Mild Steel Corrosion Inhibition by Aqueous and Ethanolic Gmelina arborea Leaf Extracts Prepared via Ultrasound-Assisted Extraction

Abstract

Corrosion of mild steel in acidic environments is a significant industrial and environmental challenge, underscoring the need for effective, environmentally sustainable corrosion inhibitors. This study evaluates the corrosion-inhibitory performance of aqueous and ethanolic leaf extracts of Gmelina arborea on mild steel in 1.0 M HCl. Phytochemical screening revealed alkaloids, flavonoids, tannins, and phenols in both extracts, whereas saponins were present only in the aqueous extract. Corrosion inhibition was assessed using weight loss and potentiodynamic polarization techniques. Results showed that inhibition efficiency increased with extract concentration (0.25, 0.50, and 1.0 g·L⁻¹), with the ethanolic extract demonstrating superior performance, achieving a maximum inhibition efficiency of 95.93% (weight loss) and 51.73% (potentiodynamic polarization) at 1.0 g·L⁻¹. Potentiodynamic polarization indicated that both extracts function as mixed-type inhibitors. Langmuir adsorption isotherm modeling confirmed monolayer adsorption on the metal surface, with Gibbs free energy values of –23.44 to –27.44 kJ·mol-1 indicating a mixed physisorption–chemisorption mechanism. These findings suggest that G. arborea leaf extract, especially its ethanolic extract, is a promising green corrosion inhibitor for mild steel in acidic environments.