Effect of Temperature and Dissolved Oxygen on the Corrosion Behavior of Carbon Steel in High-Temperature Water

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 (ru), and the rate of accumulation of iron in the oxide (r,) were obtained by differentiating the time variations in W, WF, and Wo.°S 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, r T increases with increasing DO and T, r, 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 y-Fe2O3, and above DO = 1.0 ppm, appreciable amounts of a-Fe203 cover the magnetite oxide layer. The rT and rD valuesare not related to the presence of yFe2O3 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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“…These results are in agreement with those obtained in bulk water in the temperature ranging from 333 to 523 K by Mabuchi et al [6], who identified iron (III) oxides and hydroxides for oxygen contents as low as 500 ppb.…”

“…Experimental results and fitted curves. thickness reported by Mabuchi et al[6] after 4 days experiments in bulk water at 333 K in the presence of 2.5 ppm of dissolved oxygen. The thickness of 1.5 lm estimated by these authors is about 4 times higher than in this work (inFig.…”

“…A protective film is likely to be present at the uncorroded area, and to hinder any reaction between the iron surface and the water film. According to former studies [2,6], this protective film should be a thin compact magnetite layer. It has been reported [9] that surface properties such as roughness have a substantial influence on water adsorption characteristics.…”

“…Mabuchi et al [6] have estimated the rate of dissolution and the film thickness formed on iron after 4 days of immersion in water at high temperature. They found that when temperature increases from 333 to 523 K, the rate of corrosion passes through a minimum depending on oxygen content.…”

“…Nevertheless electrochemical measurements in very thin electrolyte layers (<1000 lm) are very difficult to realize, particularly at 363 K. Thus in the literature, the corrosion behaviour of steel at high temperature has often been investigated using analytical methods such as Raman spectroscopy, infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy [2][3][4][5], or using weight-mass measurements [6]. In these references, high temperature studies have generally been performed in bulk solutions and at high pressure and do not reproduce the behaviour of steel under a thin layer electrolyte.…”