Evaluation of Laurus nobilis Essential Oil as a Corrosion Inhibitor for Mild Steel in HCl: Electrochemical Measurements and DFT Analysis

Background: This study investigates the potential of Laurus nobilis essential oil (LNEO) as a corrosion inhibitor for steel in a 1 M hydrochloric acid (HCl) solution. Materials and Methods: Gas Chromatography-Mass Spectrometry (GC-MS) analysis was performed to determine the chemical composition of LNEO, revealing key constituents, including 1.8-cineole (33.47%), α-terpinyl acetate (17.39%), and sabinene (9.18%). Corrosion inhibition efficiency was evaluated through electrochemical techniques, and adsorption behavior was analyzed using the Langmuir isotherm model. Thermodynamic parameters were also assessed to elucidate the adsorption mechanism. Results: The inhibition efficiency reached a maximum of approximately 93.4% at an optimal concentration of 2 g/l. Langmuir adsorption isotherm studies confirmed a strong interaction between LNEO and the steel surface, with an adsorption free energy (ΔGads = -21.2 kJ/mol) and an adsorption equilibrium constant (Kads = 5.21 L/g), indicating a physisorption mechanism with partial charge transfer. Thermodynamic analyses showed an activation energy (Ea = 44.7 kJ/mol), enthalpy change (ΔHa = 42.1 kJ/mol), and entropy change (ΔSa = -69.3 J/mol.K), supporting a spontaneous and endothermic adsorption process. Discussion: Density Functional Theory (DFT) calculations and Scanning Electron Microscopy (SEM) analyses confirmed the adsorption mechanism, highlighting the protective film formation on the steel surface. Conclusion: These findings demonstrate that Laurus nobilis essential oil is an effective corrosion inhibitor for steel in acidic media, with strong adsorption properties and high inhibitory efficiency.