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.