Ferritic Stainless Steel Interconnects Research Articles

Electrophoretic Deposition and Physicochemical Properties of Ni- and Fe-Doped Cu–Mn Spinel Coatings Enhanced with Ce0.9Y0.1O2 Nanoparticles on Fe16Cr Ferritic Stainless Steel Interconnects for SOEC Applications

Cu- and Mn-based spinel coatings are currently among the most promising materials for solid oxide electrolyzer cell (SOEC) interconnects due to their high electrical conductivity and ability to eliminate environmentally hazardous cobalt, which is included in the most widely used coatings. However, their properties are affected by the presence of the Cr2O3 scale and high-temperature corrosion, and to mitigate this they could be doped with elements such as Ni and Fe. In addition, the electrical properties of such steel/shell layered systems can be further improved using rare-earth element nanoparticles (Ce0.9Y0.1O2). In this work, low-chromium steel was modified using nanoparticles and/or spinel coatings and oxidized in air atmosphere at 800 °C for 2000 hours. The oxidized systems were then characterized using diffraction studies, microstructural observations and electrical measurements. Electrical studies in particular showed a significant reduction in area-specific resistance for steels modified using a combination of both nanoparticles and spinel coatings.

The Preparation and Properties of Ti(Nb)-Si-C Coating on the Pre-Oxidized Ferritic Stainless Steel for Solid Oxide Fuel Cell Interconnect.

Cr2O3 scale growth and volatilization are the main cause of the performance degradation of solid oxide fuel cells (SOFCs) with an Fe-based ferritic stainless steel (FSS) interconnect. In this work, an amorphous Ti(Nb)-Si-C coating is prepared on the pre-oxidized SUS430 with D.C. magnetron sputtering as the protective coating. The amorphous Ti(Nb)-Si-C coated alloy exhibits significantly enhanced oxidation resistance, and the oxidation kinetics obey the parabolic law with a low parabolic rate of 9.36 × 10-15 g2·cm-4·s-1. A dual-layer oxide scale is formed composed of an inner layer rich in Cr2O3 and an outer layer rich in rutile TiO2 and amorphous SiO2. MnCr2O4 appears at the interface between the inner and outer oxide layers. Meanwhile, the amorphous Ti(Nb)-Si-C coating also effectively blocks the outward diffusion of Cr. In addition, the coated steel presents good electrical properties with an area-specific resistance (ASR) of 13.57 mΩ·cm2 at 800 °C after oxidation at 800 °C in air for 500 h.

Cu- and Ce-doped MnCo2O4 spinel coatings on ferrite interconnects by electrophoretic deposition

MnCo2O4 spinel coating is considered as one of the most promising protective coatings on the surface of ferritic stainless steel interconnects in solid oxide fuel cells (SOFCs). And the doping of metallic elements and active rare earth elements may further improve the electrical conductivity and antioxidant properties of spinel coatings. In this work we firstly prepare the Cu-doped and Ce-doped MnCo2O4 spinel coatings (abbreviated as MCCu, MCCe) on the surface of SUS430 substrate by Sol-Gel and electrophoretic deposition methods. The optimal doped ratio of Cu and Ce based on the results of oxidized mass gain and area-specific resistance (ASR) tests are determined respectively. Then we deposit Cu-Ce co-doped MnCo2O4 spinel coatings (abbreviated as MCCuCe) accordingly. The successful deposition of the target modified spinel coating on the substrate surface is confirmed using SEM-EDS, XRD, XPS characterization. The performance test results of the MCCuCe coating on the SUS430 surface show that the mass gain after 500 h oxidation at 800 °C in air is 1.3564 × 10−3 mg cm−2, and the ASR at 800 °C after 500 h oxidation is 9.80mΩ cm2, which is much lower than those of the bare SUS430 substrate and the substrate with deposited MnCo2O4 spinel coating. And the MCCuCe protective coating plays a better role in preventing the diffusion of Cr from the substrate. Further analysis prove that the doping Cu mainly contributes to the electrical conductivity and adhesion to the substrate while the doping Ce mainly improve the anti-oxidation and Cr diffusion resistance. This work determines the optimal doped ratio of Cu and Ce and provides a simple way to prepare the Cu-Ce co-doped MnCo2O4 spinel coatings on the ferritic interconnects, which effectively improve comprehensive performances of MnCo2O4 spinel coatings and helps to accelerate the industrial application of SUS430 ferrite in the SOFCs.

