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The oxidation of Al-particles down to nano-scale was investigated by TG, SEM and in-situ X-ray diffraction. Al particles are usually coated by a 2 ± 4 nm layer of Al 2 O 3 which can be derived from the degree of weight increase on complete oxidation by TGcurves. The low temperature oxidation of Al particles occurs at least in two steps. The first step builds a layer of 6 to 10 nm thickness composed of crystallites of the same size independent on the initial particle size. This reaction is dominated by chemical kinetics and converts a substantial fraction of the particle if the particle sizes decrease below 1 mm, an effect carefully to be taken into account for nano-particles because of safety reasons. The second step combines diffusion and chemical reaction and proceeds therefore slowly, the slower the bigger the particles are. The kinetic parameters of these two steps can be obtained by a model taking into account both reaction steps, chemical kinetics and diffusion for spherical particles when fitting it to TG-curves. X-ray diffraction shows that particles smaller than 1 mm build gand q-Al 2 O 3 in the first step with nano-crystalline structures which are then transformed to a-Al 2 O 3 .

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B2-FeAl-based aluminide coatings were elaborated on P92 ferritic-martensitic steels by slurry. Under atmospheric pressure of steal and 650°C, a thin protective α-Al 2 O 3 oxide layer formed and was maintained until 5000 h. With increasing pressure to 300 bar, the morphology of α-Al 2 O 3 changed markedly. Iron oxide clusters started to appear at the surface of the scale after 5000 h of exposure and kept on growing till 10000 h. It appears that pressure increased the outward diffusion of Fe through the coating and through the oxide scale and fostered the propagation of the tensile cracks of the coating.

The high temperature oxidation of P-NiAI, undoped and doped with Ce, Y and Hf was studied in situ by thermogravimetry in He with p(0,) = 5 . W h bar at 1000°C and by high temperature X-ray diffraction at 950 and 1000°C in air. After the in situ experiments the samples were analysed by optical microscopy and SEM with EDX.It was observed by thermogravimetry that the weight gain for p-NiAl+Hf is lower than for undoped p-NiAl, whereas the weight gain for p-NiAl+Y was similar to that for 6-NiAl. For p-NiAI+Ce an enhanced increase of mass gain was observed. The in situ X-rayexperiments show that in the first hours of the oxidation process the metastable 0-Al2O3 is formed, which transforms to the protective a-A1203 after different periods of time, depending on the dopant element and the temperature. The analysis of the samples shows that the ternary phases along the grain boundaries were oxidized, whereas the ternary phase precipitates within the grains remained unoxidized. The extend of the grain boundary oxidation increases from P-NiAl+Hf and p-NiAl+Y to P-NiAl+Ce.Die Hochtemperaturoxidation von P-NiAI, undotiert und dotiert rnit Ce, Y und Hf wurde mittels Thermogravimetrie in He mit p(02) = 5.10-6 bar bei 1000°C und mittels Hochtemperatur-Rontgendiffraktrometrie bei 950 und 1000°C in Luft untersucht. Nachuntersuchungen der Proben erfolgten mit Hilfe optischer Mikroskopie sowie Rasterelektronenrnikroskopie (REM) mit EDX.Thermogravimetrische Messungen ergaben fur p-NiAl+Hf eine niedrigere und fur P-NiAl+Y eine ahnliche Massenzunahme wie fur undotiertes P-NiA1. Bei p-NiAI+Ce wurde eine deutlich hohere Massenzunahme beobachtet. Die in situ-Versuche zeigten, daB in den ersten Stunden des Oxidationsprozesses das metastabile 0-AI2O3 gebildet wird, das sich im weiteren Verlauf der Oxidation zu dem stabilen a-A1203 umwandelt. Der Zeitraum, in dem 0-A1,03 zu beobachten ist, hangt vom Dotierungselement und von der Temperatur ab. Die mikroskopische Untersuchung zeigte, daR die ternaren Phasen an den Korngrenzen oxidieren, die ternaren Phasen innerhalb der Korner aber keine Oxidation aufweisen. Die Korngrenzenoxidation nimmt in der Reihenfolge P-NiAIjHf, p-NiAl+Y und P-NiAl+Ce zu.