Effect of Mn-Co spinel coating for Fe-Cr ferritic alloys ZMG232L and 232J3 for solid oxide fuel cell interconnects on oxidation behavior and Cr-evaporation

Metallic materials, especially Fe-Cr ferritic alloys, are promising as interconnect materials of solid oxide fuel cells (SOFCs) operated at around medium temperatures. ZMG232L is one of the developed Fe-Cr ferritic alloys for SOFC metallic interconnects.These metallic materials are usually machined or pressed into various shapes of interconnect parts, and thickness of these parts is often thin. However, the oxidation rate of thin sheet was much higher than that of thick one because Cr content decreased under oxide layer of edge part of thin sheet. Such accelerated oxidation behavior could be improved by reducing Mn, increasing Cr, and adding W in ZMG232L.It is also very important to reduce Cr-evaporation from the oxidized surface of ferritic alloys in cathode side. The aim of this study is to reduce the Cr-evaporation from oxidized alloy surface in air by coating with Mn-Co spinel oxide. In this study, oxidation behavior and Cr-evaporation of ZMG232L and improved Fe-Cr alloy, 232J3, coated with Mn-Co spinel oxide were investigated at elevated temperature in air. MnCo₂O₄ spinel coating on the pre-oxidized Fe-Cr ferritic alloy surface improved oxidation resistance and Cr-evaporation.     

The effect of Mn on the oxidation behavior and electrical conductivity of Fe-17Cr alloys in solid oxide fuel cell cathode atmosphere

Four Fe-17Cr alloys with various Mn contents between 0.0 and 3.0 wt.% are prepared for investigation of the effect of Mn content on the oxidation behavior and electrical conductivity of the Fe-Cr alloys for the application of metallic interconnects in solid oxide fuel cells (SOFCs). During the initial oxidation stage (within 1 min) at 750 °C in air, Cr is preferentially oxidized to form a layer of Cr₂O₃ type oxide in all the alloys, regardless the Mn content, with similar oxidation rate and oxide morphology. The subsequent oxidation of the Mn containing alloys is accelerated caused by the fast outward diffusion of Mn ions across the Cr₂O₃ type oxide layer to form Mn-rich (Mn, Cr)₃O₄ and Mn₂O₃ oxides on the top. After 700 h oxidation a multi-layered oxide scale is observed in the Mn containing alloys, which corresponds to a multi-stage oxidation kinetics in the alloys containing 0.5 and 1.0 wt.% of Mn. The oxidation rate and ASR of the oxide scale increase with the Mn content in the alloy changes from 0.0 to 3.0 wt.%. For the application of metallic interconnects in SOFCs, Mn-free Fe-17Cr alloy with conducting Cr free spinel coatings is preferred.     

Development of a Fe-Cr alloy for interconnect application in intermediate temperature solid oxide fuel cells

The oxidation behavior and electrical property of a newly designed Fe-Cr alloy with addition of 1.05 wt.% Mn, 0.52 wt.% Ti, 2.09 wt.% Mo and other elements, such as La, Y and Zr have been investigated isothermally or cyclically at 750 °C in air for up to 1000 h. With a coefficient of thermal expansion matched to SOFC cell components, the alloy demonstrates excellent oxidation resistance and low area specific resistance of the oxide scale. The thermally grown oxide scale presents a multi-layered structure with conductive Mn-Cr spinel in-between the underneath Cr₂O₃ and the top Mn₂O₃. The oxidation rate constants obtained under both isothermal and cyclic oxidation condition are in the range of 5.1 × 10⁻¹⁴ to 7.6 × 10⁻¹⁴ g² cm⁻⁴ s⁻¹, and the measured area specific resistance at 750 °C after 1000 h oxidation is around 10 mΩ cm², lower than that of the conventional Fe-Cr stainless steels and comparable with that of the Ni-based alloys. Thermal cycling seems to improve the oxide scale adherence and promotes the formation of the highly conductive Mn₂O₃, and in turn, to enhance the oxidation resistance and electrical property.     

