Environmental Engineering Reference
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Figure 6.46 Comparative plot of cation diffusion coefficients in Al 2 O 3 , MgO, CaO,
Cr 2 O 3 , NiO, CoO, and oxygen diffusion in stabilized zirconia and SiO 2 [2].
NiO and CoO, which are characterized to possess appreciable nonstoichiometry,
exhibit high rates of diffusion due to their high concentrations of point defects.
Similarly, stabilized zirconia with a large concentration of oxygen point defects
provides a poor diffusion barrier. To the contrary, the transport of molecular
oxygen through SiO 2 is much lower than Al ions through Al 2 O 3 above 1773 K.
Accordingly, in generalized form, it may be stated that simple oxides with large
ionic character and small deviations from stoichiometry are expected to act as
better protective layers. To the contrary, oxides like FeO, NiO, and CoO cannot
act as effective diffusion barriers unless their point defect concentrations are de-
creased by doped element oxides.
During high-temperature exposure of alloys in oxidizing environments, it is
a common experience that various types of spinels are also formed along with
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