Emulsion polymerization for the fabrication of poly(o-phenylenediamine)@multi-walled carbon nanotubes nanocomposites: characterization and their application in the corrosion protection of 316L SS

Abstract

Recently, the corrosion of metals is an important academic and industrial concern which has received significant attention. Poly(o-phenylenediamine) (PoPDA) based nanocomposites with unfunctionalized multi-walled carbon nanotubes (MWCNTs) and functionalized multi-walled carbon nanotubes (FMWCNTs) were prepared through emulsion polymerization using sodium dodecyl sulfate (SDS) as an emulsifier and ammonium persulfate (APS) as an oxidant. Fourier transform infrared (FT-IR) spectra confirmed the construction of the nanocomposites. The morphology of the synthesized nanocomposites was characterized using scanning electron microscopy (SEM). X-ray diffraction (XRD) patterns of the nanocomposites indicated a more crystalline nature than that of bare PoPDA. The thermal stability of the nanocomposites was improved relative to bare PoPDA. The corrosion protection performance of the coatings containing PoPDA@MWCNTs, PoPDA@FMWCNTs and PoPDA on steel was evaluated using potentiodynamic polarization, electrochemical impedance spectroscopic (EIS) and open circuit potential (OCP) measurements in 3.5% NaCl solution. The obtained results of the potentiodynamic polarization, EIS and OCP showed that the PoPDA@MWCNT nanocomposite coated steel had excellent corrosion inhibition behavior in saline solution.

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Introduction

Polymer based nanocomposites have received great amounts of research and development owing to their widespread applications in numerous fields, such as in the removal of heavy metal ions, sensors and anticorrosion coatings.^1–3 Specifically, the preparation of nanocomposites involving carbon nanotubes (CNTs) and conducting polymers (CPs) has been extensively studied in recent years due to the synergistic effects resulting from the combination of these materials.^4–7

CPs based on aromatic diamines have been synthesized via the polymerization of the corresponding diamine monomers in the presence of a peroxy-initiator under acidic conditions. They displayed more novel multifunctionality than PANI due to one pendent free amino group per repetitive unit on the polymer chains. In addition, they have shown good solubility in common organic solvents compared to PANI, but their conductivity is poor.^8–10

Among them, poly(o-phenylenediamine) (PoPDA) containing the 2,3-diaminophenazine or quinoraline repeating unit has gradually become an important member in the family of conductive polymers and it could be used in many fields.^11,12 However, PoPDA has poor physical properties. One method to improve the physical properties of CPs is the use of inorganic nanoparticles in PANI-derived nanocomposites.

CNTs have gained considerable attention over the last decade due to their unique physical properties such as large surface area, good corrosion resistance, high temperature stability, and good mechanical properties.¹³ The formation of nanocomposite materials through the incorporation of CNTs into the polymer matrix (CNTs@polymer) is a valuable approach to enhance the mechanical properties, thermal stability, electrical conductivity, solvent resistance, and optical properties of the materials relative to their individual components. For this purpose, a number of methods have been developed for the fabrication of CNTs@polymer composites based on the types of polymeric matrices.^14,15

One of the suitable methods for preparation of nanocomposites bearing polymer matrices is emulsion polymerization. Emulsion polymerization is the most commonly used method for the production of a wide range of polymers. This method allows particles to transfer into micelles through the emulsifier template and increase the molecular weight.¹⁰ CPs@CNT composite films are expected to act as excellent coating materials in corrosion protection applications.

Recently, the corrosion of metals is an important academic and industrial concern which has received significant attention. Many corrosion control methods use coatings and resistant layers that contain toxic and environmentally hazardous materials, especially chromium compounds. Hence, efforts should be made to develop pinhole-free coatings that satisfy environmental concerns in order to avoid the use of heavy metals. CPs and CP based nanocomposites are promising options for the protection of metals against corrosion. Recently, many researchers have reported the ability of CPs and CP based nanocomposites to protect against corrosion.

