Electrochemical Synthesis of PHO-SiO₂/Al₂O₃ Nanocomposite Coatings for Corrosion Protection of Low-Carbon Steel Alloy

Low carbon steel (LCS) corrosion has become one of the most important industrial problems, thanks to its application in the petroleum, marine, and chemical industries. The electrochemical deterioration of LCS surfaces is expedited through aggressive environments with chloride ion-heavy environmental conditions, resulting in serious economic and environmental problems due to the rapid electrochemical loss of the steel surface with corrosion. As a result, developing efficient, protective coatings with high corrosion resistance to the environment is one of the most relevant areas of research [1].

Conductive polymers prepared with electrochemical polymerization have attracted a great deal of attention because they possess good adhesion, electrochemical activity, and anti-corrosion effect [2]. Moreover, conductive polymers can serve as selective protective barriers that reduce the penetration of corrosive species onto metallic surfaces and make them more durable in hostile environments [3]. Polymeric coatings are widely applied in preventing corrosion due to their convenience in preparation and their good chemical, thermal, and mechanical properties [4].

Conventional polymeric coatings have shortcomings, however. For instance, poorly-packed microstructures and coating defects are present, allowing corrosive ions to penetrate more easily and reducing the lasting benefits of the process [5]. Recent studies have thus shifted towards integrating nanoparticles into polymer matrices to enhance coating compactness, barrier properties, and corrosion resistance. Polymer-based nanocomposites are promising because of synergistic interactions between polymer matrices and nanomaterials [6].

Mathai and Shaji [6] reviewed polymer-based nanocomposite coating approaches and found that nanocomposite coatings showed superior barrier performance, corrosion resistance, and mechanical stability when compared to conventional polymer coatings.

However, they also reported that the long-term coating stability under extreme conditions remains a great challenge. Alqudsi and Saleh [7] applied electrochemical polymerization to eugenol and evaluated the corrosion protection of stainless steel 304 L. They also proved that electropolymerized coatings could effectively lower corrosion current density (I_corr) as well as promote corrosion resistance in harsh media.

Huang et al. [8] prepared electroactive epoxy-SiO₂ hybrid nanocomposite coatings and found that SiO₂ nanoparticles increased coating compactness and enhanced anticorrosive behavior through decreased diffusion pathways of corrosive species.

Yeganeh and Keyvani [9] studied mesoporous silica nanocontainment as part of a polymer coating, where silica nanoparticles are found to have a corrosion resistance effect by limiting the corrosive mediums from penetrating to the defective and damaged coating regions. Wang et al. [10] studied epoxy/alumina nanocomposites, and the addition of Al₂O₃ nanoparticles resulted in increased morphology and mechanical stability of the polymer matrix and enhanced coating performance.

Balaraju et al. [11] studied Ni-P-Al₂O₃ composite coatings and found that alumina nanoparticles substantially increased the hardness, compactness, and corrosion resistance of the coating because of improved microstructures. Tallman et al. [12] established a novel approach for corrosion protection in developing nano-composite coatings with polypyrrole and alumina nanoparticles.

The results showed that the stability and protective performance of polymer conductive coatings under corrosive environments [12] was improved by Al₂O₃ nanoparticles.

Previous results showed the excellent enhancement during the process of electropolymerizing poly[(Z)-4-((4-hydroxyphenyl) amino)-4-oxobut-2-enoic acid] (PHO) on LCS surfaces. In fact, although there has been considerable improvement in the performance of conductive polymer nanocomposite coatings, little work on such coatings has been conducted on the electropolymerization of PHO.

Moreover, not enough work has been conducted on the comparative effects of SiO₂ and Al₂O₃ nanoparticles on the electrochemical properties, thermal stability, and surface morphology of PHO coatings at different temperatures. Thus, in this work, we intend to explore the synthesis of PHO- and PHO-based nanocomposite coatings on LCS surfaces by means of electropolymerization and assess their corrosion protection potentials against 3.5 wt.% NaCl solution with varying temperature options.

3.1 Cyclic voltammetry and electro-polymerization of the (Z)-4-((4-hydroxyphenyl) amino)-4-oxobut-2-enoic acid monomer

The electrochemical growth behavior of 0.1 gm HO monomer in 3.5% NaCl solution. Because the solution exhibited low conductivity, three drops of concentrated H₂SO₄ were added to improve conductivity, as shown in Figure 2. The evolution of the electropolymerization process was performed by cyclic voltammetry via sweeping potential in the range: −2000 to +2000 mV vs. SCE at a scan rate of 550 mV/s.

