remains one of the most challenging and potentially
detrimental corrosion mechanisms to manage in pipeline
integrity. It occurs as a result of electrical current flowing
in paths other than the intended path. There are two types
of DC interference: static and dynamic. Static interference
currents are characterised by a constant amplitude and
direction of flow. Dynamic interference currents are
characterised by changing amplitude and direction of flow.
Interference is caused by the collection and discharge of
stray current from the pipe surface into the surrounding
environment (electrolyte). The rate at which corrosion occurs
is a function of the magnitude and duration of charge transfer
according to Faradays Law.
Potential sources of static stray DC interference current
include:
)
Cathodic protection from foreign sources.
)
High voltage DC transmission systems.
Sources of dynamic DC interference current include:
)
DC traction power systems.
)
Industrial plant.
)
Welding operations.
)
Mining operations.
)
Telluric currents.
Mears routinely evaluates the impact of DC interference
for liquid and gas pipeline operators, and has recently been
instrumental in negotiating and implementing a co-operative
investigative and mitigation programme involving DC
interference caused by a metropolitan light rail DC powered
transit system. Mears engineers from the Ohio Project
engineering office, Plain City, developed a four phase
investigation programme that involved: screening piping
infrastructure that was within a 2000 ft corridor of the light
rail traction system; implementing a targeted field testing
programme; identifying pipeline segments that were at risk
of accelerated corrosion; and designing and implementing
remedial measures that are intended to eliminate the
detrimental interference.
A key component of the project was Mears’ ability to
demonstrate the impact of the operation of the light rail
system to the operator of the system and work co-operatively
with them to gain their financial support for the investigative
programme and for the design and implementation of the
remedial measures implemented on the gas system.
Stray current interference may result in coating damage
at the point of stray current collection (pick-up), where the
pH at the metal interface is sufficiently increased and results
in a decrease in coating adhesion (cathodic disbondment),
and can result in electro-endosmosis, where water molecules
migrate through the coating membrane and become entrapped
under the coating film. These mechanisms are shown in
Figure 1. Current discharge from the pipe surface can result in
significant metal loss, as shown in Figure 2.
Methods of mitigation for DC interference currents
include:
)
Consultation with owners and/or Electrolysis
Co-ordinating Committee.
)
Removal or relocation of interfering current source.
)
Providing a metallic return path for the interfering current.
)
Counteraction of the effect of interfering current by
application of supplemental CP.
)
Prevention of the pick up or limitation of the interfering
current.
Mears engineers work with owners and operators to
custom design and implement targeted solutions that achieve
the most cost-effective and technically feasible solution.
A combination of methods is often required to achieve
satisfactory results.
AC interference
Pipelines sharing, paralleling or crossing HVAC transmission line
right-of-ways may be subjected to electrical interference from
capacitive, electromagnetic induction and conductive effects.
Electromagnetic induction is the primary effect of the HVAC
Figure 1.
Cathodic disbondment at stray current pick-up site.
Figure 2.
Stray DC corrosion at stray current discharge site.
38
World Pipelines
/
SEPTEMBER 2014
