Thermodynamics and adsorption behaviour of Sclerocarya birrea leaf extract as a potential green corrosion inhibitor for mild steel in a simulated seawater (3.5 % NaCl) environment

This study explores the thermodynamic behavior and adsorption mechanism of Sclerocarya birrea leaf extract as a sustainable corrosion inhibitor for mild steel in simulated seawater (3.5 % NaCl). Corrosion inhibition efficiency and corrosion rate were assessed using the weight-loss method over a temperature range of 303–323 K. Thermodynamic parameters such as activation energy (Ea), enthalpy (ΔH°), entropy (ΔS°), and Gibbs free energy (ΔG°) were calculated to evaluate the adsorption nature and spontaneity of the inhibition process. Adsorption isotherms (Langmuir, Temkin, and Freundlich) were applied to model the inhibitor-metal surface interactions. The results show that increasing extract concentration enhanced inhibition efficiency, while increasing the temperature reduced it, indicating a temperature-sensitive physisorption process. Inhibited samples exhibited Ea values below 80 kJ/mol and positive ΔH° values, confirming endothermic adsorption. ΔG° values ranged from -6.653 to -21.765 kJ/mol, consistent with spontaneous physical adsorption. Among the isotherm models evaluated, the Temkin isotherm best described the adsorption behavior (R² = 0.932 at 303 K). These findings demonstrate that Sclerocarya birrea extract offers a viable, eco-friendly approach to corrosion mitigation through thermodynamically favorable physisorption.