Corrosion inhibition mechanism of a functionalized schiff base–derived quaternary ammonium salt for carbon steel in 1 M HCl: electrochemical, adsorption, and theoretical studies

Abstract

To address costly corrosion and material degradation, we report the development of a new quaternary ammonium salt (Q-Ar) via functionalization of a Schiff base with benzyl chloride through a quaternization reaction, intended as a potential corrosion inhibitor for carbon steel (C-steel) in 1 M HCl. FT-IR and ¹H NMR spectroscopy confirmed the chemical structure of Q-Ar. Electrochemical potentiodynamic polarization (PDP) studies revealed that Q-Ar acted as mixed-type inhibitor and achieved corrosion inhibition efficiency 93.94% at 35 ppm. Electrochemical impedance spectroscopy (EIS) confirmed strong adsorption layer with charge transfer resistance (R_ct) of 737.3 Ω.cm². The adsorption behavior followed the Langmuir isotherm model, suggesting chemical adsorption reaction of Q-Ar at room temperature over C-steel surface according to standard adsorption energy values (∆G_ads) of -43.2 kJ.mol^{- 1} for Q-Ar. Activation thermodynamic parameters of C-steel reaction in presence of Q-Ar suggested its inhibition action forming barrier film over C-steel surface as reflected by the increase in activation energy (Eₐ) from 21.60 kJ mol^{- 1} for the uninhibited system to 42.79 kJ mol^{- 1} in the presence of Q-Ar. Scanning electron microscopy (SEM) confirmed the protective effect of Q-Ar, showing that the C-steel surface remained largely free of corrosion products compared with the severely corroded blank sample. Energy-dispersive X-ray (EDX) analysis further verified this observation by revealing a marked reduction in iron oxide formation in the presence of Q-Ar. Theoretically, the lower energy gap (ΔE_gap) of the optimized Q-Ar molecule and the higher adsorption energy (E_ads) of the Q-Ar/Fe(110) system indicate strong interactions between the Q-Ar and the C-steel surface, corroborating the experimental observations.