1-(2-Pyridyl)-2-thiourea (TP) and 2-(imidazol-2-yl)-pyridine (IP) are described here for the first time as inhibitors of mild steel corrosion in acidic medium based on investigations such as weight loss tests, potentiodynamic polarization measurements and electrochemical impedance spectroscopy (EIS). The experimental results revealed that 1-(2-pyridyl)-2-thiourea and 2-(imidazol-2-yl)-pyridine are effective corrosion inhibitors for mild steel in acidic medium, and their maximum corrosion inhibition efficiencies at 4 × 10⁻⁴ M are 93.57% and 96.66%, respectively. TP and IP are determined as mixed-type inhibitors based on polarization studies, and their adsorption on the mild steel surface follows the Langmuir adsorption isotherm and physical adsorption is dominant. The formation and characteristics of the protective layer on the steel surface were verified by scanning electrochemical microscopy (SECM), UV-visible spectroscopy, FT-IR spectroscopy and X-ray photoelectron spectroscopy (XPS) methods. Besides, the correlation between the inhibition efficiency and the molecular structure of inhibitors was theoretically studied via quantum chemical calculations.

The inhibition performance of three pyrido[1,2-a]benzimidazoles, namely, benzo[4,5]imidazo[1,2-a]pyridine (BIP), 2-methylbenzo[4,5]imidazo[1,2-a]pyridine (MBIP), and 2-(trifluoromethyl)benzo[4,5]imidazo[1,2-a]pyridine (TBIP), for steel corrosion in hydrochloric acid solution were researched through a weight loss test, electrochemical techniques, surface morphology (SEM, XPS) studies, and theoretical methods. The experimental results revealed that TBIP, BIP, and MBIP had excellent inhibition performance, and their maximum inhibition efficiencies at 0.25 mmol L⁻¹ were 95.48%, 97.65%, and 98.96%, respectively. The properties of these inhibitors are considered to be mixed-type, and their adsorption mode belongs to the Langmuir isothermal type. The formation and properties of an adsorbed film on a steel surface were investigated using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Quantum chemical calculations provided insightful quantitative information to conclude the correlation between the molecular structures and inhibition performance. Molecular dynamics (MD) simulations were carried out to explore the configurationally adsorption behavior of these pyrido[1,2-a]benzimidazoles on Fe(110) surface, and the binding energy of these three inhibitors followed the order MBIP > BIP > TBIP, which is consistent with the experimental evaluation results.