Evaluation of electrodeposited Fe-Ni alloy on ferritic stainless steel solid oxide fuel cell interconnect

Fe-Ni alloy is electrodeposited on ferritic stainless steel for intermediate-temperature solid oxide fuel cell (SOFC) interconnects application. The oxidation behavior of Fe-Ni alloy coated steel has been investigated at 800 °C in air corresponding to the cathode environment of SOFC. It is found that the oxidation rate of the Fe-Ni alloy coated steel becomes similar to that of the uncoated steel after the first week thermal exposure, although the mass gain of the coated steel is higher than that of the uncoated steel. Oxide scale formed on the uncoated steel mainly consists of Cr₂O₃ with (Mn,Cr)₃O₄ spinel. However, a double-layer oxide structure with a Cr-free outer layer of Fe₂O₃/NiFe₂O₄ and an inner layer of Cr₂O₃ is developed on the Fe-Ni alloy coated steel. The scale area specific resistance (ASR) for the Fe-Ni alloy coated steel is lower than that of the scale for the uncoated steel.     

Interfacial failure via encapsulation of external particulates in an outward-growing thermal oxide

A Cr₂O₃-forming Ni-base superalloy and this alloy coated with a Pt-modified aluminide coating were exposed to SiO₂ powder and cyclically oxidized at 950 °C. The uncoated alloy showed a considerable amount of spallation and buckling whereas the Pt-NiAl coated alloy remained protective throughout hundred 1 h-cycles. The interfacial failure is mainly ascribed to the increased thermal strain by the encapsulation of external SiO₂ particulates in an outward-growing Cr₂O₃ layer. However, the particles were not embedded in the thermally grown oxide of the Pt-NiAl coated alloy due to the slow inward-growing characteristics of Al₂O₃ scales. The buckling of the Cr₂O₃ scale with embedded SiO₂ was analyzed with (1) a classical buckling criterion using the instantaneous coefficients of thermal expansion of the constituents, and (2) finite element analyses (FEA) to estimate the local interfacial shear stresses. It turns out that the thermal strain with embedded SiO₂ is larger than the experimentally determined critical thermal strain (?_b) explaining the buckling of the oxide scale observed in the experiment. The FEA results demonstrate that local shear stresses at the metal/oxide interface are significantly amplified near the SiO₂ particles showing that the buckling of oxide can be readily initiated especially in the vicinity of the embedded particles.     

CuFe2O4 protective and electrically conductive coating thermally converted from sputtered CuFe alloy layer on SUS 430 stainless steel interconnect

An inexpensive CuFe alloy layer with an atomic ratio (1:2) of Cu to Fe is coated on SUS 430 stainless steels via magnetron sputtering for solid oxide fuel cells interconnect application. The coated steels are thermally exposed to air at 800 °C for 15 weeks. The CuFe alloy layer is converted to CuFe₂O₄ spinel layer atop Cr₂O₃ layer developed from steel substrate. The outer layer of CuFe₂O₄ spinel not only retards Cr outward migration and reduces oxidation rate but also significantly lowers area specific resistance of the surface scale which is predicted for solid oxide fuel cells lifetime by a parabolic law. The sputtered CuFe alloy layer demonstrates a promising prospect for the application of steel interconnects coatings.     

Protective coatings for Cr2O3-forming interconnects of solid oxide fuel cells

Cr₂O₃ evaporation from Cr₂O₃-forming metallic interconnects during operation of the solid oxide fuel cells (SOFC) can poison other cell components and cause degradation. Protective NiFe₂O₄ spinel coatings on interconnect alloys were developed by electroplating and screen printing, respectively. Results indicate that NiFe₂O₄ coatings can significantly improve the oxidation resistance of the alloy while providing effective conducting path with inherent low resistance, and also are expected to serve as a diffusion barrier to effectively reduce the Cr₂O₃ evaporation. Two coating techniques were evaluated in terms of the performances of the coatings. A very interesting and smart coating structure was reported.     

NiO/NiFe2O4 dual-layer coating on pre-oxidized SUS 430 steel interconnect

A Ni/NiFe₂ dual-layer coating is deposited on 50-h pre-oxidized SUS 430 steel by magnetron sputtering for solid oxide fuel cell (SOFC) interconnects application, followed by thermal exposure in air at 800 °C for 1680 h. The thermally grown oxide scales exhibit tri-layer structure with inner Cr₂O₃ layer, middle NiO layer and outer NiFe₂O₄ spinel layer. The oxide coating converted from Ni/NiFe₂ coating not only inhibit the growth of Cr₂O₃ and the outward diffusion of Cr species but also improve the electrical performance of the surface scale. In addition, pre-oxidation treatment for the steel before Ni/NiFe₂ coating deposition prevents the interdiffusion between steel substrate and coating in the oxidation process.     

