Second-harmonic generation technique for in situ study of passive film formation on carbon steel surfaces in aqueous solutions

The aim of this work was to revisit second-harmonic generation (SHG) as a technique for the in situ analysis of oxide film formation on steel samples immersed in aqueous solutions and during electrochemical experiments. The measurements were compared to active reflectance spectroscopy (ARS), which is a more established in situ spectroscopy technique. Carbon steel electrodes were subjected to polarization in pH neutral and mild-alkaline electrolytes to provoke the formation of passive films. In situ SHG measurements allowed monitoring of compositional changes in the formed film, as the SHG intensity increased in the domain of stability of Fe3O4 and decreased in the domain of stability of Fe2O3 during potentiodynamic polarization. ARS, on the other hand, is suitable to monitor film thickness. ARS absorption increased along with film growth when moving in the anodic direction during potentiodynamic polarization. During 30 min cathodic polarization at -0.9 V-Ag/AgClsat. KCl in initially pH neutral the combination of SHG and ARS revealed the presence of an oxide film that was primarily composed of Fe3O4 and with a thickness up to 0.7 nm.