Synergistic inhibition effect of Mikania micrantha extract with KI on steel corrosion in H2SO4 solution

Mikania micrantha extract (MME) was prepared by reflux extraction method with 40% C2H5OH water solution. The synergistic inhibition effect of MME and potassium iodide (KI) on cold rolled steel (CRS) in 0.50 M H2SO4 solution was studied by weight loss and electrochemical methods for the first time. The CRS surfaces were fully characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Meanwhile, the main effective compounds in MME were discussed in depth by high performance liquid chromatography (HPLC). The results show that either MME or KI acts as a moderate inhibitor on CRS in 0.50 M H2SO4 solution, and the maximum inhibition efficiency is below 80%. The mixture of MME/KI has a pleasant inhibition performance with the maximum inhibition efficiency of 94.4%. There is an evident synergistic effect between MME and KI, and the corresponding synergism parameter is higher than 1. Regardless of the extra addition of KI, the adsorption of MME obeys the Langmuir adsorption isotherm. The presence of KI enhances the adsorption of MME owing to larger value of adsorption equilibrium constant for MME/KI mixture. MME is a cathodic inhibitor, while MME/KI can be arranged as a mixed-type inhibitor, and the inhibition of both cathodic and anodic reactions is further enhanced by the MME/KI combination. Nyquist diagram mainly exhibits a capacitor loop that is controlled primarily by charge transfer. As the concentration of corrosion inhibitor increases the capacitance arc length gradually increases, especially for MME/KI mixture. The charge transfer resistance increases more than 10 times and shows excellent corrosion inhibition. SEM and AFM clearly confirm that the presence of MME/KI has a significant effect on inhibiting the corrosion of CRS, and the inhibition is expected to follow the order: MME/KI > KI > MME. XPS reveals that the MME/KI mixture can adsorb on steel surface to achieve inhibitive film. Most important, there is also a synergistic inhibition of effective components in MME (chlorogenic acid, ferulic acid) with KI.