Synergistic effect of KI on corrosion inhibition of carbon steel by Styphnolobium japonicum (L.) Schott in H2SO4 solution.

• The inhibition efficiency following the order: KI<SE<SE+KI. • Corrosion inhibitors were tested for carbon steel in 0.5 mol/L H 2 SO 4 solution. • The adsorption process of corrosion inhibitor was studied theoretically. • The adsorption model conforms to Langmuir isotherm. This work explored the synergistic effect of Styphnolobium japonicum (L.) Schott extract (SE) and potassium iodide (KI) on the corrosion inhibition of carbon steel (CS) in 0.5 mol/L H 2 SO 4 solution. Firstly, the functional groups in the SE were identified using FTIR. The results of UPLC–Q–TOF–MS analysis confirmed the major phytoconstituents presence in SE were Glutaric acid, 2,3-Dihydroxybenzoic acid, Oxoadipic acid, Isoleucine, 2-Oxovaleric acid, l-phenylalanine, Indoline, and Trigonelline. Further a comprehensive investigation of anticorrosive properties of SE in the absence and presence of KI was conducted employing various techniques, including electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), ultraviolet visible spectroscopy (UV‒vis), and X-ray photoelectron spectroscopy (XPS) and theoretical studies like quantum chemical calculations (QC) and molecular dynamics simulations (MS). The results demonstrated that the SE+KI blend outperforms SE or KI individually, achieving an appreciable corrosion inhibition of 94.6 % for CS in H 2 SO 4 solution. SEM analysis confirmed that the CS surface was smoother when SE+KI was added in the aggressive solution. Theoretical calculations were carried out to determine the dipole moments, Mulliken charges, and Fukui parameters of the eight major molecules found in SE. The results showed that SE can inhibit corrosion. [ABSTRACT FROM AUTHOR]