Environmental Engineering Reference
In-Depth Information
partial pressure of oxygen under consideration) are formed in a proportion given
by the composition of the bulk alloy. Oxide nuclei with high intrinsic growth
rates due to high level of point defect concentrations, such as FeO, CoO, NiO,
Cu
2
O, etc., overgrow the nuclei of the slower growing oxides and spinels (e.g.,
Cr
2
O
3
,Al
2
O
3
, SiO
2
, FeCr
2
O
4
, NiCr
2
O
4
, etc.). The rapid kinetics of overgrowth
formation contribute to the high initial rate of oxidation. While the overgrowth
is continuing, the underlying nuclei of the slower growing and usually more stable
oxides grow laterally. Eventually they impinge one another and form a continuous
layer, or may remain as isolated particles or precipitates in the faster growing
oxide matrix. Which one of the two materializes is decided by a number of factors
involved in determining the steady-state configuration of the scale, e.g., percent-
age of reactive solute in the alloy, chemical potential of the oxidant in the gas
phase, interdiffusion coefficients of the elements in the alloy, surface finish and
pretreatment of the alloy, oxidation procedure, etc.
Eventually, the transient oxidation stage gives way to a steady-state scale for-
mation stage, which essentially means that the morphology and composition of
the scale remain independent of exposure time. Generally, the overall oxidation
rate is governed by the transport of one or more ionic species through a particular
layer in the total scale. As a consequence, the kinetics approximately conforms
to a parabolic rate law. This further implies that the interface concentrations and
indeed the concentration profiles through the scale and alloy, when expressed as
a function of
√
t
, are independent of exposure time.
From a practical point of view, the duration of this steady-state period is the
most critical factor for oxidation-resistant alloys. Any oxidation-resistant alloy
depends on this period for continued protection. Accordingly, the steady-state
period should be as long as possible. However, it cannot last indefinitely, since
selective oxidation of one of the alloying elements of the alloy is taking place,
and as a consequence, the effective service life will end when the supply of
alloying element in the alloy is exhausted. Severity of operating conditions, such
as higher temperature, rapid thermal fluctuation or cycling, higher gas flow rate,
and higher level of mechanical or thermal stresses, will shorten this period. The
steady-state scaling behavior of some commercially important binary and ternary
alloy systems is discussed in Sec. 6.5.
The ultimate (equilibrium) state of an alloy under oxidizing conditions is an
oxide scale containing the alloy components in the same ratio as in the original
alloy. Usually the end of the steady-state period occurs before all of the selec-
tively oxidizable elements have been consumed by some type of mechanical fail-
ure of the oxide scale. This last stage is often related to mechanical influences,
such as adherence, spallation, thermal or mechanical stresses, and void formation.
Since after mechanical rupture of the scale, the alloy is of different composition
than originally, it can no longer regain its original steady-state oxide composition
and morphology, thereby leading to a more devastating situation for the depleted
ξ
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