The present study investigates oxidation at 600 °C of alloy Sanicro 25 (42Fe22Cr25NiWCuNbN) in dry and wet O2 environments. The exposure time was 1–168 h. The oxidized samples were analyzed by grazing incidence X-ray diffraction, glow discharge optical emission spectroscopy, scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. Alloy Sanicro 25 showed protective oxidation behaviour under the present conditions. Initially, a thin and smooth corundum-type single layer base oxide formed, featuring a Cr-rich bottom part and a Fe-rich top. With time, double-layered oxide nodules form consisting of inward- and outward-growing parts. Below the oxide scale a 100–200 nm thick oxidation-affected zone formed in the alloy, which was depleted in Cr and enriched in Ni. In this region the chromium carbides and copper-rich particles present in the bulk alloy were dissolved. In O2 + H2O environment, chromium volatilized from the surface, causing the chromium content of the oxide to be lower than after oxidation in dry O2. However, under present experimental conditions, the Cr depletion of the scale was not enough to trigger accelerated corrosion of the alloy.
Oxidation Water vapour effect Chromia evaporation Chromia former Sanicro 25
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