On the Development of an Intelligent Poly(aniline-co-o-toluidine)/Fe₃O₄/Alkyd Coating for Corrosion Protection in Carbon Steel

Featured Application

Smart corrosion protection systems.

Abstract

The corrosion of metals and alloys presents a significant challenge in many industries, demanding constant maintenance, and thereby increasing costs. In response to this problem, the smart corrosion protection coating has emerged as a promising solution. By enabling the immediate detection of, and response to, environmental changes, such as in the temperature and pH, these smart coatings contribute significantly to extending a material’s lifespan, and reducing maintenance expenses. In this study, nanomagnetic [poly(aniline-co-o-toluidine)/Fe₃O₄] systems were prepared and used as a self-healing corrosion inhibitor, mixed with alkyd paint at different weight percentages (5–25%). The composites were used as a coating on carbon steel (C1010), and their corrosion protection performance was tested in 0.1 mol/L HCl, using electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM), and FTIR analyses. The results showed an adequate corrosion inhibition performance for the developed composites, compared to the alkyd paint alone, reaching an inhibition efficiency of 80% at 20 wt.% of composite. Adding increasing weight percentages of the developed composites to the paints led to a significant increase in the corrosion resistance, accompanied by a remarkable decrease in the double-layer capacitance. Thus, these developed composites show excellent potential as a corrosion protection formulation in paints.

1. Introduction

Due to its favorable mechanical and physical properties, low-carbon steel is widely used in a variety of applications. Its excellent weldability and machinability, and relatively low cost, make it a first-choice material in several applications, such as making pipes and structural components in the construction industry; and producing various components, such as beams, frames and supports in the automotive industry [1,2,3].

Coatings are a widely used corrosion protection method for many metals and alloys [4,5,6,7]. Coatings have several advantages and features that make them a better choice to protect metal from corrosion. Coatings can form a physical barrier that separates the metal from its environment, thus limiting the access to the metal surface of corrosive particles, such as oxygen, chlorides, and other corrosive species [8,9]. Moreover, conversely to other methods, coatings can be applied to a wide range of metallic substrates, such as steels, aluminum, copper, and alloys, and can be customized to meet specific requirements, providing long-term corrosion protection even in highly corrosive environments [8,9,10,11]. Furthermore, coatings have the advantage of being cost-effective, particularly when compared to expensive methods, such as material replacement and the repairing of corroded metallic substrates.

Alkyd paints are one of the most used corrosion protection coatings, due to their excellent durability and low-cost. These oil-based paints can be applied to a variety of surfaces using several methods, such as spray, roller, or brush [12]. However, alkyd paint suffers from a low corrosion protection performance, especially when exposed to sunlight and humidity [8,13]. One practical solution to improve the corrosion protection performance of alkyd paints is to incorporate organic or inorganic compounds into the coating formulation [14,15]. Such a practice can significantly improve the barrier properties of the coating, making it more effective in preventing corrosive particles from reaching the metallic substrate. Additives such as nanoparticles can remarkably improve the hardness, adhesion, and durability of the alkyd paint, granting it an excellent resistance to abrasion and scratching, and reduced cracking.

Many researchers have reported smart coatings that show response to a stimulant [16,17,18]. Smart paint can include different responses, such as self-healing, self-cleaning, corrosion-sensing, and anti-fouling, among others [19]. Among smart-coating technologies, the concept of self-healing coating was introduced as a new approach to achieving corrosion protection function. The automatic corrosion response can be designed according to various factors, such as abrasions, pH change, surface tension, and temperature. Self-repairing organic and inorganic coatings can increase the functional life of the resulting coating structure. The self-healing function can be transported, due to the presence of nano/microcontainers of polymers and inorganic particles in paint structures [20].

Magnetic nanoparticles are of great importance to researchers because of their wide range of applications, which cover many research fields, including catalysts, biotechnology, biomedicine, and data storage [14,21,22]. They possess excellent characteristics, such as a small size, high surface area-to volume ratio, and biocompatibility, making them the best option for many applications. The successful application of these particles depends on the stability of molecules under different conditions. For instance, the performance of magnetic nanoparticles is better when the particle size is less than the critical value.

