Despite a relatively mature technology for its control, corrosion caused by stray current from electrified rapid-transit systems was estimated in 1995 to cost the USA approximately $500 million annually. Part of that cost is the result of corrosion of the electrified rapid-transit system itself, and part is the result of corrosion on neighboring infrastructure components, such as buried pipelines and cables. (reference)
Stray current induced corrosion damage has been associated with North American DC rail transit systems for more than a century. In the USA alone, there are more than twenty transit authorities operating electrified rail systems in major urban centers. Stray current corrosion problems continue to affect several North American cities where the transit systems are typically installed in high-density urban areas. Obviously such urban areas are associated with underground cables and piping (water and gas) systems, that can also be highly susceptible to this form of corrosion damage. (reference)
Solidly grounded systems were historically used on older transit systems, and used the "tie everything together and let the current flow" philosophy of the late 1800s. The major characteristics of a solidly grounded system are direct metallic connection of the AC rectifier negative buses to the earthing mats at the substations and the absence of insulation on the running rails. Such a design allows stray current to flow totally unrestricted between the rectifier negative bus and any available underground metallic path. Consequently, stray-current corrosion occurs frequently on the transit rails, rail fasteners, tunnels, bridges and other transit structures. The only advantage of a solidly grounded system is that the negative return voltage is at the same voltage as the earth ground, which eliminates the hazard of having electric potentials develop between station platforms and the earth ground. Electric potentials can vary from zero to 150 volts and can represent a hazard for passengers. (reference)
Ungrounded systems represent the other extreme of traction power system design. An ungrounded system has no direct metallic connection between earth and the rectifier bus at the substations. Rail fastener insulation is also important so that high, rail-to-earth resistances are maintained. In theory, stray currents from an ungrounded system should be low as long as rail shorts are not allowed to develop along the line. Practically, however, because of the thousands of fasteners in parallel on the system, an earth ground does exist. In addition, special trackwork is often difficult to isolate completely, and represents areas where grounding can occur. The one disadvantage of an ungrounded system is that sufficiently high electric potentials can develop between platforms and earth ground.
Diode-grounded systems represent a compromise between a solidly grounded and ungrounded system. They are often used to eliminate the problems of stray-current corrosion from a solidly grounded system, but also to keep electric potentials to a safe level. Diode-grounded systems contain a direct metallic connection of the rectifier bus to the earthing mats at substations, but through a diode circuit. The diode circuit allows current to flow from the earthing mat to the negative bus when a certain threshold voltage is reached. The threshold can be as low as 10 volts or as high as 50 volts depending on the conditions at the substation. In this way, electric potentials are dissipated and not allowed to build up to unsafe levels. Stray-current corrosion can still occur on diode-grounded systems, especially on the rails and rail fasteners where low rail-to-earth resistances are seen. In addition, because of the diode-ground circuit path, the return rails periodically discharge current when a threshold voltage is exceeded. It has been observed that a rail designed for 35 years life on a diode-grounded transit system had to be replaced in seven years due to stray-current corrosion and rail cracks.
Stray current problems stem from the fundamental design of electrified rail transit systems, whereby current is returned to substations via the running rails. The ground surrounding the rails can be viewed as a parallel conductor to the rails. The magnitude of stray current flow in the ground conductor will increase as its resistivity decreases. Any metallic structure buried in ground of this nature will tend to “attract” stray current, as it represents a very low resistance current path. The highest rate of metal dissolution occurs where the current leaves the structure and undesirable overprotection effects can occur at the points of current pick-up. (reference)
The stray currents tend to be very dynamic in nature, with the magnitude of stray current varying with usage of the transit system and relative position and degree of acceleration of the electrified vehicles. Fundamentally, the following factors all have an effect on the severity of stray currents: magnitude of propulsion current, substation spacing, substation grounding method, resistance of the running rails, usage and location of cross bonds and isolated joints, track-to-earth resistance and the voltage of the traction power system. At a particular location on an affected structure, the presence of stray currents can be identified when fluctuating pipe-to-soil potentials are recorded with time. The older DC transit systems generally produce the worst stray current problems due to the following factors:
In modern system designs stray current problems are ameliorated with two fundamental measures:
(i) Decreasing the electrical resistance of the rail return circuit and
(ii) Increasing the electrical resistance between the rails and ground.
The first measure makes current return through the ground less likely. Steps taken in this direction include the use of heavier rail sections, continuously welded rails, improved rail bonding and reduced spacing between substations. It is desirable to combine substations with passenger stations. At passenger stations current flow is highest due to acceleration of trains. This combination ensures that these peak currents have a very short return path. The rail to soil resistance can be increased by using insulators placed between the rails and concrete or wooden ties and by using insulated rail fasteners. Stray current concerns are particularly relevant when older rail systems are integrated with newer designs. The higher current demand of modern, high-speed vehicles poses increased stray current risks in the older sections. (reference)
On the object of stray current corrosion see also: DC traction, Cathodic protection, Coating, Contour plots, Definition, Detection, Examples, External currents, Historical perspective, Impressed current, Interference, Mechanisms, Modeling. Pipeline, Potential distribution, Prevention, Stray fields and leakage, Transit systems
M. Szeliga, Stray-Current Corrosion: The Past, Present and Future of Rail Transit Systems. Houston, TX, NACE International,1994.
T.J. Barlo and A.D. Zdunek, Stray Current Corrosion in Electrified Rail Systems, May 1995 (Internet reference 96)