Effect of narrow diameter polyaniline nanotubes and ... - UMExpert

Progress in Organic Coatings 75 (2012) 301–308

Contents lists available at SciVerse ScienceDirect

Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat

Effect of narrow diameter polyaniline nanotubes and nanofibers in polyvinyl butyral coating on corrosion protective performance of mild steel M.R. Mahmoudian a,∗ , Y. Alias a , W.J. Basirun a,b a b

Department of Chemistry, University of Malaya, Kuala Lumpur 50603, Malaysia Nanotechnology & Catalysis Research Centre (NanoCat), Institute of Postgraduate Studies, University Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e

i n f o

Article history: Received 12 January 2012 Received in revised form 31 July 2012 Accepted 1 August 2012 Available online 24 August 2012 Keywords: Polyaniline Morphology Nanomaterials Corrosion

a b s t r a c t This study shows the effect of narrow diameter polyaniline (PAn) nanotubes and nanofibers in polyvinyl butyral coating on corrosion protective performance of mild steel. The PAn nanotubes and nanofibers were synthesised by chemical oxidative polymerisation in the presence of dodecylbenzene sulfonic acid (DBSA) as a bulky dopant acid. The molar ratios of monomer to DBSA used for the nanotube and the nanofiber synthesis are 1:1 and 4:1, respectively. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) results confirm the narrow diameter range of 180–230 nm for the PAn nanotubes and nanofibers. Electrochemical impedance spectroscopy (EIS) confirms that the resistance of the coating containing PAn nanotubes is three times higher than the coating containing PAn nanofibers after 30 days of immersion. This effect can be explained because of the higher surface area of the nanotubes compared to nanofibers with the same mass. This difference increases the ability of PAn to interact with the ions liberated during the corrosion of the steel and increases the rate of cathodic reduction of oxygen on the surface of PAn. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Polyaniline (PAn) is among the group of conducting polymers that has been most widely used due to its good electrical conductivity and environmental stability. Because of its applications, facile syntheses of PAn have been developed from a range of aqueous and non-aqueous solvents [1,2]. Over the past decades, PAn has been considered to be one of the best candidates for corrosion control without the use of heavy metals. Many studies since the mid-1980s have shown that PAn provides sufficient protection for various types of metals such as mild steel [3], aluminium [4], stainless steel [5] and copper [6]. Recently, a few studies have reported the anticorrosion performance of PAn with different nanostructures [7]. Recent investigations have shown that the nanostructure morphology of PAn is able to influence its anticorrosion performance [8]. The majority of researchers have reported that an increase in surface area is the primary factor for the enhanced anticorrosion ability of the PAn nanostructure [7,9]. Different polymerisation methods, such as interfacial polymerisation [10], oligomer-assisted polymerisation [11], hard templates [12] and soft templates, could be employed to obtain 1-D (1

∗ Corresponding author. Tel.: +60 173928320. E-mail address: M R [email protected] (M.R. Mahmoudian). 0300-9440/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.porgcoat.2012.08.004

dimensional) PAn nanostructures with different morphologies. One of the most effective methods that can be used to synthesise 1-D PAn nanostructures is soft-templates, such as surfactant [13], bulky dopant acid [14], liquid crystalline [15] and biological templates [16]. Dodecylbenzenesulfonic acid (DBSA) is a bulky dopant acid that is widely used in the synthesis of PAn. It has been reported that PAn polymerisation carried out in high concentrations of DBSA promotes the formation of granular-like PAn [17] or PAn nanoparticles [18]. Therefore, the molar ratio of the aniline monomer to DBSA is an important factor in the synthesis of PAn nanostructures in the presence of DBSA. In this work, the synthesis of PAn nanotubes and nanofibers with a narrow diameter range in the presence of DBSA and the comparison of their anticorrosion performance are described. The effect of the surface area of synthesised PAn nanostructures on the anticorrosion performance of two different coatings containing narrow-diameter nanotubes and nanofibers of PAn is investigated.

2. Experimental 2.1. Synthesis and characterisation of PAn nanofibers and nanotubes The molar ratios of the aniline monomer to DBSA and the HCl concentration have a significant effect on the morphology and size of the PAn nanostructures [17,18]. We synthesised PAn with

