W. J. Quadakkers

Metallic interconnectors for solid oxide fuel cells – a review

Abstract For planar solid oxide fuel cell (SOFC) designs, ceramic as well as metallic materials are being considered as construction materials for the interconnectors. Compared to the ceramics, mostly compounds on the basis of La-chromite, metallic materials have the advantage of easier fabricability, lower costs as well as higher heat and electrical conductivity. Based on the requirements in respect to oxidation resistance, low thermal expansion coefficient and electrical conductivity of surface oxide scales, Cr-based alloys and high-Cr ferritic steels seem to be the most promising metallic interconnector materials. Whereas Cr-based alloys have recently especially been developed for SOFC application, a large number of ferritic steels are commercially available in a wide range of compositions. However, it seems that the specific combination of properties required for a SOFC interconnector will necessitate the development of a new, specifically designed steel or the modification of an existing commercial steel composition.

Role of Water Vapor in Chromia-Scale Growth at Low Oxygen Partial Pressure

The oxidation behavior of pure chromium and ODS-Cr alloys in Ar-H2-H2O and Ar-O2-H2O was studied at 1000°C. At high oxygen potentials, the addition of H2O to the gas had negligible effect on the scaling behavior. However, at low oxygen potentials, when the pH2O/pH2 ratio was held constant, the oxidation rate increased with water partial pressure. Increasing values of pH2O/pH2 led to more rapid rates. At fixed pH2O values, the rate increased with increasing pH2. Compact scales were formed under all conditions. In addition Cr2O3 blades grew on the scale surface when pure chromium was reacted with H2O/H2 mixtures, but not in reaction with O2/H2O. These blades did not form when Y2O3 dispersion-strengthened material was reacted. A model, in which oxide growth was sustained by diffusion of chromium vacancies and adsorption of H2O on oxide exposed to low oxygen-activity gas led to the formation of hydroxyl species, explained most of the complex effects of gas composition on scale growth and blade formation. However, it failed to account for the observed increase in scaling rate with pH2 at fixed pH2O. The latter effect is ascribed to alteration of an additional contribution to diffusion from chromium interstitials.

Long-term oxidation tests on a re-containing MCrAlY coating

Abstract The oxidation and interdiffusion properties of Re-containing NiCoCrAlY-coatings for application in gas turbines were studied in the temperature range from 950 °C to 1000 °C. For this purpose cylindrical specimens of IN 738 were coated with a number of NiCoCrAlYs by vacuum plasma spraying and subsequently exposed in air under isothermal as well as thermocycling conditions. After exposure, the oxidation products, subsurface depletion layer as well as interdiffusion zone, with the IN 738 substrate were analysed by optical metallography and SEM/EDX. Very high amounts of Re led to extensive precipitation of Cr-rich phases, causing an embrittlement of the coating. Optimized coatings in respect to Re-, Cr-, and Al-content appeared to possess excellent performance even up to exposure times of 20 000 h.

The formation of protective alumina-based scales during high-temperature air oxidation of γ-TiAl alloys

The effect of Ag additions on the oxidation behavior of γ-TiAl hasbeen studied. The materials investigated containing 47–50 at.% Aland 0–5 at.% Ag were tested with respect to oxidation resistanceduring exposure in air at 800°C. The exposures up to around 1600 hrshowed that suitable Ag additions can promote formation of long-term,protective, alumina scales on γ-TiAl alloys. Extensive analysesof the oxidation products using optical metallography SEM, XRD, EPMA,and SIMS revealed that Ag stabilizes the Z-phase (Ti5Al3O2) in thesubscale-depletion layer thereby preventing formation of α2-Ti3Alas well as Ti-rich nitrides, which are responsible for the destructionof alumina scales in common γ -TiAl alloys. The best results wereobtained for the alloy Ti–50Al–2Ag; even during exposures aslong as around 1600 hr, this alloy still appeared to form a stable aluminalayer. It was found that high Ag additions of 5% were detrimental afterlonger exposure times due to extensive Ag precipitationat the interface between the alloy and depletion layer, resulting inlocalized formation of rapidly growing, mixed-oxide scales.

Studies concerning the effect of nitrogen on the oxidation behavior of TiAl-based intermetallics at 900°C

The oxidation behavior of the titanium aluminides Ti-50Al and Ti-48Al-5Nb has been investigated in Ar+20%O2 and in air at 900°C. Thermogravimetric studies in combination with structural analyses using optical metallography, SEM/EDX and X-ray diffraction show a marked influence of nitrogen on the composition and growth rate of the oxide scales. For a more detailed study concerning the effect of nitrogen on the scale-growth kinetics, thermogravimetrical analyses were carried out during which the gas atmosphere was changed from air to Ar−O2, and vice versa, without intermediate cooling of the specimen. The results show, that nitrogen adversely affects the formation of the initially formed alumina scale and that it enhances the growth rate of the rapidly growing Ti-rich oxide. This effect was observed in both alloys investigated, although the thermogravimetric results at first sight indicated an opposite effect for the Nb-containing alloy. This apparent contradiction is caused by internal oxidation which occurs in this alloy during exposure in Ar−O2.

