Influence of Alloy Microstructure on Oxide Growth in HCM12A in Supercritical Water
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1125-R06-05
Influence of Alloy Microstructure on Oxide Growth in HCM12A in Supercritical Water Jeremy Bischoff1, Arthur T. Motta1, Lizhen Tan2 and Todd R. Allen2 1 Department of Mechanical and Nuclear Engineering, Pennsylvania State University, 227 Reber Building, University Park, PA, 16802, USA. 2 Department of Engineering Physics, University of Wisconsin-Madison, 1500 Engineering Drive, Madison, WI 53706, USA. ABSTRACT HCM12A is a ferritic-martensitic steel alloy envisioned for cladding and structural material in the Generation IV Supercritical Water Reactor (SCWR). This alloy was oxidized in 600ºC supercritical water for 2, 4 and 6 weeks, and the oxide layers formed were analyzed using microbeam synchrotron radiation and electron microscopy. The oxide layers show a three-layer structure with an Fe3O4 outer layer, an inner layer containing a mixture of Fe3O4 and FeCr2O4 and a diffusion layer containing FeCr2O4 and Cr2O3 precipitates along ferrite lath boundaries. The base metal microstructure has a strong influence on the advancement of the oxide layers, due to the segregation at the lath boundaries of chromium rich particles, which are oxidized preferentially. INTRODUCTION The Supercritical Water Reactor is one of the six Generation IV nuclear power plant designs and was envisioned for its high thermal efficiency and simplified core [1]. This reactor is designed to function at high outlet temperature (between 500ºC and 600ºC), which requires cladding and structural materials with good corrosion resistance. Because of their radiation and stress corrosion cracking resistance, ferritic-martensitic steels, such as HCM12A, are candidates for the supercritical water reactor [2]. The oxide layers formed on HCM12A exposed to supercritical water have been previously studied using scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD) [2-4]. These studies have shown that HCM12A forms a dual layer structure at 500ºC with Fe3O4 in the outer layer and spinel (Fe,Cr)3O4 in the inner layer with some evidence of Cr2O3 [4]. At 600ºC an internal oxidation layer or diffusion layer is also observed, showing evidence of FeO [2]. The implantation of an yttrium coating prior to oxidation appears to enhance the corrosion resistance of the alloy [2]. In the present study, HCM12A samples were exposed to 600ºC SCW for three different exposure times: 2, 4 and 6 weeks. The 4-week sample was implanted with an yttrium surface coating prior to oxidation to investigate the influence of this coating on the corrosion resistance but these results are not shown in this article. The oxide layers formed on HCM12A samples were characterized using scanning and transmission electron microscopy (SEM and TEM), and microbeam synchrotron radiation diffraction and fluorescence. The focus is on determining the phases during the formation and evolution of the oxide layer and to understand the influence of the ferritic-martensitic lath structure of the base metal on the advancement of the oxide layer.
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