Because of the significant losses that result from the high corrosion rate of metals and alloys in corrosive acidic environments, the mechanism of corrosion protection in alloys and metals has gained a high importance, due to its durability in these environments [23]. Consequently, the protection of metals and alloys using organic coatings is of great importance to reducing corrosion [24,25,26,27]. Microsize commercial pigments are used in the coating systems, and have an essential effect on the organic coating properties. However, these coatings are not guaranteed to be free of defects and problems [28]; for example, ineffective adhesion problems, reduced coating elasticity, shock resistance loss, lower wear resistance, and low resistance to corrosion. Therefore, to overcome these defects and improve the coating’s performance, nanopolymers can be involved in the coating; this is considered to be a modern method. These materials show a tremendous improvement in properties compared to traditional coatings, because of the small size of their particles. Moreover, the nanomaterials utilized in the coatings and the production of the coatings provide multiple functions [29].

Riaz et al. have reported the development of nanostructured polyaniline (PANI) and poly(1-naphthylamine) (PNA) incorporated into soya oil alkyd, and their corrosion protection performance [30]. Authors have monitored the anti-corrosion properties of developed composites using weight loss, electrochemical tests, and scanning electron microscopy. Electrochemical tests showed that the developed PNA/Alkyd composite exhibited the highest corrosion protection performance. Jadhav et al. have studied the synthesis of nanopolyaniline (PANI) and poly-o-anisidine (POA), using the emulsion polymerization method in a micellar solution of SDS, and then their impregnation into alkyd paint, to improve its corrosion protection ability [31]. The anti-corrosion studies using the weight loss method showed that the nano PANI/Alkyd composite demonstrated excellent corrosion protection compared to POA/Alkyd coatings. The authors also reported a significant improvement in the scratch hardness of both coatings, compared with the pristine alkyd coating. On the other hand, Jeyasubramanian et al. [15] studied the preparation and characterization of nano-iron oxide, and its impregnation into alkyd coatings for corrosion protection improvement. The authors reported excellent anti-corrosion properties in the developed coating formulation for steel in a 3.5% NaCl solution. Other successful applications of metallic nanoparticles incorporated into alkyd coatings have been reported in a recent publication by Ifijen et al. [14].

In this research, an acrylic polyurethane (APU) coating, which consists of two components, an isocyanate in combination with nanomagnetite particles, was used. Magnetite, which has a high magnetic susceptibility, is highly adherent to the metal surface [32]. Magnetite is a magnetic oxide that can be prepared in various ways, including the co-precipitation method. It can also be produced from the interaction of iron and trioxide, as shown in the following equation:

$$\begin{array}{ccccc}\mathrm{FeO}& +& \mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3& \to & \mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_4\\ \mathrm{Ferrous}\mathrm{Oxide}& & \mathrm{Ferric}\mathrm{Oxide}& & \mathrm{Magnetite}\end{array}$$

Drawing inspiration from similar studies on nanocomposites, the mechanical characteristics of these developed composites could potentially include an enhanced fracture toughness, superior strength-to-weight ratio, high tensile properties, increased fatigue resistance, and improved corrosion resistance to severe environments [33,34,35]. Additionally, based on analogous studies, as the weight percentage of nanomagnetic systems increases, there could be a significant enhancement in the material’s mechanical behavior, similar to the linearly increasing Young’s modulus and flexural modulus observed in certain studies when filler materials were added [36,37,38,39].

