Oxidation Behaviour of Sanicro 25 (42Fe22Cr25NiWCuNbN) in O2/H2O Mixture at 600 °C

The present study investigates oxidation at 600 A degrees C of alloy Sanicro 25 (42Fe22Cr25NiWCuNbN) in dry and wet O-2 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 O-2 + H2O environment, chromium volatilized from the surface, causing the chromium content of the oxide to be lower than after oxidation in dry O-2. However, under present experimental conditions, the Cr depletion of the scale was not enough to trigger accelerated corrosion of the alloy.