Corrosion inhibiting properties of polysaccharide extracted from Lepidium meyenii root for mild steel in acidic medium: Experimental, density functional theory, and Monte Carlo simulation studies

Abstract

A polysaccharide obtained from maca (Lepidium meyenii) root extract (MREP) was evaluated as a novel, inexpensive, biocompatible corrosion inhibitor for mild steel (MS) in 1 M HCl. Fourier transform infrared (FTIR) and nuclear magnetic resonance studies were conducted to characterize the inhibitor. Gravimetric, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization (PDP) techniques were applied to assess the corrosion inhibition potential of the inhibitor. Gravimetric studies were conducted over the temperature range of 30–60 °C. The inhibition efficiency of MREP increased with the concentration and temperature. The maximum inhibition efficiency of MREP was 94.78% at 700 ppm and 60 °C. EIS analysis showed that as the MREP concentration increased, the Rp value obtained from the Nyquist plot increased due to the formation of MREP barrier film at the MS/solution interface. PDP analysis showed that MREP had a mixed type inhibition mechanism. The inhibition efficiencies calculated based on gravimetric, EIS, and PDP analyses were similar. Ultraviolet–visible, FTIR, X-ray photoelectron spectroscopy, atomic force microscopy, and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDAX) analyses demonstrated the formation of a protective film on the MS surface. MREP adsorbed on MS according to the Langmuir adsorption isotherm. The calculated density functional theory parameters indicated the formation of a strong bond between MREP and the unoccupied d-orbital of iron atoms in MS. By contrast, Monte Carlo simulations indicated the adsorption of the MREP inhibitor on the MS surface at the atomic level.