MIL-HDBK-1004/10
Table 2
Current Requirements for Cathodic Protection of Coated Steel
ENVIRONMENT
MILLIAMPERES PER
SQUARE FOOT (mA/ft)
Pipeline, epoxy or other high performance coating
0.001
-
0.005
Pipeline, reinforced coal tar
0.005
-
0.025
Pipeline, grease coating with wrapper
0.05
-
0.150
Pipeline, asphalt mastic 1/2" thick
0.001
-
0.005
Pipeline, old asphalt, or other deteriorated coating
0.05
-
0.35
Pipeline, old paint coating
0.10
-
0.30
Tank bottoms
0.05
-
2.0
Tanks for cold potable water
0.05
-
2.0
Tanks for cold seawater
0.05
-
4.0
Turbulent cold water or hot potable water tanks
0.30
-
3.0
Steel sheet piling fresh water side
0.10
-
1.5
Steel sheet piling seawater side
0.10
-
2.0
Steel sheet piling soil side
0.05
-
1.0
a) Soil resistivity is the single most important characteristic
used in the design of cathodic protection systems for buried structures. As
will be shown in sections to follow, protective current requirements,
sacrificial anode outputs, and impressed current anode bed resistance are all
dependent upon soil resistivity. Formerly, soil corrosivity and soil
resistivity were considered to be closely related with the highest corrosivity
being associated with low soil resistivity. However, due to frequent
exceptions to this relationship, soil resistivity alone is no longer
considered to be sufficient for establishing soil corrosivity.
b) pH is a measure of the acidity or alkalinity of a water
solution. Soil pH is a measurement of the pH of the water contained in the
soil. pH values range from 0 to 14 with 0 to 7 being acidic, 7 being neutral,
and 7 to 14 being alkaline.
c) Sulfide, sulfate, chloride, and other chemical constituents in
the soil may be important in the design of a cathodic protection system.
Unless experience in the area has shown otherwise, analysis for sulfate,
sulfide, and chloride should be performed on soil samples from the site prior
to system design.
Current Requirement Tests. Current requirements for buried
4.2.2.3
structures are best determined by field tests. In these tests, a temporary
cathodic protection system is installed. Measurements of the currents
supplied and the structure-to-electrolyte potentials achieved are used to
establish the required current to protect the structure.
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