Anticorrosive Effect of New Polymer Composite Coatings on Carbon Steel in Aggressive Environments by Electrochemical Procedures

Abstract

In this investigation, electrochemical deposition procedures were reported to synthesize a novel composite polymer, 3-methylpyrrole-dodecyl sulfate sodium/3,4-ethylenedioxythiophene (3MPY-SDS/EDOT) coatings, on OL 37 samples for anticorrosion protection. The anionic surfactant dodecyl sulfate sodium used in deposition can have a relevant action on the protective capacity. These coatings were considered by cyclic voltammetry (CV), Fourier transform infrared (FT-IR) spectroscopy, and scanning electron microscopy (SEM) methods. The protective attributes of OL 37 coated with P3MPY-SDS/PEDOT have been examined by potentiostatic and potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) procedures in 0.5 M H₂SO₄. The corrosion rate of the P3MPY-SDS/PEDOT-coated OL 37 sample was found to be approximately nine times lower than that of the uncoated sample. The protective layers of these composites demonstrate an effectiveness of over 90%. The optimal efficiency is obtained by electrochemical deposition of P3MPY-SDS/PEDOT, performed at applied potentials of 1.0 V, 1.2 V, and 1.4 V, with current densities of 3 mA/cm² and 5 mA/cm² and a molar ratio of 5:3 at 20 min. The influence of electrochemical polymerization parameters—applied potential, current density, scan rate, cycle number, and monomer ratio—on the protective behavior of P3MPY-SDS/PEDOT layers was analyzed, identifying optimal synthesis conditions. Corrosion examinations confirmed that P3MPY-SDS/PEDOT coatings provide effective protection for OL 37 in a corrosive environment.

1. Introduction

Metals such as iron and its alloys (carbon steel) have been used in numerous technological practices, such as in engineering, construction, machinery, and other industrial applications, because they have outstanding functional characteristics and their cost is comparatively low. Even so, these materials suffer from a disadvantage which is corrosion. Exposing carbon steel to various severe industrial circumstances can change the rate of corrosion. The acidity of the aggressive environment is possibly one of the most significant elements to analyze. Recently, the protection of the surfaces of metallic materials by utilizing conductive polymers as advanced coating materials has emerged as an extremely interesting field of research [1,2,3,4]. Polymer coating on metals and their alloys is one of the areas developed in the application of anticorrosion protection [5,6,7,8]. In technological practices, the materials used are exposed to aggressive acids and alkaline environments, which provoke corrosion and considerable destruction. The degradation of materials poses a considerable economic and technical concern. Some explorations carried out for the defense of metallic materials in the sphere of engineering have determined that the use of polymer composites is the most efficacious and easy way to impede the degradation of these metallic samples in corrosive solutions [9,10,11,12]. The acquisition of novel composites to different monomers has been accomplished to develop the physico-chemical characteristics of composite coating, to extend long-term shielding, to raise adhesion, and to enhance the electrochemical particularities. These composite coatings regularly increase the protection of metals against some corrosive agents. The performance of these shielding layers that provide a surface barrier can involve various elements: the type of conducting polymer, the electrodeposition procedure that was used on the substrate, and the corrosive solutions [13,14,15,16,17]. The new polymer coatings work as a physical barrier by obstructing the movement of corrosive factors to the metal surface under the coatings. The defensive activity of conductive polymers has been supported by various assumptions, such as their ability to generate an electric field on the material’s surface, preventing electron transfer from the metal to the oxidizing environment. Additionally, these polymers form a dense, low-porosity layer on the sample surface, serving as a barrier between the metallic material and the aggressive solution. Furthermore, they facilitate the formation of a protective metal oxide film on the sample’s substrate [18,19,20,21,22]. Many studies have confirmed that these polymers under various circumstances can recover their original mechanical and electrical properties without significant deterioration [23,24,25,26,27]. Conducting polymers such as polythiophene, polypyrrole, and polyaniline and its derivatives (3-methylpyrrole, poly(3,4-ethylenedioxythiophene)), provide a distinctive combination of physicochemical, electrochemical, and optical attributes that make them encouraging for use as protective coatings, chemical and biological sensors, supercapacitors, and organic photovoltaic devices [28,29,30,31,32,33,34]. This is the principal cause of the intense investigation and development of such materials. One of the principal problems related to the use of conducting polymers for inhibiting metal corrosion is their permeability to water, which can lead to the transport of corrosive components to the metal surface. It can be said that the polypyrrole derivatives (3MPPY, NMPY) have better protective attributes than pyrrole due to the hydrophobicity of the methyl group. Polypyrrole and its derivatives are especially relevant because they have a good environmental steadiness, including conductivity, owing to heterocyclic monomeric units in them which possess a similar construction to protective inhibitors [35,36,37,38,39,40]. The monomer 3,4-ethylene-dioxythiophene (EDOT) is of particular importance due to the prospect of polymer production with linear chains and fewer deficiencies due to obstruction of the 3, 4 sites of thiophene ring [41,42,43,44]. The presentation of the ethylenedioxy bridge also reduces the oxidative doping potential of the polymer and establishes its doping form (situation). However, PEDOT is considered to typically be the steadiest organic conducting polymer. PEDOT has attracted great attention in the biomedical domain owing to its remarkable environmental stability as well as observable biocompatibility, facile synthesis, high charge mobility, and thermal steadiness with high electrical redox characteristics [43,44,45,46]. It has been established that PEDOT, either alone or combined with other materials, can provide excellent corrosion protection for metals such as carbon steel, magnesium, aluminum, copper, and titanium alloys. A. Madhan Kumar et al. analyzed the impact of surface treatment on the newly developed TiNbZr alloys and their subsequent PEDOT coatings. Corrosion tests confirmed that the PEDOT coatings on treated TNZ substrates exhibited significantly improved corrosion protection. D. Aradilla et al. studied the modification of stainless-steel substrates using self-assembled monolayers of octanethiol and dodecanethiol for electrodeposition of PEDOT and evaluated the corrosion protection provided by PEDOT films on treated substrates and compared it with films deposited on bare SS electrodes and the findings revealed that corrosion inhibition in a 3.5% NaCl solution was significantly higher when PEDOT was deposited on the treated electrodes. Some investigations have indicated that conducting polymers (polythiophene, pyrrole, and aniline and its derivatives) can be readily electropolymerized in water and an aqueous micellar medium on different substrates (aluminum, copper, iron, platinum, and carbon steel) [47,48,49,50,51,52]. N. Özcicek Pekmez et al. studied the electropolymerization of pyrrole and bithiophene on stainless steel in the presence of SDS in oxalic acid. Their results demonstrated the best corrosion protection performance (in 3.5% NaCl) in terms of anticorrosive effectiveness. B. Zeybek et al. carried out the electrosynthesis of a PNMPy-dodecylsulfate coating on AISI 304 stainless steel using the potentiodynamic technique in aqueous oxalic acid; the PNMPy-DS film showed excellent adhesion to the metal and provided strong corrosion protection for the stainless steel. The reason for this study is to acquire a novel composite appropriate for the anticorrosion protection of numerous materials, as well as to expand appropriate electrodeposition processes that will ensure the capability to achieve homogeneous, dense, and adhesive coatings on metallic materials substrates and the optimizing of these new coatings with excellent defensive possessions for some materials in aggressive media.