A promising NiCo2O4 protective coating for metallic interconnects of solid oxide fuel cells

The NiCo₂O₄ spinel coating is applied onto the surfaces of the SUS 430 ferritic stainless steel by the sol-gel process; and the coated alloy, together with the uncoated as a comparison, is cyclically oxidized in air at 800 °C for 200 h. The oxidation behavior and oxide scale microstructure as well as the electrical property are characterized. The results indicate that the oxidation resistance is significantly enhanced by the protective coating with a parabolic rate constant of 8.1 × 10⁻¹⁵ g² cm⁻⁴ s⁻¹, while the electrical conductivity is considerably improved due to inhibited growth of resistive Cr₂O₃ and the formation of conductive spinel phases in the oxide scale.     

High-temperature oxidation process analysis of MnCo2O4 coating on Fe-21Cr alloy

Even though the operation temperature of solid oxide fuel cells (SOFCs) stacks has been reduced (∼750 °C), stainless steel interconnect within the stacks still requires protection by high conductive coatings to delay the growth of oxide scales and reduce chromium evaporation. Manganese cobaltite spinel protective coating with a nominal composition of MnCo₂O₄ was produced on Fe-21Cr stainless steel. Electrical, microstructural and compositional analysis were performed to investigate the interfacial reaction of MnCo₂O₄ protective coating with the stainless steel substrate during 750 °C oxidation process. The spinel coating not only acts as a barrier to Cr outward transport, but also improves the electrical conductivity of the alloy interconnect during long-term oxidation. The coated alloy demonstrates good electrical conductivity with an area specific resistance (ASR) of about 5 mOhm cm² after oxidation for 1000 h at 750 °C, which is about 1/4 of the ASR of bare Fe-21Cr alloy. The reduction of ASR might be caused by the fact that Cr migrated from the steel substrate interact with MnCo₂O₄ coating and generated Mn-Co-Cr spinel phase, which has higher electrical conductivity than that of Cr₂O₃.     

Heat resistant alloys as interconnect materials of reduced temperature SOFCs

Heat-resistant alloys, Haynes 230 and SS310, were exposed to air and humidified H₂ at 750 °C for up to 1000 h, respectively, simulating the environments in reduced temperature solid oxide fuel cells (SOFCs). The oxidized samples were characterized by using SEM, EDS and X-ray diffraction to obtain the morphology, thickness, composition and crystal structure of the oxide scales. A mechanism for the formation of metallic Ni-rich nodules on top of the oxide scale in Haynes 230 sample oxidized in humidified H₂ was established. Thermodynamic analysis confirmed that MnCr₂O₄ is the favored spinel phase, together with Cr₂O₃, in the oxide scales.     

Performance evaluation of several commercial alloys in a reducing environment

Several commercial alloys including Ebrite, Crofer 22 APU, Haynes 230 and Haynes 242, which are candidates for intermediate-temperature solid oxide fuel cell (SOFC) interconnect materials, were isothermally and cyclically oxidized at 900 °C in the reducing atmosphere of Ar + 5 vol.% H₂ + 3 vol.% H₂O corresponding to the SOFC anode environment. Results indicate that these alloys exhibited good scale spallation resistance with the Ni-base alloys possessing better oxidation resistance over the Fe-base alloys. Both Mn–Cr spinel and Cr₂O₃ were formed in the oxide scales of these alloys. For Crofer 22 APU and Haynes 242, a continuous protective MnO and Mn–Cr spinel layer formed outside on the inner layer of Cr₂O₃. The increase in scale ASR after longer-term thermal exposure in the reducing environment was relatively slower for the Ni-base alloys than for the Fe-base alloys.     

