Railway Stray Current Corrosion: Pipeline Protection Methods

Stray Current Corrosion in Pipelines Near Railways

Understanding Stray Current Corrosion

It is difficult to prevent a significant proportion of current from leaving the rails and dissipating into the ground near substations, instead of returning directly to the track. This occurs because the negative terminal of the substation is connected to the rails, which are typically ungrounded.

These stray currents seek paths of least electrical resistance, often utilizing metal pipes such as water or gas pipelines, and telecommunications cable sheaths.

In places where these currents exit the pipes to re-enter the ground, destructive effects manifest themselves due to electrolytic corrosion.

Factors Influencing Corrosion

• These hazards are generally common in DC facilities.
• If alternating current (AC) is superimposed on the direct current (DC), it compounds the corrosive effects.
• Therefore, for pipelines running parallel to the railway track, the most vulnerable areas are in the vicinity of substations.
• These effects can occur even at significant distances from the railway line, potentially hundreds of meters away in moderately conductive ground.

Mitigation Strategies for Stray Currents

Stray currents can be reduced through several methods:

• Isolating the rails from the ground as much as possible.
• Using clean ballast.
• Ensuring a low electrical resistance in the traction return circuit.

Alternatively, pipelines can be protected either directly (using non-metal pipes) or indirectly through various electrical methods:

Indirect Pipeline Protection Methods

1. Electrical Drainage

This method involves making an electrical connection between the pipeline and the rail. This provides stray currents that have entered the pipeline with an alternative path, preventing them from exiting through the pipe-soil interface.

This method has a drawback: it is absolutely necessary that the rail potential is always negative relative to ground at the chosen connection point. Otherwise, if the polarity changes (e.g., during substation decommissioning), there is a risk of current flowing into the pipeline from the ground at that point, causing damage.

To avoid this risk, the output connection is equipped with a device that allows current to flow in only one direction. This is known as a polarized electrical drain.

2. Forced Drainage

This involves connecting an external power source between the pipeline and the rail to raise the pipeline's electrical potential relative to its surroundings. This mitigates the risk of reverse current flow associated with simple electrical drainage.

However, this method can, in some cases, disrupt the functioning of certain track circuits. Therefore, the system explained below (Cathodic Protection) is often preferred.

3. Cathodic Protection

Cathodic protection is a system similar to forced drainage, but the external power source is connected not to the track, but to a sacrificial ground bed (anode) designed to corrode sacrificially. This anode must be of an appropriate material and monitored regularly.

Specifically, the pipeline is connected to the negative terminal of a DC power source, and the sacrificial metal mass (anode) is connected to the positive terminal. This impresses a negative potential on the pipeline relative to the surrounding ground, thereby protecting its metal.

The metal mass connected to the positive pole acts as the anode and bears the brunt of the corrosion. The pipeline, now acting as the cathode, is protected from corrosion, hence the name of the system.