Egbert Wessel

Chromium Poisoning of Perovskite Cathodes by the ODS Alloy Cr5Fe1Y2O3 and the High Chromium Ferritic Steel Crofer22APU

The alloys Cr5Fe1Y 2 O 3 and the ferritic steel Crofer22APU are typical alloys used as solid oxide fuel cell (SOFC) interconnect materials. Alloy Cr5Fe1Y 2 O 3 is an oxide dispersion strengthened (ODS) alloy developed by Plansee, Reutte, Austria, for use at high temperature. A typical material for medium-temperature SOFC, is the high chromium ferritic steel Crofer22APU supplied by Thyssen Krupp VDM, Germany. The two alloys form different oxide scales which affect chromium poisoning. Chromium vaporization as source term and electrochemical degradation of La 1-x Sr x MnO 3 (LSM) and La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) describing the poisoning were studied for the two alloys. The dynamics of the chromium deposition in porous perovskite cathodes was studied by a dc method and impedance spectroscopy. Electrical degradation of the LSM cathode by alloy Cr5Fe1Y 2 O 3 was significantly higher than for Crofer22APU. The microstructure of the cells was studied after measurements by scanning and energy filtering transmission electron microscopy. Significant amounts of chromium were observed at the TPB in the functional layer of cells, with the LSM cathode giving insight into the degradation mechanism. Cells tested with the LSCF cathode clearly show Cr poisoning. Formation of large SrCrO 4 crystals was observed on the surface of the LSCF cathode.

Chromium Vaporization from High-Temperature Alloys I. Chromia-Forming Steels and the Influence of Outer Oxide Layers

The vaporization of chromium species from chromia scales limits the applicability of chromia-forming steels at high temperatures and is one of the major reasons for degradation in the development of planar solid oxide fuel cells (SOFCs). Cr(VI) vaporized from the interconnector is reduced at the cathode and deposits in the form of solid Cr(III)-oxide, thereby inhibiting the electrochemical processes. This work presents the first systematic study on the Cr vaporization of Cr-, Fe-, Ni-, and Co-based alloys in air and in H 2 atmospheres at high temperatures. The influence of outer oxide layers of (Cr,Mn) 3 O 4 , (Fe, Cr) 3 O 4 , Co 3 O 4 , TiO 2 , and Al 2 O 3 on the Cr vaporization is investigated. It is shown that the Cr vaporization of chromia-forming steels can be reduced by more than 90% by alloying. An estimate of the expected degradation effects on planar SOFC designs for the use of uncoated interconnector materials is used to show that in order to achieve the desired lifetimes for SOFC systems, additional Cr-retention coatings are necessary. Additionally, equilibrium vaporization measurements are carried out for pure Cr 2 O 3 (s) in humid air in order to elucidate controversies in the literature concerning the thermodynamic data of CrO 2 (OH) 2 (g).

Oxidation behaviour of ferrous alloys used as interconnecting material in solid oxide fuel cells

Under operating conditions in the solid oxide fuel cell (SOFC), metallic interconnect plates form electrically insulating or poor-conducting oxide scales (e.g. Cr2O3, Al2O3) at their surface which increase the contact resistance from one fuel cell membrane to the next. In order to minimize electric losses in a fuel cell stack, the formation of oxide scales on the interconnect surface must either be prevented or the oxide scale formed must have sufficient electrical conductivity. In the present work, investigations were carried out on the corrosion behaviour of different FeCrAl and FeCrMn alloys, some of which were coated with nickel (Ni). Information about ageing of these alloys on the anode side of the fuel cell was obtained by means of contact resistance measurements and scanning electron microscopy. The results reveal that FeCrMn(LaTi) alloys and Ni-coated interconnects exhibit low ageing rates and are thus suitable for use on the anode side of SOFCs.

Corrosion and Interdiffusion in a Ni/Fe–Cr–Al Couple Used for the Anode Side of Multi-Layered Interconnector for SOFC Applications

Under operating conditions in solid-oxide fuel cells (SOFC), metallic-interconnect plates form oxide scales (e.g., Cr2O3, Al2O3) on their surface, which have an electrically insulating effect and increase the contact resistance between the interconnect and the electrodes. In order to ensure high electrical conductivity between the electrodes and the interconnects, the formation of oxide scales on the interconnect surface must be prevented or minimized. The present work shows possibilities for improved contacts at the anodic side in solid-oxide fuel cells by plating an Fe–Cr–5.7Al interconnect with a Ni foil. Contact-resistance measurements and microscopic studies show the electrical behavior and corrosion of materials used at 800°C under an Ar/4 vol.% H2/3 vol.% H2O atmosphere. The results reveal that the interconnects coated with nickel exhibit low aging rates in the investigations performed and are thus suitable for use on the anode side.

