ISSN:
1573-4889
Source:
Springer Online Journal Archives 1860-2000
Topics:
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes:
Abstract The oxide formed over polished iron-carbon alloys at 300°C and in a 100 Torr dry oxygen atmosphere was primarily due to the oxidation of the ferrite phase, but a thin protective oxide film (∼ 150 Å) was formed on the carbide phase. The protective nature was attributed to the existence of a kinetic barrier of CO and CO2 at the carbide-oxide interface. The initial oxide film formed on the carbide phase comprised many small, randomly oriented crystallites; approximately 70 Å in size, of γ-Fe2O3. This film then underwent a grain growth process via a strain-induced grain boundary migration accompanied by a phase transformation from γ-Fe2O3 to α-Fe2O3. This transformation produced a tensile stress in the oxide film which was later relieved by the generation of cracks along the newly grown grain boundaries. These cracks exposed fresh carbide surface, released the gas barrier at the carbide-oxide interface, and provided rapid diffusional paths for further oxidation. As a result, the oxide film (α-Fe2O3) was reduced to Fe3O4 and raised ridges appeared at the crack sites. Once the cracks were healed, the film again became protective.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00614623
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