302

M.R. Mahmoudian et al. / Progress in Organic Coatings 75 (2012) 301–308

different molar ratios of aniline monomer to DBSA in different HCl concentrations. In the next section, we introduce varying molar ratios and HCl concentrations used in the synthesis of the PAn nanotubes and nanofibers with a narrow diameter range. 2.1.1. PAn nanofibers All the chemicals used were from Aldrich. The pure aniline (An) monomer was always stored in the absence of light before the synthesis experiments. The chemical polymerisations was performed by dissolving 4:1 aniline (0.54 g, 6.0 mmol) and DBSA (0.49 g, 1.5 mmol) in 80 mL of distilled water in a reaction vessel and then continuously stirring at 500 rpm with a mechanical stirrer for 30 min at room temperature. A solution of 10 mL of 0.05 M HCl was added to the solution, and the stirring was continued until the solution turned cloudy. The mixture was stirred for 20 min at room temperature before 10 mL of 0.5 M ammonium persulfate (APS) solution was added to the mixture in a single portion. The polymerisations were performed under static conditions for 24 h at 0–5 ◦ C. The precipitate was filtered, washed with distilled water repeatedly and dried in a vacuum oven at 50 ◦ C for 36 h. 2.1.2. PAn nanotube The synthesis of the PAn nanotubes was performed in a typical process as follows: chemical polymerisation was performed by dissolving DBSA (3 g, 0.09 mol) in 50 mL of distilled water, the mixture was stirred slowly until a homogeneous solution was formed, 9 g of aniline (0.09 mol) monomer was added, and the mixture was stirred until it became emulsified. Subsequently, 9 mL of 1 M HCl was mixed with the solution. It is notable that the molar ratio of the aniline monomer to DBSA in this case was 1:1. The mixture was stirred for 10 min at room temperature and 30 mL of 0.03 M APS solution (kept at low temperature in a freezer for 6 h) was added to the mixture in a single portion with vigorous stirring using a magnetic stirrer. The polymerisations were performed under static conditions for 3 h at room temperature. The precipitate was filtered, washed with distilled water repeatedly and dried in vacuum oven at 50 ◦ C for 36 h [14]. 2.1.3. Characterisations Transmission electron microscopy (TEM) (Philips CM200 with an operating voltage of 200 kV) and field emission scanning electron microscopy (FESEM) (JEOL JSM-840A) were used to estimate the size of the two different PAn morphologies. Fourier-transform infrared spectroscopy (FT-IR) spectra of the polymer powders were taken using KBr pellet with Nicolet 380 FT-IR instrument. 2.2. Preparation of PAn dispersion formulations for coating 2 g of polyvinyl butyral (PVB) (Sigma–Aldrich, with M.W. 60,000) was dissolved in 20 mL of methanol with continuous stirring for 24 h. The PAn powders with different morphologies were mixed separately in the PVB solution with continuous stirring for 24 h. The PAn content in the incorporated PVB coating was 1 wt.%. The elemental composition analysis of the steel plate substrate in weight% was: 2.71% C, 0.49% Si and 94.79% Fe. Steel panels of 10 cm × 5 cm × 0.2 cm were sand-blasted to obtain a near white surface profile, according to Swedish specification SA 2.5 [19]. The substrates were dip coated for 30 s in the PAn–PVB dispersion, dried at room temperature for 10 min and baked in an air-circulating oven at 50 ◦ C for 12 h. In addition, a set of panels was coated with the PVB and without the PAn, which was used as a control for these experiments. The thickness of the coating was measured using mechanical profiler (KLA-Tencor, P-6) equipment. Panels with a coating thickness of 24 ± 2 ␮m were selected for corrosion studies. A glass tube with a 2 cm diameter and length of 4 cm was fixed on the steel coating. The steel-coating specimen was used as the

