Practical Application of Cathodic Protection

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Since cathodic protection depends on the energy of electrons and
their tendency to flow only from an area of high (negative) potential to one
of lower (negative) potential, the principle of cathodic protection can also
be demonstrated through a hydraulic analogy (see Figure 4).  In this analogy
the surge tank is the metal to be protected.  Flow from the surge tank is
prevented by coupling the tank to a supply of water at higher pressure,
leaving the tank full.
Practical Application of Cathodic Protection.  Cathodic protection
is only one of many methods of corrosion control.  Cathodic protection should
be evaluated as one alternative method to control corrosion in an overall
corrosion control program.  Application of cathodic protection should be
evaluated on the basis of technical feasibility, economic analysis, and system
functional requirements such as reliability and consequence of failure.  In
some cases (e.g., underground pipelines), field experience has shown that
cathodic protection is such an effective means of providing the required
levels of safety in the operation of the systems that cathodic protection is
required by Federal regulation.
When Cathodic Protection Should Be Considered.  Cathodic protection
should be considered, possibly in conjunction with other forms of corrosion
control such as the application of protective coatings, wherever the system is
exposed to an aggressive environment in such a manner that cathodic protection
is technically and economically feasible.
Where Feasible.  Cathodic protection is primarily feasible when the
surfaces to be protected are buried or submerged.  External surfaces of buried
metallic structures, surfaces of metal waterfront structures such as sheet
pilings or bearing piles, and the internal surfaces of tanks containing
electrolytes such as water are applications where cathodic protection is
usually technically feasible and is commonly utilized in protecting such
structures.  Cathodic protection has limited applicability on internal
surfaces of small diameter pipelines and other areas where ion flow in the
electrolyte is restricted by electrolyte resistance.
When Indicated By Experience.  When construction of a new buried or
submerged system is being planned, the corrosivity of the environment should
be considered as one of the factors in the design of the system.  If
experience with similar systems in the vicinity of the construction site has
shown that the site conditions are aggressive based upon leak and failure
records, cathodic protection should be provided as a means of controlling
corrosion on the new system.  Cathodic protection is one of the few methods of
corrosion control that can be effectively used to control corrosion of
existing buried or submerged metal surfaces.  Thus, if leak records on an
existing system show that corrosion is occurring, cathodic protection may be
applied to stop the corrosion damage from increasing.  Cathodic protection
can, however, only stop further corrosion from occurring and cannot restore
the material already lost due to corrosion.
As Required By Regulation.  Regulations by the Department of
Transportation (DOT) have established standards for the transportation of
certain liquids and compressed gas by pipelines in order to establish minimum
levels of safety.  These regulations require that these pipelines be protected
by cathodic protection combined with other means of corrosion control such as


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