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In this experimental study, electrolyte-supported solid oxide fuel cells (SOFCs) with two different anodes were investigated. Specifically, the stability of cells with a nickel/8 mol % yttria-stabilized zirconia (Ni/8YSZ) cermet anodes was compared to those based on nickel/40 mol % gadolinia-doped ceria (Ni/CG40). For this, the cells were characterized by impedance spectroscopy as well as by four-point electrical conductivity measurements. A high frequency process was observed in the Ni/8YSZ anode, which was not detected in the Ni/CG40 anode. After eight redox cycles at 950°C , the cell with the Ni/8YSZ anode showed an increase in the polarization resistance mainly in the high frequency domain. However, the cell with the Ni/CG40 anode showed an increase in both ohmic and polarization resistances, the latter mainly in the low frequency domain. Compared with Ni/CG40, the degradation in Ni/8YSZ upon redox cycling was higher at 850°C but lower at 950°C . For the Ni/8YSZ anode, a significant degradation was seen in the first 3 h after a redox cycle. The increase in the ohmic resistance of the Ni/CG40-based cell is believed to correlate with a decrease in the electrical conductivity of the anode. The latter showed a strong decrease upon a subsequent redox cycling at 950°C . For the Ni/CG40 anode, the degradation in both the conductivity and electrochemical performance significantly improved by decreasing the operation temperature from 950 to 850°C .

Recent experimental investigations have widened the understanding of metal dusting significantly. Microscopic observations have been used to dissect dusting mechanisms. Iron dusts by growing a cementite surface scale, which catalyses graphite nucleation and growth. The resulting volume expansion leads to cementite disintegration. Cementite formation on iron can be suppressed by alloying with germanium. Nonetheless, dusting occurs via the direct growth of graphite into the metal, producing nanoparticles of ferrite. This process is faster, because carbon diffusion is more rapid in a-Fe than in Fe 3 C. Austenitic materials cannot form cementite, and dust via formation of graphite at external surfaces and interior grain boundaries. The coke deposit consists of carbon nanotubes with austenite particles at their tips, or graphite particles encapsulating austenite. TEM studies demonstrate the inward growth of graphite within the metal interior. It is therefore concluded that the dusting mechanism of austenitic materials like high alloy Cr-Ni steels and Ni base materials is one of graphite nucleation and growth within the near surface metal. In all alloys examined, both ferritic and austenitic, the principal mass transfer process is inward diffusion of carbon. Alloying iron with nickel leads to a transformation from one mechanism with carbide formation to the other without. Copper alloying in nickel and high nickel content stainless steels strongly suppresses graphite nucleation, as does also an intermetallic Ni-Sn phase, thereby reducing greatly the overall dusting rate. A surface layer of intermetallic Ni-Sn Fe-base materials facilitates the formation of a Fe 3 SnC surface scale which also prevents coking and metal dusting. Current understanding of the roles of temperature, gas composition and surface oxides on dusting rates are summarised. Finally, protection against metal dusting by coatings is discussed in terms of their effects on catalysis of carbon deposition, and on protective oxide formation.

The paper critically reviews the state of the art on the oxidation of TiAl. At the beginning differences compared to the oxidation of conventional Fe, Ni and Co alloys are discussed. The Knowledge of the TiAlO phase diagram as a basis for an understanding of the processes occurring during oxidation is very incomplete. Three different scale structures can be distinguished as functions of time or scale thickness. In the sub‐surface zone two phases with high oxygen solubilities can be identified after a certain time. These phases do not follow from the existing phase diagram. Nitrogen can have two different effects depending on the time it is added to the oxidation environment. The micro‐structure also affects the oxidation behaviour significantly, so this effect can lead to a misinterpretation of the influence of other parameters such as alloying elements. The influence of alloying elements is not yet understood. Mechanical loading leads to scale cracking, however, the system has a significant scale repair capacity, though about twothirds of the scale grows by inwards transport.

