UFC 3-570-06
JANUARY 31 2003
Figure 2-18. Direct Attachment Galvanic (Sacrificial) Cathodic Protection System
Test Lead
Metallic connection
Anode
TANK
Anode
(Cathode)
Current Flow in the Earth
Current Flow in the Earth
Metallic Anode Selected from Galvanic Series (Usually Magnesium or Zinc)
For the protection of underground structures such as pipelines, the anodes
are not usually attached directly to the structure, but are placed in the soil, evenly
distributed a short distance from the pipeline, and are connected to the pipeline by a
wire, usually through a test station. This type of sacrificial anode system is shown in
Figure 2-19.
The application of galvanic anodes is limited by the small potential difference
(normally less than 1 volt DC) that can be obtained. Galvanic systems generally can
only be economically used on small or well-coated structures in low resistivity
electrolytes. Refer to Tables 2-8 and 2-9 for available voltage and estimated output of
various sacrificial anodes in different resistivity electrolytes. Since the amount of
cathodic protection is dependent on the current density supplied to the protected
structure, the electrolyte resistivity determines the amount of current that the limited
voltage will supply. The amount of metal exposed to the electrolyte determines the
amount of current required. Uncoated (bare) structures may require an exorbitant
number of anodes for adequate protection. In higher resistivity electrolytes, the small
anode-structure voltage difference would yield (Ohms law) an extremely small amount
of anode current, hence requiring a large amount of anodes. High purity magnesium
anodes have the highest potential available, but in high resistivity soil there would not be
sufficient current to protect a structure unless it had a very good coating (Table 2-9).
2-29
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