Conducting poly(o-anisidine) nanofibre dispersed epoxy-siloxane composite coatings: synthesis, characterization and corrosion protective performance

Abstract

Conducting polymers (CPs) exhibited a promising ability of corrosion inhibition and find application in the formulation of new generation smart anti-corrosive coating materials. CPs not only act as an active barrier for corrosive ions but also provide protection to the metal substrate through the redox mechanism. In view of this, the present article reports the synthesis, structural, physico-chemical, physico-mechanical characterization and corrosion protective performance of tartaric acid (TA) and functionalized protonic acid-dodecyl benzenesulphonic acid (DBSA) doped poly(o-anisidine) nanoparticles (TA–DBSA-POA) and these nanoparticles dispersed epoxy-siloxane (ES) nanocomposite coatings (TA–DBSA-POA–ES) on carbon steel (CS). The structure, size and morphology of TA–DBSA-POA nanoparticles and those of coatings were investigated by FT-IR, XRD, TEM and SEM analysis. The conductivity of TA–DBSA-POA nanoparticles (2.09 S cm⁻¹), and TA–DBSA-POA–ES nanocomposites (5.02 × 10⁻³ S cm⁻¹) as well as their thermal stability were investigated with the help of the four-probe method and the TGA technique. The physico-mechanical properties of these coatings were evaluated using standard laboratory methods. To the best of our knowledge, the corrosion protective performance of these coatings was investigated for the first time in our laboratory, using the salt spray test (SST), potentiodynamic polarization (PDP), and electron impedance spectroscopy (EIS) techniques at varying concentrations of NaCl viz. 3.5 wt%, 5.0 wt% and 7.0 wt% NaCl. SEM-EDAX and Raman studies revealed the presence of a passive ferric oxide layer. These studies revealed that nanocomposite coatings show far superior thermal stability, physico-mechanical and corrosion protective performance than plain ES and other such CP reinforced epoxy coating systems.

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1. Introduction

Corrosion poses a serious economic and industrial threat as well as latent danger to humans. Furthermore, it is important to mention that corrosion cannot be prevented but can be controlled through different strategies, which retards the corrosion kinetics by altering its mechanism using cathodic protection, protective coatings and corrosion inhibitors or combination thereof.^1–4 In recent years nano-conducting⁵ polymers (Cps) have not only been used as corrosion inhibitors but also as fillers in doped and undoped states⁶ in polymer nanocomposite coatings. The nano-conducting polymer composite coatings exhibit very promising physico-mechanical and thermal resistant anti-corrosive properties, which has helped in the processing of new generation smart coating materials. These coatings sometimes also act as good corrosion sensors for inhibition.⁷

Generally, chromium passivation techniques were used for the pre-treatment of the metal surface to enhance the adhesion between the polymer coatings and the metal surface, which can be correlated with its strong oxidative nature in the hexavalent state that provides extra corrosion inhibition ability to coatings. However, because of highly toxic nature, hexachromate has been banned worldwide by the act of environmental legislation.⁸ Thus, there is an urgent need to develop some green alternative surface pre-treatment technique to replace the present carcinogenic hexavalent passivation technique. The literature revealed that in the field of corrosion protection CPs act as green^9,10 primers, e.g. PANI and its various derivatives like poly(o-anisidine) could act as promising alternatives to chromate passivation techniques owing to their ease of synthesis, availability, cost effectiveness and stability.^11,12 The doping of CPs with organic acids reasonably improved properties like stability,¹³ solubility and conductivity.^5,14,15 The literature also reports that doped POA nanoparticles exhibit enhanced mechanical stability with low percolation threshold and high conductivity as compared to undoped conventional POA. PANI doped para-toluenesulphonic acid-DGEBA nanocomposites showed good thermomechanical and dielectric properties.¹⁶ Similarly, tartaric acid doped PANI exhibited enhanced spectral, thermal, optical and conducting properties.¹⁷ Interestingly, in addition to this a number of studies on PANI and its derivatives with reference to their corrosion inhibition activity have also been reported.^{1,2,8,11,18–25} However, there is no report available on the effect of two coupled dopants i.e. organic surfactant-acid dopants on synthesis, physico-mechanical and anti-corrosion protection properties of poly(o-anisidine) nanoparticle dispersed epoxy-siloxane nanocomposite coatings.

Among the conventional polymers, epoxy resins find extensive industrial applications due to their low shrinkage, ease of cure, processing, higher adhesion strength, excellent moisture, solvent and chemical resistance.^2,20,26–37 However, the shortcomings of epoxy such as low fracture energy, low pigment holding ability, poor hydrophobicity weathering resistance, discolouration upon UV exposure and poor impact strength prevail, which limit their application as a topcoat material in the field of paints and coatings. The use of modifiers, as second component in epoxy has helped in combating these shortcomings. Silicone in epoxy is considered to be the best among various modifiers like rubber, clay, fibres etc. In order to improve further, the addition of hydroxyl terminated poly(dimethylsiloxane) (HPDMS) in the epoxy-siloxane blend further improved the properties as per our reported earlier method.²⁷ Secondly, it has been reported that the dispersion of nanoCP fillers^10,14,38 in the epoxy matrix lead to the formation of nanocomposites which have tailored properties and improve all the above cited shortcomings of a neat epoxy matrix.

In view of this, the present article reports the synthesis of bi-dopant (TA and DBSA) doped POA nanoparticles and their dispersed epoxy siloxane (ES) nanocomposite coatings, and spectral, physic-mechanical and electrochemical corrosion characterization were also reported. To the best of our knowledge these studies have not been reported to date. The corrosion resistance performances of these coatings were investigated by potentiodynamic, electrochemical impedance spectroscopy and salt spray techniques. These studies revealed that nanocomposite coatings showed superior corrosion protective performance as compared to plain ES and other conducting nanocomposite coatings.^{11,20,24,39,40}

2. Experimental

2.1 Materials

The ortho-anisidine monomer (MW 123.16 g mol⁻¹ and density 1.0923 g cm⁻³) (Merck, Germany) was distilled twice under reduced pressure and was stored in a refrigerator prior to use, epoxy (DGEBA, D.E.R 332, epoxy equivalent 180–185, sp. gr. 1.2306, refractive index 1.5685, viscosity about 10 000 CP) DOW Chemicals, dodecylbenzenesulphonic acid (DBSA), phosphoric acid, ammonium peroxydisulphate (APS) (S.D fine chem., India), tartaric acid (MW 150.087, density 1.7 g ml⁻¹), ethyl methyl ketone, hydroxyl-terminated poly(dimethyl siloxane) (HPDMS), (viscosity 90–150 cSt, refractive index 1.4057, density 0.970 g cm⁻³ at 25 °C) and hydrochloric acid (HCl) (Sigma-Aldrich, USA) were used as received.

