Aniline and 2-methylaniline are monomers, and an aqueous sodium tartrate solution was used as the supporting electrolyte. The LCS substrate was polished with polishing paper, cleaned, and washed with acetone and deionised water.

Initially, the polyaniline (PANI), poly-2-methylaniline (PMA), and poly(aniline-co-2-methylaniline) 1:1 copolymer (PAMA) coatings were synthesized on low-carbon steel (LCS) in an aqueous tartrate solution. During the overall synthesis process, the concentration of each monomer and tartrate solution was kept constant at 0.2 M. The electrochemical polymerization (ECP) was accomplished by using a three-electrode configuration, working electrode (LCS), counter electrode (platinum), and reference electrode (saturated calomel electrode (SCE)).

The synthesis was carried out under cyclic voltammetry by scanning the electrode potential in the range −0.5 to 1.5 V at a scan rate of 20 × 10⁻³ V/Sec. The CV was recorded using a PARSTAT 2363-1, EG and G, Princeton Applied Research (U.S.A.) in triplicate to ensure reproducibility.

Afterwards, the PAMA copolymer coatings were synthesized at various reaction temperatures by keeping the same experimental conditions as explained above, except for the temperature. The higher temperatures, i.e., 60˚C and 40˚C, were kept constant by a hot plate on a magnetic stirrer. While the lower temperature of 15˚C was maintained by an ice bath, 0˚C was retained by adding salt to the ice bath.

The coatings were characterized by cyclic voltammetry (CV), scanning electron microscopy (SEM) with a Leica (United Kingdom) Cambridge 440 microscope., and corrosion resistant characteristics are characterized by Tafel plots by scanning the electrode voltage in range −0.25 to 0.25 V at the scan rate of 2 × 10⁻³ V/s by PARSTAT 2363-1 in triplicate to ensure reproducibility.

The polyaniline (PANI), poly-2-methylaniline (PMA), and poly(aniline-co-2-methylaniline) 1:1 copolymer (PAMA) coatings were synthesized on LCS in an aqueous tartrate solution, showing the same cyclic voltammetry results as reported in Pawar etal. . The electroanalysis of the first scans of PAMA copolymer coatings at different reaction temperatures is shown in Figure 1. These cyclic voltammograms are characterized by three anodic peaks: A, B, and C. The anodic peak A represents the dissolution of the reactive LCS electrode surface, which produces Fe2+ ions in its vicinity. These ions interact with the tartrate counter-ions of the electrolyte to form insoluble iron (II) tartrate (FeC₄H₄O₆), which adheres to the electrode surface, thereby forming an iron tartrate film. Oxidation peak B represents the oxidation of the monomer(s) and formation of radical cations, which are rapidly consumed in subsequent reactions to yield dimers, trimers, tetramers, and so forth. Anodic peak C is assigned to the oxidation of the tartrate electrolyte .

Figure 1. First scan of CV recorded during the synthesis of PAMA copolymer coatings at different reaction temperatures.

Nevertheless, a critical observation of 1^st scan of the CV represents that the area of the anodic peak A of PAMA copolymer diverges substantially depending on the reaction temperature, shown in Figure 2 and it obeys the sequence 60˚C > 40˚C > 27˚C (AT) > 15˚C > 0˚C so highest for 60˚C and lowest for 0˚C while intermediate for AT. In the second scan of cyclic voltammograms of PAMA shown in Figure 3, recorded at different reaction temperatures, the anodic peak A which represents dissolution of LCS, is not observed. The growth of PAMA copolymer occurs in the second scan as current densities of oxidation peaks are increased, and it follows the order 60˚C > 40˚C > 27˚C > 15˚C > 0˚C as shown in Figure 4. After the second scan, the current density corresponding to the anodic peaks decreased gradually with the number of scans.

Figure 2. Variation area of the anodic peak A of the PAMA copolymer with reaction temperature.

Figure 3. A second scan of CV was recorded during the synthesis of PAMA copolymer coatings at different reaction temperatures.

Figure 4. Variation of anodic peak C of PAMA copolymer coatings with reaction temperature during the 2^nd scan of CV.

(a) 0˚C (b) 15˚C

(c) 27˚C (AT) (d) 40˚C

(e) 60˚C

Figure 5. The SEM images of PAMA coatings synthesized at reaction temperatures: (a) 0˚C, (b) 15˚C, (c) 27˚C (AT), (d) 40˚C, (e) 60˚C.

