In situ Raman Spectroscopic Study of NaCl Particle-Induced Marine Atmospheric Corrosion of Carbon Steel

The study of NaCl particle-induced atmospheric corrosion is facilitated by in situ Raman spectroscopic analysis through real time identification of corrosion products. Droplet formation on carbon steel by the deliquescence of pre-deposited NaCl particles that were exposed to high relative humidity (RH) (>80%), and corrosion initiation under the droplets were observed using an optical microscope. Pitting initiated within several minutes under the NaCl droplets. In situ and ex situ Raman spectra from the various rust species that formed during the corrosion process show that green rust (GR) formed close to the corrosion pit (i.e., anodic site) while lepidocrocite (γ-FeOOH) clusters precipitated outside of the GR region. A small amount of magnetite (Fe3O4) was also detected, mostly in the transitional region from GR to lepidocrocite. Upon exposure to an ambient atmosphere, GR transformed to lepidocrocite. This study confirms that the initial rust formed in NaCl particle-induced marine atmospheric corrosion of carbon steel is GR and lepidocrocite. While revealing the initiation mechanisms of NaCl particle-induced marine atmospheric corrosion of carbon steel, this study also provides insights for future studies involving wet-dry cycle exposures, which are more akin to the real atmospheric corrosion process.