MIL-HDBK-1005/16
facilities, the wastewater has a nearly neutral pH (7.0). This
pH is not significantly aggressive to concrete surfaces.
However, most ferrous metals (ductile iron and carbon steel) will
be corroded in this water unless other methods of corrosion
protection are employed.
12.1.2.1 Concrete Structures. When totally immersed in
wastewater, concrete structures are generally uneffected and
seldom require additional protection. However, wherever
wastewater is agitated (overflowing a weir; falling in a drop-box
or manhole), hydrogen sulfide can be released, which creates an
acidic environment. This agitation is more prevalent in the
primary clarifiers and other equipment upstream of the primary
clarifiers (grit basins, headworks, influent sewer lines).
In open structures, the release of hydrogen sulfide to
the atmosphere will not always create a corrosion environment.
However, covers, overhanging structures, or other equipment that
can impede the release of the hydrogen sulfide to the atmosphere
can lead to corrosion of the concrete. This often occurs when
odor control systems are added, covering equipment that generates
the release of hydrogen sulfide. Concrete surfaces in these
situations generally require some form of protection against the
acidic environment generated by the hydrogen sulfide.
12.1.2.2 Metallic Structures. Metallic structures that are
submerged in wastewater will corrode unless they are of alloy
construction. Carbon steel, ductile iron, and cast iron will
corrode in this environment, which can be accelerated in aeration
basins, because of the higher oxygen content present. In most
submerged applications, protective coatings should be provided on
the exterior surfaces of carbon steel, ductile iron, and cast
iron. Equipment made of these materials includes piping, sluice
gates, wall penetrations, and pipe thimbles.
Stainless steel alloys are preferred for immersion
applications. In most applications, the 300-series stainless
steels are preferred.
12.1.3
Atmospheric Exposure. The atmosphere around most
wastewater treatment plants is considered to be rather corrosive
because of the presence of hydrogen sulfide and high humidity,
particularly in confined areas. Often, if the hydrogen sulfide
can readily pass into the atmosphere, little corrosion occurs on
plant equipment. However, if the release of hydrogen sulfide to
the atmosphere is prevented by equipment covers or other
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