10.
Other Protective Methods.
a. Galvanic (or Sacrificial) Anode Cathodic Protection.
(1) Advantages:
(a) Self-powered -- normally do not depend on an outside source of power.
(b) Low maintenance requirements.
(c) Minimum stray current -- probability of interference on other underground structures
minimum.
NOTE: The pipe is the "cathode", which is connected to the "anode" placed along side the metal structure. An
insulated cable connects the pipe (cathode) to the anode. The anode must be composed of a metal listed higher on
the metal activity series list. D-C current flows from the anode through the earth (electrolyte) to the cathode
(pipeline) then back to the anode by way of the insulated cable.
NOTE: The galvanic/sacrificial anodes are installed not along the full length of pipeline but only at points where
additional corrosion protection is needed due to high corrosion probability. This installation will cause points on
the pipeline that are anodic to become cathodic (areas that are already cathodic will remain cathodic), thereby
stopping corrosion.
(2) Limitations:
(a) Low driving voltage.
(b) Relatively low current output capacity.
NOTE: The protective current generated by galvanic anodes depends upon the inherent potential between the
anodes and the structure to be protected.
1 If the structure is made of iron or steel, any metal which is more active (higher) in the
electromotive series can be used as anode material.
2 Materials which are used for galvanic anodes are zinc and magnesium. Magnesium has a
much wider use due to its higher voltage. Aluminum is also used, but primarily where seawater is encountered.
3 In underground applications, these anodes are usually surrounded with a special backfill
mixture. The backfill is usually a mixture of gypsum, bentonite and sodium sulfate. There are a number of
reasons that the backfill serves;
Uniform environment for the anode thereby uniform corrosion.
QM5200
16-8
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