The combination of the APU coating with nanomagnetic poly(aniline-co-o-toluidine) can be performed via copolymer, which is considered as a novel morphology, with better thermal stability and higher solubility compared to its homopolymers [40]. Juliet et al. reported the characterization of copolymerization for aniline-co-o-toluidine and V₂O₅ nanocomposites [41]. Its unique properties have been reported, such as an easy preparation method at a low cost [42], and its high resistance to corrosion. The prepared composites were characterized using scanning electron microscopy (SEM) and FTIR analyses. Furthermore, electrochemical impedance spectroscopy (EIS) was used to evaluate the effects of adding copolymer poly(aniline-co-o-toluidine)/iron oxide magnetic nanoparticle composites to the alkyd paint on the corrosion protection of carbon steel, in an acidic medium. The prepared composites were added in different weight percentages, ranging from 5 to 25 wt.%.

2. Materials and Methods

2.1. Raw Materials

Several chemicals from various companies were used in this study, such as aniline, Ferric chloride hexahydrate and ferrous sulfate heptahydrate, ortho toluidine, hydrochloric acid, and sodium persulfate. The alkyd paint and carbon steel alloy (Grade C1010) were supplied by the local market. The physical properties of the alkyd paint and carbon steel composition are shown in

Table 1. The physical properties of the alkyd paint used in the present work.

Shade No./Color	Finish	Solids Volume, %	Theoretical Spreading Rate	Flashpoint
Grey 12170	Matt	Grey 12170: 48%	10.8 m2/L - 0.035 mm	>35 °C
	Surface dry	Through-dry	Volatile organic compounds content	Specific gravity
	4 h (10 °C)	12 h (10 °C)	405 g/L	1.26 kg/L

and

Table 2. The chemical composition of the carbon steel used (in wt.%).

C%	S%	Mn%	P%	Si%	Cr%	Ni%	Cu%	As%	Fe%
0.083	0.002	0.50	0.003	0.011	0.019	0.0111	0.0183	0.097	99.1

, respectively. Aniline and o-toluidine monomers were distilled before use, under reduced pressure. No further treatments were carried out for the other used reagents. All chemicals were supplied by BDH chemicals (Poole, UK), except for FeCl₃·6H₂O and FeSO₄·7H₂O, which were sourced from Merck Co. (Darmstadt, Germany). The aniline and O-Toluidine were freshly distilled before polymerization.

2.2. Synthesis of Copolymer Poly(aniline-co-o-toluidine)

Firstly, 0.27 mol of aniline monomer was dissolved in 27 mL of 1 mol/L HCl, and 0.27 mol of o-toluidine monomer was dissolved in 27 mL of the same acid. Then, the monomers were placed in a 250 mL round-bottom flask. Next, 32 mL of sodium persulfate (SPS) solution was added dropwise to the monomer mixture. The reaction mixture was continuously stirred for two hours. The reaction mixture temperature was set between 0 °C and −5 °C. The copolymer obtained from the previous test was washed by distilled water and filtered. A dark green precipitate was acquired and then dried at 60 °C in a vacuum [43].

2.3. Synthesis of Magnetite Nanoparticles (Fe₃O₄)

Iron oxide nanoparticles were prepared based on the co-precipitation of Fe³⁺ and Fe²⁺, with a molar ratio of 2:1, respectively [44], and 2.7 g of FeCl₃·6H₂O and 1.2 g of FeSO₄·7H₂O, were dissolved in 100 mL of deionized water under nitrogen. The mixture was vigorous stirred, using a magnetic stirrer, at 80 °C for one hour. Then, 15 mL of 32% ammonium hydroxide was added to the mixture gradually, dropwise, with the reaction mixture constantly stirred. After the ammonium hydroxide was added, a precipitate was produced instantly [45]. The dark precipitate was washed six times, with the distilled water to remove excess ammonium ions. Finally, the humid suspension was evaporated, and dried for 24 h.

