Stray current corrosion

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When stray current corrosion refers to a form of electrochemical corrosion . The cause of stray current corrosion are electrical currents that are part of the operating currents from systems operated by direct current and are referred to as stray currents, more rarely as stray currents.

Basics

Stray currents flow back to their power source on undesired paths . In addition to the ground, such paths primarily include systems laid underground as metallic structures such as pipelines, tank containers, reinforced cable jackets and reinforced concrete structures, which are consequently subject to a risk of corrosion from stray currents and also affect the rails of direct current railways. This risk arises from:

The leakage current is made possible by an electrical (often resistive) connection between the influencing DC system and the affected installation. Such electrical connections can be:

  • Electrode as an impressed current anode in a KKS system
  • Running rails of a direct current railway that have a finite value of the bedding resistance to earth or building earth
  • Electrode of an HVDC system
  • by mistake

Electrochemical process

Stray current corrosion takes place in places where the stray current passes from the metal into the ground as an electrolyte solution . Electrochemically, this process is referred to as the anodic partial reaction :

The electrons remain in the metal lattice, while the positively charged metal ions (mostly iron, steel) go into solution. The cathodic partial reaction takes place in the opposite direction when the stray current enters :

No corrosion takes place here. If there is a lack of oxygen (in anaerobic soils), additional hydrogen is produced. Both partial reactions always take place at the same time, but can be spatially far apart. Faraday's laws give information about the theoretical amount of material removal with 9.13 kg for iron per ampere in one year. Corrosion processes also depend on the pH value of the electrolyte solution. Conclusions about the risk of corrosion can be described with the Nernst equation and Pourbaix diagrams .

Description of the influence

Affected objects

Potential limit values ​​are specified to protect affected systems. A distinction is made between cathodically protected and non-cathodically protected systems.

Cathodically protected systems

Stray current corrosion is to be expected if the object / soil potential on cathodically protected systems becomes more positive than the minimum protection potential of −850 mV. In the case of potential fluctuations due to stray currents that change over time, the daily mean value of the object / ground potential must be used. In unfavorable soil conditions (acidic soils with a low pH value), the minimum protection potential is even −950 mV. Cathodic protection is only guaranteed if there is an ingress of current at all faults in the cathodically protected system.

Systems not cathodically protected

There is also the risk of stray current corrosion on systems that are not cathodically protected. Limit values ​​for anodic influence (positive potential shift) are also specified for them.

Material of
the system
 
specific resistance ρ of
the electrolyte
in Ωm
maximum positive
potential
shift Δ U in mV with IR component
maximum positive
potential
shift Δ U in mV without IR component
Steel, cast iron <15 20th 20th
15 to 200 1.5 • ρ / [Ωm] 20th
> 200 300 20th
lead   1 • ρ / [Ωm]  
Steel in concrete   200  

The limit values ​​depend on the type of metal and the specific soil resistance. They apply to constant as well as fluctuating stray currents. The IR component is an ohmic voltage drop in the ground caused by galvanic element currents, protective or stray currents.

Influencing DC systems

Cathodic corrosion protection systems

Cathodically protected pipelines with a bitumen coating have a lower coating resistance than plastic-coated pipes. In order to achieve the necessary protective potential, higher protective currents are necessary and can only be achieved with an external current anode system. There is a risk that the protective current will adversely affect external, buried installations as stray current. As a result of the formation of cathodic and anodic areas, part of the protective current, in its effect as stray current, passes from the unprotected pipeline into the cathodically protected pipeline in the approach area. This is an influence that is constant over time.

Principle of stray current corrosion through a cathodic corrosion protection system

DC railways

Stray current corrosion can be a problem, especially with direct current railways , since the running rails are mostly used to return traction current and are only insulated to a limited extent from earth or building earth. The rail return current causes a longitudinal rail voltage drop between the rail vehicle and the substation. As a result, there is a rail potential against reference ground or building ground, which is the cause of stray currents. In areas with a positive rail potential, the stray current is transferred from the running rails into the ground or onto an object laid in the ground, e.g. B. as a pipeline to the railway line running parallel or crossing. In areas with a negative rail potential, the stray current is transferred from the pipeline or from the ground back into the running rails. Anodic areas with the risk of stray current corrosion and cathodic areas are formed on both objects.

