Comparative Experimental and Theoretical Studies of Three Different Expired Pharmaceutical Compounds as Green Corrosion Inhibitors of Mild Steel in Different Acidic Solutions

Mild steel (MS) corrosion is a crucial industrial problem and thus efficient inhibitors for MS corrosion are highly demanded. Here, the inhibition of MS corrosion in 1.0 M HCl or H2SO4 solutions utilizing three pharmaceutical compounds expired for human uses (as green inhibitors of MS corrosion) at different exposure times (2 h., 1 day, 2 days and 4 days). These pharmaceutical compounds named as metoclopramide hydrochloride (MCP), chlorpromazine hydrochloride (CPZ) and mebeverine hydrochloride (MEB) and it was studied using electrochemical technique (Potentiodynamic polarization). With raising expired drug concentrations up to 700 ppm, the inhibitory effectiveness of these drugs was improved. Potentiodynamic polarization suggests that at this concentration, CPZ is the most efficient antioxidant after 4 days' exposure time. Potentiodynamic polarization indicates a combined-type inhibition strategy, where the compounds inhibit both anodic and cathodic reactions. Langmuir adsorption isotherm governs the adsorption of investigated inhibitors on MS surface, and the adsorption equilibrium constant (Kads) and free energy of adsorption (ΔGºads) were computed. Density functional theory (DFT) utilizing Becke's three parameter exchange functional (B3LYP) at 6-311G (d,p) level was utilized to compute quantum chemical descriptors of the investigated green corrosion inhibitors and their geometric and electronic structures. The theoretical results derived are in good accord with the obtained practical data. Additionally, scanning electron microscope (SEM) was also employed to explore MS surface before and after immersion in acidic media whether it contained the specified corrosion inhibitor or not.