2.2 Synthesis of TA–DBSA-POA nanoparticles

TA–DBSA doped POA nanoparticles were synthesized using an in situ emulsion polymerization technique. The freshly double distilled o-anisidine monomer (1.0 g) and tartaric acid (0.2 g) were injected to the reaction flask having DBSA (0.5 g) in 100 ml of 0.1 M HCl solution under constant stirring. The APS (as initiator) solution was prepared in 10 ml of 0.1 M HCl and added dropwise to the reaction mixture at room temperature. The total volume of the solution was maintained upto 110 ml. The reaction mixture was continuously stirred at 20–25 °C (as the Kraft temperature of DBSA is known around 15 °C) on a mechanical stirrer for 14 h. The brownish colour residue of tartaric acid doped POA was filtered and washed several times with methanol, and dried in a vacuum oven at 50 °C for 2 days resulting in the formation of brown colour precipitates as per Fig. 1. The yield of TA–DBSA-POA nanoparticles was found to be 65%, comparable with those of earlier studies of POA synthesis. The successful formation of TA–DBSA-POA nanoparticles was confirmed by FT-IR analysis.

Fig. 1 Scheme for the formulation of TA–DBSA-POA–ES nanocomposite where A represents dopants.

2.3 Synthesis of TA–DBSA-POA–ES nanocomposite coating formulations

ES was synthesized as per our earlier reported method.²⁷ DGEBA resin was pre activated at 120 °C for 30 min. The 4 : 1 ratio of pre-activated DGEBA and HPDMS was taken in a three necked flat bottom flask fitted with a magnetic stirrer, a thermometer and a nitrogen inlet. 0.02 g of phosphoric acid that acts as a catalyst was added into a reaction mixture. The reaction mixture was continuously stirred on a magnetic stirrer at 80 °C, for 45 min. The reaction between DGEBA and HPDMS was carried out by the SN² mechanism and thus resulted in the formation of a highly crosslinked ES network structure.

Then, different wt% of synthesised TA–DBSA-POA nanoparticles (0.5, 1 and 1.5% by weight) were dispersed in 10.0 g of the ES matrix through sonication, using an ultrasonic wave sonicator at 30 °C for a period of 30 min followed by mechanical stirring for 5 h at room temperature, resulting in the formation of homogeneously dispersed solution of TA–DBSA-POA–ES nanocomposites. The nanocomposite solution was kept under observation for fortnight to see whether any separation occurred or not. However, no separation was observed. TA–DBSA-POA nanoparticle loading in ES showed a uniform dispersion of up to 1.5 wt% however, beyond this loading agglomeration and phase separation appeared. The coatings of nanoparticle dispersed TA–DBSA-POA solutions were prepared and applied by the brush technique on the finally polished and degreased surface of CS specimen of standard sizes. The coatings get cured within 30 min at room temperature and are formed with an average thickness of 120 μm.

2.4 Characterization

Fourier transform infrared (FT-IR) spectra of TA–DBSA-POA, ES matrix and TA–DBSA-POA–ES nanocomposites were taken on a PerkinElmer 1750 FT-IR spectrophotometer (PerkinElmer Instruments, Norwalk, CT) in the form of KBr pellets or with the help of a NaCl cell. The particle sizes and its surface morphology analysis were demonstrated with the help of transmission electron microscopy (TEM) model Morgagni 268-D TEM, FEI, USA operated at an accelerated voltage of 120 kV and scanning electron microscopy on Scanning Electron Microscope model FEI Quanta 200F with Oxford-EDS system IE 250 X Max 80. The crystalline nature and phase purity were demonstrated on a Philips X-ray diffractometer model Philips W3710 using CuKα radiation. Thermo-gravimetric analysis (TGA) was performed using the SII EXSTAR 6000 thermal analyzer (Japan) in the range 40 °C to 800 °C in a nitrogen gas environment at a flow rate of 10 °C min⁻¹. The specific gravity, epoxy equivalent, inherent viscosity and refractive index of these composite solutions were measured with the help of standard ASTM methods. The coatings of ES and TA–DBSA-POA–ES composites were prepared with the help of the brush technique on the surface of 70 × 30 × 1 mm size CS strips for the preparation of coatings for the determination of their specular gloss at 45° using a gloss meter (model RSPT-20, digital instrument Santa Barbara, CA), scratch hardness (BS 3900), bend test on 1/8” inch conical mandrel (ASTM D 3281-04) and impact resistance (IS: 101 par 5/sec-31988). For each test, five samples were used and their average values were determined using error bars representing standard deviation. Conductivity was measured by the standard four probe method using a Keithley DMM 2001 and EG and G Princeton Applied Research potentiostat model 362 as a current source. For each composite, three specimens were used for the measurement of conductivity. The mean values of conductivity were taken. Corrosion resistance performance of ES and TA–DBSA-POA–ES coated CS strip specimens was evaluated by potentiodynamic polarization in different wt% of NaCl (3.5, 5.0 and 7.0 wt%) at room temperature (30 °C) using micro Autolab type III with the FRA unit (μ3AVT 70762, Netherlands) potentiostat. The Tafel plots in the presence of these corrosive media were obtained using a three-electrode electrochemical cell (EG&G 362) containing platinum gauze as a counter electrode, Ag/AgCl as reference and test specimens (coated and uncoated mild steel specimens) as working electrodes. The 1.0 cm² area of the working electrode was exposed to the solution. Prior to potentiodynamic polarization and the EIS test, the working electrode was allowed to stabilize for 20 min and then its open circuit potential (OCP) was recorded as a function of time for 600 s. After OCP stabilization, impedance measurements were performed at respective corrosion potentials (E_corr) over a frequency range of 100 kH–0.1 Hz, with a signal amplitude perturbation of 10 mV. The potentiodynamic polarization tests were carried out in the potential range ±100 mV (with respect to OCP) at a 0.001 mV s⁻¹ scan rate. Nova 1.8 software was used for data fitting and calculation of the results obtained. The impedance and Tafel parameters were determined by curve fitting programme available in the above mentioned software. Each test was run in triplicate to verify the reproducibility of the data.