The cyclic voltammogram of HO exhibits an anodic peak at approximately 2.936 V during the forward scan, which may be associated with the oxidation of the HO monomer. During the reverse scan, a cathodic peak appears, indicating the reduction of electroactive species formed during the anodic process. The observed changes in the voltammetric response after successive scans suggest the possible formation of a surface layer on the electrode. These results are consistent with previous reports describing the electrochemical oxidation behavior of HO and the potential formation of polymeric films on electrode surfaces. However, additional surface and structural characterization techniques are required to conclusively confirm electropolymerization and film deposition mechanisms [15], as shown in Figure 2.

3.2 Proposed polymerization mechanism of (Z)-4-((4-hydroxyphenyl) amino)-4-oxobut-2-enoic acid on low-carbon steel electrode

The electropolymerization of the HO monomer on the LCS electrode surface to form the PHO polymer coating was performed through cationic polymerization, which is carried out in the following phases [16].

1. When anodic potential is applied to monomer solutions, electrons are extracted from monomers to electrodes, forming cationic radicals on the electrode surface (Steps 1-2 of Figure 3). The functional groups of monomers and Fe atoms (from LCS) interact to facilitate adsorption between monomers and electrodes; therefore, there is a lack of electrons on the electrode surface, resulting in the physical interaction of cationic radicals to form cationic radicals on lead electrodes.
2. To form larger molecules, the cationic radicals formed react with nearby monomers that are uncharged. This is accomplished via a cationic addition mechanism at the oxidized monomers' charged end (refer to steps 3 through 4 in the figure provided).
3. During the coupling process shown in Steps 3 to 4, protons (H⁺) are released from the monomer units. This step is crucial for the formation of stable covalent bonds between the units, allowing the polymer chain to grow effectively on the surface. This process continues with the addition of more monomer units to the growing chain. Anions from the electrolyte solution stabilize the cationic centers, ensuring the continuous buildup of the polymeric structure across the LCS electrode surface.

3.3 Fourier transform infrared spectroscopy characterization of (Z)-4-((4-hydroxyphenyl) amino)-4-oxobut-2-enoic acid electropolymerization to form poly[(Z)-4-((4-hydroxyphenyl) amino)-4-oxobut-2-enoic acid]

The Fourier transform infrared spectroscopy (FTIR) spectra of the HO monomer and the PHO polymer, as seen in Figure 4, give insight into the functional groups found in the polymer and its overall structure as applied onto the LCS electrode. The FTIR spectrum for the monomer (Figure 4(A)) shows that there are bands attributed to carboxylic O–H, N–H, and C–H stretching at 2983.67 cm^-1, and that the C = O bands for C = O carboxylic acid and C =O amide were just overlapped/appeared in the same region at 1699.17 cm -1; and that there was a broad band at 1573.81 due to a C = C ring. The disappearance of the olefinic C=C bands in Figure 4(B) indicates the formation of the PHO polymer. Due to the form of international chains covering the new area of dispersion for polymer PHO, a very wide transmission peak was created [17].

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3.4 Analysis of Atomic Force Microscopy

Utilizing atomic force microscopy (AFM), the surface morphology of LCS discs with and without the polymer coating is assessed through an examination under the microscope. 3D images of the polymer and polymer nanocomposites created in the presence and absence of nanoparticles can be seen in Figure 5, respectively. The average roughness (Sa), root mean square roughness (Sq), and primary grain size values obtained from this study are shown in Table 1 and can be used to quantitatively describe the surface characteristics of the materials being researched.

Table 1. The parameters of AFM of PHO-coated LCS with and without nanoparticles

The results indicate that the incorporation of nano metal oxides significantly influenced the surface morphology and grain distribution. For the PHO/Al₂O₃ nanocomposite, a noticeable decrease in surface roughness was observed (Sa = 28.11 nm) compared to the pure PHO film (Sa = 31.97 nm). This improvement in surface smoothness is attributed to the reduction in the main grain size from 161.2 nm to 89.91 nm, suggesting that Al₂O₃ nanoparticles effectively filled the polymer matrix voids and promoted a more compact arrangement. On the other hand, the PHO/SiO₂ nanocomposite exhibited a different trend, where the surface roughness increased to Sa = 50.32 nm. Despite the reduction in grain size to 94.87 nm, the higher Sa and Sq values suggest the formation of a more complex surface texture or slight nanoparticle aggregation.

In conclusion, the significant reduction in grain size observed for both nanocomposites confirms a high degree of integration within the polymer chains. This modification in surface topography is expected to enhance the barrier properties of the coating, where the more compact and refined grain structure provides a denser protective layer against corrosive environments [18].

3.5 Corrosion studies

Typical polarization curves for coated and uncoated LCS with produced polymer before and after utilizing nanomaterial in a 3.5% sodium chloride (NaCl) solution with a temperature range of (298-318) K are shown in Figure 6. The protective efficacy is calculated using the following Eq. (1) [19].

