Corrosion in Supercritical carbon Dioxide: Materials, Environmental Purity, Surface Treatments, and Flow Issues

Abstract

Executive SummaryThe supercritical CO 2 Brayton cycle is gaining importance for power conversion in the Generation IV Fast Reactor system because of its high power conversion efficiencies. When used in conjunction with a Sodium Fast Reactor for example, the supercritical CO 2 cycle offers additional safety advantages, by eliminating potential sodium-water interactions that may occur in a steam cycle. In power conversion systems for Generation IV Fast Reactors, supercritical CO 2 temperatures could be in the range of 30oC to 650oC or higher, depending on the specific component in the system. Materials corrosion particularly at the higher temperatures will be an important issue. Therefore, the corrosion performance limits for materials at various temperatures in supercritical CO 2 must be established. Additionally, it is also important to gain a fundamental understanding of mechanisms of materials corrosion in this environment that can guide the selection of alloys for supercritical CO 2 Brayton cycle applications and aid the development of models for long-term corrosion prediction.In this project, the corrosion behavior of various candidate alloys was investigated in supercritical CO 2 at temperatures ranging from 450oC to 650oC for exposure durations of up to 1000 hours (in some cases up to 1500 hours). Most tests were performed at a pressure of 20MPa, although select tests were also performed at 8.27MPa to investigate the role of pressure on corrosion. Alloys investigated included ferritic steels NF616 (Grade T92) and HCM12A (Grade T122), and austenitic alloys IN800H, 347 stainless steel, and three grades of AFA alloys (alumina forming austenitics). AFA alloys, that contain small amounts of aluminum to promote alumina surface layer formation, were developed at Oak Ridge National Laboratory (ORNL) for high temperature oxidation resistance. Tests were performed in research grade (very high purity) and industrial grade CO 2 . Three types of surface treatments, namely, deposition of thin films (~500nm) of aluminum and yttrium, and shot peening were also investigated with the goal of enhancing corrosion resistance.Evaluation of corrosion was performed using weight change measurements, scanning electron microscopy in conjunction with energy dispersive spectroscopy (SEM-EDS), x-ray diffraction (XRD), and for select samples by transmission electron microscopy (TEM). These results are discussed in great detail in the report and in technical papers that have been written or being written on this work. In general ferritic steels exhibited the least corrosion resistance and at best suitable for the lower temperature regime of the temperature range investigated. AFA alloys exhibited better corrosion resistance and in general suitable for the intermediate temperature range, although there was a notable difference in the performance of the three AFA alloys tested. 347 stainless steel and alloy 800H were superior to the other alloys tested with alloy 800H exhibiting the best corrosion resistance. All three surface treatments r...

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