Mathematical modeling and experiment verification for the Solid oxide Fuel Cell Mn1.5Co1.5O4 interconnect coating

This study presents a new mathematical modeling study for interconnect oxidation analysis. A protective coating Mn1.5Co1.5O4 with different thicknessessare applied on ferritic stainless steel interconnect to address the chromium poisoning issue. Different Mn1.5Co1.5O4 coating thicknesses are applied. The coatings are assessed by surface morphology and phase structure. Developing coatings with various thicknesses is successful while maintaining consistent crystalline phases and coating morphology. The average coating thickness achieved at 35v for deposition times of 20, 30, and 40 s are 53.38 μm, 68.13 μm, and 85.13 μm, respectively. The coating thickness, 68.13 μm yields the lowest area specific resistance of 0.0469 Ω cm2 after 400 h of oxidation at 800 °C, compared to 0.0532 Ω cm2 and 0.0477 Ω cm2 for 53.38 μm and 85.13 μm The weight gains of the coated samples are recorded mathematically.This modeling approach results in a low weight gain and low oxidation kinetic rate of 0.139 mg cm−2 and 21.55 × 10−15 g2 cm−4 s−1, respectively for 68.13 μm coating thickness It predicts an area-specific resistance of 0.0451 Ωcm2 for the 35v 30 s coating, which is close to 0.0469 Ωcm2 experimental data.

Co-WO3 composite coating on ferritic stainless steel interconnects

In order to apply Co-WO3 composite coating to SUS 430 steel interconnects for solid oxide fuel cell (SOFC) interconnects, a combination of electrophoresis and electroplating is used, followed by thermal energy exposure to ambient temperature at 800 ℃ for up to 5 weeks. The thermal growth of the oxide scale displays a tri-layer structure with a Cr2O3 layer in the inner part, a CoWO4 layer in the middle and (Co,Fe)3O4 and Co3O4 layer in the outer part. Co-WO3 coating produces an oxide layer that not only prevents Cr2O3 development and outward transport of Cr species but also enhances the electrical behavior of the surface scale.

Development of Mn-Co and Mn-Co-CeO2 Coatings on Crofer 22 APU Steel for Solid Oxide Fuel Cell Interconnects

The present study compared the performance of uncoated, Mn-Co and Mn-Co-CeO2-coated Crofer 22 APU ferritic stainless steel interconnects. The samples were oxidized for 500 h in an electric furnace at 800 °C. The surface morphology and phase structure of the samples before and after oxidation were examined using FESEM microscopy and X-ray diffraction (XRD) analysis, respectively. The electrical conductivity evaluation of the samples was also conducted by measuring the area specific resistance (ASR). The results indicated that the weight gain of the uncoated, Mn-Co and Mn-Co-CeO2-coated samples after 500 h of oxidation was 0.55, 0.58 and 0.27 mg.cm−2, respectively. Additionally, a comparison of the oxidation kinetics of the experimental samples revealed that the oxidation rate constant of the Mn-Co-CeO2-coated steel is 10 and 40 times lower than that of the Mn-Co-coated and uncoated steel, respectively. XRD analysis of the samples after oxidation confirmed the presence of MnCo2O4, Co3O4, and (Mn,Cr,Co)3O4 in the oxidized Mn-Co-coated, MnCo2O4, Co3O4, and CeO2 in the oxidized Mn-Co-CeO2-coated, and the chromium-containing phases (FeCr2O4, MnCr2O4, and Cr2O3) in the oxidized uncoated samples. The presence of phases with high electrical conductivity in the oxidized coated samples reduced their activation energy for conduction compared to the oxidized uncoated sample.

