Raman Investigation of Anodic Undermining of Coated Steel During Environmental Exposure

The atmospheric corrosion of coated steel samples with artificial defects was characterized to determine the environmental variables that are important in the corrosion process. Coated steel samples were exposed to different climates in Hawaii, Florida, and Ohio. The iron oxides formed under the coating were identified with Raman spectroscopy, which allowed the corrosion mechanisms to be deduced. Those mechanisms were influenced by the time of wetness (TOW) and the deposition rate of chloride and sulfur that were measured at the different sites. Two corrosion morphologies were observed: bead-like corrosion was formed in the defect vicinity and filiform corrosion (FFC) was initiated from the bead-like corrosion. Bead-like corrosion was influenced by the TOW and the salt deposition, whereas FFC grew in the humid environment, but its mechanism was independent of the salt deposition. At almost all of the sites tested, chloride was the only salt that participated in the corrosion process.