Environmental Engineering Reference
In-Depth Information
the early stages for simplicity even though they may play an important role, but
this phase is incorporated at a later stage to illustrate the reaction between NiO
and Cr
2
O
3
. For the alloy Ni-20-40%Cr, the NiO overgrows the more slowly
developing Cr
2
O
3
. As the grains coarsen and the scale becomes more uniform,
lattice transport tends to become rate determining in the growth of the nuclei and
alloy interdiffusion starts playing a greater role. The Cr
2
O
3
starts growing later-
ally as Cr at the alloy-oxide interface enters these nuclei directly accompanied
by possible reduction of NiO in contact with the alloy by a displacement reaction.
Densely distributed internal oxide particles of Cr
2
O
3
behind the NiO nuclei even-
tually form a complete layer of Cr
2
O
3
at the scale base by coalescing and linking
up with the initially developing Cr
2
O
3
nuclei. Once the Cr
2
O
3
layer is complete,
it continues to thicken, there being little further Ni incorporation into the scale,
since the oxygen potential at the alloy-oxide interface is lower than the dissocia-
tion pressure of NiO. Ultimately, the protective doped Cr
2
O
3
becomes the rate-
determining layer. However, Cr
2
O
3
can react with the outer NiO to form NiCr
2
O
4
also, and if this layer turns out to be sufficiently thick and compact, it can become
rate determining.
In comparision, the behavior of Ni-5%Cr alloy is different because its low Cr
content never favors development of a complete Cr
2
O
3
layer at the scale base,
even though the initial stages of oxide formation are similar. Accordingly, NiO
in the vicinity of the alloy-oxide interface partly dissociates, supplying the alloy
with atomic oxygen, which diffuses inward, producing a front of internal Cr
2
O
3
particles. During scale thickening, these are incorporated into the inner layer as
NiCr
2
O
4
particles. The inner layer may be a porous one due to vacancy coales-
cence and void formation, and oxygen gas transport may become operative, sup-
plied by the dissociation of oxide.
Thus, a steady-state scale for Ni-5%Cr alloy consists of a two-phase surface
oxide with NiCr
2
O
4
in a matrix of NiO and internal oxide of Cr
2
O
3
. The NiO
and Cr
2
O
3
may be doped as in the case of higher Cr(20-40%)-containing alloys
and such doping will influence the scaling rate. However, it should be noted that
a complete layer of equilibrium-doped Cr
2
O
3
is not formed on low Cr-containing
alloys because of the spatial and diffusional factors.
The alloy interdiffusion coefficient undoubtedly plays a vital role in the early
stages of oxidation, determining how readily and preferentially the less noble
oxidizable element can be supplied to the alloy-oxide interface. Giggins and
Pettit [22] have demonstrated that the development of Cr
2
O
3
on Ni-Cr alloy grain
boundaries occurs more readily and rapidly than over the bulk areas of the grains.
The higher diffusion rate of Cr along the grain boundaries permits an easy transi-
tion from internal oxidation to external scale formation with respect to Cr
2
O
3
formation at these locations. Thus, finer grain alloys have been found to approach
the steady-state configuration at a faster rate. Further, smaller alloy grain size
