Evaluation of synthesized polyaniline nanofibres as corrosion protection film coating on copper substrate by electrophoretic deposition

Aniline (C₆H₅NH₂), ammonium persulphate (APS), hydrochloric acid (HCl), formic acid (HCOOH), acetonitrile (C₂H₃N) of analytical grade were used as received; aniline was double distilled under vacuum pressure; deionized water was used throughout this work.

List of chemicals and base fluid used in the experimental study:

Polyaniline was synthesized chemically using aniline monomer and ammonium persulphate [(NH₄)₂S₂O₈] used as oxidant in a ratio of 4:1 for monomer/oxidant. 40 ml of the aniline monomer was dissolved in 80 ml (1 M/L) HCl under ice bath below 15 °C. In parallel, 320 ml of 1 M/L HCl was dissolved in 24 g [(NH₄)₂S₂O₈]. The solution containing [(NH₄)₂S₂O₈] was carefully and slowly added dropwise into the solution containing aniline for a period of 1 h under constant stirring. The mixture was continuously stirred for another 45 min. The mixture was kept in a room temperature for 24 h to fully polymerize. The fully polymerized solution was filtered and washed with distilled water, followed by 1 M HCl solution. The obtained solid polymer was dried in an oven for 24 h [21]. The surface properties of the copper sheet are tabulated in Table 1.

Stock solution (1 mg/mL) of formic acid (HCOOH) and PANI was prepared. PANI powder was dissolved in HCOOH aqueous solution and stirred under stirrer for 15 min for uniform mixture. Colloidal suspension of PANI was then prepared by dissolving 35 ml of the stock solution into 15 ml of acetonitrile to obtain 50 mg/mL. The mixture was sonicated for 5 min to fully dispersed the PANI colloids [22]

A copper sheet of 99% purity that is 1 mm thick was purchased from a local market in Alexandria, Egypt. The sheets were cut at 4 cm × 1 cm, length by width, respectively. These sheets were polished manually with a grit paper made from silicon carbide (grit size range 2000) to give a smooth surface and also to remove an oxide layer and also to activate the surface for electron activity during the EPD process. The sheets were sonicated using (model: 08895-83, Cole-Parmer, USA) in acetone for 20 min, rinsed with copious amount of distilled water, and finally dried in a room temperature [23].

During the electrophoretic deposition process, the prepared PANI was first dispersed in formic acid and acetonitrile under ultrasonication (Probe sonicator) for 15 min. PANI nanoparticles were completely dispersed in formic acid and acetonitrile, forming deep green colloidal suspension. The potential of the colloidal suspension measured using Zeta-sizer analyser (3000HSa, Malvern Instruments Ltd., UK) was positive potential (+ 32.9 mV). Consequently, cathodic electrophoretic deposition process shown in Fig. 1 was used for coating PANI on the Cu sheet electrode. A copper sheet (1.5 cm × 2 cm) was used as the cathode, and two sheets of the sizes were used as the anode. This is due to the fact that both sides of the sheet were deposited by the PANI colloidal suspension. The three electrodes were immersed vertically in 150-ml beaker containing the colloidal suspension with a standard distance of 1 cm separated between each of the electrodes and connected to a DC power supply (Model: TB160V22A1080W, Matsusada, Japan) as shown in Fig. 2. Consequently, the electrophoretic deposition (EPD) process was conducted using various conditions at different deposition times and applied voltages. The deposition time was fixed at 180 s when the voltage applied was adjusted and fixed at 15 V [24]. The coated dark green samples were dried at room temperature overnight [24, 25]

The prepared PANI was utilized to prepare the colloidal suspension in the presence of formic acid (HCOO) and acetonitrile. The PANI in the emeraldine form (base) was initially dissolved in the formic acid forming a stock solution after sonication. Dispersion was formed from the stock solution using the acetonitrile and sonicated for 20 min, thereby forming a suspension of colloids. The rationale behind utilizing these mixtures for the suspension is that there is solubility of formic acid in PANI in the emeraldine base form; as a result, the formic acid isolates the random chains of the PANI which has intertwined and breaks down the PANI in its chains level. The proton addition to the polymer’s atom at its amine site on the polymer chains concurrently is carried out by the HCOO, consequently transforming PANI from emeraldine base to a conducting emeraldine salt. Notwithstanding this, there is solubility of acetonitrile (C₂H₃N) in HCOO but not in PANI. Consequently, dispersing C₂H₃N in HCOO dissipate as a result the HCOO shrinks in size into the medium of its surrounding. This causes compression of the chains inside the polymer in spherical shape, and the polymer starts to split up in the di-electric medium into ions simultaneously. Thus, rendering the colloids of the polymer carrying positive charges and also due to the electrostatic repulsion occurring between the spherical particles causes the stability of the colloidal suspension as shown in Fig. 3.

