Environmental Engineering Reference
In-Depth Information
tures. The gases may be present in metals as interstitially dissolved atoms; as
molecular gas in voids, microchannels, and microcracks; or as separate phases
such as oxides, nitrides, sulfides, etc.
Dissolution and diffusion of oxidant in metals may become an important factor
for assessing the high-temperature oxidation behavior of metals, particularly for
metals belonging to groups IVA (Ti, Zr, Hf) and VA (V, Nb, Ta) of the periodic
table. These metals are very much prone to readily dissolve relatively large
amounts of oxygen. Dissolved gases greatly affect the mechanical properties of
metals used for various structural applications at high temperatures and may be-
come a crucial factor determining the life span and usefulness of such metals
and their alloys.
The solubility of gases in metals may qualitatively be correlated with the posi-
tion of metals in the periodic table. As mentioned earlier, oxygen is readily solu-
ble in the transition metals belonging to groups IVA and VA. However, the solu-
bility in group VI metals (e.g., Cr, Mo, W, Se, Te) is extremely small and that
in noble metals (e.g., Au, Ag, Pt) is quite high. As a measure of the solubility
it may be referred that the oxygen solubility just below 1173 K, i.e., in the
α
phase of Ti, Zr, and Hf amounts to about 30 at. %, 28.5 at. %, and 20 at. %,
respectively. The solid solubility of oxygen in the corresponding
phases is
comparitively small; however, with an increase of temperature, solubility in-
creases in both phases. In contrast, group VA metals show relatively smaller
solubility which also increases with temperature and for Nb and Ta it amounts
to hardly 5 at. % at 1773 K [57].
β
5.9.1 Diffusion of Oxygen in Metals
In the lattice structures of metals, interstitially dissolved atoms may occupy two
types of interstitial sites: octahedral and tetrahedral. Octahedral sites are larger
and can accommodate larger atoms than the tetrahedral sites. Thus, carbon, nitro-
gen, and oxygen atoms are expected to occupy octahedral interstitial sites.
Diffusion of interstitial solute atoms in metals takes place by the interstitial
mechanism in which solute atoms successively jump from one interstitial site to
another. Empirically the temperature dependence of the diffusion coefficient of
any species in a matrix phase can be given by the relation:
Q
RT
D
D
0
exp
(5.120)
where
D
is the diffusion coefficient and
D
0
is a preexponential function. Theoreti-
cal relations for the diffusion coefficient have been developed by Wert and Zener
[57] on the assumption of random motion of the solute atoms and thus negligible
interactions between interstitial atoms and substitutionally dissolved impurity
atoms or parent lattice atoms. It has been observed that these relations are in
