The role of ethanolic extract of Stachys byzantina's leaves for effective decreasing the mild-steel (MS) degradation in the acidic solution; coupled theoretical/experimental assessments

Abstract

In the current research, the anti-corrosion performance of MS in the HCl solution was investigated when potentially corrosion inhibitive green molecules, extracted from the Stachys byzantina (SB) leaves, were added to the solution. Potentiodynamic polarization (PP) along with the EIS tests were carried out. Also, surface characterization was examined using FE-SEM, contact angle measurements, AFM, and EDS. The film composition formed on the MS surface was explored by Ultraviolet-visible spectroscopy (UV–Vis), GIXRD, FT-IR, and Raman spectroscopy. EIS test demonstrated that the MS surface exposed to the 1 M HCl solution into which 1200 ppm SB extract was incorporated yielded the highest resistance against corrosion after 4 h resulting in the inhibition efficiency of 96%; the optimum concentration was also substantiated with PP test. SB extract showed a mixed-inhibition performance, and the best-fitted adsorption isotherm was calculated to be Langmuir; furthermore, the inhibitor adsorption was a combination of physisorption and chemisorption. Surface studies showed that the inhibited surface exhibited a smoother texture with lower roughness because of the inhibitor adsorption. The presence of several SB extract's functional groups such as OH, CH, C=C, and C=O on the surface was confirmed through FT-IR spectroscopy and UV–Vis analysis. Via Raman spectroscopy, a graphene-like layer was observed on the surface. The results, as a whole, proved the presence of a protective layer, which adsorbed on the surface and subsequently prevented further corrosion attack. Finally, the interactions taking place between metal surface and corrosion inhibitor were examined from the theoretical point of view using molecular dynamics (MD), Monte Carlo (MC), and density functional theory (DFT). The results were suggestive of the formed complex affinity toward the surface resulting in corrosion prevention.