Acid-induced water solubility in a phenyl-rich thiosemicarbazone Schiff Base enables highly efficient inhibition of mild steel corrosion

Limited aqueous solubility restricts the application of highly conjugated organic corrosion inhibitors in acidic media. To address this limitation, six phenyl-rich thiosemicarbazone Schiff bases ( 2a – 2f ) were synthesized from substituted diphenyl α,β-unsaturated ketones and 4-phenylthiosemicarbazide and characterized using spectroscopic, thermal, and computational methods. Among them, the dimethylamino -substituted derivative 2a was fully soluble in 1 M HCl at the concentrations used for electrochemical testing without requiring an organic co-solvent, a behavior attributed to acid-induced protonation of its amino groups. At 30°C, 2a inhibited mild-steel corrosion in 1 M HCl in a concentration-dependent manner. At 1.0 × 10 -3 M, the corrosion current density decreased from 1.69 to 0.098 mA cm -2 , whereas charge-transfer resistance increased from 10.5 to 235 Ω cm 2 , corresponding to inhibition efficiencies of 94.2% by potentiodynamic polarization and 95.5% by electrochemical impedance spectroscopy. The small corrosion-potential displacement (≤19 mV) identified 2a as a mixed-type inhibitor. Adsorption was spontaneous (ΔG ° ads ≈ −41 kJ mol -1 ) and consistent with a mixed physicochemical mechanism dominated by chemisorption. Surface characterization and molecular dynamics simulations further supported the formation of a protective adsorbed layer. These findings establish acid-responsive protonation as an effective molecular-design strategy for reconciling extended π-conjugation with aqueous compatibility in metal-free corrosion inhibitors.