Environmental Engineering Reference
In-Depth Information
oxide layer from the substrate at some locations can be attributed to void coales-
cence as a result of the arrival of vacancies. Moreover, these gaps cause a slower
rate of reaction by impeding metal transport (at such moderate temperatures,
metal vapor transport through the gaps is negligible due to very low volatility).
The oxidation kinetics of pure Ni, Cr, and Fe have provided ample evidences
to the fact that prior oxide grain size and hence surface pretreatment or adopted
oxidation procedure strongly affect the subsequent scale growth process. So long
as fine-grained oxide is produced and maintained, the oxidation rate is rapid be-
cause of enhanced cation transport via oxide grain boundaries. At times the effect
of initially formed oxide grain size on subsequent oxidation rate can be large.
For example, the parabolic rate constant of Ni oxidation at 873 K is
10
4
times
smaller for the growth of monocrystalline NiO than that for fine-grained polycrys-
talline nickel oxide. These features of oxidation tests clearly imply some practical
utility considering the fact that proper choice of surface preparation, pretreatment,
or specific oxidation procedure can be conducive to protective and adherent oxide
layer growth.
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