Oxidation of Iron in 288°C, Oxygen-Containing Water

In situ Raman spectroscopy, surface-enhanced Raman spectroscopy, scanning electron microscopy (SEM), and measurements of corrosion potential were used to investigate the films formed on iron in 288°C water with dissolved oxygen concentrations that ranged from 0 to 208 ppb. The film that formed on iron immersed in water containing 0 to 22 ppb oxygen consisted mostly of faceted, loosely packed particles of magnetite (Fe₃O₄). It was inferred from SEM and Raman spectra that the film on iron was composed of two layers. The faceted particles of Fe₃O₄ made up the outer layer and an inner conformal layer of Fe₃O₄ formed the inner layer. Passivation of the iron surface began at dissolved oxygen concentrations of ≈8 ppb and was a two-step process. First, between 8 ppb and 22 ppb oxygen, there was an increase in the protectiveness of the Fe₃O₄. Second, between 22 ppb and 53 ppb, the combination of SEM and Raman spectra suggested that the outer surface of Fe₃O₄ transformed to α-Fe₂O₃, which completed passivation.