Polyorthotoluidine dispersed castor oil polyurethane anticorrosive nanocomposite coatings

Abstract

The present article reports the synthesis and corrosion protection performance of polyorthotoluidine (POT) nanoparticles dispersed castor oil polyurethane (COPU) nanocomposite coatings in acid and saline environments. The structure of POT/COPU was confirmed by FTIR and UV-visible spectroscopic techniques. The nanostructure of the pristine POT nanoparticles and those in the composite was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The physico-mechanical properties were analyzed using standard methods. The corrosion protective performance was evaluated by weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy in an acid medium. While in the saline environment the corrosion protective performance was investigated in a salt mist chamber using a 5% NaCl solution for 30 days. The morphological behavior of the corroded and un-corroded coated specimen was investigated by SEM studies. The stipulated acid and saline corrosion mechanism was also discussed.

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1. Introduction

For the last few years a growing interest has been developed in the application of conducting polymers and their nanostructures, because of their promising physico-mechanical, conducting magnetic, electrochromic and corrosion inhibition properties.^1–7 The nano-conducting polymers, due to a high surface volume ratio and unique morphology, exhibit an easy and improved dispersion behavior in a non conducting polymer matrix.^8,9 Hence, these polymers find wide applications in the fields of microelectronics, electroluminescent, electrochromic devices, sensors, corrosion inhibitors and coatings.^10–15 Nano-conducting polymers have been dispersed in various polymer matrices such as polyester, epoxy, polyurethane etc. and are used as corrosion protective composite coating materials.^16–18 Among the various conducting polymers, polypyrrole (PPy) and polyaniline (PANI) are considered to be the most promising ones used for corrosion protection.^{13–15,19,20} Wang et al.²¹ have developed PANI dispersed epoxy-polyamide coatings on mild steel. Chaudhari and Patil et al.²² reported a poly-orthotoluidiene (POT)–CdO nanocomposite coating for MS. Advincula et al.²³ synthesized a cross-linked poly(vinylcarbazole) (PVK)-conjugated polymer network (CPN) using the electro-polymerization technique on ITO glass. An investigation has been made on the corrosion protective performance of W doped PANI–vinyl coatings on mild steel by Sathiyanarayanan et al.¹³ However, major drawbacks associated with the conducting polymer coatings are their solubility and processing.²⁴ In order to improve these properties different derivatives of PANI such as N,N-dimethyl aniline and o- and m-toluidine have been used and their modifications with metals and metalloids as well as their copolymers and IPNs with other conducting polymers have been reported.^13–16,24

Most of the conducting polymer based composite coatings are formulated with the help of synthetic non conducting polymers, which are derived from petrochemical resources involving the use of organic solvents, which produce volatile organic components (VOCs). VOCs produce prominent health hazards and harmful effects to the society and environment. Thus it is important to overcome the problem of VOCs and to meet the requirements of various environmental acts implemented in this regard. The usage of some alternative coating materials has been proposed and developed via high solid coating materials, UV curable, waterborne and low molecular weight vegetable oil based polymeric coatings.^25–30 In context to these VOC free materials, vegetable oils may act as a potential alternatives. Vegetable oils such as linseed, pongamia, jatropa, soya, castor etc. have been employed for the synthesis of different types of polymeric coating materials such as oil based epoxy, alkyd, polyesteramide, PU etc.^31–34 Among these coating materials PUs exhibit promising physico-mechanical and corrosion resistant properties that provide PU coatings an opportunity to be utilized in the automobile industry. However, there is still scope of research for the development of high performance oil based PU coating materials, which may find applications in various advance industries like medical, construction, marine, appliances etc.¹⁸

Considering the promising corrosion protective performance of conducting polymers, here we report the synthesis and formulation of nanostructured POT, a derivative of PANI, dispersed castor oil polyurethane (COPU) nanocomposite coatings on mild steel, their physico-mechanical characterization and corrosion protective performance. The corrosion resistance measurements were carried out by weight loss measurements, potentiodynamic polarization, electrochemical impedance and salt spray tests. These studies revealed that the nano-composite coatings exhibit a superior corrosion protective performance as compared to those of other reported coatings.^35–37

2. Experimental

Materials

Castor oil (C₅₇H₁₀₄O₉, hydroxyl number: 159 mg KOH per g, mol. wt.: 932 g mol⁻¹), ethyl methyl ketone, (mol. wt: 72.11 g mol⁻¹), methanol and ammonium per sulphate (mol. wt. 228.18 g mol⁻¹) were obtained from Merck, India. 2,4-Toluylene diisocyanate (TDI) (mol. wt. 174.06 g mol⁻¹; Merck, Germany) was used as received. ortho-Toluidine monomer (Sigma, Aldrich, USA) was double distilled prior to use.

The synthesis of nanopolyorthotoluidine (POT)

POT nanoparticles were synthesized as per our earlier reported method.³⁸ The o-toluidine monomer (0.1 M) was dissolved in methyl alcohol (50 ml) at room temperature (30 °C). The oxidant solution was prepared by dissolving APS (10 g) in an aqueous HCl solution (100 ml, 1 M). Both solutions were precooled to 0–5 °C in an ice bath. The monomer solution was placed in a round bottom flask and inserted in an ice bath on a magnetic stirrer. An APS solution was then added to the o-toluidine solution dropwise over a period of 30 min with constant stirring. The mixture was stirred in an ice bath for 5 h to ensure complete mixing. A dark green precipitate of POT was obtained, which was filtered, repeatedly washed with distilled water to remove the excess of acid and impurities and dried under vacuum at 80 °C for 72 h. The TEM studies confirmed the nanosize of the POT particles.

The synthesis of castor oil polyurethane [COPU]

Castor oil polyurethane was prepared by a reported method.³⁹ Castor oil was reacted with TDI (NCO/OH ratio 1 : 2) in a three-necked round bottom flask under nitrogen gas purge. The reaction was carried out at room temperature (30 °C) with continuous stirring for 2 h. The polymer of COPU was isolated as a viscous liquid.