Electrodeposited Ni/CeO2 mulriple coating on SUS 430 steel interconnect

Ni/CeO₂ mulriple coating has been fabricated on SUS 430 steel via electrodepositing approach. 100-h initial and 3-week long-term thermal exposing to air at 800 °C has enunciated that the oxide scale grown on the Ni/CeO₂ coated steel contains an external oxide layer of NiFe₂O₄ spinel, a middle oxide layer of NiO and an internal oxide layer of Cr₂O₃. Simultaneously, dispersive CeO₂ particles embed in the oxide scale. Compared to the Ni coated steel on which the same tri-layer oxide structure without discrete CeO₂ particles grows in the same exposing environment, growth rate of the internal Cr₂O₃ layer on the Ni/CeO₂ coated steel has been profoundly suppressed, which subsequently lowers the oxide scale area specific resistance (ASR). Enhancement of the oxidation resistance and reduction of the oxide scale ASR are attributed to the presence of CeO₂.     

Effect of Ti addition on the electric and ionic property of the oxide scale formed on the ferritic stainless steel for SOFC interconnect

Ferritic stainless steels with Ti addition are considered as promising candidates for SOFC interconnect application. In this study, the effect of Ti addition on the electrical conductivity and Cr evaporation resistance was discussed in terms of microstructure and ionic property of the oxide scale by using TEM analysis and asymmetry polarization method. Ti addition induced the generation of ionic defects in the oxide layer and modified the growth kinetics of Cr₂O₃ and MnCr₂O₄, but in different manner depending on Ti amount. Ti content in a range of 0.05–0.07 wt% was effective for reducing the oxidation rate and electrical resistance. Addition of 1 wt% Ti promoted fast Cr₂O₃ growth due to the excess ionic defect in Cr₂O₃ matrix. However, the formation of the outermost MnCr₂O₄ layer was accelerated by Ti segregation near the scale/alloy interface and it reduced Cr evaporation effectively. Co-addition of a small amount of Ti and La enhanced Ti segregation without generation of excess ionic defect and improved both the electric conductivity and Cr evaporation resistance.     

NiMn2O4 spinel as an alternative coating material for metallic interconnects of intermediate temperature solid oxide fuel cells

In an effort to improve the performance of SUS 430 alloy as a metallic interconnect material, a low cost and Cr-free spinel coating of NiMn₂O₄ is prepared on SUS 430 alloy substrate by the sol-gel method and evaluated in terms of the microstructure, oxidation resistance and electrical conductivity. A oxide scale of 3-4 μm thick is formed during cyclic oxidation at 750 °C in air for 1000 h, consisting of an inner layer of doped Cr₂O₃ and an outer layer of doped NiMn₂O₄ and Mn₂O₃; and the growth of Cr₂O₃ and formation of MnCr₂O₄ are depressed. The oxidation kinetics obeys the parabolic law with a rate constant as low as 4.59 × 10⁻¹⁵ g² cm⁻⁴ s⁻¹. The area specific resistance at temperatures between 600 and 800 °C is in the range of 6 and 17 mΩ cm². The above results indicate that NiMn₂O₄ is a promising coating material for metallic interconnects of the intermediate temperature solid oxide fuel cells.     

Influence of preoxidation on high temperature behavior of NiFe2 coated SOFC interconnect steel

NiFe₂O₄ spinel coating is promising for solid oxide fuel cell (SOFC) steel interconnects application. In this work, NiFe₂ alloy coating was sputtered on bare steel and preoxidized steel (100 h in air at 800 °C), respectively, followed by exposing in air at 800 °C for up to 15 weeks in order to investigate the influence of steel preoxidation on high temperature behaviors of the coated steels. The results indicated that an outer NiFe₂O₄ spinel layer atop an inner Cr₂O₃ layer formed on the coated samples after oxidation. The preoxidation enhanced the oxidation resistance of the coated sample and reduced Cr out-migration to NiFe₂O₄ spinel layer. After 15 weeks, the area specific resistance (ASR) of surface scale on the coated preoxidized steel was much lower than that on the coated bare steel. The mechanisms of the preoxidation influence on oxidation behavior and surface scale electrical property of the coated steels were discussed.     

