UFC 3-570-06
JANUARY 31 2003
7-14
INTERFERENCE TESTING PROCEDURES. When current flows through an
electrolyte, it follows the path of least electrical resistance. If a metal structure is
immersed in the current-carrying electrolyte, it is likely to present a low resistance path for
the flow of current. At the point where electrons enter the structure from the electrolyte,
corrosion will be accelerated. Where electrons leave the structure, corrosion will be
reduced. This phenomenon is called cathodic interference. Cathodic interference is
commonly encountered on buried structures. Cathodic interference can be detected by
electrical measurements and can be controlled. Cathodic protection sometimes causes
undesirable effects on structures not connected to the protection system. This occurs
because some current is picked up by these "foreign" structures at one point, and in
attempting to return to the source and complete the circuit, is discharged at another
point. Corrosion occurs at the current discharge point. Current can cause corrosion
damage by passing between two structures or across high-resistance joints in the same
structure. Sources of interference currents may be constant or fluctuating. Constant
current sources have essentially constant direct current output, such as cathodic
protection rectifiers and thermoelectric generators. Fluctuating current sources have a
fluctuating direct current output, such as direct current electrified railway systems, coal
mine haulage systems and pumps, welding machines, direct current power systems,
and telluric currents. See paragraph 2-2.4 for explanations and sources of interference
current.
Mitigation can usually be accomplished by installing an electrical bond
between the unprotected structure (at the location of maximum discharge) and the
protected structure. Other mitigation methods include breaking the continuity of the
foreign pipeline, coating the foreign pipeline pickup area, coating the protected pipeline
in the discharge area of the foreign pipeline, and using galvanic anodes to apply current
to the discharge area of the foreign pipeline (and also provide an alternate current
discharge point). See paragraph 5-6 for information on interference corrosion control.
7-14.1
Interference from Cathodic Protection Rectifiers. Cathodic protection
systems are a major source of stray current on other metallic structures. Structures not
electrically connected to the protected structures are considered to be "foreign"
structures." A foreign structure may provide an alternate path for the current flowing
from the impressed current anodes to a protected structure. If this path is of sufficiently
low resistance, significant current flow will occur. Since there is no metallic return path,
the current will discharge from the surface of the foreign structure to the electrolyte to
return to its source, resulting in severe corrosion.
Testing requires cooperation by the owners of the structures involved. Such
cooperation is best effected by a corrosion coordinating committee; all companies
operating underground or underwater structures, and particularly those under cathodic
protection, should be members of such a committee. A list of most existing committees
may be obtained from the National Association of Corrosion Engineers, P.O. Box
218340 Houston, TX, 77218-8340. Interference testing is usually performed when new
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