Reactive Air Aluminizing of Nicrofer-6025HT for Use in Advanced Coal-Based Power Plants
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luminizing is a widely used technique to improve the oxidation resistance of nickel- and iron-based alloys for high-temperature applications. Components commonly aluminized include turbine blades used in aerospace industries and for stationary power-generation units.[1,2] The aluminizing process is a diffusion-coating technique in which the elements or compounds involved in the coating formation (in this case aluminum) and the substrate elements mutually diffuse and develop a compact scale or coating. This technique not only aids in the development of a protective scale on the substrate by selective oxidation, but also it provides a reservoir of the protective element and thus improves the long-term high-temperature stability.[1] More recently, aluminizing has been suggested as a potential coating forming ferritic stainless steels used in solid oxide fuel cells (SOFC) interconnect components.[2] Aluminizing of these interconnect materials (e.g., the sealing surfaces) results in the development of an aluminized scale on these alloys, which eventually
VINEET V. JOSHI, Research Associate, JUNG-PYUNG CHOI, JENS DARSELL, and K. SCOTT WEIL, Scientists, are with the Pacific Northwest National Laboratory, Richland, WA 99354. Contact e-mail: [email protected] ALAN MEIER, Assistant Professor, is with the Department of Metallurgical and Materials Engineering, Montana Tech, Butte, MT 59701 Manuscript submitted July 6, 2011. Article published online June 8, 2012 S188—VOLUME 44A, JANUARY 2013
yields an alumina layer during the sealing process. Choi et al.[2] observed that aluminizing the sealing surfaces not only helps in mitigating the detrimental effects of the chromia scale on the SOFC cathode material (a mixed ionic and electronic conductors [MIECs]) but also enhances the joint properties between the two faying surfaces. Reactive air brazing (RAB)[3] is one such novel joining technique used to join the two dissimilar materials used in SOFCs. Prevost et al.[4] air brazed different nickel-based alloys to zirconia-toughened alumina, a MIEC, using Ag-CuO binary filler material. It was observed that their joint strength was a function of the oxide scale formed on nickel alloys at the elevated brazing temperature. The results indicated that the nickel-based alloys that formed an outer alumina oxide scale showed the highest strength compared with the alloys that developed chromia scale. Nicrofer-6025HT (Ni-60 wt pct, Cr-25 wt pct Fe11 wt pct, and Al-2 wt pct) has been evaluated as a potential manifold material for the MIEC-based gas separation units.[5] After oxidation, this alloy forms an outer chromia scale and an inner scale of alumina.[6] To join this alloy to the MIEC effectively by air brazing it is necessary to aluminize the faying surface of Nicrofer6025HT. Aluminizing nickel-based alloys causes interdiffusion of aluminum into the nickel substrate resulting in the creation of a stable b-NiAl layer. This b-NiAl formed on the surface oxidizes to form transient phases of alumina and a subsurface of a stable compact alumina layer,
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