Environmental Engineering Reference
In-Depth Information
Al required to develop the protective layer of Al
2
O
3
, reduces to only 5-6 wt %.
Here Cr acts as an oxygen getter, inhibiting internal oxidation.
6.5 SCALING OF BINARY AND TERNARY ALLOYS
Oxidation of alloys involves the same general phenomena as described for
pure metals in the preceding chapter. However, alloy oxidation is generally much
more complex as a result of the following:
1.
Alloys in general contain two or more oxidizable constituents having differ-
ent affinities for oxygen reflected by different free energies of formation of
the respective oxide.
2.
The reacting metal atoms do not diffuse at the same rates either in the oxide
or in the alloy phases. As a result, the oxide scales on the alloys will not
contain the same relative amounts of the alloy constituents as does the alloy
phase.
3.
A degree of solid solubility between the oxides may exist.
4.
Ternary and higher oxides are likely to be formed.
5.
Dissolution of oxygen into the alloy may result in subsurface precipitation
of oxides of one (or more) alloying element (internal oxidation).
6.
The composition and structure of the oxide scales on alloys often change as
oxidation proceeds. The oxidation kinetics, in turn, often markedly deviates
from the ideal simple rate equations.
7.
The scales can also crack, spall, develop voids, sinter, and produce multiple
irregular layers.
Accordingly, there can be no unified theory of alloy oxidation, only a set of
special cases can be treated to varying extents in a fundamental and quantitative
way.
Pure metals have poor engineering properties and are rarely used as construc-
tion materials. Therefore, oxidation of alloys, particularly the methods to achieve
improved oxidation resistance, are in a practical sense the most important aspects
of high-temperature oxidation of metals. Because of technological importance,
numerous experimental studies on the oxidation behavior of different alloys are
reported in literature. Since the field of alloy oxidation is vast, no attempt is made
here to give a complete survey of the extensive literature on the topic; rather,
examples to illustrate the important fundamentals are presented.
6.5.1 Preliminary Classification
The oxidation of alloys under normal conditions leads to formation of an external
scale. However, for many alloys, depending on reaction conditions, simultaneous
occurrence of internal oxidation also takes place. The composition and micro-