2.4. Synthesis of Copolymer Poly(aniline-co-o-toluidine)/Iron Oxide Magnetic Nanoparticle Composites

Poly(aniline-co-o-toluidine)/Fe₃O₄ magnetic nanoparticles composites were prepared [46] by adding 1 mol/L of pre-prepared co-polymer poly (aniline-co-o-toluidine) to the reaction mixture containing 2 mol/L of aqueous ferric chloride, 1 mol/L of aqueous ferrous sulfate, and 100 mL of distilled water. The mixture was placed in a three-necked round-bottom flask, equipped with a mechanical stirrer. The reaction was carried out at a temperature of 80 °C, for one hour, in an inert atmosphere. Then, the mixture was cooled, and 15 mL ammonium hydroxide solution at 32% concentration was added to the mixture as drops, gradually, and the nanomagnetic particles were precipitated. In order to remove excess ammonium ions, the precipitate was washed several times in distilled water. The humid suspension eventually evaporated, and dried for 24 h. After the drying process, copolymer/Fe₃O₄ nanocrystals were ground. In the same way, the copolymer poly(aniline-co-o-toluidine)/Fe₃O₄ magnetic nanoparticles were prepared. This method is called co-precipitation and, through it, the magnetic nanoparticles of the copolymer were obtained.

2.5. Preparation of the Carbon Steel

In this study, the carbon steel alloy C1010 was used; its components are shown in Table 2. It was washed with distilled water several times, then washed with ethanol and acetone to remove traces of water. Next, the alloys were polished using very fine silicon carbide sheets (600–3000) and, in the presence of alumina (Al₂O₃), a smooth and shiny surface was obtained for the alloy. Then, the ingot was cleaned using a cloth, and washed with distilled water and ethanol once again. It was immersed in acetone for a short time, then raised and left to dry in the air, and then kept in a glass dryer.

2.6. Coating of the Carbon Steel

The alloy was first coated with alkyd paint alone; a weight of 0.3 g of paint was taken, then distributed homogeneously over a 1 cm² area of the alloy’s surface. A small area of the alloy was left without coating, for electrical contact with the working electrode. Then, the second model was coated with Fe₃O₄, taking a weight of 5%w/w, which corresponds to a weight of 0.015 g of Fe₃O₄, and 0.285 g of the alkyd paint, mixed homogeneously, and this was repeated with the rest of the percentages (10, 15, 20, and 25%w/w) of the coating mixture. Then, with the same percentages as above, the alkyd paint was mixed with the prepared copolymer [poly(aniline-co-o-toluidine)/Fe₃O₄] MNPs, while being mixed at (25%w/w) with the copolymer poly (aniline-co-o-toluidine). The coating thickness of the samples was maintained, at an average of 40 µm, with a tolerance of ±5 µm. To ensure uniformity across all samples, the same amount of coating material was applied to each. Furthermore, the samples were dried under identical conditions, and they were placed on a level surface, to maintain consistency during the drying process.

2.7. Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) is a non-destructive technique used to study the electrical properties of materials in electrochemical systems. It involves applying a small AC voltage to the system, and measuring the resulting current response, over a range of frequencies. By analyzing the impedance data, EIS can provide information on parameters such as the resistance, capacitance, and charge transfer processes in the system, making it a powerful tool for studying corrosion and other electrochemical processes. To perform electrochemical measurements, a classical setup, consisting of a single-compartment glass cell and a three-electrodes system, was used. The system was composed of a reference electrode (saturated calomel (SCE)), a counter electrode made from a platinum grid with a large surface area, and a working electrode made from carbon steel, employing the Corrtest potentiostat/galvanostat model CS350, controlled by CS Studio5software. The electrochemical impedance spectroscopy (EIS) tests for the carbon steel immersed in 0.1 mol/L HCl, both with and without the formulations, were carried out, applying an alternating current within the frequency range of (0.01–105) Hz, and using an AC signal of 10 mV. Prior to the EIS tests, the open circuit potential (OCP) was monitored, until a stable potential was achieved, ensuring that the system had reached a steady state, and thereby providing a more accurate reflection of the corrosion behavior under equilibrium conditions.

