Coatings 2022, 12, 1701. https://doi.org/10.3390/coatings12111701 www.mdpi.com/journal/coatings Article Evaluating the Corrosion Inhibition Efficiency of Pyridinium-Based Cationic Surfactants for EN3B Mild Steel in Acidic-Chloride Media Rabia Talat 1 , Muhammad Adeel Asghar 1, *, Irsa Tariq 1 , Zareen Akhter 1 , Faroha Liaqat 1 , Laiba Nadeem 1 , Ali Haider 1,2, * and Saqib Ali 1,2, * 1 Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan 2 Pakistan Academy of Sciences, 3-Constitution Avenue Sector G-5/2, Islamabad 44000, Pakistan * Correspondence: madeel@chem.qau.edu.pk (M.A.A.); ahaider@qau.edu.pk (A.H.); saqibali@qau.edu.pk (S.A.) Abstract: 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 im- pedance 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 Kads 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 sur- face 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. Den- sity 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 syn- thesized 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. Keywords: anticorrosion; corrosion protection; charge transfer resistance; langmuir adsorption; 3- methylpyridine; DFT 1. Introduction The corrosion process is a gradual destruction of metal surfaces owing to attack by chemical or electrochemical environments. It is the oxidative degradation of metals in the presence of oxidants such as oxygen or sulphur, dissolution of metals, or oftentimes, the reprecipitation of corrosive products to the surface [1,2]. Corrosion badly affects the char- acteristic properties of metals, and the economic effects are also well reported. About 3– 4% of the GDP of industrialized countries is spent dealing with the damages and losses of corrosion in industrial, infrastructure and household sectors [3]. Mild steel can be regarded as the most widely used engineering material, having major industrial importance, with particular applications in the fields of construction, pe- troleum (production and refining), metal-processing equipment, chemical processing, Citation: Talat, R.; Asghar, M.A.; Tariq, I.; Akhter, Z.; Liaqat, F.; Nadeem, L.; Haider, A.; Ali, S. Evaluating the Corrosion Inhibition Efficiency of Pyridinium-Based Cationic Surfactants for EN3B Mild Steel in Acidic-Chloride Media. Coatings 2022, 12, 1701. https:// doi.org/10.3390/coatings12111701 Academic Editor: Andrzej Miszczyk Received: 10 October 2022 Accepted: 2 November 2022 Published: 8 November 2022 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institu- tional affiliations. Copyright: © 2022 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https://cre- ativecommons.org/licenses/by/4.0/).