Electrochemical and computational study of the epicatechin-3-gallate isolated from green tea leaves as a corrosion inhibitor for 1018 carbon steel in sulfuric acid

Camellia sinensis has been previously studied as a green corrosion inhibitor; most studies have focused on using the extract on low-carbon steels. However, this study aims to examine epicatechin-3-gallate, isolated from the group of catechins abundant in Camellia sinensis, demonstrating its greater efficacy as a corrosion inhibitor than green tea leaf extract. The almost complete isolation of epicatechin-3-gallate is easily reproducible and inexpensive, its main advantage compared to other corrosion inhibitors: it is easy to use and obtain, non-polluting, and completely biodegradable.

The compound was characterized by FTIR, ${}^1$H-NMR, and Gas Chromatography-Mass Spectrometry, which corroborated the isolated catechins. The results in Electrochemical Impedance Spectroscopy (EIS) highlight the pseudo-inductive effect that decreases with increasing inhibitor concentration, thus increasing the resistance to charge transfer and inhibitory effectiveness. Potentiodynamic polarization curves (PPC) show passivations at 80 and 100 ppm that continue when polarized to higher potentials. The gravimetric test (wt ) was used to determine the corrosion rate, and the values obtained were used to obtain the adsorption isotherms. It was found that the adsorption of epicatechin-3-gallate follows the Langmuir method through physisorption; this finding was confirmed by scanning electron microscopy, which highlighted the high oxygen and carbon contents in the samples at 100 ppm. The optimized structure of Epicatechin-3-gallate was obtained using Density Functional Theory (DFT). The values obtained in the HOMO and LUMO orbitals showed that the inhibitor is most likely an electron donor concerning the metallic surface. The theoretical and experimental results will facilitate the application of epicatechin-3-gallate to inhibit the corrosion of 1018 carbon steel in acidic media.