STRAY CURRENT CORROSION
Stray current refers to the current that flows elsewhere rather than along the intended current path. It is an important cause of corrosion and leakage of underground metal pipelines . Because of the high electrical conductivity of buried steel pipelines, potential differences with the less conductive environment are formed when stray current flows through the pipe effectively creating a corrosion cell. The corrosion caused by stray current is more serious than soil corrosion under normal conditions.
Stray current
stray current has a great effect on corrosion, and so affects the service life and safe use of buried pipelines. Therefore, it is important to mitigate stray current corrosion.
Identification and measurement of DC stray currents should include the following:
- Recording of potentials
- DC line current measurements/recordings
- Line current measurement by current clamp method
- Interference between CP systems
- Comparison of patterns of recordings on the stray current source and on the pipeline itself
Mitigation is key
stray current has a great effect on corrosion, and so affects the service life and safe use of buried pipelines
Sacrificial Anodes Or Bonds
Sacrificial or galvanic anodes may be used to mitigate stray current effects in situations where small current flows or small voltage gradients exist. In effect, a potential gradient field produced by the galvanic anode(s) counteracts the interference current. The effect is a net current flow to the interfered with structure.
Another consideration when using a galvanic anode system to overcome stray currents is the expected life of the anodes. As the anodes dissipate, their resistance to ground increases. The increased resistance reduces the current flow from the anode and decreases the resulting voltage gradients. Sacrificial anodes should be sized to provide a sufficient anticipated life span. As with any other stray current mitigation procedure, the anodes should be placed on an active monitoring schedule.
Galvanic anode drains are commonly used in lieu of bonds where small drain currents are involved. In areas of large current drains, the use of galvanic anode drains would be impractical due to the high consumption rate of the anode material; frequent anode replacement would be required. Galvanic anodes would also be impracticable where voltage gradients are encountered that are greater than those galvanic anodes can produce.
AC Interference Corrosion
Recommendation
- Monitor the current density rather than just AC voltages to determine AC current related corrosion hazards .
- Monitor the AC current density by installing coupon test stations (CTS) along the affected portion of the pipeline . Coupon test stations can be used to measure AC current density and not just current flow in the ground. Also, based on the severity rating of the multiple interacting variables, client should consider installing AC test stations at specific areas.
- The mitigation criteria should put the emphasis on mitigating the AC current density, rather than just the AC voltage.
- Record AC pipe – to – soil potentials along with the DC pipe – to – soil potentials during the annual cathodic protection survey on sections where AC interference threats may exist. This can provide information, should the power transmission company change its operating parameters, or unexpected changes occur between the pipeline and transmission line.
- Request power line loads corresponding to the time of AC pipe – to – soil potential measurement to provide thorough understanding of the interference measurements .
- Measure soil resistivity at locations where AC interference threats may exist. This data can be used with the measured AC potentials to estimate theoretical AC current density for specific locations in the absence of coupons.
