Engineered Coatings for Ni Alloys in High Temperature Reactors
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A leading concept in GenIV nuclear plants is the Hecooled very high temperature reactor (VHTR) with ultimate target temperatures in the primary gas loop ‡1223 K (950 C) and a plant design service life requirement of 60 years.[1] In addition to the reactor, a critical component in the primary He loop is the intermediate heat exchanger (IHX) that transfers the energy to a secondary working fluid for use in power generation, chemical processes, etc. While the VHTR reactor concept is generally acknowledged as technically viable,[1] implementation and full exploitation of its potential are critically limited by materials that can withstand the extreme combination of temperature, thermal/mechanical stresses, and a He environment containing low levels of deleterious impurities (O2, CO/CO2, H2/H2O, CH4). Candidate materials of current interest are based on Ni, typified by alloy 617 (UNS N06617).[1–3] Previous work has shown that Ni alloys in the impure He environment at temperatures of order 1173 K to 1273 K (900 C to 1000 C) can be susceptible to oxidation, carburization, and/or decarburization,[4–6] with deleterious effects on their toughness[7] and creep strength.[8] Moreover, the interplay between the environment and thermo-mechanical cycles arising from fluctuations in power or flows of the heat exchange fluids can lead to ELIZABETH A. CLARK, Graduate Student, JAMES Y. YANG, Research Specialist, and CARLOS G. LEVI, Professor, are with the Materials Department, University of California, Santa Barbara, CA 93106-5050. Contact e-mail: [email protected] DEEPAK KUMAR, Graduate Student, is with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2104. GARY S. WAS, Professor, is with the Department of Nuclear Engineering and Radiological Sciences, University of Michigan. Manuscript submitted October 25, 2011. Article published online September 26, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
cracking by the creep-fatigue phenomena.[9] In the absence of suitable environmental protection, the leading candidate alloys are unlikely to maintain their structural integrity at temperatures above ~1123 K (850 C). The success of the VHTR concept depends critically on the development of strategies that improve the stability of alloys in impure He for temperatures up to 1273 K (1000 C) and for long exposure times. Effective approaches to environmental protection of alloys at high temperatures are generally based on thin surface layers of dense, stable, and thermo-chemically compatible oxides. Ni alloys of interest are natural chromia formers, but this oxide is susceptible to reduction and/or conversion to carbide depending on the relative O and C potentials.[4] Alumina is considerably more stable than chromia against hydro- and carbo-thermic reduction at all temperatures of interest, e.g.[10] From a kinetic perspective, alpha alumina (a-Al2O3) is well established as the most efficient diffusion barrier to protect Ni- and Fe-based alloys against oxidation and corrosion in extreme environme
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