3. Results and discussion

Tartaric acid and dodecylbenzene sulphonic acid doped poly(o-anisidine) nanoparticles were synthesized using an in situ emulsion polymerization technique. The epoxy-siloxane (ES) matrix was synthesized by the reaction between DGEBA and HPDMS by the SN² mechanism and thus resulted in the formation of a highly crosslinked ES network structure. Then, different wt% of synthesised TA–DBSA-POA nanoparticles (0.5, 1 and 1.5% by weight) were dispersed in 10.0 g of ES matrix through sonication followed by mechanical stirring. These nanoparticles and nanocomposites were characterized by different techniques for the determination of their structure, size, morphology, composition, conductivity etc. The coatings of nanoparticle dispersed TA–DBSA-POA solutions were prepared and applied by the brush technique on the finally polished and degreased surface of the CS specimen of standard sizes. These nanocomposite coatings were further analysed for their anti-corrosive performance by various physico-mechanical, potentiodynamic polarization and electrochemical impedance spectroscopic techniques at varying concentration of NaCl (3.5 wt%, 5 wt% and 7 wt%) media.

3.1 FT-IR analysis

The FT-IR spectrum of pristine TA–DBSA-POA (Fig. 2) showed the presence of C=N (ν_str) and C=C (ν_str) stretching modes for the quinonoid (Q) and benzenoid (B) rings at 1595 cm⁻¹ and 1505 cm⁻¹ respectively. The peak at 1256 cm⁻¹ showed the C–O aromatic peak of the benzene ring and C–H bending vibration during protonation was confirmed by the peak at 1112 cm⁻¹ and the C–O–C ether peak was observed at 1115 cm⁻¹. The band at 750 cm⁻¹ revealed the presence of 1, 2 substitutions of the NH₂ group at the 1st position and the OCH₃ group at the 2nd position respectively. The peak at 650 cm⁻¹ is a characteristic peak of DBSA, which confirms doping with DBSA.

Fig. 2 FT-IR of (a) TA–DBSA-POA (b) ES matrix (c) 1.0% TA–DBSA-POA–ES nanocomposite.

The FT-IR spectrum of ES (Fig. 2) showed a characteristic band at 3449 cm⁻¹ associated with the broad OH stretching of hydroxyl groups, which confirmed the condensation reaction between epoxy and siloxane²⁷ corresponding to CH₂ asymmetric stretching (v_assym), and the peak at 1670 cm⁻¹ is attributed to the aromatic stretching vibration (v_ar). The peaks appearing in the range of 1296–1184 cm⁻¹ were correlated with the C–O–C of aryl alkyl ether, while the peak at 1095 cm⁻¹ was attributed to the aryl alkyl ether symmetric stretching (sym). The peaks corresponding to the oxirane ring appeared at 910–830 cm⁻¹ and those corresponding to Si–O vibration occurred at 660 cm⁻¹ and 575 cm⁻¹.

The spectrum of 1.0% TA–DBSA-POA–ES (Fig. 2) showed the peak corresponding to OH stretching vibration at 3460 cm⁻¹ with a broader wavelength, which gets shifted by 11 cm⁻¹ as compared to the ES matrix. The peaks at 1390 cm⁻¹, 1438 cm⁻¹ and 1508 cm⁻¹ correspond to the bezenoid and quinonoid stretching modes of POA. The interaction between PANI chains and DBSA was confirmed by the peak appearing at 1028 cm⁻¹. The benzenoid and quinoid vibration peaks were very intense and pronounced indicating strong interaction between TA–DBSA-POA and ES, which was observed even at a lower loading of the TA–DBSA-POA filler. These bands suggested a conjugated pi-bond system, which may be attributed to the doped state of TA–DBSA-POA. Doping improves the conductivity levels forming polaron/bipolaron structures resulting in the increase of charge transfer in TA–DBSA-POA and higher electronic delocalization. The presence of conjugated double bands in the benzenoid and quinonoid rings permits electron mobility throughout the TA–DBSA-POA chains responsible for more electron delocalization, which resulted in the conducting nature of nanocomposites. The NH stretching vibration band of TA–DBSA-POA overlaps the OH stretching vibration, which reveales an electrostatic interaction of the NH group of TA–DBSA-POA and the OH group of ES via strong hydrogen bonding (Fig. 3). All other characteristic peaks of ES did not exhibit any major shift. Hence, the formation of TA–DBSA-POA–ES as nanocomposites was confirmed.

Fig. 3 Structure of TA–DBSA-POA–ES showing the interaction between TA–DBSA-POA and ES where A⁻ shows doping with TA and DBSA.