For instance, Sathiyanarayanan et al. prepared a polyaniline/TiO₂ composite for the protection of steel.¹⁶ Kumar et al. investigated the corrosion protection of a PANI@FMWCNT nanocomposite.¹⁷ Ionita et al. used polypyrrole@carbon nanotube composites as anti-corrosive coatings.¹⁸ Ganash studied the anticorrosive properties of poly(o-phenylenediamine)@ZnO nanocomposites.¹⁹ Olad et al. considered polypyrrole nanocomposites with organophilic and hydrophilic montmorillonite as a corrosion protector for iron.³ Lenz et al. investigated the application of polypyrrole@TiO₂ composite films for corrosion protection.²⁰ Emamgholizadeh et al. investigated the corrosion inhibition of steel with a EP/PpPDA/SiO₂ nanocomposite.²¹

The goal of this study is the preparation of poly(o-phenylenediamine) based nanocomposites with functionalized and unfunctionalized multi-walled carbon nanotubes via emulsion polymerization. Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM) were applied in the characterization of the synthesized nanocomposites. Finally the corrosion inhibition of the synthesized nanocomposites against 316L stainless steel in 3.5% NaCl solution was investigated.

Experimental

Materials

ortho-Phenylenediamine (oPDA), ammonium persulfate (APS), sodium dodecyl sulfate (SDS) and all solvents were purchased from Merck Company (Germany) and were used without further purification. Multi-walled carbon nanotube (MWCNT) particles (diameter 20–50 nm, length 5–20 μm) were supplied from Sigma-Aldrich Company.

Characterization

Fourier transform infrared (FTIR) spectra were recorded on a Bruker Tensor 27 spectrometer (Bruker, Karlsrohe, Germany). X-ray diffraction (XRD, Shibuya-ku, Tokyo, Japan) patterns were obtained in the 2 theta range of 10–70° using a Rigaku D/Max-2550 powder diffractometer with a scanning rate of 5° min⁻¹ at room temperature. Thermal gravimetric analyses (TGA) of the prepared polymers and composites were determined using a LENSES STAPT-1000 calorimeter (Linseis STA PT1000, Selb, Germany) by scanning up to 700 °C with a heating rate of 10 °C min⁻¹. Scanning electron microscopy (SEM) was conducted on a Hitachi S4160 instrument (Tokyo, Japan). Potentiostat polarization and open-circuit potentials were recorded using an Autolab 302 N (Netherlands). Also, electrochemical impedance spectroscopy was carried out using Palmsense (PS Trace software version 4.2.2, Netherlands).

Functionalization of multi-walled carbon nanotubes (FMWCNTs)

MWCNTs were functionalized according to our previous work.⁵ MWCNTs (1 g) were preserved in a 200 mL oxidizing mixture of H₂SO₄/HNO₃ (3 : 1 v/v) (6 M) to prevent the massive structural destruction of MWCNTs. Then the mixture was dispersed using a sonication bath for 8 h at 50 °C. Then, the functionalized MWCNTs were neutralized to pH 7.0, centrifuged and washed with water/acetone (1 : 1) three times and separated with centrifugation. The filtered product was then dried under vacuum at 40 °C in an oven for 12 h.

Preparation of poly(o-phenylenediamine) based nanocomposites with un-functionalized and functionalized multi-walled carbon nanotubes

