Dual Atmosphere Research Articles

Accelerated High-Temperature Oxidation Behavior of Ferritic Stainless Steel under Air/H2+H2O Dual Atmosphere

Improvement in durability is one of the important subjects for utilizing solid oxide electrolyzer cell (SOEC) systems widely. In SOEC stacks, separators are exposed to complexed dual atmosphere (i.e. hydrogen - steam mixed gases and oxygen rich air are on each side). In this study, the degradation phenomena of separator materials (SUS430, ZMG232G10, Crofer22APU) in SOEC dual atmosphere were investigated. The samples were exposed in SOEC dual and simple atmospheres at 700 °C for 500 h. Then, their cross sections were analyzed by X-ray diffraction and scanning electron microscopy/energy-dispersive X-ray analysis. The degradation mechanisms of the separator materials in the SOEC stack operating conditions were discussed. ZMG232G10 and Crofer22APU with high Cr content showed thin oxide layer under both the dual or single atmosphere, while SUS430 with low Cr content showed significant oxidation in the H2-steam atmosphere and on the air side under the dual atmosphere condition. The significant oxidation on the air side of the dual atmosphere of SUS430 observed in this study is a unique phenomenon under the dual atmosphere condition and not observed in a single air atmosphere.

Temperature-Dependent Sheet and Contact Resistivity Measurements on Ag and Ag-Ni Circuit Pastes

The metal circuits and brazes used in electronic, energy conversion, and/or energy storage devices often have difficulty wetting and adhering to ceramic and/or ceramic-passivated metal substrates. Here, a novel Particle Interlayer Directed Wetting and Spreading (PIDWAS) technique is demonstrated that utilizes screen printing, the low wetting angle of silver on nickel, and the high work of adhesion between nickel and various ceramics to produce well-adhered, self-assembled silver patterns on ceramic and/or stainless-steel substrates not normally wet by silver. The resulting Ag-Ni circuits have higher sapphire adhesion strengths (up to 30 MPa on sapphire), higher densities (>97% on sapphire), and similar high-temperature electronic resistivities to those made using commercially-available Heraeus C8710 or DAD-87 silver circuit pastes. Similarly, Ag-Ni brazes between a variety of ceramic and/or stainless-steel substrates have better microstructural stability with rapid thermal cycling, reduction-oxidation cycling, and dual atmosphere isothermal aging than conventional Ag-CuO brazes. In addition, the residual Ni in these Ag-Ni circuits and brazes can be used to chemically getter surface segregated Al, ensuring low contact resistances on a variety of chromia- and/or alumina-passivated stainless steels.Reference:[1] Hu G, Zhou Q, Bhatlawande A, Park J, Termuhlen R, Ma Y, Bieler TR, Yu HC, Qi Y, Hogan T & Nicholas JD. Patterned Nickel Interlayers for Enhanced Silver Wetting, Spreading and Adhesion on Ceramic Substrates. Scripta Materialia, 2021; 196, 113767. 10.1016/j.scriptamat.2021.113767[2] Park J, Phongpreecha T, Nicholas JD & Qi Y. Enhanced Liquid Metal Wetting on Oxide Surfaces via Patterned Particles. Acta Materialia, 2020; 199, 551-560. 10.1016/j.actamat.2020.08.037