3.2 Mechanism of synthesis of ES and TA–DBSA-POA based TA–DBSA-POA–ES nanocomposites

In emulsion polymerization of a conducting polymer, solubilisation locus, reactant monomers and their position in the micelles significantly affect the reaction kinetics, selectivity, yield of the product and the site for the incorporation of solubilizates in micelles. Kim et al. explained that in the case of non-polar, highly hydrophobic reactants the solubilizates are located in the hydrocarbon core of the micelles.⁴¹ It has been well accepted that the polar or surface active molecules are solubilized at the micelle–water interface. ortho-Anisidine exists in the form of the anisidinium cation in acidic aqueous solution. Furthermore, Kim et al. have reported that in the synthesis of polyaniline nanoparticle, anilinium cations are adsorbed on the micellar surface by electrostatic interaction with anionic SDS molecules being fully exposed to the aqueous phase.⁴¹ Similarly, anisidinium cations in the o-anisidine monomer, these cations are adsorbed on the micellar surface by electrostatic interaction with anionic DBSA molecules being fully exposed to the aqueous phase as in the case of PANI. However there are some portions of Na⁺ as counter-ions would have interacted with the micelles, followed by the reduction of the charge density of micelles. Thus, referring to the literature, we assumed that most of the ortho-anisidine monomers were solubilized at the micelle–water interface. Obviously, some of them adsorb on the micellar surface and some in the aqueous phase. The solubilized o-anisidine or o-anisidinium molecules were oxidatively polymerized by APS present in the aqueous phase. Mainly, the reaction occurs at the micelle–water interface adjacent to the surfactant head groups, as hydrated APS molecules could not penetrate into the micellar surface, as micelles⁴¹ are not static and rigid entities. Probably they are in a dynamic equilibrium state with surfactant monomers in the solution and were dissociated into monomers, which were continuously associated into micelles. Thus all segments of the surfactant and solubilizates could be exposed to water. As doping is the process by which polymers that are insulators are exposed to the charge transfer agent. Here, in our work DBSA and TA were used as a doping agent which provided electrons during the mechanism of formation of POA nanoparticles. TEM studies confirmed that the presence of TA on the periphery of POA nanoparticles (Fig. 5c). Finally, o-anisidine molecules present in the aqueous phase would be incorporated readily into the micelles and grow into dimer, trimer or tetramer due to their increased hydrophobicity.

After the completion of reaction, synthesized TA–DBSA-POA nanoparticles are stabilized by adsorbed and incorporated DBSA molecules by electrostatic repulsive interactions. The mechanism of formation of the TA–DBSA-POA–ES nanocomposite using the solution blending method is purely based on electrostatic interactions between the nanoconducting polymer (filler) and the matrix. The increase in viscosity with the increase in loading of TA–DBSA-POA loading was due to the presence of Si–O–Si bonds between DGEBA backbones in a heterogeneous manner that resisted the terminal epoxy group with the amine bond of TA–DBSA-POA, which can be attributed to their ionic character. However, the presence of bulky methoxy pendant groups further prevented the curing reaction between the NH groups of TA–DBSA-POA with the OH group of the ES matrix. Hence, TA–DBSA-POA acted as a reinforcing part by forming a network like structure through intermolecular hydrogen bonding between the OH group of ES and the NH group of TA–DBSA-POA establishes the physical nature of interaction rather chemical bonding (Fig. 3).

3.3 X-Ray diffraction analysis

X-ray diffraction was used to explore the crystalline structure and phase identification of nanocomposites. The X-ray diffractogram of ES showed a diffraction peak spanning from 20 to 30°. The presence of this broad hump predominantly indicated the amorphous nature (Fig. 4). The size of virgin nanoparticles and that of nanocomposites was calculated using the Debye–Scherrer equation

D = 0.89λ/β cos θ

(where D, λ and β represent the size of crystallite, wavelength, full width half maximum respectively while θ represents the diffraction angle). The XRD diffractogram of TA–DBSA-POA showed a pronounced peak at 20° which can be correlated with (0 2 0) reflection.⁴² The θ value evaluation using the Scherrer equation revealed TA–DBSA-POA nanoparticles with an average particle size of 25–35 nm. The small angle XRD peak of the TA–DBSA-POA–ES composite revealed the presence of microcrystalline domains and the diffraction pattern of composites with 2θ = 25°. This peak exhibited a shift of 5° due to the formation of a composite structure with a difference in the structure and confirmation relative to TA–DBSA-POA nanoparticles. Thus confirming the semi-crystalline nature of composites which is further confirmed by TEM studies (Fig. 4).

Fig. 4 XRD analysis of (a) ES matrix (b) TA–DBSA-POA (c) 1.0% TA–DBSA-POA–ES nanocomposite.

3.4 Solubility test

The solubility behaviour of the matrix, the nanoparticle filler and their composites was investigated in various polar and non-polar solvents at room temperature. In the case of nanocomposites (TA–DBSA-POA), the solubility was measured in terms of the extent of their dispersibility and formation of the most miscible colloidal solution in polar and non-polar solvents. TA–DBSA-POA nanoparticles were well dispersed in polar solvents such as methyl alcohol, ethyl alcohol, DMSO, DMF and NMP while they fail to show their dispersion in non-polar solvents. TA–DBSA-POA contains oxygen atoms which are more electronegative and have a lone pair of electrons, which induce electrostatic interaction between oxygen and hydrogen atoms of the constituent moieties.⁴³ Hence, the hydrogen atoms of organic solvents are attracted to the lone pair of electrons on the negatively polarized oxygen atom of TA–DBSA-POA, forming a hydrogen bond.⁵ The lone pair of electrons of the oxygen atom are projected into the space away from the positively charged nuclei, promoting a considerable charge separation; therefore, dispersion was better in polar solvents than non-polar solvents. Similarly, for the ES matrix and the TA–DBSA-POA–ES nanocomposite, which have polar hydroxyl groups, oxirane, silicone and amino moieties, show more solubility in polar solvents than in non-polar solvents (Table 1).

