Solution: Pipeline protection

Corrosion is always caused by potential differences between a metallic material (e.g., a pipe) and its surroundings. It is an electrochemical process in which material is removed from the metal surface. Passive protective measures such as casings or coatings do not provide sufficient protection in practice, as even the smallest damage or scratches in the casing can lead to severe corrosion damage. Therefore, comprehensive protection is only achieved by combining passive and active protective measures, such as cathodic corrosion protection.

By applying an electrical voltage between the metal and the ground, the corrosion process can be actively influenced. Cathodic corrosion protection causes a potential reduction, thereby reducing the corrosion removal rate to a technically negligible value (approx. 0.01 mm per year).

Corrosion due to galvanic action, similar to corrosion caused by ventilation elements, is caused by different resting potentials at various locations. Unlike ventilation elements, however, the potential differences in contact elements are not due to different soils but to different metallic materials. Pipelines that are metallically connected to the steel reinforcement of concrete structures are particularly affected by contact element corrosion.

Steel in concrete has a potential that is several hundred mV more positive than that of pipes buried in the ground. Due to the large difference in surface area between the reinforced concrete structure and the small defect in the pipe, severe corrosion damage can occur within just a few years. Corrosion rates of up to 1 mm per year are possible.

Stray currents are caused by direct current (DC) systems, such as streetcars, subways, or buses, and can cause significant damage to underground metal structures. Rails are electrically conductive and connected to the ground via the track ballast. As a result, a portion of the current flowing back through the rails enters the ground as stray current. Since current always follows the path of least resistance, it utilizes metal installations such as pipes, steel reinforcement, or cables as it travels through the ground.

Due to the polarity of the railway power supply (positive pole on the contact wire and negative pole on the rail), the stray current primarily occurs near railway substations and causes material erosion there, which can very quickly lead to pitting corrosion.

Pipelines that, due to local conditions (e.g., mountain valleys, dense development), run for long distances under or near high-voltage overhead lines or railway facilities are affected by alternating currents. These induced alternating voltages not only cause severe corrosion damage to the pipeline but also dangerously high touch voltages that require special protective measures. Therefore, in the case of both stray current corrosion and alternating current corrosion, it is necessary to install surge arresters on the affected pipelines in addition to active corrosion protection. Only in this way can the risk to both people and equipment be minimized.