Corrosion mitigation of mild steel in acid media using a potential hydrazide derivative; experimental and theoretical investigation

This study evaluated the corrosion mitigation performance of the hydrazide derivative 2-methyl-3-phenylallylidene hydrazinecarbothioamide (MPHT) for mild steel (MS) in 0.5 M HCl and 0.5 M H₂SO₄ solutions over the temperature range 303–323 K. Electrochemical analysis using potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) demonstrated that inhibition efficiency increased progressively with MPHT concentration, achieving maximum efficiencies of 91% in HCl and 89% in H₂SO₄ at 303 K with 5 mM MPHT. The protective effect diminished at elevated temperatures, indicating predominant physisorption, while adsorption behaviour followed the Temkin isotherm. MPHT acted as a mixed-type inhibitor. Surface characterisation via SEM and AFM confirmed a significant improvement in surface morphology upon inhibitor addition. Complementary theoretical studies, including HOMO–LUMO analysis, Mulliken charges, and Fukui indices, elucidated the electronic properties governing MPHT–metal interactions. Overall, the results establish MPHT as a highly effective corrosion inhibitor for mild steel in acidic media.