Development of Alumina-Forming Austenitic Alloys for Solid Oxide Fuel Cell Balance of Plant Components
Authors: Brady Michael P , Yamamoto Yukinori , Leonard Donovan N , Meyer Harry M , Momono Shogo , Fukada Shinichiro , Uehara Toshihiro , Ashary Adil , Zhou Lingfeng , Liu Xingbo 
Published: July 2021

Solid oxide fuel cells (SOFC) are of great interest for cleaner energy production. As the technology moves to more widespread commercial adoption, balance of plant (BOP) components such as heat exchangers and other structural components are becoming increasingly important [1]. These components operate at ~600-950°C in the presence of aggressive species such as water vapor. Currently used chromia-forming Fe-based stainless steels and Ni-based alloys suffer from accelerated oxidation under these operation conditions due to enhanced internal oxidation and Cr oxy-hydroxide volatility [1,2]. Further, the volatilization of Cr species from BOP alloys results in Cr poisoning of the SOFC stack and a concomitant reduction in performance [1]. The use of alumina-forming alloys in SOFC BOP applications is a promising approach to reducing Cr contamination of the SOFC stack [3]. Alumina scales offer 1 to 2 orders of magnitude lower oxide growth rate than chromia and are far more stable in water vapor. Most alumina-forming alloys also make use of Cr alloying additions to promote the formation of the protective alumina; however, studies suggest > 10x reduction in Cr release can be achieved despite the presence of Cr in these alumina-forming alloys [3].
Alumina-forming austenitic (AFA) alloys are a new class of stainless steels that offer the potential for improved oxidation resistance via an alumina scale combined with good creep resistance due to their austenitic structure, in a lower cost Fe-based alloy formulation [4]. The alloy design compromises to achieve both creep resistance and alumina formation result in a loss of protective oxidation and a transition to internal oxidation of Al with increasing temperature, typically in the range of ~750-900°C for wrought 20-25Ni mass % based AFA alloys depending on composition [5]. (A 35Ni-based cast grade of AFA alloy capable protective alumina formation to 1100-1200°C is available but is not suitable for SOFC BOP components) [6].
The goal of the present work was to develop a lower cost, 25 Ni mass % grade AFA alloy capable of long-term SOFC BOP use at ~800-950°C in water vapor containing environments. The composition range of interest was based on Fe-25Ni-(13-18)Cr-4Al-(1-2.5)Nb-(0.5-2)Mn-(0.15-0.5)Si-(0-2)W-(0-2)Mo-(0-1)Cu-(0-0.1)Ti-(0.1)V-(0.05-0.2)Zr-(0.05-0.2)Hf-(0-0.1)Y-(0.02-0.2)C-(0.005-0.015)B mass %. The effort focused on systematic variation of C, Cr, and Nb levels ± Hf, Y, and Zr, which impact oxidation resistance, manufacturability, and cost. Key findings to achieve oxidation resistance at ≥850-900°C in air with 10% H2O without loss of creep resistance included:

Protective alumina formation was enhanced by use of Zr additions, rather than previously demonstrated, more costly additions of Hf and Y.

Levels of Nb as low as 1.5 mass % were sufficient to achieve protective alumina formation, as compared to previously used 2.5 mass % Nb levels, which reduces cost and aids manufacturability.

The Cr addition levels were critical to pushing protective alumina formation past 900°C, with the critical transition occurring between ~14.5 and 16 mass % Cr.

Protective alumina formation was achieved with lower than expected C addition levels, as low as 0.02 to 0.03 mass % C, which aids manufacturability to thin product forms by reducing primary coarse NbC formation.

Details of the alloy design strategy and oxidation behavior out to 10,000 h of exposure at 900-1000°C in air with 10% H2O will be presented. Insights for the oxidation mechanism relative to Cr evaporation behavior will also be discussed.
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