Oxidation Behavior of Carbon Steel: Effect of Formation Temperature and pH of the Environment
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JMEPEG (2017) 26:5312–5322 DOI: 10.1007/s11665-017-3027-6
Oxidation Behavior of Carbon Steel: Effect of Formation Temperature and pH of the Environment Vivekanand Dubey and Vivekanand Kain (Submitted May 9, 2017; in revised form August 28, 2017; published online October 25, 2017) The nature of surface oxide formed on carbon steel piping used in nuclear power plants affects flowaccelerated corrosion. In this investigation, carbon steel specimens were oxidized in an autoclave using demineralized water at various temperatures (150-300 °C) and at pH levels (neutral, 9.5). At low temperatures (< 240 °C), weight loss of specimens due to dissolution of iron in water occurred to a greater extent than weight gain due to oxide formation. With the increase in temperature, the extent of iron dissolution reduced and weight gain due to oxide formation increased. A similar trend was observed with the increase in pH as was observed with the increase in temperature. XRD and Raman spectroscopy confirmed the formation of magnetite. The oxide film formed by precipitation process was negligible at temperatures from 150 to 240 °C compared to that at higher temperatures (> 240 °C) as confirmed by scanning electron microscopy. Electrochemical impedance measurement followed by Mott–Schottky analysis indicated an increase in defect density with exposure duration at 150 °C at neutral pH but a low and stable defect density in alkaline environment. The defect density of the oxide formed at neutral pH at 150300 °C was always higher than that formed in alkaline environment as reported in the literature. Keywords
corrosion and wear, EIS, FAC, magnetite, oxidation, semiconductors
1. Introduction Carbon steel is extensively used as structural material for majority of the components in the secondary circuit of pressurized heavy water reactors (PHWRs). Feed water system of Indian PHWRs is made of carbon steel ASTM A 106 Gr B (Ref 1, 2). The major mode of degradation of these secondary circuit components/pipelines is flow-accelerated corrosion (FAC). Flow-accelerated corrosion causes thinning of carbon steel components/pipelines carrying high-velocity water/steam. The secondary circuit components are exposed to temperatures up to 200 °C, and the process fluid is deaerated alkaline water (pH adjusted by ethanolamine (ETA) or ammonia). It is known that the mechanism of FAC is electrochemical dissolution of the protective magnetite film (formed on the internal surfaces of carbon steel pipelines/components) at a faster rate in a highvelocity environment (Ref 3-6). FAC has caused a large numbers of failures in piping used in all types of fossil, industrial steam and nuclear power plants (Ref 1, 2, 6-8). The parameters that affect FAC are environmental, material and hydrodynamic (Ref 8, 9). Environmental parameters include water chemistry, e.g., pH, dissolved oxygen (DO) and temperature of the process fluid. Water chemistry especially pH affects the FAC rate by altering the solubility of the oxide film
Vivekanand Dubey and Vivekanand Kain, Materials Processing an
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