Molybdenum effect on oxidation resistance and electric conduction of ferritic stainless steel for SOFC interconnect

The effect of molybdenum addition on the oxidation, electric property and Cr evaporation of Fe–22Cr–0.5Mn ferritic stainless steel is investigated in terms of mass gain, area specific resistance and Cr evaporation rate. Addition of 0.1–2 wt% Mo reduces the oxidation rate and especially area specific resistance of Fe–22Cr–0.5Mn steel. Mo addition of these contents increases the activation energy and suppresses the inward diffusion of oxygen, which indicates the defect chemistry of oxide scale is altered. This results in the increase of oxidation resistance and electric conductivity. When more than 4 wt% Mo is added, the oxidation rate increases after 300 h of oxidation at 800 °C in ambient air. The evaporation of volatile Mo species reduces the stability of protective chromia, so that rapid growing Fe-rich spinel is formed after 300 h of oxidation. The evaporation rate of Cr is similar in all alloys.     

Effect of reactive elements on oxidation behaviour of Fe–22Cr–0.5Mn ferritic stainless steel for a solid oxide fuel cell interconnect

Ferritic stainless steel has become a promising material for metallic interconnects for solid oxide fuel cells (SOFCs) operating in an intermediate temperature range (650–800 °C). Ferritic stainless steels containing reactive elements (REs) such as Crofer22APU and ZMG232 have been developed for SOFC interconnects. Nevertheless, the effectiveness of REs on the growth kinetics of the chromia-rich scale that forms on the ferritic stainless steels is not yet well understood. The current study focuses on the investigation of the effect of REs such as Y, Ce and La on the oxidation behaviour and scale properties of Fe–22Cr–0.5Mn stainless steel. The results show that Y is the most effective reactive element for reducing the scale growth kinetics and area-specific resistance of the chromia scale which forms on this stainless steel. The growth kinetics of the chromia-rich scale can be effectively reduced by the dominant segregation of Y at the interface between the oxide scale and alloy substrate, and by the formation of a thin SiO₂ and MnO layer underneath the Cr₂O₃-rich oxide.     

Evaluation of Nb- or Mo-alloyed ferritic stainless steel as SOFC interconnect by using button cells

The Fe–22Cr–0.5Mn ferritic stainless steel alloyed with Nb or with Mo is evaluated in the button cell configuration at 750 °C in terms of degradation in ohmic resistance and cathodic polarization resistance. STS444 and Crofer22 APU are also evaluated for comparison. Each polarization element is separated by equivalent circuit analysis on the electrochemical impedance spectroscopy data. Cr deposition on the button cell cathode is also analyzed both qualitatively by transmission electron microscope and quantitatively by inductively coupled plasma. The Nb- or Mo-alloyed ferritic stainless steel shows comparable performance with Crofer22 APU in terms of the increase rate in ohmic resistance and Cr evaporation rate, even without the addition of reactive element such as La. When the same amount of Cr is deposited on the cathode, the cathode performance deteriorates more at the high Cr evaporation rate than at the low Cr evaporation rate.     

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.     

Investigation of iron–chromium–niobium–titanium ferritic stainless steel for solid oxide fuel cell interconnect applications

As part of an effort to develop cost-effective ferritic stainless steel-based interconnects for solid oxide fuel cell (SOFC) stacks, both bare AISI441 and AISI441 coated with (Mn,Co)₃O₄ protection layers were studied in terms of its metallurgical characteristics, oxidation behavior, and electrical performance. The addition of minor alloying elements, in particular Nb, led to formation of Laves phases both inside grains and along grain boundaries. In particular, the Laves phase which precipitated out along grain boundaries during exposure at intermediate SOFC operating temperatures was found to be rich in both Nb and Si. The capture of Si in the Laves phase minimized the Si activity in the alloy matrix and prevented formation of an insulating silica layer at the scale/metal interface, resulting in a reduction in area-specific electrical resistance (ASR). However, the relatively high oxidation rate of the steel, which leads to increasing ASR over time, and the need to prevent volatilization of chromium from the steel necessitates the application of a conductive protection layer on the steel. In particular, the application of a Mn_1.5Co_1.5O₄ spinel protection layer substantially improved the electrical performance of the 441 by reducing the oxidation rate.     

