Leszek Niewolak

Anode Side Diffusion Barrier Coating for Solid Oxide Fuel Cells Interconnects

During the operation of solid oxide fuel cells (SOFCs) the Ni base anode and/or Ni-mesh is in direct contact with the ferritic steel interconnect or the metallic substrate. For assuring long-term stack operation a diffusion barrier layer with high electronic conductivity may be needed to impede interdiffusion between the various components. A preoxidation layer on the ferritic steel turned out to be not viable as a barrier layer since a Ni-layer tends to dissociate the oxide scale. Therefore the potential of ceria as a diffusion barrier layer for the anode side of the SOFC was estimated. The barrier properties of a ceria coating between the Ni and the ferritic steel Crofer 22 APU were tested for 1000 h in Ar-4H 2 -2H 2 O at 800°C. Conductivity experiments were performed in the same atmosphere at different temperatures. After long-term exposures no indication of interdiffusion between Ni and ferritic steel could be detected, however, sputtered coatings on ferritic steel substrates showed signiftcantly lower conductivities than bulk ceria samples because of void formation between the ceria and the oxide on the steel surface. The latter could be prevented by an intermediate copper layer, which resulted in overall area specific resistance values lower than 20 mΩ cm 2 after 100 h exposure at 800°C.

Effect of Alloying Additions in Ferritic 9-12%Cr Steels on the Temperature Dependence of the Steam Oxidation Resistance

The oxidation behaviour of a number of commercially available 9 – 12 % Cr steels as well as model alloys with a systematic variation of selected alloying elements was studied at temperatures between 550 and 650°C. For characterizing the oxidation behaviour, the results of gravimetric studies were combined with data obtained from a number of analysis techniques, such as optical metallography, scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX), Raman spectroscopy (LRS) and sputtered neutrals mass spectrometry (SNMS). The critical Cr-content for obtaining protective oxidation appeared to depend on presence of minor alloying addition and probably alloy microstructure. In the mentioned temperature range some of the studied materials exhibited a decreasing oxidation rate with increasing temperature. Introduction A number of high strength, ferritic 9-12%Cr steels have recently been developed to be used as construction materials in advanced, high efficiency steam power plants [1]. The materials possess substantially higher creep strength than conventional low-alloy steels however, a large variety of long-term tests have illustrated, that the oxidation resistance of the steels in water-vapour rich gases is worse than that in air or oxygen [2, 3]. A further problem in respect to component design using these materials is their anomalous temperature dependence of the oxidation rates. This means, that in the envisaged temperature range of applications, i.e. 550 to 650°C, the maximum oxidation rate is in most cases not found for the highest temperature, however it frequently occurs at 600 or even 550°C. Experimental The compositions of the studied materials are listed in Table 1. Experimental details were given elsewhere [4, 5]. After exposure the oxidation products were characterised by optical metallography, scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX), sputtered neutrals mass spectrometry (SNMS) and Raman spectroscopy (LRS). Materials Science Forum Online: 2004-08-15 ISSN: 1662-9752, Vols. 461-464, pp 791-798 doi:10.4028/www.scientific.net/MSF.461-464.791

Subsurface Depletion and Enrichment Processes During Oxidation of a High Chromium, Laves-Phase Strengthened Ferritic Steel

During oxidation of a Laves-phase strengthened ferritic steel at 800 C in Ar/H2/H2O a precipitate-depleted zone occurs as a result of subscale chromium depletion. In parallel, an enrichment of Laves-phase is found in the immediate vicinity of the scale/alloy interface. Modeling the phase equilibria and diffusion processes for an Fe-Cr-Nb model alloy using Thermo-Calc and DICTRA revealed that this combined enrichment/depletion process can be explained by the influence of alloy chromium concentration on the Nb activity. VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3590258] All rights reserved.

Why the Growth Rates of Alumina and Chromia Scales on Thin Specimens Differ from those on Thick Specimens

For a number of chromia and alumina forming high temperature alloys and coatings, recent studies revealed, that in some cases the specimen/component or coating thickness may substantially affect the growth rates of the surface oxides. For the alumina formers the thickness dependence is mainly governed by depletion of oxygen active elements such as Y, Zr, Hf, Mg which are either intentionally added alloying elements or manufacturing related alloy impurities. In the case of the chromia forming materials, which tend to exhibit a more substantial dependence of oxidation rate on specimen/component thickness, depletion of minor alloying additions is also an important factor to be considered. However, for these alloys relaxation of oxide growth stresses by plastic deformation of the metallic substrates seems to be the dominant parameter which governs the observed behaviour.

Oxidation and reduction kinetics of iron and iron based alloys used as storage materials in high temperature battery

Abstract The background of the present studies relates to the development of a novel high temperature energy storage system based on a solid oxide cell. The energy is stored in a metal/metal oxide system which is part of the fuel side of the cell. The aim of the present study was to evaluate the suitability of pure iron and iron based model alloys as possible energy storage material for this type of high temperature battery system at a service temperature of 800°C. For this purpose the oxidation and reduction behaviour of iron in Ar–H2–(H2O) environments has been examined. The reduction process in Ar–2%H2 of the wüstite scale formed on pure iron in Ar–H2–H2O was hampered by the formation of a continuous, gas tight metallic iron layer on its surface. Possible approaches to increase oxidation and reduction kinetics by optimised alloy compositions for the Fe storage are discussed.