2.8. Characterization Techniques

Fourier-transform infrared spectroscopy analysis was performed using a Perkin Elmer Spectrum 100 FT-IR Spectrometer (PerkinElmer, Waltham, MA, USA). In this technique, infrared radiation was passed through a sample, and the absorbance was measured at various wavelengths, to yield an infrared spectrum. The spectra were recorded in the wave-number range of 4000–400 cm⁻¹, at a resolution of 4 cm⁻¹. Scanning electron microscopy was used to examine the morphological structure and topography of the coatings. The analyses were conducted using a Zeiss EVO MA10 (Carl Zeiss AG, Oberkochen, Germany) scanning electron microscope.

3. Results and Discussion

3.1. FT-IR Analysis

The FT-IR spectra of Fe₃O₄ MNP_S and [Poly(aniline-co-o-toluidine)/Fe₃O₄] MNP_S are shown in Figure 1a,b, respectively. It can be seen from Figure 1a that the characteristic absorption of the Fe-O bond is at 570 cm⁻¹, whereas in the copolymer, it appears at 582 cm⁻¹. As shown in Figure 1b, the characteristic peak at 3321 cm^–1 is related to the N-H stretching of the primary amine, and proves the amino group copolymer [43]. The peak at 2920 cm^–1 is attributed to the C-H stretching vibration of the methyl group of the copolymer. The absorption peak at 1589.40 cm^–1 is assigned to the quinoid ring stretching for Poly(aniline-co-o-toluidine) MNPs. The peak observed at 1496 cm^–1 corresponds to the presence of the C=C stretching vibration in the benzenoid ring in the copolymer. The sharp peaks at 1149.61 cm^–1 and 813.99 cm^–1 correspond to the C-H bending in-plane and out-plane, respectively, in the copolymer. The peak at 1300 cm^–1 corresponds to the C-N stretching vibrations of the primary aromatic amine in the Poly(aniline-co-o-toluidine) MNP_s. The peak at 1242 cm^–1 is attributed to the C-N stretch of the benzenoid ring C-N-C.

3.2. SEM Analysis

Figure 2a,b present the SEM images of Fe₃O₄ MNPs and [Poly(aniline-co-o-toluidine)/Fe₃O₄] MNPs, respectively. Upon analysis, the nanoparticle sizes appear to fall within the range of 29.95 nm to 49.60 nm. Significantly, the size distribution is narrow, pointing toward a controlled synthesis and a uniformity in the particle size. Furthermore, a close examination of the particle morphology shows a consistent and well-defined shape throughout the samples. This uniform morphology could potentially contribute to enhanced physical and chemical properties in the resulting composite materials, making them suitable for corrosion protection. The detailed features highlighted in the SEM images underline the material’s potential for use as an effective corrosion protection material.

3.3. Corrosion Resistance Performance

The corrosion protection capabilities of the materials prepared can be effectively evaluated using electrochemical impedance spectroscopy (EIS) tests. The EIS data is graphically represented via Nyquist and Bode plots, as depicted in Figure 3 and the Supplementary Materials. To further scrutinize the electrochemical impedance characteristics, the collected data were processed utilizing CS Studio, specifically employing the Z-view function to extract additional pertinent information. This technique yields two types of visual outputs: Nyquist and Bode plots. These plots depict the behavior of the carbon steel alloy coated with specific weight ratio mixtures of Fe₃O₄ MNPs and [Poly(aniline-co-o-toluidine)/Fe₃O₄] MNPs, combined with alkyd resin, in a 0.1 mol/L HCl solution.

The analysis of the experimental EIS results involves multiple variables, all of which are included in

Table 3. The EIS parameters for the coated alloy in the presence of [poly(aniline-co-o-toluidine)/Fe₃O₄] MNPs and magnetite with alkyd paint in 0.1 mol/L HCl at 298 K.

