Compatibility of Ni- and Fe-based alloys in high-temperature $supercritical-CO_2$ environment

Abstract

The supercritical-carbon dioxide $(S-CO_2)$ Brayton cycle is being considered as alternative working fluid of energy conversion system replacing the conventional steam Rankine cycle in the next generation nuclear reactors (Gen-IV) such as sodium-cooled fast reactor (SFR) and other applications. In structural mate-rial point of view, material compatibility in $S-CO_2$ environment should be evaluated to assure the long-term integrity of the system. In this thesis study, corrosion and carburization behavior of chromia-forming alloys were investigated in high-temperature $S-CO_2$ environment for the application of $S-CO_2$ to the SFR. Ni-based alloys (Alloy 600 and Alloy 690) and high-Cr containing Fe-based alloy (Alloy 800HT) were corroded in $S-CO_2$ at $550-650^\circ C$ (20 MPa) for 1000 h. For all test alloys, superior corrosion resistance was observed by the formation of thin and continuous chromia $(Cr_2O_3)$ layer. Meanwhile, despite the presence of continuous chromia layer, an existence of carburized region as an amorphous C layer was identified at the chro-mia/matrix interface. In addition, below the amorphous C layer, Cr-rich $M_{23}C_6$ carbides were extensively formed in the matrix of Alloy 800HT but not in Alloy 600 or Alloy 690. It could be said that the carburiza-tion resistance in the matrix was strongly dependent on whether the matrix is Ni-based or Fe-based rather than Cr content since chromia layer was formed in all test conditions. It is important to mention that the ex-istence of Cr-rich $M_{23}C_6$ carbides in Alloy 800HT caused additional loss of ductility as compared to bulk aged specimens. On the other hand, alumina $(Al_2O_3)$ layer is known to have a better corrosion and carburization re-sistance than chromia although it has various oxide types depending on exposure temperatures. Corrosion and carburization behavior of alumina-forming Ni-based alloy was evaluated in SFR-relevant $S-CO_2$ environment. First, in order to form alumina-forming Ni-based alloy, an Al-rich surface layer was developed on Alloy 600 by Al deposition and a subsequent high energy electron beam (EB) remelting. As a result of the EB surface treatment, an Al enriched (5-7 wt.%) micro-alloying zone (40 μm) was produced. When the EB surface-treated Alloy 600 was corroded in $S-CO_2$ at $600^\circ C$ (20 MPa) for 500 h, the surface oxide layer mostly consisted of chromia with small amount of transition alumina. In addition, a carburized region of an amor-phous C layer inter-mixed with the alumina was observed at the oxide/matrix interface similar to as-received Alloy 600. Meanwhile, when the EB surface-treated specimen was pre-oxidized in helium at $900^\circ C$ , α-alumina layer was formed on the surface, which showed superior corrosion and carburization resistance in $S-CO_2$ environment. Therefore, it could be said that the presence of Al-rich surface layer alone is not enough to provide sufficient corrosion and carburization resistance in $S-CO_2$ environment at $600^\circ C$ unless pre-oxidation at higher temperature is applied to form a more protective α-alumina on the surface. Finally, for the better understanding of corrosion and carburization behavior in high pressure $S-CO_2$ environment (20 MPa), the effect of $CO_2$ pressure on those properties were also evaluated. Ni-based alloys (Alloy 600 and Alloy 690) and Fe-based alloy (Alloy 800HT) were corroded in high-temperature $CO_2$ (or $S-CO_2$) at 550-650 oC (0.1 and 10 MPa) for 1000 h. Combining with the results tested in 20MPa, following conclusions could be drawn. First of all, higher $CO_2$ pressure accelerated corrosion rate in terms of weight gain and oxide thickness while the same oxides were formed regardless of $CO_2$ pressure. In addition, carbu-rized region of the amorphous C layer was also formed at the oxide/matrix interface even in 0.1 MPa with increasing the thickness at higher $CO_2$ pressure. However, the accelerated corrosion and carburization kinet-ics are less than a factor of 2-3, which was not significant compared to pressure ratio up to 200. Moreover, for Alloy 800HT, the depth of precipitate-containing region (Cr-rich $M_{23}C_6$ carbides) was similar in all $CO_2$ pres-sures from 0.1 to 20 MPa, resulting in similar changes in tensile property at a given temperature. Those indi-cate that since the changes in mechanical property of chromia-forming alloys in $S-CO_2$ environment (20 MPa) were similar to those in lower $CO_2$ pressures, various test results from low $CO_2$ pressure could be uti-lized as a reference with limited number of high pressure tests for the system design of $S-CO_2$ cycle applica-tions.