Green Synthesis of Novel Schiff Bases as Eco-friendly Corrosion Inhibitors for Mild Steel in Hydrochloric Acid

Two novel Schiff bases, (E)-N-(4-hydroxy-3-methoxybenzylidene)lysine (1) and (E)-N-(4-hydroxy-3-methoxybenzylidene)arginine (2) were synthesized by using natural products of amino acid and vanilline, and their inhibitive effect on the corrosion of mild steel in 0.5 mol⋅L⁻¹ HCl solution was evaluated by various experimental methods such as electrochemical impedance spectroscopy (EIS), potentiodynamic polarization curves, scanning electron microscope (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Moreover, quantum chemical calculations and molecular dynamics simulations were used to theoretically explore the internal relation between inhibitory efficiency and molecular structure. The results of EIS show the polarization resistance of mild steel is increased from 6.592 Ω⋅cm² to 36.46 Ω⋅cm² and 74.78 Ω⋅cm² in the presence of 1 and 2, respectively. Tafel curves indicate HMBL and HMBA act as good inhibitors for iron corrosion in hydrochloric acid and that 2 has the better performance than 1. XPS analysis confirms the adsorption of inhibitor molecules on iron surface by forming inhibitor-Fe complex, which is well fitted by Langmuir model.