Corrosion inhibition of mild steel in 1 M HCl solution by N-(4-methoxyphenyl)piperazine-1-acetamide: potentiodynamic polarization, weight loss, SEM analysis, and DFT computational studies

• H.M. Luaibi¹, M.A. Ahmed², N. Betti³, S. Al-Taweel⁴ and A. Alamiery⁵

¹ Alkarkh University of Science, College of Science, Department of forensic Sciences, P.O. Box: 10001, Baghdad, Iraq
² Alkarkh University of Science, College of Energy and Environmental Sciences, Department of Renewable Energy Sciences, P.O. Box: 10001, Baghdad, Iraq
³ Materials Engineering Department, University of Technology-Iraq, P.O. Box: 10001, Baghdad, Iraq
⁴ Al-Naji University, P.O. Box: 10001, Baghdad, Iraq
⁵ Al-Ayen Scientific Research Center, Al-Ayen Iraqi University, AUIQ, An Nasiriyah, P.O. Box: 64004, Thi Qar, Iraq

Abstract: This study investigates the corrosion inhibition performance of N-PAP (N-(4-methoxyphenyl)piperazine-1-acetamide) for mild steel protection in 1.0 M hydrochloric acid solution through an integrated experimental and theoretical approach. Potentiodynamic polarization measurements revealed that N-PAP functions as a mixed-type corrosion inhibitor, achieving a maximum inhibition efficiency of 80.3% at 0.5 mM concentration. The compound systematically reduced corrosion current density from 117.4 µA·cm^-2 (blank solution) to 23.1 µA·cm^-2 and decreased the corrosion rate from 56.3 mpy to 10.3 mpy, representing an 82% reduction in material loss. Gravimetric studies confirmed enhanced protection with extended exposure time and elevated temperatures, reaching 97.4% efficiency at 333 K and 1.0 mM concentration. The inhibitor adsorption followed the Langmuir isotherm model with excellent correlation (R²=0.9956), indicating monolayer formation through predominantly physisorption mechanisms. The calculated standard free energy of adsorption (ΔG⁰_ads=–13.06 kJ/mol) confirmed spontaneous adsorption. Scanning electron microscopy analysis demonstrated significant surface protection, showing smooth, intact morphology in inhibited samples compared to severe corrosion damage in uninhibited specimens. Density functional theory calculations provided molecular-level insights, revealing favorable electronic properties including HOMO energy (–7.873 eV), LUMO energy (0.533 eV), and optimal electron transfer fraction (ΔN=0.397) that support strong metal-inhibitor interactions. The proposed inhibition mechanism involves coordinate bond formation through nitrogen atoms in the piperazine ring and additional anchoring via the para-methoxy group, creating a protective organic film that blocks both anodic metal dissolution and cathodic hydrogen evolution reactions. These findings establish N-PAP as an effective, environmentally compatible corrosion inhibitor with practical applicability for mild steel protection in aggressive acidic environments.

Keywords: piperazine, corrosion inhibitor, potentiodynamic polarization, DFT, mechanism

Int. J. Corros. Scale Inhib., 2026, 15, no. 1, 351-381
doi: 10.17675/2305-6894-2026-15-1-18

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