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
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.
Stray currents, can be classified into three categories: 1. - Direct currents 2. - Alternating currents 3. - Telluric currents
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 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.
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 • 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 • 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 • 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
Stray Current Associated with DC Transit System • Stray current due to electrified transit system might be illustrated from the following figure:
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 • 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. • 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 • 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.