working electrode (WE), the graphite rod as the counter electrode (CE), and a saturated calomel electrode (SCE) was used as the reference. An iCamscope-305A (magnification 40×) was used to take the images of the samples. 2.3. Adhesion measurement An adhesion test was designed to assess the adherence strength of the film according to ASTM D3359-93. A criss-cross lattice pattern was made with spacing of 1 mm. After making the required cuts, the film was brushed lightly with a soft brush to remove any detached chips of coating. A piece of pressure-sensitive tape approximately 75 mm long was placed over the grid and smoothed into place manually. The tape was rubbed firmly with an eraser to ensure sufficient contact with the film. 2.4. Testing of corrosion resistance properties Electrochemical impedance spectroscopy (EIS) spectra was performed over a frequency range of 100 kHz to 10 mHz with an acquisition of 10 points per decade and with a signal amplitude of 5 mV around the open circuit potential (OCP). A potentiostat/galvanostat model PGSTAT-302N from Autolab that was controlled by a USB IF030 interface and by FRA.EXE software was used to perform these experiments. Each surface was exposed to 3.5% NaCl aqueous solution. The polarisation curves were obtained starting from the open circuit potential (OCP) and varying the potential up to 850 mV in a set of experiments (anodic region of the Tafel plot) and down to −850 mV in another set of experiments (cathodic region of the Tafel plot). The polarisation curves were recorded with a scan rate of 5 mV s−1 in 3.5% NaCl solution. 3. Results and discussion 3.1. FESEM and TEM of nanotubes and nanofibers PAn powders The FESEM images for the PAn powders are shown in Fig. 1(a) and (b) for the nanotubes and the nanofibers, respectively. The inset of Fig. 1(a) shows a high-magnification image of the nanotubes. The FESEM images of the PAn nanotubes and nanofibers confirm the homogeneity of the synthesised PAn. As seen from Fig. 1(a) and (b), the diameter ranges of the PAn nanotubes and nanofibers are approximately 180–230 nm. These results are confirmed by the TEM images of the PAn nanotubes (Fig. 2(a)) and nanofibers (Fig. 2(b)). 3.2. FT-IR spectroscopy The FT-IR spectrum of the PAn nanotubes and nanofibers is shown in Fig. 3. The peak at 3434 cm−1 in Fig. 3(a) and (b) is attributed to N H bonds. A large descending baseline appears in the region from 1700 to 2800 cm−1 , which indicates the free-electron conductivity of PAn. Previous FT-IR reports of PAn synthesised on the surface of Pt confirm these results [20]. The peaks at 1570 and 1297 cm−1 in the PAn nanotube spectra and the peaks at 1569 and 1298 cm−1 in the PAn nanofiber spectra, when taken together with the peaks attributed to the N H bond, confirm that PAn polymerisation occurs through the formation of C N C and C NH C bonds, which result from head-to-tail coupling polymerisation of the aniline monomers [21]. In addition, the peaks at 1128 cm−1 (Fig. 3(a)) and 1130 cm−1 (Fig. 3(b)) are related to SO3 . The peaks at 688 and 1037 cm−1 in the FT-IR of the PAn nanotubes are attributed to the S O stretch. The peaks at 685 and 1040 cm−1 in FT-IR of the PAn nanofibers are also attributed to the S O stretch [22,23].


303

Fig. 1. FESEM of the (a) PAn nanotubes and (b) PAn nanofibers that were synthesised by chemical oxidative polymerisation in the presence of dodecylbenzene sulfonic acid (DBSA) as a bulky dopant acid.

Fig. 2. Transmission electron microscopy (TEM) image of: (a) PAn nanotubes and (b) PAn nanofibers synthesised by chemical oxidative polymerisation in the presence of dodecylbenzene sulfonic acid (DBSA) as a bulky dopant acid.

3.3. Corrosion study

Fig. 3. FT-IR spectra of (a) PAn nanotubes and (b) PAn nanofibers by chemical oxidative polymerisation in the presence of dodecylbenzene sulfonic acid (DBSA) as a bulky dopant acid.

3.3.1. Open circuit potential studies The variation of open circuit potentials (OCP) with exposure time (30 days) for the steel coated with (a) PVB + PAn nanotubes, (b) PVB + PAn nanofibers and (c) PVB in 3.5% NaCl is shown in Fig. 4 (inset is the variation of OCP during 48 h exposure time). The coatings containing the PAn nanotubes and nanofibers are able to maintain higher noble potential values compared to the coated steel with PVB. As can be seen from Fig. 4, three main stages are observed in the time-course of PVB + PAn nanotubes. In the stage I, the OCP decreases from −0.018 V to −0.083 V after 9 days immersion time which can be attributed to the attack of the corrosive media to the metal substrate. The result shows that with the increase of reaction time the anodic protection of PAn nanotube can oxidize the surface of mild steel again and consequently the potential increases from −0.083 V to −0.025 V. The anodic protective mechanism of PAn is discussed later. In the stage II, the OCP variation of PVB + PAn nanotubes shows little changes between 17 and 27 days immersion times which can confirm the effect of PAn nanotube protection. In the stage III with the increase of immersion time to 30 days, the OCP decreases from −0.07 V to −0.15 V

304


Fig. 4. Plots of open circuit potential against exposure time (day) in 3.5% NaCl solution for steel coated with (1) PVB + PAn nanotubes and (2) PVB + PAn nanofibers (inset, plots of open circuit potential against exposure time (min) in 3.5% NaCl solution of steel coated with (a) PVB + PAn nanotubes, (b) PVB + PAn nanofibers and (c) PVB).