The preparation of the poly(o-toluidine) (POT) nanoparticles dispersed castor oil–polyurethane nanocomposite (POT/COPU)

The POT/COPU nanocomposites were formulated by mixing different amounts of POT (0.25 wt%, 0.5 wt%, 1.0 wt%) in a 10 wt% solution of COPU in ethyl methyl ketone. The POT was found to be homogeneously dispersed in COPU up to 1.0 wt%, beyond which, the POT failed to disperse in a COPU solution, which can be attributed to the low volume and higher surface area ratio of the POT nanoparticles, hence, the loading of POT beyond 1.0 wt% was not attempted. The POT dispersed COPU mixture was continuously stirred for a period of 4–5 hours at room temperature (30 °C) to ensure the uniform and stable dispersion of POT in COPU. The constituent mixture was then placed in a rotary evaporator to remove the solvent. The reaction scheme for the same is given in Fig. 1. The formation of the POT/COPU nanocomposites was confirmed by FT-IR, UV-visible spectroscopy and TEM analysis.

Fig. 1 The synthesis of COPU and the POT/COPU composite.

Characterization

Spectral analysis. The FTIR spectra of the polymer nanocomposite (POT/COPU) were determined on a Perkin-Elmer 1750 FTIR spectrophotometer (Perkin-Elmer Instruments, Norwalk, CT) with the help of an NaCl cell. The UV-visible spectra were taken on a Perkin-Elmer-LAMDA-ez-221 in the solution form.

Thermal analysis. The thermo-gravimetric analysis (TGA) of POT and POT/COPU was performed using the SII EXSTAR 6000 analyzer (Japan) from 40 °C to 800 °C in a nitrogen atmosphere at the rate of 20 °C min⁻¹.

Size and morphological analysis. The X-ray diffractograms were recorded on a Philips X-ray diffractometer model Philips W3710 using copper Kα radiation. Transmission electron micrographs (TEM) were taken on a Morgagni 268-D TEM, FEI, USA. The samples were prepared by placing an aqueous drop of POT and POT/COPU on a carbon-coated copper grid and subsequently drying in air before transferring it to the microscope operated at an accelerated voltage of 120 kV.

Physico-mechanical characterization. The specific gravity and refractive index of the COPU and POT/COPU nano-composite coating materials were determined using the ASTM D792 and ASTM D542 methods, respectively. The specular gloss at 60° (gloss meter, model RSPT-20; digital instrument Santa Barbara CA), the scratch hardness (BS 3900) and the impact resistance (IS: 101 par 5/sec-31988) were determined on COPU and POT/COPU coated 70 × 30 × 1 mm size mild steel strips. The five coated samples were tested and their mean average values were determined using error bars representing the standard deviation.

Electrical conductivity measurements. The conductivity of the POT/COPU films was measured by the standard four probe method using Keithley DMM 2001 and EG&G Princeton Applied Research potentiostat model 362 as the current source. For each composite, three specimens were taken and their mean average was reported.

Coating preparation on mild steel. The 70 wt% solution of COPU and the POT/COPU nanocomposites using different weight percentages of POT (i.e., 0.25POT/COPU, 0.5POT/COPU and 1.0POT/COPU, where the prefix indicates the percentage of POT) in ethyl methyl ketone, were applied by the brush technique, on finely polished and degreased mild steel strip (IS: 6240 HR: carbon 0.16%, manganese 0.30%, silicon 0.25%, sulphur 0.030%, phosphorous 0.030%, aluminium 0.02%, Fe 98.0%) of 70 mm × 30 mm × 1 mm and 25 mm × 25 mm × 1 mm sizes used for physico-mechanical and corrosion resistant tests, respectively. The coating thickness of COPU and POT/COPU were found in the range of 102–106 μm.

Corrosion resistance measurements.
Corrosion rate analysis by the weight loss method. The corrosion resistance performance of COPU and the POT/COPU coated MS with reference to uncoated MS were investigated in an acid (3.5 wt% HCl) environment using the ASTM G31 method. The rough edges and faces of the uncoated specimen have been grounded to a 120-grit finish on a water cooled polishing unit and marked for identification with a vibratory tool. After washing with 10% nitric acid for 10 minutes at 50 °C, the sample was lightly scrubbed with a slurry of Alconox cleaner using a soft bristle brush. Furthermore, the finely finished samples were rinsed with distilled water, methanol, acetone, and then dried. The length, width, and thickness of the dried samples were accurately measured. The test was performed by weighing samples before and after the immersion exposure in corrosion flasks. Each flask was equipped with an Allihn condenser to prevent evaporation. The test samples were suspended using a fluoropolymer filament within the flasks in such a way that they did not touch either the side wall of the vessel or each other. The total surface area of the test sample per flask was controlled so that the minimum recommended solution volume to surface area ratio (0.2 ml mm⁻²) was maintained. The flasks were kept in the static condition without agitation or aeration. The corrosion cells were visually monitored periodically during the immersion time.

The protective behavior of the coatings against the dissolution of the MS was evaluated by calculating the corrosion protection rate (CR) for each one of the samples. The calculation was performed using the following expression.³⁹

where Δg is the change in weight loss of the specimen, A is the area of the specimen, t is the time (year), and d is the density of the specimen. The results were given in milli-inches per year (MPY).

The potentiodynamic and electrochemical impedance spectroscopy (EIS) measurement. The corrosion resistance performance of the COPU and POT/COPU coated as well as the uncoated MS strips was evaluated by potentiodynamic and electrochemical impedance spectroscopy (EIS) measurements in HCl (3.5 wt%) at room temperature (25 °C) using micro Autolab type III with FRA unit (μ3AVT 70762, Netherlands) potentiostat for the period of 20 days. The Tafel polarization curves in 3.5 wt% HCl were obtained using a three-electrode electrochemical cell (EG&G Flat cell) containing platinum gauze as the counter electrode, Ag/AgCl as the reference electrode and the test specimen (coated and uncoated mild steel specimen) as the working electrodes. The test specimens were fitted in an electrochemical cell with 1 cm² area of the sample exposed to the corrosive media. The potentiodynamic tests were performed in the potential range of −0.4 V to 1.4 V at a sweep rate of 0.5 mV s⁻¹.

