The effect of surface treatment on the oxidation of ferritic stainless steels used for solid oxide fuel cell interconnects

Ferritic stainless steels are candidate interconnect materials for solid oxide fuel cells (SOFC); however, the oxidation resistance of commercial stainless steels within the operating temperature range of 700–800 °C is not adequate. A relatively thick, poorly conducting oxide layer forms on the surface of the stainless steel interconnect, decreasing cell performance. One way of modifying the oxidation behaviour of an alloy is through surface treatment. The aim of this work is to perform a systematic study of the effect of surface treatment (sandblasting and cold rolling) on the oxidation behaviour of three different ferritic stainless steels at 800 °C in air. Oxidized specimens are characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). In addition, specimens oxidized under the same conditions for 15 min are examined using secondary ion mass spectrometry (SIMS) depth profiling and X-ray photoelectron spectroscopy (XPS) depth profiling.     

Effect of electrical current on the oxidation behavior of electroless nickel-plated ferritic stainless steel in solid oxide fuel cell operating conditions

Planar solid oxide fuel cell (SOFC) systems often employ metallic interconnects, which separate and connect individual cells in electrical series to create a stack. Coated and uncoated ferritic stainless steels (FSSs), are reported among the most promising materials currently being investigated for interconnect applications. In this study, FSS AISI 441 samples coated with electroless nickel (～25 μm) were subjected to intermediate temperature IT-SOFC operating conditions at 700 °C for 500 h with and without the application of electrical current (0.5 Acm^?2). The application of the electric current promotes Fe migration on both the cathode and the anode side. This phenomenon results in the formation of a ～4 μm thick Fe₂O₃ on the anode side responsible for increased ASR values. Comparative analyses of the current and no current exposures and resultant surface oxide layers, along with suspected mechanisms and implications are presented and discussed.     

Degradation analysis of commercial interconnect materials for solid oxide fuel cells in stacks operated up to 18000 hours

Despite being a mature technology, solid oxide fuel cell (SOFC) devices are still limited by lifetime issues. In SOFC stacks, cell/interconnect interaction is the main responsible for voltage degradation at the oxygen electrode side. Corrosion and chromium evaporation might in fact increase ohmic and charge transfer losses. This study presents the evolution of the degradation phenomena inside four SOFC short-stacks tested respectively for 45, 2700, 4800 and 10000 hours. An additional stack which underwent 124 thermal cycles is also analyzed to assess the mechanical reliability of the interconnect/ceramic coupling. Metal interconnect was made of K41/AISI441 ferritic stainless steel coated with MnCo₂O₄ porous barrier layer. Scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) characterization is applied to examine the degradation process. Observations indicate that despite a harsh initial red-ox interaction between the cathode materials and the interconnect, after 5000 h of operation the kinetic of the degradation process in the electrical contact areas slows down dramatically. An empirical model based on the scale thickness at different interconnect location gives estimation for the oxide thermal growth for a stack lifetime period. From the mechanical properties point of view, no spallation was observed and local delamination was mainly due to the sample preparation process.     

Oxidation of ferritic stainless steel interconnects: Thermodynamic and kinetic assessment

Planar solid oxide fuel cells with yttria-stabilized zirconia electrolytes typically operate at temperatures in the range of 700-850 °C. The maximum temperature is limited by the use of ferritic stainless steel interconnects which offer significant advantages such as low cost and high thermal and electronic conductivity over traditional ceramic interconnects. However, these alloys rely on the formation of a protective chromia scale for oxidation resistance that results in an increase in ohmic resistance and can volatilize leading to a loss of cathode catalytic activity. To better understand the oxidation behavior of chromia forming ferritic stainless steels, thermodynamic modeling was performed in conjunction with experimental oxidation testing and empirical kinetic evaluation. The phase stability and oxidation behavior of the Fe-Cr-O ternary system were assessed using thermodynamic calculations. Calculated ternary phase diagrams were validated against the experimental oxidation data of Fe-20Cr and Fe-18Cr ferritic stainless steels, GE-13L and AL-441HP, respectively. Results indicate that the use of accelerated testing, such as exposing the system to higher temperatures, can lead to changes in phase equilibria and the oxidation kinetics of the alloys. Through combined thermodynamic assessment and controlled oxidation experiments, the oxidation behavior of high-chromium ferritic stainless steels is presented and discussed.     