Principle of stray current corrosion by a direct current path

Rail vehicles are portable loads. Therefore, due to accelerations, stationary driving and braking processes including the feedback, there are constant rail potential changes with polarity changes. With direct current railways, the positive pole is usually on the contact line and the negative pole on the rail. In the area of ​​the substation, the rail potential is more negative on average over time and more positive at half the distance between two substations. If the polarity is reversed on the overhead contact line and rail, the anodic and cathodic areas also change because the direction of the current is reversed. These circumstances must be taken into account when using directional stray current discharges or Soutiragen.
The stray currents that occur through direct-current railways are referred to as influencing that fluctuates over time. The level of the stray current depends on two parameters:

  • the rail potential (voltage running rails - structure)
  • the discharge layer as the reciprocal of the length of the bedding resistance of the running rails

There are two requirements for operators of direct current railways to protect against stray current corrosion:

  • the corrosion on the running rails should not reduce the intended service life as determined by the driving operation
  • no negative influence on the building earth and foreign installations that are metallically laid in the earth

The running rails are intended for an operational service life of around 25 years. So that this period is not reduced by stray current corrosion, a maximum current of 2.5 mA / m per track applies. Based on a potential shift of 1 V in the positive direction, the following maximum permissible discharge coverings are obtained for a track:

  • 0.5 S / km in open bedding
  • 2.5 S / km in closed bedding

In the case of a double-track line, the values ​​for discharge surfaces double.

HVDC systems

Similar effects can occur on HVDC systems in the vicinity of the grounding electrodes .

measuring technology

Stray current corrosion takes place at the phase interface of the metal in the ground or a comparable medium as an electrolyte solution and cannot be observed directly. A functional impairment up to complete failure of the component or the system is therefore difficult to predict. In order to be able to assess the risk of corrosion due to stray currents, potential measurements are carried out. Furthermore, stray currents cannot be measured directly. Copper / copper sulfate electrodes
are preferred for corrosion protection measurement technology . They cannot be polarized and have an equilibrium potential of +320 mV compared to the standard hydrogen electrode . They can be buried in the ground as a permanent reference electrode or used as a portable reference electrode for installation on the surface. Since a measurement at the metal / electrolyte solution phase boundary is also not possible, the object / soil potential is of great importance for cathodic corrosion protection and for protective measures against the corrosive effects of stray currents.

remedy

  • Conversion of the system to alternating current (usually hardly practicable for railways, since the electrical equipment of the vehicles would have to be fundamentally changed and scattering alternating currents can have an unfavorable effect on signaling and telecommunications systems)
  • No use of the running rails for traction current return (2nd overhead contact line or 2nd conductor rail). Rarely used variant, because of increased technical effort and increased risk of short circuits in points with two-pole overhead lines. One example is the London Underground with a second busbar between the tracks.
  • Use of fully welded rails or bridging brackets at the joints
  • Track bed with sufficiently high bedding resistance between the running rails and earth or building earth
  • The lowest possible rail potential of the running rails against reference ground or building ground
  • Electrically conductive through-connection of reinforced concrete structures (tunnels, bridges or company buildings) as well as metallic installations (e.g. bus stop facilities or fences) to earthing systems along the railway line
  • metal-free parts or corrosion-resistant metals (if possible and economical)
  • Sheathing of pipelines with plastic coatings instead of bitumen as a passive corrosion protection measure
  • Exclusive construction of bipolar HVDC systems
  • Placement of the grounding electrodes of HVDC systems far away from places where metallic parts are in the ground

literature

  • Friedrich Kiessling, Rainer Puschmann, Axel Schmieder: Contact lines for electric trains. Planning - calculation - execution - operation. 3. Edition. Publicis Publicing Verlag, Erlangen 2014, ISBN 978-3-89578-407-1 , p. 390.
  • Hans-Burkhard Horlacher, Ulf Helbig (Hrsg.): Pipelines 2. Use - laying - calculation - rehabilitation. 2nd Edition. Springer Vieweg, 2018, ISBN 978-3-662-50354-6 .
  • G. Wranglen: Corrosion and Corrosion Protection . Basics - processes - protective measures - testing. Springer Verlag, Berlin / Heidelberg 1985, ISBN 3-540-13741-6 .
  • Ulrich Bette: Measures to reduce the risk of corrosion due to stray currents on grass tracks of DC railways - Research report FE-No. 70348/90 . Wuppertal 1993.

Norms

  • EN 50162 (VDE 0150): 2004-08, protection against corrosion caused by stray currents from direct current systems
  • EN 50122-2 (VDE 0115-4): 2010-10, Railway applications - Fixed installations - Electrical safety, earthing and return line - Part 2: Protective measures against the effects of stray currents from DC railways

Guidelines

  • DVGW worksheet GW 21 (same text as AfK recommendation no. 2): 2014-02 Influence of underground metallic systems by stray currents from direct current systems .
  • VDV -Schrift 501-1 and -2: 1993-04 Reduction of the risk of corrosion due to stray currents in tunnels of direct current railways with current return via rails .

Web links

Individual evidence

  1. Ulrich Bette, Markus Büchler: Pocket book for cathodic corrosion protection . 9th edition, Vulkan Verlag, 2017, ISBN 978-3-8027-2867-9 , pages 35 and 36.
  2. DIN EN 50162: 2004-08, Protection against corrosion caused by stray currents from DC systems, table 1, page 9