Oxidation behavior of (Co,Mn)3O4 coatings on preoxidized stainless steel for solid oxide fuel cell interconnects

Ceramic coatings are being explored to extend the lifetime of stainless steel interconnects in planar Solid Oxide Fuel Cells (SOFCs). One promising coating is Co_1.5Mn_1.5O₄ spinel, which is deposited using various techniques, resulting in different coating thicknesses, compositions and microstructures. In this study, stainless steel 441HP samples were subjected to three levels of preoxidation (0, 3, 10 and 100 h in 800 °C lab air) prior to coating. Samples were coated with 2 μm CoMn alloy using magnetron sputtering and were subsequently annealed in 800 °C air for 0, 10, 100 or 1650 h. Oxidation behaviors were evaluated as a function of these exposures, as well as in dual atmospheres and during area specific resistance (ASR) measurements in 800 °C lab air. Preoxidation was found to inhibit Fe and Cr transport from the stainless steel into the coating and preoxidized samples exhibited a substantially thinner surface layer after oxidation. After ASR testing for 1650 h in 800 °C air, the trend of the preoxidized sample values remained level while trend of the non-preoxidized sample values showed an increase. Observed oxidation behaviors, their possible mechanisms, and implications for SOFC interconnects are presented and discussed.     

Studies on elements diffusion of Mn/Co coated ferritic stainless steel for solid oxide fuel cell interconnects application

The spinel structure of manganese cobalt oxide (Mn,Co)₃O₄ is one of the most promising coatings for solid oxide fuel cell (SOFC) stainless steel interconnects. The stoichiometric Mn_1.5Co_1.5O₄ composition has properties that are preferable to other Mn/Co ratios, for example a higher conductivity and a thermal expansion coefficient that matches the typical steel substrate. However, previous work showed the Mn/Co ratio changes during operation due to the diffusion of Mn from the substrate. The results presented here are on three coatings with different compositions (namely; pure Co, Mn₂₀Co₈₀, and Mn₄₀Co₆₀) with each coating composition deposited to a thickness of 800 nm, 1500 nm, and 3000 nm. The coatings were applied by DC magnetron sputtering and then machine cut into coupons for isothermal annealing at 800 °C in air using a batch-type furnace for 2, 10, 50, 250, and 1000 h. The morphology, chemical composition (including surface and cross sections of the layers) and structures of the oxides formed were analyzed by SEM, EDS and XRD. Analysis of the element diffusion (Mn, Co, Cr, Fe) shown here points to an optimized coating recipe of Mn₄₀Co₆₀.     

Magnetron-sputtered Mn/Co(40:60) coating on ferritic stainless steel SUS430 for solid oxide fuel cell interconnect applications

With recent progress in lowering the operating temperature, chromia-forming ferritic stainless steels are considered as promising interconnect materials for solid oxide fuel cells (SOFCs). To block the chromium evaporation and chromium poisoning, coatings with Mn/Co (40:60) was tested as the optimized recipe to maintain Mn_1.5Co_1.5O₄ composition after long term operation due to Mn diffusion from substrate. In order to study the coating thickness effect, Mn/Co (40:60) coatings were fabricated in thickness of approximately 800 nm, 1500 nm, 3000 nm on ferritic stainless steels SUS430 using magnetron sputtering. Oxidation behavior of sputtered samples was investigated after oxidized at 800 °C in air for 2 h, 250 h, 500 h, 1000 h, respectively. SEM, EDS, XRD and FIB are used to analyze the surface morphology, chemical composition and structures of the coatings. Area specific resistance measurement indicated the sputtered samples in thickness of 800 nm, 1500 nm, 3000 nm at 800 °C for various hours in air are in range of 15–36 mΩ cm², 12–25 mΩ cm², 10–23 mΩ cm² respectively. Eventually the optimized thickness of Mn/Co (40:60) coatings was suggested.     