Development of MnCoO coating with new aluminizing process for planar SOFC stacks

Chromia-forming ferritic stainless steels find widespread use as interconnect materials in SOFCs at operating temperatures below 800 °C, because of their thermal expansion match and low cost. However, volatile Cr-containing species originating from this scale can poison the cathode material in the cells and subsequently cause power degradation in the devices. To prevent this, a conductive manganese cobaltite spinel coating has been developed, but unfortunately; this coating is not compatible with glass-based seals between the interconnect or cell frame components and the ceramic cell due to reactions between the coating and the glass. Thus, a new aluminizing process has been developed to improve the stability of the sealing regions of these components, as well as for other metallic stack and balance-of-plant components.     

Ferritic stainless steel interconnects for protonic ceramic electrochemical cell stacks: Oxidation behavior and protective coatings

Protonic ceramic fuel or electrolysis cells (PCFC/PCEC) have shown promising performance at intermediate temperatures. However, these technologies have not yet been demonstrated in a stack, hence the oxidation behavior of the metallic interconnect under relevant operating environments is unknown. In this work, ferritic stainless steels 430 SS, 441 SS, and Crofer 22 APU were investigated for their use as interconnect materials in the PCFC/PCEC stack. The bare metal sheets were exposed to a humidified air environment in the temperature range from 450 °C to 650 °C, to simulate their application in a PCFC cathode or PCEC anode. Breakaway oxidation with rapid weight gain and Fe outward diffusion/oxidation was observed on all the selected stainless steel materials. A protective coating is deemed necessary to prevent the metallic interconnect from oxidizing.To mitigate the observed breakaway oxidation, state-of-the-art protective coatings, Y₂O₃, Ce_0.02Mn_1.49Co_1.49O₄, CuMn_1.8O₄ and Ce/Co, were applied to the stainless steel sheets and their oxidation resistance was investigated. Dual atmosphere testing further validated the effectiveness of the protective coatings in realistic PCFC/PCEC environments, with a hydrogen gradient across the interconnect. Several combinations of metal and coating material were found to be viable for use as the interconnect for PCFC/PCEC stacks.     

Determination of interfacial adhesion strength between oxide scale and substrate for metallic SOFC interconnects

The interfacial adhesion strength between the oxide scale and the substrate is crucial to the reliability and durability of metallic interconnects in solid oxide fuel cell (SOFC) operating environments. It is necessary, therefore, to establish a methodology to quantify the interfacial adhesion strength between the oxide scale and the metallic interconnect substrate, and furthermore to design and optimize the interconnect material as well as the coating materials to meet the design life of an SOFC system. In this paper, we present an integrated experimental/analytical methodology for quantifying the interfacial adhesion strength between the oxide scale and a ferritic stainless steel interconnect. Stair-stepping indentation tests are used in conjunction with subsequent finite element analyses to predict the interfacial strength between the oxide scale and Crofer 22 APU substrate.     

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.     

Review: Influence of alloy addition and spinel coatings on Cr-based metallic interconnects of solid oxide fuel cells

Solid oxide fuel cell (SOFC) is the modern eco-friendly technology of fuel cell power generation system. It generates electricity from a redox chemical reaction without producing hazardous gases. It consists of anode, cathode and electrolyte. It is operated in the form of stack connected by interconnects to boost-up power output. The recent development of low-temperature (600 °C–800 °C) brings an opportunity to use metallic interconnects over ceramics. Cr-based metallic interconnects are one of the prominent metallic interconnects. They offer chemical inertness, thermal stability, compatible coefficient of thermal expansion and highly dense structure. However, the Cr-migration towards the cathode side is the major problem in them which adversely affect the SOFCs performance. Therefore a good oxidation resistance without sacrificing electrical conductivity is required. To resolve this issue, several alloying elements and spinel coatings have experimented. These spinel coatings are the thin solid films of Mn, Co, Cu and rare earth metals. This review concluded that the Mn–Co based spinal coating showed excellent performance in reducing the Cr-migration in specially designed expensive Crofer 22 APU interconnect. However, the emerging low-cost ferritic interconnects also show their best results with Cu–Fe based spinel coating. Among them, the SUS-430 interconnect shows the equivalent performance of Crofer 22 APU interconnect after surface treatment and appropriate Cu–Fe based spinel coating. Therefore, it can replace the Crofer 22 APU interconnect on a cost basis.     

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.     

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.     