Poly(o-phenylenediamine)@unfunctionalized multi-walled carbon nanotube (PoPDA@MWCNT) and poly(o-phenylenediamine)@functionalized multi-walled carbon nanotube (PoPDA@FMWCNT) nanocomposites were prepared by emulsion polymerization at room temperature as follows: in a typical experiment, 7.5% (0.081 g) of CNTs (optimum amount), 2.5 g (8.66 mmol) of SDS, and 30 mL of CHCl₃ were added into 30 mL of distilled water and the mixture was dispersed using a sonication bath at room temperature for about 2 h. Then, 30 mL of oPDA solution (1 g in 30 mL of HCl (1 M)) was added to the above solution. An APS solution (1.5 g of APS in 20 mL deionized water) used as an initiator was added dropwise into the reaction medium over 40 min and the reaction was carried out at room temperature for 24 h. Afterward, the mixture was poured into acetone to terminate the reaction. Finally, the obtained precipitate was filtered and washed with distilled water and methanol several times and was then dried under vacuum at 50 °C for 24 h. For comparative study, bare PoPDA was also synthesized via a radical oxidation polymerization of oPDA in an acidic medium according to our previous report.¹⁰

Corrosion tests

Tafel tests were carried out using a conventional three-electrode electrochemical cell with platinum wire as the counter electrode, Ag/AgCl as the reference electrode, and the PoPDA, PoPDA@MWCNT and PoPDA@FMWCNT nanocomposites coated on 316L stainless steel (SS), and uncoated 316L SS samples, as the working electrodes.

To prepare the electrodes, firstly 2 g of polyvinyl butyral (PVB) (an adhesion agent) was dissolved in 20 mL of methanol and then 20 wt% (0.5 g) of powder sample was added. The mixture solution was kept in a sonication bath for 30 min to obtain a uniform dispersion of sample (PoPDA, PoPDA@MWCNTs and PoPDA@FMWCNTs) in the PVB solution. A steel electrode with a surface area of 0.07 cm² was then dip coated with the synthesized materials/PVB in a methanol solution and dried at 60 °C for 50 min. A NaCl 3.5% (w/w) electrolyte was used as the corrosive environment.

Electrochemical impedance spectroscopy (EIS) measurements were done with frequencies ranging from 20 to 10 kHz with the amplitude of the superimposed AC signal set at 10 mV. Polarization curves for the uncoated and polymer coated SS specimens were recorded under potentiodynamic conditions in the potential range of ±250 mV with respect to OCP at a sweep rate of 2 mV s⁻¹.

Results and discussion

Nowadays, the use of polymers and polymer matrix based nanocomposites for the inhibition of metal corrosion is one of the most applied approaches. Scheme 1 presents a typical procedure for the fabrication of PoPDA@FMWCNT nanocomposites using the emulsion polymerization technique and electrode construction pathway.

Scheme 1 The synthesis procedure for the fabrication of PoPDA@FMWCNT nanocomposites using the emulsion polymerization technique and electrode construction pathway.

FTIR analysis

FTIR spectra were used to characterize the functional groups of the synthesized PoPDA@MWCNT and PoPDA@FMWCNT nanocomposites, which are presented in Fig. 1.

Fig. 1 FTIR spectra of the PoPDA@MWCNT and PoPDA@FMWCNT nanocomposites.

To the best of our knowledge no significant functional groups were detected in the FTIR spectra of the MWCNTs. Thus, the observed peaks at around 3325 and 1220 cm⁻¹ are ascribed to the presence of hydroxyl groups of adsorbed water on the surface of the MWCNTs, which could have appeared either from the humidity bound to the MWCNTs or during the purification of the raw material.²² On the other hand, the FMWCNTs exhibit characteristic peaks at 3356, 1680 and 1080 cm⁻¹ that can be attributed to the stretching vibrations of O–H, the C=O of the carboxylic acid groups and C–O, respectively.⁵ The presence of these peaks suggests that the oxidation of the MWCNTs has introduced COOH groups on the surface of MWCNTs. In the FTIR spectra of PoPDA, the peaks at 3380, 1630 and 1500 cm⁻¹ can be attributed to the N–H stretching vibration of the secondary amine group in the polymer chain, quinoid and benzenoid stretching vibrations, respectively.¹² The FTIR spectra of the MWCNTs, FMWCNTs and PoPDA are presented in Fig. S1 in the ESI.†

