Long term, cyclic oxidation studies of three high -Cr, ferritic steels were carried out at 800 8C and 900 8C in air. It was found that with decreasing sample thickness the life time of the mentioned alloys decreases due to breakaway phenomena. This effect is caused by faster exhaustion of the chromium reservoir from the bulk alloy in case of thinner components. The observed life time limits can be predicted with reasonable accuracy by a theoretical model, using oxide growth rate parameters, initial alloy Cr content and critical Cr content required for protective chromia scale formation. In the calculation it has, however, to be taken into account that the oxidation rates of the steels increase with decreasing specimen thickness.

Numerous chromia-forming austenitic steels and nickel-base alloys contain chromium-rich strengthening precipitates, e.g. chromium-base carbides. During high temperature exposure the formation of the chromia base oxide scale results in chromium depletion in the alloy matrix and consequently in dissolution of the strengthening phase in the sub-surface zone. The present study describes the oxidation induced phase changes in the chromium depletion layer in case of alloy 625, a nickel base alloy in which the strengthening precipitates contain hardly any or only minor amounts of chromium. Specimens of alloy 625 were subjected to oxidation up to 1000 h at 900 and 1000°C and analyzed in respect to oxide formation and microstructural changes using light optical microscopy, scanning electron microscopy, energy and wavelength dispersive analysis, glow discharge optical emission spectroscopy, and X-ray diffraction. In spite of the fact that the alloy precipitates d-Ni 3 Nb and/or (Ni, Mo) 6 C contain only minor amounts of chromium, the oxidation induced chromium depletion results in formation of a wide sub-surface zone in which the precipitate phases are depleted. However, in parallel, substantial niobium diffusion occurs towards the alloy surface resulting in formation of a thin layer of d-Ni 3 Nb phase adjacent to the alloy/oxide interface. By modeling phase equilibria and diffusion processes using Thermo-Calc and DICTRA it could be shown that the phase changes in the sub-scale zone are governed by the influence of alloy matrix chromium concentration on the thermodynamic activities of the other alloying elements, mainly niobium and carbon. The d-phase depletion/ enrichment process is caused by a decreasing niobium activity with decreasing chromium concentration whereas the (Ni,Mo) 6 C dissolution finds its cause in the increasing carbon activity with decreasing chromium content.A. Chyrkin (&) Á P. Huczkowski Á V. Shemet Á L. Singheiser Á W. J. Quadakkers

The oxidation behaviour of two ferritic steel variants was studied at 800 °C in air and the results were compared with data obtained for two Cr‐based materials. The mechanisms of scale formation were investigated for oxidation times ranging from a few minutes up to 6,000 hours. A number of conventional analysis techniques such as optical metallography, scanning electron microscopy, and X‐ray diffraction were used for scale characterization, in combination with two‐stage oxidation studies using an 18O‐tracer. It was found that the growth rates of the scales on the two steels were not only governed by the main scale forming alloying elements, Cr and Mn, but to a substantial extent by minor additions of Si and Al. At the test temperature of 800 °C these latter elements affect scale formation, although they are not directly incorporated in the surface scales. Independent of a detailed alloy composition, the conductivities of the scales on the ferritic steels were found to be higher than those of the surface scales formed on the Cr‐based materials studied.

In an oxyfuel plant, heat exchanging metallic components will be exposed to a flue gas that contains substantially higher contents of CO2, water vapor, and SO2 than conventional flue gases. In the present study, the oxidation behavior of the martensitic steel P92 was studied in CO2‐ and/or H2O‐rich gas mixtures with and without addition of SO2. For this purpose, the corrosion of P92 at 550 °C up to 1000 h in Ar–H2O–SO2, Ar–CO2–SO2, Ar–CO2–O2–SO2 and simulated oxyfuel gas (Ar–CO2–H2O–O2–SO2) was compared with the behavior in selected SO2‐free gases. The oxidation kinetics were estimated by a number of methods such as optical microscopy, scanning electron microscopy with energy and wave length dispersive X‐ray analysis, glow discharge optical emission spectroscopy, X‐ray diffraction as well as transmission electron microscopy. The experimental results revealed that the effect of SO2 addition on the materials behavior substantially differed, depending on the prevailing base gas atmosphere. The various types of corrosion attack affected by SO2 could not be explained by solely comparing equilibrium activities of the gas atmospheres with thermodynamic stabilities of possible corrosion products. The results were found to be strongly affected by relative rates of reactions of the various gas species occurring within the frequently porous corrosion scales as well as at the scale/gas‐ and scale/alloy interfaces. Whereas SO2 addition to Ar–CO2 resulted in formation of an external mixed oxide/sulfide layer, the presence of SO2 in oxyfuel gas and in Ar–H2O–SO2 resulted in Fe‐sulfide formation near the interface between inner and outer oxide layer as well as Cr‐sulfide formation in the alloy. In the latter gases, the presence of SO2 seemed to have no dramatic effect on oxide scale growth rates.

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