What is Cathodic Protection?
- What is Cathodic Protection?
Cathodic protection is a procedure used to protect an object from corrosion
making it a cathode. To make a tank a cathode, you have to attach an anode
it. Both have to be in an electrolyte such as earth or water. To protect a
propane tank, you would attach an anode consisting of a 17 pound bar of
magnesium metal packed inside a cloth bag containing 28 pounds of conductive
earth backfill with a wire that is attached to the magnesium and to the
The magnesium metal is a source of electrons for the iron atoms and it is
where d.c. current originates. As long as enough electrons flow from the
magnesium through the wire and the connections to the tank, the tank will
corrode. At the same time, a small amount of direct current flows from the
magnesium metal, through the earth to the tank. The current is less than one
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 -0.85
or greater, the tank will be protected. If the voltage is below -0.85 volts,
partial but not complete protection will exist.
To illustrate what was said above, assume that a new tank is installed and
protected with magnesium anodes. As long as a combined voltage of at least
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 voltage
shifted from -0.50 volts to at least -0.85 volts, the pits should not become
deeper and the tank would be protected from further corrosion. Therefore, if
corrosion has damaged a tank but not caused it to leak, cathodic protection
be installed and the tank does not have to be replaced.
Due to the difference in the oxygen content of the soil around the tank
to the less oxygen in the undisturbed earth under the tank, corrosive
will exist under the tank. If any of the answers to the above questions are
"yes" then the tank will be corroded.
This also applies to buried piping. Because the voltage of underground
cannot be measured with absolute accuracy over 100% of the surface area,
generalizations are made. It is not possible for corrosion engineers to
guarantee complete protection over every square inch of the tank's surface