Sr-doped LaFeO3 thin coatings for protection of ferritic stainless steel interconnects in solid oxide cells: A study on Cr-barrier and electrical properties

A high-temperature conversion process in molten carbonate baths was applied to produce thin Sr-doped LaFeO3 perovskite coatings on the surface of a K41 steel for interconnect applications in Solid Oxide Cells. Two conversion coatings with identical structure and phase composition, but slightly different thickness were produced in nitrate-accelerated baths containing the nitrate additive at two concentration levels. The as-prepared coatings consisted of a dual-layered perovskite-spinel structure with a top sub-micrometric Sr-doped LaFeO3 layer grown onto a micron-range thick inner Fe-Cr spinel oxide layer. Both coatings showed high protection against oxidation and chromia scale formation at 700°C. Further, excellent Area Specific Resistance (ASR) behavior was found for both coatings at 700°C, although an optimal balance of ASR and Cr-diffusion barrier properties was achieved with the coating produced in the lower nitrate content bath (N-02 coating). The causes for the enhanced barrier properties of the N-02 coating were ascribed not to a different coating thickness, but rather to the evolution of the pristine dual-layer into a more impervious three-layer structure during oxidation aging at 700°C. The evolved structure was formed by decomposition of the pristine sub-coating Fe-Cr spinel into two new spinel sublayers: an outer ferrite and an inner chromite layer, whereas the top thin-film perovskite layer remained essentially unaltered. Partial change in the Fe oxidation state and other possible causes involved in the observed decomposition of the pristine Fe-Cr spinel layer during the thermal aging are analyzed and discussed. Measurements of ASR activation energy indicated that the contact resistance behavior of the thin perovskite coatings was dominated by the internal spinel layer conductivity.

Experimental review of the performances of protective coatings for interconnects in solid oxide fuel cells

Ferritic stainless steel interconnects are used in solid oxide fuel cells; however, coatings are required to improve their performance. Although several types of coatings have been proposed, they have been scarcely investigated under similar conditions. This study compares the characteristics of uncoated Crofer 22 APU and eight different coatings on Crofer 22 APU for up to 3000 h at 800 °C. The coatings were deposited at various research laboratories around the world, and the experiments were performed at Chalmers University of Technology, Sweden. Cross-sections of the samples were analysed using scanning electron microscopy and energy-dispersive x-ray spectroscopy. The (Co,Mn)-based coated steels showed more than 50-fold lower chromium evaporation and at least 3 times thinner Cr2O3 scale thickness compared to uncoated steel. The coated steel samples showed lower area-specific resistance (ASR) values than the uncoated steel after 3000 h of exposure, irrespective of the coating thickness, composition and deposition method.

Investigation of coated FeCr steels for application as solid oxide fuel cell interconnects under dual-atmosphere conditions

Dual-atmosphere conditions are detrimental for the ferritic stainless steel interconnects used in solid oxide fuel cells, resulting in non-protective oxide scale growth on the air side. In this paper, low-cost steels AISI 441 and AISI 444 and the tailor-made Crofer 22 APU, were investigated at 800 °C and 600 °C under dual-atmosphere conditions: air-3%H2O on one side and Ar-5%H2-3%H2O on the other side. At 800 °C, the uncoated and Ce/Co-coated steels formed protective layers of (Cr,Mn)3O4/Cr2O3 and (Co,Mn)3O4/Cr2O3 respectively on the air side after 336 h. However, at 600 °C, the Ce/Co-coated AISI 441 and AISI 444 showed ∼20–25 μm thick Fe2O3/(Fe,Cr)3O4 oxide scale on the air side after 336 h. Ce/Co coated Crofer 22 APU remained protective after 772 h at 600 °C, indicating better resistance to the dual-atmosphere. The effect of Ce/Co coatings on the air side and the need for coatings on the fuel side are discussed and compared with experimental data.

