Gravimetric, electrochemical, and theoretical investigation of thiourea-based corrosion inhibitors for mild steel in 15% HCl at low concentrations

Mild steel is susceptible to heavy corrosion during acid pickling through 15 % HCl. Corrosion inhibitors are hence mixed with the acid solution to mitigate the effects of corrosion. In this study, two thiourea derivatives, namely 1-((2-chloroquinolin-3-yl)methylene)-3-(5-phenyl-1,3,4-thiadiazol-2-yl)thiourea (CMPT) and 1-((2-chloroquinolin-3-yl)methylene)-3-(5-(4-nitrophenyl)-1,3,4-thiadiazol-2-yl)thiourea (CMNT), have been assessed as corrosion inhibitors. At 50 μM, CMPT and CMNT showed maximum inhibition efficiencies of 99.1 % and 95.6 %, respectively. Electrochemical impedance spectroscopy (EIS) presented enhanced total charge transfer resistance (137 Ωcm² for CMPT, 69 Ωcm² for CMNT) as opposed to the blank 4.9 Ωcm²), while potentiodynamic polarization presented a decrease in corrosion current density from 676 μA cm^–2 (blank) to 18.9 μA cm^–2 (CMPT) and 49.3 μA cm^–2 (CMNT). Adsorption experiments established Langmuir adsorption isotherm behavior, with high equilibrium adsorption constants and free energy values (ΔG_ads = -42.03 kJ mol^–1 for CMPT, −41.56 kJ mol^–1 for CMNT), suggesting spontaneous strong adsorption. Density functional theory (DFT) computations and Monte Carlo simulations also supported the adsorption mechanism. CMPT showed better inhibition owing to its increased electron-donating capability and improved surface coverage.