MIL-HDBK-1138
appearance of the corroded metal. In most cases, the forms can
be identified with the naked eye, but magnification can help.
Not every form of corrosion is likely to be seen in a WWTP, but
it is helpful to know each form and be able to recognize it in
case it occurs.
The eight forms of corrosion are unique but are often
interrelated:
a)
Uniform corrosion
b)
Galvanic corrosion
c)
Crevice corrosion
d)
Pitting corrosion
e)
Intergranular corrosion
f)
Selective leaching
g)
Erosion corrosion
h)
Stress corrosion
8.1.2.1
Uniform Corrosion. Uniform corrosion is one of
the most common forms of corrosion and probably accounts for the
greatest corrosion loss in the world. It is generally
characterized by an electrochemical reaction that proceeds
uniformly over all exposed surfaces. Uniform corrosion can be
predicted with simple tests, and accurate estimates of equipment
life may be made. It is easily controlled by selection of proper
materials and by use of protective coatings, inhibitors, and
cathodic protection (discussed in par. 8.2). In a WWTP, examples
of uniform corrosion would include unpainted steel tanks and
structures that are allowed to corrode in the atmosphere,
resulting in heavy, flaky rusting of all exposed surfaces.
Galvanic Corrosion. Often referred to as two-
8.1.2.2
metal corrosion, galvanic corrosion involves two or more metals
that are electrically connected together in an electrolyte.
Galvanic corrosion is the second most common form of corrosion.
Whenever two metals are connected together, one behaves as an
anode and is always corroded, while the other behaves as a
cathode and is not corroded. A typical galvanic corrosion cell
is shown in Figure 8. This figure is very similar to Figure 7,
except that the anodes/cathodes are replaced with specific
metals. Which metal becomes the anode and which the cathode
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