The potentiodynamic studies were made after exposing the specimen in electrolyte for 30 min. The percent inhibition efficiency IE (%) was calculated, using the following equation.⁴⁰

Theimpedance measurements were carried out at a frequency range of 100 kHz to 0.1 Hz with an AC amplitude of 10 mV after 30 minutes of immersion in the said corrosive environment up to 20 days.

3. Results and discussion

Spectroscopic analysis

FT-IR spectra of POT and POT/COPU. The FTIR spectra of the POT/COPU (Fig. 2) nanocomposite showed a significant negative shift of −15 and −16 cm⁻¹ in the –NH and –NHCO stretching bands, respectively, i.e. from 3356 cm⁻¹ of pristine POT to 3341 cm⁻¹ for the –NH stretching and for the –NHCO peak from 1744 cm⁻¹ to 1728 cm⁻¹.³⁹ The negative shift in the –NH and urethane of POT/COPU indicates that the urethane group of COPU interacts strongly with that of the –NH group of the POT.³⁹ The broadness of the NH peak is attributed to the presence of strong hydrogen bonding in between the –NH and –NHCO groups. The carbonyl of the POT/COPU absorption peaks in the nanocomposites of POT/COPU observed at 1728 cm⁻¹ and 1650 cm⁻¹ exhibit a shift of −16 and −15 cm⁻¹ as compared to that of the pristine COPU, which can be further correlated to the intense hydrogen bonding between the –NH of POT and the –NHCO of COPU (Fig. 1). The peak at 2253 can be attributed to the CN stretching vibration.

Fig. 2 FTIR spectra of POT/COPU.

UV-visible spectroscopy. The UV-visible spectra of POT and POT/COPU are given in Fig. 3. The absorption peak at 330 nm can be correlated to the π–π* transition of the benzene ring, which was related to the extent of the conjugation of the adjacent phenylene rings in the polymer chain and the charge transfer from the valence band to the conduction band.⁴⁵ The absorption peak in the visible region (550–700 nm), showed the excitation of the imine units present in the backbone of POT, induced donor–acceptor interactions between benzenoid–quinonoid rings leading to n–π* transitions.⁴¹ The broad absorption peak around 650–700 nm observed in pristine POT can be correlated to the presence of highly delocalized electrons. The spectra of the POT/COPU composites showed a blue shift of 100 nm in the polaronic transition peak, which was observed at 600 nm. The increase in the intensity of the polaronic transition peaks upon a higher loading of POT in COPU and the increase in the shifting of the polaronic transition peaks confirmed the restriction in the delocalization of the polarons induce in the POT chains, this can be attributed to the electrostatic interaction between the carbonyl group of COPU and the amide of POT. The physical interaction of the POT chains with the COPU segments hinders the path for the charge conduction, causing a significant decrease in the conductivity of the POT from 5.01 × 10⁻³ S cm⁻¹ to 5.7 × 10⁻⁴ S cm⁻¹.

Fig. 3 The UV-vis spectra of POT and different compositions of POT/COPU.

TEM analysis. The TEM micrograph of 1.0-POT/COPU, Fig. 4 exhibits the chain like dispersion of POT particles of average size 20–30 nm which led to the formation of a two phase system. The dark particles correspond to the POT nanoparticles while the bright phase can be correlated to the COPU matrix (Fig. 4). The POT nanoparticles within the COPU matrix appear to be globular and highly agglomerated upon increasing (more than 1.5%) the loading of POT.

Fig. 4 TEM micrograph of POT/COPU.

X-ray diffraction analysis. The X-ray diffractogram of COPU was reported in our earlier paper,³⁹ where as in case of POT (Fig. 5a) a pronounced peak was observed at 20 Å, corresponding to a full width at half maxima (FWHM) of 2.5, having an average particle size of 20 nm, calculated by the Scherrer equation.⁴² This peak confirm the semicrystalline behavior of POT. The significant impact of the nanostructured POT on the amorphous nature of the COPU matrix has been observed (Fig. 5b). The diffraction pattern in the POT/COPU nanocomposite showed the presence of a prominent peak (a hump) at 25 Å. The peak exhibited a positive blue shift of 5 Å, which can be corroborated to the encapsulation of POT within the amorphous COPU matrix. The encapsulation also led to a reasonable decrease in the peak of POT.^46,47 This peak further confirms the dispersion of the POT nanoparticles in the coating.

Fig. 5 X-ray diffraction patterns of POT (a) and POT/COPU (b).

Thermal analysis. The thermograms of POT, COPU and the POT/COPU nanocomposites revealed that the dispersion of POT in the COPU matrix increased the thermal stability (Fig. 6). The thermograms of POT, COPU and the POT/COPU nanocomposite showed a similar decomposition pattern (Fig. 6). The 10 wt% decomposition of pristine COPU was found at 115 °C and 20 wt% at 205 °C while the 50 wt% decomposition was observed at 325 °C. The pristine POT exhibits 10 wt%, 20 wt% and 50 wt% decompositions at 228 °C, 320 °C and 500 °C, respectively. For various compositions of the POT/COPU nanocomposites, the thermal decomposition temperatures for 10 wt%, 20 wt% and 50 wt% were found to be higher than that of pristine COPU but lower than that of pristine POT (Fig. 6). It was further observed that the thermal stability of the nanocomposites was increased with an increase loading of POT in COPU. The thermal stability study revealed the following trend: pristine POT > 1-POT/COPU > 0.5-POT/COPU > 0.25-POT/COPU > COPU. The increase in the thermal stability of the composites can be correlated to the hydrogen bonding between the C=O of COPU and the NH of POT (FTIR spectra Fig. 2) and the loading of POT in COPU.

Fig. 6 The TGA thermogram of POT, COPU and POT/COPU.