The objective of this paper is to examine the anticorrosion performance of new P3MPY-SDS/PEDOT composite coatings in acidic medium. The novel composite was electrodeposited on OL 37 (equivalents EN 10025—S235J2; DIN 17100—St37-3N) electrodes by potentiostatic and galvanostatic processes from 0.1 M 3-methylpyrrole, 0.1 M 3,4-ethylenedioxythiophene, and 0.02 M dodecyl sulfate sodium in 0.3 M H₂C₂O₄ solution. The investigation of these coatings was carried out by FT-IR spectroscopy, cyclic voltammetry, and SEM procedures. Corrosion examinations of OL 37 coated with P3MPY-SDS/PEDOT were evaluated by potentiodynamic polarization and EIS methods in 0.5 M H₂SO₄. These new P3MPY-SDS/PEDOT composite coatings were found to exhibit excellent protection performance on OL 37 in 0.5 M H₂SO₄ solution. This study is a continuation of a previous paper evaluating suitable composite coatings for inhibition of corrosion of metallic materials in corrosive solutions.

2. Experimental

2.1. Materials and Methods

OL 37 was used as the working electrode for corrosion analysis. The structure of OL 37 was: C 0.15%, Si 0.09%, Mn 0.4%, Fe 99.293%, P 0.023%, S 0.02%, Al 0.022%, Ni 0.001%, and Cr 0.001%. The corrosive solution was 0.5 M H₂SO₄ that was made by diluting AG 96% H₂SO₄ (from Merck) with bidistilled water. All substances were reagent grade, and 3-methylpyrrole (3MPY), 3,4-ethylenedioxythiophene (PEDOT) sodium dodecylsulfate (C₁₂H₂₅NaSO₄-SDS), and oxalic acid (H₂C₂O₄ 2H₂O) were from Aldrich (>98%). In all experimental determinations, synthesis solutions were achieved with bidistilled water. In this investigation, all electrochemical techniques (electrochemical polymerization, cyclic voltammetry, potentiostatic, potentiodynamic polarization, and the electrochemical impedance spectroscopy) were employed to investigate the corrosion protection behavior of carbon steel using a PGZ 402 potentiostat/galvanostat, which was controlled by Voltamaster 4 software. The measurements were taken using a conventional three-electrode cell configuration, where the operational working electrode took the form of a disk electrode. A platinum plate functioned as the opposing electrode alongside, and a calomel electrode was used as the reference. The working samples (carbon steel) were mechanically polished using sandpapers of varying grits (600–4000) to achieve a mirror-like finish. The electrodes were first cleaned with benzene to remove greasy residues, then rinsed with double-distilled water, dried at room temperature, and finally placed in the electrochemical cell.

The poly (3-methylpyrrole-dodecyl sulfate sodium/3,4-ethylenedioxythiophene) coatings have been electrodeposited from 0.1 M 3-methylpyrrole, 0.02 M SDS, 0.1 M 3,4-ethylenedioxythiophene, and 0.3 M H₂C₂O₄ on the OL 37’s passivated surface by potentiostatic and galvanostatic procedures (Scheme 1). The electropolymerization was carried out by potentiostatic application at the applied potentials 1.0 V, 1.2 V, and 1.4 V and by galvanostatic operation at constant current densities 3 mA/cm² and 5 mA/cm² and in distinct molar ratios (5:3 and 3:5) for 10 and 20 min.

The adhesion of this composite layer was effectuated using an extended version of the “standard sellotape test”, specifically standard ASTM D3359—Method B (cross-cut). This method involves cutting the film into small equal squares, applying the sellotape to the surface, allowing it to settle and adhere to the coating, and then peeling it off. The adherence is assessed by dividing the number of unaffected squares by the total number of squares (see Scheme 2) [7,21,35,36,45]. The electrochemical behavior of the polymeric composite was examined in 0.3 M H₂C₂O₄ by the cyclic voltammetry method. The inhibition of corrosion of the composite coating and bare samples was analyzed by potentiostatic and potentiodynamic polarization methods and by electrochemical impedance spectroscopy in sulfuric acid. Exploration of Tafel polarization curves was performed by potential translation from cathodic to anodic potential for the OCP. All potentials were registered with respect to the reference electrode. To provide complete data, all experiments were repeated three times, allowing results to be averaged and diminishing the effect of any random errors or measurement fluctuations. Electrochemical impedance spectroscopy (EIS) assessments were conducted at the OCP of the sample–solution junction, covering frequencies from 100 kHz down to 10 mHz. The magnitude of the applied voltage was established (set) at 10 mV, with ten data points acquired per logarithmic decade. All electrochemical determinations were repeated at least three times to ensure reproducibility.

2.2. Instruments

A VoltaLab PGZ 402 potentiostat/galvanostat setup was employed in all electrochemical determinations. Composite coating was analyzed with a Bruker optics FT-IR spectrometer (ATR) in the spectral range 4000–650 cm⁻¹ at a resolution of 4 cm⁻¹. The morphologies of the covered samples have been explored by scanning electron microscopy (SEM). The evolution of the composite film on the samples was investigated using morphological and microcompositional analysis, which was performed with an FEI Inspect F50 scanning electron microscope (FEI Company, Brno, Czech Republic) equipped with an EDAX APEX 2i energy dispersive X-ray spectrometer (EDS) with an SDD Apollo X detector (EDAX Inc., Ametek MAD Mahwah, NJ, USA). To minimize charging effects and obtain high-quality images, the samples were sputter-coated with a thin layer of gold using the SC7620 Mini Sputter Coater/Glow Discharge System (Quorum Technologies, Laughton, East Sussex, UK). SEM images were acquired at magnifications of ×1000, ×10,000, and ×20,000 using an accelerating voltage of 15 kV, a working distance of 10–11 mm, a spot size of 3.0, and a dwell time of 30 µs. For microcompositional analysis, EDS spectra were collected to identify and quantify the elements present on the surface, with a target of up to 1200 counts to achieve an appropriate signal-to-noise ratio, ensuring reliable detection of target elements.

