Electrochemical, thermodynamic and theoretical study on anticorrosion performance of a novel organic corrosion inhibitor in 3.5% NaCl solution for carbon steel

The theoretical and electrochemical performance of a novel organic corrosion inhibitor 3,4-dihydro-3-[2-mercaptothiazolidine]indol-2-one (DMI), for API 5L Grade B carbon steel in 3.5% NaCl, was evaluated by potentiodynamic polarization (Tafel), electrochemical impedance spectroscopy (EIS) and density functional theory (DFT) for quantum chemical studies. Potentiodynamic studies confirmed that DMI was a mixed organic corrosion inhibitor type which specially affects the cathodic branch. The inhibition efficiencies of reactants, DMI and acetylcysteine followed the following order at 25 degrees C and 200ppm: DMI (87%) > isatin (71%) > 2-thiazoline-2-thiol (62%) > acetylcysteine (54%). EIS measurements illustrated the charge transfer controlled corrosion process. The Langmuir adsorption isotherm model of DMI was adopted. Surface studies were performed using scanning electron microscopy. Activation and adsorption thermodynamic parameters of DMI were computed. The magnitude of Gads degrees and the sign of Hads degrees concluded that the adsorption occurred through chemisorption. Quantum chemical calculations of four corrosion inhibitors were used for investigating the molecular structure effect on inhibition efficiency.