Identification of Potential Green Inhibitors Extracted from Thymbra capitata (L.) Cav. for the Corrosion of Brass in 3% NaCl Solution: Experimental, SEM–EDX Analysis, DFT Computation and Monte Carlo Simulation Studies

The purpose of this study is to identify the green corrosion inhibitors of brass in a 3% NaCl solution. Potential corrosion inhibition compounds extracted from the essential oil (EO) and the hydrosol (HY) of Moroccan Thymbra capitata (L.) Cav. ( Lamiaceae ) were tested and characterized. Gas chromatography coupled with mass spectrometry (GC–MS) method was used to identify the chemical composition in EO and HY. The electrochemical analysis was conducted using potentiodynamic polarization curves and impedance spectroscopy (EIS) techniques. The results obtained show a high percentage of carvacrol for both EO (85.96%) and HY (98.04%). In addition, the corrosion rate of brass decreased with increasing concentrations of EO and HY in the presence of 3% NaCl. Unexpectedly, the maximum inhibition efficiency was recorded for HY (93.04%) then for EO (84.90%) in a 3% NaCl medium. The protective film adsorbed on the brass surface when using the two green extracts was analyzed using scanning electron microscope (SEM), coupled with energy-dispersive X-ray analysis (EDX). The surface morphology showed a good inhibition for HY compared to that of EO. Computational studies, using density function theory (DFT) and Monte Carlo (MC) simulations, confirmed the experimental results, this allowed us to understand that the high inhibition efficacy of HY is likely associated with the synergistic effect between carvacrol and eugenol despite this latter low percentage (0.50%) in our extract. This study also enabled us to suggest an inhibition mechanism of carvacrol and the synergistic effect between carvacrol and eugenol, which increase the corrosion inhibiting effect of HY.