Theoretical and experimental studies for corrosion inhibition of thiosemicarbazide derivatives

4-(2-Phenhylethyl)-3-thiosemicarbazide, 4-(2-Bromophenyl)-3-thiosemicarbazide, 4-(4-Bromophenyl)-3-thiosemicarbazide, Exo-2-Norbornyl-3-thiosemicarbazide, 4- (Cyclohexyl)-3-thiosemicarbazide, 4- (2-Methoxyphenyl)-3-thiosemicarbazide, 4-Phenyl-3-thiosemicarbazide, 4-Benzyl-3-thiosemicarbazide and 4- (3-methoxyphenyl) -3-thiosemicarbazide (TSC) were synthesized by the reaction of hydrasine hydrate and isothiocyanate derivatives. The inhibitory effects of the synthesized thiosemicarbazide derivatives on the corrosion behavior of mild steel in 1.0 M HCl were investigated in 293 K by using potentiodynamic polarization, electrochemical impedance spectroscopy and linear polarization resistance techniques. The inhibitory activity of all studied TSC derivatives increased with increasing concentration of inhibitor. In addition, the inhibition effect of 4-(Cyclohexyl)-3-thiosemicarbazide in acidic environment was investigated by weight loss method and at 293 K -303 K by electrochemical methods. Thiosemicarbazide derivatives acted as mixed-type inhibitors. The adsorption isotherm on the mild steel surface of all the inhibitory molecules studied and the thermodynamic parameters of the 4-(Cyclohexyl)-3-thiosemicarbazide were calculated. TSC derivative inhibitor molecules were adsorbed on the electrode surface according to the Langmuir adsorption isotherm. The thermodynamic parameters of 4-(Cyclohexyl)-3-thiosemicarbazide showed that the inhibitor molecule was adsorbed physically to the surface of the mild steel. Quantum chemical parameters were calculated by B3LYP/6-311++G (2d,2p) and B3LYP/6-311G (d,p) method. The calculated parameters are energy of the highest occupied molecular orbital energy (EHOMO), the lowest unoccupied molecular orbital energy (ELUMO), the energy gap between ELUMO and EHOMO (ΔE), Chemical Potential (μ), hardness (η), softness (s), electronegativity (χ), global electrophilicity(ω), nucleofugality (ΔEn) and electrofugality (ΔEe). The relationship between the parameters calculated by B3LYP/6-311++ G(2d,2p) and B3LYP/6-311G(d,p) method and the dependent variable impedance were examined.