt%, 0.25 wt%, 0.5 wt%, 0.75 wt%, 1.0 wt%, 1.25 wt% and 1.5 wt% of PANI/RGO composites were prepared as follows: first, 3 g of HMDI was added into 100 g of WPU with stirring for 2 min. Then, the required amount of PANI/RGO composite was dispersed in WPU with a glass rod to get a uniform mixture. Subsequently, 0.5 g of dispersing agent and 0.3 g of defoamer for water-based coating were added into WPU and stirred with a homo-mixer at 8000 rpm for 30 min. Finally, the mixture was coated on the surface of tin plates by a wire bar coater at 50 μm and 150 μm. The coatings of 50 μm were used for salt spray test, while the coatings of 150 μm were used for coating mechanical properties test. The coatings were cured for 4 hours at room temperature. The fabrication of composite WPU anticorrosive coatings is illustrated schematically in Fig. 1.

Fig. 1 Schematic illustration of the preparation of PANI/RGO composites and composite WPU anticorrosive coatings.

3. Results and discussion

3.1. Structural properties

FTIR spectroscopy was utilized to investigate the reaction GO and RGO in the process of chemical modification. The FTIR spectra of GO, RGO, and PANI/RGO composites in the range of 500–4000 cm⁻¹ are presented in Fig. 2. In Fig. 2(a), a broad and strong stretching vibration peak of –OH group is seen at 3430 cm⁻¹. The absorption peaks at 1725, 1630, 1220 and 1055 cm⁻¹ correspond to C=O stretching vibration of carboxyl or carbonyl, C–OH bending vibration, C–O bending vibration of hydroxyl and C–O–C vibration, respectively. GO, prepared by a modified Hummers method, is proved to contain –OH, –COOH, C–O–C and –C=O, four oxygen-containing functional groups, according to these peaks. Compared with GO, the spectrum of RGO shows only a weak absorption peak of –OH group at around 3500 cm⁻¹. GO is confirmed by hydrazine hydrate reduction in full. In the spectrum of PANI/RGO composites, the absorption peaks at 3450, 1564 and 1477 cm⁻¹ should be assigned to the stretching vibration of –N–H group, quinoid ring (N=Q=N) and benzenoid ring (N–B–N), respectively. Meanwhile, the doublet at 1296 and 1238 cm⁻¹ can be attributed to –C–H stretching vibration. Moreover, the absorption peaks at 1100 and 799 cm⁻¹ are due to bending vibration of quinone ring and 1,4-disubstituted benzene ring out-of-plane bending. However, the characteristic absorption band of RGO does not appear on curve (c) due to overlap of the characteristic absorption band of PANI. Therefore, these results can fully prove that PANI/RGO composites were successfully synthesized.¹⁹

Fig. 2 FTIR spectra for (a) GO, (b) RGO, and (c) PANI/RGO composites.

Raman spectra of the RGO powder and PANI/RGO composite powder are shown in Fig. 3. Both RGO and PANI/RGO composites display two typical peaks at 1340 and 1570 cm⁻¹, which correspond to G band and D band respectively. Moreover, PANI/RGO composites show a 2D band at around 2865 cm⁻¹.²⁰ It is well known that G band and 2D band are characteristic of sp² hybridized carbon–carbon bonds in graphene (E_2g vibrational mode).^20,21 D band refers to symmetry breakdown at the edge of graphene sheets, which signifies the defects in the graphitic domain and the intensity ratio of D and G bands (I_D/I_G) is considered as a unique characteristic tool to measure the density of defects and probe the degree of graphitization.²² After chemical reduction, the I_D/I_G increases relative to that of RGO. Raman spectra provide further evidence of the successful preparation of RGO and PANI/RGO composites.

Fig. 3 Raman spectra for (a) RGO and (b) PANI/RGO composites.

XRD patterns of flake graphite, GO, RGO and PANI/RGO composites are displayed in Fig. 4. From curves (a) to (c), a structural change of carbon materials is observed. The XRD pattern of flake graphite shows a sharp (002) peak at 26.3° with a typical d spacing of 0.336 nm calculated by the Bragg formula.^22,23 While GO's diffraction peak markedly shifts to a lower value of 10.4°, corresponding to a layered structure with a basal spacing of 0.82 nm. Compared with the raw material of flake graphite, the interlayer spacing increases obviously after oxidation, illustrating the successful introduction of oxygen-containing functional groups such as –OH, –COOH, C–O–C and –C=O groups between the graphite layers, and thus results in enlargement of interlayer spacing. Moreover, a broad peak for RGO appearing at 26.1° with an interlayer spacing of 0.368 nm is observed, indicating that GO is adequately reduced by hydrazine hydrate. PANI/RGO composites display four typical peaks at 8.7, 14.9, 20.3 and 25.3°; however, the diffraction peak of PANI is too strong and led to the disappearance of RGO's diffraction peak. The conclusion is well consistent with FTIR spectra.

