MIL-HDBK-1138
e)
Introduce a third metal into the circuit that is
more anodic than either of the original metals (lower in the
galvanic series). This form of cathodic protection is discussed
in detail in MIL-HDBK-1004/10, Electrical Engineering Cathodic
Protection.
8.1.2.3
Crevice Corrosion. Crevice corrosion is a very
intense form of corrosion that is localized within a small
crevice or shielded area. Crevice corrosion typically develops
in gasketed surfaces, within lap joints, under sludge deposits,
or under bolt heads. The crevice must be wide enough to permit
the fluid to enter but small enough to create a stagnant
condition.
Metals that rely on oxide films for their corrosion
resistance (stainless steels and aluminum) are prone to
developing crevice corrosion. If chlorides are present in the
wastewater (possible in WWTPs near sea-coast installations), they
will tend to accelerate breakdown of the oxide film and lead to
crevice corrosion. The use of Type 316 stainless steel instead
of Type 304 will provide improved resistance to chloride-
initiated crevice corrosion.
Prevent or minimize crevice corrosion using the
following techniques:
a)
Use welded connections instead of bolted
connections.
b)
Use continuous welds instead of stitch welds.
c)
Do not allow solids to settle out in vessels.
d)
Use gaskets that do not wick or absorb fluids.
e)
For buried structures, provide uniform, clean
backfill material.
Pitting Corrosion. This form of corrosion is
8.1.2.4
extremely localized and is very destructive because a deceptively
small area of the metal surface is corroded. However, pitting
corrosion can penetrate the metal wall and lead to leaks.
Pitting corrosion is difficult to predict because the conditions
that initiate pitting may not always be present (they may be
intermittent or may suddenly arise). The rate of material loss
in the pit generally increases as the pitting continues, causing
pit depth to increase rapidly. Pitting corrosion is more easily
initiated in stagnant or low-flow conditions. Often pitting will
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