Insights into the corrosion inhibition mechanism of mild steel in 1 M HCl by Cymbopogon citratus E.O.: A combined electrochemical, surface, and multi-scale computational study

This study thoroughly evaluates the efficacy of Cymbopogon citratus essential oil ( C. citratus E.O.), along with its major constituents, Citral and Alpha-pinene, as corrosion inhibitors for mild steel in 1M HCl medium. The experimental approach included the characterization of the E.O. by Gas Chromatography-Mass Spectrometry (GC-MS), which revealed a high content of Geranial (50.3%) and Neral (31.2%). The inhibitory performances were investigated using electrochemical techniques: Electrochemical Impedance Spectroscopy (EIS) and potentiodynamic polarization (PDP) curves. These measurements demonstrated that the tested substances act as mixed-type inhibitors, with inhibition efficiencies reaching 95.95% for C. citratus E.O., 94.45% for Citral, and 88.71% for Alpha-pinene via PDP, and comparable efficiencies via EIS (95.04%, 94.39%, and 89.68%, respectively). The influence of immersion time and temperature on corrosion was also examined. Surface morphological analysis by Scanning Electron Microscopy (SEM) coupled with Energy Dispersive X-ray Spectroscopy (EDX) confirmed the formation of a protective film in the presence of the inhibitors. To elucidate the interaction mechanisms at a molecular level, theoretical studies were conducted. Density Functional Theory (DFT) calculations were used to determine various quantum chemical descriptors (EHOMO, ELUMO, ΔEgap, ΔN, etc.) and to analyze reactive sites via the Molecular Electrostatic Potential (MEP). Monte Carlo (MC) simulations were employed to model the adsorption of inhibitors on the Fe(110) surface and to calculate adsorption energies, showing a strong affinity of Citral for the surface. The combined results from experimental and theoretical approaches suggest an effective adsorption of the inhibitors, primarily of a physical and chemical nature, following the Langmuir adsorption isotherm, and indicate a possible synergistic effect within the E.O. The novelty of this work lies in the multi-level correlation established between macro-scale electrochemical behavior and molecular-scale interactions, highlighting a significant synergistic effect within the natural oil matrix compared to its isolated major constituents.