Physico-mechanical characterization. The values of the physico-mechanical properties for COPU and POT/COPU nanocomposites coatings (Table 1) revealed that the specific gravity of the POT/COPU nanocomposites increases with the increased loading of POT in COPU. The linear decrease in the refractive index and gloss values were observed with the increased concentration of POT in the nanocomposite (Table 1) which can be correlated to the opaque nature of the conducting polymer POT.³⁹

Table 1 The physico-chemical and physico-mechanical properties of the COPU, 0.25POT/COPU, 0.5POT/COPU and 1.0POT/COPU coatings

Resin code	COPU	0.25POT/COPU	0.5POT/COPU	1.0POT/COPU
Physico-chemical characterizations
Specific gravity (g ml^−1)	1.22	1.32	1.42	1.51
Inherent viscosity (dl dg^−1)	0.732	0.953	1.085	1.164
Refractive index	1.53	1.52	1.50	1.48
Physico-mechanical characterizations
Gloss at 45°	94	62	61	58
Scratch hardness (kg)	1.0	7.5	7.8	8.2
Impact resistance (lb per inch)	150	200	200	200
Bending (1/8 inch)	Pass	Pass	Pass	Pass
Drying time (min)	25	15	12	10
Coating thickness (μm)	104	104	105	104

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The nanocomposite coatings showed the decrease in the dry to touch and dry to hard times compared with that of the COPU coatings. However, with the increase in the loading of POT in COPU the dry to touch and dry to hard time was increased (Table 1). The scratch hardness value of the COPU coating was found to be 1 kg, while those of the POT/COPU (0.25% to 1.0% POT) nanocomposite coatings were found to be much higher (from 7.5 kg, to 8.2 kg). The increase in the scratch hardness values of the nanocomposites can be attributed to the synergistic effect of the POT/COPU nanocomposite, and the presence of a POT (conducting polymer) induced stiffness in the nanocomposite coatings, further enhanced the mechanical properties of these coatings. The impact resistance values in the case of the POT/COPU nanocomposite coatings (from 150 to 200 lbs per inch) were found higher than those of the COPU coatings (100 lbs per inch). As a characteristic of oil based polymeric coatings, all these coatings passed the 1/8′′ conical mandrel bend test.³⁹ The physico-mechanical properties of POT/COPU were found much higher than those of the other reported POT based coatings.^41,43

Conductivity measurement. The conductivity measurements were carried out at room temperature (30 °C). The value for the conductivity of POT (Fig. 7) was found to be 5.01 × 10⁻³ S cm⁻¹, while upon loading the 0.25 wt% of POT in COPU, the conductivity decreased by an order of magnitude (5.96 × 10⁻⁴ S cm⁻¹). On further increasing the loading of POT from 0.25 to 1.0 wt%, a slight increase in conductivity was observed (6.23 × 10⁻⁴ S cm⁻¹ for 0.5-POT/COPU and 6.89 × 10⁻⁴ S cm⁻¹ for 1.0POT/COPU). The conductivity data have not exhibited any well defined percolation threshold. Since, the transition from the insulating state of the COPU matrix to the conducting state of the nanocomposites was observed at ∼0.5 wt% loading, it can be concluded that the percolation limit may fall below the 1.0 wt% loading of POT.

Fig. 7 A plot showing the change in conductivity with the increased loading of POT in COPU.

Corrosion resistant measurement.
Corrosion rate analysis using weight loss measurement. The corrosion resistance performance of COPU and POT/COPU coated and uncoated mild steel strips (Fig. 8) was investigated for a period of 480 h in a 3.5 wt% HCl solution. After 480 h of immersion, the coatings of POT/COPU showed no visual deterioration or dissolution. The uncoated MS showed the dissolution of metal in HCl with a corrosion rate of 3.0 × 10⁻¹ mpy. The pristine COPU coatings started deteriorating after 48 h and were completely leached out within 96 h, which revealed a rapid deterioration with the corrosion rate of 9.0 × 10⁻² mpy. However, the corrosion rates for the 0.25, 0.5 and 1.0POT/COPU nanocomposite coated MS were found to be 5.2 × 10⁻³, 4.2 × 10⁻³ and 3.7 × 10⁻³ mpy, respectively. The high corrosion resistance performance of the 1.0POT/COPU nanocomposite coatings in the HCl medium can be attributed to the intimate and homogeneous dispersion of POT, which acted as an efficient barrier.²² The presence of POT further provided resistance to the permeability of corrosive ions at the coating–metal interface. The interaction between the NH of POT and the C=O of COPU led to the formation of the highly compact and well adhered coatings of the POT/COPU nanocomposites, which has effectively induced protection to the metallic surface. Mobin et al. have find out that poly(aniline-co-o-toluidine) and poly(pyrrole-co-o-toluidine) coated steel under 0.1 M HCl show corrosion rates of 5.02 and 4.11 mpy, respectively, which are much higher than that of the POT/COPU coatings.³⁷ The 2.0-PANI/COPU coating in HCl shows a corrosion rate of 0.38 mpy, which is also much higher than that of 1.0-POT/COPU (3.7 × 10⁻³ mpy). This further confirmed that the interaction of POT with COPU makes a strong adherence on mild steel which induces far superior corrosion resistance properties in the POT/COPU coatings.

Fig. 8 The corrosion rate of MS, COPU and POT/COPU in 3.5% HCl.