due to the breakdown of the passivity. The compression results of the OCP variation of PVB + PAn nanotubes and PVB + PAn nanofiber confirm that the time (day) required for the breakdown of the passivity in the coating containing the PAn nanotubes is more than PAn nanofiber. The breakdown of the passivity of the coating containing of PAn nanofiber occurs after 25 days immersion time while this time is longer for the coating containing PAn nanotube. In addition, the results show that the OCP of the coatings containing PAn nanotube and nanofiber reaches to −0.15 V and −0.35 V, respectively which can confirm the better performance of PAn nanotube in comparison with PAn nanofiber. Three possible mechanisms have been proposed for the corrosion protection of the conducting coatings in the literature; 1. The conducting polymer contributes to the formation of an electric field at a metal surface, thus restricting, electronically, the flow of electrons from metal to the oxidant [24]. 2. The conducting polymer forms a dense, adherent, low-porosity film, and maintains a basic environment on the metal substrate, thus restricting the access of oxidants and forcing the corrosion reaction in the direction of the unoxidised metal [25]. 3. The conducting polymer coatings force the formation of protective layers of metal oxides on a metal surface thus preventing corrosion [26,27]. Based on the third mechanism, we can make an interpretation about the comparison of the ability of PAn nanotube and nanofiber to prepare a passive layer on the surface of the steel. The OCP variation shows that the PAn nanotubes are able to passivate the steel better than the PAn nanofibers. This effect can be explained by the higher surface area of the nanotubes compared to nanofibers with the same mass. The increase of PAn surface area can directly improve the above factors and can increase the performance of the coating against corrosion. 3.3.2. Potentiodynamic polarisation Fig. 5 shows the potentiodynamic polarisation curves for steel with coated PVB, steel coated with the PAn nanotube–PVB (NTPAP), steel coated with the PAn nanofiber–PVB (NFPAP) and the PAn bulk powder–PVB samples in 3.5% NaCl solution after 15 days of immersion time. The corresponding corrosion potential (Ecorr ), corrosion current density (Icorr ), polarisation resistance (Rp ) and corrosion rate (CR) are listed in Table 1. The shift of the

Fig. 5. Polarisation curves of steel coated with (1) PVB, (2) PAn bulk (powder) + PVB, (3) PAn nanofibers + PVB (NFPAP) and (4) PAn nanotubes + PVB (NTPAP) after 15 days of immersion in 3.5% NaCl solution.

corrosion potential for steel coated with (NTPAP) and (NFPAP) to more positive regions compared to the steel coated with PVB and the PAn bulk powder–PVB shows the inhibition of the corrosion process by the (NTPAP) and (NFPAP) coatings. In contrast, the corrosion current decreases from 7.40 × 10−6 (steel coated with PVB) to 1.38 × 10−10 A cm−2 (steel coated with (NTPAP)) and 6.26 × 10−9 A cm−2 (steel coated with (NFPAP)). These results show that the corrosion rate of steel coated with (NTPAP) is lower than that of steel coated with (NFPAP). The corrosion results from potentiodynamic measurements indicate that the PAn nanofibers and nanotubes show better protection against steel corrosion than PAn bulk powder. These results confirm that the nanostructure and morphology of PAn are able to influence its anticorrosion performance. 3.3.3. EIS study EIS was used to study the corrosion protection performances of the coating containing the PAn nanotubes and nanofibers that was incorporated with PVB on steel panels and then immersed in 3.5% NaCl solution. Fig. 6 shows the Nyquist plots of steel coated with (NTPAP) (Fig. 6(1)), (NFPAP) (Fig. 6(2)) and PVB (inset) after 15 days of immersion time. The results show that the Zre of (NTPAP) is approximately double the Zre of (NFPAP) after 15 days of immersion time. This result clearly shows the effect of morphology on the protective properties of PAn for corrosion control; the PAn nanotubes have a better corrosion protection performance than the PAn nanofibers on mild steel. Fig. 7 shows the Bode plot and the phase angle frequency diagrams of coated steel with (NTPAP) (a1 and

Fig. 6. Nyquist plots of steel coated with (1) PAn nanotubes + PVB (NTPAP), (2) PAn nanofibers + PVB (NFPAP) and inset coated steel with PVB after 15 days of immersion in 3.5% NaCl solution.


305

Table 1 The Ecorr , Icorr , Rp and corrosion rate (CR) values of the steel coated with PAn nanotubes + PVB (NTPAP), PAn nanofibers + PVB (NFPAP), PAn bulk powder + PVB and PVB after 15 days of exposure time (t) in 3.5% NaCl solution. Electrode

t (days)

Ecorr (V)

bc (V/dec)

ba (V/dec)

Icorr (A cm−2 )

Rp (ohm cm2 )

Corrosion rate (mm/year)

Coated steel with PVB + PAn (nanotube) Coated steel with PVB + PAn (Nanofiber) Coated steel with PVB + PAn (bulk powder) Coated steel with PVB

