STRAY CURRENT STRAY CURRENT CORROSIONCORROSION

Stray currents are currents Stray currents are currents flowing in the electrolyte from flowing in the electrolyte from external sources. Any metallic external sources. Any metallic structure, for example a pipe structure, for example a pipe

line, buried in soil represents a line, buried in soil represents a low resistance current path and low resistance current path and

is therefore fundamentally is therefore fundamentally vulnerable to the effects of stray vulnerable to the effects of stray

currentscurrents

Stray current tends to enter a buried Stray current tends to enter a buried structure in a certain location and leave structure in a certain location and leave it in another. It is where the current it in another. It is where the current leaves the structure that severe leaves the structure that severe corrosion expected. corrosion expected.

Overprotection might also occur at a Overprotection might also occur at a location where the high current density location where the high current density of stray current enter a structure. of stray current enter a structure.

There are a number of source of There are a number of source of undesirable stray currents, including undesirable stray currents, including foreign cathodic protection installations, foreign cathodic protection installations, dc transit systems such as electrified dc transit systems such as electrified railways, subway systems, and railways, subway systems, and streetcars, welding operations, and streetcars, welding operations, and electrical power transmission systems.electrical power transmission systems.

Stray currents, can be Stray currents, can be classified into three classified into three categories:categories:

1. - Direct currents1. - Direct currents

2. - Alternating 2. - Alternating currents currents

3. - Telluric currents3. - Telluric currents

DIRECT STRAY CURRENT DIRECT STRAY CURRENT CORROSIONCORROSION

Direct stray currents come from Direct stray currents come from foreign cathodic protection foreign cathodic protection systems, transit systems, and dc systems, transit systems, and dc high voltage transmission line. high voltage transmission line.

Direct stray current can cause :Direct stray current can cause :

1. Anodic interference1. Anodic interference

2. Cathodic interference2. Cathodic interference

3. Combined interference3. Combined interference

Anodic InterferenceIt 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.

In the current pickup region, the In the current pickup region, the potential of the pipeline will shift potential of the pipeline will shift in the negative region. It receives a in the negative region. It receives a boost of cathodic protection boost of cathodic protection current locally. This local current current locally. This local current boost will not necessarily be boost will not necessarily be beneficial, because a state of beneficial, because a state of overprotection could be created. overprotection could be created. Excess of alkaline species Excess of alkaline species generated can be harmful to generated can be harmful to aluminum and lead alloys.aluminum and lead alloys.

Cathodic Interference

Cathodic interference is produced in Cathodic interference is produced in relatively close proximity to a polarized relatively close proximity to a polarized cathode.cathode.

Current will flow away from the structure in Current will flow away from the structure in region in close proximity to the cathode. region in close proximity to the cathode. The potential will shift in the positive The potential will shift in the positive direction where the current leaves this direction where the current leaves this structure, and this area presents the highest structure, and this area presents the highest corrosion damage risk.corrosion damage risk.

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

Combined Anodic and Combined Anodic and Cathodic InterferenceCathodic Interference

Current pickup occurs close to an anode, Current pickup occurs close to an anode, and current discharge occurs close to a and current discharge occurs close to a cathodically polarized structure.cathodically polarized structure.

The degree of damage of the combined The degree of damage of the combined stray current effects is greater than in stray current effects is greater than in the case of anodic or cathodic the case of anodic or cathodic interference acting alone.interference acting alone.

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

Controlling Stray Current Controlling Stray Current CorrosionCorrosion

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

Removal of the stray current source or Removal of the stray current source or reduction in its output current.reduction in its output current.

Use of electrical bondingUse of electrical bonding Cathodic shieldingCathodic shielding Use of sacrificial anodesUse of sacrificial anodes Application of coatings to current pickup Application of coatings to current pickup

areasareas

Use of a drainage bond Use of a drainage bond

Cathodic shielding Cathodic shielding

Use of sacrificial anodes

Stray Current Associated Stray Current Associated with DC Transit Systemwith DC Transit System

Stray current due to electrified transit Stray current due to electrified transit system might be illustrated from the system might be illustrated from the following figure:following figure:

The rail has been grounded, however The rail has been grounded, however remote from the substation, due to remote from the substation, due to the voltage drop in rail itself, the rail the voltage drop in rail itself, the rail will tend to be less negative relative will tend to be less negative relative to earth and stray current flows onto to earth and stray current flows onto the pipeline. the pipeline.

Close to the substation, the rails are Close to the substation, the rails are highly negative relative to earth, and highly negative relative to earth, and stray current will tend to leave the stray current will tend to leave the pipeline and induce corrosion pipeline and induce corrosion damage. damage.

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

There are two dominant There are two dominant mechanisms by which stray mechanisms by which stray currents associated with currents associated with powerlines transmssion can be powerlines transmssion can be produced in buried pipelines:produced in buried pipelines:

1. Electromagnetic 1. Electromagnetic inductioninduction

2. Transmission line faults 2. Transmission line faults

ALTERNATING STRAY CURRENT FROM POWERLINES

A voltage is induced in a buried A voltage is induced in a buried structure under the influence of the structure under the influence of the alternating electromagnetic field alternating electromagnetic field surrounding the overhead surrounding the overhead transmission line. The effect is similar transmission line. The effect is similar to the coupling in a transformer, with to the coupling in a transformer, with the overhead transmission line acting the overhead transmission line acting as primary transformer coil and the as primary transformer coil and the buried structure acting as the buried structure acting as the secondary coil.secondary coil.

The magnitude of the induced voltage The magnitude of the induced voltage depends of the factors such as the depends of the factors such as the separation distance from the power separation distance from the power line, the relative position of the line, the relative position of the structure to the powerlines, the structure to the powerlines, the proximity to other buried structures, proximity to other buried structures, and the coating quality.and the coating quality.

Such induced voltages can be Such induced voltages can be hazardous to anyone who comes in hazardous to anyone who comes in contact with the pipeline or its contact with the pipeline or its accessories.accessories.

The second mechanism is one of The second mechanism is one of resistive coupling, whereby AC resistive coupling, whereby AC currents are directly transmitted to currents are directly transmitted to earth during transmission line faults. earth during transmission line faults. Usually such faults are of very short Usually such faults are of very short duration, but due to the high currents duration, but due to the high currents involved, subtantial physical damage involved, subtantial physical damage to coated structures is possible.to coated structures is possible.

TELLURIC EFFECTTELLURIC EFFECT The stray current are induced by transient The stray current are induced by transient

geomagnetic activity. The potential and geomagnetic activity. The potential and current distribution of buried structures can current distribution of buried structures can be influenced by such disturbances in the be influenced by such disturbances in the earth’s magnetic fields. Such effects, often earth’s magnetic fields. Such effects, often assumed to be greatest significance in closer assumed to be greatest significance in closer proximity to the poles, has been observed to proximity to the poles, has been observed to be more intense during periods of intensified be more intense during periods of intensified sun spot activities. In general, harmful sun spot activities. In general, harmful influences on structure are of limited influences on structure are of limited duration and do not remain highly localized duration and do not remain highly localized to specific current pickup and discharge to specific current pickup and discharge areas. Major corrosion problems as a direct areas. Major corrosion problems as a direct result of telluric effects are therefore relative result of telluric effects are therefore relative rare.rare.