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The oxidation behaviour of the binary alloys Fe?C2.25Cr, Fe?C10Cr, Fe?C18Cr and Fe?C25Cr (wt%) in dry and wet O2 at 600???C is investigated by isothermal exposures of carefully polished samples for up to 168?h. The oxidized samples are investigated gravimetrically and the oxides formed are studied by X-ray diffraction. X-ray photoelectron spectroscopy is used for depth profiling of the thin oxides. The scale surface is imaged by SEM. Cross-sections through the scale are analyzed by SEM/EDX for imaging and for measuring the chemical composition. The oxidation behavior of the four FeCr alloys is intermediate between those of iron and chromium. Fe?C2.25Cr oxidizes in a way similar to iron in both environments, forming a poorly protective scale consisting of FeCr spinel at the bottom, magnetite in the middle and a hematite cap layer. In dry O2, Fe?C10Cr, Fe?C18Cr and Fe?C25Cr form a thin and protective (Fe,Cr)2O3 oxide similar to the chromia film formed on pure chromium. In wet O2, Fe?C10Cr, Fe?C18Cr and Fe?C25Cr initially form the same kind of protective oxide film as in dry conditions. After an incubation time that depends on alloy chromium content, all three alloys go into breakaway oxidation and form thick, poorly protective scales similar to those formed on Fe?C2.25Cr. Breakaway oxidation in wet O2 is triggered by the evaporation of CrO2(OH)2 from the protective (Fe,Cr)2O3 oxide.  相似文献   
2.
The oxidation of iron in dry and wet O2 at 400-600 °C has been re-investigated using gravimetry, SEM/EDX, XRD and FIB. In the presence of O2, water vapour accelerates iron oxidation at 500 and 600 °C. At 400 and 500 °C the magnetite layer is duplex and exposure to water vapour results in the formation of blades on top of a fine-grained hematite layer. At 600 °C it results in a surface without needles and blades. The increased oxidation rate at 500 and 600 °C is attributed to a smaller grain size in the hematite layer resulting in faster ion transport.  相似文献   
3.
The initial oxidation of iron and the effect of different O2/H2O mixtures have been investigated through ESEM in situ exposure and analysis at 500 °C. In dry and wet air a two-layered magnetite forms beneath a thin, fine-grained hematite layer, while only a two-layered magnetite forms in H2O. The two-layered magnetite is separated by a straight interface (in all environments), which is suggested to be the original metal surface. The presence of low levels of water vapour (1% H2O) produces a thicker hematite layer (compared to dry air), exhibiting a large number of whiskers. At least four different factors are suggested to influence the local growth rate: the surface of the metal grain, the thickness of the hematite layer, the oxide grain size and the exposure environment. The ESEM in situ exposures have in addition been shown to have high quality and reproducibility.  相似文献   
4.
The oxidation of chromium in dry O2 and in O2 + 10%H2O at 600 and 700 °C is studied. Scale morphology is investigated by several methods, including scanning electron microscopy (SEM) of cross sections prepared by focussed ion beam milling (FIB). In O2 + H2O at 600 and 700 °C, chromium forms a duplex scale consisting of an inner barrier oxide and a discontinuous outer oxide made up of blade-shaped crystals. Thermogravimetric (TG) measurements show that water vapour influences chromium oxidation by causing vaporization of the protective oxide, resulting in paralinear oxidation kinetics. An extension of the original treatment by Tedmon is deduced, which allows for the determination of the evaporation rate constant k s and the parabolic oxidation rate constant k d from TG data acquired during short exposures. The results show that k d is the same in dry O2 and in O2 + 10%H2O. Equivalently, the transport properties of chromia are the same in the two environments. The equilibrium constant of CrO2(OH)2 formation from chromia is reported. The activation enthalpy of the vaporization reaction is determined.  相似文献   
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