Synergistic corrosion inhibition of 4-hydroxypyridine and halogen ions: Insights from interfacial nonlinear spectroscopy.

This article focuses on the molecular-level investigation of the synergistic corrosion inhibition mechanism of 4-hydroxypyridine (4-HPy) combined with halogen ions (Cl⁻, Br⁻, I⁻) on N80 low-carbon steel in 0.5 mol/L hydrochloric acid. Using in situ second harmonic generation (SHG) spectroscopy alongside electrochemical methods, Raman spectroscopy, atomic force microscopy, x-ray photoelectron spectroscopy, and UV–vis spectroscopy, the study reveals that halogen ions enhance 4-HPy adsorption by reducing its molecular orientation angle, leading to denser and more upright adsorption films that improve corrosion resistance. The synergistic efficacy follows the order I⁻ > Br⁻ > Cl⁻, attributed to differences in electronegativity and ionic radii, with iodide ions forming complexes with 4-HPy in solution prior to co-adsorption on the steel surface, while chloride and bromide ions primarily adsorb on the metal surface first. These findings provide insights for designing more effective, environmentally compatible corrosion inhibitors for industrial applications involving acidified carbon steel.