stray current corrosion

8/2/2019 Stray Current Corrosion

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STRAY CURRENTCORROSION

Stray currents are currents flowing in theelectrolyte from external sources. Any

metallic structure, for example a pipe line,

buried in soil represents a low resistancecurrent path and is therefore fundamentallyvulnerable to the effects of stray currents


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Stray current tends to enter a buried structure ina certain location and leave it in another. It iswhere the current leaves the structure thatsevere corrosion expected.

Overprotection might also occur at a locationwhere 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 suchas electrified railways, subway systems, andstreetcars, welding operations, and electricalpower transmission systems.


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Stray currents, can be classified into

three categories:

1. - Direct currents

2. - Alternating currents

3. - Telluric currents


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DIRECT STRAY CURRENT CORROSION

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 interference3. Combined interference


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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. Excessof alkaline species generated can be harmful

to aluminum and lead alloys.


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Cathodic Interference

Cathodic interference is produced in relativelyclose proximity to a polarized cathode.

Current will flow away from the structure in

region in close proximity to the cathode. Thepotential will shift in the positive directionwhere the current leaves this structure, and thisarea presents the highest corrosion damage risk.

Current will flow onto the structurew over alarge area at farther distance from the cathode.


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Combined Anodic and Cathodic

Interference

Current pickup occurs close to an anode, andcurrent discharge occurs close to a cathodicallypolarized structure.

The degree of damage of the combined straycurrent effects is greater than in the case ofanodic or cathodic interference acting alone.

The damage in both the current pickup(overprotection effects) and discharge regions(corrosion) will be greater.


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Controlling Stray Current Corrosion

In implementing countermeasures against straycurrent effects, the nature of stray currents hasto be considered. For mitigating dc interference,the following fundamental steps can be taken:

Removal of the stray current source or reductionin its output current.

Use of electrical bonding

Cathodic shielding Use of sacrificial anodes

Application of coatings to current pickup areas


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Use of a drainage bond


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Cathodic shielding


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Use of

sacrificial

anodes


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Stray Current Associated with DC

Transit System

Stray current due to electrified transit system

might be illustrated from the following figure:


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The rail has been grounded, however remotefrom the substation, due to the voltage drop

in rail itself, the rail will tend to be lessnegative relative to earth and stray currentflows onto the pipeline.

Close to the substation, the rails are highlynegative relative to earth, and stray currentwill tend to leave the pipeline and inducecorrosion damage.

The presence of stray currents can thus canthus usually be identified when fluctuatingpipe-to-soil potential are recorded with time.


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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

ALTERNATING STRAY CURRENT

FROM POWERLINES


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A voltage is induced in a buried structureunder the influence of the alternatingelectromagnetic field surrounding the

overhead transmission line. The effect issimilar to the coupling in a transformer, withthe overhead transmission line acting as

primary transformer coil and the buriedstructure acting as the secondary coil.

The magnitude of the induced voltagedepends of the factors such as the separation

distance from the power line, the relativeposition of the structure to the powerlines,the proximity to other buried structures, andthe coating quality.


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Such induced voltages can be hazardous to

anyone who comes in contact with the

pipeline or its accessories.

The second mechanism is one of resistive

coupling, whereby AC currents are directlytransmitted to earth during transmission line

faults. Usually such faults are of very short

duration, but due to the high currentsinvolved, subtantial physical damage to

coated structures is possible.


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TELLURIC EFFECT

The stray current are induced by transientgeomagnetic activity. The potential and currentdistribution of buried structures can be influenced bysuch disturbances in the earths magnetic fields.

Such effects, often assumed to be greatestsignificance in closer proximity to the poles, has beenobserved to be more intense during periods ofintensified sun spot activities. In general, harmfulinfluences on structure are of limited duration and donot remain highly localized to specific current pickupand discharge areas. Major corrosion problems as adirect result of telluric effects are therefore relativerare.

stray current corrosion

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