Transient oxidation of alumina forming Ti–Al–Ag-based alloys and coatings studied by SEM, AFM, XPS and LRS

γ-TiAl based intermetallics possess poor oxidation properties at temperatures above approximately 700°C. Previous studies showed that protective alumina scale formation on γ-TiAl can be obtained by small additions (around 2 at.%) of Ag. Recently, this type of materials has therefore been proposed as oxidation resistant coatings for high strength TiAl alloys. In the present study, a number of cast Ti–Al–Ag alloys and magnetron sputtered Ti–Al–Ag coatings were investigated in relation to transient oxide formation in air at 800°C. After various oxidation times the oxide composition, microstructure and morphology were studied by combining a number of analysis techniques, such as SEM, ESCA, AFM and LIOS-RS. The γ-TiAl–Ag alloys and coatings appear to form an α-Al2O3 oxide scale from the beginning of the oxidation process, in spite of the relatively low oxidation temperature of 800°C. The formation of metastable alumina oxides seems to be related to the presence of Ag-rich precipitates in the alloy matrix.

Oxidation behaviour and phase transformations of an interconnect material in simulated anode environment of intermediate temperature solid oxide fuel cells

Abstract The oxidation behaviour and the phase transformations associated with high temperature exposure of a commercial ferritic interconnect steel, Crofer 22 H, was studied in a simulated solid oxide fuel cell (SOFC) anode atmosphere at 700 °C. Special emphasis was placed on the formation of the intermetallic sigma phase. No sigma phase was detected in the bulk alloy after 500 h of exposure of bare specimens. However, specimens which were pre-coated with a layer of nickel showed formation of an interdiffusion zone after as little as 2 h of exposure and sigma phase formation occurred after 10 h. The presence of the nickel layer, which simulates the contact between ferritic steel interconnects and a nickel mesh in a SOFC results in the formation of an austenitic zone and accelerated formation of a σ-phase rich layer at the ferrite/austenite interface. The ferritic steel is transformed into austenite due to the inward diffusion of nickel, σ-phase started to nucleate at the transformed austenite grain boundaries. The nucleation is enhanced by an increased Cr/Fe-ratio at that interface due to more pronounced diffusion of Fe, compared to Cr, in the direction of the Ni-layer. Different possible mechanisms for the nucleation and growth of σ-phase were identified. The experimental results led to the conclusion that sigma nucleates in the austenite and grows following an isothermal eutectoid-like decomposition. The kinetics of σ-phase formation and the depth of the interdiffusion zone were found to follow a traditional diffusion relationship. It was observed that as the Ni-concentration increases the sigma-phase re-dissolves and thus the zone which, contains sigma phase moves deeper into the ferritic steel with exposure time. Interdiffusion processes between the nickel layer and the ferritic steel result not only in accelerated formation of σ-phase but also in the formation of Cr-rich oxides within the nickel layer.

The Effect of Copper and Ceria Additions on the High Temperature Oxidation of Nickel

In this study, the effects of alloying with Cu and external doping with CeO2 on the oxidation of nickel were evaluated. The materials studied were pure Ni, Ni-5wt% Cu, and Ni with the surface doped with CeO2 by pulsed laser deposition (PLD). The oxidation kinetics were measured using thermogravimetric analysis (TGA). The oxidation microstructures were observed by scanning electron microscopy (SEM) compositional analysis was performed with energy dispersive x-ray analysis (EDS) and sputtered neutrals mass spectrometry (SNMS). Phase identification was performed using X-ray diffraction (XRD). The Cu additions had a negligible effect on the oxidation kinetics but Cu was found to be present in the outer portions of the scale in significant concentrations. Doping with CeO2 resulted in a significant decrease in the NiO growth rate. The scales on doped Ni grew primarily inward whereas those on the undoped Ni grew primarily outward. Deposition of the CeO2 dopant onto Ni with a thin, preformed NiO layer produced a similar reduction in the subsequent NiO growth rate. This suggests that the poisoned interface model, proposed by Pieraggi and Rapp, does not describe the effect of the CeO2 dopant. Mechanisms are presented to attempt to describe the above observations.

Influence of Alloying Elements on the Behavior of Different Ferritic Steels as Candidate Materials for SOFC Interconnect

The efficiency of a solid oxide fuel cell (SOFC) can be improved by using a Ni-mesh between the ferritic steel interconnect and the Ni/YSZ anode. However, interdiffusion processes can lead to internal oxidation within the Ni-mesh and some microstructural changes, i.e., formation of an austenite zone and accelerated formation of σ-phase at the ferrite/austenite interface. These changes may adversely affect the performance of the cell during long-term operation. The present work focused on the influence of certain alloying elements on the overall behavior of the ferritic steel under simulated SOFC operating conditions to define conditions to minimize σ-phase formation without degrading the oxidation resistance and mechanical properties of the steel. The experimental results indicate that decreasing the amount of Cr and adjusting the amount of Nb, Si and W of the steel is a possible way to achieve that goal.

Erratum to: Analysis of the Reactive Element Effect on the Oxidation of Ceria Doped Nickel

The effects of external doping with CeO2 on the oxidation of nickel have been evaluated. The materials studied were pure Ni and Ni with the surface doped with CeO2 by pulsed laser deposition. The oxidation kinetics were measured using thermogravimetric analysis. The oxidation microstructures were observed by scanning electron microscopy and cross-sectional transmission electron microscopy. Compositional analysis was performed with energy dispersive X-ray analysis and sputtering neutrals mass spectrometry. Phase identification was performed using X-ray diffraction. Doping with CeO2 resulted in a significant decrease in the NiO growth rate at intermediate temperatures, e.g. 800 C. The scales on doped Ni grew primarily inward whereas those on the undoped Ni grew primarily outward. Deposition of the CeO2 dopant onto Ni with a thin, preformed NiO layer produced a similar reduction in the subsequent NiO growth rate. The CeO2 dopant did not reduce the growth rate at high temperature (1,300 C). The results indicate that the CeO2 dopant influences grain boundary transport in the NiO. Mechanisms are presented to attempt to describe the above observations.