Experimental and theoretical studies of Inula viscosa extract as a novel eco-friendly corrosion inhibitor for carbon steel in 1 M HCl

In the present study, the extract of Inula viscosa leaves (IVE) was used as an environmentally-friendly and green corrosion inhibitor for X70 carbon steel in 1 M HCl. The experimental results were endorsed by computational approaches. The corrosion inhibition effect of the extract was investigated by weight loss measurements, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. Besides the surface characterization by SEM/EDS (scanning electron microscopy/energy dispersive X-ray spectroscopy) and AFM (atomic force microscopy), the mechanism of inhibition was assessed through FTIR and UV/Vis studies. The results of weight loss measurements and electrochemical experiments showed that IVE was an excellent corrosion inhibitor, as the corrosion inhibition efficiency was found to increase with the concentration of plant extract and exceeded 92% at a concentration of 600 mg L-1. This excellent inhibitory efficiency was attributed to the efficient adsorption of this green inhibitor onto the steel surface and its mixed-type behavior; the adsorption data were fitted to the Langmuir isothermal model. The studies at an optimum concentration and for an immersion time of 120 h revealed the stability of adsorbed inhibitor layer on the X70 surface in acidic media. The success of the adsorption was confirmed by analysis of the organic protective film formed by SEM/EDS and FTIR tools. AFM showed a significant reduction of surface roughness of the susbtrate in the presence of IVE. Moreover, the adsorption-inhibition relationship was discussed in light of the quantum chemical parameters calculated for the main components of the plant extract using the density functional theory (DFT).