Environmental Engineering Reference
In-Depth Information
ers have concluded that oxygen diffusion predominates below 1173 K, whereas
diffusion of titanium ions becomes more important at higher temperatures.
Parabolic oxidation of Ti above 873-973 K comprises of two simultaneous
processes—oxygen dissolution and oxide scale formation. At such high tempera-
tures, the oxide grains are larger and better developed with faceting than those at
lower temperatures and correspondingly volume diffusion becomes the dominant
diffusion process. During the parabolic oxidation period of the metal, attempts
have been made to estimate the solubility of oxygen by using conventional metal-
lography, microhardness, and x-ray diffraction techniques. Measurement of lat-
tice parameter at the metal-oxide interface as a function of time suggests that,
at 923 K and 973 K, oxygen is readily dissolved in Ti up to concentrations of
14-15 at. %, which corresponds to a composition Ti
6
O, at which order-disorder
transformation may occur. Similar studies by Kofstad et al. [59] indicate that the
oxygen concentration in an outer layer of the metal tends to a limiting value of
TiO
0.35
when titanium is oxidized at 1173 K. It is further reported that 80% of
the reacting oxygen gets dissolved in the metal during the initial parabolic oxida-
tion at 1173 K, whereas Stringer [60] has reported a corresponding value of only
45% at 1223 K and admitted that scatter in the estimated values increases with
temperature reaching a maximum at 1223 K. The activation energy for oxidation
of titanium in the temperature range of 873-1273 K has been reported to lie
between 209-230 kJ/mol, whereas at lower temperature, a smaller value has been
suggested which is attributed to enhanced grain boundary migration.
It is also important to note that Ti undergoes a phase transformation from
α
-Ti
to
-Ti at 1155 K. It is argued that oxygen probably diffuses faster in the more
open bcc structure, but at the same time it is to be remembered that oxygen is
an
β
stabilizer. So during oxidation above the transformation temperature, there
will always be an outer layer of
α
-Ti. On this basis it has been suggested that
the phase change may be of minor importance with regard to the oxidation mecha-
nism except during the very initial stages of the reaction. Once the parabolic
oxidation changes to linear kinetics, the protective behavior is lost. Such transi-
tion from parabolic to linear oxidation takes place at shorter times, the higher
the temperature of exposure.
The linear oxidation rate is reflected through an increased rate of oxide forma-
tion whereas dissolution of oxygen continues at a rate governed by the prevailing
oxygen gradient and the diffusion coefficient of oxygen in the metal. It is reported
[57] that during linear oxidation the oxide has a lamellar structure parallel to the
metal surface and the color of the thick scale formed at 1173 K for 17 h in 1
atm O
2
ranges from yellow to white. Pt-wire marker study has shown its position
at the top of the scale, i.e., at the oxide-oxygen interface. The more or less
uniformly light color of the entire scale suggests that there is no change in non-
stoichiometry of the oxide across the scale. This leads to the logical conclusion
that this portion of the scale is nonprotective and porous to oxygen which might
α
