protective coatings and electrical insulation. These regulations provide
excellent guidelines for the application of cathodic protection to buried and
submerged pipelines. The pertinent sections of these regulations are included
herein as Appendix E.
Due to the safety and environmental consequences of system failure,
there are also increasing numbers of federal, state, and local governmental
regulations regarding the storage and transportation of certain materials that
require corrosion control. Many of these regulations either make the
application of cathodic protection mandatory on existing facilities as a
primary means of corrosion control or allow it to be selected as a means for
the mandatory control of corrosion on new facilities.
Functional Requirements for Cathodic Protection. In order to be
technically feasible, cathodic protection requires that the protected
structure be electrically continuous and immersed in an electrolyte of
sufficient volume to allow the distribution of current onto the structure.
Continuity. Electrical continuity of the structure to be protected
may be through metallic continuity provided by bolting, or welding of the
structure. Continuity is often achieved or insured by means of electrical
connections installed specifically to insure the effectiveness of cathodic
protection. These connections are commonly called "bonds."
Electrolyte. The electrolyte is commonly water or the water
contained in moist earth. The conductivity of the electrolyte is an important
factor in the determination of the need for cathodic protection and in the
design of cathodic protection systems.
Source of Current. Cathodic protection also requires the presence
of a source of electrical current at the proper voltage or potential to
prevent attack on the structure. These sources of current are commonly called
"anodes." As described below, the anodes may be fabricated from an active
metal such as magnesium, or zinc which provides a high potential source of
electrons through corrosion on its surface. The anodes may also be fabricated
from a relatively inert material which has the ability to pass current from
its surface without being consumed at a high rate but which requires the use
of an external energy source to increase the potential of the electrons
supplied to the structure being protected. Anodes made from active metal are
commonly called "sacrificial" or "galvanic" anodes, as the anode material is
sacrificed to protect the structure under protection. The inert anodes are
commonly called "impressed current" anodes as the external energy source is
used to impress a current onto the structure under protection.
Connection to Structure. The anodes must be electrically connected
to the structure through a metallic connection in order to complete the
circuit of the electrochemical cell responsible for the protection of the
Sacrificial Anode Systems. Cathodic protection in the sacrificial
anode system is essentially a controlled electrochemical cell (see Figure 5).
Corrosion on the protected structure is shifted to the anode. The anode is
consumed in the process but is designed and installed so that it is easily
replaced when consumed. Anode life of 10 to 15 years is common. Anode life
is dependent upon the amount of current emitted by the anodes and their size.
If the cathodic protection system is properly designed and installed, and if
it is properly maintained (including periodic replacement of anodes as
necessary), the structure being protected is essentially immune to corrosive
attack and its lifetime is limited by other factors such as mission
requirements or mechanical damage.