Table 1 Solubility test

Solvent	Solubility of ES	Solubility of TA-POA	Solubility of TA–DBSA-POA–ES
Xylene	Insoluble	Insoluble	Insoluble
DMF	Soluble	Soluble	Soluble
DMSO	Partly soluble	Soluble	Soluble
Toluene	Insoluble	Partially soluble	Insoluble
CCl4	Insoluble	Insoluble	Insoluble
THF	Insoluble	Insoluble	Insoluble
Acetone	Partially soluble	Partially soluble	Partially soluble
Benzene	Insoluble	Insoluble	Insoluble
Ethylmethyl ketone	Soluble	Soluble	Soluble
Ethanol	Insoluble	Soluble	Partially soluble
Methanol	Soluble	Soluble	Soluble
NMP	Insoluble	Soluble	Partially soluble

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3.5 TEM and SEM of TA–DBSA-POA, ES and TA–DBSA-POA–ES

The TEM micrograph of ES revealed the formation of the two phase matrix i.e. the dark phase of silane (contrast) which was homogeneously dispersed in (Fig. 5a) the grey phase of epoxy. The TEM micrographs of ES showed the compatibility of silane with the DGEBA matrix. The synthesis of fibular POA nanoparticles containing fine and small white particles of ca. 25–35 nm diameter was confirmed by TEM (Fig. 5b) analysis. These non-agglomerated particles were modified by TA and DBSA dopants. The TEM micrograph (Fig. 5c) illustrated the transformation of the two phase morphology of doped POA nanoparticles from rod to spherical fibres. This can be attributed to the encapsulation of the POA rod in TA and DBSA (Fig. 5c-i and ii). These doped nanoparticles are also acquiring an intermingling long chain structure. Fig. 5d shows a fine granular structure of ES containing well-dispersed nanoparticles of varying shapes, i.e. spherical to cubic fibres. This exhibits the formation of homogeneous, compact, uniformly dispersed nanofiller composite coatings, which can be correlated with the improved properties of coating. SEM studies (Fig. 5e) were in concordance with TEM analysis. The SEM micrograph of TA–DBSA-POA revealed the presence of nanofibres of doped POA.

Fig. 5 TEM micrographs of (a) ES matrix (b) POA (inset enlarge view of POA nanofibres) (c) showing change in morphology (rod to spherical) of POA on doping with TA and DBSA (d) TEM micrographs of 1.0% TA–DBSA-POA–ES. (e) SEM micrographs illustrating surface morphology of nanofibres of POA upon doping with TA and DBSA.

3.6 Thermal gravimetric analysis

Fig. 6 represents the TGA curves of ES and nCP dispersed ES nanocomposite coatings. The thermal behaviour of TA doped POA (Fig. 6a) shows a three step weight loss process. The first weight loss (5–10%) below 100 °C is attributed to the loss of water and the second weight loss (20–50%) ranging from 200 to 300 °C is believed to be due to the elimination of the acid dopant. The third weight loss starting at around 400 °C is assigned to the thermal decomposition of the TA and DBSA doped POA backbone chains.

Fig. 6 TGA of (a) TA-POA (b) ES (c) (b) 0.5% TA–DBSA-POA–ES nanocomposite (d) 1.0% TA-POA/ES nanocomposite (e) 1.5% TA–DBSA-POA–ES nanocomposite.

Furthermore, the effect of incorporation of nanoparticles in the ES matrix on the thermal stability of ES was investigated by TGA as shown in Fig. 6b–e. 5–10 wt% weight loss occurred in the temperature range of 100 °C–170 °C due to the loss of absorbed solvent molecules. The first weight loss occurred at around 280 °C (Fig. 6b), due to the de-crosslinking of epoxy and siloxane. The second (350 °C–450 °C) and third weight loss step (470 °C–550 °C) could be assigned to the decomposition of the silica network. However, in the case of nanocomposite coatings, degradation in materials at higher temperature was observed in comparison to ES coatings. High thermal stability in nanocomposite coatings may be attributed to the uniform dispersion of TA–DBSA-POA-nanoparticles throughout the matrix and the strong interaction with the organic matrix. Besides, the presence of hydrogen bonding as evident from infrared spectroscopy was responsible for an increase in the thermal stability of the nanocomposites.

3.7 Physico-chemical and physico-mechanical characteristisation

Fig. 7a–d provides information about physico-chemical and physico-mechanical properties of the ES matrix and TA–DBSA-POA–ES nanocomposites. The decrease in the epoxy equivalent was observed with the loading of TA–DBSA-POA nanoparticles in the ES matrix. The values of epoxy equivalent (EE) for ES, TA–DBSA-POA–ES, 1.0 TA–DBSA-POA–ES and 1.5 TA–DBSA-POA–ES were found to be 181, 179, 175 and 173 respectively, (Fig. 7a). It indicated that the consumption of the oxirane ring content increases with the increase in TA–DBSA-POA interaction with the ES matrix, which increased with the increase in loading. The specific gravity of the ES matrix and TA–DBSA-POA–ES nanocomposites increased with the increase in loading. The value of specific gravity for ES, 0.5% TA–DBSA-POA–ES, 1.0% TA–DBSA-POA–ES and 1.5% TA–DBSA-POA–ES was 1.4, 1.6, 1.65 and 1.8 respectively. This also supported the formation of composites having higher density.

Fig. 7 Graphs showing variation of (a) oxirane content with loading (b) gloss with loading (c) DTT and DTH with loading (d) conductivity.

The gloss value had an inverse relationship with the loading of TA–DBSA-POA. The gloss values for 0.5% TA–DBSA-POA–ES, 1.0%TA–DBSA-POA–ES and 1.5% TA–DBSA-POA–ES were 42°, 39° and 36° respectively. This can be attributed to the increase in TA–DBSA-POA loading in ES which lead to the formation of a dense structure and to the opaque nature of TA–DBSA-POA (Fig. 7b) The dry-to-touch and dry-to-hard times of TA–DBSA-POA–ES composites exhibited a large decrease (Fig. 7c).

The dry-to-touch time for pristine ES was 6 h while the dry to-hard time was 14 h. The inclusion of DBSA–TA-POA resulted in a gradual decrease in dry-to-touch time and dry to-hard times, which was found to be 5 h and 12 h for 0.5% TA–DBSA-POA–ES, 4 h and 10 h for 1.0% TA–DBSA-POA–ES, 2 h and 8 h for 1.5% TA–DBSA-POA–ES, respectively; the trend can be attributed to the extent of crosslinking occurring in the cured structure of TA–DBSA-POA–ES. It was found that the scratch hardness values increased from 4 to 12 kg as the loading of TA–DBSA-POA in ES increased from 0.5 to 1.5 wt%. The increase in scratch hardness values was assigned to an intimate mixing of two components and the increase in adhesion at the interface of TA–DBSA-POA–ES nanocomposite coating and the surface of the substrate. The presence of strong hydrogen bonding between the ES matrix and the TA–DBSA-POA was also responsible for the increase in adhesion. The nanocomposite coatings with different loadings of TA–DBSA-POA passed the 1/8 inch bend (flexibility) test and the impact resistance test (150 lb per inch). The coatings were found to be flexible as they bend without any damage or fracture. The higher scratch hardness, flexibility and impact resistance can be correlated with the polar hydroxyl groups, oxirane and siloxane moieties present on the backbone of the polymeric chain of nanocomposites and also with the presence of strong hydrogen bonding. The MEK double rub cycle test values of all the coated CS were higher than 400 cycles.