In several high temperature processing environments the presence of chlorine may significantly reduce the life-time of the components. Although metallic materials have been widely used under such conditions there is still a need for data on the role of the different alloying elements in commercial alloys. In the present work this role was investigated in detail at temperatures between 650 and 1000 8C in synthetic air containing up to 2 vol.% Cl 2 . Before starting the experimental investigation a detailed literature evaluation on chlorine high temperature corrosion was performed followed by a thermodynamic assessment of the stability and the partial pressures of the phases formed by the reaction between alloy and environment. The results of this "theoretical approach" are presented in the following first part of the publication while the experimental work will be reported in the second part appearing in a later issue of this journal. Already the results of the "theoretical approach" yield a clear picture of which alloying elements play a detrimental role and which elements are beneficial. These results can be used as a tool for a general assessment of metallic alloys with regard to their performance in oxidizing/chloridizing high temperature environments. In the second part of this work the results from this "theoretical approach" will be compared with the behavior of 14 commercial materials, where the content of the major alloying elements was varied in a systematic manner.In einer Reihe von Hochtemperatur-Prozeßumgebungen führt die Anwesenheit von chlorhaltigen Verbindungen zu einer deutlichen Verkürzung der Lebensdauer der Anlagenbauteile. Obwohl in diesen Anlagen metallische Legierungen bereits in großem Umfang eingesetzt werden, besteht immer noch ein ausgeprägter Bedarf an Daten zur Rolle des Verhaltens der verschiedenen Legierungselemente unter diesen Bedingungen. In der vorliegenden Arbeit wurde diese Rolle im Detail bei Temperaturen von 650 bis 1000 8C in Luft mit Chlorgehalten bis zu 2% vol. untersucht. Vor der Durchführung des experimentellen Teils der Untersuchungen wurde eine umfangreiche Literaturauswertung vorgenommen, gefolgt von thermodynamischen Rechnungen zur Stabilität und zum Partialdruck der Phasen, die durch die Reaktion der Legierungen mit der Prozeßumgebung gebildet werden können. Die Ergebnisse dieses "theoretischen Ansatzes" werden im ersten Teil einer zweiteiligen Veröffentlichung vorgestellt, während über die Ergebnisse der experimentellen Untersuchungen im zweiten Teil berichtet wird, der in einer späteren Ausgabe dieser Zeitschrift erscheint. Bereits die Ergebnisse des "theoretischen Ansatzes" erlauben eine klare Einschätzung, welche Legierungselemente eine positive und welche eine negative Rolle bezüglich der Korrosionsbeständigkeit von metallischen Legierungen in diesen Umgebungen spielen. Diese Ergebnisse können als Werkzeug für die Bewertung des Verhaltens kommerzieller Werkstoffe in oxidierend/chlorierend wirkenden Atmosphären eingesetzt werden. Im zweiten Teil dieser Arbeit we...

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In recent years a new group of ferritic-martensitic chromium steels for the use in fossil power stations has been developed with chromium contents between 9 and 12%. Typical representatives of these steels are P91, E911 and Nf616, which are nowadays widely used in the more advanced power plants. In the development phase the focus was on the mechanical properties of these steels but when taking them to practical operation conditions it turned out that much of the life-time of the materials and components is determined by their oxidation properties. Oxidation resistance is first of all a function of alloy composition. For the steels of this group it is chromium, silicon, manganese and molybdenum that decide their oxidation performance and since the contents especially of the four elements can be significantly different for the different steels there can be clear differences in oxidation behaviour. One of the most important issues from this point of view is how the concentrations of these elements change in the metal subsurface zone during operation/oxidation since if their level drops below a critical level oxidation resistance of the steels will be lost. In the work to be reported the influence of alloy composition and metal subsurface zone concentration as a function of oxidation time up to 10000 h was investigated in dry air and air up to 10% water vapour at 650 °C. The investigations comprised several of the advanced commercial 9% Cr steels including P91, E911, Nf616 and six laboratory melts of Nf616 with different amounts of silicon. As the results of the investigations show humidity, which is omnipresent in combustion environments, can dramatically accelerate oxidation. Silicon as an alloying element reduces the detrimental effect of water vapour significantly while molybdenum has a negative effect. The effects of the key alloying elements in these steels was quantified for conditions with and without water vapour in the environment including the role of mechanical load and recommendations were developed on how to guarantee the optimum oxidation resistance of these steels