Oxidation behavior of ferritic stainless steel containing Nb,Nb–Si and Nb–Ti for SOFC interconnect

The effect of Nb on the oxidation kinetics, electrical conductivity and Cr evaporation behavior of FSS has been discussed depending on the Nb content and oxygen active element such as Ti and Si. Nb in ferritic stainless steel is saturated during heat treatment as NbO₂ at the outermost oxide scale and as both Nb₂O₅ and Laves phase near the oxide scale/alloy interface. Excess Nb (>4.7 wt%) suppresses precipitation of Nb₂O₅, because of rapid Laves phase growth. Nb enhances selective Ti oxidation, whereas Ti retards Nb₂O₅ precipitation near the scale/alloy interface. On the other hand, Si suppresses Nb enrichment near the scale/alloy interface and it reduces the precipitation of both Nb₂O₅ and Laves phase. Nb also suppresses Si enrichment and the formation of continuous Si oxide at the scale/alloy interface. Co-addition of Nb and Ti is effective to decrease the electrical resistance and Cr evaporation rate of oxide scale.     

Conducting oxide formation and mechanical endurance of potential solid-oxide fuel cell interconnects in coal syngas environment

The oxidation properties of potential SOFCs materials Crofer 22 APU, Ebrite and Haynes 230 exposed in coal syngas at 800 °C for 100 h were studied. The phases and surface morphology of the oxide scales were characterized by X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray analysis (EDX). The mechanical endurance and electrical resistance of the conducting oxides were characterized by indentation and electrical impedance, respectively. It was found that the syngas exposure caused the alloys to form porous oxide scales, which increased the electrical resistant and decreased the mechanical stability. As for short-term exposure in syngas, neither carbide nor metal dusting was found in the scales of all samples.     

High-temperature tensile and creep properties of a ferritic stainless steel for interconnect in solid oxide fuel cell

The purpose of this study is to investigate the high-temperature mechanical properties of a ferritic stainless steel (Crofer 22 APU) for use as an interconnect material in planar solid oxide fuel cells (pSOFCs). Tensile properties of the Crofer 22 APU steel are evaluated at temperatures of 25-800 °C. Creep properties are evaluated by constant-load tests at 650-800 °C. Several creep lifetime models are applied to correlate the creep rupture time with applied stress or minimum creep rate. Experimental results show the variation of yield strength with temperature can be described by a sigmoidal curve for different deformation mechanisms. The creep stress exponent, n, has a value of 5 or 6, indicating a power-law creep mechanism involving dislocation motion. The apparent activation energy for such a power-law creep mechanism is estimated as 393 kJ mol⁻¹ through some thermally activated relations. Creep rupture time of the Crofer 22 APU steel can be described by a Monkman-Grant relation with a time exponent, m = 1.11. The relation between creep rupture time and normalized stress is well fitted by a universal simple power law for all of the given testing temperatures. Larson-Miller relationship is also applied and shows good results in correlating the creep rupture time with applied stress and temperature for the Crofer 22 APU steel. Fractographic and microstructural observations indicate most of the creep cavities are nucleated along grain boundaries and a greater amount of cavities are formed under high stresses.     

Influence of precipitation on initial high-temperature oxidation of Ti–Nb stabilized ferritic stainless steel SOFC interconnect alloy

Oxidation phenomena on Laves phase forming Ti–Nb stabilized ferritic stainless steel (EN 1.4509) were studied at 650 °C by electron microscopic and electron spectroscopic methods. These investigations reveal a strong competition between Nb and Si for interfacial oxidation at the oxide–metal interface that is affected by different segregation rates of Nb and Si at elevated temperatures. In particular, formation of Si containing Laves (FeNbSi)-type intermetallic compounds in the bulk results in non-uniform distribution of Si oxide at the interface. This has direct implications to the electrical properties of this alloy in solid oxide fuel cell (SOFC) applications. Furthermore, these results provide better understanding to the controversial role of second phases (e.g. Laves, chi) on high-temperature oxidation (as recently discussed by Dae Won Yon, Hyung Suk Seo, Jae Ho Jun, Jae Myung Lee, Do Hyuong Kim, Kyoo Young Kim in Int J Hydrogen Energy 2011;36:5595–5603).     

