Amino/Keto Derivatives of Pyrazole as Effective Corrosion Inhibitors for Mild Steel in an Acidic Medium: Surface Morphological, Electrochemical, and Computational Studies

The corrosion inhibition performance of two pyrazole derivatives, 3-methyl-1<i>H</i>-pyrazol-5-amine (MPA) and 3-methyl-1<i>H</i>-pyrazol-5-(4<i>H</i>)-one (MPO), on mild steel (MS) in 1 M H₂SO₄ was examined through weight loss, electrochemical methods (polarization and impedance), and quantum chemical analyses. The inhibition efficiency improved with an increasing concentration, achieving 92.28% for MPA and 88.14% for MPO at 0.8 mg/L and 303 K. The adsorption process adhered to the Freundlich model, indicating a heterogeneous interaction on the surface. Electrochemical tests revealed that MPA exhibited superior inhibition at 96.47%, compared to MPO's 85.55% at 0.8 g/L. UV-vis spectroscopy indicated that MPA decreased the band gap energy from 5.44 to 5.28 eV, suggesting a stronger interaction with the metal. Density functional theory further supported a higher adsorption potential of MPA due to its lower energy gap and increased softness. Although the presence of C=O groups in MPO improved adsorption through metal coordination, MPA formed a more stable protective layer, leading to better corrosion resistance. Further modifications with the polar groups on the pyrazole skeleton can lead to the formation of some potent corrosion inhibitors.