Stability of Glass Ceramic Sealants in Atmospheres with Increased Water Contents

For Solid oxide fuel cells (SOFC), operating at temperatures above 800 °C, glass ceramic sealants are state of the art. The properties of the glass ceramics are adapted to the metallic and ceramic join partners, operation conditions, as also to the atmospheres: fuel gas and air. In SOFCs, glass ceramic sealants offer reliable, long term stable operation over several 10,000 hours of service. The demand to operate solid oxide cells (SOC) in fuel cell- as also in electrolysis cell mode increases the demands of the stability of the sealants against atmospheres. In the electrolysis mode, the SOC-stacks were impinged by gas with high water contents to produce H2. Water contents of up to 95 % and temperatures above 800 °C are hard conditions for all applied materials of SOC stacks. In addition, glass ceramic sealants have to electrical insulator under applied electric voltage. In the electrolysis mode, the applied voltage is even higher than in fuel cell mode. In conclusion, the electrolysis mode is much harsher as fuel cell mode and glass ceramic sealants have to be adapted to this. Glasses in the system BaO-CaO-Al2O3-B2O3-SiO2 with La2O3-additions have the potential to form glass ceramics with extraordinary high coefficient of thermal expansion (CTE)values, which are suitable for use as sealants compatible with Crofer 22 APU interconnect. Their characteristics have been generally described in various studies and are also in focus of our own work and have proven their suitability for use under SOFC operating conditions. However, achieving such high CTE values is especially challenging when these glasses must be designed for use in stacks operating with electrolyte supported cells because sealing and operating temperature are usually higher than in other stack designs. Their behavior under simulated electrolysis conditions together with a direct correlation of test by using these glasses as sealants in real stack designed to operate with electrolyte supported cells has not been investigated yet and presents the scope of this study.For this study, a series of glasses has been prepared as glass powders and characterized in terms of relevant thermophysical properties such as softening behavior, thermal expansion and formation of crystalline phases. La2O3-free sealing glasses established as standards for use in commercial stacks served as references and results of both groups of sealing glasses have been compared. The new La2O3-containing glasses showed CTE-values up 10.8 ppm K-1 at a glass transition temperature ranging between 640°C and 650°C for a partially crystallized microstructure. The softening and crystallization characteristics analyzed by hot stage microscopy and DSC/TGA allow sealing processes of stacks components in the temperature range between 900°C and 950°C. Helium-leakage rates of sealed sandwich type samples made of Crofer 22 APU showed values below 10-8 mbar l-1 s-1. From suitable glasses sandwich type sample sealings made of two Crofer 22 APU coupons have been prepared and exposed to a dual atmosphere in a special test rig under simulated electrolysis conditions at 850 °C with different electrical voltages applied across the sealants and a water content of 50 % in the steam atmosphere. In this out-of-stack environment, the resistance of the glass sealants can be recorded while being treated with high water atmosphere and electric voltage of up to 5 V at 850 °C. The development of resistance over 300 hours of aging show which glass composition fit to the conditions and which exhibit high degradation rates. Afterwards cross sections have been prepared and analyzed by SEM. Effects observed in the interfacial regions glass-Crofer 22 APU and glass-atmosphere have been correlated with the electrical measurements. A 30 layers stack was tested under typical SOEC conditions for approx. 3200 hours. Four different glass types were used and compared in terms of their degradation rate and the ASR value. The selection of the glasses was made according to their thermal expansion and softening behavior as well as their insulating properties.

Spatially Heterogeneous Chemistry Observed using NIRTI on SOFC Anodes

Solid oxide fuel cells (SOFCs) are high temperature electrochemical conversion devices which are challenging to study due to their high operating temperature and dual atmosphere requirements. Traditional tools to study SOFCs are unable to resolve any spatial dependent chemistry which may be occurring on the cell. This study used Near Infrared Thermal Imaging (NIRTI) to identify spatially varying temperature changes indicative of heterogeneous chemistry occurred during C remediation reactions using gas phase oxidants and electrochemical oxidation on anode-supported SOFCs. The spatial dependences observed are probably due to kinetics and transport of the oxidizing species involved, which may provide insight for modeling SOFC kinetics and operation.

Stability of Glass Ceramic Sealants in Atmospheres with Increased Water Contents

Operating solid oxide cells (SOC) in the fuel cell moder as well as in electrolyser mode increases the demands on the stability of glass-based sealants due to higher cell voltages and water contents in the steam atmospheres. Glasses in the system BaO-CaO-Al2O3-SiO2 with B2O3 and La2O3-additions form glass ceramics with extraordinary high CTE-values, suitable as sealants compatible with Crofer 22 APU interconnect in electrolyte supported stacks. The intrinsic properties of a set of glasses are characterized. Sandwich type sealing samples are exposed to a dual atmosphere at 850 °C with an electric voltage of 1,3 V applied across the sealants and a water content of 50 % in the steam atmosphere. In this out-of-stack environment, the resistance of the sealants is recorded. Their resistance over aging time indicates degradation phenomena like interface reactions. By SEM analysis, reactions between sealants and steel interconnect can be correlated with insulating characteristics.

Controlled thermal pre-treatment of ZMG232G10® for corrosion mitigation under simulated SOFC interconnect exposure conditions

The IT-SOFC candidate interconnect material ZMG232G10® was thermally pre-treated in air and in H2 – 3% H2O to mitigate dual atmosphere corrosion. Bare and pre-treated steel samples were exposed to a dual atmosphere exposure condition (dry air/metal/4% H2 – N2 (bal.) + 3% H2O) for 500 h at 600 °C. Corrosion products and oxide morphology developed on the air-exposed surfaces were analyzed. Pre-treatments of the metal in oxidizing (air) and reducing (H2 – 3% H2O) gas atmospheres provide improved passivation against dual atmosphere corrosion compared to bare steel. The uniform and iron-free oxide scale developed during the pre-treatment of alloy in the low pO2 H2 – 3% H2O gas atmosphere provides an effective diffusion barrier against the outward transport of cations and oxidation of iron observed under dual atmosphere conditions.

