Environmental Engineering Reference
In-Depth Information
the process, Mo gets accumulated at the alloy interface; accordingly, a liquid
MoO
2
-MoO
3
mixture (eutectic melting point is 778
C) or a more complex liquid
mixture may be formed, which may penetrate the scale along grain boundaries
to the external oxide surface, where MoO
3
gets evaporated producing a porous
nonprotective oxide.
It is further reported [19] that increased content of nickel decreases the ten-
dency for catastrophic oxidation. Thus, an Fe-Ni-Cr-Mo alloy with 20% Mo and
40% Ni did not demonstrate catastrophic oxidation. However, no suitable expla-
nation has been provided for such observation.
Similar types of attack are also encountered in alloy components when ex-
posed to combustion products or ash from fuel oils containing vanadium. The
combustion products may contain V
2
O
5
,SO
2
-SO
3
, etc., and, with the simultane-
ous presence of NaCl, oxygen, and water vapor, low-melting mixtures of Na
2
SO
4
and (Na
2
O)
x
V
2
O
5
may be formed, which get deposited on the alloy surfaces.
Such a situation leads to accelerated attack and degradation of the constructional
materials. This type of corrosion damage is termed ''hot corrosion,'' to be dis-
cussed in Sec. 6.7.
°
6.4 SEQUENCES IN ALLOY OXIDATION
Morphologies of the scales formed on alloys are time-dependent. The three gener-
ally observed stages in alloy oxidation are presented schematically in Fig. 6.8,
where stages I, II, and III represent transient, steady-state, and break-away oxida-
tion, respectively.
Upon initial exposure of an alloy to an oxidizing environment at a fixed tem-
perature and oxygen pressure, the oxides of essentially every reactive element
(for which
∆
G
values of oxide formation are negative at the temperature and
Figure 6.8
Stages in alloy oxidation; I, transient stage. II, steady state. III, break-away
stage.