Fig. 4 XRD patterns for (a) flake graphite, (b) GO, (c) RGO, and (d) PANI/RGO composites.

3.2. Morphological properties

The morphology of RGO and PANI/RGO composites can be identified by TEM observations, as shown in Fig. 5. The image of RGO at low magnification shows that the RGO sheet is thin and transparent, while the high-magnification image shows that RGO prepared by chemical reduction method has a few layers and plenty of folds on the surface. The formation of the folds is due to disorderly layer overlap in the process of reaction. From the images of PANI/RGO composites, a fine uniform dispersion of PANI on the surface of RGO is observed. However, the acting force of aniline polymerization reaction results in the stacking of RGO. After accumulation, RGO still is in a state of a few layers, and a phenomenon that a lot of slice layers overlay together disappeared.

Fig. 5 TEM images for (a and b) RGO and (c and d) PANI/RGO composites.

SEM micrographs of the surfaces of WPU coatings with different added composite dosages of 0 wt%, 0.25 wt%, 0.5 wt%, 0.75 wt%, 1.0 wt%, 1.25 wt% and 1.5 wt% are shown in Fig. 6. From the low-magnification image of neat WPU coating (Fig. 6(a)), relatively smooth surfaces are observed. Moreover, with the addition of composites, the surfaces of composite coatings exhibit more and more micro-scaled mastoids and pinhole depressions. However, many little cracks running through the coatings are observed from the high-magnification image, implying that there are gaps on the surfaces of neat WPU coatings, even if over smooth surfaces. When the PANI/RGO composite content increases to 0.5 wt%, the surfaces of the composite coatings become filled due to repair by the PANI/RGO composites, indicating good dispersion and compatibility of the PANI/RGO composites in the WPU matrix. Meanwhile, dark lines in the image represent PANI/RGO platelets in the coatings, while the bright area represents the WPU matrix. Furthermore, when the PANI/RGO composite content increased to 1.25 wt%, the phenomenon of uneven dispersion of fillers in the matrix is evidenced. Moreover, there are some conditional parameters, which are expected to promote the dispersion of PANI/RGO composites, including the high stirring speed during the coating preparation, the low viscosity of the coatings and elevated temperature during solvent evaporation.

Fig. 6 SEM micrographs of surfaces of WPU coatings with different added composite dosages: (a) 0 wt%, (b) 0.25 wt%, (c) 0.5 wt%, (d) 0.75 wt%, (e) 1.0 wt%, (f) 1.25 wt%, and (g) 1.5 wt%.

3.3. Mechanical properties

A coating can be used as a physical isolation to protect a metal base material. Therefore, whether or not a coating has excellent mechanical properties will have a great influence on its anti-corrosion effect. Hardness, adhesion, flexibility and impact resistance of different mass fractions of composite WPU coatings are shown in Table 1. With an increase of filler amount, hardness increased from 5 H to 6 H, and adhesion decreased from 0 to 2 grade. The best grade is 0 grade, which indicates the edges of the incision are smooth and coatings are not peeled off. However, the addition of PANI/RGO composites has no effect on flexibility and impact resistance of coatings. PANI/RGO composites, based on the data, should not be added too much, and better control is between 0.25 and 1.0 wt%.

Table 1 Mechanical properties for WPU coatings with different added composite dosages of 0 wt%, 0.25 wt%, 0.5 wt%, 0.75 wt%, 1.0 wt%, 1.25 wt% and 1.5 wt%

Content of composites	Hardness	Adhesion	Flexibility	Impact resistance
0 (wt%)	5 H	0	1 mm	50 cm
0.25 (wt%)	6 H	1	1 mm	50 cm
0.5 (wt%)	6 H	1	1 mm	50 cm
0.75 (wt%)	6 H	1	1 mm	50 cm
1.0 (wt%)	6 H	1	1 mm	50 cm
1.25 (wt%)	6 H	2	1 mm	50 cm
1.5 (wt%)	6 H	3	1 mm	50 cm

---

3.4. Anti-corrosion properties

Potentiodynamic polarization curves (Tafel polarization curves) of different specimens are shown in Fig. 7. The specific data of a series of electrochemical measurements (i.e., anodic slope (b_a), cathodic slope (b_c), corrosion potential (E_corr), polarization resistance (R_p), and corrosion current (I_corr) measured in a corrosive medium (3.5 wt% aqueous NaCl electrolyte)) are listed in Table 2. Information about E_corr and I_corr can be obtained by the point of intersection of cathodic and anodic polarization curves. In general, a material which has a higher E_corr and R_p and a lower I_corr and corrosion rate (CR) has a better corrosion resistance.

