High temperature oxidation of ultra-high purity Fe-Cr-Al model alloys

Abstract

A field emission gun scanning electron microscope (FEGSEM), equipped with an EDX analysis system was used to investigate the oxidation behaviour of Fe-20Cr-5Al based ultra-high purity alloys, which were specifically prepared with controlled levels of Zr, Ti, Hf, La, Si, and Y additions. The oxidation temperature was up to 1200°C for 3000 hours. Numerous studies dedicated to this field have shown that impurities and additions strongly affect the oxidation/spallation behaviour in many different ways. In this pilot study, the effect of carefully controlled combinations of the added elements on α-alumina scale formation, growth and spallation have been examined. The model alloy which contained approx. 300-450 ppm Hf, Ti and Y behaved the best with respect to scale spallation, whereas, the alloy containing only 250 ppm of La spalled the most. Another interesting finding is that in the case of the alloys which contain Hf+Ti, Zr+Ti and Zr+Hf in addition to Y, the scale fractured/spalled in a cohesive manner, whereas in the case of the alloys containing Si +Y and La the scale spalled at the scale/metal interface (adhesive failure). Also a “sunflower” type oxide grain structure was observed in the spalled regions in the case of the commercial alloy YHfAl and the model alloys containing Zr+Hf, and Zr+Ti in addition to Y.In this paper the different oxidation behaviour of the alloys will be discussed, in order to shed light on the influence of the various selected combinations of additional elements.