Environmental Engineering Reference
In-Depth Information
have penetrated to the oxide-metal interface. Thus, the linear rate is attributed
to phase boundary-controlled processes like nucleation and growth of oxide. It
is further postulated that the lamellar structure is possibly due to repetitive exfoli-
ation of a very thin outer surface layer of the oxygen-saturated metal core, which
is readily converted to rutile. During the linear oxidation the metal-oxide inter-
face tends to move inward at a linear rate, which will eventually become faster
than, and will begin to overtake, the parabolic displacement of the oxygen gradi-
ent into the metal. However, when this happens, the oxygen gradient into the
metal becomes steeper and causes a faster oxygen diffusion into the metal. As
a result, a steady-state oxygen gradient should eventually be approached during
the linear oxidation.
After an extended period of oxidation beyond 1173 K, the rate begins to de-
crease with time and at 1473 K the linear stage is not detected at all. Some
researchers have suggested that the decreasing rate is due to rate-limiting oxygen
diffusion through the porous scale for which a model has also been proposed in
which the pores are identified with screw dislocations. In contrast, Kofstad et al.
[57] attributed this decreasing rate to a sintering process promoting grain growth
of the outer surface scale. Once a compact oxide layer is formed, it can act as
a solid state diffusion barrier causing decrease in reaction rate. The sintering and
grain growth phenomena further suggest that the mobility of Ti in rutile becomes
important at high temperature, which has been supported by Pt-wire marker study
conducted at 1478 K in 1 atm 0
2
where the marker is positioned within the scale
close to the metal-oxide interface.
Zirconium
The reaction between zirconium and oxygen always leads to dissolution of oxy-
gen in the metal and formation of ZrO
2
films or scales. Lower oxides of zirconium
are not known.
Similar to titanium, the oxidation of zirconium is logarithmic below 573-673
K. At higher temperatures, the reaction behavior changes and the general rate
equation can be described by
W
n
kt
, where the value of
n
has been found to
vary from 3 to 2 depending on temperature, i.e., the oxidation kinetics vary from
cubic to parabolic. Some investigators have proposed that such change in kinetics
may be attributed to the surface preparation of samples because oxidation of
mechanically abraded specimens could best be fitted to a cubic law, whereas
chemically polished specimens conform to a parabolic relationship.
From a number of studies on the oxidation behavior of zirconium it has been
concluded that high-temperature protective oxidation deviates from ideal diffu-
sion-controlled behavior, particularly during early stages of reaction. But after
extended exposure, the oxidation tends to become approximately parabolic. After
protective behavior, the oxidation rate suddenly increases (breakway oxidation)