15 15 15 15

−0.079 −0.252 −0.479 −0.752

0.232 0.268 0.214 0.225

0.186 0.280 0.344 0.249

1.38 × 10−10 6.26 × 10−9 2.58 × 10−8 7.40 × 10−6

1.36 × 108 5.20 × 106 4.872 × 106 3.28 × 103

1.60 × 10−6 7.27 × 10−5 8.74 × 10−5 8.59 × 10−2

a2, respectively) and (NFPAP) (b1 and b2, respectively) at different immersion times in a 3.5% NaCl solution. The results show that the log |Z| of steel coated with (NTPAP) is higher than (NFPAP) after 30 days of immersion time. These results also confirm that better corrosion protection is provided by the coating containing the PAn nanotubes than the PAn nanofibers as the immersion time is increased. In addition, the results show that the log |Z| decreases as the electrolyte penetration increases through the coating over longer immersion times. The phase angle variation of steel coated with (NTPAP) and (NFPAP) at 10 kHz during 30 days of immersion time is shown in Fig. 7(a2) inset and Fig. 7(b2) inset, respectively. The results show that the phase angle for steel coated with (NFPAP) and (NTPAP) decreases to −83.1◦ and −87.5◦ , respectively. These results confirm that the decrease of resistive behaviours of the (NFPAP) coating is more than that of (NTPAP) after 30 days of exposure. The EIS parameters of the (NTPAP)/electrolyte and (NFPAP)/electrolyte coatings were evaluated by employing the ZSimpWin software. An excellent agreement is obtained between the experimental results and the parameters for the R (RQ) equivalent circuit model, when the chi-squared (x2 ) is minimised at 10−4 (Fig. 7(a) and (b) inset). The R (RQ) equivalent circuit model was used to simulate the impedance behaviour of the coating/electrolyte from the experimentally obtained impedance data. The model was built using series components; the first component is the bulk solution resistance of the electrolyte Rs , the second is the parallel combination of the constant phase element (CPE) of the coating capacitance (Cc ), and the final component is Rc , which is the coating resistance. Instead of a pure capacitor, a constant phase element (CPE) was introduced into the fitting procedure to obtain a good agreement between the simulated and experimental data. The simulation data of steel coated with (NTPAP) and (NFPAP) are given in Tables 2 and 3, respectively. The impedance of CPE is defined as ZCPE = 1/Q (jω)n , where Q is the combination of properties related to both the surface and the electroactive species independent of frequency, “n” is related to a Table 2 Electrochemical parameters obtained by simulation of the EIS results of the steel coated with PAn nanotubes + PVB (NTPAP) as a function of immersion time in 3.5% NaCl solution.

Fig. 7. Bode plots and the equivalent electrical circuit used for the simulation of the EIS results of coated steel with (a) PAn nanotubes + PVB (NTPAP) and (b) PAn nanofibers + PVB (NFPAP) at different immersion times in 3.5% NaCl solution.

Time (days)

Rc (M cm2 )

Q/Yo (n−1 sn cm−2 )

n

Cc (nF cm−2 )

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 30

163.1 88.3 151.5 115.4 127.7 67.6 118.0 137.0 151.0 99.6 123.0 137.4 149.3 125.6 109.5 62.9

0.9635 1.1110 1.2090 1.8780 0.6250 1.0640 1.7140 0.7128 0.8151 0.6122 1.1340 0.6617 0.6591 0.9603 1.2060 2.5620

0.8849 0.9360 0.8682 0.8470 0.9368 0.9476 0.8420 0.9137 0.9030 0.9261 0.8731 0.9288 0.9205 0.8847 0.8826 0.8350

0.6481 0.8652 0.7914 1.2963 0.5027 0.8990 1.0313 0.5295 0.5967 0.4910 0.7541 0.5528 0.5104 0.6455 0.8495 1.6528

306


Table 3 Electrochemical parameters obtained by simulation of the EIS results of the steel coated with PAn nanofibers + PVB (NFPAP) as a function of immersion time in 3.5% NaCl solution. Time (days)

Rc (M cm2 )

Q/Yo (n−1 sn cm−2 )

n

Cc (nF cm−2 )

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 30

74.1 73.6 71.4 61.1 58.4 60.0 61.7 67.5 70.4 68.1 68.3 55.8 57.0 19.1 11.2 19.9

0.0921 0.0943 0.0927 0.8470 0.0940 0.1114 0.0984 0.0798 0.0797 0.0956 0.0990 0.1042 0.0890 0.1747 0.2327 0.1834

0.9469 0.9452 0.9469 0.9562 0.9467 0.9306 0.9412 0.9619 0.9612 0.9445 0.9417 0.9370 0.9523 0.8899 0.8643 0.8855