Fig. 7 Tafel polarization curves (a) for bare steel, PANI and PANI/RGO composites with different ratios of 1 wt%, 2 wt%, 3 wt%, 4 wt% and 5 wt%, and (b) for bare steel, WPU and WPU coatings with different added composite dosages of 0.25 wt%, 0.5 wt%, 0.75 wt%, 1.0 wt%, 1.25 wt% and 1.5 wt%.

Table 2 Electrochemical corrosion measurements of different specimens

Specimens	Electrochemical corrosion measurements					CR (mm per year)	PEF (%)
	Anodic slope	Cathodic slope	Ecorr (V)	Rp (kΩ cm^2)	Icorr (μA cm^−2)
Bare steel	7.905	2.087	−0.857	12.20	58.76	6.85 × 10^−1	—
PANI	7.916	2.950	−0.847	69.22	12.76	1.49 × 10^−1	4.67
1 wt% RGO/PANI	7.831	3.502	−0.794	94.66	11.71	1.36 × 10^−1	6.75
2 wt% RGO/PANI	7.783	4.125	−0.735	103.21	11.34	1.32 × 10^−1	7.46
3 wt% RGO/PANI	6.536	8.274	−0.686	145.44	10.91	1.27 × 10^−1	10.92
4 wt% RGO/PANI	3.857	7.905	−0.611	157.75	7.14	8.31 × 10^−2	11.93
5 wt% RGO/PANI	7.083	5.616	−0.714	112.01	12.14	1.40 × 10^−1	8.18
WPU	4.345	5.690	−0.552	515.98	2.07 × 10^−3	2.41 × 10^−5	41.29
0.25 (wt%)	11.674	3.616	−0.492	847.45	1.41 × 10^−3	1.64 × 10^−5	68.46
0.5 (wt%)	6.697	4.859	−0.378	1043.14	1.17 × 10^−3	1.36 × 10^−5	84.49
0.75 (wt%)	5.154	5.244	−0.349	3057.52	3.69 × 10^−4	4.30 × 10^−6	249.61
1.0 (wt%)	6.052	4.323	−0.362	1661.98	6.59 × 10^−4	7.68 × 10^−6	135.22
1.25 (wt%)	6.829	5.060	−0.436	989.52	1.28 × 10^−3	1.49 × 10^−5	80.11
1.5 (wt%)	5.566	5.438	−0.455	863.17	1.38 × 10^−3	1.61 × 10^−5	69.75

---

The R_p values are evaluated from the Tafel plots, according to the Stern–Geary equation:^14,24

The CR is calculated as:²⁵

where K is constant of 3270, M is the molecular weight, D is the density and V is the valence. The protection efficiency (P_EF%) is estimated using the following equation:²⁶

It is easy to determine that PANI/RGO composites with a ratio of 4 wt% and composite WPU coatings with addition of 0.75 wt% filler have better anti-corrosion property than the others as evidenced by the higher value of E_corr and the lower value of I_corr from Fig. 7(a) and (b), respectively. According to the experimental results, we deduce that the addition of RGO enhances the dispersion of PANI, the best content being 4 wt%, and PANI/RGO composites in WPU coatings can provide a more perfect barrier; however, too much addition may cause obvious deterioration of shielding property.

EIS as a kind of effective means is used to evaluate the dielectric properties of a medium.^3,27 Fig. 8(a) and (b) present the EIS spectra (Nyquist plots) of the measured samples. In general, the slope of a curve is inversely proportional to I_corr.¹⁹ On the basis of Fig. 8(a) and (b), there is seen obviously two sequences of anti-corrosion performances as follows: PANI < 1 wt% PANI/RGO < 2 wt% PANI/RGO < 3 wt% PANI/RGO < 4 wt% PANI/RGO > 5 wt% PANI/RGO; and WPU < 0.25 wt% composite WPU coatings < 0.5 wt% composite WPU coatings < 0.75 wt% composite WPU coatings > 1.0 wt% composite WPU coatings > 1.25 wt% composite WPU coatings > 1.5 wt% composite WPU coatings. The obtained results are consistent with the trends of Tafel polarization curves.

Fig. 8 EIS spectra (a) for PANI and PANI/RGO composites with different ratios of 1 wt%, 2 wt%, 3 wt%, 4 wt% and 5 wt%, and (b) for WPU and WPU coatings with different added composite dosages of 0.25 wt%, 0.5 wt%, 0.75 wt%, 1.0 wt%, 1.25 wt% and 1.5 wt%.

