Cationic Surfactants and Schiff Bases as Corrosion Inhibitors for Protection of Mild Steel in Acidic/Chloride Media Electrochemical and Morphological Examinations

Abstract

In this work, newly designed corrOSIOn inhibitors have been introduced for the protection of EN3B mild steel using two different approaches for water soluble and insoluble inhibitors. For this purpose, a series of cationic pyridinium based surfactants namely, N-(n-alkyl)-3-methylpyridinium bromide [Py(n), n = 6 to 17] and six new Schiff bases, namely 5-( diethylamino )-2-[ {(R)imino} methy l]phenol where R represents 3,5-dimethylphenyl (DM), diphenylmethyl (BH), 4-isopropylphenyl (IP), n undecyl (11), n-tetradecyl (14) and n-hexadecyl (16) groups [NS(n), n = DM, BH, IP, 11 , 14, 16], have been synthesized and characterized by multiple techniques. These compounds were analyzed in a strong corrosive media (3.5% NaCI, pH 1.5) by a variety of electrochemical, morphological, and theoretical studies. The results of the electrochemical measurements, i.e., Tafel polarization, linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) revealed a high inhibition potential of Py(n) and NS(n) inhibitors, and the maximum inhibition efficiency (IE) achieved with Py(n) and NS(n) reached 85 % and 99.8 %, respectively. The process of adsorption ofPy(n) inhibitors followed the Langmuir adsorption isotherm, and a higher value of Kads (equilibrium constant for the process of adsorption) highlighted the existence of strong interaction between inhibitors and the EN3B mild steel surface. Furthermore, the values of LlG:dswere calculated to be -32 kJ mol-J for Py8 and -33 kJ mol-I for Py12 and Py16, indicating the coexistence of both physisorbed and chemisorbed molecules. The adsorption ofNS(n) inhibitors from ethanol on the surface of mild steel was detected via RAIRS (Reflection Absorption Infrared Spectroscopy) prior to the electrochemical and morphological examinations. Afterwards, 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 Py(n) and NS(n) in chloride media further supported the evidence of an efficient inhibition process. The minimum surface roughness of EN3B mild steel surface was observed in the presence of NS16 and the root mean square (RMS) roughness was found to be 10.41 nm which is very close to the RMS roughness of polished steel i. e., 9.12 nm, thereby highlighting an efficient inhibition system. The experimental findings were complemented by density functional theoretical (DFT) calculations revealing that the electron donating power of substituents is directly related to the inhibition potential ofthe inhibitors. Furthermore, a possible mechanism for the