Transcript STRAY CURRENT CORROSION - Universiti Sains Malaysia

STRAY CURRENT
CORROSION
Stray currents are currents flowing in the
electrolyte from external sources. Any
metallic structure, for example a pipe line,
buried in soil represents a low resistance
current path and is therefore fundamentally
vulnerable to the effects of stray currents
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Stray current tends to enter a buried structure in
a certain location and leave it in another. It is
where the current leaves the structure that
severe corrosion expected.
Overprotection might also occur at a location
where the high current density of stray current
enter a structure.
There are a number of source of undesirable
stray currents, including foreign cathodic
protection installations, dc transit systems such
as electrified railways, subway systems, and
streetcars, welding operations, and electrical
power transmission systems.
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Stray currents, can be classified into
three categories:
1. - Direct currents
2. - Alternating currents
3. - Telluric currents
DIRECT STRAY CURRENT CORROSION
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Direct stray currents come from foreign
cathodic protection systems, transit systems,
and dc high voltage transmission line.
Direct stray current can cause :
1. Anodic interference
2. Cathodic interference
3. Combined interference
Anodic Interference
It is found in relatively close proximity to a buried anode.
At location close to anode the pipeline will pick up current. This
current will be discharged at a distance farther away from the
anode.
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In the current pickup region, the potential
of the pipeline will shift in the negative
region. It receives a boost of cathodic
protection current locally. This local current
boost will not necessarily be beneficial,
because a state of overprotection could be
created. Excess of alkaline species
generated can be harmful to aluminum and
lead alloys.
Cathodic Interference
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Cathodic interference is produced in relatively
close proximity to a polarized cathode.
Current will flow away from the structure in
region in close proximity to the cathode. The
potential will shift in the positive direction
where the current leaves this structure, and this
area presents the highest corrosion damage risk.
Current will flow onto the structurew over a
large area at farther distance from the cathode.
Combined Anodic and Cathodic
Interference
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Current pickup occurs close to an anode, and
current discharge occurs close to a cathodically
polarized structure.
The degree of damage of the combined stray
current effects is greater than in the case of
anodic or cathodic interference acting alone.
The damage in both the current pickup
(overprotection effects) and discharge regions
(corrosion) will be greater.
Controlling Stray Current Corrosion
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In implementing countermeasures against stray
current effects, the nature of stray currents has
to be considered. For mitigating dc interference,
the following fundamental steps can be taken:
Removal of the stray current source or
reduction in its output current.
Use of electrical bonding
Cathodic shielding
Use of sacrificial anodes
Application of coatings to current pickup areas
Use of a drainage bond
Cathodic shielding
Use of
sacrificial
anodes
Stray Current Associated with DC
Transit System
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Stray current due to electrified transit system
might be illustrated from the following figure:
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The rail has been grounded, however remote
from the substation, due to the voltage drop
in rail itself, the rail will tend to be less
negative relative to earth and stray current
flows onto the pipeline.
Close to the substation, the rails are highly
negative relative to earth, and stray current
will tend to leave the pipeline and induce
corrosion damage.
The presence of stray currents can thus can
thus usually be identified when fluctuating
pipe-to-soil potential are recorded with time.
ALTERNATING STRAY CURRENT
FROM POWERLINES
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There are two dominant mechanisms by which
stray currents associated with powerlines
transmssion can be produced in buried
pipelines:
1. Electromagnetic induction
2. Transmission line faults
A voltage is induced in a buried structure
under the influence of the alternating
electromagnetic field surrounding the
overhead transmission line. The effect is
similar to the coupling in a transformer, with
the overhead transmission line acting as
primary transformer coil and the buried
structure acting as the secondary coil.
The magnitude of the induced voltage
depends of the factors such as the
separation distance from the power line, the
relative position of the structure to the
powerlines, the proximity to other buried
structures, and the coating quality.
Such induced voltages can be hazardous to
anyone who comes in contact with the
pipeline or its accessories.
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The second mechanism is one of resistive
coupling, whereby AC currents are directly
transmitted to earth during transmission line
faults. Usually such faults are of very short
duration, but due to the high currents
involved, subtantial physical damage to
coated structures is possible.
TELLURIC EFFECT
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The stray current are induced by transient
geomagnetic activity. The potential and current
distribution of buried structures can be influenced by
such disturbances in the earth’s magnetic fields.
Such effects, often assumed to be greatest
significance in closer proximity to the poles, has been
observed to be more intense during periods of
intensified sun spot activities. In general, harmful
influences on structure are of limited duration and do
not remain highly localized to specific current pickup
and discharge areas. Major corrosion problems as a
direct result of telluric effects are therefore relative
rare.