The TEM image was employed to evaluate the structural and morphological features of the produced PANI. In general, Fig. 4 depicts the production of PANI with nanofibre morphology that is twisted and agglomerated into an interconnected network, rather than bundle. The image explains very clearly that the PANI has a typical 3D structure with a wrinkled morphology. The dark areas point to the multilayer PANI, while low-density layers result from the nanostructure exfoliation highlighted by the transparent areas. Notwithstanding these, the dark areas are more visible and this shows that most layers of the PANI are well exfoliated.

Morphological and particle size analysis were carried out on the synthesized organic acid. PANI-doped thin films were investigated with scanning electron microscopy (SEM) micrographs from 10 to 25 Kx magnification separating the images along with accelerating a voltage of 20 kV, and their respective photographs are illustrated in Fig. 4. Considering the SEM images, all the PANI thin films exhibit a homogenous phase with uniform matrix [26] with Fig. 5a depicting SEM image of the polished copper substrate at X 40 magnification. Figure 5b that was taken at X 5000 magnification demonstrates the SEM image of pure PANI powder with an irregular busted small inured piece of ice rock and also appears like a coral reef surface morphology with 12 µm as the size of the particle. The PANI-coated layers in Fig. 5c, which were in the same magnification scale of Fig. 5b, appeared as a dense speck colossal structure with irregular broken ice rock-like morphology with 5 µm dimension. Notwithstanding these, the PANI-coated layer with 1-µm scale exhibits a characteristic of layer-by-layer twisted fibre with some tiny particles as shown in Fig. 5d, which was taken at X 10,000 magnification. Considering the SEM images, we could observe different morphologies for Fig. 5b–d. This is as a result of the formic acid and the acetonitrile that causes the PANI powder to be stable and forms colloids for the EPD process. The formic acid separates the random chains of the PANI and protonation which occurs at the amine site on the polymer chain causing changes in PANI’S morphology. Acetonitrile effect on causing formic acid to shrink into its surrounding causes the compression of the PANI chains in spherical shape which amounts to the breakup of the PANI into ions in the dielectric medium. Consequently, charging the PANI colloids being positively as a result of the electrostatic force of repulsion occurring between these spherical particles also alters the morphological properties of the PANI. Nevertheless, the obtained image is in accordance with the results obtained by [27, 28].

Figure 6 shows the XRD patterns for pure PANI powder and PANI-coated layer on copper substrate. The observed diffraction peaks are 2θ = 15°, 21°, 25°, 44°, 53°, 74° which correspond to (011), (020), (200), (022), (213), (222), respectively [29, 30]. The patterns of the XRD as observed are analogous to the ICDD data number 00-053-1891 confirming the formation of polyaniline emeraldine salt in a doped form. As a result of murkiness in the chains of the polymer, it appears semi-crystalline [31] or amorphous [32] in nature dependent on experimental conditions. The semi-crystalline polymers exhibit well and tighter arrangement of its molecular chains which combines the strength of crystalline polymers with the non-crystalline flexibility. The observed intense or sharp peaks of the XRD patterns correspond to the region of crystallinity while the broader peaks show the region of non-crystallinity in the chain of the polymers. The semi-crystallinity is observed as a result of the repetitive arrangement of the quinoid and the benzenoid rings in the polymer chain. This result has been reported by Butoi et al. [33], Olivera et al. [34], Mitra et al. [35], and Elnagar et al. [30]. The observed peaks at 2θ = 53° and 74° are due to the presence of the metallic substrate. The XRD peaks at 2θ = 21° for the two samples, the structural parameters, namely micro-strain (ε), interplanar distance (d), dislocation density (D), interchain separation (R), and distortion parameter (g), were calculated using the following five formulae. The results are summarized in Table 2.

From Table 3, it is evidenced that an increase in the number of cycles, microstrain, and interplanar distance decreases the size of crystallites. As the deposition of cycles increases, the reconstruction of the molecules/atoms takes place resulting in a decrease in the size of the crystals. Also, as the size of the crystals decreases, full width at half maximum, β (FWHM), increases with an increment in the number of cycles. Lattice misfit results in the production of micro-strain, ε, dependent on the condition of growth [36]. Interplanar distance and micro-strain thus increased in PANI-coated layer compared to the pure PANI powder. Increase in the number of cycles increases the density of the atom/molecules causing internal stress which introduces deformations in the materials. Dislocation thus occurs when deformation increases. Relatively high structural disorders and defects in the PANI-coated layer sample promoted better process of oxidation–reduction. Quite a number of researchers have reported a better electrochemical performance for an active material of small crystal size coupled with structural defects and disorders [37, 38], but our results are in agreement with the research done by Suman et al. [39].