0.0624 0.0639 0.0635 0.0619 0.0904 0.0647 0.0647 0.0603 0.0599 0.0623 0.0653 0.0665 0.0627 0.1611 0.0733 0.0749

slope of the log |Z| vs. log f in the Bode plot, and ω is the angular frequency [28]. CPE is then given as both the parameter Q and the exponent “n”; it should be stressed that Q is often considered as capacitance for simplicity. In the case of a ‘classical’ depressed semicircle (CPE in parallel with resistance), Hsu and Mansfeld [28] gave the following equation for the conversion of Q into the ‘true’ capacitance, Cc : Cc = Q (ω

m)

n−1

(1)

is the angular frequency at where Cc is the coating capacitance, ωm which Z imaginary is at a maximum, and the units of Cc (farad or S-s) and Q (S-sn ) are different from one another. An estimation of the change in coating deterioration can be obtained by considering the variation of Rc and Cc as a function of immersion time. Fig. 8(a) and (b) shows the variation of Rc and Cc with the immersion time of steel coated with (NTPAP) and (NFPAP), respectively. There is an initial decrease in Rc up to the 3rd day of immersion for the steel coated with (NTPAP), and it increased until the 5th day of immersion. The decrease and minimum of Rc values have been associated with the formation and rupture of blisters, and the subsequent increase and maximum of Rc is due to the deposition of corrosion products in the blisters. The variation of Rc of the coated steel with (NFPAP) (Fig. 8(b)) shows that the magnitude of

Fig. 8. The variation of the polymer coating capacitance, Cc , and the coating resistance, Rc , of steel coated with (a) PAn nanotubes + PVB (NTPAP) and (b) PAn nanofibers + PVB (NFPAP) as a function of immersion time in 3.5% NaCl solution.

Table 4 The results of the adhesion test of steel coated with PAn nanotubes + PVB and PAn nanofibers + PVB. Sample

Test image

Classification and description

Coated steel with PVB + PAn (Nanotube)

5B: The edges of the cuts are completely smooth; none of the squares of the lattice is detached. Good and acceptable

Coated steel with PVB + PAn (Nanofiber)

3B: Small flakes of the coating are detached along edges and at intersection of cuts. The area affected is 5–15% of the lattice. Good and acceptable

Rc of this coating is generally smaller than that of (NTPAP) during immersion; the Rc of (NTPAP) is three times larger compared to the Rc of (NFPAP) at the end of 30 days of immersion (Table 4). The variation of Cc in Fig. 8(a) and (b) is another way to understand the coating behaviour. As the coating degrades, the coating resistance decreases with the increase in coating capacitance. The increase of the capacitance value is an indication of the increase in the uptake of water (electrolyte) [29]. Comparatively, there is agreement between the results of Rc and Cc in Fig. 8(a) and (b). The higher value of Rc is in accordance with the lower capacitance value. Notably, the value of Cc of (NTPAP) is larger than (NFPAP), whereas the value of Rc confirmed the coating quality of (NTPAP) and is better than (NFPAP). The reason for this phenomenon can be explained by the intrinsic nanotube capacitance properties [30]. Due to the Faradaic reactions, the energy density of a supercapacitor consisting of conducting polymers with several structures is expected to be higher than that of double-layer capacitors. Therefore, conducting polymers, such as PAn and its derivatives with a large degree of ␲-orbital conjugation and various oxidation structures have been considered as electroactive materials for the application of supercapacitors [31,32]. Recent reports confirm that PAn nanotubes show higher capacitance values and more symmetrical charge/discharge cycles due to their increased available surface area [33]. Fig. 9 shows the effect of a 3.5% NaCl solution on the surface of steel coated with (a) PVB, (b) PVB + PAn nanofibers and (c) PVB + PAn nanotubes before (up) and after 30 days of the corrosion test (down). The increase in the corrosion resistance of steel coated with PVB + PAn nanofiber and PVB + PAn nanotube coatings could be related to the effect of PAn and the lower permeability of the coatings against corrosive species when compared to the steel coated with only PVB. In addition, a comparison between all coating images shows that the effect of the NaCl solution and the number of pores on the surface of steel coated with PVB + PAn nanotubes is lower than other coatings at the end of 30 days of immersion. The potentiodynamic polarisation and EIS results show that the performance of (NTPAP) is better than (NFPAP). The increased surface area of the nanotubes compared to the nanofibers can be considered the primary reason for the performance improvement of the coating containing PAn. Other researchers have reported that the performance of an organic coating for corrosion protection can be described by several factors: barrier effects that prevent oxygen and moisture from reaching the metal substrate, reactions with the corrosive ions, and the formation of a passivation layer [34]. The


307

Fig. 9. The images of steel coated with (a) PVB, (b) PVB+ PAn nanofibers and (c) PVB + PAn nanotubes before the corrosion test (up) and after 30 days of immersion in 3.5% NaCl solution (down).

