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

4-Hydroxypiridine (4-HPy) is a green chemistry corrosion inhibitor for low-carbon steel, valued for its environmental compatibility and low toxicity. Despite lower initial effectiveness than 4-mercapto/4-aminopyridine, 4-HPy's performance is markedly enhanced by halogen ions. By employing second harmonic generation (SHG) spectroscopy combined with electrochemical methods, Raman spectroscopy, atomic force microscopy, x-ray photoelectron spectroscopy, and in situ UV spectroscopy, this study elucidates the synergistic enhancement mechanism of 4-HPy with Cl-, Br-, and I- in 0.5 mol/L HCl solution. Time-dependent SHG measurements showed a two-step process of rapid adsorption and subsequent orientation change, with a proposed mechanism to interpret the temporal changes in SHG intensity. Deducing the adsorption kinetic equations and their application to the experimental data yields the adsorption rate (kad) and orientation change rate (Kre). Halogens reduce the orientation angle of 4-HPy, facilitating its adsorption on the substrate surface and effectively inhibiting corrosion via distinct mechanisms. Cl- and Br- ions primarily adsorb onto the metal surface, forming an adsorption film that not only enhances the subsequent adsorption of 4-HPy but also provides a protective effect for the metal surface. Conversely, I- forms mainly complexes with 4-HPy in solution, co-adsorbs onto the metal surface, and demonstrates a significant synergistic effect. This study revealed the synergistic efficacy hierarchy among halogen ions, with the order 4HPy + NaCl < 4HPy + NaBr < 4HPy + NaI. This study enhances our molecular-level understanding of the synergistic mechanism between halogen ions and corrosion inhibitors and provides valuable insights for designing and developing effective corrosion inhibitors.