3.8 Conductivity studies

Conductivity measurements were carried out under ambient conditions. The conductivity value of pristine POA was found to be 5.02 × 10⁻³ S cm⁻¹ and that of TA–DBSA-POA (Fig. 7d) was 2.09 S cm⁻¹. Upon loading of 0.5 wt% of TA–DBSA-POA in ES, the conductivity decreases in regard of the insulating nature of the matrix. Upon increasing loading, a slight increase in conductivity occurred (0.5 to 1.5 wt%).

The conductivity values for TA–DBSA-POA–ES-0.5, TA–DBSA-POA–ES-1.0 and TA–DBSA-POA–ES-1.5 were found to be 4.8 × 10⁻⁴ S cm⁻¹, 5.2 × 10⁻⁴ S cm⁻¹ and 3.1 × 10⁻⁴ S cm⁻¹ respectively. The threshold value for conductivity was found in the case of TA–DBSA-POA–ES-1.0 nanocomposites.

3.9 Salt spray test

The salt spray test (SST) was conducted for a period of 15 days in NaCl solution. The uncoated CS specimen was tested as a control. Initially, the coated and uncoated specimens show a glossy and shiny surface, with the exception of a greenish colour on the TA–DBSA-POA–ES coated specimen. After 48 h of the SST the uncoated specimens lost their glossy shiny appearance and the entire surface was covered with dark grey areas, exhibiting pitting and gazing at the surface along with the significant corrosion damage while the ES coated specimens show loss in gloss after 144 h, while TA–DBSA-POA–ES nanocomposite coated samples lost their gloss only after 288 h and no degradation was observed even after 320 h for TA–DBSA-POA–ES coating while ES coated CS show some deterioration after this period. This clearly shows that the TA–DBSA-POA–ES nanocomposite protects the CS from corrosion in saline medium.

3.10 SEM-EDAX

After 360 h of the SST, the SEM images showed the deposition of salt (NaCl) on the surface, however, no cleavage of the matrix was visually observed. The EDAX measurement of the coated surface exhibited no metallic iron, only the presence of slight deposition of chloride ions was observed. The improved corrosion protection performance of nanocomposite coatings was synergistically correlated with the generation of hydrophobic surface, barrier and redox protection due to dispersed POA nanoparticles (optical micrograph Fig. 8a and b) and after the salt spray test.

Fig. 8 (a) Optical micrograph of TA–DBSA-POA–ES coated CS. (b) TA–DBSA-POA–ES coated CS after SST.

They are responsible for increased crosslink density and the formation of the compact coating surface, which did not allow the penetration of corrosive ions to the coating-metal interface (Fig. 9), thus improving the corrosion resistance performance of TA–DBSA-POA–ES coating as compared to plain ES coatings (Fig. 10).

Fig. 9 SEM micrograph of TA–DBSA-POA–ES coated CS and EDAX spectra.

Fig. 10 SEM micrograph of TA–DBSA-POA–ES coated CS and EDAX spectra after SST.

3.11 Potentiodynamic polarization studies

The polarization measurements are used to monitor the electrochemical corrosion rate and the mechanism of anodic and cathodic partial reactions as well as the identification of the effect of an additive on the partial reaction. The potentiodynamic polarization curves and the corresponding electrochemical parameters were recorded for ES coated and TA–DBSA-POA–ES composite coatings on CS at different corrosive ion concentrations (3.5, 5.0 and 7.0 wt% NaCl solution) are shown in Fig. 11 and Table 2(a–c) respectively. Corrosion potential and corrosion current density were obtained using the extrapolation method in which the intersection of anodic and cathodic curves was extrapolated at the point of intersection. The well-known Stern–Geary equation was used to calculate the polarization resistance R_p:

Fig. 11 PDP curves of TA–DBSA-POA–ES in (i) 3.5 wt% NaCl (ii) 5.0 wt% NaCl (iii) 7.0 wt% NaCl medium where ((A) ES (B) 0.5-TA–DBSA-POA–ES (C) 1.0-TA–DBSA-POA–ES (D) 1.5-TA–DBSA-POA–ES).

Table 2 (a) Electrochemical parameters obtained from PDP and EIS studies for uncoated and coated CS in 3.5% NaCl at room temperature. (b) Electrochemical parameters obtained from PDP and EIS studies for uncoated and coated CS in 5% NaCl at room temperature. (c) Electrochemical parameters obtained from PDP and EIS studies for uncoated and coated CS in 7% NaCl at room temperature

