Environmental Engineering Reference
In-Depth Information
5.
Protective scale failure that occurs by lifting and cracking is less prevalent
in Ni-Cr alloys than in Fe-Cr alloys. This is partly due to keying action on
the scale by internally oxidized Cr
2
O
3
. Subscale and associated internal oxide
particle formation within the alloy are never observed behind the protective
Cr
2
O
3
scale on Fe-Cr and Co-Cr alloys.
6.
A basic difference between the doped Cr
2
O
3
scales growing on Fe-Cr and
Ni-Cr alloys is their greater adhesion on the latter. This is related to the more
convoluted alloy-scale interface and intergranular penetration of fingers of
Cr
2
O
3
into the Ni-Cr alloy base. Such convolutions are produced by the inter-
play of stress development in the oxide and substrate and by its relief through
plastic deformation. An oxide scale of uniform thickness is stable only if
diffusion of the less noble metal in the alloy phase is relatively fast compared
to its diffusion in the oxide phase. Consequently, the greater the ratio of
the oxidation rate constant to the alloy interdiffusion coefficient, the more
convoluted and irregular the alloy-oxide interface is likely to be. This ratio is
higher for Ni-Cr than for Fe-Cr alloys in the appropriate composition range. It
is therefore significant to note that although an increase in alloy interdiffusion
coefficient usually promotes the rapid development of a protective surface
scale, if it is too large it may lead to generation of a flat alloy-oxide interface,
which is detrimental to scale adhesion, at least on cooling.
7.
Break-away oxidation, in which there is a dramatic increase in the oxidation
rate after an induction period, follows from the lifting and cracking open of
the scale on alloys that are eventually somewhat more dilute in Cr and follow-
ing the exposure of the Cr-depleted substrates to the hot gas. Such break-
away occurrences are less evident on Ni-Cr alloys than on Fe-Cr alloys, at
least during isothermal exposures. Potentially, break-away is more likely
with Ni-Cr alloys because the Cr depletion at the alloy-oxide interface is
greater. However, the overgrowth and undercutting rates of NiO are so low
in comparison to FeO that oxidation of Ni-Cr alloys is never catastrophic
and a protective Cr
2
O
3
layer is readily reestablished. This occurs by coales-
cence of densely precipitated internally oxidized Cr
2
O
3
particles yielding a
healing layer at the scale base.
8.
It is difficult to form a protective layer of Cr
2
O
3
on Co-Cr alloys with Cr
content less than 30%, other than at low oxygen partial pressures or after
preferential oxidation treatments. This is due to the interplay of the different
governing parameters, which is least favorable for this alloy system. Al-
though CoCr
2
O
4
, spinel phase has a modest composition range but is gener-
ally insufficient to produce a complete layer and prevent the rapidly growing
CoO from overgrowing and undercutting the Cr
2
O
3
nuclei. The alloy interdif-
fusion coefficient for FCC Co-Cr alloys (Co-40% Cr, 6
10
12
cm
2
/s at
1273 K [20]) is an order of magnitude lower than for Ni-Cr alloys and two
orders of magnitude lower than that of ferritic Fe-Cr alloys. Co-Cr alloys