3. Results and Discussion

3.1. Electrochemical Polymerization of PNMPY-SDS/PEDOT Coating on OL 37 Substrate

Electrodeposition of 3MPY-SDS and EDOT monomers was achieved by the potentiostatic and galvanostatic methods on the passivated OL 37 surface electrode. The passivation of the OL 37 surface was performed in oxalic acid solution by the cyclic voltammetry method at a range from −0.4 V to 1.5 V vs. SCE at 20 mV/s and for four cycles (this procedure was presented in previous works [7,47,51]). The insoluble elements obtained by the passivation action are complexes of iron oxides and iron oxalates such as Fe (Ox) (FeO, Fe₂O₃) and FeC₂O₄ that prevent the dissolution of the material without blocking the electrodeposition [7,34,35,36,37,38,39,40]. By improving the polymerization attributes (parameters) we can obtain the P3MPY-SDS/PEDOT composite which has a remarkable protective result. The electropolymerization of 3MPY and EDOT monomers has been realized on passivated OL 37 substrate. After the passivation action, the electrochemical polymerization process of the monomers was carried out without affecting the polymerization characteristics. Deposition of a successful coating requires the acquisition of a passivated layer that is substantial in stopping the dissolution of the oxidizable metal without impeding the admission of monomers and its further (following) oxidation [7,38,39,40,47,48,49,50,51]. The final result of electrodeposition is a homogeneous, compact, and adherent coating. The P3MPY-SDS/PEDOT composite covering was accomplished from 0.3 M oxalic acid, 0.1 M 3-methylpyrrole, 0.02 M sodium dodecylsulfate, and 0.1 M 3,4-ethylenedioxythiophene by the potentiostatic process at 1.0 V, 1.2 V, and 1.4 V applied potentials and by a galvanostatic procedure at current densities 3 mA/cm² and 5 mA/cm² and in certain molar ratios.

Electrochemical polymerization of the monomers was allowed for 10 and 20 min. From Figure 1 one can see the “current density–time” curves of the formation of P3MPY-SDS/PEDOT composite coatings on OL 37 at 1.0 V, 1.1 V, and 1.4 V applied potentials in various molar ratios. After 600 s and 1200 s of oxidation, the primary shape of the “current density–time” graph while assessing the electrodeposition of the polymer indicates that this deposition was initiated by “nucleation and growth” on the OL 37 substrate. At startup, the current suddenly decreased due to the electroadsorption of the H₂C₂O₄ and 3MPY-EDOT monomers. After almost 40 s, the current increased and this fact is due to the dissolution of the passive layer and development of the polymer on the OL 37 surface. At the end of the electrochemical oxidation process, the current remained steady (constant) as the polymer layer was made on the OL 37 substrate. The transient change in the upper part is linear with the gradient of the nucleation interval, the electrochemical inspection, and the difference in the molar ratio of 3MPY: EDOT. At an application potential of 1.0 V, 1.2 V, and 1.4 V at 5:3 and 3:5 molar ratios, the current is about constant at 2 mA/cm² and 4 mA/cm², the current was greater than in other potentiostatic electrochemical deposition conditions, and the composite layer attained was homogeneous, dense, and adhesive (adherent). It can be noted from Figure 1 that the potentiostatic electrochemical deposition at 1.0 V and 1.2 V applied potentials at a 3:5 3MPY:2MT molar ratio has a longer induction range for acquiring of P3MPY-SDS/PEDOT coating and was less advantageous for the development of the polymer with superior attributes. Electrochemical characteristics, including the applied potential, have been found to have a large effect on the “induction time”. It can be remarked from Figure 1 that the composite covering obtained at 1.2 V and 1.4 V applied potentials in 5:3 and 3:5 molar ratios (3MPY-SDS: EDOT) has a short induction range for the development of the electrodeposited layer and was estimated to achieve the best-quality coating. The surfactant SDS as a dopant ion employed in electropolymerization (in 3MPY) can have a substantial effect on ion broadening selectivity by providing polymer conductivity. Visional examination of the OL 37 substrate following electrochemical polymerization indicates the buildup of a black layer of P3MPY-SDS/PEDOT. The coating is compact, homogeneous, and adherent to the OL 37 substrate. Coating adhesion as assessed by the “standard sellotape test” was evaluated to be ~85%.

Figure 2 shows the “potential–time” curves of poly (3-methylpyrrole-sodium dodecyl sulfate/3,4-ethylenedioxythiophene) polymer film on the OL 37 surface (P3MPY-SDS/PEDOT) at various current densities and at varied molar ratios. Through the electrochemical oxidation range of 600 s and 1200 s, the initial shape of the “potential–time” curves through the electrodeposition process of the polymer shows that the composite coating was accomplished by “nucleation and growth” on OL 37 substrate. In Figure 2, at a current density of 3 mA/cm² and 5 mA/cm² at certain molar ratios, an induction range of less than 5 s was noticed in the deposition of the composite of 3MPY-SDS: 2MT and its value was decreased by raising the molar ratio and increasing the “nucleation potential”. The electropolymerization (deposition) potential is different between 0.6 V, 0.7 V, and 0.98 V compared to SCE for current densities 3 mA/cm² and 5 mA/cm² in 5:3 and 3:5 molar ratios for 3MPY-SDS and EDOT. Composite coatings are the most homogeneous and adherent at current densities of about 0.5 mA/cm². Visual examination of the OL 37 sample covered with a black layer of P3MPY-SDS/PEDOT was carried out. Coating adherence assessed by the “standard sellotape test” was approximately ~80%–85%.

3.2. Electroanalysis of P3MPY-SDS/PEDOT Composite Coating

Figure 3 shows the “electrochemical” improvement of P3MPY-SDS/PEDOT/OL 37 in 0.3 M H₂C₂O₄ (no monomer) at the potential interval of −0.4 to +1.5 V vs. SCE and a scan speed of 20 mV/s. The cyclic voltammetry process was analyzed in the extended potential interval to consider all the “physical and electrochemical” characteristics of this composite layer. In Figure 3, it can be affirmed that the electrochemical behavior of the coating is impacted by the number of cycles and the parameters of electrochemical deposition. The stability of any conductive polymer in a “reduced and oxidized” form is a significant attribute for many procedures. The fundamental element that determines the “lifetime of a conducting polymer” is the permanent chemical presence of the matrix itself [7,32,38,39,40,47]. The strength of the P3MPY-SDS/PEDOT composite was considered by cyclic voltammetry (no monomer) in oxalic acid (see Figure 3).

As a result, this polymer composite can be repeatedly transitioned between its “oxidized” and “reduced” states without substantial deterioration. The current density declines with each cycle and eventually reaches a stable value. The polymer composite that exhibits a minimal reduction in current density during repeated cycles demonstrates remarkable electrochemical stability.