The FTIR spectra of the PoPDA@MWCNT nanocomposite showed almost identical characteristic peaks at 3360, 1660, 1497, and 1210 cm⁻¹, respectively, which were in accordance with previous reported composites with unfunctionalized MWCNTs.²³ This confirmed that the PoPDA@MWCNT nanocomposites had been successfully synthesized. In the FTIR spectra of the PoPDA@FMWCNT nanocomposite most of the FMWCNT signals have been overlapped by those of PoPDA. The characteristic peaks of PoPDA detected at 3390, 1630, and 1500 cm⁻¹ were slightly shifted to lower wavenumbers and became very weak. A reasonable explanation for this may be attributed to the formation of hydrogen bonding between the amino groups of PoPDA and hydroxyl of the carboxylic groups in the FMWCNTs.

X-ray diffraction (XRD)

X-ray diffraction (XRD) is a rapid analytical technique primarily used for the phase identification of a crystalline material. In the XRD patterns of the MWCNTs and FMWCNTs, three main peaks were observed at 2θ = 25.9°, 41.02°, and 55.11° which indicated their crystalline nature (Fig. 2).^24,25 Meanwhile in the XRD pattern of PoPDA, three broad characteristic peaks appeared at 2θ = 21°, 24° and 27° revealing that the local crystallinity may be caused by periodicity perpendicular to the polymer chain.¹¹ The partial crystallinity may be a result of the long range ordering of polymer chains in the PoPD backbone.¹¹ The XRD patterns of the MWCNTs, FMWCNTs and PoPDA are presented in Fig. S2 in the ESI.†

Fig. 2 XRD patterns of the PoPDA@MWCNT and PoPDA@FMWCNT nanocomposites.

The XRD patterns of the PoPDA@MWCNT and PoPDA@FMWCNT nanocomposites exhibit two clear phases: the polymer phase and the MWCNT phase, which has several sharp peaks. It specifies that the PoPDA@MWCNT and PoPDA@FMWCNT nanocomposites are more crystalline than the bare PoPDA. When comparing the PoPDA@MWCNT and PoPDA@FMWCNT nanocomposites, the crystalline structure of the PoPDA@FMWCNT nanocomposite showed an improvement relative to the PoPDA@MWCNT nanocomposite. This good crystalline nature probably appeared due to intramolecular interactions between the carboxyl groups of the FMWCNTs and the amine groups of PoPDA.

SEM micrographs

Scanning electron microscopy (SEM) is extensively used in the surface analysis of synthesized materials. Fig. 3 demonstrates the SEM micrographs of the PoPDA@MWCNT and PoPDA@FMWCNT nanocomposites.

Fig. 3 SEM micrographs of the PoPDA@MWCNT (a) and PoPDA@FMWCNT (b) nanocomposites.

As seen in Fig. S3a,† it was revealed that the PoPDA particles are irregular granules with a fairly different size distribution of around 100–150 nm in average diameter. The exhibited MWCNT morphology was boundless and twisted, with a smooth surface, and the diameter of each nanotube is about 20–50 nm (Fig. S3b†). The SEM of the FMWCNTs showed the extrication of the FMWCNTs, and a slight reduction in the length of the nanotubes was observed after oxidation (Fig. S3c†). The SEM images of the MWCNTs, FMWCNTs and PoPDA are presented in Fig. S3 in ESI.†

As evidenced from Fig. 3a, there is an indication of a distribution of MWCNTs and disordered granular particles of PoPDA which represents the absence of interaction between the MWCNTs and PoPDA in the nanocomposite. In the SEM image of the PoPDA@FMWCNT nanocomposite (Fig. 3b), a tubular layer of coated PoPDA film is observed, and the diameter of the nanocomposite is increased by several tens of nanometers compared with the FMWCNTs, depending on the PoPDA content. This can be attributed to the coating formation of PoPDA taking place only at the outer surface of the FMWCNTs. In fact, the formation of the PoPDA coated tubular nanocomposite is believed to arise from the strong interaction between the amine groups of PoPDA and the carboxyl groups of the FMWCNTs.

