Corrosion inhibition of carbon steel by Roselle extract in hydrochloric acid solution: electrochemical and surface study

The inhibiting effect of Roselle extract (Krd) on carbon steel in 0.5 M HCl solution was investigated by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), electrochemical frequency modulation (EFM), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) methods. It was found that Krd extract acts as a good inhibitor for carbon steel corrosion in the acid solution. The results showed that Krd extract simultaneously retarded the hydrogen evolution on the cathode and the metal dissolution on the anode, which indicated that this inhibitor is an excellent mixed-type (cathodic/anodic) inhibitor. The dissolution rate decreases with increasing Krd extract concentration and increases with increasing the temperature. The adsorption of Krd extract on the active cathodic sites was found to follow the Langmuir adsorption isotherm. The maximum inhibition approached 91.0 % in the presence of 500 ppm Krd using the EFM technique. The formation of a protective film on the carbon steel surface in the presence of Krd was shown and confirmed by SEM results. There are good agreements between EIS, EFM, the potentiodynamic polarization data, and SEM/EDX results.