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Various embodiments relate to interconnects for solid oxide fuel cells (“SOFCs”) comprising ferritic stainless steel and having at least one via that when subjected to an oxidizing atmosphere at an elevated temperature develops a scale comprising a manganese-chromate spinel on at least

Various embodiments relate to interconnects for solid oxide fuel cells (“SOFCs”) comprising ferritic stainless steel and having at least one via that when subjected to an oxidizing atmosphere at an elevated temperature develops a scale comprising a manganese-chromate spinel on at least a portion of a surface thereof, and at least one gas flow channel that when subjected to an oxidizing atmosphere at an elevated temperature develops an aluminum-rich oxide scale on at least a portion of a surface thereof. Other embodiments relate to interconnects comprising a ferritic stainless steel and having a fuel side comprising metallic material that resists oxidation during operation of the SOFCs, and optionally include a nickel-base superalloy on the oxidant side thereof. Still other embodiments relate to ferritic stainless steels adapted for use as interconnects comprising ≦0.1 weight percent aluminum and/or silicon, and >1 up to 2 weight percent manganese. Methods of making interconnects are also disclosed.

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We claim: 1. An interconnect for solid oxide fuel cells comprising a gas-impermeable body, the gas-impermeable body being formed from a ferritic stainless steel and including: (a) a fuel side comprising a via and a gas flow channel, and (b) an oxidant side opposite the fuel side, the oxidant side c

We claim: 1. An interconnect for solid oxide fuel cells comprising a gas-impermeable body, the gas-impermeable body being formed from a ferritic stainless steel and including: (a) a fuel side comprising a via and a gas flow channel, and (b) an oxidant side opposite the fuel side, the oxidant side comprising a via and a gas flow channel; wherein a metallic material that forms an oxide having a dissociation pressure greater than a partial pressure of oxygen proximate the fuel side of the interconnect during operation of the solid oxide fuel cells is connected to at least a portion of the ferritic stainless steel on the fuel side of the body, and wherein at least a portion of the oxidant side of the interconnect is electropolished so that when the oxidant side is subjected to an oxidizing atmosphere at a temperature of at least 650° C., a surface region of the via develops a scale comprising a manganese-chromate spinel, and a surface region of the gas flow channel develops an aluminum-rich oxide scale comprising iron and chromium and having a hematite structure. 2. The interconnect of claim 1 wherein the interconnect has an average coefficient of thermal expansion no greater than 17 ppm/K. 3. The interconnect of claim 1 wherein the ferritic stainless steel comprises from 18 to 35 weight percent chromium, greater than 1 up to 2 weight percent manganese, and less than 0.1 weight percent silicon. 4. The interconnect of claim 1 wherein the ferritic stainless steel comprises, in weight percent, from 0.3 to 1 aluminum, from 0 to less than 0.1 silicon, from 21 to 35 chromium, greater than 1 to 2 manganese, from 0.002 to 0.1 carbon, from 0 to 0.04 nitrogen, from 0 to 1 molybdenum, from 0 to 0.5 nickel, from 0 to 0.05 lanthanum, from 0 to 0.1 cerium, from 0 to 0.1 zirconium, from 0 to 0.5 titanium, from 0 to 0.1 tantalum, from 0 to 0.2 niobium, iron and impurities. 5. The interconnect of claim 1 wherein the ferritic stainless steel comprises, in weight percent, from 0.3to 1 aluminum, from 0 to 0.05 silicon, from 21 to 24 chromium, greater than 1 to 2 manganese, from 0.002 to 0.1 carbon, from 0 to 0.04 nitrogen, from 0 to 1 molybdenum, from 0 to 0.3 nickel, from 0.02 to 0.04 lanthanum, from 0 to 0.1 zirconium, from 0 to 0.1 titanium, from 0 to 0.1 tantalum, from 0 to 0.1 niobium, cerium, iron and impurities, wherein a sum of the weight percent cerium and the weight percent lanthanum ranges from 0.03 to 0.06. 