Environmental Engineering Reference
In-Depth Information
accounts for enhanced scale plasticity, even though there exists no direct evidence
to support this. More experimentations are needed in this domain to ascertain
the exact reason for enhanced scale plasticity.
It is to be recognized that the REE phenomenon, which was realized nearly
six decades ago by chance, still continues to allure the scientific community in
search of a unified theory that can explain the effects so far reported in the litera-
ture. Often it is proposed that several mechanisms are operative simultaneously.
6.7 HOT CORROSION
Various Ni-, Co-, and Fe-based alloys used in high-temperature structural compo-
nents, such as superheater supports; superheater and reheater tubes of boilers,
and first-stage nozzles, blades, and vanes of gas turbines in industrial, marine, and
aircraft engines, are often subjected not only to complex thermal and mechanical
stresses but also simultaneously to combustion gases generated through burning
fossil fuels containing small amounts of impurities. The environment in a gas
turbine is not that of a clean combustion gas. The source of the impurities can
be either fuel or intake air. Sodium and vanadium are often present in the fuel
as oil-soluble compounds. Sodium in the air can be present as an aerosol of sea
salt. In seawater, the majority of the sodium is present as sodium chloride, but
approximately 11% is present as sodium sulfate. Sulfur is present in the fuel,
which may account for up to 0.4 wt % in aviation kerosenes even though the
average is near 0.1%. Industrial turbines may burn light oils with similar sulfur
contents; heavy distillates may contain up to 2 wt % of sulfur or more. On thermo-
dynamic considerations, it has been established [54] that sodium chloride is unsta-
ble in the presence of even small concentrations of sulfur in an oxidizing environ-
ment and the following reaction goes virtually to completion:
1
2
O
2
2NaCl
SO
3
Na
2
SO
4
Cl
2
(6.32)
Therefore, when metals/alloys are exposed to combustion environments, de-
position of salts rich in sodium sulfate and/or vanadium compounds on the alloy
or oxide surface is a common experience. Experiences have proved beyond doubt
that the nature of degradation of the metallic component by combustion gases
in the presence of a salt deposit is definitely different from that occurring in the
absence of deposit. The salt deposits are usually liquid or semiliquid at the op-
erating temperature and cause enhanced metal deterioration. This type of severe
attack, which can be catastrophic at times, is referred to as hot corrosion. There-
fore, the process of hot corrosion can be defined as an accelerated attack/degrada-
tion in a high-temperature gaseous environment (containing oxygen, sulfur, alkali
salts, vanadium, and a host of other contaminants) of a metallic material (metal/