3.3. FT-IR Explorations

The new P3MPY-SDS/PEDOT/OL 37 composite was analyzed by FT-IR spectroscopy (see Figure 4) in the interval 4000–650 cm⁻¹ at a resolution of 4 cm⁻¹. FT-IR mechanisms can be used to highlight the type of bonding utilized to accomplish a new composite. The characteristic peaks in the transmittance spectrum of P3MPY-SDS/PEDOT/OL 37 are exhibited in graph 4. The FT-IR tests report the apparition of notable “absorption bands” observed in P3MPY and PEDOT electrochemically deposited on the OL 37 surface. The data of FT-IR determination are comparable to the opinions related in [37,38,39,40,41,42,46,47,50,51,52] where corresponding peaks were observed. The obvious bands in the spectra of 3MPY and EDOT that are introduced in Figure 4a,b is the result: the spectrum of 3MPY shown in Figure 4a, where the appreciable bands for the aromatic ring in 3MPY are located at 1568 and 1451 cm⁻¹ for C=C “stretching”, is evidently remarkable. The characteristic peaks at 1378 and 2941 cm⁻¹ are assigned to the N-H “stretching vibration” of the pyrrole ring and the CH₃ stretch of the 3-methylpyrrole moieties. The peaks that can be assigned as “in-plane and out-of-plane” of the CH chains at 1107, 1071, and 673 cm⁻¹ are displayed in the polymer. In Figure 4b the EDOT spectrum is displayed, and the peaks at 1683 cm⁻¹ and 1547 cm⁻¹ are assigned to the C-C and C=C stretch of the “quinoidal structure” of the “thiophene ring”. Vibrational bands typical of the ethylenedioxy group are indicated at 1224 cm⁻¹ and 1082 cm⁻¹, which are related to the C-O-C stretch. The bands at 980 cm⁻¹, 826 cm⁻¹, and 669 cm⁻¹ indicate the C-S “stretching vibration” of the “thiophene ring”. The P3MPY-SDS/PEDOT/OL 37 deposition spectra by “potentiostatic and galvanostatic” techniques are indicated in Figure 4c–e. The peaks present at 3403 and 3201 cm⁻¹ are related to the N-H “stretching vibration” in the polymer. The small peaks exhibited at 3503 and 3422 cm⁻¹ correspond to the OH stretching of the counterions. The peaks from 3117–2927 cm⁻¹ are associated with the CH₃ stretching of 3-methylpyrrole elements. The band observed at 1241 cm⁻¹ depicts the C-N of the pyrrole ring. Significant assignments of the “aromatic ring” peaks in P3MPY are noticeable at 1569 and 1430 cm⁻¹ for C=C stretch, being evidently established.

The “absorption bands” situated at 1598 and 1430 cm⁻¹ are described as the “stretching vibration of the quinoid rings” (Figure 4c–f). The correlated peaks at 1396 and 1316 cm⁻¹ are allocated to N-H “stretching vibration” of the methylpyrrole ring; the interval from 1660 to 1620 cm⁻¹ is related to the C=C stretching. In P3MPY-SDS/PEDOT deposition, spectra specific for asymmetric and symmetric C=C stretching vibrations of the quinoidal structure of the thiophene ring are shown at 1598 cm⁻¹ and 1443 cm⁻¹ (Figure 4c–f) [37,38,39,40,41,42,46,47,48,49,50,51,52]. The peaks appearing at approximately 1052 cm⁻¹ and 761 cm⁻¹ represent the C-S-C stretching vibration of the “thiophene ring” and the peak at 1528 cm⁻¹ is correlated to the C=C “stretching vibration” (Figure 4c–f). The existence of bands at 1221 cm⁻¹ and 1079 cm⁻¹ corresponds to the “stretching modes” of the ethylenedioxy group in PEDOT. The peaks at 1430 and 1316 cm⁻¹ are correlated with the “stretching vibration” of the CH₂ and CH₃ constituents of the surfactant and the peaks present at 1082 cm⁻¹ and 667 cm⁻¹ are assigned to the S=O “stretching vibration” (of anionic surfactant). The presence of C=O and CH bond is designated by “stretching vibration” at 1683 cm⁻¹ and 1249 cm⁻¹, which were supposed to be connected to the development of surfactant in the “polymer matrix” (Figure 4c–f). The presence of bands at around 1050–760 cm⁻¹ corresponds to the “in-plane” and “out-of-plane” C-H of the aromatic rings and to “out-of-plane” vibration of C-H doping of the PMPY polymer with oxalic acid solution (Figure 4c–f) [37,38,39,40,41,42,46,47,48,49,50,51,52]. Analyzing Figure 4a,b with Figure 4c–f, it can be supposed that the P3MPY-SDS/PEDOT coating is deposited on the OL 37 electrode. The presented monomer bands (MPY and EDOT) existed in the spectrum of the P3MPY-SDS/PEDOT composite coating on the carbon steel substrate.

3.4. Electrochemical Examination

3.4.1. Potentiodynamic Polarization Procedure

The protective performance of P3MPY-SDS/PEDOT polymeric composite was analyzed in 0.5 M H₂SO₄ by a potentiodynamic polarization process and electrochemical impedance spectroscopy. The polarization curves of bare and coated P3MPY-SDS/PEDOT/OL 37 in 0.5 M H₂SO₄ are shown in Figure 5 and Figure 6. The polarization behavior of the OL 37 sample was examined using the P3MPY-SDS/PEDOT composite layer prepared by potentiostatic and galvanostatic practices at some current densities and potentials in certain amounts and for distinct electrodeposition times.

In this study, we considered that one of the best methods of protecting OL 37 steel in corrosive environments is the use of composite polymer films where they were tested for corrosion by anodic or cathodic processes or both. The surfaces coated with the P3MPY-SDS/PEDOT composite showed a remarkable diminution of the cathodic and anodic currents, which resulted in the reduction of the cathodic and anodic processes. The electrochemical measurements were performed in 0.5 M H₂SO₄ medium to determine the corrosion protection performance of the polymer composite. Figure 5 and Figure 6 showed that both the anodic dissolution of metal and the cathodic reduction of hydrogen were hindered by these P3MPY-SDS/PEDOT composite coatings in the aggressive solutions. This phenomenon divulged that this coating had appreciable activity of cathodic and anodic mechanisms of the electrochemical application. The Tafel branches of the anodic and cathodic curves of OL 37 substrate covered by the P3MPY-SDS/PEDOT composite were extrapolated to the corrosion potential (E_corr) and the corrosion current density (i_corr) and anodic and cathodic Tafel slopes (b_a, b_c) were considered. All electrochemical corrosion parameters are presented in

Table 1. Corrosion kinetic parameters of uncoated and P3MPY-SDS/PEDOT-coated OL 37 samples (by galvanostatic procedure) in 0.5 M H₂SO₄ solution at 25 °C.