Oxidation behavior of metallic interconnect coated with La–Sr–Mn film by screen painting and plasma sputtering

Two Fe-based alloys, specific company developed and designated as Crofer22APU and ZMG232, have been extensively evaluated and considered as outstanding metallic interconnect materials. Both these alloys contain significant and minute amounts of elemental Cr and La, respectively. In this study, they are coated with films of La–Sr–Mn (LSM) using two methods, screen painting and plasma sputtering, to determine the effect of LSM on corrosion resistance and electrical resistivity of Crofer22APU and ZMG232. They were then treated in a simulated oxidizing environment at 800 °C for 200 h. Analytical results indicate that the LSM film changed the oxidation behavior of the base alloys, Crofer22 APU and ZMG232. The bare alloys formed Cr₂O₃, while the coated alloys produced (Mn, Fe) Cr ₂O₄. The electrical resistance of the former oxide at high temperature is several thousand times higher than that of the latter oxide. This remarkable effect of the LSM film on the electrical characteristics warrants further in-depth research.     

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.     

Evaluation of several low thermal expansion Fe–Co–Ni alloys as interconnect for reduced-temperature solid oxide fuel cell

Several low thermal expansion Fe–Co–Ni alloys including HRA 929C, Thermo-Span, EXP 4005 and Three-Phase were evaluated as interconnect for reduced-temperature solid oxide fuel cell (SOFC). The isothermal oxidation behaviors of the four alloys were determined at 800 °C in air corresponding to the SOFC cathode environment. The results indicate that the mass gains of HRA 929C and Thermo-Span increased continuously with oxidation time, and were higher than those of EXP 4005 and Three-Phase, both of which exhibited low oxidation rate after the first-week exposure due to the formation of a semi-continuous Al₂O₃ inner layer. Compared to the ferritic alloy Crofer 22 APU, these low thermal expansion alloys exhibited inferior oxidation resistance; however, the area specific resistance (ASR) of the oxide scales thermally grown on these alloys was lower than that for Crofer 22 APU, as a result of the formation of a highly-conductive, Cr-free surface spinel layer. The promises and problems of these low thermal expansion alloys were discussed with regard to SOFC interconnect application.     

Seal glass compatibility with bare and (Mn,Co)3O4 coated Crofer 22 APU alloy in different atmospheres

To prevent gas mixing and leakage during solid oxide fuel/electrolyzer cell operation, the interconnect/seal glass interface should bond well and remain stable. A SrO-La₂O₃-Al₂O₃-SiO₂ (SABS-0) seal glass has been bonded to bare Crofer 22 APU alloy and (Mn,Co)₃O₄ coated Crofer 22 APU alloy. The stability of the interconnect/SABS-0 interface has been studied in air and H₂/H₂O atmospheres at 800 °C for 1000 h. The interconnect/seal glass interaction involves the oxidation of the bare and (Mn,Co)₃O₄ coated Crofer 22 APU alloy surfaces, inter-diffusion of elements, chemical reaction, and the devitrification of the SABS-0 glass. The study shows that the thermal treatment atmosphere greatly affects the interfacial stability of both bare Crofer 22 APU/SABS-0 and (Mn,Co)₃O₄ coated Crofer 22 APU/SABS-0 samples. The interfacial stability is better in the H₂/H₂O atmosphere for both samples. The instability of the (Mn,Co)₃O₄ coating under the thermal treatment conditions degrades the interfacial compatibility of the (Mn,Co)₃O₄ coated Crofer 22 APU/SABS-0 sample.     

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.     

Evaluation of SmCo and SmCoN magnetron sputtering coatings for SOFC interconnect applications

Cobalt or cobalt containing coatings are promising for SOFC interconnect applications because of their high conductivity. We have investigated SmCo and SmCoN coatings deposited by magnetron sputtering from a SmCo (5% Sm) target on to Crofer 22 APU substrates. The composition, structure, surface morphology, and electrical conductivity of the coated substrates were characterized by SEM/EDX, XRD and ASR measurements. Addition of Sm enhances the oxidation resistance and the Cr retention capability of the coatings. The use of nitride as a precursor stabilizes Sm during oxidation of the films, thus inhibiting diffusion of Fe, resulting in a more compact coating and lowering ASR. The combined advantages of Sm addition to cobalt and the use of a nitride as a precursor, makes SmCoN coatings a promising new interconnect coating material.     

