UFC 3-570-06
JANUARY 31 2003
2-5
INTRODUCTION TO CATHODIC PROTECTION. Cathodic protection is the
prevention of corrosion by making a metal, which would ordinarily behave like an anode
and corrode, behave like a cathode and be free from corrosive attack. Essentially, CP is
predetermining the anode in the corrosion cell, or making a large corrosion cell to
overcome the other smaller corrosion cells. In cathodic protection this is achieved in
one of two basic ways.
The first way is by using the galvanic series (Table 2-2) to select a more
active metal, install that metal in the electrolyte and provide a metallic path. This
method is called sacrificial cathodic protection, or galvanic cathodic protection. The
galvanically more active metal (anode) is installed to sacrifice itself to protect the
structure (cathode). The voltage (and resultant current) is merely the potential
difference of the two different types of metal. The second basic method of cathodic
protection is applying a source of DC current that forces the current to flow from an
installed anode(s) to the structure, causing the entire structure to be a cathode. This
method is called impressed current cathodic protection. A rectifier, solar cell, battery,
generator, or some other DC power supply is installed in the circuit. The selection of
anode material is free from the galvanic series considerations and anodes are chosen
which are economical, or metals which have a small weight loss per ampere year of
current (Table 2-1).
The current required for cathodic protection depends upon the metal being
protected and the environment. The potentials required to determine adequate
protection (criteria) are given in Chapter 6 of this handbook. To achieve these
protective potentials, current must flow from the anode to the structure being protected.
The amount of current required to protect a given structure is proportional to the area of
the structure that is exposed to the electrolyte. Therefore, current requirements are
usually given as current densities in units of amperes or milliamperes (0.001 amperes)
per square meter (foot) of exposed surface. Coatings are dielectric in nature, and to
some degree, isolate the structure from the electrolyte. For coated structures, the
amount of current required is much lower than for bare structures, as only those areas
where the coating has been damaged or has deteriorated require or will receive current.
The coating efficiency is the percent of the structure that is effectively isolated from the
electrolyte. Efficiencies of coatings can vary greatly due to the type of coating, quality of
surface preparation, quality of application, structure handling, structure installation,
backfill techniques, and backfill material used. Coatings efficiencies normally range
from about 80 percent up to 99.7 percent. Current requirements for coated structures
are best determined by actual testing after the structure is installed. The current
densities required for cathodic protection depend on the metal being protected and the
environment. Typical values for the current densities required for cathodic protection of
bare steel structures are shown in Table 2-3. Typical values of the current densities
required for cathodic protection of coated steel structures are shown in Table 2-4.
Table 2-3. Current Requirements for Cathodic Protection of Bare Steel
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