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Iron‐chromium alloys (1–15 w/o [weight per cent] Cr) oxidized in dry carbon dioxide at 700°, 900°, and 1100°form duplex scales which are similar in structure to those generated in H2O‐normalAr atmospheres, i.e., a detached but continuous outer layer of iron oxides (predominantly normalFeO ) and a porous inner layer of normalFeO and Fe‐Cr spinel particles. Carburization accompanies this oxidation at all temperatures, and the resulting microstructural changes, the carbides formed, the hardness increases and the carbon content of homogeneous specimens are related to the equilibria of the Fe‐Cr‐C system. The carburization and the continued high oxidation rates, in spite of the detached duplex structure of the scale, require that a gaseous transfer of oxygen and carbon,occur within the voids separating the outer and inner oxide layers. This requirement suggests a carbon permeation of the dense outer scale and the generation of a CO2‒CO atmosphere in these voids which is carburizing as well as oxidizing to the alloy.

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The oxidation of Fe-Cr alloys (1 to 15 wt % Cr) in a 0.1H20-0.9Ar atmosphere at 700~176 produces a characteristic two-layer scale overlaying a thin subscale. The outer ~s t i t e scale is extensively detached from the i n n e r scale containing an Fe-Cr spinel phase in a wfistite matrix. Phase identification by x -r a y 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 F e -C r -O constitution diagram. The extensive detachment of the outer w/istite 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.

A study is made of the deep oxidation of iron at high temperatures and in an oxygen atmosphere. Large cavities are always found in specimens whose total oxygen content has been brought to that of FeO. The scale thickness of such specimens, along with other evidence, implies that FeO is plastic in the temperature range in which it is stable, while one or both of the higher oxides is relatively rigid. Evidence is presented which indicates that iron is transported to its surrounding scale with little or no body diffusion path; the mechanism involved is not identified. An expression, analogous to the parabolic rate law for scaling of plane specimens, is developed for application to cylindrical geometry. 3.2.3