TGA

Thermogravimetric analysis (TGA) is a useful technique to measure the thermal stability of synthesized materials. Fig. 4 shows the TGA thermograms of the PoPDA@MWCNT and PoPDA@FMWCNT nanocomposites.

Fig. 4 TGA thermograms of the PoPDA@MWCNT and PoPDA@FMWCNT nanocomposites.

As seen from the TGA thermograms of the MWCNTs and FMWCNTs (Fig. S4†), the COOH content in FMWCNTs is about 5.6 wt%, a reasonable value for acid treated MWCNTs.²⁶ The TGA thermogram of PoPDA (Fig. S4†) has a two-step thermal transition that leads to weight loss. The first thermal transition from 100 to 315 °C with a weight loss of ∼11% corresponds to the removal of residual water, dopants and the loss of low molecular weight oligomers.¹² The second transition which is observed between 315 °C to 600 °C with a weight loss of ∼45% can be attributed to the degradation of the backbone units of PoPDA (benzenoid and quinoid units).¹¹ The char yield at 800 °C is ∼45%. The TGA thermograms of the MWCNTs, FMWCNTs and PoPDA are presented in Fig. S4 in the ESI.†

The comparison of the PoPDA, PoPDA@MWCNT and PoPDA@FMWCNT nanocomposites showed an improvement in the thermal stability of the nanocomposites relative to PoPDA, where the residual weight at 800 °C of the PoPDA@MWCNT and PoPDA@FMWCNT nanocomposites was ∼55% and ∼50%, respectively.

Potentiodynamic polarization studies

In this study polyvinyl butyral (PVB) was used as a binder for coating the synthesized material on the steel electrode surface. The obtained results indicated that the PVB had no anticorrosion activity against steel. To gain a better understanding the Tafel plots of uncoated and PVB coated steel in 3.5% w/w NaCl solution are given in Fig. S5 in the ESI.†

Fig. 5 illustrates the Tafel plots for PoPDA, PoPDA@FMWCNT and PoPDA@MWCNT nanocomposite coated steel samples and uncoated steel in 3.5% w/w NaCl solution under potentiodynamic polarization conditions. The corrosion protection of the synthesized material coated steel can be observed from the values of the corrosion potential (E_corr), corrosion current (I_corr), and polarization resistance (R_p) as listed in Table 1; generally, a higher E_corr and a lower I_corr indicate better corrosion protection. As seen from the Tafel plot of the PoPDA@FMWCNT nanocomposite coated steel, the I_corr of the PoPDA@FMWCNT nanocomposite was 222, which was higher than that of the PoPDA@MWCNT nanocomposite coated steel and lower than that of the PoPDA coated steel. Thus, it was found that the incorporation of FMWCNTs in the PoPDA matrix improves the anticorrosive efficiency of PoPDA@FMWCNT nanocomposite coatings on the steel sample. The anticorrosion activity of the nanocomposites was more favorable at an optimum content of MWCNTs and FMWCNTs because the adhesion strength of the nanocomposite coating on the steel decreases with increasing MWCNT content. Hence, in this study we carried out anticorrosion activity experiments just for the optimized nanocomposites. Furthermore, it is observed that the E_corr of the synthesized material coated steel is shifted in the positive direction compared to that of uncoated steel. These positive shifts of −184 mV to −113 mV in E_corr indicate the protection of the steel surface with the synthesized material coatings, where the best results were obtained for PoPDA@MWCNT coated steel with a E_corr = −113. A reasonable explanation is that the presence of π electrons in the aromatic ring and quaternary nitrogen atom in the PoPDA emeraldine salt and good conjugation in the MWCNTs are important factors to effectively prevent steel against corrosion.

Fig. 5 Tafel plots for bare steel, PoPDA, PoPDA@MWCNT, and PoPDA@FMWCNT coated steel electrodes measured in 3.5 wt% NaCl aqueous solution.