Novel SrO-Containing Glass-Ceramic Sealants for Solid Oxide Electrolysis Cells (SOEC): Their Design and Characterization under Relevant Conditions.

This study presents results on the development of strontium oxide (SrO) containing glass sealants used to join Crofer22APU to yttria-stabilized zirconia (3YSZ), in which the main glass components, that is, silicon oxide (SiO2), strontium oxide (SrO), calcium oxide (CaO) and aluminum oxide (Al2O3), have been varied appropriately. Certain properties, such as the crystallization behavior, the coefficient of thermal expansion, adhesion, and reactivity of the sealants in contact with Crofer22APU, have been reviewed and discussed. The optimized glass composition (with CTE in the 9.8–10.3 × 10−6 K−1 range) results in a good joining behavior by hindering the formation of undesirable strontium chromate (SrCrO4) on contact with the Crofer22APU steel after 1000 h. at 850 °C. High specific resistivity values of about 106 Ohm.cm have been obtained, thus demonstrating good insulating properties at 850 °C under an applied voltage of 1.6 V. A negligible degradation in the electrical resistivity trend was measured during the test up to 1000 h, thus excluding the presence of detrimental reactions of the glass-ceramic sealant in contact with Crofer22APU under a dual atmosphere, as confirmed using SEM-EDS post-mortem analyses.

Internal Oxidation of a Fe–Cr Binary Alloy at 700–900 °C: The Role of Hydrogen and Water Vapor

Internal oxidation of Fe–2.25Cr has been studied in Fe/FeO Rhines pack (RP) and H2/H2O gas mixtures at 700–900 °C. A novel exposure technique allowing RP experiments in dual atmosphere conditions was developed. No measurable effect of hydrogen on lattice oxygen permeability in ferrite could be detected: neither in single nor in dual atmosphere conditions. The H2/H2O atmosphere was found to induce stronger oxidation attack at alloy grain boundaries resulting in a morphology similar to intergranular stress corrosion cracking often reported in nuclear technology. The intergranular oxidation attack was demonstrated to be independent of the dual atmosphere effect, i.e., hydrogen dissolved in the alloy.

Investigation of a Metallic Interconnect Extracted from an SOFC Stack after 40,000 h of Operation

An in-depth investigation was performed on a metallic interconnect extracted from an SOFC stack operated for 40,000 h. The characterization was performed on the surface and the cross-section, paying attention to the evolution of the materials due to the interaction with the dual atmosphere of the stack under operating parameters. The interaction between materials (i.e., metal substrate, coatings and atmospheres) and stack components (i.e., current collectors and MIC) generated several modifications that affected the surface and, in some cases, the bulk of the interconnect. The careful metallographic preparation allowed for the performance of an intensive microscopical characterization of the cross-sections all along the interconnect profile, from the inlet to the outlet of the fuel stream. The formation of thermal grown oxides on both sides and their evolution were studied and described. The interconnect, after 40,000 h, was still suitable for operation, but the few bulk changes due to the diffusion of Ni and the TGO that formed at the fuel side suggest the introduction of fuel side coatings to increase the life expectations of the whole stack.

Quantitative Analysis of the Calcium Hydroxide Content of EVA-Modified Cement Paste Based on TG-DSC in a Dual Atmosphere.

The calcium hydroxide (Ca(OH)2) content is one of the main indices of cement hydration degree. In order to accurately determine the calcium hydroxide content of ethylene and vinyl acetate (EVA) copolymer-modified cement paste, a dual atmosphere thermogravimetric method (first in an oxidizing atmosphere and then in an inert atmosphere) was used to track the mass loss and change in enthalpy by TG-DSC (simultaneous thermogravimetry and differential scanning calorimetry). The results showed that using the dual atmosphere thermogravimetric method, the source of mass loss can be distinguished. The exothermic peaks in an oxidizing atmosphere show the oxidation reactions of EVA, while the endothermic peak in an inert atmosphere is due to the pyrolysis reaction of EVA and the decomposition of the calcium hydroxide. The influence of EVA on cement hydration was investigated. The results showed that the polymer powder can be dispersed in water, forming a kind of composite membrane. The test method of dual atmosphere thermogravimetry to measure the calcium hydroxide content of polymer-modified cement pastes is more accurate and convenient than those previously applied.