Formulation	Wt.% of Inhibitor	Rp (Ω cm2)	n	Y° × 10−5 (Ω−1 Sn cm−2)	Cdl × 10−7 (µF cm−2)	τ (s)	fmax (Hz)	% IE
Alkyd	-	577.2	0.5707	7.598	1.04 × 103	0.67	236.69	-
Fe3O4 MNPs	5	986.1	0.4287	10.437	5.23	0.12	1263.7	41
	10	1073	0.5286	0.024	2.79 × 10−3	0.02	5441.2	46
	15	1143	0.7363	0.022	7.94	0.18	856.72	50
	20	1235	1.201	0.001	1.76 × 103	0.21	734.38	53
	25	1304.7	1.079	0.004	1.06 × 103	0.10	1508	56
[Poly(aniline-co-o-toluidine)/Fe3O4]	5	1167	1.294	0.026	6.98 × 104	0.61	258.61	50
	10	1195	0.1138	5.117	1.64 × 10−28	7.96	20.934	52
	15	2651	0.2900	1.158	3.58 × 10−8	3.90	40.792	78
	20	2810	0.6795	0.039	1.31	2.77	23.365	80
MNPs	25	2749	0.8284	0.040	68.7	0.39	404.57	79

. These variables include the solution resistance (R_s), the polarization resistance of the metal surface (R_p), which signifies protective properties, and the constant phase element (CPE) [47]. The phase shift (n) serves as a measure of the metal surface roughness. The double-layer capacitance (C_dl) forms a layer due to the coating on the solution’s surface, and as calculated using Equation (1) [48,49,50,51]:

C_dl = 1/(2πR_pf_max)

(1)

The efficacy of the prepared coatings can be evaluated from the corrosion protection efficiency (η%), calculated using Equation (2):

$$\mathsf{\eta }(\%)=\left(\frac{{\mathrm{Rp}}_{\mathrm{inh}}-{\mathrm{R}}_{\mathrm{p}}}{{\mathrm{Rp}}_{\mathrm{inh}}}\right)\times 100$$

(2)

where R_pinh and R_p represent the polarization resistance with and without the presence of particles, respectively. The relaxation time (τ) values can be computed using Equation (3) [52]:

τ = C_dlR_P

(3)

The EIS data depicted in Figure 3a, Figures S1a and S2a (Supplementary Material) show that the Nyquist plots’ semicircles of the copolymer[poly(aniline-co-o-toluidine)/Fe₃O₄] MNPs combined with alkyd paint at different weight proportions (5–25% w/w) are significant, compared to the plots of magnetite (Fe₃O₄) and non-magnetized poly(aniline-co-o-toluidine) when mixed with alkyd paint. This signifies an enhanced corrosion protection efficiency for [poly(aniline-co-o-toluidine)/Fe₃O₄] MNPs. An increase in polarization resistance implies an increased corrosion protection efficiency. Nyquist plots display only a single capacitive semicircle in the entire frequency range, indicating a singular time constant associated with the charge transfer process [53,54,55,56]. Corresponding Bode plots confirm this one-time constant behavior. The application of the coating on the carbon steel yields an increased value of phase angle, suggesting an excellent anti-corrosion efficiency [57].

The addition of magnetite and [Poly(aniline-co-o-toluidine)/Fe₃O₄] MNPs to the alkyd coating at various weight percentages does not alter the corrosion mechanism. Instead, it amplifies the surface coverage, and enhances the coating’s efficiency and corrosion protection ability. This improves the coating’s barrier properties, and boosts the adhesive force of the coating on the metal surface, hence physically isolating the metal alloy from the corrosive medium [58,59]. The physical separation signifies the coating’s efficiency when combined with magnetic nanoparticles, effectively impeding the penetration of corrosive particles, such as water molecules, oxygen, and other ions [60]. A higher percentage of corrosion inhibitor improves the inhibitory efficiency. The values of R_p also rise with an increased percentage of Fe₃O₄ MNPs and [Poly(aniline-co-o-toluidine)/Fe₃O₄] MNPs, as indicated in Table 3. This elevation in R_p values results in a corrosion reduction and efficiency augmentation [61]. However, a decreased inhibitory efficiency may be noticed at certain inhibitor weight percentages, due to the potential deterioration of the coating layer caused by surface roughness, heterogeneity, and possible structural deformation [62]. The adsorption of the Fe₃O₄ inhibitor and the copolymer [Poly(aniline-co-o-toluidine)/Fe₃O₄] MNPs on the corroded alloy surface impedes charge transport, and obstructs corrosion-prone active sites, leading to an R_p increase from 577.2 kΩ cm² (with the alkyd alone) to 1304.7 and 2749 kΩ cm² (with 25 wt.% of Fe₃O₄ and the copolymer [Poly(aniline-co-o-toluidine)/Fe₃O₄] MNPs, respectively). This significant effect contributes to a substantial corrosion rate reduction, thus providing effective protection for the steel surface.