Exploration of alloy 441 chemistry for solid oxide fuel cell interconnect application

Alloy 441 stainless steel (UNS S 44100) is being considered for application as an SOFC interconnect material. There are several advantages to the selection of this alloy over other iron-based or nickel-based alloys: first and foremost alloy 441ss is a production alloy which is both low in cost and readily available. Second, the coefficient of thermal expansion (CTE) more closely matches the CTE of the adjoining ceramic components of the fuel cell. Third, this alloy forms the Laves phase at typical SOFC operating temperatures of 600-800 °C. It is thought that the Laves phase preferentially consumes the Si present in the alloy microstructure. As a result it has been postulated that the long-term area specific resistance (ASR) performance degradation often seen with other ferritic stainless steels, which is associated with the formation of electrically resistive Si-rich oxide subscales, may be avoidable with alloy 441ss. In this paper we explore the physical metallurgy of alloy 441, combining computational thermodynamics with experimental verification, and discuss the results with regards to Laves phase formation under SOFC operating conditions. We show that the incorporation of the Laves phase into the microstructure cannot in itself remove sufficient Si from the ferritic matrix in order to completely avoid the formation of Si-rich oxide subscales. However, the thickness, morphology, and continuity of the Si-rich subscale that forms in this alloy is modified in comparison to non-Laves forming ferritic stainless steel alloys and therefore may not be as detrimental to long-term SOFC performance.     

Chromium transport by solid state diffusion on solid oxide fuel cell cathode

Iron-chromium ferritic stainless steel is widely used in solid oxide fuel cell (SOFC) components. At 650-800 °C, stainless steels form a protective chromia oxide scale. This low conductivity catalytic compound can degrade SOFC cathode performance. The migration of Cr species onto the cathode occurs through vapor transport and/or solid state diffusion, and electrochemical reactions may affect the migration.It is important to understand the relative Cr transport and reaction rates to evaluate the most viable commercially available cathode material. This study characterizes the migration of Cr species through solid state diffusion and vapor deposition. Chromia blocks and chromia-forming stainless steel interconnects were held in contact with LSM (Lanthanum Strontium Manganese Oxide), LSCF (Lanthanum Strontium Cobalt Ferrite) and LNF (Lanthanum Nickel Ferrite) perovskite pellets in Cr-saturated air at 700 °C for 300 h. XRD (X-ray Diffraction), SEM (Scanning Electron Microscope), EDS (Energy Dispersive X-ray Spectroscopy) and Ion Milling by FIB (Focused Ion Beam) were used to detect Cr on and within the perovskite pellets. Cr transport and reaction on LSCF is the most severe, followed by LSM. Cr transport is observed on LNF, but without noticeable reaction.     

High-temperature (800 °C) dual atmosphere corrosion of electroless nickel-plated ferritic stainless steel

Planar solid oxide fuel cell (SOFC) systems often employ metallic interconnects, which separate and connect individual cells in electrical series to create a stack. Coated and uncoated ferritic stainless steels (FSSs), are reported among the most promising materials currently being investigated for the interconnect application. In this study, FSS AISI 441 samples coated with electroless nickel (∼15 μm) were subjected to both single (moist air) and dual atmosphere (moist air/moist hydrogen) exposures at 800 °C for 100 h to simulate short-term SOFC interconnect operation. Single-atmosphere exposures induced a uniform and dense surface oxide layer of approximately 5 μm total thickness, comprised of a dense and uniform Ni-rich oxide layer above a mixed layer of Fe, Cr and Mn-rich oxides. In contrast, the air-side of dual atmosphere exposed samples consisted of a mixed, porous and delaminated surface layer comprised of Fe, Cr, Mn and Ni metals and oxides, with over 30 μm in total thickness. Comparative analyses of the single and dual atmosphere exposures and resultant surface oxide layers, along with suspected mechanisms and implications are presented and discussed.     

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.     

