Modifying Ferritic Stainless Steels for Solid Oxide Fuel Cell Applications
One of the most important problem areas associated with the solid oxide fuel cells is selection of a cost effective material for use as the interconnect component of the cell. Metals are now being considered as materials for this component, with ferritic stainless steels being the leading candidate. This work evaluates methods to combat the problem areas, namely rapid growth rate and vaporization of the oxide scale, that hinder the use of these materials. Oxidation experiments have been performed in dry and wet single atmosphere exposures as well as a dual environment exposure to simulate the conditions in a working SOFC. Measurements of the electrical properties of the oxides that formed were also performed. Commercial alloys, E-Brite and Crofer 22APU, were tested to form a baseline and resultant oxidation and electrical behaviors match those found in the literature. Isothermal oxidation tests for short exposure times have also led to a possible mechanism for the formation of the MnCr2O4 layer on Crofer. All of these tests were then replicated on a series of experimental Fe-22Cr-XTi (X=0-4) alloys. These alloys are shown to form a rutile layer analogous to the MnCr2O4 layer on Crofer. While this layer does prevent some chromia vaporization, the consequences due to the presence of Ti in the chromia include increased growth rate, decreased resistivity, extensive internal oxidation and nitridation of Ti, and a change of the growth direction of the chromia. The alloys containing ~2 – 3 wt%Ti appear to offer the best combination of oxidation, electrical, and mechanical properties. Coatings of lanthanum chromites and ferrites were also tested and shown to be very sensitive to exposure condition, resulting in the formation of pores, and to coating thickness, where thicker coatings are subject to cracking. Finally, reactive element oxide doping was attempted to slow the oxide growth rate for E-Brite (CeO2 doping) and for the Fe-Cr-Ti alloys (CeO2 and La2O3 doping). A significant effect was observed for E-Brite, while both dopants produced little change in the amount of oxide formed on the Ti bearing alloys.
Advisor:Dr. G.H. Meier; Dr. J. A. Barnard; Dr. F.S. Pettit; Dr. C. Johnson; Dr. J. P. Leonard
School:University of Pittsburgh
School Location:USA - Pennsylvania
Source Type:Master's Thesis
Keywords:materials science and engineering
Date of Publication:06/12/2007