Images of the different coatings after the salt spray test are shown in Fig. 9. Large areas of corrosion appear after 120 h exposure for pure steel. Compared with pure steel, small amounts of rust appear on the steel coated with neat WPU coating. Moreover, for the steel coated with composite WPU coatings, anti-corrosion performance has improved significantly, although there are some blisters and busts in the images of Fig. 9(c), (f)–(h), indicating that the composite WPU coatings have better anti-corrosion properties than neat WPU coating. In addition, the steel coated with composite WPU coatings which have different added dosages of 0.5 wt% and 0.75 wt% is kept relatively intact. However, the precision of salt spray test is not compatible with electrochemical testing, resulting in a similar corrosion resistance.

Fig. 9 Images of different coatings subjected to the salt spray test after 120 h for (a) pure steel, (b) neat WPU coating, and (c–h) composite WPU coatings with different added dosages of 0.25 wt%, 0.5 wt%, 0.75 wt%, 1.0 wt%, 1.25 wt%, and 1.5 wt%, respectively.

The change of specimen weight is also an indication of the degree of corrosion, and the more serious the corrosion, the more the weight loss. Changes of the steel coated with different coatings before and after the salt spray test are shown in Table 3. On the basis of Table 3, it is clarified that the specimen coated with the composite WPU coating with added dosage of 0.75 wt% showed the smallest weight loss, and an increase or decrease of composite dosage would augment the loss of weight. The conclusion is in agreement with the specimen images.

Table 3 Changes of weight and surface for steel coated with different coatings after salt spray test

Specimens	Mass before test	Mass after test	Weight loss	Phenomenon
Bare steel	11.5527 g	11.4070 g	0.1457 g	Rusting
WPU coating	11.3827 g	11.2827 g	0.0958 g	Local rusting
0.25 wt% composite WPU coating	11.6172 g	11.5676 g	0.0496 g	Local rusting
0.50 wt% composite WPU coating	11.4793 g	11.4650 g	0.0143 g	Integrity
0.75 wt% composite WPU coating	11.7267 g	11.7185 g	0.0082 g	Integrity
1.00 wt% composite WPU coating	11.6571 g	11.6472 g	0.0099 g	Blistering
1.25 wt% composite WPU coating	11.6128 g	11.5939 g	0.0189 g	Blistering
1.50 wt% composite WPU coating	11.5253 g	11.4874 g	0.0379 g	Blistering

---

To sum up, PANI/RGO composites as planar filler can provide barrier properties for WPU composite coatings. The appearance or not of effect depends on whether cracks have been restored. According to the above results, the mechanism in Fig. 10 is shown to intuitively explain the function of PANI/RGO composites in the WPU matrix during the corrosion process. For pure WPU, corrosive mediums (H₂O and O₂) can penetrate the coating easily due to the minute crevices of the surface.¹⁹ Therefore, pathways of corrosive mediums are straight. However, after adding PANI/RGO composites into the WPU matrix, the tortuosity of the diffusion pathway increases to a great extent. First, well-dispersed PANI/RGO composites as anti-corrosion barrier repair the cracks of the WPU coating, and improve the integrity of the WPU coating. Second, PANI reacts with steel to form a dense layer of Fe₃O₄ passive film. As a result of the existence of this passive film, the phenomenon by which corrosive mediums penetrate the coating is reduced substantially. Consequently, the anti-corrosion properties of the WPU composite coatings are improved substantially.

Fig. 10 Schematic representation of corrosion medium following paths through a WPU coating and a composite WPU coating.

4. Conclusions

In conclusion, a series of WPU anti-corrosion coatings containing polyaniline/graphene composites were fabricated by using an in situ polymerization technique, and well-dispersed composites with a relatively high aspect ratio in WPU coatings were characterized by their morphologies enhancing the barrier effect. When the graphene content in the composites was 4 wt% and the addition of composites was 0.75 wt%, the anti-corrosion properties of the composite WPU coatings were improved as compared with neat WPU coatings due to there being a tortuous path for an electrolyte to penetrate through the coatings. Furthermore superior anti-corrosion properties were testified by Tafel polarization curves, EIS and salt spray test. In addition, analysis of images of specimens after 120 h salt spray test indicated that under-painting corrosion did not occur.

Acknowledgements

This work was supported by the Natural Science Foundation of China (21401014), Technology Support Program of Zhenjiang City (GY2014037) and Natural Science Funds of Jiangsu Province (BK20130247).