6. The interconnect of claim 1 wherein the ferritic stainless steel comprises, in weight percent, from 0.3 to 1 aluminum, from 0 to 0.05 silicon, from 23 to 27 chromium, greater than 1 to 2 manganese, from 0.002 to 0.1 carbon, from 0 to 0.04 nitrogen, from 0 to 1 molybdenum, from 0 to 0.3 nickel, from 0 to 0.05 lanthanum, from 0 to 0.1 cerium, from 0 to 0.1 zirconium, from 0 to 0.5 titanium, from 0 to 0.1 tantalum, from 0.05 to 0.2 niobium, iron and impurities. 7. The interconnect of claim 1 wherein the ferritic stainless steel comprises, in weight percent, from 0.3 to 1 aluminum, from 0 to 0.05 silicon, from 23 to 27 chromium, greater than 1 to 2 manganese, from 0.002 to 0.1 carbon, from 0 to 0.04 nitrogen, from 0.75 to 1 molybdenum, from 0 to 0.3 nickel, from 0 to 0.05 lanthanum, from 0 to 0.1 cerium, from 0 to 0.05 zirconium, an amount of at least one of titanium, tantalum and niobium, wherein the amounts of titanium, tantalum and niobium satisfy the equation: 0.4 weight percent≦[% Nb+% Ti+½(% Ta)]≦1 weight percent, iron and impurities. 8. The interconnect of claim 1 wherein the metallic material that forms an oxide having a dissociation pressure greater than a partial pressure of oxygen proximate the fuel side of the interconnect during operation of the solid oxide fuel cells is nickel or a nickel alloy, copper or a copper alloy, or a nickel-copper alloy. 9. A planar solid oxide fuel cell comprising an interconnect according to claim 1. 10. An interconnect for solid oxide fuel cells comprising a composite body, the composite body comprising: (a) an oxidant side comprising a gas-impermeable body being formed from a ferritic stainless steel; and (b) a fuel side opposite the oxidant side, the fuel side comprising a gas-impermeable body being formed from a metallic material that forms an oxide having a dissociation pressure greater than a partial pressure of oxygen proximate the fuel side of the interconnect during operation of the solid oxide fuel cells, wherein at least a portion of the oxidant side of the composite body is electropolished so that when the oxidant side is subjected to an oxidizing atmosphere at a temperature of at least 650° C., a portion of a surface of the oxidant side develops a scale comprising a manganese-chromate spinel, and a different portion of a surface of the oxidant side develops an aluminum-rich oxide scale comprising iron and chromium and having a hematite structure. 11. The interconnect of claim 10 wherein the ferritic stainless steel comprises from 18 to 35 weight percent chromium, greater than 1 up to 2 weight percent manganese, and less than 0.1 weight percent. 12. The interconnect of claim 10 wherein the ferritic stainless steel comprises, in weight percent, from 0.3 to 1 aluminum, from 0 to less than 0.1 silicon, from 21 to 35 chromium, greater than 1 to 2 manganese, from 0.002 to 0.1 carbon, from 0 to 0.04 nitrogen, from 0 to 1 molybdenum, from 0 to 0.5 nickel, from 0 to 0.05 lanthanum, from 0 to 0.1 cerium, from 0 to 0.1 zirconium, from 0 to 0.5 titanium, from 0 to 0.1 tantalum, from 0 to 0.2 niobium, iron and impurities. 13. The interconnect of claim 10 wherein the ferritic stainless steel comprises, in weight percent, from 0.3 to 1 aluminum, from 0 to 0.05 silicon, from 21 to 24 chromium, greater than 1 to 2 manganese, from 0.002 to 0.1 carbon, from 0 to 0.04 nitrogen, from 0 to 1 molybdenum, from 0 to 0.3 nickel, from 0.02 to 0.04 lanthanum, from 0 to 0.1 zirconium, from 0 to 0.1 titanium, from 0 to 0.1 tantalum, from 0 to 0.1 niobium, cerium, iron and impurities, wherein a sum of the weight percent cerium and the weight percent lanthanum ranges from 0.03 to 0.06. 