UFC 3-570-06
JANUARY 31 2003
current required for cathodic protection is supplied by the corrosion of an active metal.
Sacrificial anode systems depend upon the differences in corrosion potential that are
established by the corrosion reactions that occur on different metals or alloys. For
example, the natural corrosion potential difference of iron referenced to a copper/copper
sulfate reference electrode is commonly found to be between -0.4 and -0.6 volts DC.
The natural corrosion potential of zinc referenced to a copper/copper sulfate reference
electrode is about -1.1 volts. Thus, if the two metals are electrically connected, the
potential difference between the iron and the zinc is approximately 0.5 to 0.7 volts DC,
and the corrosion of the zinc becomes the source of current and prevents corrosion of
the iron cathode. This is illustrated in Figures 18 and 19. Zinc, magnesium, and
aluminum alloys all have potentials that are sufficiently more negative than iron or steel
and may be useful for the protection of those structures in many environments. Other
metals such as copper and copper alloys have a lower potential than iron or steel and
are easily protected by steel (and many other metals). Materials such as aluminum
alloys that have a higher potential than iron or steel are more difficult to protect, but
even aluminum alloys can be effectively protected by magnesium alloys or commercially
pure magnesium. In the process of providing electrons for the cathodic protection of a
less active metal, the more active metal (anode) corrodes. The more active metal
(anode) is sacrificed to protect the less active metal (cathode). The amount of corrosion
is dependent on the metal being used as an anode and is directly proportional to the
amount of current supplied. Another factor is the anode efficiency, which accounts for
the anode's self-corrosion rate and the corrosion rate for the amount of cathodic
protection current. To provide a uniform electrolyte around an anode in soil, maintain
moisture, and lower the resistance of anode-to-earth, a special backfill is used. This
backfill is normally 75 percent gypsum, 20 percent bentonite, and 5 percent sodium
sulfate. The anodes in galvanic cathodic protection systems must be periodically
inspected and replaced when consumed. In many cases, when the sacrificial anodes
have failed, the entire system is replaced with an impressed current system. Sacrificial
anode cathodic protection systems are fundamentally very simple. The simplest
systems consist of an anode fabricated from an active metal such as zinc that is directly
connected to the structure in an area where it will be exposed to the same environment
as the structure being protected. This type of system is widely used in the protection of
ships and waterfront structures.
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