References

1. S. Cui, X. Yin, Q. Yu, Y. Liu, D. Wang and F. Zhou, Corros. Sci., 2015, 98, 471–477.
2. B. Wessling, Adv. Mater., 1994, 6, 226–228.
3. M. Inagaki, Carbon, 2012, 50, 3247–3266.
4. H. Li, X. Wang, L. Zhang and B. Hou, Corros. Sci., 2015, 94, 342–349.
5. O. C. Compton, S. Kim, C. Pierre, J. M. Torkelson and S. T. Nguyen, Adv. Mater., 2010, 22, 4759–4763.
6. S. Stankovich, D. A. Dikin, G. H. Dommett, K. M. Kohlhaas, E. J. Zimney, E. A. Stach, R. D. Piner, S. T. Nguyen and R. S. Ruoff, Nature, 2006, 442, 282–286.
7. L. Gu, S. Liu, H. Zhao and H. Yu, ACS Appl. Mater. Interfaces, 2015, 7, 17641–17648.
8. M. D. Stoller, S. J. Park, Y. W. Zhu, J. H. An and R. S. Ruoff, Nano Lett., 2008, 8, 3498–3502.
9. W. Chen, L. Yan and P. R. Bangal, Carbon, 2010, 48, 1146–1152.
10. K. S. Kim, Y. Zhao and H. Jang, et al., Nature, 2009, 457, 706–710.
11. Y. Shao, M. F. El-Kady, L. J. Wang, Q. Zhang, Y. Li, H. Wang, M. F. Mousavi and R. B. Kaner, Chem. Soc. Rev., 2015, 44, 3639–3665.
12. H. Kim, Y. Miura and C. W. Macosko, Chem. Mater., 2010, 22, 3441–3450.
13. Z. Tian, H. Yu, L. Wang, M. Saleem, F. Ren, P. Ren, Y. Chen, R. Sun, Y. Sun and L. Huang, RSC Adv., 2014, 4, 28195.
14. C.-H. Chang, T.-C. Huang, C.-W. Peng, T.-C. Yeh, H.-I. Lu, W.-I. Hung, C.-J. Weng, T.-I. Yang and J.-M. Yeh, Carbon, 2012, 50, 5044–5051.
15. Y. Li, Z. Yang, H. Qiu, Y. Dai, Q. Zheng, J. Li and J. Yang, J. Mater. Chem. A, 2014, 2, 14139.
16. T. K. Chen, Y. I. Tien and K. H. Wei, Polymer, 2000, 41, 1345–1353.
17. B. C. Thompson, E. Murray and G. G. Wallace, Adv. Mater., 2015, 27, 7563–7582.
18. Q. B. Zheng, W. H. Ip, X. Y. Lin, N. Yousefi, K. K. Yeung, Z. G. Li and J. K. Kim, ACS Nano, 2011, 5, 6039–6051.
19. M. Mo, W. Zhao, Z. Chen, Q. Yu, Z. Zeng, X. Wu and Q. Xue, RSC Adv., 2015, 5, 56486–56497.
20. S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen and R. S. Ruoff, Carbon, 2007, 45, 1558–1565.
21. F. Tuinstra and J. L. Koenig, J. Chem. Phys., 1970, 53, 1126–1130.
22. P. Cui, J. Lee, E. Hwang and H. Lee, Chem. Commun., 2011, 47, 12370–12372.
23. Z. H. Sheng, L. Shao, J. J. Chen, W. J. Bao, F. B. Wang and X. H. Xia, ACS Nano, 2011, 5, 4350–4358.
24. T.-C. Huang, Y.-A. Su, T.-C. Yeh, H.-Y. Huang, C.-P. Wu, K.-Y. Huang, Y.-C. Chou, J.-M. Yeh and Y. Wei, Electrochim. Acta, 2011, 56, 6142–6149.
25. T.-C. Yeh, T.-C. Huang, H.-Y. Huang, Y.-P. Huang, Y.-T. Cai, S.-T. Lin, Y. Wei and J.-M. Yeh, Polym. Chem., 2012, 3, 2209.
26. T.-C. Huang, T.-C. Yeh, H.-Y. Huang, W.-F. Ji, Y.-C. Chou, W.-I. Hung, J.-M. Yeh and M.-H. Tsai, Electrochim. Acta, 2011, 56, 10151–10158.
27. K. C. Chang, M. H. Hsu, H. I. Lu, M. C. Lai, P. J. Liu, C. H. Hsu, W. F. Ji, T. L. Chuang, Y. Wei, J. M. Yeh and W. R. Liu, Carbon, 2014, 66, 144–153.

---