increased surface area can directly improve the above factors and increase the performance of the coating against corrosion. One of the special properties of electroactive polymers is their ability to interact with the ions liberated during the corrosion reaction of steel in the presence of NaCl [35]. Therefore, their performance increases with the increase of surface area. It is reasonable that the surface area of the PAn nanotubes is larger than that of the PAn nanofibers at the same mass quantity. Notably, from the TEM results, the diameter of the PAn nanotubes and the PAn nanofibers used in this work are within the same range. In addition, the hypothesis that the inner surface of the nanotubes may react with the ions liberated during the corrosion reaction of steel in the presence of NaCl, similarly to the outer surface, can be proposed but needs further investigation. It is well known that corrosion protection of conducting polymers is essentially a form of anodic protection [36]. Recent investigations have shown that a substrate metal coated with a charged conducting polymer will place the electrode potential in the passive range in the absence of any redox reaction [37]. However, the discharge of the conducting polymer film can be observed during a redox reaction, which is predictable even in the passive range. Thus, the potential will convert to a negative value. The length of time during which the potential is maintained in the passive region depends on the total charge stored in the polymer and the rate of the reaction. Therefore, for continued protection, the polymer film must be charged continuously. This charging is performed by the cathodic reduction of oxygen on the polymer [38,39]. Therefore, the increase of the surface area may increase the rate and probability for the cathodic reduction of oxygen on PAn. The PAn nanotube morphology can provide a higher surface area compared to that of the PAn nanofibers with the same mass quantity; therefore, under the same conditions, it is reasonable that the rate of oxygen reduction in the PAn nanotube coating is higher than that of the PAn nanofiber coating. The comparison of the variation of the Rc values of the coatings supports this hypothesis. The increase in adhesion of the electroactive polymer to the metal substrate can improve the protective properties. It is well known that a smaller diameter range of PAn increases its adherence on the steel surface [40]. Because the surface area of the PAn nanotubes is higher than that of the nanofibers in the same quantity, we conclude that PAn adherence on the steel surface is higher for the nanotube coating compared to the nanofiber coating. The results of the adhesion test are given in Table 3.

This result confirms that the adhesion of coating containing the PAn nanotubes is more than the PAn nanofibers. The increase of adhesion can be seen in the increase of the compact passive layer on the surface of the mild steel for anti-corrosion. The compact layer inhibits the cathodic reaction and leads to the negative shift of the Ecorr . In contrast, PAn adsorption on the steel surface occurs through ␲-electrons in the conjugated aromatic ring, and lone-pair electrons in the nitrogen atoms can decrease the anodic dissolution of iron by forming a coordination bond with the empty orbital of the iron [40]. The presence of the PAn nanotubes increases the available surface area; therefore, the amount of adsorption and the coordination bonding increase compared to the PAn nanofibers.

4. Conclusions Polyaniline nanotubes and nanofibers were synthesised by chemical oxidative polymerisation in the presence of dodecylbenzene sulfonic acid as a bulky dopant acid. The molar ratios of 1:1 and 4:1 of the monomer to DBSA were used for the synthesis of the PAn nanotubes and the PAn nanofibers, respectively. TEM and FESEM confirmed the narrow diameter range of 180–230 nm of the PAn nanotubes and nanofibers. From the potentiodynamic polarisation, EIS results, adhesion test and the higher available surface area of the PAn nanotubes compared to the PAn nanofibers, it can be concluded that the adhesion of PAn on the surface of the steel, the ability to interact with the ions liberated during the corrosion reaction of steel in the presence of NaCl solution, the rate of cathodic reduction of oxygen on the surface of PAn and the number of coordination bonds between the empty orbital of iron and the lone-pair of electrons on the nitrogen of PAn are improved. Therefore, the resistance of the PAn nanotube coating became three times higher than the PAn nanofiber coating after 30 days of immersion.

Acknowledgments The authors wish to thank Mojdeh Yeganeh for valuable discussion. This work has been supported by the University of Malaya Centre for Ionic Liquids (UMCiL) and Ministry of Higher Education, grant nos.: HIR-MOHE F000004-21001 and FP039-2010B.

