Mechanistic investigation of mild steel corrosion inhibition by Codiaeum variegatum extract: weight loss, EIS, and DFT perspectives

Abstract

The corrosion inhibition efficacy of Codiaeum variegatum leaf extract (CVLE) on mild steel in 1 N HCl was assessed utilizing weight-loss (WL), potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS) methods at different temperatures 298 K, 308 K, 318 K, 328 K and 338 K. The study revealed that CVLE is an environmentally friendly and sustainable corrosion inhibitor, attaining an impressive inhibition efficacy of 91.83 ± 0.04% at 1000 ppm concentration at 298 K. A significant reduction in corrosion rate (C_R) from 21.264 ± 0.02 to 1.735 ± 0.05 mmpy and corrosion current density (i_corr) from 157.63 ± 0.01 to 4.02 ± 0.02 μA cm⁻² was observed with increasing CVLE concentration, highlighting its protective efficacy. The adsorption of CVLE followed the Langmuir adsorption isotherm on the mild steel surface, indicating monolayer adsorption behavior. Potentiodynamic polarization studies classified CVLE as a mixed-type inhibitor, influencing both cathodic and anodic reactions. The mechanism of inhibition occurring on mild steel was discovered using the XPS method. Scanning electron microscopy (SEM) confirmed the creation of a layer of protection by CVLE on the mild steel surface, corroborating its inhibitory action. EIS analysis further demonstrated that the inhibiting efficacy was elevated with higher concentrations of CVLE, consistent with a rise in adsorption equilibrium constant (K_ads), indicating stronger inhibitor–metal interactions. Theoretical studies supported these findings by showing a low energy gap (ΔE), signifying strong adsorption and excellent anti-corrosion potential of CVLE at the molecular level. This inclusive study establishes CVLE as an effective and eco-friendly inhibitor for mild steel corrosion in acidic environments.