MIL-HDBK-1138
Section 8:
CORROSION CONTROL
8.
8.1
Causes of Corrosion. Most corrosion present
around a WWTP is called aqueous corrosion because it occurs in a
wet or damp environment. Other forms of corrosion, such as high
temperature corrosion, are not usually found in a WWTP. Aqueous
corrosion can occur on metal surfaces that are a) submerged in
water, b) buried in the earth, or c) wet from surface moisture,
such as rain water, condensation, or high humidity. The rate at
which the corrosion occurs depends on the amount of moisture
present and what chemicals or other contaminants are present in
that moisture. Aqueous corrosion is also called electrochemical
corrosion.
8.1.1
Electrochemical Corrosion. Electrochemical
corrosion derives its name from the electrical current
("electro") that is flowing and the chemical reactions that occur
at the same time. The flow of current is critical, since the
amount of current flowing in a corrosion cell will affect the
amount of metal that will be corroded. Corrosion currents are
direct currents (DC) controlled by Ohm's Law. Ohm's Law states
that the voltage (E, volts) is equal to the current (I, amperes)
times the resistance (R, ohms). The current flow will be
directly affected by the resistance of the corrosion circuit.
Wastewater is more conductive than distilled water; therefore, a
corrosion cell in wastewater will be more active than in
distilled water.
For an electrochemical corrosion cell to exist, four
elements must be present: a) an anode or corroding surface;
b) a cathode or noncorroding surface; c) a metallic connection
between the anode and cathode surfaces; and d) a common
electrolyte, which contains both the anode and cathode surfaces.
The anode/cathode surfaces can exist on the same piece of metal
and can be very close to each other or at a great distance from
each other. The connection, in a single piece of metal, is the
metal itself. The typical electrochemical corrosion cell is
shown on Figure 7. The electrolyte can be any media capable of
conducting an electrical current. If the metal is buried, the
electrolyte is the soil; if the metal is submerged in wastewater,
the wastewater acts as the electrolyte.
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