Corrosion of Chromia-Forming and Alumina-Forming Ferritic Stainless Steels under Dual Atmosphere Exposure Conditions

The surface morphology and chemistry of oxide scales formed on select chromia-forming and alumina-forming ferritic steels have been studied after exposure to a dual atmosphere of hydrogen and air. Localized Fe-rich oxide nodules with surface whiskers/platelets form at the onset of corrosion. The initiation and growth of localized nodules and breakdown of passivation are attributed to the presence of hydrogen, inclusion of iron oxide in the passivating scale, and subsequent growth of iron-rich oxide due to the establishment of redox (H2-H2O) atmosphere and modification of oxide defect chemistry.

Materials challenges in hydrogen-fuelled gas turbines

ABSTRACT With the increased pressure to decarbonise the power generation sector several gas turbine manufacturers are working towards increasing the hydrogen-firing capabilities of their engines towards 100%. In this review, we discuss the potential materials challenges of gas turbines fuelled with hydrogen, provide an updated overview of the most promising alloys and coatings for this application, and highlight topics requiring further research and development. Particular focus is given to the high-temperature oxidation of gas turbine materials exposed to hydrogen and steam at elevated temperatures and to the corrosion challenges of parts fabricated by additive manufacturing. Other degradation mechanisms such as hot corrosion, the dual atmosphere effect and hydrogen diffusion in the base alloys are also discussed.

The Influence of Humidity Content on Ferritic Stainless Steels Used in Solid Oxide Fuel Cell Under Dual Atmosphere Conditions at 600°C

In the last few years, the interest in the development and application of green energy harvesting, storage and production technologies, boosted the research activities to fulfill the demand of better performing, more reliable and cost-effective materials. In the field of Solid Oxides Fuel Cells (SOFC), many efforts have been spent on the interconnects, that are used to electrically connect single cells to obtain stacks for higher output voltages. Interconnects are exposed simultaneously to fuel gas (i.e., H2) and oxidizing (humid air) atmospheres (dual atmosphere) at a working temperatures in the range between 600°C and 800°C. Nowadays, chromia forming ferritic stainless steel (FSS) has been selected as material for interconnects due to its ease of fabrication, affordability and electrical conductivity. In air the material forms a protective chromia layer that reduces further corrosion. However, in dual atmosphere conditions, FSS commonly show fast and unexpected degradation, on the air side, especially at 600°C.The mechanism behind the dual atmosphere effect is still not fully clear, but previous works highlighted the importance of pre-oxidation time, exposure temperature, humidity and surface finishing. Until now, major efforts have been spent in studying the long-term behaviour, with the aim to identify a long-lasting interconnect material. To successfully overcome the effects of dual atmosphere-induced corrosion, it is of fundamental importance to investigate how the corrosion phenomenon starts and evolves. In this work, the initial stages of dual atmosphere breakaway corrosion at 600°C were investigated for selected FSS alloys. The samples were exposed under controlled conditions which resemble those of SOFC (Ar-5%H2 + 3%H2O//FSS//air + 3%H2O). The oxide scales were further investigated by means of Scanning Electron Microscope (SEM) and Energy Dispersive X-ray (EDX) analysis. Experiments with different exposure times were performed (i.e., up to 12 hours) and the influence of temperature ramping was studied.Several parameters were evaluated, like the effects of chromium content, the role of sample thickness and the influence of trace elements in different alloys and different batches of the same alloy. Implications of grain orientation at the surface, as well as of pre-oxidation treatment were investigated too.

The Influence of Humidity Content on Ferritic Stainless Steels Used in Solid Oxide Fuel Cell Under Dual Atmosphere Conditions at 600°C

In Solid Oxide Fuel Cells (SOFC), interconnects are simultaneously exposed to dual atmosphere conditions in a range of temperature between 600 and 900°C. The presence of dissolved hydrogen in the steel can cause accelerated corrosion on the side exposed to air compared to exposures in air only. Moreover, the interconnect is subject to different humidity levels on the fuel side depending on fuel utilization. It has been already observed that a protective layer of chromia (Cr2O3) at the fuel side can mitigate the dual effect at the airside acting as a barrier for hydrogen permeation into the material. In this work, AISI 441 and Crofer 22 APU samples were exposed to dual atmosphere at 600°C under controlled humidity levels at the fuel side (20% vs. dry). Analysis performed showed how the humidity content on the fuel side largely affects the breakaway corrosion of the samples at the airside.

