TM 5-811-7
CHAPTER 2
CATHODIC PROTECTION DESIGN
resistivity increases, the corrosion rate decreases
2-1. Required information.
(table 2-1). Resistivity can be measured either in a
Before deciding which type, galvanic or impressed
laboratory or at the site with the proper instru-
current, cathodic protection system will be used
ments. Appendix A explains the methods and
and before the system is designed, certain prelimi-
equipment needed to complete a soil resistivity
nary data must be gathered.
survey. The resistivity data will be used to calculate
a. Physical dimensions of structure to be pro-
the sizes of anodes and rectifier required in
tected. One important element in designing a
designing the cathodic protection system.
cathodic protection system is the structure's phys-
ical dimensions (for example, length, width, height,
and diameter). These data are used to calculate the
Table 2-1. Corrosivity of soils on steel based on soil resistivity
surface area to be protected.
b. Drawing of structure to be protected. The
Soil resistivity range (ohm-cm)
Corrosivity
installation drawings must include sizes, shapes,
material type, and locations of parts of the structure
0 to 2000
Severe
to be protected.
2000 to 10,000
Moderate to severe
c. Electrical isolation. If a structure is to be
10,000 to 30,000
Mild
protected by the cathodic system, it must be elec-
Above 30,000
Not likely
trically connected to the anode, as figure 1-2
shows. Sometimes parts of a structure or system
U.S. Air Force.
are electrically isolated from each other by insula-
g. Electrolyte pH survey. Corrosion is also pro-
tors. For example, in a gas pipeline distribution
portional to electrolyte pH (see glossary for defini-
system, the inlet pipe to each building might
tion of pH and other terms). In general, steel's
contain an electric insulator to isolate inhouse
corrosion rate increases as pH decreases when soil
piping from the pipeline. Also, an electrical
resistivity remains constant.
insulator might be used at a valve along the pipeline
h. Structure versus electrolyte potential survey.
to electrically isolate one section of the system
For existing structures, the potential between the
from another. Since each electrically isolated part
structure and the electrolyte will give a direct
of a structure would need its own cathodic
indication of the corrosivity. According to NACE
protection, the locations of these insulators must be
Standard No. RP-01, the potential requirement for
determined.
cathodic protection is a negative (cathodic) poten-
d. Short circuits. All short circuits must be
tial of at least 0.85 volt as measured between the
eliminated from existing and new cathodic protec-
structure and a saturated copper-copper sulfate
tion systems. A short circuit can occur when one
reference electrode in contact with the electrolyte.
pipe system contacts another, causing interference
A potential which is less negative than -0.85 volt
with the cathodic protection system. When
would probably be corrosive, with corrosivity in-
updating existing systems, eliminating short circuits
creasing as the negative value decreases (becomes
would be a necessary first step.
more positive).
e. Corrosion history of structures in the area.
i. Current requirement. A critical part of design
Studying the corrosion history in the area can
calculations for cathodic protection systems on
prove very helpful when designing a cathodic pro-
existing structures is the amount of current re-
tection system. The study should reinforce predic-
quired per square foot (called current density) to
tions for corrosivity of a given structure and its
change the structure's potential to -0.85 volt. The
environment; in addition, it may reveal abnormal
current density required to shift the potential indi-
conditions not otherwise suspected. Facilities per-
cates the structure's surface condition. A well
sonnel can be a good source of information for
coated structure (for example, a pipeline well
corrosion history.
coated with coal-tar epoxy) will require a very low
f. Electrolyte resistivity survey. A structure's
current density (about 0.05 milliampere per square
corrosion rate is proportional to the electrolyte
foot); an uncoated structure would require high
resistivity. Without cathodic protection, as electro-
current density (about 10 milliamperes per square
lyte resistivity decreases, more current is allowed to
foot). The average current density required for
flow from the structure into the electrolyte; thus,
cathodic protection is 2 milliamperes per square
the structure corrodes more rapidly. As electrolyte
2-1
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