MIL-HDBK-1138
Cast iron and, to a lesser degree, ductile iron
sometimes show the effects of selective leaching, called
graphitization. This is the result of selective leaching of the
iron matrix in the cast iron, leaving a weak, brittle, graphite
structure. This type of corrosion is often seen with buried or
submerged pipelines, sluice gates, and other cast structures in a
WWTP. Graphitization can be readily detected by a soft graphite
structure that can be cut with a knife. The use of protective
coatings and cathodic protection easily controls this form of
corrosion.
8.1.2.7
Erosion Corrosion. Erosion corrosion is usually
the result of movement between a corrosive media and a metal
surface. Most metals are susceptible to erosion corrosion,
depending upon their critical velocity. If particles are
entrained in the fluid stream (grit streams or sludge streams),
erosion corrosion will be accelerated. Typical locations for
erosion corrosion are areas of higher velocities, such as:
a)
Piping systems, especially at elbows and tees
b)
Across valves, especially throttling valves
c)
Pumps
d)
Propellers, mixers, impellers
e)
Orifices and nozzles
f)
Inlets to heat exchanger tubes
Other forms of erosion corrosion are fretting corrosion
and cavitation. Control erosion corrosion by minimizing the
velocities of fluids, selecting materials that are more
resistant, or providing special coatings (such as hard-facing
metals or ceramics) or rubber linings.
Stress Corrosion. This type of corrosion has a
8.1.2.8
number of different names: stress corrosion cracking (SCC),
chloride stress cracking (CSC), caustic embrittlement, and season
cracking. Stress cracking failures can be very sudden, dramatic,
and serious, partly because there are often no outward signs of
failure. Stress corrosion occurs under tensile stress in a
corrosive environment. The stresses can be applied stresses,
residual stresses (from fabrication), thermal stresses, or
localized stresses from welding operations. With stainless
steels, chlorides can be an initiator; with carbon steel, sodium
hydroxide can initiate failure (caustic embrittlement).
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