Adsorption and thermodynamic studies of the corrosion Inhibition effect of Desmodium adscendens (Swartz) extract on carbon steel in 2 M HCl

This study examined the thermodynamic parameters and adsorption mechanism of Desmodium adscendens (Swartz) extract (DAPE) as a corrosion inhibitor for carbon steel in 2.0 M HCl. DAPE's chemical structure was analyzed using FT-IR spectroscopy, while the inhibition efficiency and corrosion rate were determined via gravimetric analysis. Thermodynamic parameters such as activation energy, enthalpy, and entropy changes were calculated using relevant equations. Adsorption isotherms were used to assess Gibbs free energy change. DAPE, according to the FT-IR spectra contained O-H, C = O, C = C, and C-O functional groups, typical of organic corrosion inhibitors. Higher concentrations of DAPE resulted in an increase in activation energy (from 45.36 to 65.26 kJmol^{- 1}) and enthalpy of activation (from 42.61 to 62.51 kJmol^{- 1}), along with a decrease in entropy of activation (from - 0.163 to -0.114 kJmol^{- 1}), indicating the formation of a film barrier through physisorption on the metal surface. Spontaneous adsorption of DAPE was validated by negative ∆G_ads values (-19.52 to -20.41 kJmol^{- 1}), and negative ∆H_ads (-12.55 kJmol^{- 1}) revealing an exothermic process. The Langmuir isotherm was the most suitable model, with high R² values (0.9982 to 0.9995) and an equilibrium constant of absorption (K_ads) of 32.57 Lg^{- 1} indicating monolayer adsorption. In addition, DAPE achieved an inhibition efficiency of 87.61%, which demonstrated its promise as a potent and cost effective corrosion inhibitor for carbon steel in acidic environments.