Synergistic mixture of Eupatorium adenophora spreng leaves extract and KI as a novel green inhibitor for steel corrosion in 5.0 M H3PO4

The synergistic inhibition effect of Eupatorium adenophora Spreng leaves extract (EASLE) and potassium iodide (KI) on the corrosion of cold rolled steel (CRS) in 5.0 M H3PO4 solution was investigated by weight loss, electrochemical techniques, surface characterizations and quantum chemical calculations. The results demonstrate that there is a synergism between EASLE and KI, and the maximum inhibition efficiency is 97.9%. The presence of KI facilitates the adsorption of EASLE owing to higher adsorption equilibrium constant. The synergistic mixture of EASLE/KI is arranged as a mixed inhibitor through “geometric blocking effect”, and further retards the cathodic reaction compared with either EASLE or KI. EIS is one time constant with a dispersive capacitive loop in Nyquist plot and one peak in Bode phase plot. The charge transfer resistance becomes more larger in the presence of synergistic mixture of EASLE/KI, while electrical double layer capacitance is dropped. The synergistic inhibited CRS surface by EASLE/KI appeared non-corroded through scanning electron microscope (SEM) and atomic force microscope (AFM) images, and more hydrophobic with higher contact angle. X-ray photoelectron spectroscopy (XPS) confirms that the organic compounds in EASLE can synergistically adsorb on CRS surface with KI. The phenylpropanoids in EASLE of chlorogenic acid, caffeic acid and ferulic acid exhibit synergistic inhibition with KI, but their inhibition is much lower than that of EASLE. The adsorption site of these compounds is phenylpropanoid backbone structure.