Stray Current Corrosion
According to the definition, stray current is understood as a partial current that unintentionally flows from the power cables of electrical facilities into the surrounding soil.
Stray currents generated by DC railway systems (e.g. tram or underground systems) are particularly critical because of their negative effect on metal installations in the soil. The track bed facilitates an electrically conductive connection between the tracks and the surrounding soil, so that a portion of the current flowing back into the tracks flows instead into the soil. The track system's polarity (positive terminal to the contact wire and the negative terminal to the tracks) causes the stray current to flow back into the tracks near the rail substation.
On its way through the soil, the stray current makes use of any buried metal installations (e.g. steel reinforcement, pipelines or cable sheaths). At the places where it exits the installations mentioned, the stray current removes material, which in turn can very quickly lead to corrosion damage (pitting).
Recording and accurately assessing stray current activity (pipe/soil potential as evaluation criteria) are almost impossible without the help of state-of-the-art metrology technology. Depending on the measurement results, various measures to control the effects of stray current can be taken, often in combination:
- Drainage connections between the pipeline and the railway system that allow the stray current to return galvanically.
- Cathodic protection systems with potential-regulated rectifiers that produce protective currents to compensate for what are often highly variable stray currents.
- Isolation of the pipeline network by installing insulating joints to reduce longitudinal conductivity and to isolate the earthing system in order to prevent stray currents from spreading over long distances.
- Galvanic discharge of stray currents using AC arresters and limiters.