A Comparative Study on Inhibitory Actions of TCE, GIE, and CSE on the Corrosion of 6063 Al Alloy in H₃PO₄ Medium

Investigations of Terminalia chebula Ritz. extract (TCE), Garcinia indica Choisy extract (GIE), and Coriandrum sativum L. extract (CSE) as corrosion inhibitors for 6063 Al alloy in phosphoric acid (H₃PO₄) of 0.5 M, 1.0 M, and 2.0 M using potentiodynamic polarization and electrochemical impedance spectroscopy techniques were carried out. The optimal inhibitor concentrations were varied in the temperature range of 30–50 °C at 5 °C interval. Scanning electron microscope/energy-dispersive X-ray spectroscopy (SEM/EDX) technique was used to characterize the coupon surfaces before and after corrosion tests. Inhibition efficiency results with the optimal concentrations of inhibitors were 95.52% for the addition in 0.4 g L⁻¹ of GIE at 50 °C, 91.64% for the addition in 0.4 g L⁻¹ of TCE at 30 °C, and 90.06% for the addition in 0.5 g L⁻¹ of CSE at 30 °C. The activation and adsorption parameters in the presence of inhibitor suggested the adsorption mechanisms were of chemisorption of GIE and physisorptions of TCE and CSE. Langmuir adsorption isotherm was followed. A suitable inhibition mechanism was proposed. The SEM revealed cracks and roughness in the sample without inhibitor, while the surface at an optimal concentration of 0.4 g L⁻¹ of inhibitor had a smooth surface which was accomplished by EDX studies.