14. The interconnect of claim 10 wherein the ferritic stainless steel comprises, in weight percent, from 0.3 to 1 aluminum, from 0 to 0.05 silicon, from 23 to 27 chromium, greater than 1 to 2 manganese, from 0.002 to 0.1 carbon, from 0 to 0.04 nitrogen, from 0 to 1 molybdenum, from 0 to 0.3 nickel, from 0 to 0.05 lanthanum, from 0 to 0.1 cerium, from 0 to 0.1 zirconium, from 0 to 0.5 titanium, from 0 to 0.1 tantalum, from 0.05 to 0.2 niobium, iron and impurities. 15. The interconnect of claim 10 wherein the ferritic stainless steel comprises, in weight percent, from 0.3 to 1 aluminum, from 0 to 0.05 silicon, from 23 to 27 chromium, greater than 1 to 2 manganese, from 0.002 to 0.1 carbon, from 0 to 0.04 nitrogen, from 0.75 to 1 molybdenum, from 0 to 0.3 nickel, from 0 to 0.05 lanthanum, from 0 to 0.1 cerium, from 0 to 0.05 zirconium, an amount of at least one of titanium, tantalum and niobium, wherein the amounts of titanium, tantalum and niobium satisfy the equation: 0.4 weight percent≦[% Nb+% Ti+½(% Ta)]≦1 weight percent, iron and impurities. 16. The interconnect of claim 10 wherein the metallic material that forms an oxide having a dissociation pressure greater than a partial pressure of oxygen proximate the fuel side of the interconnect during operation of the solid oxide fuel cells is nickel or a nickel alloy, copper or a copper alloy, or a nickel-copper alloy. 17. The interconnect of claim 10 wherein a layer of a nickel-base superalloy is connected to at least a portion of the oxidant side of the interconnect. 18. A planar solid oxide fuel cell comprising an interconnect according to claim 10. 19. An interconnect for solid oxide fuel cells comprising a body formed from a ferritic stainless steel and including: (a) a fuel side, and (b) an oxidant side opposite the fuel side, wherein a gas-impermeable layer of metallic material that forms an oxide having a dissociation pressure greater than a partial pressure of oxygen proximate the fuel side of the interconnect during operation of the solid oxide fuel cells is connected to at least a portion of the ferritic stainless steel on the fuel side of the body, and wherein at least a portion of the oxidant side is electropolished so that when the oxidant side is subjected to an oxidizing atmosphere at a temperature of at least 650° C., a portion of a surface of the oxidant side develops a scale comprising a manganese-chromate spinel, and a different portion of a surface of the oxidant side develops an aluminum-rich oxide scale comprising iron and chromium and having a hematite structure. 20. The interconnect of claim 19 wherein the metallic material that forms an oxide having a dissociation pressure greater than a partial pressure of oxygen proximate the fuel side of the interconnect during operation of the solid oxide fuel cells is nickel or a nickel alloy, copper or a copper alloy, or a nickel-copper alloy. 21. An interconnect for solid oxide fuel cells comprising a composite body, the composite body comprising: (a) an oxidant side comprising a gas-impermeable body being formed from a ferritic stainless steel, and (b) a fuel side opposite the oxidant side, the fuel side comprising a gas-impermeable body being formed from a metallic material that forms an oxide having a dissociation pressure greater than a partial pressure of oxygen proximate the fuel side of the interconnect during operation of the solid oxide fuel cells, wherein at least a portion of the oxidant side of the composite body is electropolished so that when the oxidant side is subjected to an oxidizing atmosphere at a temperature of at least 650° C., a portion of a surface of the oxidant side develops a scale comprising a manganese-chromate spinel, and a different portion of a surface of the oxidant side develops an aluminum-rich oxide scale comprising iron and chromium and having a hematite structure. 22. The interconnect of claim 21 wherein the metallic material that forms an oxide having a dissociation pressure greater than a partial pressure of oxygen proximate the fuel side of the interconnect during operation of the solid oxide fuel cells is nickel or a nickel alloy, copper or a copper alloy, or a nickel-copper alloy.