Corrosion inhibition of ductile iron in hydrochloric acid using 5-amino-1,3,4-thiadiazole-2-thiol: electrochemical and computational studies

The corrosion inhibition performance of 5-amino-1,3,4-thiadiazole-2-thiol (5ATT) toward ductile iron in 1.0 M HCl solution was systematically investigated using complementary experimental and theoretical approaches, including weight loss measurements, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and surface characterization (SEM/EDX), supported by density functional theory (DFT) calculations and Monte Carlo (MC) simulations. The results show that the inhibition efficiency increases markedly with inhibitor concentration, reaching ~ 81%, which indicates effective adsorption of 5ATT molecules on the ductile iron surface. Electrochemical measurements revealed a marked decrease in corrosion current density and a significant increase in charge transfer resistance, confirming the formation of a protective adsorbed film, which was further supported by surface analysis. Importantly, the present study provides a clear correlation between the molecular electronic properties and adsorption behavior of 5-ATT and its experimentally observed inhibition performance. Adsorption studies indicated strong interaction between the inhibitor molecules and the metal surface, while thermodynamic parameters suggested a mixed physisorption–chemisorption mechanism. Furthermore, theoretical calculations supported the experimental findings, demonstrating that both the electronic structure and adsorption configuration of 5-ATT play a key role in its corrosion inhibition efficiency. Therefore, 5-ATT exhibits high inhibition performance and strong adsorption capability, highlighting its potential as an effective corrosion inhibitor for ductile iron in acidic environments.