Highly dense (Mn,Co)3O4 spinel protective coating derived from Mn–Co metal precursors for SOFC interconnect applications

The major degradation issues of solid oxide fuel cells (SOFC) are associated with the Cr2O3 scale growth and Cr diffusion of the Cr-based ferritic stainless steel (FSS) interconnects. Although (Mn,Co)3O4 has been proved as a suitable material for protecting FSS interconnects, the porous structure of the coatings prepared with the pre-synthesized spinel weakens the protective capability of the coatings. In this paper, the widely-used pre-synthesized spinel is replaced with metal precursors (Mn and Co powders). Due to the low melting point (≤1290 °C) and the volume expansion during oxidation, the metal precursors, can be effectively sintered at 900 °C in a reducing atmosphere and form dense, well-protective coatings at 850 °C in the air. The samples are characterized with X-ray diffraction (XRD), scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), and a 4-probe area-specific resistance (ASR) test. Compared with the coatings derived from pre-synthesized spinel, the metal-derived coatings present denser structures with better electrical conductivity (ASR = 5.76 mΩ cm2). The weight gain and ASR measurement results indicate that the metal-derived coatings significantly mitigate the increase of weight gain and ASR by inhibiting scale formation and growth, showing better protective capability for SOFC applications.

Protective Coatings for Ferritic Stainless Steel Interconnect Materials in High Temperature Solid Oxide Electrolyser Atmospheres

Stainless steel interconnect materials used in solid oxide fuel cells and electrolysers need to be coated to improve oxidation resistance and to mitigate Cr-vaporization. This work aimed to explore the optimal steel/coating combinations suitable for use in reversible solid oxide stacks and evaluated (Co,Mn)3O4 spinel, LaFeO3 perovskite, Ce/Co and Y-based coatings, on AISI441 and Crofer 22 APU steels. The coatings were evaluated based on measurements of mass gain and oxide scale thickness after exposure at 700 and 800 °C to fuel side (90 vol.% H2O/10 vol.% H2) and air/oxygen side (pure O2) atmospheres. In pure O2, the most efficient coatings for limiting oxide scale formation and Cr evaporation, compared to the bare steel, were (Co,Mn)3O4 and CeCo on Crofer 22 APU. In 90 vol.% H2O/10 vol.% H2, the Y-based coating showed the largest improvement in oxidation resistance.

Comparison of electroplating and sputtering Ni for Ni/NiFe2 dual layer coating on ferritic stainless steel interconnect

Ni/NiFe2 dual layer coating is fabricated to improve the performance of SUS 430 steel as solid oxide fuel cells (SOFCs) interconnects by a combined approach. Ni layer is deposited by electroplating or magnetron sputtering, and subsequently the NiFe2 layer is sputtered on it. The coated steels are evaluated at 800 °C in air corresponding to SOFC cathode environment. It is found that the coating with electroplated Ni and sputtered NiFe2 results in breakaway oxidation after 5 weeks. Preoxidation treatment in 900 °C air for electroplated Ni steel prior to depositing NiFe2 layer improves oxidation resistance. The sputtered Ni and NiFe2 sample exhibits superior oxidation resistance and inhibition against Cr outward migration while maintaining a low area-specific resistance (ASR) of surface scale. Comparative analyses of the coating preparation and resultant oxidation behavior with possible mechanisms and implications are discussed.

Electrically Conductive and Protective Coating Reaction with LSCF Material

Ferritic stainless steel interconnect materials are used in intermediate or high-temperature SOFCs because of their oxidation resistance, high formability, and low cost. Their corrosion resistance is due to the formation of a protective chromium oxide scale during operation. However, the protective oxide layer produces Cr-containing volatile species at SOFC operating temperatures and conditions, which can cause cathode poisoning. Electrically conducting spinel coatings have been developed to prevent cathode poisoning and maintain an electrically conductive pathway through SOFC stacks. Generally, Mn-Co-O spinel is used for this protective coating in SOFCs. To remove the Co and enhance the electrical conductivity, PNNL has developed a Mn-Cu-O spinel for this protective coating. This paper will focus on the electrically conductive coating’s compatibility with LSCF in high temperature operating environments.

