MIL-HDBK-1138
Prevent stress corrosion by providing stress relief,
using more resistant materials, applying cathodic protection, or
using inhibitors. Corrosion fatigue is another form of stress
corrosion that occurs after repeated, cyclic stresses. In WWTPs,
stress corrosion can occur in sodium hydroxide facilities or in
incinerator units without proper stress relief.
8.2
Control and Minimization of Corrosion. With a
basic understanding of electrochemical corrosion, it is easier to
understand how various methods of corrosion control work. One of
the primary methods to control corrosion is to isolate the
structure from its environment (electrolyte) by applying
protective coating systems (the terms "coating" and "painting"
will be used interchangeably in this document; both refer to
high-performance products). However, even with a high-quality
coating system, some deficiencies can exist, requiring the
application of cathodic protection to certain structures. The
combination of good protective coatings and adequate cathodic
protection can provide good, long-term performance in the
aggressive environment of a WWTP.
8.2.1
Protective Coatings. The most important component
of a coating is the vehicle, or organic polymer base. This is
the material that forms the continuous film over the substrate
and protects against corrosion. Common polymer bases include
alkyd, epoxy, polyurethane, acrylic, and silicone. These
vehicles may be modified with other materials to form
combinations, such as coal-tar epoxy. See MIL-HDBK-1110, Paints
and Protective Coatings for Facilities, for more information on
selection and application of protective coatings.
When dealing with coating systems in an existing WWTP,
it is very important to know the generic vehicle type used in the
coating. Compatibility between coatings is critical.
Incompatible coatings can lead to failures. Generic coatings
commonly used in WWTP include the following:
a)
Coal-tar Epoxy. Used extensively in immersion
applications in wastewater and mild chemical exposures, coal-tar
epoxy is generally considered to be a self-priming material,
applied in two coats of 8 mils (0.008 inches) each.
b)
Polyamide Epoxy. Available as a primer,
intermediate, or finish coats, Polyamide epoxy is used in more
aggressive environments. Primers are generally about 2 mils
thick; intermediate coats may be 4 to 6 mils thick; finish coats
2 to 4 mils thick. Epoxy coatings tend to chalk when exposed to
sunlight.
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