Pyrazole‐Based Chelators as Corrosion Inhibitors for Mild Steel in 1 M HCl and Seawater: EIS, SEM/EDX, and DFT Insights

Abstract

ABSTRACT Four pyrazole‐based ligands (L1–L4) were synthesized in a one‐step procedure and evaluated as corrosion inhibitors for mild steel in 1 M HCl and artificial seawater (ASW). In 1 M HCl, electrochemical impedance spectroscopy (EIS) shows that ligand L1 exhibits the highest inhibition efficiency, 94.92% at 300 ppm, whereas L2 and L3 afford only moderate protection, and L4 is the least effective. In ASW, the halogenated ligand L2 displays the best performance, 73% inhibition after five days of immersion; under these conditions, L1 acts as a moderate inhibitor, while L3 and L4 show weak or negligible protection. Gravimetric tests corroborate the EIS findings, with the corrosion rate decreasing from 14.3 to 1.64 mpy at 300 ppm of L1. Scanning electron microscopy (SEM) and energy dispersive X‐ray spectroscopy (EDX) reveal a smooth, pit‐free surface in the presence of the inhibitors, accompanied by a significant drop in oxygen content from 52.11% (wt%) to 5.59% (wt%), confirming the formation of a protective film. Density functional theory (DFT) calculations rationalize the experimental trends, indicating that L1 possesses high polarity and can form two Fe–N chelate rings with short Fe–N distances (2.53–2.62 Å), while L2 combines suitable electronic properties and halogen substitution that favour inhibition in ASW.