Potentiodynamic polarization measurement. The corrosion protective performance of the COPU and POT/COPU nanocomposite coated and uncoated MS was further evaluated using the potentiodynamic polarization technique in 3.5 wt% HCl at different intervals of time (Fig. 9). The values for the corrosion potential (E_corr), the corrosion current density (I_corr), the beta cathodic (βc), the beta anodic (βa) and the inhibition efficiency (%IE) of the COPU and POT/COPU nanocomposite coatings were obtained from their potentiodynamic polarization curves (Table 2). The E_corr for the COPU (−1.091 V) coated MS showed a positive shift as compared to that of the bare MS (−1.173 V). The positive shift in the E_corr confirmed that the COPU coatings led to the decrease in the anodic current of the corrosion reaction and offered resistance to the MS against corrosion.⁴³ The POT/COPU nanocomposite coatings showed a further increase in their E_corr values with the increased loading of POT in COPU (−0.902, −0.812 and −0.738 V for the 0.25POT/COPU, 0.5POT/COPU and 1.0POT/COPU coatings, respectively). The I_corr of the COPU coated MS was found to be lower than that of the uncoated MS, which further decreased with the increased loading of POT (Table 2). This observation from the above trend suggested that the coated MS controls both the cathodic and anodic reaction and thus provides protection to the MS substrate and the protection provided by the POT/COPU coating was more than that of the COPU coating. Among the different compositions of the POT/COPU coatings, 1.0POT/COPU showed the best corrosion resistant properties as exhibited by its highest value of E_corr (−0.738 V) and lowest value of I_corr (8.30 × 10⁻⁸ A cm⁻²) after 0.5 h. The inhibition efficiency of 1.0POT/COPU was 99.0% which was found to be higher than those of the 0.25POT/COPU and 0.5POT/COPU coated MS (97% and 98%), respectively. The higher corrosion resistant performance of the POT/COPU coating as compared to the COPU coating can be correlated to the fact that the conducting polymer acts as a barrier, which decrease the amount of water, oxygen, and corrosive ions to reach the metal–coating interface and form a protective passive layer on the stainless steel surface because of its redox catalytic properties.⁴⁴ The conducting polymer acts as a hydrophobic material, which also induces protection to the metal through the reflection of the wet corrosive species.⁴⁴ Therefore, as the amount of POT in COPU increases its corrosion resistant properties were also increased. It was further observed that as the exposure time increases a drop in E_corr (Table 2) and an increase in I_corr for both the COPU and POT/COPU coated MS was observed (Table 2). This observation suggested that the coating began to deteriorate on a prolonged immersion. The COPU coating completely leached out after 240 h, however, the POT/COPU coating provides protection to the MS for up to 480 h, although their protective efficiency decreased from 99% to 84% for 1.0POT/COPU. This can be attributed to the fact that with prolonged immersion corrosive ions accumulated at the coating–metal interface causing the deterioration of the coatings. Hence, the potentiodynamic studies revealed that the corrosion protective performance of these coatings was found to increase with the increased loading of POT in COPU, The highest corrosion protective performance was recorded in the case of the 1.0POT/COPU nanocomposite coating (Table 2).

Fig. 9 The Tafel plots of the uncoated, COPU and POT/COPU coated MS in 3.5% HCl.

Table 2 The potentidynamic polarization parameter for the uncoated, COPU and POT/COPU coated MS substrate in 3.5% HCl

Sample	β-Anodic (V dec^−1)	β-Cathodic (V dec^−1)	Ecorr (V)	Icorr (A cm^−2)	IE%
MS	0.081	0.085	−1.173	6.08 × 10^−5	—
COPU (0.5 h)	0.137	0.114	−1.091	5.14 × 10^−6	91.44
COPU (240 h)	0.139	0.161	−1.162	2.86 × 10^−5	53.59
0.25POT/COPU (0.5 h)	0.175	0.523	−0.902	9.90 × 10^−8	99.88
0.25POT/COPU (480 h)	0.807	0.877	−0.978	6.48 × 10^−6	89.34
0.50POT/COPU (0.5 h)	0.465	0.399	−0.812	2.92 × 10^−7	99.52
0.5POT/COPU (480 h)	0.585	0.387	−0.898	9.58 × 10^−7	96.84
1.0POT/COPU(0.5 h)	0.152	0.199	−0.738	8.30 × 10^−8	99.86
1.0POT/COPU(480 h)	0.167	0.265	−0.811	7.19 × 10^−8	99.76

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The corrosion protective performance of the POT/COPU coating is remarkably higher than those reported by Mobin et al.,³⁷ who found that the poly(aniline-co-o-toluidine) and poly(pyrrole-co-o-toluidine) coated steel after one month of immersion in 0.1 M HCl shows an inhibition efficiency only up to 78.41 and 78.45 mpy, respectively, which was much lower than that of 1.0-POT/COPU (99.76%). No other work had been reported on the corrosion resistance performance of POT in HCl by potentiodynamic polarization studies. However, Patil et al.³⁶ have electro polymerized POT on steel and found out that in 3.0% NaCl, the E_corr and I_corr values were 0.089 V and 5.11 × 10⁻⁸ A cm⁻², respectively, which were slightly lower than those of the POT/COPU coating (I_corr = 7.19 × 10⁻⁸ A cm⁻²) (Fig. 10).

Fig. 10 The E_corr versus time curve in 3.5% HCl.

Electrochemical impedance spectroscopy (EIS) measurement. Electrochemical impedance spectroscopy (EIS) was employed to investigate the corrosion characteristics of the uncoated, COPU and POT/COPU coated MS specimen, the Nyquist and bode plots for these systems are given in Fig. 11–13, respectively. Based on the impedance plots, an appropriate equivalent circuit was proposed (Fig. 14). The equivalent circuit consists of two resistance parameters; polarization resistance (R_p) and pore resistance (R_pore), two capacitances; one component (C_cl) in series with R_por, corresponding to one of the three resistance of the outer layer of coatings, and C_dl is the inner layer/interface capacitance of the coating. The circuit elements calculated from the fitting results of the COPU and POT/COPU coatings are given in Tables 3–6. The impedance values were reproducible ±2–3%. FRA Software was used for plotting, graphing and fitting the data.

