Understanding the Corrosion Inhibition Mechanism of a Carbon Steel in Hydrochloric Acid Medium by an Isatin Derivative: Electrochemical Characterizations and Thermodynamic Approach

In this work, a new isatin derivative, namely 1-[2-(diethylamino)ethyl]-1H-indole-2,3-dione (5-HEI), was synthesized and its inhibitory effect on carbon steel in hydrochloric acid solution was estimated by gravimetric measurements, electrochemical impedance spectroscopy (EIS) and electrochemical potentiodynamic polarization. The steel surface was analyzed by SEM. The results obtained showed that this isatin derivative acts as an effective corrosion inhibitor for carbon steel in 1 M HCl, with an inhibition efficiency of 93.9% achieved with 1 mM after 6 hours of immersion at 303 K. Polarization curves suggested that 5-HEI acted as a mixed inhibitor. Charge transfer resistance values increased with increasing inhibitor concentration, as demonstrated by electrochemical impedance spectroscopy. The adsorption of 5-HEI onto the steel surface obeyed El-Awady's kinetic/thermodynamic model. Thermodynamic studies have indicated that the main mechanism controlling corrosion inhibition by 5-HEI involves a chemisorption process.