Experimental, Theoretical Modeling and Optimization of Inhibitive Action of Ocimum basilicum Essential Oil as Green Corrosion Inhibitor for C38 Steel in 0.5 M H2SO4 Medium

In this study, the chemical composition of Ocimum basilicum essential oil (OBEO) was investigated using gas chromatography (GC) and GC-mass spectrometry (GC/MS). The inhibiting power of this oil for the corrosion of C38 steel (CS) in 0.5 M H2SO4 was similarly studied using weight loss (WL) measuring, open-circuit potential analysis and electrochemical tests such as potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) techniques. The interactive effects of temperature (25–55 °C), the concentration of inhibitor (0.5–2.00 g/L), and immersion time (2–6 h) were optimized for a maximum response of inhibition efficiency (IE%) using the Response Surface method (RSM). 21 components were identified accounting for 96.0% of the total oil, with Estragol (67.2%) and Linalol (14.8%) were the major compounds. The optimum conditions were temperature of 40 °C, inhibitor concentration of 1.25 g/L, and immersion time of 4 h. Under these optimum conditions, the optimal IE% estimated was 88.10%. The findings suggest that the experimental and the predicted data are in reasonable agreement, showing that the quadratic model was the fittest for the optimization of the inhibition process. PDP showed that OBEO functioned as a mixed-mode inhibitor. According to EIS experiments, the observed increase in charge transfer resistance is due to the adhesion of a film layer on the metal surface via the substitution of the water molecules by the molecules of the tested oil. The thermodynamic study revealed that the adsorption process obeyed the Langmuir isotherm and that the inhibition mechanism involved a combination of both physisorption and chemisorption. Theoretical results through using density functional theory and Monte Carlo simulation were performed to understand the contribution of the main components and their protonated form in the inhibitory effect of OBEO and to explain their adsorption configurations on top of the Fe (110) surface.