Corrosion inhibition, surface adsorption and computational studies of new sustainable and green inhibitor for mild steel in acidic medium

The inhibitory effect investigation of aqueous and ethanolic extracts of Origanum elongatum Plant (will be referred to in this paper, respectively, as "OEAq and OEEt ") on mild steel corrosion in HCl solutions has been evaluated. The analysis and characterization of both products extracted (OEAq and OEEt) were performed using HPLC-UV-ELSD-MS techniques. Various electrochemical techniques was used such as weight loss measurements, potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) in order to advance in the inhibition process. An appreciable inhibitory effect for both tested inhbitory has been obtained from all the combined results. Experimental results revealed that OEAq and OEEt can be considered as good green inhibitors and that OEAq is more efficient than OEEt with a maximum inhibition efficiency of 94.1% at 0.4 g/L at 303 K. The inhibition efficiency of each inhibitor increases sharply with increasing concentration and a decreasing of efficiency was observed with increasing temperature for both inhibitors. Both OEAq and OEEt were seen to act as a mixed type inhibitors from PDP study and according to EIS technical, their corrosion process is charge transfer controlled. Thermodynamic and activation parameters were determined to discuss the inhibitory behaviour of tested extracts molecules. The two extracts OEAq and OEEt obey to Langmuir adsorption isotherm model as a result of their adsorption on mild steel surface. Morphology of the metal specimen surface without and with inhibitors was investigated using Scanning electron microscopy (SEM) and EDX spectrum. The electronic characteristics of major compounds of OEAq and OEEt (MC-OEAq and MC-OEEt) were studied using the DFT method (density functional theory), the Fukui indices and MD (molecular dynamics) modelling in order to predict the spontaneous adsorption.