Expired fexofenadine hydrochloride acts as a high-performance sustainable corrosion inhibitor for copper in MSF desalination applications

This study evaluates the repurposed application of an expired non-sedating antihistamine drug (ENSAD), fexofenadine hydrochloride, as a high-performance “green” corrosion inhibitor for copper in 1.0 M HCl. Gravimetric results demonstrate a concentration-dependent inhibition efficiency reaching 96.4% at 120 ppm, with remarkable long-term stability (> 92.5% efficiency after 72 h). Adsorption behavior followed the Langmuir isotherm model, indicating the formation of a stable monolayer. The calculated Gibbs free energy (∆Goads =-33.8 kJ mol− 1) confirms a comprehensive physicochemical adsorption mechanism involving both electrostatic attraction and chemical coordination. Thermodynamic investigations revealed that the addition of ENSAD increased the activation energy from 30.36 to 53.89 kJ mol⁻¹, creating a substantial energy barrier against metallic dissolution. Electrochemical studies (PDP and EIS) confirmed ENSAD as a mixed-type inhibitor that significantly enhances charge transfer resistance (Rct). Quantum chemical parameters, including a low energy gap (ΔEg = 2.361 eV) and high softness (S = 0.424 eV− 1), corroborate the high reactivity and electron-donating capability of the molecule’s heteroatoms (N, O) and π-systems. Surface characterization (SEM/EDX) visually and chemically confirmed the presence of a robust organic film. These findings position ENSAD as a technically viable, thermally stable, and sustainable alternative for corrosion protection in industrial acid cleaning and low-temperature desalination stages.