Fourier transform infrared spectroscopy (FT-IR) is the spectroscopic technique widely used in analysing the polymers [40]. They are of great importance since the FTIR could collect the infrared spectrum information of polymers and provide the structural information to identify the molecules. The FTIR spectroscopy could provide detailed chain structure information of polymers [41, 42]. The spectrum of FT-IR of polyaniline is shown in Fig. 7. The main characteristic properties of the polyaniline bands follow [43]: the band at approximately 1700 cm⁻¹ is because of stretching of N–H mode, C=C and C=N stretching modes for benzenoid and quinoid rings occurring at approximately 1550 cm⁻¹ and 1600 cm⁻¹, respectively [44‐46]. C–N stretching vibrations of the quinoid imine site units’ band are at approx. 1300 cm⁻¹. In this region, a characteristic band of the conducting polyaniline protonation appears as well indicating a characteristic of emeraldine salt (ES) phase of polyaniline. The bands between 850 and 825 cm⁻¹ correspond to the vibrational flexions out of the C–H bonds plane [44]. The band at approximately 1150 cm⁻¹ is assigned to 1–4 substitution on the benzene ring. The bands at 782 cm⁻¹ show ortho substitution. Finally, vibrations of the bonds, C–C and C–H, in the aromatic ring appears at 740–650 cm⁻¹ [45]. The spectra of the FT-IR study show that polymerization of the aniline monomer occurred and resulted in the formation of the polyaniline. Table 4 shows the peaks observed for polyaniline.

The UV–Visible spectrum of the polyaniline was recorded using Hitachi U-3900 double-beam spectrophotometer in the range of 300–900 nm. Electronic spectra of polyaniline were recorded by dispersing the polyaniline in organic solvent with an ultrasonication. Electronic absorption of the conducting polymer is useful in investigating the oxidation and doping state of the polymer backbone. The optical absorption spectra of the polyaniline and PANI-coated layer are presented in Fig. 8 depicting the absorbance as a function of wavelength (nm) derived from a graph appearing with two peaks. Polyaniline peak at wavelength of 340 nm corresponds to the π–π* [47] transition of the benzenoid rings. The second peak appearing at a wavelength of 310 nm is due to the π‐π* transition from the nitrogen oxide containing nonatomic electrons to the conducting polymer, while the shoulder at 465 nm is attributed to the localized polarons which are characteristic of the protonated polyaniline [48]. The broad band increases the absorption band at a higher wavelength at 800 nm in approximation with a free tail carrier confirming the existence of conducting form of emeraldine salt (ES) of the PANI [49].

Electrophoretic mobility is a measurement used to determine the zeta potential value which depends on the nanoparticles and the medium property [50]. Zeta potential controls the deposition type and its velocity [51]. Determination of suspension’s stability depends largely on zeta potential value. Suspensions with zeta potential are more than either + 30 mV or − 30 mV [52] because of the polyaniline functional groups [sulphonated poly(aniline-co-o-aminophenol), s-copolymer]; the zeta potential value of the polyaniline attained a positive charge as a result of the functional group; and confirmation was made via measurement of the zeta potential and was equal to + 32.9 mV as shown in Fig. 9. This caused the colloidal suspension during EPD process to be deposited on the copper sheet of the cathode. The value also indicates the stability of the suspension [53]. Particle size distribution shown in Fig. 10 indicates 545.2 nm as the average particle size.

Potentiodynamic polarization analysis was used to extrapolate the rate of corrosion of the PANI-coated specimen in 3.5 wt% NaCl. Open-circuit potential, OCP, and Tafel polarization curves of the bare Cu and PANI-coated Cu are shown in Figs. 11, 12, respectively. EPD-PANI indicated a significant decrease in the cathodic and the anodic currents. This confirms the protection of PANI to the copper. The shift in corrosion potential from − 259 to − 199 mV indicating the cathodic barrier PANI provided, consequently, decreasing the tendency of the copper corroding by the release of electrons. Table 5 displays the calculated parameters from the Tafel polarization curves with its respective corrosion potential (E_corr), corrosion current (I_corr), and the corrosion rate. The corrosion rate (0.1006 mpy) recorded for the coated Cu sample was higher in magnitude as compared to the corrosion rate of the bare copper with a value of 1.422 mpy, which indicates the absorption of the PANI’s colloidal suspension onto the surface of the Cu, thereby inhibiting the occurrence of corrosion. Additionally, the π-electrons of the heteroatoms and the aromatic rings of the polyaniline which contains lone pairs of electrons: oxygen (O) and nitrogen (N) atoms, were contributory factors for the strong absorption and adhesion of the PANI onto the Cu, which led to the efficient resistance of the Cu in the corrosive media. EPD-PANI recorded low corrosion rate at 0.1006 Mils per year (mpy) compared to the corrosion rate of the copper (1.422 mpy) with positive in corrosion potential of 60 mV compared to the bare copper. The corrosion potential shows its susceptibility to corrosion, but the positive demonstrates the corrosion resistance of the copper as a result of the PANI coating at the expense of chlorine ions (Cl⁻) albeit speeding the dissolution of the copper [54]. The presence of the polymer on the surface of the conducting substrate decreased the activity of the Cl⁻ in reaching the surface of the metal, consequently decreasing the rate of metal dissolution in the electrolyte.