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.     

Characteristic of (La0.8Sr0.2)0.98MnO3 coating on Crofer22APU used as metallic interconnects for solid oxide fuel cell

This study reports the high temperature oxidation kinetics, area specific resistance (ASR), and interfacial microstructure of metallic interconnects coated by (La_0.8Sr_0.2)_0.98MnO₃ (LSM) in air atmosphere at 800 °C. An efficient LSM conductive layer was fabricated on metallic interconnects for solid oxide fuel cells (SOFCs) by using a wet spray coating method. The optimum conditions for slurries used in the wet spray coating were determined by the measurement of slurry viscosity and coated surface morphology. The surface roughnesses of the substrates were increased through sandblast treatment. The adhesive strength of the interface between the coated layer and the metal substrate increased with increased surface roughness of the metallic interconnects. The electrical conductivities of the coated substrates were measured by using a DC two-point and four-wire method under air atmosphere at 800 °C. Of note, the Crofer22APU treated at 1100 °C in N₂ with 10 vol.% H₂ showed long-term stability and a lower ASR value than other samples(heat-treated at 800 °C and 900 °C). After an 8000-h oxidation experiment the coated Crofer22APU substrate, the ASR showed a low value of 23 mΩ cm². The thickness of the coated conductive oxide layer was about 10-20 μm. These results show that a coated oxide layer prevents the formation and the growth of scale (Cr₂O₃ and (Mn, Cr, Fe)₃O₄ layer) and enhances the long-term stability and electrical performance of metallic interconnects for SOFCs.     

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.     

CoFe2O4 spinel protection coating thermally converted from the electroplated Co-Fe alloy for solid oxide fuel cell interconnect application

CoFe₂O₄ has been demonstrated as a potential spinel coating for protecting the Cr-containing ferritic interconnects. This spinel had an electrical conductivity of 0.85 S cm⁻¹ at 800 °C in air and an average coefficient of thermal expansion (CTE) of 11.80 × 10⁻⁶ K⁻¹ from room temperature to 800 °C. A series of Co-Fe alloys were co-deposited onto the Crofer 22 APU ferritic steel via electroplating with an acidic chloride solution. After thermal oxidation in air at 800 °C, a CoFe₂O₄ spinel layer was attained from the plated Co_0.40Fe_0.60 film. Furthermore, a channeled Crofer 22 APU interconnect electrodeposited with a 40-μm Co_0.40Fe_0.60 alloy film as a protective coating was evaluated in a single-cell configuration. The presence of the dense, Cr-free CoFe₂O₄ spinel layer was effective in blocking the Cr migration/transport and thus contributed to the improvement in cell performance stability.     

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.     

Optimization of the electrical properties of Ti–Nb stabilized ferritic stainless steel SOFC interconnect alloy upon high-temperature oxidation: The role of excess Nb on the interfacial oxidation at the oxide–metal interface

Interfacial oxidation of Nb and Si at 650 °C on Laves phase forming Ti–Nb stabilized ferritic stainless steel (Fe–19Cr–0.9Si–0.2Nb–0.1Ti (at.%), grade EN 1.4509) was studied by electrochemical impedance spectroscopy and photoelectron spectroscopy. It was found that excess Nb efficiently hinders the formation of electrically resistive SiO₂ layer at the oxide–metal interface. The beneficial role of Nb was attributed to its high segregation rate and the formation of conductive oxides at the interface. However, the oxidation was strongly influenced by age-precipitation of the Laves (FeNbSi)-type intermetallic phase, which removed free Nb from the alloy solution and thus allowed SiO₂ layer to form more easily. These results can be applied to optimize the oxide scale composition by Nb alloying of the ferritic stainless steel to maintain high performance under various operation conditions, particularly in solid oxide fuel cell applications.     

