In Situ Raman Spectroscopic Analysis on Carbon Steel, Immersed in Aqueous Solutions at Different pH and Anions

In situ Raman spectroscopic studies on carbon steel specimens immersed in aqueous solutions showed the presence of Fe hydroxy salt, specifically green rust (GR), as the initial corrosion product in acidic, neutral and alkaline conditions. With increasing exposure time, GR was transformed to oxy-hydroxides, predominantly lepidocrocite (gamma-FeOOH) in the upper layer and magnetite (Fe3O4) in the oxygen-deficient lower layer. The stability of GRs and their transformation are found to be dependent on the pH of solutions and anions present. The de-convoluted Fe peak in the Fe 2p(3/2) region of XPS spectra of the corrosion products, formed on CS specimen immersed in tap water for 80 h, indicate the presence of lepidocrocite phase (gamma-FeOOH), which was also evident from the laminar morphologies seen in SEM micrograph. The predominance of lepidocrocite over magnetite phase, in the presence of sulfate ions and in the low pH conditions, was in good agreement with the highest corrosion rate observed.