Environmental Engineering Reference
In-Depth Information
It is natural that liquid oxide phase cannot provide protection to the underlying
substrate alloy and may lead to excessively fast oxidation and eventual disintegra-
tion of the alloy. This phenomenon is commonly termed catastrophic oxidation,
as postulated in 1948 by W. C. Leslie.
Important examples of catastrophic oxidation are found during oxidation of
metals and alloys in the presence MoO
3
(m.p. 795
°
C), V
2
O
5
(mp. 674
°
C), Bi
2
O
3
(m.p. 820
C). Leslie and Fontana [17] investigated the
special features of this type of oxidation for such systems as Fe, Ni, Cr, stainless
steel, Inconel, Hastelloy, and other commercial high-temperature alloys in the
presence of MoO
3
and V
2
O
5
. In all cases, increased oxidation rates were observed
compared to those in dry air. Since rapid oxidation took place, the oxide scales
were invariably porous, spongy, or nonadherent to the substrate alloy. Rathenau
and Meijering [18] also studied the oxidation behavior of a few pure metals,
binary and ternary alloys when buried in MoO
3
powder. They too observed accel-
erated attack above certain critical temperatures depending on the metal or alloy
system. However, they [18] were the first to point out the importance of a liquid
oxide phase in such aggressive attack. They reported that accelerated attack oc-
curred at the eutectic temperatures of the binary or ternary oxides involved. For
Cr-containing alloys, the onset of catastrophic oxidation occurred near the tem-
perature at which liquid MoO
3
dissolves Cr
2
O
3
.
Unfortunately, no conclusive mechanism for this type of degradation has been
elucidated. In the presence of oxide vapors, the liquid phase is probably initially
formed on the scale surface. But when the metal forming the low-melting oxide
is present as an alloying element, rapid attack may be initiated at the scale-alloy
interface. Accordingly, it may be hypothesized that the liquid phase penetrates
the scale along the initial oxide grain boundaries or microchannels or microcracks
to the alloy surface where rapid degradation takes place. Presence of a liquid
phase along the grain boundaries may also act as easy diffusion paths for both
cations and anions, leading to accelerated attack.
An elaborate study by Brenner [19] on ternary alloys like Fe-Ni-Mo and Fe-
Cr-Mo has shown that when Ni and Cr are added to binary Fe-Mo alloys, cata-
strophic oxidation occurs in certain concentration regions, as illustrated in Fig.
6.6, where low-melting ternary or quaternary compounds might form. From these
results, the mechanism of such attack has been suggested for Fe-Mo-Cr alloys
as depicted in Fig. 6.7. During initial oxidation, Fe, Ni, and Cr are preferentially
oxidized, and the oxidation is protective in nature. With progress of oxidation,
Mo becomes enriched at the alloy interface, leading to formation of an inner
layer of MoO
2
. Catastrophic oxidation is initiated by the formation of a crack in
the scale due to some sort of stress development; MoO
2
becomes oxidized to
molten MoO
3
, which penetrates along the alloy-scale interface. As Mo is less
noble than the other alloying elements, MoO
3
will be reduced to a lower oxide
of Mo or even to Mo. Simultaneously, molten MoO
3
may exert dissolving action
°
C), and PbO (m.p. 888
°