Oxidation-Resistant Aluminide Coatings on γ-TiAl

The long-term application of TiAl alloys based on the γ-phase at temperatures above 750–800°C requires suitable surface coatings to provide the needed oxidation resistance. Without a coating, these alloys, containing large amounts of titanium, suffer from rapid oxidation attack at elevated temperatures. The pack-cementation coating process was used to aluminize the surface region of a Ti–50 at.% Al alloy to TiAl3, the most promising, oxidation-resistant phase in the Ti–Al system. The isothermal oxidation behavior of the coated alloy was studied in the temperature range 800–1000°C in air for up to 300 hr. The aluminide coating greatly improves the oxidation resistance of γ-TiAl, forming a protective alumina scale. The rapid aluminum interdiffusion between the TiAl3 coating and the γ-TiAl substrate determined the effective life of the coating. In addition, the oxidation behavior of the TiAl2 phase formed by interdiffusion of the coating system was studied by oxidation of cross sections.

Steam oxidation of ferritic steels – laboratory test kinetic data

Abstract The oxidation resistance of 9–12% chromium steels in steam-containing environments simulating the service conditions of steam power plant has been investigated for exposure times ranging from 1 h up to 10 000 h. In the long-term experiments the steels were evaluated by the mass changes determined in exposure tests, which were interrupted every 250 h, thus providing information concerning the scale growth and spalling characteristics. The morphologies of the oxide scales were examined using optical microscopy, scanning electron microscopy, secondary neutrals mass spectrometry and Raman spectroscopy. Different mechanisms of oxidation were observed for the various steels in different temperature regimes and exposed for different durations. For some steels, the steam oxidation resistance increased with increasing exposure temperature. The scale thickness at which spalling was observed varied according to the exposure temperature and the interval between thermal cycles, so that a critical scale thickness for spalling cannot be derived. The cracking and spallation of scales was correlated with the type, morphology and growth of pores and voids in the scale and could be influenced by the steel microstructure.

A mathematical model describing carburization in multielement alloy systems

Previously, a finite difference model was set up describing the diffusion of carbon in high-temperature model alloys and its chemical reaction with one of the alloy components. The model described the formation of up to three different chromium carbides, two of which could coexist. This paper describes the further development of the model for application to commercial alloys. The model was extended to enable treatment of an arbitrary number of (a) metallic carbide-forming components in the alloys, (b) carbides which may form, (c) components out of which each carbide may be composed, and (d) carbides which may coexist. With this model, carbon concentration profiles and distribution profiles of precipitated carbides occurring during carburization of binary, ternary, and quaternary Ni-based alloys were calculated. Kinetic and thermodynamic data needed for the calculations were obtained by combining literacture data with experimental results and by fitting measured with calculated concentration profiles. The resulting calculated carbon profiles and carbide distributions were in good agreement with the experimental results.

Effect of water vapour on growth and adherence of chromia scales formed on Cr in high and low pO2-environments at 1000 and 1050°C

Abstract The oxidation behaviour of pure Cr at 1000 and 1050°C was studied in Ar–O2 and Ar–H2–H2O mixtures. It was found that in the low-pO2 gases the oxide scales exhibited higher growth rates than in the high-pO2 gases. The scales formed in the low-pO2 gases showed substantially better adherence during cooling, than scales formed in the high-pO2 gases. These differences in growth rate and adherence can be correlated with differences in size and location of the in-scale voids formed during the isothermal exposure. Exposures in Ar-O2-H2O mixtures revealed that the differences in scale growth rates as well as in scale void formation and growth are not primarily related to differences in the oxygen partial pressure of the atmosphere but to the presence of water vapour in the test gas. At sufficiently high H2O/O2-ratios, water vapour promotes oxide formation at the scale/metal interface thereby suppressing excessive growth of existing voids, and also as a consequence improved scale adherence. Whether the enhancement of inward scale growth is related to transport of H2O- or H2-molecules or due to OH− ions, cannot be derived with certainty from the present results.

Current Thoughts on Reactive Element Effects in Alumina-Forming Systems: In Memory of John Stringer

In memory of John Stringer (1934–2014), one of the leaders in studying the reactive element (RE) effects, this paper reviews the current status of understanding of the effect of RE dopants on high-temperature oxidation behavior, with an emphasis on recent research related to deploying alumina-forming alloys and coatings with optimal performance in commercial systems. In addition to the well-known interaction between indigenous sulfur and RE additions, effects have been observed with C, N, and O found in commercial alloys and coatings. While there are many similarities between alumina-forming alloys and coatings, the latter bring additional complicating factors such as the effects of O incorporation during thermal spraying MCrAlY coatings, coating roughness, and heat treatments that must be considered in optimizing the beneficial dopant addition. Analogies can be seen between RE effects in alloys and in the substrates beneath diffusion M–Al coatings. Recently, there has been more interest in the influence of mixed oxidant environments, since these may modify the manifestation of the RE effect. Finally, some thoughts are provided on optimizing the RE benefit and modeling oxidation of RE-doped alloys.