The first step to implementing stray current corrosion counter measures is to identify the root cause(s) of the issue.
Foreign cathodic protection installations; DC transit systems such as electrified railways, subway systems, trams; and High Voltage AC (HVAC) electrical power transmission systems are all sources of undesirable stray currents.
Stray currents can be classified into three categories:
- Direct (DC)
- Alternating (AC)
Pipelines sharing, paralleling or crossing a high voltage AC (HVAC) transmission line right of ways, for example, may be subjected to electrical interference from capacitive, electromagnetic inductive and conductive effects. Electromagnetic induction is the primary effect of the HVAC transmission line on the buried pipeline during normal (steady state) operation. This form of interference is due to the magnetic field produced by AC current flowing in the phase conductors of the transmission line coupling with the pipeline and inducing a voltage.
Conductive interference can also result in hazardous voltages and currents on buried pipelines when the pipeline is in close proximity to the transmission line and an electrical fault conducts current into the soil.
If these electrical effects are great enough during steady state normal operation, a potential shock hazard exists for anyone that touches an exposed part of the pipeline such as a valve, CP test point or other above ground appurtenance of the pipeline.
During steady state, normal transmission line operation, AC current density at a coating defect, above a certain threshold, may cause accelerated external corrosion damage to the pipeline. In addition, damage to the pipe or its coating can occur if the voltage between the pipeline and surrounding soil becomes excessive during a fault condition.
AC mitigation can be designed and installed to decrease induced voltage on the pipeline and simultaneously help prevent AC corrosion.
Similarly, Stray DC current can present corrosion problems. Direct stray currents come from foreign cathodic protection systems, transit systems, and high voltage transmission lines. They can cause anodic interference, cathodic interference and combined interference.
CP System BD offers all services relating to DC & AC Stray Current Mitigation and LFI/EPR in accordance with AS/NZS 4853:2012 so that you can be proactive against the threat of stray current corrosion. These services include site audits, testing, modelling and recommendations as well as design and implementation of mitigation systems.