Table 1 Electrochemical corrosion parameters of prepared coating materials

Samples	Bare steel	PoPDA	PoPDA@MWCNTs	PoPDA@FMWCNTs
E corr (mV)	−184	−163	−113	−153
I corr (µA cm^−2)	2.51	0.562	0.012	0.028
R p (kΩ cm^2)	0.43	0.83	0.96	3.88

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On the other hand, the polarization resistance (R_p) values were calculated from the Tafel plots, using the Stern–Geary equation²⁷

where I_corr, b_a and b_c are the corrosion current density, anodic and cathodic Tafel slopes, respectively. The results showed that both the nanocomposite and PoPDA have R_p values higher than bare steel metal.

The obtained electrochemical corrosion parameter results of the PoPDA@MWCNT and PoPDA@FMWCNT nanocomposites are compared with other compound coated steel on the basis of I_corr and E_corr found in the literature and are summarized in Table 2. According to this data, the PoPDA@MWCNT and PoPDA@FMWCNT nanocomposites have low I_corr and relatively high E_corr in comparison with some previously reported coatings based on aniline derivatives. This shows that PoPDA based nanocomposites can be good candidates for applications related to corrosion protection.

Table 2 Comparison of the electrochemical corrosion parameters for various sample coated steel in 3.5% NaCl solution

Sample	E corr (mV)	I corr (µA cm^−2)	Reference
PoPDA@MWCNTs	−113	0.012	Present work
PoPDA@FMWCNTs	−153	0.028	Present work
Poly(o-phenylenediamine) nanofibers	−174	0.114	12
Polyaniline/FMWCNT	−505	0.034	17
Poly(o-phenylenediamine)/ZnO	−36	0.010	19
Polyaniline/MWCNTs	−564	12.6	28
Polyaniline/graphene	−584	1.38	29
Nano-colloidal polyaniline	−505	0.034	30

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Electrochemical impedance spectroscopy

The other way to evaluate the protection of the steel with the nanocomposite films that are deposited on the metal surface is by using Electrochemical Impedance Spectroscopy (EIS). The surface resistance of uncoated and coated steel in NaCl solution (3.5% w/w) was investigated using the EIS technique and reported here in terms of Nyquist plots.

The Nyquist plots of uncoated steel, PoPDA, PoPDA@FMWCNT, and PoPDA@MWCNT nanocomposite coated steel samples after immersion in NaCl solution (3.5% w/w) are shown in Fig. 6. The shapes of the Nyquist plots for all samples showed a depressed semicircle in the high frequency region. The diameter of the depressed semicircle shows the charge transfer resistance (R_ct). The R_ct values for the PoPDA@MWCNT nanocomposite coated steel (120 kΩ) appeared to be significantly higher than those for the PoPDA@FMWCNT nanocomposite (74 kΩ), and PoPDA (14 kΩ) coated steel and uncoated steel (5 kΩ). The higher R_ct in the PoPDA@MWCNT nanocomposite coated steel is probably ascribed to the existence of MWCNTs in the nanocomposite matrix which blocks the access of the aggressive electrolyte to the reactive metal surface. Also, it seems that the lower impedance observed in the PoPDA@FMWCNT nanocomposite coated steel is due to the less hydrophobic nature of the coating. The percentage of inhibition efficiency (IE%) of the synthesized material coated steel was calculated as follows:³⁰