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$P E \%=\frac{i_{\text {corr}, \text { uncoated}}-i_{\text {corr}, \text { coated}}}{i_{\text {corr}, \text { uncoated}}} \times 100$      (1)

The current density of the corrosion current (I_corr) in coated and uncoated LCS, i.e., (I_corr) unc. and (I_corr) c., respectively, is determined as the extrapolation of the corrosion potential (E_corr) of the anodic and cathodic Tafel lines. The polarization resistance (R_p) can be computed using the rearranged Stern-Geary equation [20].

$R_p=\frac{B_a\left|B_c\right|}{2.303\left(B_a+\left|B_c\right|\right) i_{\text {corr}}}$     (2)

The protective performance of electrodeposited coatings is evaluated using R_p measurements, as R_p values are inversely proportional to the i_corr. Consequently, a higher R_p indicates a lower corrosion rate and enhanced coating durability. R_p represents the material's kinetic resistance to anodic oxidation under an applied external voltage, providing a quantitative measure of the specimen's ability to withstand degradation in polarized environments.

The electrochemical properties of uncoated LCS and samples coated with PHO and its nanocomposites (Al₂O₃ and SiO₂) at three different temperatures (298, 308, and 318 K) are shown in Table 2. The uncoated alloy had a much worse corrosion resistance than the uncoated LCS (LCS/PHO), but adding nano-fillers (SiO₂ and Al₂O₃) improved the LCS/PHO's protective performance significantly. The corrosion resistance (Rp) data at 298 K confirms the lowest Rp value (145.5 Ω/cm²) of the uncoated LCS alloy, indicating a very high corrosion rate in the saline region. The Rp significantly increased to 2142 Ω/cm² when coating the alloy with just polymer (LCS/PHO). The increase can be ascribed to the development of a barrier layer preventing the passage of ions and corrosive particles to the metal surface. It was revealed that the corrosion resistance was enhanced further by the addition of nano-oxides into the polymer coating. The Rp value obtained was 2563 Ω/cm² when aluminum nano-oxide (Al₂O₃) was added, and the highest value, measured to be 3517 Ω/cm², was recorded when silicon nano-oxide (SiO₂) was added to the mixture. This increase implies that nanoparticles had a satisfactory effect of providing a density of coating, and to eliminate pores and micro-cracks, enhancing the protective barrier properties and the corrosion resistance efficiency, especially nano-silicon oxide, which showed the most effective performance of all types of models studied. At 298 K, the PHO/SiO2 nanocomposite exhibited higher Rp than the PHO/Al₂O₃ nanocomposite, with a Rp State clearly: SiO₂ gives higher Rp, whereas Al₂O₃ gives slightly higher PE% at 298 K, due to having the ability of nanoparticles to fill submicron-sized pores and defects within the PHO matrix and forming a denser and tighter barrier layer that inhibits corrosion of the metal surface by the influx of corrosive chloride ions. The findings of this study align with the findings of previous investigations into the performance of nanocomposite coatings that were produced using silica-based fillers. Several studies have demonstrated improved corrosion resistance and barrier properties of nanocomposite coatings, which can be attributed to the homogenous distribution of nano-sized particles in the coating matrix [21].

Table 2. The properties of corrosion in the case of the LCS coated and uncoated with PHO at different temperatures in the presence of a saline solution (3.5% NaCl) with and without nanomaterials

Researchers looked at the impact of raising the temperature from 298 K to 318 K on the protective films' durability. Across the board, they found that as the temperature rose, the Rp and PE% for each of the samples went down. This is usually attributed to two factors: one, that the corrosion process is happening faster because of increased temperatures; and two, that a portion of the protective layer is being removed from the surface of the metal either through corrosion or due to heat alone. However, even at elevated temperatures, the nanocomposite coated samples demonstrated high levels of corrosion protection. The authors noted that while SiO₂-based coatings outperformed all other coatings at lower temperatures, the PHO/Al₂O₃ nanocomposite performed the best at 318 K with a PE% of 85.87% compared to the SiO₂-based nanocomposite (84.37%) and neat PHO coatings (77.86%). The authors also noted that it appears as though the addition of Al₂O₃ nanoparticles to the PHO coating enhanced both the adhesion and chemical stability of the PHO matrix at elevated temperatures [22]. Overall, the present findings confirm that the incorporation of nanomaterials into polymer coatings is an effective strategy for corrosion protection, in agreement with recent studies reporting the superior anticorrosive performance of SiO₂-based nanocomposite coatings [21].