Electrically Conductive and Protective Coating Reaction with LSCF Material

Ferritic stainless-steel interconnect materials are used in intermediate or high-temperature SOFCs because of their oxidation resistance, high formability, and low cost. Their corrosion resistance is due to the formation of a protective chromium oxide scale during operation. However, the protective oxide layer produces Cr-containing volatile species at SOFC operating temperatures and conditions, which can cause cathode poisoning. Spinel coatings have been developed to prevent cathode poisoning while maintaining an electrically conductive pathway through SOFC stacks. Generally, Mn-Co-O spinel has been the material of choice, but PNNL has developed an Mn-Cu-O spinel protective coating to remove the costly Co and enhance the electrical conductivity. This paper will focus on the electrically conductive coating’s compatibility with LSCF in high-temperature operating environments.

Microstructure and area specific resistance of cathodic half cells for solid oxide fuel cells composed of perovskite-type cathodes and Co-alloy-coated ferritic stainless steel interconnects

In order to prevent oxidation and Cr poisoning, a Co-W coating has been applied on ferritic stainless steel (FSS), which is used as the interconnect parts of solid oxide fuel cells (SOFCs). However, the electrical properties of the Co–W-coated stainless steels have not yet been evaluated. In this study, cathodic half-cells were experimentally manufactured with La0.8Sr0.2MnO3 (LSM) and La0.8Sr0.2Co0.8Fe0.2O3 (LSCF) as cathodes, and uncoated, Co-coated, and Co–W-coated FSSs as interconnects, respectively. Then their oxidation property and area-specific resistance (ASR) were evaluated at 800 °C. Based on phase identification and microstructural analysis, the Co–W-coating was confirmed to effectively suppress the Cr poisoning of the cathode. The ASR values of the Co–W-coated steel with LSM and LSCF as cathodes were 102 and 97 mΩ cm2, respectively, which are adequate for SOFC application. Furthermore, when the LSM was applied as a cathode material, the formation of Mn spinels enhanced the adherence between the cathode and interconnected parts. Thus, the combination of Co–W-coated FSS steel with LSM as the cathode material exhibited the optimal combination of electrical conductivity and microstructural stability.Graphic abstract.

The Effects of Dual Atmosphere Exposure Duration and Temperature on the Growth of Iron Oxide on Ferritic Stainless Steels

Literature within the past two decades has highlighted the ongoing issue of dual atmosphere corrosion in solid oxide fuel cell (SOFC) systems. This corrosion phenomenon occurs as a result of simultaneous exposure of ferritic stainless steel interconnects to the anodic fuel gas on one side and cathodic air stream on the opposing side. In this, the air-exposed side of the metal interconnect undergoes fast, anomalous corrosion and metal loss in the form of iron oxide overgrowth with whisker and platelet-type morphology. Studies have indicated that the corrosion is favored at lower temperatures in comparison to temperatures above 800 °C. This poses an issue for the current trend of lowering SOFC operating temperatures and utilizing cost-effective ferritic stainless steels as interconnect materials. In this study, the oxidation behavior of two ferritic stainless steels is examined under dual atmosphere exposure conditions. A series of experiments with varying time, temperature, and steel compositions were carried out to better understand the effects of these variables on severity and morphology of corrosion products. A mechanism which describes the localized overgrowth of iron oxide in platelet and whisker form and the porous oxide sub-layer is proposed. The transition from a thin, protective, and dense chromia scale under nominal ambient conditions to porous, non-protective scaling under dual atmosphere conditions will be discussed.

