Quinoline derivatives as corrosion inhibitors for mild steel in acid medium: Deeper insights from experimental, ab initio density functional theory modeling, and in silico ecotoxicity investigations

Abstract

Two (E)-2-(((2-((7-chloroquinolin-4-yl)amino)ethyl)imino)methyl)-4-R-phenol Schiff bases derivatives CMN (R = NO2) and CMP (R = H) were successfully prepared. Then, CMN and CMP were evaluated as corrosion inhibitors for 1020 mild steel in 1.0 mol L–1 HCl using different methods. Gravimetric experiments have shown that CMN and CMP reach efficiencies of 80 and 91% at 1.45 mmol L–1. Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) measurements have indicated the formation of a protective adsorbed film on the mild steel surface. Electrochemical Impedance Spectroscopy (EIS), Linear Polarization Resistance (LPR), Potentiodynamic Polarization (PP), and Electrochemical Frequency Modulation (EFM) have also shed some light on electrochemical behavior of compounds. These data have depicted better polarization resistance and lower corrosion current density in the presence of both organic molecules, also proving that the process is controlled by charge transfer and that CMN and CMP are mixed-type corrosion inhibitors. ab initio Density Functional Theory (DFT) investigated how two quinoline-based molecules (CMN and CMP) prevent corrosion on iron surfaces. Simulations revealed CMP bonds more strongly to iron than CMN, leading to better protection. Simulations also showed that these inhibitor molecules form a protective layer on the iron, slowing down the movement of corrosive substances. Environment risk assessment through QSAR-based models have indicated that the corrosion inhibitors do not have the potential to bioaccumulate in fish. Still, the acute and chronic toxicity values at the different trophic levels investigated call for attention for environmentally friendly structural optimization.