Understanding Cathodic Protection
- Understanding Cathodic Protection
Cathodic protection is a procedure used to protect an object from
making it a cathode. To make a tank a cathode, you have to attach an
it. Both have to be in an electrolyte such as earth or water. To protect
isolated coated tank, you would attach an anode consisting of a 17 lb.
magnesium metal packed inside a cloth bag containing 28 Ibs. of
backfill with a wire that is attached to the magnesium and to the tank.
magnesium metal is a source of electrons for the iron atoms and it is
d.c. current originates. As long as enough electrons flow from the
through the wire and the connections to the tank, the tank will not
the same time, a small amount of direct current flows from the magnesium
through the earth to the tank. The current is less than one amp.
Steel has an average voltage of about -0.50 volts referenced to a copper
electrode. Magnesium has a voltage of -1.75 volts. By connecting the two
together, an average voltage is created. If the combined voltage is
or greater, the tank will be protected. If the voltage is below -0.85
partial but not complete protection will exist.
To illustrate what was said above, assume that a new tank is installed
protected with magnesium anodes. As long as a combined voltage of at
volts can be measured all around the tank, it meets the criteria for
protection. If anodes are attached to an old pitted tank, and the
shifted from -0.50 volts to at least 0.85 volts, the pits should not
deeper and the tank would be protected from further corrosion.
corrosion has damaged a tank but not caused it to leak, cathodic
be installed and the tank does not have to be replaced. This also
buried piping. Because the voltage of underground structures can not be
with absolute accuracy over 100%of the surface area, generalizations are
It is not possible for corrosion engineers to guarantee complete
every square inch of the tank's surface area.