3.6 Kinetic and thermodynamic corrosion activation parameters

Temperature is one of the important factors influencing the corrosion behavior of LCS in 3.5 wt.% NaCl medium, whether the steel surface is protected by a coating or left uncoated. Therefore, electrochemical tests, particularly potentiodynamic polarization measurements, were performed within the temperature range of 298-318 K for bare LCS, PHO-coated LCS, and PHO/metal oxide nanocomposite-coated LCS. These measurements were carried out to investigate the temperature dependence of the corrosion process and to determine the corresponding activation energy. The obtained data are summarized in Table 3. The Arrhenius relationships, represented by Eqs. (3) and (4), were applied to describe the formation of the activated complex at the transition state and to calculate the activation parameters related to corrosion [23]. Using the corrosion rate values measured at various temperatures, plots of log C.R against 1/T were generated for the uncoated, PHO-coated, and PHO/metal oxide nanocomposite-coated LCS samples, as illustrated in Figure 7. Furthermore, transition-state plots of log (C.R/T) versus 1/T were used to evaluate the thermodynamic activation parameters, including ΔH^* and ΔS^*, as shown in Figure 8.

Table 3. The kinetic and thermodynamic activation characteristics for uncoated and PHO-coated LCS discs with and without nanometal oxides at different temperatures

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Figure 8. Log (C.R/T)–1/T plots for bare and coated LCS in 3.5 wt.% NaCl

$\log C . R=\log A-\frac{E_a}{2.303 R T}$       (3)

$\log \frac{C . R}{T}=\log \left(\frac{R}{N h}\right)+\frac{\Delta S^*}{2.303 R}-\frac{\Delta H^*}{2.303 R T}$               (4)

where, C.R = the corrosion rate that is equivalent to the I_corr, Ea = the apparent effective activation energy, R = the molar gas (JK^-1mol^-1) constant, A = the Arrhenius preexponential factor, and T = the absolute temperature (K), N = Avogadro’s number, h = Planck constant.

Results indicated that nanocomposites made using both PHO and PHO metal oxides covering LCS discs had greater values for thermodynamic activation parameters (Ea and H) as compared with uncoated LCS discs, indicating an increase in the energy barrier associated with these reactions. The coated and uncoated LCS entropy of activation (ΔS‡) values are both negative and change from negative to lower negative, meaning that the rate-limiting step associated with the formation of the activated complex from reactants was produced through the process of association rather than dissociation. There was also found to be less disorganization during the reaction between the reactants and the active complex [24].

3.7 Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) was employed to investigate the electrochemical response of the corroding metal surface when subjected to a low-amplitude alternating potential over a range of frequencies. The impedance results were interpreted using Nyquist diagrams, in which the imaginary component of impedance (Z) is plotted as a function of the real component (Z). In addition, Bode representations were used to describe the frequency-dependent changes in the phase angle (o) and the impedance modulus (Z). From the Nyquist response, important corrosion-related parameters, such as impedance, capacitance, and Rp, can be determined. Moreover, the coating capacitance (C) of the polymer film and the uncompensated solution resistance (Rs) between the working and reference electrodes were estimated using the following equations [25, 26]:

$C_C=\varepsilon \varepsilon^0 \frac{A}{d}$        (5)

where, Ɛ⁰ is the dielectric constant in vacuum (8.85 × 10-12 A/cm), E is the dielectric constant of the polymer, d is the film thickness, and A is the area covered by the alloy.

Figure 9. Nyquist plots of PHO coated and uncoated LCS at different temperatures in a 3.5% NaCl solution

Figures 9-10 depict the experimental data of EIS of the LCS in the 3.5% NaCl solution under the investigation of uncoated and coated with PHO at three temperatures (298, 308, and 318 K) using the Nyquist plots and the Bode plots.

The results of EIS spectrum and the corresponding impedance parameters indicate that PHO coating hold significant corrosion resistance toward LCS discs in 3.5% NaCl solution via impeding the permeation of corrosive ions and water to a metal interface, does have a good physical barrier properties; The effectiveness of this protective layer is clearly highlighted by the very large increase in Rp for the coated system (LCS\PHO) compared to uncoated steel, with Rp values reaching upwards of 2000 Ω. cm² at 298 K, however these values are usually decrease with the increase in temperature since the thermal acceleration of the corrosion process.

In addition, the Bode plot analysis further confirms that the characteristic of coating performance is reflected in stable Rs and certain capacitive behavior (CPE), which validates that the polymer film can enhance barrier performances effectively by providing strong prevention of electrochemical reactions at the coating/metal interface [27]. They had the same equivalent circuit, as shown in Figure 11.

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The analogous circuit yielded the following electrochemical parameters [28]:

Resistant electrolyte (Rs).

Rct = charge transfer resistance or Rp.

Constant phase element magnitude (CPE-Q).

CPE-n = phase shift or empirical exponent.

These parameter values are listed in Table 4.

Table 4. The electrochemical properties of an equivalent circuit of the PHO polymer-coated and untreated LCS discs