Evaluation of pulse electroplated cobalt/yttrium oxide composite coating on the Crofer 22 APU stainless steel interconnect

Ferritic stainless steels have been evaluated as favorable materials for utilization in SOFC interconnects. However, there are difficulties in utilizing these metallic interconnects, including the quick decrease of their electrical conductivity and cathode poisoning due to the evaporation of Cr species. In this work, Co and Co/Y₂O₃ composite coatings have been deposited onto Crofer 22 APU stainless steels by the pulse current electrodeposition method and the oxidation and electrical properties of uncoated and coated steels have been evaluated. Cyclic oxidation was performed in air at 800 °C for 500 h, oxidation rates were calculated, and oxide layer microstructures were examined. SEM–EDS and XRD investigations exhibited the created oxide layer on both coated samples made up of two scale after oxidation. The internal thin scale was composed of Cr and O and the external scale comprised of Co, Mn, Cr, Fe, and O. Y₂O₃ was observed as dispersed particles in the external oxide scale after the cyclic oxidation test. The thicknesses of internal oxide scale were reduced and oxidations rates also were meaningfully decreased for Co/Y₂O₃-coated steels relative to uncoated and Co-coated steels. Finally the ASR values of coated and uncoated substrates was also tested as a function of temperature and time in air. Results showed that the ASR value of the Co/Y₂O₃-coated steel was 13.1 mΩ cm² after 500 h of cyclic oxidation at 800 °C, which was significantly lower than that of bare steel and the Co-coated sample.     

Improved oxidation resistance of a nanocrystalline chromite-coated ferritic stainless steel

An economical dip coating process was developed to synthesize uniform, crack-free, and adherent thin nanocrystalline LaCrO₃ films on a ferritic stainless steel substrate for the solid oxide fuel cell interconnect applications. LaCrO₃ perovskite phase was formed after annealing in air at 800 °C for 1 h for both the LaCrO₃ and La₂O₃ precursors. The effectiveness of the coating in improving the oxidation resistance of the alloy was demonstrated by both isothermal and cyclic oxidation tests. The LaCrO₃ coatings were found to cause a pronounced reduction in oxidation rate of the alloy, especially with low La-content precursors. The area-specific resistance of the oxide scales formed on the bare and coated alloy substrates was also evaluated and discussed.     

A review of recent progress in coatings, surface modifications and alloy developments for solid oxide fuel cell ferritic stainless steel interconnects

Ferritic stainless steels have become the standard material for solid oxide fuel cell (SOFC) interconnect applications. The use of commercially available ferritic stainless steels, not specifically designed for interconnect application, however, presents serious issues leading to premature degradation of the fuel cell stack, particularly on the cathode side. These problems include rapidly increasing contact resistance and volatilization of Cr from the oxide scales, resulting in cathode chromium poisoning and cell malfunction. To overcome these issues, a variety of conductive/protective coatings, surface treatments and modifications as well as alloy development have been suggested and studied over the past several years. This paper critically reviews the attempts performed thus far to mitigate the issues associated with the use of ferritic stainless steels on the cathode side. Different approaches are categorized and summarized and examples for each case are provided. Finally, directions and recommendations for the future studies are presented.     

Evaluation of Ni–Cr-base alloys for SOFC interconnect applications

To further understand the suitability of Ni–Cr-base alloys for solid oxide fuel cell (SOFC) interconnect applications, three commercial Ni–Cr-base alloys, Haynes 230, Hastelloy S and Haynes 242 were selected and evaluated for oxidation behavior under different exposure conditions, scale conductivity and thermal expansion. Haynes 230 and Hastelloy S, which have a relatively high Cr content, formed a thin scale mainly comprised of Cr₂O₃ and (Mn,Cr,Ni)₃O₄ spinels under SOFC operating conditions, demonstrating excellent oxidation resistance and a high scale electrical conductivity. In contrast, a thick double-layer scale with a NiO outer layer above a chromia-rich substrate was grown on Haynes 242 in moist air or at the air side of dual exposure samples, indicating limited oxidation resistance for the interconnect application. With a face-centered-cubic (FCC) substrate, all three alloys possess a coefficient of thermal expansion (CTE) that is higher than that of candidate ferritic stainless steels, e.g. Crofer22 APU. Among the three alloys, Haynes 242, which is heavily alloyed with W and Mo and contains a low Cr content, demonstrated the lowest average CTE at 13.1 × 10⁻⁶ K⁻¹ from room temperature to 800 °C, but it was also observed that the CTE behavior of Haynes 242 was very non-linear.     

