Corrosion behavior of an alumina forming austenitic steel exposed to supercritical carbon dioxide
Introduction
The goal of this study was to investigate materials corrosion issues in high temperature sections of the supercritical carbon dioxide (SC-CO2) Brayton cycle for power conversion system in a Generation IV Fast Reactor [1], [2]. The temperatures of SC-CO2 in the recuperator, reactor, turbine, and generator sections can be quite high (about 400–650 °C). Additionally, the SC-CO2 operated in a closed-loop recompression Brayton cycle also offers the potential of equivalent or higher cycle efficiency versus supercritical or superheated steam cycles at temperatures relevant for concentrating solar power (CSP) applications [3]. CSP plants are required to operate at 600–900 °C to improve the power conversion efficiencies. Materials corrosion primarily at these high temperatures will be an important issue, particularly because the expected life times of these components will be 20 years or higher. Additionally, the formation of thick corrosion product oxide layers can impede heat transfer capability in components such as the heat exchanger. Therefore, a detailed knowledge of the corrosion mechanism and rates of microstructure degradation is important to estimate the lifetime of the component and define mitigation strategies for improved corrosion performance of alloys.
The alumina forming austenitic (AFA) steels were developed at Oak Ridge National Laboratory, which exhibit a unique combination of high-temperature creep strength through the formation of stable nano NbC and submicron B2-NiAl and Fe2Nb base Laves precipitate and oxidation resistance via protective alumina scale formation [4], [5], [6], [7], [8], [9]. Since Al2O3 scales have lower growth rates and show better thermodynamically stability when compared to Cr2O3 scales [6], which form on conventional stainless steels, AFA steels hold the potential to permit significantly increased operating temperatures in high-temperature oxidizing environments. Also, the Al2O3 scales have proven to be particularly protective in the presence of aggressive carbon- or sulfur-species and water vapor [10], [11], [12]. It follows that AFA steels show good corrosion resistance in various simulated environments (sulfidation-oxidation, metal dusting, steam and air with 10% water vapor) for chemical processing and energy production applications [13], [14]. It has also been shown that the oxidation resistance of the AFA steels in supercritical water was superior to other austenitic alloys, 800H, D9, and 316 stainless steels tested under similar conditions due to the formation of a protective Al–Cr–Fe-rich oxide layer [15].
In this work, the corrosion behavior of an AFA steel was studied in SC-CO2 at 450–650 °C and 20 MPa, and its multi-scale microstructure and composition were investigated by scanning electron microscope (SEM) and advanced analytical transmission electron microscopy (TEM).
Section snippets
Corrosion tests
The composition (in wt.%) of AFA-OC 6 steel (referred to as AFA hereon) is listed in Table 1. Raw material was provided in plate form and was solution heat-treated. The material was cut using the electrical discharge machining (EDM) technique to yield square test coupon geometry of 12.7 mm × 12.7 mm × 1.5 mm. A 3 mm hole was drilled through the upper corner of each specimen for test mounting purposes. Prior to corrosion testing, all surfaces of the coupons were carefully ground with 800 grit SiC paper.
Weight gain and morphology
At 450 °C, weight changes due to oxidation were low in magnitude, but obeyed the parabolic growth rate law, typically characteristic of the presence of an inner compact and protective oxide layer (Fig. 2). However, the kinetics of oxidation transformed from a parabolic to a linear growth rate law as the test temperature increased. The oxidation curve at 550 °C indicates a trend that is initially linear through an inflection point at 600 h and grows exponentially thereafter. This particular
Discussion
At the beginning of the oxidation of the AFA steel, the rapid uptake of oxygen converts the surface layer to oxides. As shown in Fig. 7 and Table 2, this layer is composed of (Fe, Al, Cr, Mn)-containing oxides. As the oxidation reaction continues, diffusional processes in the AFA begin to affect the oxidation reaction. Since the oxygen activity required to oxidize Cr and Al in the AFA steel is less than that established over the surface of the steel by the (Fe, Al, Cr, Mn)-containing oxides,
Conclusions
At low temperatures or short exposure times the oxide scale was mainly composed of thin and continuous Al2O3 and (Cr, Mn)3O4. As the temperature and exposure time increased, continuity in Al2O3 scale is lost and instead a multilayer structure mainly composed of non-protective Fe3O4, (Cr, Fe)3O4, NiFe/FeCr2O4/Cr2O3/Al2O3, FeCr2O4/Al2O3, NiFe/Cr2O3/Al2O3 forms. The weight gain and scale thickness increases rapidly and the alloy suffers breakaway oxidation.
The mechanism of the breakaway oxidation
Acknowledgements
The authors would like to thank Michael P. Brady, from the Materials Science and Technology Division at Oak Ridge National Laboratory, USA for providing the test AFA steels. This work has been supported by the U.S. Department of Energy through NEUP and by the National Renewable Energy Laboratory and completed in part using the NSF-supported shared facilities at the University of Wisconsin.
References (36)
- et al.
Medium temperature carbon dioxide gas turbine reactor
Nucl. Eng. Des.
(2004) - et al.
Corrosion of alumina-forming austenitic steel Fe–20Ni–14Cr–3Al–0.6Nb–0.1Ti in supercritical water
J. Nucl. Mater.
(2010) - et al.
Corrosion of a stainless steel and nickel-based alloys in high temperature supercritical carbon dioxide
Corros. Sci.
(2013) - et al.
Corrosion of austenitic alloys in high temperature supercritical carbon dioxide
Corros. Sci.
(2012) - et al.
Corrosion of austenitic and ferritic–martensitic steels exposed to supercritical carbon dioxide
Corros. Sci.
(2011) - et al.
Synthesis of Fe3O4 thin films by selective oxidation with controlled oxygen chemical potential
Scripta Mater.
(2011) - et al.
Spinel/metal interfaces in laser coated steels: a transmission electron microscopy study
Acta Metall. Mater.
(1991) - et al.
Hydrogen emission during the steam oxidation of ferritic steels: kinetics and mechanism
Corros. Sci.
(1989) - et al.
Oxidation mechanism of an Fe–9Cr–1Mo steel by liquid Pb-Bi eutectic alloy at 470 °C (Part II)
Corros. Sci.
(2008) - M.Z. Podovski, K.J. Kimball, Proceedings of Conference on Supercritical CO2 Power Cycle Symposium, RPI, Troy, NY, USA,...