In Situ Spectroscopies Unraveling the Molecular Mechanisms of H+ and Cl– on Pyridine Inhibition of Low-Carbon Steel Corrosion

This study utilized a variety of in situ spectroscopic techniques to investigate the corrosion inhibition mechanism of three pyridine-based inhibitors on carbon steel in HCl and NaCl solutions. The results demonstrated that the adsorption and orientation of the inhibitor molecules play key roles in the corrosion inhibition mechanism studied using second harmonic generation (SHG). The number density of molecules adsorbed on the substrate is a crucial factor affecting the inhibition efficiency. A two-step physical model elucidates the SHG signal time evolution, revealing initial rapid inhibitor adsorption and subsequent slow dynamic adsorption optimization. Our research indicated that, in HCl solution, H⁺ protonates the pyridine inhibitors, and the protonated species adsorb onto the positively charged substrate bridging, forming a compact chemisorbed layer that effectively blocks corrosive ion access. In contrast, in NaCl solution, Cl^– promotes the formation of a porous corrosion product film, which synergizes with adsorbed inhibitors to mitigate metal degradation. This finding offers insights into molecular-level interfacial inhibition mechanisms in environments with H⁺ and/or Cl^– ions.