Applicability of extra low interstitials ferritic stainless steels for bipolar plates of proton exchange membrane fuel cells

Ferritic stainless steels can be attractive bipolar plate materials of proton exchange membrane fuel cells (PEMFC), provided that the stainless steels show sufficient corrosion resistance, for instance, by eliminating interstitial elements such as carbon and nitrogen. In the present study, thus, ferritic stainless steels (19Cr2Mo and 22Cr2Mo) with extra low interstitials (ELI) are evaluated to determine the required level of chromium content to apply them for PEMFC bipolar plates. In a simulated PEMFC environment (0.05 M SO₄²⁻ (pH 3.3) + 2 ppm F⁻ solution at 353 K), the 22Cr2Mo stainless steel showed lower current density during the polarization in comparison with the 19Cr2Mo one. The polarization behavior of the 22Cr2Mo stainless steel resembles that of the type 316 one (17Cr12Ni2Mo). Similar values of interfacial contact resistance (ICR) are observed for both ferritic stainless steels. The 22Cr2Mo stainless steel bipolar plate is found to be stable throughout the cell operation, while the 19Cr2Mo stainless steel corroded within 1000 h. After the cell operation, the 22Cr2Mo stainless steel retains the chromium enriched passive film, while the chromium enriched surface film is not found for the 19Cr2Mo one, showing iron oxide/hydroxide based film. X-ray fluorescence (XRF) analysis of the membrane electrode assemblies (MEAs) after the cell operation indicates that the 22Cr2Mo stainless steel was less contaminated with iron species. The above results suggest that the 22Cr2Mo stainless steel can be applicable to bipolar plates for PEMFC, especially 22 mass% of chromium content in ferritic stainless steel with ELI system is, at least, demanded to ensure stable cell performance.     

Ferritic stainless steels as bipolar plate material for polymer electrolyte membrane fuel cells

Both interfacial contact resistance (ICR) measurements and electrochemical corrosion techniques were applied to ferritic stainless steels in a solution simulating the environment of a bipolar plate in a polymer electrolyte membrane fuel cell (PEMFC). Stainless steel samples of AISI434, AISI436, AISI441, AISI444, and AISI446 were studied, and the results suggest that AISI446 could be considered as a candidate bipolar plate material. In both polymer electrolyte membrane fuel cell anode and cathode environments, AISI446 steel underwent passivation and the passive films were very stable. An increase in the ICR between the steel and the carbon backing material due to the passive film formation was noted. The thickness of the passive film on AISI446 was estimated to be 2.6 nm for the film formed at −0.1 V in the simulated PEMFC anode environment and 3.0 nm for the film formed at 0.6 V in the simulated PEMFC cathode environment. Further improvement in the ICR will require some modification of the passive film, which is dominated by chromium oxide.     

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.     

Yttrium,cobalt and yttrium/cobalt oxide coatings on ferritic stainless steels for SOFC interconnects

Ferritic stainless steels are being considered as potential interconnect materials for SOFCs, in part because of their low cost relative to alternatives. These materials are, however, susceptible to degradation over time. A primary source of degradation is an increase in the area specific resistance (ASR), which is due to the formation of poorly conducting oxides (Mn–Cr spinel and Cr₂O₃) on the surface. In this work, the influence of Y, Co and Y/Co oxide coatings on the oxidation behaviour of a ferritic stainless steel (16–18 wt% Cr) has been investigated. Samples were oxidized in air for up to 500 h at temperatures ranging from 700 to 800 °C. Coated and uncoated samples were characterized, before and after heat treatment, using X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric (TG) analysis and four-point probe resistance measurements.Surface morphology investigations of coated and uncoated stainless steels showed differences for Y and Co in terms of oxide formation. In all cases, Cr–Mn spinel and Cr₂O₃ were the two main surface oxides; however, the morphology of the spinel phase was dependent on the type of coating. The lowest resistances were obtained for the Y/Co-coated samples, which had ASR values up to seven times lower than corresponding uncoated ferritic stainless steels.     

Anodic behavior of high nitrogen-bearing steels in PEMFC environments

High nitrogen-bearing stainless steels, AISI Type 201 and AL219, were investigated in simulated polymer electrolyte membrane fuel cell (PEMFC) environments to assess the use of these materials in fuel cell bipolar plate applications. Both steels exhibit better corrosion behavior than 316L steel in the same environments. Type 201 steel shows similar but lower interfacial contact resistance (ICR) than 316L, while AL219 steel shows higher ICR than 316L.

X-ray photoelectron spectroscopy (XPS) analysis shows that the air-formed films on Type 201 and AL219 are composed of iron oxides, chromium oxide, and manganese oxide. Iron oxides dominate the composition of the air-formed film, specially the outer layer. Chromium oxide dominates passive films. Surface film thicknesses were estimated. The results suggest that high nitrogen-bearing stainless steels are promising materials for PEMFC bipolar plates.