Chemical and Physical Interactions of 1-Benzoyl-3,3-Disubstituted Thiourea Derivatives on Mild Steel Surface: Corrosion Inhibition in Acidic Media

1-Benzoyl-3,3-diphenylthiourea (1), 1-benzoyl-3,3-dibenzylthiourea (2), 1-benzoyl-3,3-diethylthiourea (3), 1-benzoyl-3,3-dibutylthiourea (4), 1-benzoyl-3,3-bis(2-methylpropyl)thiourea (5), and 1-benzoyl-3,3-bis(propan-2-yl)thiourea (6) were synthesized and investigated as inhibitors for the corrosion of the surface of mild steel in 1.0 M HCl by chemical and electrochemical measurements. The inhibition efficiencies obtained from different methods were in good agreement with each other. Inhibitor 2 showed a higher inhibition efficiency according to all of the methods. The Tafel polarization method revealed the mixed-mode inhibition of inhibitors with predominant control of the anodic reaction. At all studied temperatures, the adsorption of the inhibitor molecules onto the steel surface was found to follow the Langmuir adsorption isotherm. The values of the Gibbs free energy of adsorption strongly supported spontaneous chemical and/or physical adsorption of inhibitor molecules. The adsorption mechanism for inhibition was supported by ultraviolet-visible (UV-vis), Fourier transform infrared (FTIR), Raman, and scanning electron microscopy-energy-dispersive X-ray (SEM-EDS) spectroscopic methods, and adsorption isotherm measurements. The crystalline/amorphous nature of the inhibitors adsorbed onto the mild steel surface was indicated by wide-angle X-ray diffraction (WAXD) analysis.