Environmental Engineering Reference
In-Depth Information
alloy) whose surface is coated by a thin salt film. Turbine manufacturers and
users became aware of such aggressive corrosion processes in the late 1960s. In
this regard, Stringer [55] has presented an excellent review of the phenomenology
and proposed mechanisms. In most early cases of severe hot corrosion, the salt
deposits were analyzed and found to contain a substantial amount of sodium
sulfate (Na 2 SO 4 ) with different amounts of Ca, Mg, Pb, V, Zn, and chloride ion;
however, the compositions also included a mixture of calcium, sodium, and mag-
nesium sulfates. Vanadium, often present in fuel oils as metal-organic complexes
(porphyrins) whose concentration may reach as high as 500 ppm in heavy distil-
lates and residues, can also condense as molten vanadium oxide, typically on the
vanadium-rich oxide of V 2 O 5 . In industrial applications there may be situations
where the salt deposits consist of a mixture of sulfates and vanadates. Experience
has shown that the combined vanadate-sulfate attacks can be more serious than
individual attacks. Vanadates containing 10-20% Na 2 O are reported to be most
corrosive. Sodium vanadates are formed by the reaction of Na 2 SO 4 and V 2 O 5 .
But the exact composition of the vanadates may vary as (Na 2 O) x V 2 O 5 , depending
on the conditions. The molten vanadates are capable of fluxing most metal oxides,
allowing rapid diffusion of oxygen through the melt to the metal surface. The
combined attack of vanadate-sulfate deposits causing serious damage to the com-
ponents is probably due to the fact that vanadate serves as an effective fluxing
agent while the sulfate reacts according to the sulfate-sulfide mechanism. The
presence of vanadium principally modifies the nature of sodium sulfate, rendering
it more acidic so that both forms of corrosion may proceed simultaneously and
interactively. This type of highly accelerated corrosion process is often termed
vanadate-induced hot corrosion .
Furthermore, accelerated corrosion induced by other salts, such as molten al-
kali carbonates and nitrates, has also been experienced in special applications
such as molten carbonate fuel cells and solar heat exchangers at and above 873 K,
and the involved processes have been explained in the light of general mechanism
suggested for hot corrosion.
Basically, the essential ingredients for hot corrosion processes are (1) elements
in an alloy for oxidation, (2) species in the gas phase for reduction, and (3) a
salt deposit on the surface of the alloy capable of influencing the oxidation-
reduction processes.
Even though this type of corrosion takes place in the presence of various types
of salt deposits, in many practical situations it is caused by sulfates, e.g., Na 2 SO 4 ,
or mixtures of sulfates. Therefore, in the present context, it is neither the intention
nor the desire to account for such corrosion processes in the presence of various
salt deposit compositions, and accordingly the present discussion is restricted to
Na 2 SO 4 -induced processes. Furthermore, with respect to materials, the gas tur-
bine manufacturers normally choose Ni- or Co-based superalloys (for composi-
tions, refer to Tables 6.4 and 6.5), at times with oxidation-resistant coatings, and
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