Experimental and theoretical insights into 4-(1-piperazinyl)pyridine as an efficient corrosion inhibitor for mild steel in 1.0 M HCl

The corrosion inhibition performance of 4-(1-piperazinyl)pyridine for mild steel in 1.0 M HCl was systematically investigated using gravimetric measurements, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PDP), surface morphology analysis (SEM), and density functional theory (DFT) calculations. Weight loss results revealed a strong concentration-and time-dependent inhibition behavior, with inhibition efficiency (IE%) increasing from 57.69% at 0.001 M after 1 h to 95.76% at 0.005 M after 24 h. The inhibitor remained highly effective at elevated temperatures, achieving IE% values up to 95.51% at 333 K, indicating thermally enhanced performance. Adsorption followed the Langmuir isotherm with high correlation coefficients (R² > 0.99) and spontaneous adsorption free energies 0 (ΔG_ads = −18.11 to −22.82 kJ/mol), suggesting mixed physisorption–chemisorption behavior. EIS measurements showed a dramatic increase in charge-transfer resistance from 20.4 Ω·cm² (blank) to 463.7 Ω·cm² at 0.005 M, accompanied by a decrease in double-layer capacitance from 215.3 to 34.1 μF/cm² . PDP results revealed a reduction in corrosion current density from 0.708 to 0.007 mA/cm² with a maximum IE of 99.0% and minor shifts in corrosion potential (ΔEcorr ~ 40 mV), confirming mixed-type inhibition. SEM images demonstrated a smooth and compact surface in the inhibited medium. DFT calculations (ΔE = 9.594 eV; ΔN = 0.309) highlighted strong electron-donating ability of nitrogen-rich sites and favorable charge transfer toward iron. The combined experimental and theoretical results confirm that 4-(1-piperazinyl)pyridine is a highly efficient inhibitor for mild steel in acidic environments, with strong potential for industrial acid pickling and descaling applications.