MIL-HDBK-1005/16
12.3.2
Cathodic Protection. To supplement the normal
deficiencies in coating systems, cathodic protection may be
provided to coated surfaces. Two basic systems may be used:
1) galvanic anode cathodic protection and 2) impressed current
cathodic protection. Cathodic protection may be used to protect
interior surfaces of storage tanks, clarifiers, and other
metallic equipment, including rake mechanisms. Buried pipelines
and tanks may also be protected on the soil side exposure with
cathodic protection.
12.3.2.1 Galvanic Cathodic Protection Systems. This form of
cathodic protection utilizes the natural electrical potential
between two metals. Magnesium and zinc are the most common
metals used to cathodically protect steel. Providing a
sufficient amount of magnesium anodes, connected to a steel
pipeline, will provide cathodic protection of that pipeline for
many years.
Most galvanic anode cathodic protection systems for
buried pipelines are installed with the following provisions:
a)
Anodes buried with the pipeline
b)
Test stations, containing wires from the anodes
and pipeline, brought to the ground surface
c)
Dielectric insulation between cathodically
protected pipelines and other structures
The output from the anodes and the electrical potential
of the pipeline should be measured on an annual basis by
certified corrosion specialists or cathodic protection
specialists (persons may be certified by NACE International,
Houston, Texas). The effectiveness of the cathodic protection
systems also requires the protected pipelines remain electrically
isolated from other plant structures.
12.3.2.2 Impressed Current Cathodic Protection Systems. Instead
of using the natural electrical potential difference between two
metals, cathodic protection can be provided by converting
alternating current (AC) to direct current (DC) with a rectifier
unit. By forcing the DC to an anode material, the cathodic
protection will be provided. Impressed current anode systems
will be more complicated than a galvanic anode system. Typical
components for a buried pipeline may include:
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