Experimental and Computational Anticorrosion Behaviors of Pyrazole s‑Triazine/anilino-morpholino Derivatives for Steel in Acidic Solutions

The corrosion inhibition of C-steel by two s-triazine/morpholino-anilino-pyrazole derivatives, namely, 4-(3,5-dimethyl-1H-pyrazol-1-yl)-6-morpholino-N-phenyl-1,3,5-triazin-2-amine (1) and N-(4-bromophenyl)-4-(3,5-dimethyl-1H-pyrazol-1-yl)-6-morpholino-1,3,5-triazin-2-amine (2) was investigated by impedimetric and potentiometric studies. It was found that (1) and (2) acted as cathodic-type corrosion inhibitors that retard the hydrogen evolution reaction. The percent corrosion inhibition, 98.5% for compound (2) (with bromo substituent) at 80 ppm, was slightly higher than 97.8% for (1) at 100 ppm. Thus, the replacement of a –H with –Br substituent increased the corrosion inhibition properties. Compound (2) exhibited Temkin isotherm adsorption, whereas compound (1) exhibited Langmuir adsorption. Scanning electron microscopy (SEM) analysis of the steel surface indicated that the inhibitors caused protection of the surface. The weight loss experiment also proved the decrease in the corrosion rate when inhibitors were added. The difference in inhibitory efficiency between compounds (1) and (2) was investigated by density functional theory (DFT) to study neutral and protonated species in gaseous and aqueous phases. The theoretical analysis demonstrated that compound (2) exhibited higher inhibitory activity on a metal surface compared to compound (1), aligning with the experimental results. The energy associated with the metal/adsorbate arrangement, represented by dE ads/dNi , was higher for (2) (−380.91 kcal mol–1) compared to (1) (−371.64 kcal mol–1). This indicated better adsorption of (2) over (1).