Environmental Engineering Reference
In-Depth Information
being oxidized under a temperature gradient, e.g., zircalloy cladding in nuclear
reactors. When starting with a homogeneous solid solution in a temperature gradi-
ent, the solute atoms migrate and build up a concentration gradient, and a steady
state is reached when the opposing constraints of thermal and concentration diffu-
sion just balance. The steady-state condition in a temperature gradient can be
described by the expression:
Q
*
RT
ln
N
C
(5.121)
where
N
is the atomic fraction of the solute at position
ζ
along the temperature
gradient,
T
is the temperature in absolute scale at
,
Q
* is the heat of transport,
and
C
is a constant. For a positive heat of transport, the solute atoms migrate
toward the colder region as has been observed for migration of hydrogen, oxygen,
and nitrogen in zirconium.
ζ
5.9.4 High-Temperature Oxidation Behavior
of Group IV and V Metals
Many common features are observed during the oxidation of Ti, Zr, and Hf.
High-temperature exposure of these metals to oxidizing environment results in
simultaneous oxygen dissolution in the metallic phase and oxide scale formation.
During the initial stage of oxidation all the three metals exhibit protective scale
formation, whereas after extended exposures at high temperatures nonprotective
oxide scale formation takes place. In such a situation it is essential to assess the
oxidation behavior of these metals in terms of the relative importance of oxygen
dissolution and oxide scale formation under different experimental conditions.
In such studies a critical issue draws special attention to whether the oxygen
gradient in the metal beneath the oxide scale and the dissolution process would
bring about a behavioral change from protective to nonprotective. Studies on
such reactive metals have recently gained special attention considering their wide
applications in industry, especially in alloy forms.
Titanium
Whenever titanium is exposed to an oxidizing environment, thermodynamically
one should always expect the oxide scale to consist of a sequence of layers of
different stoichiometries (TiO, Ti
2
O
3
,Ti
3
O
5
, TiO
2
). However, at temperatures
below 1273 K, only the rutile modification of TiO
2
has been detected in signifi-
cant quantities.
Oxidation of Ti follows different rate equations depending on temperature and
