Two new effective corrosion inhibitors, namely N-(n-octyl)-3-methylpyridinium bromide (Py8) and N-(n-dodecyl)-3-methylpyridinium bromide (Py12), have been presented. The cationic pyridinium-based surfactants were analyzed for the corrosion protection of general purpose steel (EN3B) against a strong corrosive media (3.5% NaCl, pH 1.5). The results of the electrochemical measurements, i.e., Tafel polarization, linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) revealed a mixed-type behavior of both inhibitors, and the maximum inhibition efficiency (IE) achieved with Py8 and Py12 was 85% and 82%, respectively. The process of adsorption of synthesized inhibitors followed the Langmuir adsorption isotherm, and a higher value of K_ads highlighted the existence of strong interaction between inhibitors and the EN3B mild steel surface. Furthermore, the values of ΔG°_ads were calculated to be −32 kJ mol⁻¹ for Py8 and −33 kJ mol⁻¹ for Py12, indicating the coexistence of both physisorbed and chemisorbed molecules. The surface morphology of EN3B mild steel samples was observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM), where the reduced surface roughness in the presence of Py8 and Py12 in chloride media further supported the evidence of an efficient inhibition process. Density functional theory (DFT) calculations reveal excellent correlation with the experimental results, with Py8 showing superior corrosion inhibition potential, signifying that the alkyl chain length and intramolecular charge transfer are crucial factors in deciding the inhibition performance of the synthesized cationic surfactants. Furthermore, this study proposes the mechanism for the adsorption of the surfactant-based inhibitors over the EN3B mild steel surface, which leads to the formation of an effective and protective anticorrosive film.

This was due to the effect of the alkyl chain length on the critical micelle concentration (CMC), adsorption behavior, and corrosion inhibition of CR4 mild steel in an acidified chloride solution (3.5 wt.% NaCl, pH 1.5) of a series of pyridinium-based cationic surfactants, namely N-(n-octyl)-2-methylpyridinium bromide (MP8),N-(n-decyl)-2-methylpyridinium bromide (MP10), and N-(n-dodecyl)-2-methylpyridinium bromide (MP12). The synthesized surfactants were characterized by Fourier Transform Infrared (FT-IR) and Nuclear Magnetic Resonance (NMR) spectroscopy, and their critical micelle concentration values were determined using the conductivity measurement technique. Weight-loss measurements, Potentiodynamic polarization, Electrochemical Impedance Spectroscopy (EIS), Scanning Electron Microscope with Energy Dispersive X-ray Spectroscopy (SEM/EDX) and Density Functional Theory (DFT) calculations were used for the assessment of corrosion inhibition performance. The lower CMC was correlated with an increase in the length of the alkyl chains (MP8<MP10<MP12), and the inhibition efficiency also increased. Weight-loss tests showed that MP12 (90%), MP10 (89%) and MP8 (88%) gave better results for adsorption on the surface of steel for which a compact protective film was formed, and electrochemical tests (MP8 < MP10 < MP12) showed increasing the alkyl chain ;length from MP8 to MP12 increased the hydrophobic character of the surfactants, which promoted molecular aggregation at lower concentrations, decreased the CMC, enhanced adsorption and protective film formation on the steel surface, and consequently improved the corrosion inhibition performance and adsorption tests showed the same trend. The potentiodynamic polarization icorr (µA/cm2) results showed that the prepared surfactants inhibited in mixed type while by EIS measurements the charge transfer resistance Rct increased and double layer capacitance (Cdl) values were decreased as the inhibitor concentration increased. Surface deterioration results were confirmed by SEM/EDX test which showed less surface deterioration where the inhibitors were present. The adsorption sites were determined through (DFT) calculations assisted with the help of the Mulliken population analysis and Fukui indices and it was found that as the alkyl chain length increased the adsorption strength and corrosion inhibition was increased. The combined experimental and theoretical results obtained indicated that the critical micelle concentration (CMC) played a vital role in the mechanism of adsorption and corrosion inhibition and could help in the rational design of the surfactant-based corrosion inhibitors.