Table 3 The electrochemical kinetic parameters derived from the EIS plots of COPU in 3.5% HCl

Kinetics parameters	Immersion period in hour
	24	120	240	360	480
Rp (Ω cm^2)	5.6 × 10^5	8.2 × 10^4	1.9 × 10^4	7.8 × 10^3	6.7 × 10^2
Rpore (Ω cm^2)	4.1 × 10^5	6.3 × 10^4	9.21 × 10^3	2.1 × 10^3	5.8 × 10^2
Cc (F cm^−2)	6.4 × 10^−7	9.3 × 10^−7	5.12 × 10^−6	4.7 × 10^−5	8.2 × 10^−5
Cdl (F cm^−2)	7.8 × 10^−7	8.5 × 10^−7	1.2 × 10^−6	5.7 × 10^−5	6.1 × 10^−5

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Table 4 The electrochemical kinetic parameters derived from the EIS plots of 0.25POT/COPU in 3.5% HCl

Kinetics parameters	Immersion period in hours
	24	120	240	360	480
Rp (Ω cm^2)	3.0 × 10^7	1.5 × 10^7	3.3 × 10^6	4.5 × 10^5	3.2 × 10^5
Rpore (Ω cm^2)	2.5 × 10^7	3.2 × 10^6	4.3 × 10^5	3.9 × 10^5	2.6 × 10^4
Cc (F cm^−2)	1.8 × 10^−9	4.0 × 10^−9	3.9 × 10^−8	4.5 × 10^−7	5.2 × 10^−7
Cdl (F cm^−2)	3.2 × 10^−9	3.5 × 10^−10	4.9 × 10^−9	3.5 × 10^−9	5.4 × 10^−9

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Table 5 The electrochemical kinetic parameters derived from the EIS plots of 0.5POT/COPU in 3.5% HCl

Kinetics parameters	Immersion period in hours
	24	120	240	360	480
Rp (Ω cm^2)	2.0 × 10^9	3.5 × 10^8	1.3 × 10^7	4.5 × 10^6	1.2 × 10^5
Rpore (Ω cm^2)	2.0 × 10^9	5.2 × 10^8	3.5 × 10^8	2.9 × 10^6	5.6 × 10^5
Cc (F cm^−2)	3.8 × 10^−9	4.5 × 10^−9	3.7 × 10^−8	2.5 × 10^−7	4.9 × 10^−7
Cdl (F cm^−2)	2.0 × 10^−9	2.7 × 10^−10	2.9 × 10^−9	3.2 × 10^−9	5.0 × 10^−9

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Kinetic parameters.
(a) Polarisation resistance (R_p). The R_p value of COPU after 24 h was found 5.6 × 10⁵ Ω cm², which was found to decrease with the increased immersion time and reaches 6.7 × 10² Ω cm² after 240 h. Such a low value of R_p indicated that the COPU coating deteriorated after this period of time. However, the R_p values for the POT/COPU coatings were found much higher than those of COPU (Tables 4–6), which further increased with the increase in the loading of POT in COPU from 0.25 to 1.0 wt% (Tables 4–6). Furthermore, as the immersion time increases the R_p values were found to decrease sluggishly (Tables 4–6). The 0.25-POT/COPU nanocomposite coating has a higher initial polarization resistance (R_p > 10⁷ Ω cm²) but after 24 h it began to drop with time and reached a value of 10⁵ Ω cm² after 480 h, below which the corrosion protection ability of the coating seemed to be lost.⁴⁵ The 1.0POT/COPU coating has a very high R_p value 3.0 × 10¹⁰ Ω cm² (24 h) and drops to 5.2 × 10⁷ Ω cm² (480 h). Such a high value of R_p even after 480 h of exposure to corrosive media shows that the 1.0POT/COPU coating composition provides maximum protection to the MS.

The Nyquist and Bode plots of the COPU and POT/COPU nanocomposite coated MS exposed to a 3.5 wt% HCl solution for periods of 24, 240 and 480 h are shown in Fig. 11–13. With the increased immersion time, the decrease in the diameter of the Nyquist plot was recorded (Fig. 11–14) this can be attributed to the decrease in the R_p value.

The Bode plot in Fig. 11 shows the impedance value of the COPU coated MS, which showed that the impedance value decreased sharply from 10⁵ to 10⁴ ohm after 120 h of exposure and reached 10² ohm after 240 h in a lower frequency range. In the medium frequency zone, a linear relationship between log|Z| vs. log f with a slope of 1.3 (value of log f) has been observed after 24 h and was found to increase up to 1.8 after 240 h exposure. In the higher frequency region the impedance value decreased in the manner of 10⁴, 10³ and 10² ohm (100 Hz frequency) after 24, 120 and 240 h, respectively. These values thus represent the failure of the COPU coating, i.e., the point at which a coating no longer provided corrosion protection. For the POT/COPU coated MS initially (after 24 h) the impedance was found in the range of 10¹⁰–10⁷ ohm in the lower frequency region, which was further dropped to 10⁷–10⁵ ohm on longer exposure, which is remarkably higher than that of the COPU coated MS. In the higher frequency region (100 Hz) also the impedance value of the 0.25POT/COPU coated MS was found in the range of 10⁶ to 10⁵ ohm, which was much higher than that of the COPU coated MS. The higher impedance value of the POT/COPU coated MS in a lower as well as in a higher frequency region would imply that these coating systems provided protection to the metal substrate. The higher impedance values depicted by the Bode and Nyquist plots for the 0.5POT/COPU and 1.0POT/COPU nanocomposite coatings (Fig. 12 and 13) for all immersion periods suggested that these coatings provided superior protection against acid corrosion (Tables 4–6).

Fig. 11 The Nyquist (a) and Bode plots (b) of the COPU coated MS after different intervals of time in a 3.5 wt% HCl solution.

Fig. 12 The Nyquist impedance plots as a function of exposure time obtained in a 3.5 wt% HCl solution for 0.25POT/COPU (a) 0.50 POT/COPU (b) and 1.0POT/COPU (c).

Fig. 13 The Bode plots of (a) 0.25POT/COPU (b) 0.5POT/COPU and (c) 1.0POT/COPU in a 3.5 wt% HCl solution after 24, 240 and 480 h.

Fig. 14 The equivalent circuit for coated and uncoated mild steel (MS).

