In the present investigation, the corrosion tendency of mild steel under acidic pH was studied by employing unused expired amiodarone (EAD) drug as a potential corrosion inhibitor by adopting the weight loss measurement method. The corrosion inhibition efficiency (IE) of the formed protective film (EAD) on the steel surface was analyzed using potentiodynamic polarization and AC-impedance spectroscopy studies. The surface morphology of the mild steel before and after corrosion (in 1.0 M HCl) was analyzed via scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM–EDAX), atomic force microscopy (AFM), and thermodynamic studies. The weight loss measurement under different concentrations of EAD indicated that an excellent inhibition was displayed at a concentration of 0.001 M, and the IE was found to depend on both the concentration and molecular structure of EAD. A potentiodynamic polarization study revealed that EAD predominantly acted as a cathode inhibitor, and electrochemical impedance spectroscopy (EIS) confirmed the adsorption of EAD on the surface of mild steel, which obeyed Temkin’s adsorption isotherm model. The calculated thermodynamic parameters revealed that adsorption was spontaneous and exothermic.

The versatile chemistry of Mannich bases offers many opportunities for creating tailor-made molecular structures suited to different needs by simple and relatively inexpensive procedures that can be readily carried out. The choice of the Mannich base, namely, 1-[(4-methoxyphenyl)(morpholino-4-yl)methyl]urea (MMPU), has been primarily based on the idea of a clear picture of the role of electron-donating nature of the substituents groups in phenyl ring of the Mannich bases that chiefly govern the electrostatic interaction and chemisorption which decides the inhibition performance of the compounds. The standard weight loss studies, gasometric, potentiodynamic polarization and impedance studies form the methodology of the experimental approaches. The weight loss techniques to be studied at room temperature and elevated temperatures will be useful in determining the inhibition efficiency of the compounds. The polarization studies are intended to evaluate the corrosion potential, corrosion current and Tafel slopes, which will help to propose a suitable mechanism of inhibition. The charge transfer resistance and double-layer capacity values are to be calculated through impedance measurement. The surface coverage values calculated from the weight loss techniques will help to propose a suitable adsorption isotherm that characterizes the adsorption process which is primarily responsible for the inhibition process. The surface examination of the metal specimen both under corroded and inhibited conditions is to be made through Scanning Electron Microscopic (SEM) studies. The performance of the compounds is to be identified based on the above experimental results and correlated with the structural and electronic effects. This will help us to have a clear insight into the process of inhibition and performance of the compounds which culminate in the identification of some potential pickling inhibitors.