Environmental Engineering Reference
In-Depth Information
expected that Ni-Cr alloys would behave in a similar way to that as discussed
for Co-Cr alloys (in Sec. 6.5.2). However, solubility, diffusivity, and alloy inter-
diffusion coefficient are also important factors; consequently, the scale growth
pattern and oxidation mechanism, even between the two types of FCC alloys,
may not be exact.
The major differences in the oxidation behavior between these three types of
binary alloy system [21,25,30,31,32,33] are as follows:
1.
Fe-Cr and Ni-Cr alloys display a minimum in their oxidation rates at about
the 20% Cr level, whereas Co-Cr alloys must contain more than 30% Cr to
exhibit a similar effect.
2.
For dilute alloys (Cr-5-10%), addition of Cr increases the oxidation rates
in all three alloy systems. However, the rate for Fe-Cr alloys is several times
faster than that for Co-Cr and Ni-Cr alloys when no protective healing layer
of Cr
2
O
3
can form.
3.
A continuous layer of doped Cr
2
O
3
is more readily formed on Fe-Cr alloys
(Cr
15-30%) than on the corresponding Ni-Cr alloys, in 1 atm O
2
at 1073-
1473 K. This is basically due to the following [21]:
a.
The alloy interdiffusion coefficient for FCC. Ni-Cr (for Ni-20% Cr, 1.1-
2.8
10
11
cm
2
/s at 1273 K) is an order of magnitude lower than that for
BCC Fe-Cr alloys (for Fe-20% Cr, 6
10
10
cm
2
/s at 1273 K [20]), whereas
the solubility and diffusivity of atomic oxygen is higher in Ni-Cr alloys.
Therefore, Cr
2
O
3
tends to precipitate as densely populated internal oxide par-
ticles in Ni-Cr alloys, whereas in Fe-Cr alloys they tend to coalesce producing
a continuous layer near the alloy surface.
b.
The wide composition range of FeFe
2-
x
Cr
x
O
4
(0
2), the spinel phase
in Fe-Cr alloys, permits it to form as a relatively extensive phase. This can
prevent the rapidly growing FeO (with very high native defects) from over-
growth and undercutting the Cr
2
O
3
nuclei. Such favorable influence is less
in Ni-Cr alloys where NiCr
2
O
4
is of relatively fixed composition rich in Cr,
less extensive and mostly in particulate form. Fortunately, NiO, unlike FeO,
does grow at a relatively slow rate for which overgrowth and undercutting
by this oxide are also slow, and eventually a continuous layer of Cr
2
O
3
is
formed.
x
4.
In Cr-rich alloys of all the three systems, the main protective oxide is doped
Cr
2
O
3
and not the spinels. Doped Cr
2
O
3
is less readily established on Ni-Cr
alloys as initial scale, but it is more readily retained because of subscale
keying, which is never the case with Fe-Cr alloys. For Fe-Cr alloys, chromia
is the sole oxide except for a little dissolved iron as Fe
2
O
3
. However, for
Ni-Cr alloys, there is a considerable outer layer of NiO containing dissolved
Cr. In contrast, Co-Cr alloys form an external continuous Cr
2
O
3
scale with
dissolved Co or with CoCr
2
O
4
.
