Experimental and Quantum Chemical Studies on Corrosion Inhibition Performance of Thiazolidinedione Derivatives for Mild Steel in Hydrochloric Acid Solution

In the present investigation, two thiazolidinedione derivatives, 5-[(2-(3,4,5-trimethoxyphenyl)-6-phenylimidazo[2,1-b][1,3,4]thiadiazol-5-yl)methylidene]-1,3-thiazolidine-2,4-dione (Inh I) and 5-[2-(3,4,5-trimethoxyphenyl)-6-(4-methoxylphenyl)-imidazo[2,1-b][1,3,4]thiadiazol-5-yl)methylidene]-1,3-thiazolidine-2,4-dione (Inh II) were synthesized and investigated as inhibitors for mild steel corrosion in 15% HCl solution using the weight loss, electrochemical polarization, and electrochemical impedance spectroscopy (EIS) techniques. It was found that the inhibition efficiency of these inhibitors increased with increasing concentration. The effect of temperature on the corrosion rate was investigated, and some thermodynamic parameters were calculated. Polarization studies showed that both studied inhibitors were of mixed type in nature. The adsorption of inhibitors on the mild steel surface in acid solution was found to obey the Langmuir adsorption isotherm. Scanning electron microscopy (SEM) was performed on inhibited and uninhibited mild steel samples to characterize the surface. The semi-empirical AM1 method was employed for theoretical calculation of highest (E (HOMO)), and lowest unoccupied molecular orbital (E (LUMO)) energy levels, energy gap (E (LUMO)-E (HOMO)), dipole moment (mu), global hardness (gamma), softness (sigma), binding energy, molecular surface area, chemical potential (Pi), and the fraction of electrons transferred from the inhibitor molecule to the metal surface (Delta N). The results were found to be consistent with the experimental findings.