Effect of hydroxy group position on the corrosion inhibition performance of 2-(2-hydroxyphenyl)benzothiazole derivatives for mild steel in HCl solution

A series of 2-(2-hydroxyphenyl)benzothiazole derivatives with different hydroxyl substituent positions were synthesized, and their solubility in 1 M HCl solution was preliminarily evaluated; moreover, the corrosion inhibition performance of two derivatives with good solubility (DH4 and DH5) on mild steel was studied in detail. A multi-methodological approach was employed, including ultraviolet–visible (UV–vis) spectroscopy, potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), weight loss measurement, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), density functional theory (DFT), and molecular dynamics (MD) simulations. Among the four derivatives examined, DH5 showed the highest solubility, whereas DH4 exhibited the most effective inhibition performance. In contrast, DH3 and DH6 demonstrated both poor solubility and inferior corrosion inhibition. Adsorption isotherm modeling based on electrochemical data indicated that the protective behavior of DH4 and DH5 followed the El-Awady model. Quantum chemical calculations suggested that the position of the hydroxyl group influences the intramolecular hydrogen bonding strength, which may regulate solubility in acidic media. Molecular dynamics simulations further corroborated the distinction in corrosion inhibition efficacy between DH4 and DH5. This work underscores the critical role of hydroxyl substituent positioning in determining the solubility and corrosion inhibition performance of benzothiazole derivatives in hydrochloric acid solution.