MnCo1.9Fe0.1O4 spinel protection layer on commercial ferritic steels for interconnect applications in solid oxide fuel cells

In solid oxide fuel cells (SOFC) for operating temperatures of 800 °C or below, the interconnection plates can be made from stainless steel. This is a big economic advantage, but energy losses can be caused by undesirable reactions between the alloys and other SOFC components. The use of coatings on interconnect stainless steels can reduce this degradation. A MnCo_1.9Fe_0.1O₄ (MCF) spinel not only significantly decreases the contact resistance between a La_0.8Sr_0.2FeO₃ cathode and a stainless steel interconnect, but also acts as a diffusion barrier to prevent Cr outward migration through the coating. The level of improvement in electrical performance depends on the ferritic substrate composition. For Crofer22APU and F18TNb, with a Mn concentration of 0.4 and 0.12 wt%, respectively, the reduction in contact resistance is significant. In comparison, limited improvement is achieved by application of MCF on IT-11 and E-Brite containing no Mn. No influence of the minor additions of Si or Al is observed on contact resistance. The MCF protection layer bonds well to the stainless steel substrates under thermal cycling, but the thermal expansion difference is too large between the La_0.8Sr_0.2Co_0.75Fe_0.25O₃ contact layer used and Crofer22APU and IT-11.     

Manufacture of Co-containing coating on AISI430 stainless steel via pack cementation approach for SOFC interconnect

Stainless steel can be applied as interconnect materials in solid oxide fuel cells (SOFCs) at operating temperatures 600–800 °C. Chromium (Cr)-forming stainless steel as an interconnect plate possesses a low oxidation resistance at high temperature and electrical conductivity, and volatility of Cr oxide scale can poison the cathode material. One effective strategy is to use a surface coating to improve interconnect performance. This work is to form cobalt (Co)-containing coatings on the surface of AISI 430 ferritic stainless steel interconnect via pack cementation approach. The resultant coating is extremely effective at heightening the oxidation resistance and electrical conductivity of AISI 430 ferritic stainless steel. The area specific resistance of samples was measured as a function of time. The area specific resistance of coated sample with 2% of activator content and holding time of 2 h is 90.21 and 108.32 mΩ cm² after 450 h of oxidation in air, respectively. Additionally, the coated sample with 2% of activator content and holding time of 2 h has a weight change of merely 0.299 and 0.231 mg/cm² after 650 h of isothermal oxidation at 800 °C, separately. The results displayed that the formation of CoFe₂O₄ spinel coating enhanced oxidation resistance by inhibiting the outward diffusion of Cr cations and the inward diffusion of oxygen anions.     

Oxidation behavior of nickel coating on ferritic stainless steel interconnect for SOFC application

The oxidation behavior of nickel coated ferritic stainless steel SS441 has been investigated. A nickel coating layer is deposited on the steel which is employed in a solid oxide fuel cell stack as interconnect. The nickel film is about 8 μm thick and is topped by an additional 4 μm thick La_0.67Sr_0.33MnO₃ (LSM) film on the interconnect cathode-contacting surface for the prevention of chromium evaporation from the steel substrate. A 10,000-h 800 °C isothermal ageing on 10 × 10 mm² steel coupons shows a continuous growth of oxide scales up to ∼200 μm in thickness on the surface, consisting of a 100 μm thick iron oxide layer followed by a complex Fe–Ni–Cr spinel structure. A single-cell stack is tested at 800 °C for up to 1226 h and an average degradation rate of 7.5% kh⁻¹ is observed. Oxidation characteristics of the coating system are analyzed after testing. A Fe–Ni spinel phase is found covering most of the surface area. This is attributed to the intensive interdiffusion of iron and nickel during the stack operation and the high intersolubility of the two elements. In both the tests of the steel coupons and the stack, LSM film structures are damaged by the thermally grown Fe–Ni oxides, and the expected Cr-preventing function is limited. The Fe–Ni spinel layer initially forms an effective obstacle against Cr out-migration. However, the increasing content of iron in the spinel phase induces oxide scale spallation afterwards. Though the fast grown Fe–Ni oxide scale can serve as an effective barrier against chromium out-migration, the iron-enriched scale structure is susceptible to corrosion attacks after an extended stack operation period.     

