Robust pyrazole-based thiourea scaffolds as carbon steel corrosion inhibitors in 1 M HCl, complemented by electrochemical and theoretical approaches.

Herein, polyfunctionalized pyrazole-based thiourea scaffolds were synthesized utilizing ethanol as a top-tier green solvent and evaluated as inhibitors for carbon steel degradation in 1 M HCl through the weight loss method, potentiodynamic polarization (PP) measurements, open circuit potential (OCP) measurements, electrochemical impedance spectroscopy (EIS), test solution analysis, surface characterizations, and quantum chemical parameter analysis. Maximal inhibition achieved was 91.59% protection at 25 °C (rising to 97.21% at 55 °C) for 2-(5-(4-methoxyphenyl)-1-phenyl-1H-pyrazole-3-carbonyl)-N-phenylhydrazine-1-carbothioamide (MPPC), while N-allyl-2-(5-(4-methoxyphenyl)-1-phenyl-1H-pyrazole-3-carbonyl) hydrazine-1-carbothioamide (NAPC) reached 85.71% at 25 °C and 93.5% at 55 °C. Adsorption analysis showed good agreement with the Langmuir isotherm. PDP analysis indicated a mixed-type inhibition behavior. The current density decreased significantly from 285.0 µA cm-2 (without inhibitor) to 32.7 and 35.8 µA cm-2, while EIS confirmed an increase in the charge transfer resistance from 44.70 Ω cm2 (uninhibited medium) to 608.0 and 462.7 Ω cm2 for MPPC and NAPC, respectively, demonstrating the formation of a stable adsorbed layer on the surface. After immersion for one day, contact angle (CA) measurements, Fourier transform infrared spectroscopy (FT-IR) and atomic force microscopy (AFM) confirmed the adsorption of the inhibitor. UV-visible spectroscopy confirmed chemical interactions between MPPC/NAPC and the surface. The experimental results were additionally validated through DFT calculations, Monte Carlo (MC) simulations, and Fukui index analysis. The novelty of this study stems from the molecular design of MPPC and NAPC. These molecules combine electron-rich phenyl and pyrazole rings with extended π-conjugations, thereby boosting surface adsorption and inhibition. The tested inhibitors are cost-effective, and their impressive thermal stability and high inhibition performance in acidic environments highlight the innovative molecular structure and their potential for use under harsh conditions. The synthesized compounds effectively control corrosion, paving the way for the development of efficient, multifunctional, and eco-friendly inhibitors.