308


References [1] A. Malinauskas, J. Malinauskiene, A. Ramanavicius, Nanotechnology 16 (2005) R51–R62. [2] S.Y. Park, M.S. Cho, H.J. Choi, Curr. Appl. Phys. 4 (2004) 581–583. [3] M.R. Mahmoudian, W.J. Basirun, Y. Alias, J. Coat. Technol. Res. 9 (2012) 79–86. [4] V. Karpagam, S. Sathiyanarayanan, G. Venkatachari, Curr. Appl. Phys. 8 (2008) 93–98. [5] S.R. Moraes, D.H. Vilca, A.J. Motheo, Prog. Org. Coat. 48 (2003) 28–33. [6] R. Vera, R. Schrebler, P. Cury, R. del Río, H. Romero, J. Appl. Electrochem. 37 (2007) 519–525. [7] X. Yang, B. Li, H. Wang, B. Hou, Prog. Org. Coat. 69 (2010) 267–271. [8] R.C. Patil, S. Radhakrishnan, Prog. Org. Coat. 57 (2006) 332–336. [9] H.-C. Zhao, Q.-P. Guo, B.-C. Wang, Y.-J. Guo, Corros. Protect. 31 (2010) 674–677. [10] G.M. Do Nascimento, P.Y.G. Kobata, M.L.A. Temperini, Phys. Chem. J. B 112 (2008) 11551–11557. [11] W. Li, H.-L. Wang, J. Am. Chem. Soc. 126 (2004) 2278–2279. [12] Y. Cao, T.E. Mallouk, Chem. Mater. 20 (2008) 5260–5265. [13] X. Zhang, S.K. Manohar, Chem. Commun. 10 (2004) 2360–2361. [14] R.K. Paul, C.K.S. Pillai, J. Appl. Polym. Sci. 80 (2001) 1354–1367. [15] L. Huang, Z. Wang, H. Wang, X. Cheng, A. Mitra, J. Mater. Chem. 12 (2002) 388–391. [16] Z. Niu, J. Liu, L.A. Lee, M.A. Bruckman, D. Zhao, G. Koley, Q. Wang, Nano Lett. 7 (2007) 3729–3733. [17] B.-Z. Hsieh, H.-Y. Chuang, L. Chao, Y.-J. Li, Y.-J. Huang, P.-H. Tseng, T.-H. Hsieh, K.-S. Ho, Polymer 49 (2008) 4218–4225. [18] J. Lu, B.J. Park, B. Kumar, M. Castro, H.J. Choi, J.-F. Feller, Nanotechnology 21 (2010), art. no. 255501. [19] J.R. Venison, Structural Steel Painting: The International Decorative Paints, Allen Devices and Co., Ltd., Bristol, England, 1973, pp. 5–6.

[20] [21] [22] [23] [24] [25]

[26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40]

D. Sazou, C. Georglios, J. Electroanal. Chem. 429 (1997) 81–93. Y. Cao, S. Li, Z. Xue, D. Guo, Synth. Met. 16 (1986) 305–315. M.G. Han, Y.J. Lee, S.W. Byun, S.S. Im, Synth. Met. 124 (2001) 337–343. X. Lu, H.Y. Ng, J. Xu, C. He, Synth. Met. 128 (2002) 167–178. F.C. Jain, J.J. Rosato, K.S. Laonia, V.S. Agarwala, Corrosion 42 (1986) 700–707. A.G. MacDiarmid, C.K. Chiang, A.F. Richter, N.L.D. Somasiri, A.J. Epstein, Conducting Polymers: Special Applications, 1st ed., D. Reidel Publishing Co., Dordrecht, 1987, pp. 131–132. B. Wessling, Adv. Mater. 6 (1994) 226–228. R.L. Eslenbaumer, W-K. Lu, B. Wessling, Synth. Met. 71 (1995) 2163–2166. C.H. Hsu, F. Mansfeld, Corrosion 57 (2001) 747. B.S. Skerry, C-T. Chen, C.J. Ray, J. Coat. Technol. 64 (1992) 77–86. G.A. Snook, P. Kao, A.S. Best, J. Power Sources 196 (2011) 1–12. A. Burke, J. Power Sources 91 (2000) 37. A. Laforgue, P. Simon, J.F. Fauvarque, J.F. Sarrau, P. Lailler, J. Electrochem. Soc. 148 (2001) A1130. Y.-Z. Long, M.-M. Li, C. Gu, M. Wan, J.-L. Duvail, Z. Liu, Z. Fan, Prog. Polym. Sci. 36 (2011) 1415–1442. F.N. Wicks Jone, S.P. Pappas, Organic Coatings: Science and Technology, Wiley, New Jersey, 2007. S. Radhakrishnan, C.R. Siju, D. Mahanta, S. Patil, G. Madras, Electrochim. Acta 54 (2009) 1249–1254. D. Sazou, M. Kourouzidou, E. Pavlidou, Electrochim. Acta 52 (2007) 4385–4397. H. Zhu, L. Zhong, S. Xiao, F. Gan, Electrochim. Acta 49 (2004) 5161–5166. Z. Deng, W.H. Smyrl, H.S. White, J. Electrochem. Soc. 136 (1989) 2152–2163. F. Bedioui, M. Voisin, J. Devynck, C. Bied-Charreton, J. Electroanal. Chem. 297 (1991) 257–269. H. Bhandari, R. Srivastav, V. Choudhary, S.K. Dhawan, Thin Solid Films 519 (2010) 1031–1039.