(b) Pore resistance (R_pore). The initial R_pore value at a low frequency range for COPU was found to be 4.1 × 10⁵ Ω cm², which decreases with the increase in the immersion time and reaches 5.8 × 10² Ω cm² (after 240 h), which was reasonably low. Such a low value for R_pore suggested that the electrolyte starts to penetrate through the coating at the interface that leads to the deterioration of MS.⁴⁸

For the POT/COPU coatings the R_pore value increases with the increased loading of POT. The R_pore values after 24 h for the 0.25POT/COPU, 0.5POT/COPU and 1.0POT/COPU coatings were found to be 2.5 × 10⁷, 2.0 × 10⁹ and 2.8 × 10¹⁰ Ω cm², respectively. This increase in the R_pore can be attributed to the fact that with the increased loading of the conducting polymer the sealing of the pores in the coating occurs,²² which enhances the pore resistance properties of the POT/COPU coating. It was further observed that with time the R_pore value decreases for all the compositions of POT/COPU coatings, i.e. after 480 h the R_pore values for the 0.25POT/COPU, 0.5POT/COPU and 1.0POT/COPU were found to be 2.6 × 10⁴, 5.6 × 10⁵ and 4.6 × 10⁷ Ω cm², respectively. The decrease in the pore resistance with respect to the immersion period (Table 6) for all the compositions indicated that the pores were open and led to the penetration of the electrolyte.⁴⁹ However, the R_pore values for the 1.0POT/COPU coatings have the highest values both after 24 h and 480 h among all the compositions. This suggested that this composition of coating provides the maximum protection to the MS.

Table 6 The electrochemical kinetic parameters derived from the EIS plots of 1.0POT/COPU in 3.5% HCl

Kinetics parameters	Immersion period in hours
	24	120	240	360	480
Rpore (Ω cm^2)	3.0 × 10^10	2.5 × 10^10	2.8 × 10^9	1.5 × 10^8	5.2 × 10^7
Rp (Ω cm^2)	2.8 × 10^10	3.4 × 10^10	5.5 × 10^9	3.7 × 10^9	4.6 × 10^7
Cc (F cm^−2)	2.6 × 10^−10	3.5 × 10^−9	4.7 × 10^−9	2.8 × 10^−8	5.9 × 10^−7
Cdl (F cm^−2)	4.1 × 10^−10	5.7 × 10^−9	2.9 × 10^−9	2.2 × 10^−8	5.7 × 10^−8

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(c) Coating capacitance (C_c). The capacitance of a coating is used to measure the quantity of water absorbed at the coating–metal interface. Generally, in an aqueous environment, the coating capacitance (C_c) increases at the initial stage of exposure then after an incubation period it becomes constant. The EIS experiments showed that the COPU coatings have higher C_c values (6.4 × 10⁻⁷ and 8.2 × 10⁻⁵ F cm⁻² after 24 and 240 h, respectively) compared to those of all the compositions of the POT/COPU coatings. This can be correlated to the fact that the electrolyte easily penetrates through the pores present in the COPU coatings, while in case of the POT/COPU nanocomposites the presence of POT induces a locking effect, which did not allow the electrolyte to penetrate at the coating–metal interface.⁵⁵ The POT/COPU coatings show an increase in the C_c values with the increased exposure time (Tables 4–6), since with the prolonged immersion time the electrolyte penetrates at the coating–metal interface.^49,50
(d) Double layer coating capacitance (C_dl). The double layer capacitance is associated with an area exposed to the electrolyte (i.e. the delaminated area). Its formation occurs after the penetration of an electrolyte through the coating to the substrate.⁴⁹ The formation of a corrosion product at the coating–metal interface on the penetration of electrolytes inside the coatings reduces the area of the double layer capacitor, causing an increase in the C_dl value. The stable value for the C_dl of the coating was an indication for the formation of a stable interface.⁵¹ The presence of corrosion products at the metal–coating interface allowed more electrolytes to absorb through the coating, which led to an increase in the dielectric constant of the coating and can be attributed to the deterioration of the coating with longer immersion times. The higher C_dl values (7.8 × 10⁻⁷ F cm⁻²) for the COPU coating even after 24 h of exposure suggested that the electrolyte easily penetrates and reaches the coating–metal interface in the acidic environment. The 1.0POT/COPU coating exhibits the lowest C_dl values (4.1 × 10⁻¹⁰ and 5.7 × 10⁻⁸ F cm⁻² after 24 and 480 h, respectively) among all compositions of the POT/COPU coatings. This confirmed that the 1.0POT/COPU coatings have the highest stable coating–metal interface.⁵²

EIS also reveal that the corrosion resistant performance of POT/COPU is remarkably higher than that of other reported systems. Patil et al.^22,53 have developed POT/CdO and POT/ZrO₂ nanocomposite coatings on mild steel and found R_p values of 6.23 × 10³ Ω and 1.6 × 10³ Ω, respectively. Wei et al.⁵⁴ have developed a multi-walled carbon nanotube (MWCNT) dispersed polyurethane coating on steel and to find out its anticorrosive performance under 3.0 wt% NaCl, he observed R_p and R_ct values of 4.49 × 10³ and 6.74 × 10⁴ Ω, respectively. Wang et al.²¹ have reported the corrosion resistant performance of a PANI nanofibre containing Epoxy polyamide coating which showed R_c and R_ct values of 8.87 × 10⁶ and 2.12 × 10⁶ Ω, respectively. These values of the kinetic parameters are much lower than that of the POT/COPU coating which lies in the range of 10¹⁰ Ω. However no other work has been reported in an HCl environment.

The EIS studies revealed that the corrosion resistance performance of the coatings has increased with the increase loading of POT, which acts as a filler particle, sealed the pores on the surface of the coatings and protecting the MS from corrosion.

