JANUARY 31 2003
systems overcome all of the shortcomings of galvanic cathodic protection systems,
there are problems. The two main problems are that impressed current CP systems
require a higher level of maintenance and there is a possibility of interference corrosion
on other metallic (foreign) structures. Interference corrosion is the most serious form of
corrosion. When a metallic structure experiences interference corrosion, 9.4 kilograms
(20.7 pounds) of steel is transformed to iron-oxide for every ampere of current which
flows for one year. For a coated pipeline, this current is leaving from the holidays and
the time to failure is extremely short. Whenever signs of interference corrosion are
noted on any scheduled maintenance or leak survey, emergency steps must be taken to
preclude further damage to the structure. See paragraph 7-14 for detailed procedures
on testing for interference.
INTERFERENCE CORROSION CONTROL. Cathodic protection
interference, whether caused by the influence of cathodic protection systems or by other
current sources, can be effectively controlled. Examples of typical corrective actions are
presented in this manual to illustrate some of the methods employed and to show how to
perform field measurements to determine the continuing effectiveness of the corrective
measures. Correction of actual interference problems is beyond the scope of this manual.
When interference is suspected, assistance in correcting the problem can be obtained
through the local Engineering Field Division.
Correcting Interference. One method of correcting interference is to bond
the foreign structure to the protected structure. Thus, both are protected. Figure 5-4
shows correction of an interference problem by bonding. A test station is usually installed
at such a location to either verify the continuity of the bond, or to measure the current
flowing through the bond. Extra wires to each structure allow potential testing of individual
structures using a non-current carrying conductor results in a four-wire test station.
Current measurements are normally taken using a calibrated shunt. Other methods
include using a clamp-on ammeter (or milliammeter), or disconnecting and measuring in
Direct Bonding. Direct bonding
is often not desirable, either because the
existing cathodic protection system cannot supply enough current to protect both
structures, or the foreign structure is not owned by the same organization as the one
supplying the current, and minimization of extra current is desired. In this case, a resistive
bond is installed between the structures and adjusted so that only that amount of current
is supplied to the foreign structure which is required to bring its potential to the same level
as it would have been without the interference present. Figure 5-4 shows such an
Test stations are normally installed where resistive bonds are used in order to
facilitate testing of the corrective action and adjustment or replacement of the resistor.
Direct bonding usually is not possible if the protected structure is well coated and the
foreign structure is bare or poorly coated. Resistors may fail due to substantial