Ba0.5Sr0.5Co0.8Fe0.2O3−δ-Sm0.2Ce0.8O1.9 carbonate perovskite coating on ferritic stainless steel interconnect for low temperature solid oxide fuel cells

Ferritic stainless steel interconnect at a high temperature and a prolonged operating time is exposed to severe conditions, such as the overgrowth of chromium oxide layer on an interconnect and cathode chromium poisoning due to chromium oxide migration. In this study, a protective Ba0.5Sr0.5Co0.8Fe0.2O3−δ-Sm0.2Ce0.8O1.9 carbonate (BSCF-SDCC) perovskite coating was developed on SUS 430 stainless steel through electrophoretic deposition and subsequently sintering to overcome these problems. BSCF-SDCC coating layer was sintered from 550 °C to 750 °C and examined in terms of its crystalline phase, carbonate bonding and microstructure. Area specific resistance (ASR) was tested at each sintering temperature of BSCF-SDCC coating by using a two-probe DC technique during oxidation at 600 °C. Afterwards, the microstructure was analyzed to ensure the endurance of coating characteristics and determine chromium diffusion during oxidation. Carbonate bonding in BSCF-SDCC decreased when sintering temperature increased. The microstructure of the BSCF-SDCC coating sintered at 550°C-650 °C was uniform and dense, but a crack was observed in the coating sintered at 700 °C and 750 °C. The crystalline phase of BSCF-SDCC was verified except the coating sintered at 750 °C, indicating the secondary phase formation of BaCO3. After 500 h of oxidation, the BSCF-SDCC coating sintered at 550°C-650 °C was achieved below the required 0.1 Ωcm2, whereas the coating sintered at 600 °C exhibited the lowest ASR of 0.073 Ωcm2. The coating microstructure remained dense, compact and uniform after thermal oxidation. Therefore, a protective BSCF-SDCC coating for interconnects was successfully developed at low temperatures (400°C-600 °C).

Impact of Different Atmospheres on Oxidation and Electrical Performance of a Solid Oxide Fuel Cell Interconnect with Co-Containing Protective Coating

Chromium (Cr) evaporation of ferritic stainless steel (FSS) interconnects is considered to be the main reason for severe deterioration of solid oxide fuel cells (SOFCs). To suppress the influence o...

Oxidation behavior of ferritic stainless steel interconnect coated by a simple diffusion bonded cobalt protective layer for solid oxide fuel cells

A protective cobalt coating is fabricated on AISI 441 ferritic stainless steel interconnect by diffusion bonding method for solid oxide fuel cells. Oxidation behavior of coated and bare steels are studied. Microstructure evolution of steels during oxidation is analyzed by XRD, SEM and TEM with EDS. After 800 °C/1000 h oxidation, three layers are observed on the coated steel surface: a Co3O4 top layer, a (Co,Fe)3O4 middle layer and a Cr2O3 inner layer. The thickness of Cr2O3 of the coated steel is 2.7 μm, while that of bare steel is 10.3 μm. The out-migration of Cr is effectively suppressed.

Initial Oxidation Behavior of Ferritic Stainless Steel Interconnect with Sputtered NiFe2 Alloy Coating

Initial oxidation behavior of ferritic stainless steel with a sputtered NiFe2 coating for solid oxide fuel cells interconnect application was investigated in air at 800 °C to understand the transformation process from the NiFe2 alloy coating to NiFe2O4 spinel layer. The results indicated that the NiFe2 coating was initially converted to a surface scale with a layered structure consisting of a top Fe2O3 layer followed by NiFe2O4 mid-layer and an NiO inner layer. Cr oxide started to form at scale/steel interface before the coating was completely oxidized. The preferential growth orientation of Fe2O3 on the surface varied with time during the initial oxidation stage. Steel preoxidation prior to coating not only accelerated the oxidation of NiFe2 coating and the growth of NiFe2O4 layer, but also suppressed the orientated growth of Fe2O3. The surface scales on the coated steels were electrically conductive. The oxidation mechanism of the coated steels is discussed.