Biomedical Engineering Reference
In-Depth Information
acidic or an alkaline solution depending on the pH value in the local region.
In electrochemical dissolution, depending on the electrochemical potential,
the metal can undergo either transpassive or active dissolution. All forms of
electrochemical corrosion, require the presence of aggressive ionic species
(as reactants, products, or both), which in turn requires the existence of an
aqueous environment capable of stabilizing them.
Stainless and nickel
chromium alloys experience high corrosion rates at
supercritical pressure but not at subcritical temperatures because of transpas-
sive dissolution (Friedrich et al., 1999), where the nickel or iron cannot form
a stable insoluble oxide that protects the alloy. Under supercritical condi-
tions, the acids are not dissociated and ionic corrosion products cannot be
dissolved by the
solution
because
of
the
solvent's
low polarity.
Consequently, corrosion drops down to low values.
Electrochemical corrosion requires the presence of ionic species like
halides, nickel-based alloys, and compounds. These show high corrosion
rates which decrease at higher temperatures. High-pressure water in an
SCW reactor provides favorable conditions for this, but once the water
enters the supercritical domain the solubility and concentration of ionic
species in it decreases, although the reaction rate continues to be higher
because of higher temperatures. The total corrosion reduces because of
decreased concentration of the reacting species. Thus, corrosion in a plant
increases with temperature, reaching a peak just below the critical tempera-
ture, and then reduces when the temperature is supercritical. The corrosion
rate increases downstream, where the temperature drops into the subcritical
region.
At a relatively low supercritical pressure (e.g., 25 MPa), the salt NaCl is
not soluble. Thus, in an SCW a reaction that produces NaCl, the salt can pre-
cipitate on the reactor wall. Sometimes water and brine trapped between the
salt deposit and the metal can create a local condition substantially different
from conditions in the rest of the reactor in terms of corrosion. This is known
as under deposit corrosion.
In general, a reaction environment that is characterized by high density,
high temperature, and high ion concentration (e.g., acidic) is most conducive
to corrosion in an SCW reactor. Rather than the severity of corrosion in
terms of whether the flow is supercritical or subcritical, the density of the
water should be the major concern.
9.8.2 Prevention of Corrosion
According to Marrone and Hong (2008), corrosion prevention in an SCW
unit is broadly classified in four ways: (1) contact avoidance, (2) corrosion-
resistant barriers,
(3) process adjustments, and (4) corrosion-resistant
materials.
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