Characterization of metallic interconnects extracted from Solid Oxide Fuel Cell stacks operated up to 20,000 h in real life conditions: The fuel side

The performance of an SOFC stack is related to the evolution over time of the materials during operation. Metallic interconnects (MICs) are exposed to high temperature (ca. 850 °C) and dual atmosphere (fuel and air streams), and hence react to form an oxide scale on top. This paper focuses on the fuel side of the MIC and investigates the nature of the oxides formed in SOFC stacks field operated by the SUNFIRE GmbH Company for up to 20,000 h in the frame of the EU-H2020 ADASTRA project. This opportunity supports the understanding on how the metal (i.e., CROFER®22APU) reacts once exposed to the fuel stream by performing investigations on the surface and on the cross-section of the samples by SEM-EDS and micro-Raman spectroscopy. The results show the presence of a layered thermally grown oxide (TGO) rich in Cr at the interface with the metal. On top of the scale, a predominant presence of MnxOy is found supporting the hypothesis of a further reaction of the TGO with selective Cr depletion. The manganese oxides may represent a severe issue due to their very low conductivity.

Characterization of Metallic Interconnects Extracted from Solid Oxide Fuel Cell Stacks Operated up to 20,000 h in Real Life Conditions: The Air Side

Metallic interconnects represent the main component of a solid oxide fuel cell (SOFC) stack in terms of weight and volume. They are typically made of ferritic stainless steel (FSS) coated on the air side. At the stack operating conditions, the interconnect is exposed to a dual atmosphere: air at the cathode side; fuel (a hydrogen-rich mixture) at the anode side. The stacks considered in this study were field operated in reformed natural gas for 5000, 9000 and 20,000 h respectively. The analyzed interconnects are made from CROFER22APU and coated on the air side with Co-Mn base spinel. One interconnect has been studied for each stack by sampling and preparing cross section the inlet and outlet positions. The samples were characterized by SEM-EDXS in order to investigate the evolution of the interconnect at the air side. The interaction between the metal substrate and the coating is investigated highlighting the formation of chromia based thermal grown oxide (at the FSS/coating interface) and the solid-state diffusion of Cr and Fe from the metal into the coating. The microstructural features evolving as a function of time are also quantified.

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.

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

In this study, the oxidation behavior of two ferritic steels with different amounts of chromium is examined under dual atmosphere exposure conditions. Experiments with varying time and temperature were carried out to understand the effects of these variables and difference in chromium concentration on severity and morphology of oxidation products. Higher amounts of alloyed chromium in the steel leads to better passivation against iron oxidation. With increased exposure time, however, the passivating scale breaks down leading to iron oxide overgrowth. This phenomenon appears more pronounced at lower temperatures and complete passivation of both steels was observed at the higher temperature (700°C) after 100 h of exposure. A hypothesis with regards to the presence of hydrogen during exposure is presented along with discussion on observed oxidation behavior.

Glass-Ceramic Sealants for SOEC: Thermal Characterization and Electrical Resistivity in Dual Atmosphere

A Ba-based glass-ceramic sealant is designed and tested for solid oxide electrolysis cell (SOEC) applications. A suitable SiO2/BaO ratio is chosen in order to obtain BaSi2O5 crystalline phase and subsequently favorable thermo-mechanical properties of the glass-ceramic sealant. The glass is analyzed in terms of thermal, thermo-mechanical, chemical, and electrical behavior. Crofer22APU-sealant-Crofer22APU joined samples are tested for 2000 h at 850 °C in a dual atmosphere test rig having reducing atmosphere of H2:H2O 50/50 (mol%) and under the applied voltage of 1.6 V. In order to simulate the SOEC dynamic working conditions, thermal cycles are performed during the long-term electrical resistivity test. The glass-ceramic shows promising behavior in terms of high density, suitable CTE, and stable electrical resistivity (106–107 Ω cm) under SOEC conditions. The SEM-EDS post mortem analysis confirms excellent chemical and thermo-mechanical compatibility of the glass-ceramic with Crofer22APU.

CHAPTER 6 Development of SOFC Interconnect Stainless Steels

The chapter introduces components and working principle of solid oxide fuel cells (SOFCs). It is followed by the explanation on the choices of materials focussing on ferritic stainless steels. The review is further made on the required properties of these steels, i.e. low oxidation rate, low chromium species volatilisation rate, high electrical conductivity and good scale adhesion. For the oxidation aspect, the behaviour of stainless steel interconnect in cathode, anode (hydrogen and biogas), and dual atmospheres are described. Surface modification by pre-oxidation and coatings to improve the oxide electrical conductivity and to reduce chromium species volatilisation is finally reviewed.