The System P3MPY-SDS/PEDOT OL 37	Ecorr (mV)	icorr (mA/cm2)	Rp Ω cm2	Rmpy	Pmm/year	Kg (g/m2h)	ba (mV/ decade)	−bc (mV/ decade)	E (%)	%P
OL 37 + 0.5 M H2SO4	−507	0.902	14	419	10.63	9.45	98	95	-
P3MPY-SDS/PEDOT 3 mA/cm2 3:5 molar ratio, t = 10 min	−487	0.118	114	55.06	1.39	1.24	77	98	87	0.06
P3MPY-SDS/PEDOT 3 mA/cm2 5:3 molar ratio, t = 10 min	−454	0.096	119	44.8	1.13	1.01	63	82	90	0.08
P3MPY-SDS/PEDOT 3 mA/cm2 3:5 molar ratio, t = 20 min	−458	0.114	116	53.2	1.35	1.20	68	88	88	0.02
P3MPY-SDS/PEDOT 3 mA/cm2 5:3 molar ratio, t = 20 min	−481	0.076	180	35.46	0.90	0.80	64	103	92	0.009
P3MPY-SDS/PEDOT 5 mA/cm2 3:5 molar ratio, t = 10 min	−457	0.121	127	54.46	1.43	1.22	75	109	87	0.02
P3MPY-SDS/PEDOT 5 mA/cm2 5:3 molar ratio, t = 10 min	−487	0.087	162	40.6	1.03	0.91	83	97	91	0.04
P3MPY-SDS/PEDOT 5 mA/cm2 3:5 molar ratio, t = 20 min	−488	0.085	168	39.66	1.01	0.89	80	101	91	0.04
P3MPY-SDS/PEDOT 5 mA/cm2 5:3 molar ratio, t = 20 min	−449	0.041	304	19.13	1.48	0.43	65	96	95	0.008

,

Table 2. Corrosion kinetic parameters of uncoated and P3MPY-SDS/PEDOT-coated OL 37 samples (by potentiostatic procedure) in 0.5 M H₂SO₄ solution at 25 °C.

The System P3MPY-SDS/PEDOT OL 37	Ecorr (mV)	icorr (mA/cm2)	Rp Ω cm2	Rmpy	Pmm/year	Kg (g/m2h)	ba (mV/ decade)	−bc (mV/ decade)	E (%)	%P
OL 37 + 0.5 M H2SO4	−507	0.902	14	419	10.63	9.45	98	95	-	-
P3MPY-SDS/PEDOT 1.0 V 3:5 molar ratio, t = 10 min	−500	0.16	90	74.6	1.89	1.68	88	93	82	0.095
P3MPY-SDS/PEDOT 1.0 V 5:3 molar ratio, t = 10 min	−499	0.063	240	29.4	0.74	0.66	96	102	92	0.024
P3MPY-SDS/PEDOT 1.0 V 3:5 molar ratio, t = 20 min	−497	0.144	106	67.2	1.70	1.51	101	103	85	0.07
P3MPY-SDS/PEDOT 1.0 V 5:3 molar ratio, t = 20 min	−503	0.027	586	13.1	0.33	0.29	94	108	97	0.001
P3MPY-SDS/PEDOT 1.2 V 3:5 molar ratio, t = 10 min	−467	0.18	89	84	2.13	1.89	77	78	80	0.052
P3MPY-SDS/PEDOT 1.2 V 5:3 molar ratio, t = 10 min	−502	0.074	230	34.5	0.87	0.77	99	105	92	0.005
P3MPY-SDS/PEDOT 1.2 V 3:5 molar ratio, t = 20 min	−448	0.081	190	37.8	0.95	0.85	89	85	91	0.014
P3MPY-SDS/PEDOT 1.2 V 5:3 molar ratio, t = 20 min	−494	0.052	327	24.2	0.61	0.54	92	104	94	0.07
P3MPY-SDS/PEDOT 1.4 V 3:5 molar ratio, t = 10 min	−408	0.059	212	27.5	0.69	0.62	62	84	93	0.006
P3MPY-SDS/PEDOT 1.4 V 5:3 molar ratio, t = 10 min	−487	0.046	300	21.4	0.54	0.48	69	107	95	0.002
P3MPY-SDS/PEDOT 1.4 V 3:5 molar ratio, t = 20 min	−496	0.17	89	79.3	2.01	1.79	76	87	81	0.08
P3MPY-SDS/PEDOT 1.4 V 5:3 molar ratio, t = 20 min	−481	0.039	379	18.2	0.46	0.41	101	87	96	0.001

and

Table 3. Corrosion kinetic characteristics of the P3MPY-SDS/PEDOT-covered OL 37 in 0.5 M H₂SO₄ solutions at 25 °C at different immersion periods.

The P3MPY-SDS/PEDOT/OL 37	Ecorr (mV)	icorr (µA/cm2)	Rp Ω cm2	Rmpy	Pmm/year	Kg (g/m2h)	ba (mV/ decade)	−bc (mV/ decade)	E (%)
P3MPY-SDS/PEDOT 1.0 V 5:3 molar ratio, t = 20 min, 0 h	−503	0.028	587	13.1	0.33	0.29	94	108	97
P3MPY-SDS/PEDOT 1.0 V 5:3 molar ratio, t = 20 min 24 h	−551	0.020	684	9.38	0.24	0.22	101	102	98
P3MPY-SDS/PEDOT 1.0 V 5:3 molar ratio, t = 20 min 48 h	−574	0.029	617	13.53	0.34	0.30	105	107	97
P3MPY-SDS/PEDOT 1.0 V 5:3 molar ratio, t = 20 min 72 h	−573	0.036	544	16.81	0.43	0.38	106	93	96
P3MPY-SDS/PEDOT 1.0 V 5:3 molar ratio, t = 20 min 96 h	−572	0.045	370	21	0.53	0.47	107	103	95
P3MPY-SDS/PEDOT 1.0 V 5:3 molar ratio, t = 20 min 120 h	−581	0.055	301	25.66	0.65	0.58	113	114	94
P3MPY-SDS/PEDOT 1.0 V 5:3 molar ratio, t = 20 min 144 h	−580	0.061	291	28.46	0.72	0.64	114	116	93

. From these polarization curves it can be observed that the corrosion potential of the protected carbon steel substrate is shifted to a more positive potential in comparison with the bare OL 37. These circumstances could be caused by the attack of corrosive components that reach the pores of the layer as a result of the building of passive films that stopped the corrosion of OL 37 substrate.

The analysis of the polarization curves in Figure 5 and Figure 6 and Table 1 and Table 2 indicated that the electrochemical characteristics of uncovered and covered OL 37 at applied potentials of 1.0 V, 1.2 V, and 1.4 V and 3 mA/cm² and 5 mA/cm² current densities at 10 min and 20 min for 5:3 and 3:5 molar ratios of P3MPY-SDS/PEDOT were inferior for carbon steel in 0.5 M H₂SO₄ solution.