The SEM images of PAMA coatings synthesized at different reaction temperatures after completion of the 10^th scan shows that (Figure 5) at higher temperature such as 60˚C and 40˚C thick black colored porous nonuniform surface morphology, i.e., degradation in quality of film is observed it may result due to very high electric current densities of oxidation peaks outcome the hyper-oxidation of monomer. However, at AT, the surface morphology is uniform and featureless as the current densities of the peaks are intermediate. But at lower temperatures, such as 15˚C and 0˚C, the surface morphology is uniform and featureless, but visual observation shows that films are very thin and non-adhesive due to small current densities and do not show complete oxidation or reduction performance.

3.3.1. Corrosion Protection Performance of PANI, PMA, and PAMA Coatings

The Tafel plots recorded in aqueous 3% NaCl of PANI, PMA, and PAMA copolymer and uncoated LCS can be observed in Figure 6. The Corrosion Rate (CR) measured in mm/year for PANI (0.07), PMA (0.01), and PAMA (0.004), and uncoated LCS (0.20), and those are 3, 20, and 50 times lower than that observed for uncoated LCS. The considerable shift in E_corr is observed as compared to uncoated LCS, showing the order PAMA (−329 mV) > POT (−375 mV) > PANI (−506 mV) > uncoated LCS (−710 mV). Thus, this analysis indicates that the copolymers are more effective in protecting the LCS than the corresponding homopolymers, i.e., Polyaniline and Poly-2-methylaniline.

Figure 6. The Tafel plots of PANI, PMA, PAMA copolymer, and Bare LCS were recorded in aqueous 3% NaCl.

3.3.2. Corrosion Protection Performance of PAMA Coatings of Different Reaction Temperature

By taking previous results as a reference [19] and moving forward in it, PAMA copolymer coatings, which effectively protect the LCS, are synthesized at various reaction temperatures and evaluated for their corrosion-resistant characteristics in aqueous 3% NaCl solution by recording the Tafel plots as shown in Figure 7. The analysed values of E_corr, I_corr, and CR derived from the Tafel plot fitting are given in Table 1.

Figure 7. Tafel plots of PAMA copolymer coatings synthesized at different reaction temperatures recorded in aqueous 3% NaCl solution.

The Tafel plot analysis shows a positive shift in corrosion potential (Ecorr) and a significant decrease in corrosion current (Icorr) and CR of LCS due to these PAMA coatings at various temperatures. The corrosion potential (Ecorr) is function of reaction temperature and shows sequence 27˚C (AT) (−329 mV) > 0˚C (−369 mV) > 40˚C (−383 mV) > 15˚C (−390mV) > 60˚C (−413 mV) (with positive shift in Ecorr of (381, 341, 327, 320, 297) mV resp. as compared to uncoated LCS). In contrast, the corrosion rate of these coatings follows the order AT (0.004) > 40˚C (0.009) > 0˚C (0.0136) > 60˚C (0.014) > 15˚C (0.017), and a substantial reduction in CR 50, 22, 15, 14, 12 times, respectively. Compared to uncoated LCS. The hyper-oxidation at higher temperatures, 60˚C and 40˚C, disrupts the polymer’s conjugated structure, reducing its electrical conductivity, which is essential for effective corrosion protection. The polymer’s morphology becomes less ordered, leading to increased porosity and cracks and reduced barrier properties against corrosion. In comparison, the under-oxidation (at 15˚C and 0˚C) of conducting polymers during synthesis leads to incomplete doping and poor conductivity, which compromises the polymer’s ability to protect substrates from corrosion [28].

Thus, these corrosion performance results reveal that the PAMA copolymer coatings synthesized at ambient temperature show better corrosion protection characteristics than other reaction temperatures, with a maximum positive corrosion potential (E_corr) shift of 381 mV and also a maximum reduction in CR of 50 times as compared to uncoated LCS.

The detailed comparison of our findings with those reported in existing literature is represented in Table 2, which highlights the novelty and significance of our work, especially the identification of 27˚C as an optimal synthesis temperature for achieving enhanced corrosion protection.

Table 1.Analysis of corrosion protection performance results of PAMA copolymer synthesized at various reaction temperature in 3% NaCl.

Table 2. Comparison of the influence of synthesis temperature on corrosion resistance of conducting polymers with existing literature.