Corrosion Inhibition and Adsorption Properties of <i>N</i>-{2-[2-(5-Methyl-1<i>H</i>-pyrazol-3-yl)acetamido]phenyl}benzamide Monohydrate on C38 Steel in 1 M HCl: Insights from Electrochemical Analysis, DFT, and MD Simulations

The heterocyclic compound <i>N</i>-{2-[2-(5-methyl-1H-pyrazol-3-yl)acetamido]-phenyl}benzamide monohydrate (MPAPB) was synthesized and structurally characterized by using nuclear magnetic resonance (NMR), mass spectrometry, and infrared (IR) spectroscopy. Its corrosion inhibition performance for C38 in 1 M HCl was evaluated by using gravimetric weight loss measurements, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization (PDP) techniques. MPAPB demonstrated a high inhibition efficiency of 90.2% at a concentration of 1 mM, accompanied by a substantial decrease in the corrosion current density. Electrochemical results revealed that MPAPB acts as a mixed-type inhibitor, reducing both anodic and cathodic reactions, while increasing the charge transfer resistance (<i>R</i> _p) and decreasing the double-layer capacitance (<i>C</i> _dl), indicative of effective surface adsorption. The adsorption behavior of MPAPB was consistent with that of the Langmuir adsorption isotherm, suggesting a combination of physical and chemical adsorption mechanisms. Density functional theory (DFT) calculations and molecular dynamics (MD) simulations further elucidated the interaction between the MPAPB and the steel surface, highlighting the role of electron-donating heteroatoms and π-electron systems in adsorption. These theoretical findings were in agreement with the experimental results, confirming the formation of a protective layer that inhibits corrosion.