ISSN 3121-8733
Abstract
The adsorptive and electrochemical behaviour of Abelmoschus esculentus (okra) leaf extract as a green corrosion inhibitor for mild steel in hydrochloric acid solutions was investigated. Electrochemical measurements, including open circuit potential, linear polarization resistance, and potentiodynamic polarization, were performed in 1.0, 1.5, 2.0 and 2.5 M HCl within the temperatures of 303, 313, 323 and 333 K. FTIR and GC–MS analyses revealed the presence of tannins, organic acids, and polyphenolic compounds containing hydroxyl, carbonyl, and nitrogen-bearing functional groups responsible for surface adsorption. Inhibition efficiency increased with inhibitor concentration, reaching a maximum of 98.5% at 200 ppm in 1 M HCl, but decreased with increasing temperature, indicating predominantly physical adsorption. Thermodynamic and kinetic analyses yielded activation energy values of 31.06–42.64 kJ·mol⁻¹, with positive enthalpy and negative entropy changes, consistent with endothermic and spontaneous adsorption. Surface morphology and EDX analysis confirmed the electrochemical results. The uninhibited steel showed severe pitting and porous corrosion products, with reduced iron and increased oxygen. In contrast, the inhibited sample exhibited a smoother surface with reduced corrosion damage. EDX detected nitrogen and phosphorus species on the inhibited surface, confirming adsorption of phytochemical constituents and formation of protective organic firm. Adsorption behaviour followed the Langmuir isotherm (R² = 0.99), suggesting monolayer coverage on a homogeneous surface. Electrochemical results confirm that A. esculentus acts as a mixed-type inhibitor, suppressing both anodic and cathodic reactions through the formation of a protective adsorbed film. The results demonstrate the potential of okra leaf extract as an efficient, environmentally benign inhibitor for mild steel corrosion in acidic media.
Keywords: Corrosion inhibition; Abelmoschus esculentus; Mild steel; HCl solution; Adsorption isotherm; Electrochemical behaviour