Environmental Engineering Reference
In-Depth Information
the formation of protective SiO
2
formation and, on the other, to the formation
of trioxides of the respective metals (e.g., MoO
3
,WO
3
), which are highly vola-
tile in nature. Accordingly, the oxidation behavior of such silicides is deter-
mined by the degree to which the coating constituents are oxidized, the rate of
evaporation of the trioxides, and the structure and composition of the reaction
products.
Silicide Pest.
Oxidation of molybdenum disilicide (MoSi
2
) in the temperature
range of 723-873 K exhibits an interesting phenomenon, wherein accelerated
corrosion occurs at an almost linear rate, but the rate decreases rapidly with the
rise of temperature. This phenomenon is termed
silicide pest
, which involves not
only surface oxidation of the disilicide but mainly intercrystalline oxidation of
MoSi
2
, in which each grain eventually becomes surrounded by reaction products.
Ultimately, the silicide coating disintegrates to a voluminous heap of powder, its
individual grain being surrounded by a thin layer of oxidation products. It is
pertinent to note that the process occurs within such a temperature range where
MoO
3
is stable. The pest phenomenon is not restricted to MoSi
2
only, but all
silicides suffer from such limitations in their practical applications. Although the
onset of pest can sometimes by delayed by modification of the coating, it cannot
be prevented. The mechanism proposed for the occurrence of such phenomenon
is preferential intergranular diffusion of the reacting gas coupled with a tempera-
ture-dependent hardening reaction [82].
Degradation Through Reaction with the Environment
In general, the degradation of coatings during their performances at high tempera-
tures follows the same principles and processes by which the corresponding bulk
materials degrade. But the reaction behavior of the coating constituents with the
environment may be different from the bulk homogeneous alloy for a number
of factors. In the previous discussion it was illustrated that the composition and
microstructure of the coatings may change during service due to coating-sub-
strate interaction by diffusional processes and, as a consequence, the corrosion
behavior of the coating is also expected to be altered. Depending on the technique
used in the coating formation, a coating may contain porosities (e.g., in plasma-
sprayed coatings), which permit easy penetration of the reactive constituents of
the gaseous reactant into the coating or even beyond it to the coating-substrate
interface. This may not only result in accelerated corrosion of the coating itself
but may even cause degradation to the underlying valuable alloy, which was
otherwise thought to be well protected.
Vaporization, as well as simple evaporation, may be promoted by chemical
reactions between the coating and the environment. A previously formed protec-
tive oxide layer may dissociate when the environmental temperature is raised or
the pressure is decreased or when the product of oxidation is volatile in nature.
In both cases, changes in environmental pressure affect the vaporization rate. The