The data displayed in Table 3 exhibit a substantial reduction in C_dl (double layer capacitance) values upon application of the treated coatings, compared to using alkyd resin alone. This indicates that the incorporated inhibitors, both magnetite and [copolymer/Fe₃O₄] MNPs, are effective in hindering the corrosion rate of alloys by implementing an adsorption mechanism, and thereby mitigating the extent of metal dissolution. The decreased C_dl values associated with the highest percentages of both magnetite and [copolymer/Fe₃O₄] MNPs can be attributed to a lower dielectric constant, and an increase in the double layer thickness, due to the adsorption of these particles on the carbon steel surface. The value of C_dl correlates with the thickness of the protective layer, as expressed using the following equation:

$${\mathrm{C}}_{\mathrm{dl}}=\frac{\mathsf{\epsilon }\mathsf{\epsilon }˳\mathrm{A}}{\mathrm{d}}$$

(4)

where (ε) represents the local dielectric constant of a solution, (ε˳) is the permittivity of free space, (d) is the protective layer thickness, and (A) is the electrode surface area [63].

Furthermore, the variable n, representing phase displacement, provides information regarding the surface characteristics of the coating; a lower n value implies a rougher surface, while a higher value indicates a smoother, more uniform surface [64,65]. As evidenced in Table 3, the highest n value was found at a 25% concentration for the copolymer compound, whereas the maximum n value for the magnetite compound was observed at a 20% concentration. These atypical or irregular n values could be due to variations in the coating properties at different concentrations. Changes in thickness, uniformity, and porosity can cause deviations from the expected linear trends. The higher n values imply a denser adsorption of inhibitor particles on the alloy surface, subsequently slowing down the alloy’s corrosion process, and enhancing the inhibition efficiency, with an increase in the inhibitor weight percentage. It is important to note, however, that such irregularities in n and Q values emphasize the complexity of the system, and could also hint at variations in the coatings’ physical properties. These unexpected trends underscore the necessity for further microlevel investigations of coatings, to better understand and optimize their performance.

4. Conclusions

This research has demonstrated the effective application of magnetic [poly(aniline-co-o-toluidine)/Fe₃O₄] nanoparticles and magnetite (Fe₃O₄), in combination with alkyd paint, as a novel coating for corrosion protection in carbon steel in acidic environments. The magnetic nanoparticles and magnetite were blended with alkyd resin in varying percentages (5–25%). The electrochemical impedance spectroscopy (EIS) results revealed that this innovative coating offered superior protection against corrosion, compared to alkyd paint alone.

Furthermore, the coatings incorporating the copolymer [poly(aniline-co-o-toluidine)/Fe₃O₄] nanoparticles displayed a higher inhibition efficiency than those mixed solely with magnetite. This improved performance was attributed to an increased polarization resistance (R_p), which was particularly noticeable at the maximum inhibitor percentage. As the inhibitor percentage increased, the coating’s wear resistance was enhanced, forming a more robust protective layer on the carbon steel surface.

The novel coating also demonstrated the capacity to reduce electrical capacitance, indicating its potential for displacing adsorbed water molecules or ions on the metal surface. The adhesive force of the coating on the metal alloy surface was notably improved when using the alkyd with [poly(aniline-co-o-toluidine)/Fe₃O₄] nanoparticles, creating a physical barrier between the alloy and the corrosive medium, and inhibiting electrolyte solution penetration.

The findings from this research open promising avenues for the development of smart corrosion protection strategies for carbon steel.