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Title: Effects of Oxide Thickness on Scale and Interface Stresses under Isothermal Cooling and Micro-Indentation for Ferritic Stainless Steel Interconnect

Abstract

Interconnects in solid oxide fuel cells (SOFCs) provide the electrical connection between the individual cells, as well as separate the anode air from the cathode fuel for each cell. The recently developed anode-supported planar SOFCs design allows the cell working temperature to be 800oC and lower, which led to the proposed use of ferritic stainless steel as interconnects and current collectors in the SOFC systems. Many experimental studies have shown that metal based interconnects will go through surface oxidation under the typical operating temperature and environment of an SOFC system. Since the oxide scale is formed at the outer surface of interconnect, its microstructure (morphology, texture), properties (electrical, thermal and mechanical properties), and structural integrity/stability will have a significant impact on the performance of interconnect. In this paper, we study the oxide growth kinetics and stresses generated during scale growth the thermal cycling for Cr-Fe based alloy, e.g. Crofer 22 APU. The goal is to predict the interconnect life under typical operating conditions and thermal cycles. First, a mathematical model is developed based on the growth of scale and coupling relation between diffusion and stress field. The following phenomena are considered: 1) The predominant cationic diffusion and cation distribution withinmore » the oxide scale in transient state and steady state; 2) Chemical reaction within the scale as well as the growth kinetics of the oxide scale; 3) Stresses distribution during the oxidation, especially for mid- and long-term treatment; 4) Coupling relationship or interaction between oxidation induced stresses and mass diffusion within the scale. Next, thermal stresses generated due to temperature changes in thermal cycling are predicted using finite element analyses for different substrate and scale thickness. In general, very high compressive in-plane stresses are predicted for the oxide layer. Also, high shear stress is also predicted on the oxide/substrate interface. The predicted shear stress on the interface is used to compare with the experimentally determined bond strength to predict delamination. Finally, scale spallation and IC life prediction are predicted by comparing the compressive stress in the scale with the critical buckling load for the delaminated scale.« less

Authors:
; ; ; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
960330
Report Number(s):
PNNL-SA-52354
AA2530000; TRN: US200923%%401
DOE Contract Number:
AC05-76RL01830
Resource Type:
Conference
Resource Relation:
Conference: 30th Fuel Cell Seminar: ECS Transactions, 5(1):357-368
Country of Publication:
United States
Language:
English
Subject:
30 DIRECT ENERGY CONVERSION; 36 MATERIALS SCIENCE; ANODES; CATHODES; CHEMICAL REACTIONS; DIFFUSION; FUEL CELLS; MATHEMATICAL MODELS; MICROSTRUCTURE; MORPHOLOGY; OXIDATION; OXIDES; SOLID OXIDE FUEL CELLS; STAINLESS STEELS; STRESSES; THERMAL CYCLING; THERMAL STRESSES; THICKNESS

Citation Formats

Sun, Xin, Liu, Wenning N., Vetrano, John S., Yang, Zhenguo, Recknagle, Kurtis P., and Khaleel, Mohammad A. Effects of Oxide Thickness on Scale and Interface Stresses under Isothermal Cooling and Micro-Indentation for Ferritic Stainless Steel Interconnect. United States: N. p., 2007. Web.
Sun, Xin, Liu, Wenning N., Vetrano, John S., Yang, Zhenguo, Recknagle, Kurtis P., & Khaleel, Mohammad A. Effects of Oxide Thickness on Scale and Interface Stresses under Isothermal Cooling and Micro-Indentation for Ferritic Stainless Steel Interconnect. United States.
Sun, Xin, Liu, Wenning N., Vetrano, John S., Yang, Zhenguo, Recknagle, Kurtis P., and Khaleel, Mohammad A. Tue . "Effects of Oxide Thickness on Scale and Interface Stresses under Isothermal Cooling and Micro-Indentation for Ferritic Stainless Steel Interconnect". United States. doi:.
@article{osti_960330,
title = {Effects of Oxide Thickness on Scale and Interface Stresses under Isothermal Cooling and Micro-Indentation for Ferritic Stainless Steel Interconnect},
author = {Sun, Xin and Liu, Wenning N. and Vetrano, John S. and Yang, Zhenguo and Recknagle, Kurtis P. and Khaleel, Mohammad A.},
abstractNote = {Interconnects in solid oxide fuel cells (SOFCs) provide the electrical connection between the individual cells, as well as separate the anode air from the cathode fuel for each cell. The recently developed anode-supported planar SOFCs design allows the cell working temperature to be 800oC and lower, which led to the proposed use of ferritic stainless steel as interconnects and current collectors in the SOFC systems. Many experimental studies have shown that metal based interconnects will go through surface oxidation under the typical operating temperature and environment of an SOFC system. Since the oxide scale is formed at the outer surface of interconnect, its microstructure (morphology, texture), properties (electrical, thermal and mechanical properties), and structural integrity/stability will have a significant impact on the performance of interconnect. In this paper, we study the oxide growth kinetics and stresses generated during scale growth the thermal cycling for Cr-Fe based alloy, e.g. Crofer 22 APU. The goal is to predict the interconnect life under typical operating conditions and thermal cycles. First, a mathematical model is developed based on the growth of scale and coupling relation between diffusion and stress field. The following phenomena are considered: 1) The predominant cationic diffusion and cation distribution within the oxide scale in transient state and steady state; 2) Chemical reaction within the scale as well as the growth kinetics of the oxide scale; 3) Stresses distribution during the oxidation, especially for mid- and long-term treatment; 4) Coupling relationship or interaction between oxidation induced stresses and mass diffusion within the scale. Next, thermal stresses generated due to temperature changes in thermal cycling are predicted using finite element analyses for different substrate and scale thickness. In general, very high compressive in-plane stresses are predicted for the oxide layer. Also, high shear stress is also predicted on the oxide/substrate interface. The predicted shear stress on the interface is used to compare with the experimentally determined bond strength to predict delamination. Finally, scale spallation and IC life prediction are predicted by comparing the compressive stress in the scale with the critical buckling load for the delaminated scale.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Tue Jan 30 00:00:00 EST 2007},
month = {Tue Jan 30 00:00:00 EST 2007}
}