Comparison of oxidation behaviours among three Fe–Cr based alloys for solid oxide fuel cell interconnect

Abstract

Two metallic alloys, containing comparable amounts of Cr, underwent oxidation in hot air simulating (the solid oxide fuel cell cathode atmosphere) for various periods. The results demonstrated that the oxidation kinetics of Crofer22 APU and equivalent ZMG232 followed the parabolic rate law and oxidation rates increased with temperature. Typical oxidation rates of Crofer22 APU and ZMG232 upon annealing treatment are approximately 0·21 orders of magnitude lower than that of ZMG232. An oxide scale electron probe microanalyser, a scanning electron microscope and X-ray diffractometer were adopted to verify the applicability of Fe–Cr based alloys in the solid oxide fuel cell interconnect component. Two alloys contain comparable amount of Cr, Mn and Fe, and their surface oxides as analysed are indicated to be Cr₂O₃ and (Mn,Fe)Cr₂O₄ spinel compound. In summary, Crofer22 APU had the best oxidation resistance of any of the alloys of interest.     

Chromium evaporation and oxidation characteristics of alumina-forming austenitic stainless steels for balance of plant applications in solid oxide fuel cells

The chromium (Cr) evaporation behavior of several different types of iron (Fe)-based AFA alloys and benchmark Cr₂O₃-forming Fe-based 310 and Ni-based 625 alloys was investigated for 500 h exposures at 800 °C to 900 °C in air with 10% H₂O. The Cr evaporation rates from alumina-forming austenitic (AFA) alloys were ~5 to 35 times lower than that of the Cr₂O₃-forming alloys depending on alloy and temperature. The Cr evaporation behavior was correlated with extensive characterization of the chemistry and microstructure of the oxide scales, which also revealed a degree of quartz tube Si contamination during the test. Long-term oxidation kinetics were also assessed at 800 to 1000 °C for up to 10,000 h in air with 10% H₂O to provide further guidance for SOFC BOP component alloy selection.     

Effects of silicon concentration in SOFC alloy interconnects on the formation of oxide scales in hydrocarbon fuels

Oxide scale formations on FeCr alloy interconnects were investigated in anode gas (mixtures of CH₄ and H₂O) atmospheres for solid oxide fuel cells. The silicon concentration in FeCr alloy changed the microstructures of oxide scales, elemental distribution and oxide scale growth rates. Oxide scale is composed of the following phases from surface to inner oxides: FeMn spinel, Cr₂O₃, oxide scale/alloy interface and internal oxides of Si and Al. With decreasing the Si concentration from 0.4 to 0.01 mass%, formation of thin Si and Mn layer was observed inside the FeCr alloy. Oxide scale growth rate constants decreased by lowering the Si concentration in FeCr alloy from 4.2 × 10⁻¹⁸ to 2.1 × 10⁻¹⁸ m² s⁻¹ at 1073 K. Diffusivity of Fe and Cr was changed by the concentration of Si in FeCr alloy, which affects the growth rates of oxide scale. The electrical conductivity of oxidized FeCr alloy shows almost same level regardless the Si concentration (in the orders of 10 S cm⁻² at 1073 K).     

Evaluation of Laves-phase forming Fe–Cr alloy for SOFC interconnects in reducing atmosphere

A Laves-phase forming Fe–Cr alloy was evaluated as interconnects for solid oxide fuel cells (SOFCs) in reducing atmosphere (in H₂-H₂O). The oxide scale growth was compared between Laves-phase forming alloy and typical stainless steel (SUS430). The oxide scale growth rates were decreased in the Laves-phase forming alloy, and the area-specific resistance (ASR) of oxidized Laves-phase forming alloy showed the lower values than that of SUS430. The temperature dependence of 1/ASR for the oxidized alloy was different between Laves-phase forming alloy and SUS430. The oxygen diffusivity in the oxide scale was also evaluated by the stable isotope oxygen (¹⁸O₂) diffusion in the scale. The chemical diffusion coefficients of isotope oxygen in the oxide scale showed the smaller value for the Laves-phase forming alloy (D = 7.0 × 10⁻¹³ cm² s⁻¹) than that for SUS430 (D = 4.6 × 10⁻¹² cm² s⁻¹) at 1073 K. A relatively high diffusivity of oxygen was estimated in the Mn–Cr spinel oxide on the top surface of oxide scales. Inward diffusion of oxygen and outward diffusion of cation in the oxide scale were discussed to consider the oxide scale growth mechanism.