This P3MPY-SDS/PEDOT coating has the best protective activity because the P3MPY polymer was doped with a large anionic surfactant. The SDS surfactant as a dopant ion used in deposition can lead to a significant outcome by modifying the ion selectivity by providing polymer conductivity [7,8,24,33,38,41,47]. We can assume that the presence of the large hydrocarbon chain of the anionic surfactant that “competitively adsorbs” on the carbon steel surface, obstructing the active sites and as a consequence the SO₄²⁻ corrosive factor, is blocked from damaging the OL 37 electrode and, in this case, the protective activity is carried out. The obtained results demonstrated that the corrosion rate of P3MPY-SDS/PEDOT-coated OL 37 was around ~9 times lower than what was noted for OL 37. Consequently, we can notice that these composite coatings blocked the action of aggressive factors (H₂SO₄) on carbon steel substrate.

The results of the protective effect of these coatings during immersion are presented in Figure 7 and Table 3. By potentiodynamic polarization, the action of raising the immersion time from 0–144 h on the corrosion inhibition of P3MPY-SDS/PEDOT coatings on OL 37 in 0.5 M H₂SO₄ was analyzed. The protective performance decreases over time. After 96 h of immersion time, a small increase in the corrosion rate is shown. The decrease in protective performance is caused by the degradation of the substrate morphology by increasing submersion as an effect of the alteration of the reactive substrate and may be produced by the presence of imperfections on the protective covering that permit access of corrosive ions to the OL 37/composite interface. It was observed that the P3MPY-SDS/PEDOT layer has significantly higher corrosion protection compared to the uncoated samples. It was considered that the carbon steel surface had a significant action on the electrochemical characteristics of the covering films, and these coated electrodes were realized by the concomitant constitution of a complex oxide film and composite coverings. Examining Figure 5, Figure 6 and Figure 7 and Table 1, Table 2 and Table 3, it is stated that the least corrosion rate and the greatest protective capacity were provided by P3MPY-SDS/PEDOT covering at 1.4 V, 1.2 V, and 1.0 V at a 5:3 molar ratio, t = 20 min, at 5 mA/cm² and 3 mA/cm² applied current density and very high protection was realized from 1.0–1.4 V (at 3:5 molar ratio, t = 10 min) and 3 mA/cm² (at 3:5 molar ratio, t = 10 min) compared to the uncovered substrate in 0.5 M H₂SO₄ corrosive solution.

The corrosion action of uncovered OL 37 and covered P3MPY-SDS/PEDOT in H₂SO₄ solution can be presented as follows [7,17,26,37,38,47]:

Anodic reaction:

Dissolution of Metal (M = OL 37)

M → Mⁿ⁺ + ne⁻

P3MPY_undoped − ne⁻ → P3MPY_doped

PEDOT_undoped − ne⁻ → PEDOT_doped

Cathodic reaction:

Oxygen reduction:

$\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.O_2+H_{2}O+2e\to 2H{O}^{-}$

2H⁺ + 2e⁻ → H₂

P3MPY_doped + ne⁻ → P3MPY_undoped

PEDOT_doped + ne⁻ → PEDOT_undoped

Chemical reactions:

M²⁺ + 2OH⁻ → M (OH)₂ → M(OH)₃ → M₂O₃

In acidic environments, the metal is oxidized to a higher valence in the action of anodic dissolution. The products, dissolved in acidic environments as oxygen, have hydrogen ions reduced by electrons provided by the metal during cathodic activity.

The porosity (P) of the composite layer/P3MPY-SDS/PEDOT (see Table 1 and Table 2) is a special parameter that must be assessed when a composite layer is applicable or not to protect the surface to prevent corrosion. The porosity of the coverings was analyzed by the following correlation:

$$P={\displaystyle \frac{\mathrm{Rp}\left(\mathrm{uncoated}\right)}{\mathrm{Rp}\left(\mathrm{coated}\right)}}{10}^{-\left(\left|\Delta E_{corr}\right|/{\beta }_a\right)}$$

(1)

where:

• P = film porosity (dimensionless),
• Rp = polarization resistance (Ω cm²),
• ΔE_corr = the difference between corrosion potential for covered and uncovered sample (mV),
• β_a = anodic Tafel slope for OL 37 (mV).

The porosities of P3MPY-SDS/PEDOT-coated carbon steel by electrodeposition are 0.001, 0.002, 0.005, 0.006, 0.008, and 0.01 (at 1.4 V, 1.0 V, and 1.2 V potential applied and at 3 mA/cm² and 5 mA/cm² current density and in 5:3 molar ratio, t = 20 min). The significant magnitude of porosity in the P3MPY-SDS/PEDOT films indicates an important development in the defensive activity by blocking the access of the aggressive element (H₂SO₄) to the OL 37 substrate and also decreases the corrosion of the underlying OL 37. It was concluded that the P3MPY-SDS/PEDOT coverings reveal a lower porosity value, suggesting that the P3MPY-SDS/PEDOT layer is a compact and uniform design of the coatings.

3.4.2. Electrochemical Impedance Spectroscopy (EIS) Studies

The corrosion-protective performance of P3MPY-SDS/PEDOT covering on OL 37 in H₂SO₄ solution was considered by electrochemical impedance spectroscopy (EIS). Experimental impedance tests were performed at open circuit potential in the frequency range from 100 KHz to 40 mHz with an AC wave of ± 10 mV (peak-to-peak). The applied voltage was set to 10 mV, with ten data points collected per logarithmic decade. EIS tests contribute to understanding the defensive characteristics of the new P3MPY-SDS/PEDOT covering as a corrosion defense layer. Figure 8a–e show the Nyquist impedance determinations obtained for the OL 37 substrate coated with P3MPY-SDS/PEDOT and for the OL 37 in H₂SO₄ medium. From Figure 8, it can be seen the Nyquist graphs for the OL 37 electrode revealed a small capacitive loop, determining that the charge transfer action was dominant throughout corrosion activity. Therefore, the dimensions of the capacitance loops of the composites are wider than that of the uncoated substrate and the dimensions of these loops are enhanced with the improvement of the coverings, showing that these P3MPY-SDS/PEDOT coatings have greater protective effects on the sample in H₂SO₄. From the Nyquist graphs it can be remarked that the OL 37 impedance behavior was substantially modified by the composite coverings, demonstrating that the acquired defensive layer was instituted by using the P3MPY-SDS/PEDOT polymeric composite. We can say that these capacitive loops are not exact semicircles and this configuration is assigned to frequency dispersion, specifically attributed at the rugoses and inhomogeneities of the carbon steel surface [7,36,37,38,41,42,43,44,45,46,47,50,51,52].

Figure 8 indicates that the dimensions of the capacitance loops for coatings made at 1.4 V and 1.2 V applied potential and at 3 mA/cm² and 5 mA/cm² current densities at 5:3 and 3:5 molar ratios (for 10 and 20 min) are considerable in comparison with OL 37, suggesting a remarkable defensive effect in corrosive solutions of the OL 37. It is evident from Figure 8 that the semicircle dimensions of the P3MPY-SDS/PEDOT composite achieved by 1.0 V, 1.2 V, and 1.4 V applied potentials are more considerable than those achieved by the galvanostatic method at 3 mA/cm² and 5 mA/cm² current densities (in molar ratios of 5:3 and 3:5). As a consequence, the protective action of this composite covering is better. Evaluating the experimental results in Figure 8 and

Table 4. EIS parameters of coated by potentiostatic method and uncoated OL 37 in 0.5 M H₂SO₄ solutions at 25 °C.

