Nace CP 1, Course Manual

Page: 294 / 535

RP0169-2002
NACE International 23
which causes the corrosion, has a source foreign to the
affected structure.
Usually the interfering current is
collected from the electrolyte by the affected structure
from a DC source not metallically bonded to the
affected structure.
9.2.1.1 Detrimental effects of interference
currents usually occur at locations where the
currents transfer between the affected structures
and the electrolyte.
9.2.1.2 Structures made of amphoteric metals
such as aluminum and lead may be subject to
corrosion damage from a buildup of alkalinity at or
near the metal surface collecting interference
currents.
9.2.1.3 Coatings may become disbonded at areas
where voltage gradients in the electrolyte force
current onto the affected structure. However, as
the external coating becomes disbonded, a larger
area of metal may be exposed, which would
increase the demand for a cathodic protection
current. This disbondment may create shielding
problems.
9.2.2 The severity of external corrosion resulting from
interference currents depends on several factors:
9.2.2.1 Separation and routing of the interfering
and affected structures and location of the
interfering current source;
9.2.2.2 Magnitude and density of the current;
9.2.2.3 Quality of the external coating or absence
of an external coating on the structures involved;
and
9.2.2.4 Presence and location of mechanical
joints having high electrical resistance.
9.2.3 Typical sources of interference currents include
the following:
9.2.3.1 Direct current: cathodic protection
rectifiers, thermoelectric generators, DC electrified
railway and transit systems, coal mine haulage
systems and pumps, welding machines, and other
direct current power systems;
9.2.3.2 Alternating current: AC power systems
and AC electrified railway systems; and
9.2.3.3 Telluric current.
9.3 Detection of Interference Currents
9.3.1 During external corrosion control surveys,
personnel should be alert for electrical or physical
observations that could indicate interference from a
foreign source such as the following:
9.3.1.1 Pipe-electrolyte potential changes on the
affected structure caused by the foreign DC
source;
9.3.1.2 Changes in the line current magnitude or
direction caused by the foreign DC source;
9.3.1.3 Localized pitting in areas near or
immediately adjacent to a foreign structure; and
9.3.1.4 Damage to external coatings in a localized
area near an anode bed or near any other source
of stray direct current.
9.3.2 In areas in which interference currents are
suspected, appropriate tests should be conducted. All
affected parties shall be notified before tests are
conducted. Notification should be channeled through
corrosion control coordinating committees, when they
exist (see NACE Publication TPC 11
8
). Any one or a
combination of the following test methods can be used.
9.3.2.1 Measurement of structure-electrolyte
potentials with recording or indicating instruments;
9.3.2.2 Measurement of current flowing on the
structure with recording or indicating instruments;
9.3.2.3 Development of beta curves to locate the
area of maximum current discharge from the
affected structure (see Appendix A); and
9.3.2.4 Measurement of the variations in current
output of the suspected source of interference
current
and correlations with measurements
obtained in Paragraphs 9.3.2.1 and 9.3.2.2.
9.4 Methods for Mitigating Interference Corrosion Problems
9.4.1 Interference problems are individual in nature
and the solution should be mutually satisfactory to the
parties involved.
These methods may be used
individually or in combination.
9.4.2 Design and installation of electrical bonds of
proper resistance between the affected structures is a
technique for interference control. The bond electrically
conducts
interference current from an affected
structure to the interfering structure and/or current
source.
9.4.2.1 Unidirectional control devices, such as
diodes
or reverse-current switches, may be
required in conjunction with electrical bonds if
fluctuating currents are present.
These devices
prevent reversal of current flow.
9.4.2.2 A resistor may be necessary in the bond
circuit to control the flow of electrical current from
the affected structure to the interfering structure.