Oxidation resistance of novel ferritic stainless steels alloyed with titanium for SOFC interconnect applications

Chromia (Cr₂O₃) forming ferritic stainless steels are being developed for interconnect application in Solid Oxide Fuel Cells (SOFC). A problem with these alloys is that in the SOFC environment chrome in the surface oxide can evaporate and deposit on the electrochemically active sites within the fuel cell. This poisons and degrades the performance of the fuel cell. The development of steels that can form conductive outer protective oxide layers other than Cr₂O₃ or (CrMn)₃O₄ such as TiO₂ may be attractive for SOFC application. This study was undertaken to assess the oxidation behavior of ferritic stainless steel containing 1 weight percent (wt.%) Ti, in an effort to develop alloys that form protective outer TiO₂ scales. The effect of Cr content (6–22 wt.%) and the application of a Ce-based surface treatment on the oxidation behavior (at 800 °C in air + 3% H₂O) of the alloys was investigated. The alloys themselves failed to form an outer TiO₂ scale even though the large negative ΔG of this compound favors its formation over other species. It was found that in conjunction with the Ce-surface treatment, a continuous outer TiO₂ oxide layer could be formed on the alloys, and in fact the alloy with 12 wt.% Cr behaved in an identical manner as the alloy with 22 wt.% Cr.     

Highly conductive and stable Mn1.35Co1.35Cu0.2Y0.1O4 spinel protective coating on commercial ferritic stainless steels for intermediate-temperature solid oxide fuel cell interconnect applications

Chromia scale growth and Cr evaporation of ferritic stainless steel interconnects are known to be major causes of serious degradation of the solid oxide fuel cell (SOFC) stack. The development of suitable ceramic coating materials on the metallic interconnects has been demonstrated as an effective way to address these challenges. Herein, we developed a Mn_1.35Co_1.35Cu_0.2Y_0.1O₄ (MCCY) spinel material via a facile glycine-nitrate process as a protective coating on a metallic interconnect (SUS 441). Crystal structure and surface charge state analysis of the MCCY material revealed that co-doping of Y and Cu into the (Mn,Co)₃O₄ spinel resulted in redistribution of the Mn ions (Mn³⁺ and Mn⁴⁺) into the octahedral site, which increased the electrical conduction by enhanced small polaron hopping. Accordingly, the MCCuY-coated interconnect exhibited ∼8 times lower area specific resistance (ASR) than that of the undoped Mn_1.5Co_1.5O₄ (MCO) coated interconnect. Moreover, time-dependent ASR behavior of MCCuY-coated sample was monitored in-situ using electrochemical impedance spectroscopy at 650 °C, showing excellent stability with no observable change for >1000 h, while the ASR of the MCO-coated sample was raised by ∼71%. After 1000 h operation, we found strong adhesion between the MCCuY coating and the metallic interconnect as well as remarkably restricted Cr diffusion into the coating layer. Furthermore, the parabolic constant associated with the oxidation kinetics of the MCCuY-coated substrate (8.25 × 10⁻¹¹ mg² cm⁻⁴ s⁻¹) was ∼1 order of magnitude lower than that of the MCO-coated one (7.34× 10⁻¹⁰ mg² cm⁻⁴ s⁻¹) at 650 °C after 1000 h measurement. These results demonstrate that the MCCuY is a highly promising coating material of metallic interconnects for intermediate-temperature SOFC applications.     

Effects of La0.67Sr0.33MnO3 protective coating on SOFC interconnect by plasma-sputtering

Two metallic alloys, namely, Crofer22 APU and equivalent ZMG23 were investigated as possible interconnect materials in SOFC fuel cells. A _{La0.67Sr0.33MnO3}${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ (LSM) thin film is coated on these materials using pulsed DC magnetron sputtering. The as-deposited film is amorphous but is transformed into perovskite structure after annealing at different temperatures and times. The coating and uncoated structures and surface morphologies are analyzed using X-ray diffraction (XRD), electron Probe Micro Analyzer (EPMA), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The result shows that the LSM thin film on Crofer22 APU is good for compaction and adhesion, but there are some stresses between the equivalent ZMG232 and the coating and then create some cracks on the coating. Thereby, the coefficients of thermal expansion (CTE) of the equivalent ZMG232 may be higher than the CTE of the LSM. The cross-section of equivalent ZMG232 did not allow diffusion of Cr element. Thus, coating by plasma-sputtering could prevent the growth of oxide and the diffusion of Cr element to avoid cathode poisoning and the decline of conductivity in SOFC at high temperature.     

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.