Enhanced Oxygen Diffusion Within the Internal Oxidation Zone of Alloy 617 in Controlled Impurity Helium Environments fro
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THE high temperature gas-cooled reactor (HTGR) concept features high electrical generation efficiency (~50 pct), thermal power output (600 to 800 MWt), and fuel burn-up rates that result from the very high reactor core outlet helium temperatures. Given these features, HTGRs are suitable for the generation of electricity, hydrogen, and to process heat for both residential and commercial uses. The latest HTGR design requires an intermediate heat exchanger (IHX) to transfer the coolant helium to power a gas turbine for electricity generation and to provide heat for hydrogen processing.[1,2] The IHX system is initially designed to operate at temperatures up to 1273 K (1000 C), under cyclic stresses of 7 to 9 MPa and in purified helium.[1,2] Despite the use of advanced purification systems in previous HTGRs, helium coolants still contain CO, CO2, H2, H2O, and CH4 in ppm levels (Table I) due to the leaks in the seals and welds, or outgassing of the reactor core internals and thermal insulators.[3,4] At the designated operating temperatures, these impurities in helium are known to result in surface chromium and internal aluminum oxidation, carburization or decarburization of the IHX alloys.[2–13] The combination of these corrosion modes with cyclic stresses at high temperatures can have severe negative impact on the microstructure stability and mechanical properties of the IHX alloys. Therefore, IHX materials must exhibit superior oxidation resistance, carburization/decarburiGOKCE GULSOY, Graduate Student, and GARY S. WAS, Professor, are with the Department of Materials Science and Engineering, University of Michigan, 2355 Bonisteel Boulevard, Ann Arbor, MI 48109. Contact e-mail: [email protected] Manuscript submitted June 30, 2014. Article published online 22 January 2015 1628—VOLUME 46A, APRIL 2015
zation resistance, and good creep-rupture properties at temperatures up to 1273 K (1000 C). Based on these requirements, Ni-Cr-based Alloy 617 has been identified as the primary candidate IHX alloy.[2–8] Oxidation studies conducted in IHX helium equivalent environments in the temperature range from 1123 K to 1273 K (850 C to 1000 C) have shown that the lifelimiting corrosion processes for Alloy 617 are likely to be decarburization, at temperatures between 1173 K and 1223 K (900 C and 950 C), and selective internal oxidation of aluminum along the grain boundaries.[4,10,12,14–16] Lack of carbide stability in the alloy in decarburizing conditions results in significant reduction in the creep strength,[17] and the internal Al2O3 precipitates can increase the propensity of surface crack initiation decreasing the creep and fatigue resistance of the alloy.[17,18] Failure of Alloy 617 due to decarburization can effectively be eliminated by lowering the reactor core outlet temperatures to maintain the operating temperature range of IHX from 1023 K to 1123 K (750 C to 850 C). In this temperature regime, selective internal aluminum oxidation still remains as a major concern especially for the thin sections of the compact IHX desi
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