The System P3MPY-SDS/PEDOT/OL 37	Rs ohm·cm2	Q-Yo S·s−n·cm−2	Q-n	Rf ohm·cm2	Q-Yo S·s−n·cm−2	Q-n	Rct ohm·cm2	χ
OL 37 + 0.5 M H2SO4	1.03	0.0014	0.99	1.3	0.0026	0.75	11.6	5.7 × 10−3
P3MPY-SDS/PEDOT 1.0 V 3:5 molar ratio, t = 10 min	6.68	0.000036	0.98	21	0.00040	0.64	78	2.31 × 10−4
P3MPY-SDS/PEDOT 1.0 V 5:3 molar ratio, t = 10 min	18.6	0.000055	0.63	46	0.000416	0.64	184	4.24 × 10−4
P3MPY-SDS/PEDOT 1.0 V 3:5 molar ratio, t = 20 min	14.5	0.000531	0.60	28	0.000274	0.71	112	7.97 × 10−5
P3MPY-SDS/PEDOT 1.0 V 5:3 molar ratio, t = 20 min	22.5	0.000025	0.61	64	0.000074	0.62	522	1.40 × 10−4
P3MPY-SDS/PEDOT 1.2 V 3:5 molar ratio, t = 10 min	1.3	0.000320	0.91	22	0.000631	0.88	56	7.58 × 10−4
P3MPY-SDS/PEDOT 1.2 V 5:3 molar ratio, t = 10 min	10.36	0.000015	0.71	42	0.000091	0.68	260	2.27 x10−4
P3MPY-SDS/PEDOT 1.2 V 3:5 molar ratio, t = 20 min	14.5	0.000541	0.61	32	0.000264	0.72	110	8.18 × 10−5
P3MPY-SDS/PEDOT 1.2 V 5:3 molar ratio, t = 20 min	6.5	0.000009	0.91	38	0.000094	0.74	120	1.61 × 10−4
P3MPY-SDS/PEDOT 1.4 V 3:5 molar ratio, t = 10 min	2.46	0.000462	0.72	24	0.00008	0.92	58	5.17 × 10−4
P3MPY-SDS/PEDOT 1.4 V 5:3 molar ratio, t = 10 min	2.6	0.000039	0.64	52	0.000031	0.77	240	1.89 × 10−4
P3MPY-SDS/PEDOT 1.4 V 3:5 molar ratio, t = 20 min	1.38	0.000895	0.83	60	0.000283	0.98	108	1.33 × 10−3
P3MPY-SDS/PEDOT 1.4 5:3 molar ratio, t = 20 min	6.4	0.000093	0.64	34	0.000130	0.70	347	8.79 × 10−4

and

Table 5. EIS parameters of coated by galvanostatic method and uncoated OL 37 in 0.5 M H₂SO₄ solutions at 25 °C.

The System P3MPY-SDS/PEDOT/OL 37	Rs ohm·cm2	Q-Yo S·s−n·cm−2	Q-n	Rf ohm·cm2	Q-Yo S·s−n·cm−2	Q-n	Rct ohm·cm2	χ
OL 37 + 0.5 M H2SO4	1.03	0.0014	0.99	1.3	0.0026	0.75	11.6	5.71 × 10−3
P3MPY-SDS/PEDOT 3 mA/cm2 3:5 molar ratio, t = 10 min	6.09	0.00016	0.74	26	0.00010	0.78	72	3.81 × 10−4
P3MPY-SDS/PEDOT 3 mA/cm2 5:3 molar ratio, t = 10 min	5.6	0.000224	0.83	40	0.00017	0.72	94	1.57 × 10−3
P3MPY-SDS/PEDOT 3 mA/cm2 3:5 molar ratio, t = 20 min	3.82	0.000203	8.22	28	0.000194	8.85	66	1.27 × 10−3
P3MPY-SDS/PEDOT 3 mA/cm2 5:3 molar ratio, t = 20 min	6.2	0.000072	0.77	32	0.000065	0.70	150	7.53 × 10−4
P3MPY-SDS/PEDOT 5 mA/cm2 3:5 molar ratio, t = 10 min	2.72	0.00014	0.89	26	0.00012	0.83	88	2.05 × 10−3
P3MPY-SDS/PEDOT 5 mA/cm2 5:3 molar ratio, t = 10 min	7.24	0.000484	0.61	42	0.000496	0.98	104	2.48 × 10−4
P3MPY-SDS/PEDOT 5 mA/cm2 3:5 molar ratio, t = 20 min	4.49	0.000151	0.85	28	0.0001342	0.92	60	2.38 × 10−3
P3MPY-SDS/PEDOT 5 mA/cm2 5:3 molar ratio, t = 20 min	8.52	0.000071	0.72	44	0.000097	0.68	120	1.08 × 10−4

, it could be presumed that the P3MPY-SDS/PEDOT coatings behaved as an active physical barrier that obstructed the access of the aggressive agents to the composite coverings, reducing the charge transfer and, ultimately, blocking the corrosion activity.

The Bode data of the P3MPY-SDS/PEDOT coating (Figure 9) indicated that the impedance modulus, at low frequencies, increases with the growth of this composite, revealing that the defensive layer improves the protective effect of the OL 37 in acidic environments. It can be observed in Figure 9 that the OL 37 sample presents an approximate time constant at a phase angle of 38° at medium and low frequencies, indicating an inductive behavior through low diffusion. The Bode plots in Figure 9 indicate that the composite’s phase angle versus the frequency logarithm presents a maximum at 60–65° which is correlated with a relaxation time constant, which represents a capacitive behavior with easy diffusion. Also, in these situations the coated OL 37 substrates possess good capacitive behavior in agreement with the Nyquist tests and investigation results obtained through potentiodynamic polarization. The rise in Z_mod shows a substantial defensive effect and it is evident that Z_mod increases when the composite layer is perfected. A higher Z will result in better corrosion protection. The impedance determinations were demonstrated by fitting the data to the corresponding equivalent circuit shown in Figure 10 and several impedance attributes, such as the solution resistance (R_s), resistance of coating film (R_f), capacitance of coating film (C_f), charge transfer resistance (R_ct), capacitance of double layer (C_dl), and defensive capacity, are indicated in Table 4 and Table 5. In this exploration, a frequency interval equivalent circuit model was involved and improved to fit and take into account the achieved EIS results. In these circumstances, the constant phase element, CPE, is disclosed in the circuit as a replacement of a pure double layer capacitor (Cdl) to ensure a more precise match. The CPE is applied to appreciate the deformation of the capacitance semicircle, which ascribes the heterogeneity of the electrode from substrate roughness and impurities [7,36,37,38,41,42,43,44,45,46,47,50,51,52].

