Corrosion Mechanism of 9Cr Steel Under CO2 at 550 C Identification of Key Environmental Parameters and Technological Impacts

by Fabien Rouillard, Gervaise Moine, & Jean-Christophe Ruiz
French Alternative Energies and Atomic Energy Commission (CEA)

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In the framework of new generation of sodium fast reactor whose one prototype is expected in France in 2020 (ASTRID project), supercritical carbon dioxide used in Brayton cycle is identified as an attractive alternative energy conversion system in regard with classical Rankine steam cycle or ideal gas-like cycles. The use of this fluid improves the energetic efficiency, the cycle compactness and above all eliminates the possibility of an explosive reaction between water and sodium. In the current concept, the outlet temperature and pressure of CO2 in the intermediate heat exchanger (IHX) is about 550°C and 250 bars. Ferritic-martensitic steels (F/M) with 9-12 wt% Cr are good candidates as structural materials of IHX because they have a low coefficient of thermal expansion and high thermal conductivity. Nevertheless, a comprehensive study of its corrosion resistance in the operating conditions is necessary to ensure an IHX lifetime of at least 20 years. For this purpose, samples of T91 which is currently the reference 9wt% Cr F/M steel were exposed in CO2 at 550°C for different durations from 1h to 1000h. Tests at atmospheric pressure and 250 bars have revealed the influence of total pressure of CO2 on the corrosion behavior. Extensive analyses of the corrosion products by multiple analytical techniques (GDOES, FESEM, MET, Raman microscopy and micro-hardness, LECO analysis) coupled to time evolutions of the corrosion layer thicknesses have allowed to propose a mechanism and kinetics of corrosion damage. The total pressure of carbon dioxide and equivalent oxygen partial pressure were identified as the two main factors influencing the corrosion behavior. Possible impacts of the corrosion behavior on the cycle efficiency are initiated.