Environmental Engineering Reference
In-Depth Information
fairly good agreement with experimental results for a number of parent metals.
For diffusion of carbon, nitrogen and oxygen in bcc metals, values of
D
0
often
vary between 0.001 and 0.01. Experimentally determined values of activation
energy for oxygen diffusion in bcc metals generally vary between 105 and 150
kJ/mol. The corresponding values in hcp metals are considerably higher and in
the range of 180-220 kJ/mol. Similarly, activation energy values for carbon dif-
fusion in Fe-Ni-austenite (1173-1373 K) of various compositions are reported
to be about 120-160 kJ/mol [58].
Surface penetration of oxidant into metal matrix takes place by diffusion of
oxidant solute into the solvent metal. The driving force for such diffusion is the
chemical potential gradient from the surface (maximum oxygen activity) to the
interior (minimum oxygen activity). Except for the few noble metals, all metals
exhibit some reactivity with the oxidant that can be predicted by thermodynamic
calculations. During dissolution and diffusion, at some stage, oxygen activity
may reach a critical value finally leading to the formation of a continuous layer
of some stable oxide that may be adherent or porous depending on the physical
and chemical properties of the overlying oxide and underlying metal as discussed
in earlier sections. In the case of adherent oxide layer formation, there will not
be direct access of oxygen to the metal surface. Oxygen must diffuse through
this layer. At the metal-oxide interface, since oxygen activity is brought down
to a low level, further oxygen ingress into the metal matrix may become re-
stricted. On the other hand, if oxide layer is nonadherent, there can be deeper
surface penetration into the metal substrates. Extent of these phenomena, i.e.,
penetration of oxygen into metal matrix and simultaneous formation of a barrier
oxide layer, are decided very much by the thermodynamics and kinetics of indi-
vidual metal-oxidant reactions.
5.9.2 Grain Boundary Diffusion
Along with lattice diffusion of substitutional atoms it may be expected that an
enhanced diffusion of interstitial solutes takes place along grain boundaries,
which are supposed to be easy diffusion paths. It has generally been observed
that the activation energy for the diffusion of interstitial solutes is of smaller
values at relatively lower temperatures, and this hints that grain boundary diffu-
sion predominates at low temperatures. Such effects have been observed for oxy-
gen diffusion in zirconium, as studied by the oxygen penetration method, in the
temperature range of 773-973 K [57].
5.9.3 Thermal Diffusion
Diffusion may also occur under the influence of a temperature gradient (Ludwig-
Soret effect) in addition to migration under a concentration or electrical potential
gradient. Thermal diffusion may be of considerable importance when a metal is