Oxidation behaviour of an Alloy 617 in very high-temperature air and helium environments

The oxidation characteristics of Alloy 617, a candidate structural material for the key components in the very high-temperature gas-cooled reactor (VHTR), were investigated. High-temperature oxidation tests were conducted at 900 and 1100 °C in air and helium environments and the results were analysed. Alloy 617 showed parabolic oxidation behaviour at 900 °C, but unstable oxidation behaviour at 1100 °C, even in a low oxygen-containing helium environment. The SEM micrographs also revealed that the surface oxides became unstable and non-continuous as the temperature or the exposure time increased. According to the elemental analysis, Cr-rich oxides were formed on the surface and Al-rich discrete internal oxides were formed below the surface oxide layer. After 100 h in 1100 °C air, the Cr-rich surface oxide became unstable and non-continuous, and the matrix elements like Ni and Co were exposed and oxidized. Depletion of grain boundary carbides as well as matrix carbides was observed during the oxidation in both environments. When tensile loading was applied during high-temperature oxidation, the thickness of the surface oxide layer, the internal oxidation, and decarburization were enhanced because of the increase in diffusion of oxidizing agent and gaseous reaction products. Such enhancement would have detrimental effects on the high-temperature mechanical properties, especially the creep resistance of Alloy 617 for the VHTR application.     

Life prediction of coated and uncoated metallic interconnect for solid oxide fuel cell applications

In this paper, we present an integrated experimental and modeling methodology in predicting the life of coated and uncoated metallic interconnect (IC) for solid oxide fuel cell (SOFC) applications. The ultimate goal is to provide cell designer and manufacture with a predictive methodology such that the life of the IC system can be managed and optimized through different coating thickness to meet the overall cell designed life. Crofer 22 APU is used as the example IC material system. The life of coated and uncoated Crofer 22 APU under isothermal cooling was predicted by comparing the predicted interfacial strength and the interfacial stresses induced by the cooling process from the operating temperature to room temperature, together with the measured oxide scale growth kinetics. It was found that the interfacial strength between the oxide scale and the Crofer 22 APU substrate decreases with the growth of the oxide scale, and that the interfacial strength for the oxide scale/spinel coating interface is much higher than that of the oxide scale/Crofer 22 APU substrate interface. As expected, the predicted life of the coated Crofer 22 APU is significantly longer than that of the uncoated Crofer 22 APU.     

Interfacial study between high temperature SiO2–B2O3–AO–La2O3 (A = Sr,Ba) glass seals and Crofer 22APU for solid oxide fuel cell applications

In the present work, the adhesion and chemical compatibility of two glass compositions are investigated with interconnect Crofer 22APU as a function of different heating durations for solid oxide fuel cell applications. Initially, the selected glasses have been characterized using differential thermal analyzer and dilatometer to check their suitability as a sealing material. After that, Crofer 22APU/glass–ceramics are joined to make diffusion couples. Furthermore, these diffusion couples have been heat-treated at 850 °C for different time durations of 5, 100 and 750 h and morphologically characterized. X-ray diffraction (XRD) indicates surface crystallization of various crystalline phases formed in the glass/Crofer 22APU diffusion couple. Coefficient of thermal expansion (CTE) of both the glasses is in good agreement with the CTE of Crofer 22APU. The overall analysis of resulting microstructure by scanning electron microscopy (SEM)/electron probe microanalysis (EPMA) revealed improvement in adhesion with increase in time duration of heat-treatment. Both the sealants have not shown delamination with Crofer 22APU interface even after prolonged duration of heat-treatment. The absence of unwanted oxide products in all the diffusion couples further confirms high gas tightedness and hermeticity of seals with low embrittlement.     

Corrosion of construction materials of separator in molten carbonates of alkali metals

The mitigation of steels and alloys corrosion is one of the great challenges for application of high-temperature devices in contact with molten salts. The current research estimates corrosion resistance of the following construction materials: Crofer APU 22, austenitic steel 12Cr18Ni10Ti and two types of austenitic nickel-chromium alloy: CrNi78Ti, CrNi78Ti + 5% wt. W. The measurements are performed by exposition of samples within 24 h in the molten mixture of carbonates of lithium and potassium at 650 °C. The rates and mechanism of materials corrosion under these conditions are determined. The process occurring on the surface while contact of the materials and steels with carbonate melt are investigated.Accurate quantitative data on materials interaction with alkali carbonate melt have been obtained by gravimetrical and physico-chemical methods. The high-temperature corrosion of Fe and Ni austenitic materials has turned dramatically different from both their low-temperature corrosion and each other. There are mixed passivating oxide films on the surface of the only iron-based materials.In addition, the resulting layers of corrosion products are well bonded to the substrate and have protective properties and can be used as protective coatings in molten salts of other compositions, including alkali metal fluorides.