The ability for the EPD-PANI protecting the metal from corroding in the electrolyte is because of the structure of the coating which is less distorted and the hydrophobic nature as a result resisting the saline electrolyte from permeating. In addition, good adhesion of EPD-PANI coating as confirmed by SEM (Fig. 5b) could be a factor in enhancing the resistance of the copper to corrosion by preventing the penetration of the Cl⁻ via the interface between the coating and the bare copper. The significantly enhanced performance of the EPD-PANI coatings is due to better adhesion and the coatings hydrophobic nature compared to other coating techniques. Observation could be made from Table 5 that the anodic slope (\(\beta\)_a) of EPD-PANI coatings was much higher than that of the bare copper suggesting more adsorption of Cl⁻ ions on the PANI-coated layer than the bare copper. The corrosion process involves two half reactions: anodic and the cathodic reactions. During anodic reaction, oxidation of Cu occurs releasing electrons which forms soluble Cu²⁺ represented in equation [55].

Other reactions could also take place in NaCl solution during the process

On the cathode surface, reduction reaction occurs in which electrolyte such as O₂ or H⁺ is reduced, eliminating electrons from the copper according to the reaction

According to the corrosion assumptions, transfer of electrons at the surface of the metal reactions kinetics is manipulated by the reaction rate of the anodic and the cathodic reactions. Consequently, corrosion rate (CR) between these reactions is controlled by equilibrium. There is no net current that occurs when the reactions are in equilibrium [23]. where K = 3272 mm/A cm yr is a constant which defines the units of CR (mm/yr), EW = 31.7 g represents equivalent weight of Cu, ρ = 8.97 g/cm³ density of Cu, and A = 4 cm² area of the sample.

Furthermore, the corrosion inhibition efficiency (IE) from the Tafel polarization curves was calculated [56] using the following formula:

The extent of porosity is strongly governed by the corrosion resistance of the coating behaviour. It is therefore pertinent to determine the overall resistance the coating offered to the metal. Potentiodynamic polarization measurement was used to determine porosity of the EPD-PANI on the metal substrate. The following equation was adopted in calculating the polarization resistance (R_p) and percentage porosity (P%) of the coating [57]. where high R_p value implies high resistance of the material to corrosion and vice versa. Table 5 shows that the R_p value for the PANI-coated Cu has higher value compared to the bare Cu. Polarization resistance from Table 4 reveals increment in R_p value of the PANI-coated layer (2.165 × 104 Ω cm²) by a higher margin compared to the R_p value of the bare copper (3.142 × 103 Ω cm²). The polarization resistance acted as a resistor, consequently preventing the electrolyte from penetrating the Cu.where P represents total porosity, R_puc and R_pc are, respectively, polarization resistance of bare copper and EPD-PANI-coated layer, ΔE_corr is the difference between corrosion potentials, and b_a is the Tafel anodic slope of the bare copper and the PANI-coated layer. The value of the porosity calculated is presented in Table 5. It could be deduced that the value of P for the PANI-coated layer is very low indicating the efficacy of the coating serving as a barrier for the metal thereby decreasing the corrosion rate from 12.76 to 0.1006 mpy. The low value of the porosity in the PANI-coated layer gave rise to the enhancement of the anti-corrosion resistance thereby hindering the electrolyte access to the copper substrate.

The microscopic interaction between the PANI-coated layer and the treated copper substrate increases its mechanical and adhesion properties of the coating. The PANI powder on the surface of the copper substrate provides a barrier for diffusion of H₂O, O₂, and Cl⁻ as a result of the surface of the PANI extremely rough when coated. Again, the hydrophobic nature of the pure PANI powder also prevents moisture from penetrating via the substrate surface. According to Dhanabal et al. [58], and Chen et al. [59], corrosion prevention of polyaniline occurs by forming a passivating layer of Fe₂O₃ at the metals interface and the polyaniline coating. The tendency of passivating layer forming on the surface of coating is more pronounced in these present experiments thus shifting of the potential more to the anodic side as compared to other coating materials. The thin coating and the protection offered by copper acted as anion storage and an electrical barrier for the PANI thereby slowing the effects of electrochemical reactions responsible for corrosion of metals. Figure 13 shows the schematic diagram demonstrating the mechanism of corrosion. These factors lead to better corrosion performance of the metals and in accordance to the intended functions [60‐66].