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  • Interconnects in solid oxide fuel cells (SOFCs) provide the electrical connection between the individual cells, as well as separate the anode air from the cathode fuel for each cell. In this paper, we study the thermal stresses in the oxide scale and at the oxide/substrate interface for Cr-Fe based interconnect (IC), e.g. Crofer 22 APU, during cooling. The ultimate goal is to predict the interconnect life under typical operating conditions and thermal cycles. In general, very high compressive in-plane stresses are predicted in the oxide layer during cooling. High shear stress is also predicted on the oxide/substrate interface. The predictedmore » shear stress on the interface will be used to compare with the experimentally determined bond strength to predict delamination. Finite element analyses are also performed for indentation test: high shear stress is predicted on the oxide/substrate interface, and the interfacial crack growth is predicted to be mode II driven.« less
  • The oxidation of ferritic stainless steels has been studied under solid oxide fuel cell (SOFC) interconnect ''dual'' exposure conditions, i.e. simultaneous exposure to air on one side of the sample, and fuel (hydrogen) on the other. It was found that, under the dual exposures, the oxidation behavior of the stainless steels at the airside differed significantly from that observed during exposure to air at both sides. Increased water vapor partial pressure in the air at the airside further accelerated the anomalous oxidation, resulting in nucleation and growth of hematite in the scale that led to a localized attack. The acceleratedmore » oxidation and growth of the hematite nodules was a result of combined effects of hydrogen transport from the fuel side to the airside and the presence of increased water vapor.« less
  • This study details long-term performance data for anode-supported thin-film YSZ-based SOFCs utilizing a ferritic stainless steel cathode current collector (Crofer22 APU) coated with a protective (Mn,Co)3O4 spinel to prevent Cr volatilization. Two standard cathode compositions, La(Sr)FeO3 and La(Sr)MnO3, were considered. The coating proved effective in blocking Cr migration, which resulted in long-term stability of the manganite cathode. In contrast the ferrite cathode indicated degradation that could not be attributed to Cr poisoning.
  • As part of an effort to develop cost-effective ferritic stainless steel-based interconnects for solid oxide fuel cell (SOFC) stacks, AL 441 HPTM was studied in terms of its metallurgical characteristics, oxidation behavior, and electrical performance. Minor alloying elements (Nb and Ti) captured interstitials such as C by forming carbides, stabilizing the ferritic structure and mitigating the risks of sensitization and inter-granular corrosion. Laves phases rich in Nb and Si precipitated along grain boundaries during high temperature exposure, improving the steel’s high temperature mechanical strength. The capture of Si in the Laves phase minimized the Si activity in the steel substratemore » and prevented formation of an insulating silica layer at the scale/metal interface. However, the relatively high oxidation rate, and thus increasing ASR over time, necessitates the application of a conductive protection layer on the steel. In particular, Mn1.5Co1.5O4 spinel protection layers drastically improved the electrical performance of the ferritic stainless steel 441, acting as barriers to chromium outward and oxygen inward diffusion.« less
  • Long-term tests (>8,000 hours) indicate that AISI 441 ferritic stainless steel coated with a Mn-Co spinel protection layer is a promising candidate material system for IT-SOFC interconnect applications. While uncoated AISI 441 showed a substantial increase in area-specific electrical resistance (ASR), spinel-coated AISI 441 exhibited much lower ASR values (11-13 mOhm-cm2). Formation of an insulating silica sublayer beneath the native chromia-based scale was not observed, and the spinel coatings reduced the oxide scale growth rate and blocked outward diffusion of Cr from the alloy substrate. The structure of the scale formed under the spinel coatings during the long term testsmore » differed from that typically observed on ferritic stainless steels after short term oxidation tests. While short term tests typically indicate a dual layer scale structure consisting of a chromia layer covered by a layer of Mn-Cr spinel, the scale grown during the long term tests consisted of a chromia matrix with discrete regions of Mn-Cr spinel distributed throughout the matrix. The presence of Ti in the chromia scale matrix and/or the presence of regions of Mn-Cr spinel within the scale may have increased the scale electrical conductivity, which would explain the fact that the observed ASR in the tests was lower than would be expected if the scale consisted of pure chromia.« less