Abstract
The corrosion behavior of carbon steel in high-temperature water, and the structure and composition of the oxide film were examined as functions of dissolved oxygen concentration (DO), temperature (T), and corrosion time (t). The total amount of iron corroded (WT) was differentiated into the amounts of iron ions in the oxide (WF) and dissolved into the water (WD). The total rate of corrosion (rT), the rate of iron dissolution (rD), and the rate of accumulation of iron in the oxide (rF) were obtained by differentiating the time variations in WT, WF, and WD.
The structure and composition of the oxide film were examined by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and transmission and scanning electron microscopy. In general, rT increases with increasing DO and T, rD shows T- and DO-dependent minimum, and there is serious localized corrosion at high DO above 500 ppb. Oxide films consist of magnetite except at T=60°C, DO=50 to 200 ppb where a thin layer of hydrous ferric oxide is formed. At DO=500 ppb , the outermost part of the magnetite changes into γ-Fe2O3, and above DO=1.0 ppm, appreciable amounts of α-Fe2O3 cover the magnetite oxide layer. The rT and rD values are not related to the presence of αFe2O3 or Fe3O4 in the surface structure of the oxide, but clearly decrease as the OH−/Fe mole ratio at the oxide surface increases. The mechanism determining the corrosion rate changes is discussed.
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