Diffusion coefficient of hydrogen in a cast gamma titanium aluminide

Gamma titanium aluminides have the potential for high temperature applications because of their high specific strength and specific modulus. Their oxidation resistance is good, especially at intermediate temperatures and with suitable alloying additions, good oxidation resistance can be obtained up to 800 C. One critical area of application is in combustion engines in aero-space vehicles such as hypersonic airplanes and high speed civil transport airplanes. This entails the use of hydrogen as a fuel component and hence the effect of hydrogen on the mechanical properties of gamma titanium aluminides is of significant scientific and technological utility. The purpose of this short investigation is to use an electrochemical method under galvanostatic conditions to determine the diffusion coefficient of hydrogen in a cast gamma titanium aluminide, a typical technical alloy with potential application in gas turbines under creep conditions. This result will be then compared with that obtained by microhardness profiling of electrolytically hydrogen precharged material.

Effect of Combined Yttrium and Zirconium Additions on Protective Alumina Scale Formation on High Purity FeCrAl Alloys during Oxidation in the Temperature Range of 1200 to 1300°C

The oxidation behaviour of high purity FeCrAl alloys containing minor additions (in the 100 ppm range) of Y and a combination of Y+Zr has been studied at temperatures between 1200 and 1300°C in Ar/O2. Higher oxidation rates of the Y+Zr-doped alloy compared to those of the Ydoped alloy were found. Yttrium incorporation into the alumina scale followed by formation of Y/Al-mixed oxide compounds was observed in case of the Y-doped alloy. The Y incorporation into the scale tended to be suppressed in the Y+Zr-doped alloy. Instead, substantial Zr incorporation occurred which resulted in numerous zirconia inclusions and a rather high degree of microporosity. This leads to an enhanced contribution of molecular oxygen transport to the overall scale growth process resulting in an enhanced oxidation rate.

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.

Predicting the microstructural evolution in a multi-layered corrosion resistant coating on a Ni-base superalloy

Abstract Protective metallic MCrAlY or diffusion type (NiAl) coatings enhance the oxidation and corrosion resistance of the underlying high temperature materials employed in aeroengines and industrial gas turbines by ensuring the growth of a slowly growing protective alumina scale. However, a chromia forming coating would provide a better resistance against sulphur induced corrosive attack. A hybrid coating system combining both chromia and alumina forming coating layers would provide optimum protection in oxidising-sulphidising environments. The microstructural stability and applicability of such a coating system (SmartCoat) containing alternate layers rich in chromium and aluminium respectively on the Ni-base superalloy CMSX-4 was evaluated after various exposure times at 800 C. Scanning electron microscopy (SEM) and electron microprobe analyses (EPMA) provided the element concentrations. Phases were identified by electron backscatter diffraction, and correlated with SEM and high-resolution TEM/EDX analyses. A computational approach was employed to describe the mechanisms of the phase transformations occurring in the coating system.

Microstructure of intermetallic particle strengthened high-chromium fully ferritic steels

An improvement of power plant efficiency necessitates an increase of the process parameters and thus enables a reduction of consumed primary resources. Furthermore more efficient, sustainable, flexible and cost-effective energy technologies are strongly needed. For this reason the current research concentrates on a new concept of high-chromium fully ferritic stainless steels which are strengthened by a combination of solid-solution and intermetallic Laves phase particles. Such steels exhibit favourable creep, thermomechanical fatigue and steam oxidation behaviour up to 650°C. Based on detailed analysis by high-resolution scanning and transmission electron microscopy the particle size evolution and compositions were studied. Variations in chemical compositions were analysed experimentally and compared with thermodynamic equilibrium composition modelling results. This paper is part of a thematic issue on the 9th International Charles Parsons Turbine and Generator Conference. All papers have been revised and extended before publication in Materials Science and Technology.

Fracture Behaviour of Plasma Sprayed Thermal Barrier Coatings

Thermal barrier coatings (TBCs) of plasma sprayed yttria stabilised zirconia (YSZ) are increasingly utilised for heat exposed components of advanced gas turbines1,2. An important reason for the application of zirconia coatings is the low thermal conductivity of this ceramic material which is further diminished in a TBC by the high concentration of spraying induced microstructural defects, e.g. crack-shaped defects between and within the spraying splats. Thus with TBCs on gas cooled turbine components stiff temperature gradients can be realised as an important prerequisite for an increased thermal efficiency of the energy conversion process.