Sample code	E corr (V)	I corr (A cm^−2)	R p (Ω)	R pore (Ω)	C c (farad)
(a)
CS	–0.33285	0.00055197	33.469	2.4 × 10^3	7.1 × 10^−5
Epoxy-siloxane (ES)	–0.28424	0.00026594	967.07	6.1 × 10^3	2.9 × 10^−7
0.5-TA–DBSA-POA–ES	–0.28899	0.0000021884	3228.3	2.3 × 10^5	3.6 × 10^−9
1.0-TA–DBSA-POA–ES	–0.22403	0.00000136	1799.1	3.1 × 10^7	7.8 × 10^−10
1.5-TA–DBSA-POA–ES	–0.20508	0.00000248	2226.7	5.2 × 10^7	9.8 × 10^−10
(b)
CS	–0.24017	7.9416 × 10^−4	7032.2	1.2 × 10^3	1.8 × 10^−3
Epoxy-siloxane(ES)	–0.26254	5.9084 × 10^−6	39272	2.2 × 10^4	2.32 × 10^−4
0.5-TA–DBSA-POA–ES	–0.38021	2.4207 × 10^−7	29121	1.27 × 10^6	9.67 × 10^−6
1.0-TA–DBSA-POA–ES	–0.40885	4.3782 × 10^−8	7751.1	1.72 × 10^7	7.9 × 10^−10
1.5-TA–DBSA-POA–ES	–0.50023	5.226 × 10^−8	8872.5	2.12 × 10^7	8.19 × 10^−10
(c)
CS	–0.2217	9.9416 × 10^−4	7012.2	1.2 × 10^3	1.8 × 10^−3
Epoxy-siloxane (ES)	–0.22254	6.9084 × 10^−6	49272	2.2 × 10^4	2.32 × 10^−4
0.5-TA–DBSA-POA–ES	–0.34031	3.4207 × 10^−7	29721	1.27 × 10^6	9.67 × 10^−6
1.0-TA–DBSA-POA—ES	–0.380885	4.3782 × 10^−8	8751.1	1.72 × 10^7	7.9 × 10^−10
1.5-TA–DBSA-POA–ES	–0.340985	2.3882 × 10^−8	28771	2.31 × 10^5	1.27 × 10^−6

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In 3.5% NaCl, CS samples coated with ES exhibited the lowest corrosion potential (−0.2775 V) and the highest corrosion current density (6.7084 × 10⁻⁶ A cm⁻²). In contrast, for the composite coatings containing TA–DBSA-POA–ES, the corrosion potential significantly increased and the corrosion current decreased, which indicated that the TA–DBSA-POA–ES composite coatings on CS samples provide higher corrosion resistance. Moreover, increasing the content of TA–DBSA-POA caused a gradual increase in the corrosion potential and simultaneously showed the decrease in the corrosion current, therefore the corrosion resistance was improved.

The superior corrosion protection of TA–DBSA-POA–ES coatings to CS may be correlated with the fact that the nanofiller was homogeneously dispersed in the matrix. The polar pendant functional groups of the filler and ES induced adhesion at the coating–CS interface. These polar groups developed strong electrostatic interaction between labile positive charges present on the metal surface, which induced strong adhesion at the coating–metal interface. The other reason being the hydrogen bonding between ES and TA–DBSA-POA–ES as shown in Fig. 3, which also reduced the effective area of corrosion through the blocking of reaction sites.

Similarly, in 5.0 wt% NaCl solution, the E_corr and I_corr values follow the same trend for different compositions of TA–DBSA-POA–ES coatings as discussed in the above section. However, a large negative shift towards the active side was observed for 1.5 wt% TA–DBSA-POA–ES coatings as compared to 1.0 wt% and 0.5 wt% DBSA-POA–ES coatings, as we increased the concentration of NaCl from 5 wt% to 7 wt%. Nevertheless, the 1.5 wt% TA–DBSA-POA–ES coatings still protect the underlying metal substrate from corrosive ions relatively to plain ES coatings. The low corrosion protective performance of 1.5 wt% TA–DBSA-POA–ES coatings at a higher concentration of NaCl i.e. 7 wt% may be attributed to the destruction of the Fe₂O₃ layer formed.⁴⁴

3.12 Electrochemical impedance spectroscopy

EIS experiments were conducted in order to understand the kinetics of the electrochemical corrosion processes and the role of nanocomposite coatings in CS and how they were modified by the incorporation of nanoparticles in ES. To further study the anti-corrosive performance of composite coatings, the EIS was adopted. Fig. 13 depicts Nyquist plots of TA–DBSA-POA–ES coatings at various concentrations of NaCl (3.5%, 5.0% and 7.0% respectively). The Nyquist plots showed the formation of single semi-circle with one time constant having three components R_s as solution resistance, R_pore as pore resistance and C_c as coating capacitance. (Fig. 12). All Nyquist plots were close to a semicircle, and their diameter was equal to the charge transfer resistance (R_ct) for CS samples.

Fig. 12 Equivalent circuit model.

In the case of 3.5% NaCl medium Fig. 13(i), the R_pore value increased and the C_c value decreased for TA–DBSA-POA–ES coated CS as compared to plain ES coatings. This clearly indicated that the introduction of TA–DBSA-POA in the ES matrix enhanced the anti-corrosive performance of epoxy-PA coatings. Besides, as the loading of TA–DBSA-POA–ES was introduced in the ES matrix, corrosion protective performance was increased in the said medium. The results demonstrated that the coatings containing CPs passivate the CS substrate and thus shift the potential toward the noble direction in comparison to the plain ES. Furthermore, the literature reveals that once the CPs like PANI,¹⁴ PPy,⁴ and their derivatives⁴⁵ are introduced, coating resistance could maintain relatively higher values, indicating the better protection performance of CP containing coatings compared to pure epoxy coatings.⁴⁶ Besides, it was found that the value of R_ct gradually increased with the increase in the TA–DBSA-POA content. R_ct approached the maximum at 1.5 wt%, followed by 1.0 wt% TA–DBSA-POA–ES, 0.5% TA–DBSA-POA–ES and plain ES coatings.

Fig. 13 EIS spectra of TA–DBSA-POA–ES in (i) 3.5 wt% NaCl (ii) 5.0 wt% NaCl (iii) 7.0 wt% NaCl medium where ((A) ES (B) 0.5-TA–DBSA-POA–ES (C) 1.0-TA–DBSA-POA–ES (D) 1.5-TA–DBSA-POA–ES).

In the case of 5.0% NaCl medium Fig. 13(ii), R_pore and C_c values followed the same trend for TA–DBSA-POA–ES and ES coated nanocomposite coatings as explained above. The behaviour of all the ratios remarkably followed the same trend as above in the case of 3.5% NaCl. However, a decrease in R_pore and C_c values for 1.5 wt% TA–DBSA-POA–ES relative to other compositions was observed as we increased the concentration of NaCl from 5 wt% to 7 wt% Fig. 13(iii). Although the higher concentration of NaCl protects the CS substrate by forming a passive layer in comparison to plain ES coatings, the corrosion process is not fully protected. This result may be attributed to the presence of a higher concentration of chloride ions which destroy the passive layer. Furthermore, the literature also showed that the present conducting polymer based system exhibits far excellent corrosion resistance potential as compared to other reported systems.