where R_ct (coated) and R_ct (uncoated) are the charge transfer resistance values with and without synthesized material coatings respectively. The results showed that the coated samples exhibited higher R_ct values compared to the uncoated sample. Also, the percentage of corrosion inhibition efficiencies for the PoPDA, PoPDA@MWCNT and PoPDA@FMWCNT nanocomposite coated steel are 64%, 95% and 93%, respectively. The corrosion inhibition mechanism for the PoPDA coated steel is due to the formation of an insoluble stable iron oxide–PoPDA complex at the metal/polymer interface.¹¹ In the PoPDA nanocomposite coated steel, it is probably the presence of MWCNTs (functionalized and un-functionalized) that can form an inhibiting film which protects steel against corrosion. It seems that the mechanism of corrosion protection by nanocomposites includes the formation of a stable interface between the nanocomposite and steel which acts as a barrier against corrosive ions, and the inhibition of charge transfer from the metal surface to corrosive ions or vice versa due to the electrical conductivity of the PoPDA@MWCNT and PoPDA@FMWCNT nanocomposites. The Nyquist plots of uncoated steel and PVB coated steel in 3.5 w/w% NaCl solution are presented in Fig. S6 in the ESI.†

Fig. 6 Nyquist plots of the bare steel, PoPDA, PoPDA@MWCNT and PoPDA@FMWCNT coated steel electrodes measured in 3.5 wt% NaCl aqueous solution.

Open circuit potential (OCP) measurements

The open circuit potential (OCP) measurements of the uncoated steel (a) and PoPDA (b), PoPDA@MWCNT (c) and PoPDA@FMWCNT (d) coated steel immersed in corrosive medium (NaCl solution (3.5%)) for 300 min are given in Fig. 7. The initial values measured for the bare steel, PoPDA, PoPDA@MWCNT and PoPDA@FMWCNT nanocomposite coated steel were −0.17 V, −0.16 V, −0.15 V and −0.04 V, respectively. All measurements were cathodic potential values. After 300 min of exposure time, the E_ocp values measured for the PoPDA, PoPDA@MWCNT and PoPDA@FMWCNT nanocomposites were 0.06 V, 0.07 V, and 0.09 V, respectively, which are anodic with respect to that measured for the uncoated steel electrode, which was −0.11 V under the same conditions. Surprisingly, the measured E_ocp values of PoPDA, PoPDA@MWCNT and PoPDA@FMWCNT nanocomposites increased. It seems the increase of the potential indicates the presence of electrostatic interactions between the synthesized materials and the steel surface. These results demonstrate that PoPDA, PoPDA@MWCNT and PoPDA@FMWCNT nanocomposites show barrier behavior by limiting the motion of corrosive agents toward the underlying metal and are acting as an active coating affecting the formation of the passive oxide. Also, these results displayed that the E_ocp of the PoPDA@FMWCNT nanocomposite coated steel is lower than that of the PoPDA@MWCNT nanocomposite. It can therefore be concluded that the presence of carboxylic groups in the MWCNTs is an effective factor against steel corrosion and, as a result, the E_ocp of the PoPDA@FMWCNT nanocomposite decreased. The OCP measurements for uncoated steel and PVB coated steel in 3.5 wt% NaCl solution are shown in Fig. S7 in the ESI.†

Fig. 7 Open circuit potential measurements for the bare steel (a), PoPDA (b), PoPDA@MWCNT (c) and PoPDA@FMWCNT (d) coated steel electrodes in 3.5 wt% NaCl aqueous solution.

Conclusions

PoPDA@MWCNT and PoPDA@FMWCNT nanocomposites used as anti-corrosion protection coatings for steel were successfully fabricated via an in situ emulsion polymerization. The presence of MWCNTs and FMWCNTs in the nanocomposites enhanced the electrical conductivity, crystalline nature and thermal stability compared to that of bare PoPDA. The SEM micrographs of the nanocomposites confirmed that they were fabricated successfully. Potentiodynamic polarization, EIS and OCP studies revealed that the PoPDA based nanocomposites act as a protective layer on steel against corrosion in a 3.5 wt% NaCl solution. The corrosion rate of the PoPDA@MWCNT nanocomposite coated steel was lower than that of bare steel and the IE was found to be 95%. The higher corrosion protection ability of the PoPDA@MWCNT nanocomposite was probably due to the formation of a uniform passive film on the iron surface.

Acknowledgements

This research work has been supported by a research grant from the University of Mazandaran.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11295h

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