Environmental Engineering Reference
In-Depth Information
and their influence on the overall scaling behavior of single-phase binary and
ternary alloys.
Before proceeding to a discussion of the steady-state scaling behavior of dif-
ferent alloy systems, it is reasonable to enumerate the following pertinent parame-
ters [21] determining the establishment of a steady-state scale:
1.
The standard free energies of formation of the different simple, doped, and
complex oxides predict the occurrence of thermodynamically favored oxide.
This factor is not always predominant initially, nor does it provide a guaran-
tee for the formation of a complete layer of the favored oxide.
2.
The bulk alloy composition is of prime importance in deciding whether the
favored oxide will be formed in sufficient quantity to produce a complete,
often protective layer or merely appears as nonprotective internal oxide and
finally as a precipitate incorporated in the main scale.
3.
The alloy interdiffusion coefficient determines how quickly the preferentially
oxidizing alloy constituent can be replenished at the growing surface layer.
The rapid development of a protective oxide layer is facilitated by a high
alloy interdiffusion coefficient.
4.
The solubility and diffusivity of atomic oxygen in the alloy together with
the alloy interdiffusion coefficient determine the internal oxidation character-
istics. High values of solubility and diffusivity of atomic oxygen promote
internal oxidation, and high alloy interdiffusion coefficient may permit easy
and early transition from internal to external scale formation in achieving
protective scaling behavior.
5.
The growth rates of base metal oxides decide the relative development, over-
growth, and undercutting rates of the initial nuclei and the subsequent scale.
Rapid growth of the base metal oxide can absorb the internal oxide particles
within it before the oxide particles can coalesce to form a complete protective
layer.
6.
The microstructure of the alloy may also exert influences on factors such as
epitaxial relationship, protective layer development, and so forth.
7.
The oxidizing conditions, including temperature and partial pressure of oxy-
gen, as well as the procedure adopted for elevating the alloy to the steady
temperature, can influence the scaling behavior.
The subject of alloy oxidation is vast because there are numerous alloys of
varying compositions and their corresponding oxidation behaviors are also differ-
ent. Accordingly, it is reasonable to break the field down into various limiting
cases. The following simplified classification, based on the work of Wood [25],
is presented schematically in Fig 6.12. In such an exercise, the entire scene of
alloy oxidation is broadly subdivided into two classes, with class 1 representing
the special case of the oxidation of one alloy component and class 2 representing
the more general case where both components of the binary alloy oxidize.
