Glycyrrhiza glabra leaves extract as a green corrosion inhibitor for mild steel in 1 M hydrochloric acid solution: Experimental, molecular dynamics, Monte Carlo and quantum mechanics study

The objective of this study is to investigate the inhibition of Glycyrrhiza glabra extract, commonly known as licorice, as a green source of corrosion inhibitor against mild steel corrosion in 1 M HCl solution. The extract of Glycyrrhiza glabra has been used for mild steel inhibition in 1 M HCl solution. By means of potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) the corrosion inhibition performance and by atomic force microscopy (AFM) and contact angle test the surface characteristics were evaluated. The results of polarization test revealed that the Glycyrrhiza glabra leaves extract acted as mixed type inhibitor and retarded both anodic and cathodic reactions rates. Results revealed that by addition of Glycyrrhiza glabra leaves extract the corrosion current density of mild steel significantly decreased from 260 μA/cm2 for the sample without inhibitor to 40.2 μA/cm2 for the sample containing 800 ppm inhibitor. In addition, it was found that in the presence of Glycyrrhiza glabra leaves extract the hydrogen evolution mechanism did not change but the anodic dissolution mechanism of iron was affected in the presence of inhibitors. The EIS results showed that the increase of Glycyrrhiza glabra leaves extract concentration and immersion time resulted in the increase of corrosion inhibition efficiency. The maximum corrosion inhibition efficiency (about 88%) and surface coverage (about 72%) were obtained in the presence of 800 ppm Glycyrrhiza glabra leaves extract after 24 h immersion. Atomic force microscopy test results showed lower mild steel surface degradation in the HCl solution with 800 ppm Glycyrrhiza glabra leaves extract. Also, the decrease in surface hydrophilicity in the presence of Glycyrrhiza glabra leaves extract confirmed the adsorption of organic molecules of the inhibitors, such as Glycyrrhizin (GL), 18β-Glycyrrhetinic acid (GA), Liquritigenin (LTG), Licochalcone A (LCA), Licochalcone E (LCE), and Glabridin (GLD), on the active sites of mild steel. In addition, the modeling studies based on classical molecular dynamics (MD), Monte Carlo (MC) and quantum mechanics (QM) techniques evidenced that all corrosion inhibiting materials exist in Glycyrrhiza glabra adsorbed to steel surface, and thereby form a corrosion-protective film over the steel surface.