Oxide Structures Produced on Iron-Chromium Alloys by a Dissociative Mechanism

Abstract

The oxidation of Fe‐Cr alloys (1 to 15 wt % Cr) in a atmosphere at 700°–1100°C produces a characteristic two‐layer scale overlaying a thin subscale. The outer wüstite scale is extensively detached from the inner scale containing an Fe‐Cr spinel phase in a wüstite matrix. Phase identification by x‐ray diffraction, chemical analyses by conventional and electron microprobe techniques, and detailed metallographic studies have shown these structures to be consistent with the phase equilibria of the Fe‐Cr‐O constituion diagram. The extensive detachment of the outer wüstite scale layer and the continued high rate of oxidation of these alloys after such detachment occurs suggest that a dissociative process is responsible for the maintenance of this high oxidation rate. The experimental evidence for this process is presented, the importance of such a mechanism is discussed, and examples of other gas‐metal systems in which a similar process has been recognized are cited.