3.13 Raman spectroscopy

The presence of the passive layer was determined by Raman spectra. As shown in Fig. 14, the bands on the surface of TA–DBSA-POA–ES coated CS immerged for 15 days in 5 wt% NaCl aqueous solution were stronger in comparison to ES coated CS. The presence of 526 cm⁻¹ was assigned to chlorination of TA–DBSA-POA–ES, the absence of such modification in ES without the conducting polymer filler may be an indication that the structural changes observed in ES as were induced by the filler and they improved the corrosion protection performance.

The bands at about 216, 281 cm⁻¹, and 1293 cm⁻¹ were assigned to α-Fe₂O₃ [Fig. 14]. This indicates that the surfaces of CS coated by TA–DBSA-POA–ES formed passive layers, which are composed of α-Fe₂O₃.²³ Raman spectroscopy further depicted some kind of interaction between the ES matrix and the TA–DBSA-POA filler.

Fig. 14 Raman spectra of ES and TA–DBSA-POA–ES coated CS after SST to confirm the ferric oxide layer formed.

3.14 Mechanism of corrosion protection

Corrosion occurs in the presence of water, salts and oxygen, during which iron or steel undergoes an electrochemical process in which different locations of the iron surface act as electrodes. At the local anode, iron is oxidised to soluble Fe²⁺ and Fe³⁺ ions:

nFe(s) → mFe²⁺(aq) + (n − m)Fe³⁺(aq) + 3(n − m)e⁻ (1)

At the local cathode, hydroxide ions are formed:

O₂(aq) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq) (2)

Rust, which for CS, subsequently, is composed of FeO, Fe₂O₃, Fe₃O₄ and other mixed oxides, Fe (OH)_x, and Fe^x+ salts (chlorides, sulfates etc.). This highly irreproducible and irregular composition opens up new surfaces for self-catalytic growth – mainly Fe³⁺ salts acting as rust formation catalysts. Rust that forms this way does not adhere well on the iron surface. The mechanism of the corrosion protection of CS provided by the conducting polymers is well reported in the literature.²² It was considered that the conducting moieties of conducting polymers acted as an active coating in the reaction taking place across the polymer coated and the metal-electrolyte interface. In the presence of conducting polymer (TA–DBSA-POA) coating, a completely different galvanic process occurs in which TA–DBSA-POA replaces iron as the cathode due to its metallic properties, as it is situated like PANI slightly less noble than silver in the galvanic series. This arrangement is relatively evenly distributed over the whole surface. It involves Fe-oxidation by TA–DBSA-POA–ES (the more noble metal, Emeraldine salt ES), which is thereby reduced to the Leucoemeraldine base (LE); further both oxidation of Fe(II) to Fe(III) and reoxidation of LE to TA–DBSA-POA(ES) via the Emeraldine base EB occur by oxygen and Fe₂O₃ deposition by resulting OH⁻. This scheme shows, furthermore, that TA–DBSA-POA coating is different from conventional coatings in that it does not protect simply by offering a physical barrier but it acts as a redox catalyst, and the full catalytic cycle (ES to LE to EB and back to ES) protect the metal surface as long as the mechanical integrity of the polymer film remains intact. By TA–DBSA-POA coating, a remarkable corrosion potential shift (‘ennobling’) and iron oxide layer formation (‘passivation’) together lead to a significant anticorrosion effect. On the above, the excellent adhesion of TA–DBSA-POA–ES nanocomposite coatings, with the metal substrate due to uniform dispersion of POA nanoparticles in a matrix as evident from the SEM-EDAX study (Fig. 14), which can be corroborated for the superior anti-corrosive performance. Based on these observations, the suitable mechanism has been visualized and presented in the graphical abstract.

3.15 Chemical resistance

The results of chemical resistance of all the ratios of TA–DBSA-POA–ES and ES in different chemical environments are given in Table 3.

Table 3 Chemical resistance of the ES and TA–DBSA-POA–ES in different chemical environments after 15 days

Chemical environments	ES	0.5%	1.0%	1.5%
a The film remains intact and unaffected slight loss in gloss observed after 15 days. b The film remains intact and unaffected slight loss in gloss observed after 12 days. c The film remains intact and unaffected slight loss in gloss observed after 10 days.
Aq HCl (5%)
Aq NaOH (5%)
Aq NaCl (5%)

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The coatings showed excellent aqueous alcohol resistance as well as very good aqueous alkali and acid resistance due to the presence of high hydrogen bonding between TA–DBSA-POA and ES as well as strong chemical linkages. With an increase in the loading of TA–DBSA-POA–ES from 0.5 wt% to 1.5 wt%, the alkali and acid resistance increased, though a decrease is marginal.

4. Conclusion

Tartaric acid–dodecylbenzenesulphonic acid doped poly(o-ansisidine) nanofibres were successfully synthesized via emulsion polymerization in situ and their nanocomposites were prepared using the epoxy–siloxane matrix using the solution blending method. These composites were characterized by FTIR, XRD, UV-visible and TEM. The thermal stability of nanocomposites increases with the increase in loading of nanoparticles. The conductivity of nanoparticles and that of nanocomposites was found remarkably in the order of 10⁻³ S cm⁻¹ and 10⁻⁴ S cm⁻¹ respectively. The nanocomposite coatings exhibited promisingly enhanced physico-mechanical properties in comparison to virgin epoxy-siloxane coatings owing to the uniform dispersion of nanofibres in the matrix and intermolecular hydrogen bonding. Raman spectra confirmed the formation of a ferric oxide layer. The PDP and EIS studies revealed that with the increase in loading of nanoparticles, the corrosion resistance performance remarkably increased upto 3.5 wt%; however, at 7 wt% NaCl the increase in corrosion resistance properties observed only upto 1.0 wt% of nanocomposite coating, as a slight decrease in corrosion resistant properties was observed in the case of 1.5 wt% loading.

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