The impedance of the CPE can be given as: Z_CPE = Y₀⁻¹ (jω⁾⁻ⁿ, where ω is the “angular frequency”, “j” is the “imaginary number” (j² = −1), Y₀ is the amplitude corresponding to the capacitance, and “n” is the “phase shift”. The estimation of “n” gives elements of the inhomogeneity of the state of the metallic zone [7,36,37,38,41,42,43,44,45,46,47,50,51,52]. The best value of “n” is associated with a lesser degree of substrate roughness i.n., and the surface non-homogeneity is small (CPE can be resistance when n = 0, Y₀ = R, capacitance when n = 1, Y₀ = C, and inductance when n = −1, Y₀ = L or Warburg impedance when n = 0.5, Y₀ = W) [31,36,37,38,42,43,44,45,46,47,50,51,52].

Experimental EIS tests specify that the charge transfer resistance Rct increased and the double layer capacitance Cdl decreased by the presence of these P3MPY-SDS/PEDOT composite coatings. The EIS data highlight that, as Rct increases with the P3MPY-SDS/PEDOT improved layers, the protective action increases remarkably, which demonstrates that the composite coating presents considerable protection activity for OL 37. Reducing Cdl can be possible by reducing the local dielectric constant and/or increasing the thickness of the electrical double layer as a consequence of the phenomenon that the polymeric composites operate by adsorption at the surface of the OL 37/H₂SO₄ interface. In this exploration, the P3MPY-SDS/PEDOT coated on the OL 37 sample substrate by the electropolymerization procedure presents a defensive layer on the carbon steel sample. From the Nyquist and Bode diagrams (Figure 8 and Figure 9, Table 4 and Table 5) it can be seen that the corrosion process was blocked by the P3MPY-SDS/PEDOT composite layers and this phenomenon is executed as a diffusion barrier by a charge transfer operation.

3.5. SEM Exploration

The morphological composition of the P3MPY-SDS/PEDOT coating deposited on the surface of OL 37 was analyzed by scanning electron microscopy (SEM). SEM photographs of the electrodeposition of P3MPY-SDS/PEDOT coatings under some conditions over the OL 37 electrode are displayed in Figure 11. These micrographs of the P3MPY-SDS/PEDOT coatings obtained by potentiostatic and galvanostatic methods reveal a homogeneous and dense black layer of these composites made on the carbon steel substrate (see Figure 11c–i). SEM analysis indicated that the coatings displayed a granular morphology, characteristic of cauliflower-like structure, with uniform distribution and variations in particle size, as indicated in the specialized literature. The particle size in the coatings was found to range between 1 and 5 micrometers, indicative of successful polymer growth on the steel substrate. These SEM photographs of the P3MPY-SDS/PEDOT coatings indicated a dense and adherent film formed on the OL 37 substrate and the attributes of this film included no visible cracks, highlighting the high quality of the coating. The anionic surfactant SDS as a dopant incorporated into conducting polymers has a substantial action on the electrodeposition process and on the attributes of the new composite coating [7,49,50,51,52]. The superior coating and increased adsorption activity contribute to the exceptional protective effectiveness of the composite layer. EDS of the P3MPY-SDS/PEDOT-coated carbon steel substrate was considered and the spectra are displayed in Figure 11i–l. The existence of this composite on OL 37 is noticeable by the peaks of C, N, O, and S constituents in EDS spectra. The EDS results are in agreement with the FT-IR data of P3MPY-SDS/PEDOT coating in which the surfactant and oxalate ions are introduced into the polymer matrix. The use of a combined SEM-EDS approach provided a thorough understanding of both morphological and compositional changes on the sample surfaces, offering insights into the effects of environmental exposure and evaluating material performance under simulated conditions. According to an immersion period ranging from 0 and 144 h in acidic medium, an evident change in the morphology of the composite was found, as shown by electrochemical tests. From SEM photographs in Figure 11g–i, diffusion of corrosive (SO₄²⁻) ions in the P3MPY-SDS/PEDOT composite layer can be noticed. Additionally, elongated fragments, possibly residual deposits or impurities from the polymerization process, were observed. These impurities could potentially impact the homogeneity and electrochemical behavior of the coating, depending on its application. The morphological changes observed were linked to the immersion process, which caused slight degradation, or deposits.

4. Conclusions

• As revealed by SEM, electrochemical measurements, and adhesion tests, uniform, homogeneous, dense, and adherent P3MPY-SDS/PEDOT composite coatings were successfully deposited on the OL 37 substrate under various conditions in oxalic acid solution via electrodeposition—an efficient and cost-effective method widely used for corrosion protection. Electrochemical tests demonstrate that the P3MPY-SDS/PEDOT coating serves as a protective layer for OL 37 in a sulfuric acid medium. Furthermore, by optimizing the film composition and deposition conditions, the protective performance was enhanced and evaluated using potentiodynamic and EIS measurements in 0.5 M H₂SO₄.
• The OL 37 substrate coated with the P3MPY-SDS/PEDOT composite exhibits a corrosion rate approximately nine times lower than the uncoated sample, with a protective capacity exceeding 90%. The parameters of the anticorrosion protection of P3MPY-SDS/PEDOT are ranked as follows: 1.4 V > 1.2 V > 1.0 V > 5 mA/cm² > 3 mA/cm², indicating that these composite coatings significantly reduce corrosion activity.
• Evaluation of FT-IR spectra shows that the P3MPY-SDS/PEDOT layer is created on the OL 37 substrate. Owing to the perfection of the physical barrier action, the coating is uniform, compact through low porosity, and has greater inhibitive properties and, in conclusion, P3MPY-SDS/PEDOT composite films obtained potentiostatically at 1.4 V 5:3, t = 20 min are much better and more protective than those obtained at 1.0 V under the same conditions, and coatings obtained galvanostatically at 5 mA/cm² have higher protection efficiency than those at 3 mA/cm². The higher substrate coverage and superior adsorption activity also suggested a higher corrosion protection action of the coatings according to the experimental data presented.
• Composite coatings deposited at potentials between 1.4 V and 1.2 V offered superior corrosion protection compared to those deposited at current densities ranging from 3 mA/cm² to 5 mA/cm². In addition to deposition conditions, a 5:3 molar ratio and a longer deposition time, i.e., 20 min, produce more protective films compared to a 3:5 molar ratio and shorter deposition time (10 min).
• We can state that the composite coatings obtained under the presented conditions prevent corrosive attack on the carbon steel substrate and the new composite P3MPY-SDS/PEDOT obtained by electrochemical polymerization methods can be successfully used in technological practices for the protection of various metallic materials.