Expired Dantrolene drug as sustainable corrosion inhibitor of mild steel in acidic medium.

Corrosion of mild steel in acidic environments is a major industrial and environmental concern. In this work, expired Dantrolene was investigated as a sustainable corrosion inhibitor for mild steel in 1 M H2SO4. Potentiodynamic polarization measurements showed high inhibition efficiency, reaching 92 % at 500 ppm and 318 K, with efficiency increasing with inhibitor concentration but decreasing at elevated temperatures. Adsorption followed the Langmuir isotherm and occurred spontaneously through a mixed physisorption-chemisorption mechanism. Density Functional Theory (DFT, B3LYP/6-31G) supported these findings, revealing that nitrogen and oxygen atoms, along with the aromatic conjugation in Dantrolene, facilitate strong donor-acceptor interactions with the steel surface. Complementary surface characterization confirmed the protective role of the inhibitor: FTIR identified active functional groups (C=O, -NO2, -NH) responsible for adsorption; SEM and AFM images revealed the formation of a protective film that reduced surface damage; and EDX spectra confirmed the presence of O and S atoms on the steel surface, indicating chemical interaction with the alloy. Together, these results demonstrate that expired Dantrolene is an effective, low-cost, and eco-friendly inhibitor, offering a dual benefit of corrosion protection and pharmaceutical waste management.