Salt spray test. The salt spray test (SST) on the COPU and POT/COPU coatings was conducted for a period of 720 h in a 5.0 wt% NaCl solution. The uncoated MS specimen was tested as a control. Initially, the coated and uncoated specimen have a glossy shiny surface, with the exception of a purple color on the POT/COPU coated specimen. After 48 h of the SST the uncoated specimen lost their glossy shiny appearance. Furthermore, the entire surface of the uncoated MS was covered with dark gray patches, causing significant corrosion damage (Fig. 15e). The COPU coated specimen showed a loss in gloss after 144 h, while the POT/COPU nanocomposite coated samples show a loss in gloss after 288 h, and no damage was observed on the coating surface even after 720 h (Fig. 15g). On the other hand, the COPU coated MS showed some signs of deterioration after 240 h (Fig. 15f). The SST clearly revealed that the presence of POT in the POT/COPU nanocomposite coatings provide a high corrosion protection efficiency in saline medium. The decrease in gloss for COPU and POT/COPU with time can be attributed to the deposition of the NaCl crystals at the surface of coatings, which led to the breaking of the bond of the coating material that causes the roughness and deterioration of the surface.

Fig. 15 SEM micrographs of (a) the POT/COPU nanocomposite (b) uncoated MS after 240 h in HCl (c) COPU coated MS after 240 h in HCl (d) 1.0-POT/COPU coated MS after 480 h in HCl (e) uncoated MS after 720 h in NaCl (f) COPU coated MS after 720 h in NaCl (g) 1.0-POT/COPU coated MS after 720 h in NaCl.

Morphological studies. The surface morphology of the uncorroded POT/COPU coated and corroded COPU, POT/COPU coated as well as uncoated MS in an HCl environment was investigated by SEM. The uncorroded POT/COPU nanocomposite coatings have a uniform and compact surface structure (Fig. 15a). The uniform dispersion of the POT nanoparticles in the COPU matrix was observed, which led to the formation of homogenous single phase coatings. The dense, uniform and continuous structure of the coating can be correlated to the higher corrosion protective performance of the POT/COPU coatings.

The SEM micrograph of the acid corroded uncoated MS specimen showed intergranular corrosion along with the formation of cracks and holes, which led to the fast dissolution of the MS (Fig. 15b). The COPU coated MS under HCl after a 240 h immersion test showed the formation of pits and holes causing the degradation of the coating (Fig. 15c). In the case of 1.0-POT/COPU coated MS, the presence of POT in COPU made the coating more adherent and the coating was remained undisrupted. However, the agglomerated POT particles along with some coating materials were uniformly precipitated out in the form of particles at the surface of the coating, which remained intact within the coating (Fig. 15d). The corroded uncoated MS specimen in NaCl (Fig. 15e) showed the formation of deep cracks and pits leading to the fast dissolution of the metal. The deposition of the NaCl crystal on the corroded MS surface was also visible, while in the case of the COPU coated specimen (Fig. 15f) the corrosion products were precipitated out at a few places in the form of patches. The 1.0-POT/COPU coating in an NaCl environment showed evidence of a slightly low rate of corrosion, which can be attributed to the presence of fine cracks in the coatings and the deposition of fine crystals of NaCl on the surface of the coating was also evident (Fig. 15g).

Corrosion protection mechanism. The conducting polymer based coating provides protection through a redox and barrier mechanism in different corrosive environments. Takenouti et al.⁵⁵ established the corrosion protection mechanism of conducting polymer based coatings, using PANI and PPy, by measuring the local potential and current, and impedance spectra around the defect area of the coating. They found that in tap water the conducting polymer layer is actually able to passivate a defect area. Lee et al.⁵⁶ investigated the corrosion protection ability of a polyaniline (PANI) coating for mild steel corrosion in saline and acid by electrochemical impedance spectroscopy. The impedance behaviour is best explained by a mediated redox reaction in which PANI passivates the metal surface and reoxidizes itself by dissolved oxygen. Patil et al.²² proposed a barrier mechanism for the POT/Cdo nanocomposite coatings in a 3.5% NaCl solution. However, the corrosion protection mechanism of the conducting polymer dispersed polyurethane coating is yet not reported. The POT/COPU coatings shows the formation of a compact iron/dopant complex layer at the metal–coating interface, which acts as a passive protective layer until the POT has the capability to undergo a continuous charge transfer (reduction reaction) at the metal–coating interface, where the POT is reduced from the emeraldine salt form (ES) to an emeraldine base (EB).⁴⁹ The polar group present in COPU like the carbonyl, amide and –NH groups present in POT developed a physical interaction with the Fe²⁺ ions present in the MS. These strong interactions adhered the coating onto the metal substrate. Furthermore, the POT particles act as a nanofiller, which seals the pores on the coating surface producing a locking effect that provides further strength to the coating. In addition to this the POT nanoparticles introduce roughness at the nanoscale, creating air pockets within the COPU matrix, which induce the hydrophobicity and reduces the surface wet-ability of the coating, resulting in the remarkably higher corrosion resistant performance.⁵⁷ The presence of the –CH₃ group in POT provides flexibility and also induces stiffness to the coating material, which was evident from the high scratch hardness value (7.5 to 8.2 kg) as compared to earlier reported MO-PANI/COPU coatings (2.5–4.0 kg).³⁹ However, the accumulation of excessive corrosive ions led to a breakdown of the passive layer which resulted in the deterioration of the coatings. This type of corrosion protection usually depends on the strength of the passive oxide film and the interaction of the –NH of POT and the C=O group of COPU with the iron substrate, which impedes the penetration of the corrosive ions.

4. Conclusion

The POT/COPU nanocomposite was prepared by a solution blending technique. The minimal dispersion (0.25–1.0 wt%) of POT in COPU significantly enhances the thermal stability, physico-mechanical properties and corrosion resistance performance of these coatings. The potentiodynamic and EIS measurements further showed that the POT/COPU coated MS effectively provide protection through a barrier mechanism against acid and salt medium to the mild steel. The salt spray test also revealed the similar behavior of coatings to that of the acid environment. The POT/COPU coatings have shown a far superior corrosion protective performance as compared to that of the COPU coatings in acid and saline environments.

Acknowledgements

The author authors are also thankful to the Naval Research Board (NRB) for providing the financial assistance, vide sanction no DNRD/05/4003/155 DATED 03/10/2008. Dr Mohammad Kashif is also thankful to CSIR (New Delhi, India) for financial support through Research Associateship [RA] against Grant no. 05/466 (0160)/2K13-EMR-I.

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