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Stray current corrosion activity on rail transit system in urban areas

    
19 May 2018, Zadar, Croatia
International Conference on Road and Rail Infrastructure
  
     
Katarina Vranešić, Stjepan Lakušić, Marijana Serdar
University of Zagreb, Faculty for Civil Engineering, Zagreb, Croatia
Corrosion reaction is a consequence of metal’s natural reaction with the environment which
can’t be stopped by itself. Reactions like this are causing huge construction maintenance
costs. The most common type of corrosion is electrochemical corrosion, which includes stray
current corrosion – it is electrochemical corrosion that occurs mostly on railway structures in
urban areas. DC (direct current) transit system operators are using rails as current returning
path from the vehicle to the substation. When proper drainage of the track isn’t assured,
surrounding media in which rails are embedded becomes an electrolyte which leads to stray
current leakage and development of corrosion. At the part where stray current is entering to
the rail, rail in under cathodic protection. This is cathodic zone of rail. Corrosion occurs on
parts where current is leaking from the rail, in anodic zone where deterioration of material
under stray current corrosion eect can be notice. Dierent type of measures for reducing
stray current leakage at the source are used among operators. At that way anodic reaction of
metal (rails) with the electrolyte (media in which rails are embedded) can be stopped.
Keywords: electrochemical corrosion, stray current, anode, cathode, tram track
Corrosion reaction of metal with liquid or moisture on metal’s surface is electrochemical pro-
cess. This means that metal’s corrosion is a consequence of transmission charge between me-
tal and electrolyte (ion conductor), []. Water, wet soils and aqueous solution of acid, alkali or
salt are some types of electrolytes, []. At the metal/electrolyte interface oxygen – reduction
reaction is happening. This reaction is defined by migration of electrons through the metal or
between two metals that are immersed in electrolyte and mutually connected through metal
conductor such as copper wire, [, ].
. Corrosion cell
Reaction of oxidation can be defined as process of electron’s releases and reaction of reduc-
tion represent process of binding electron with one substance or group of substance which is
resulting with creating new substance or group of substance, [, ]. Final process of electro-
chemical corrosion can be described by the equation ():
j  ()
In redox process oxidation and reduction reactions are happening on dierent places of
metal’s surface. Area on the metal where oxidation reaction is happening is called anodic
zone and area on the metal where reduction reaction is happening is called cathodic zone, [].
   
 2018 – th International Conference on Road and Rail Infrastructure
Anodic and cathodic reactions are caused by the electron transmission through metal (Figure
). Speed of corrosion is defined by the speed of electron’s transmission. When transmission
is stopped, the electrochemical corrosion reaction is also stopped, []. This corrosion process
is analogous to the process in galvanic couple, []. Two metals that are connected and immer-
sed in some electrolyte are creating potential dierence between them, which is resulting with
the electron flow, []. Based on Faraday’s low, if corrosion process is happening in corrosion
cell, metal’s losses on anode is proportional to the electric current in the cell, []. Mass-loss
of iron due to corrosion current of  ampere in the period of one year is . kilogram, [].
Schematic view of electrochemical corrosion, [6]
. Electrolyte
Basic characteristic of electrolyte and electrolyte’s solution is to conduct electrical current.
Positively charge ions (cations) and negatively charge ions (anions) are carriers of electrical
charge in the electrolyte, [, ]. In embedded railways the media (concrete slabs, asphalt)
in which rails are places represent electrolyte, especially when, due to bad maintenance,
adequate drainage isn’t insure. In cases of bad drainage and presence of moisture and water
retention in tracks electrochemical corrosion at the interface of metal (rail) and electrolyte is
Stray current corrosion
. Stray current
Stray current is part of the return current which follows paths other than the return circuit,
[]. Leakage of current from there intendent path is caused by placing insuciently insulated
metal conductor in electrolyte (ground, water), []. Considering the source of stray current,
they can be static or dynamic. Static stray current is caused by cathodic protection on buried
pipelines and source of dynamic stray current is mostly DC transit system, [].
Due to the limitation of construction and maintenance costs, most DC transit system opera-
tors use rails as return path for the current from the vehicle to the substation, [,]. Since
the rails have limited conductivity and rail insulation cannot be completely eective, part of
the current that passes through the rails find new less resistance path and leaks from the
rails through the electrolyte to the nearest buried metal object (mostly metal pipelines). Stray
current continue its path through buried pipeline until it comes near the substation. At that
place current leaves the pipeline and returns back to the substation (Figure ), [].
    
 2018 – th International Conference on Road and Rail Infrastructure
Stray current corrosion can produce damages on railway lines and on burred pipelines near
the tram tracks. This type of corrosion is the result of external magnetic field activity on metal
that is placed in electrolyte. In cases like this stray current corrosion cell is manifested due to
external electrical field, []. Stray current corrosion cells is dierent from the regular corrosion
cell described in section . this paper because it doesn’t appear spontaneously, but under
the external influence and in these cell electrodes aren’t place near each other, [].
Mechanism of stray current corrosion, [13]
Cathodic zone is created at the place where stray current enters to the metal from electrolyte
and the anodic zone represents place where stray current leaves the metal to enter the
electrolyte, []. Distance between anodic and cathodic zone can be even few kilometre long,
[]. At the area of cathodic zone, construction is protected from corrosion by cathodic protec-
tion, but at the anodic zone the process of corrosion starts to happen. If this type of corrosion
isn’t noticed on time, material lost at the anodic zone start to happen, [, ]. The amount
of corroded material on anodic zone due to stray current influence can be calculated using
Faraday’s low, [].
. Stray current detection at the source
Based on the norm EN : [], amount of stray current and their source can be detec-
ted by measuring on buried metals object. In order to identify stray current polarity and ma-
gnitude potential gradient measurements at metallic can be carried out using two reference
electrodes, []. One electrode have to be placed above the structure and the second one at
a distance more than  m, []. By this measurement possible corrosion risk can be assess
and static and dynamic stray current on that pipeline at the measuring period can be observed
and recorded. Measured values can oscillate due to current source and dierent trac load,
which means that by measuring potential in the period of  hours source of stray current can
be detected. If the biggest amount of stray current is noticed at peak hours ( am to  am and
 pm to pm), when the trac is increased, the main assumption is that the dc transit system
is the source of stray current, [, ]. This measurement can give a good results only if the
results are compared with an external event (like tramway passing) at observed moment, [].
At the part of pipeline with the more negative potential then potential of the neighbouring
soil, current is leaving from the rail and entering to the pipeline. This area represent cathodic
zone at buried pipeline and anodic zone at the rail. In other case, if positive potential is no-
ticed at the pipeline, current is leaving from the pipeline and enters at the rail (anodic zone
on the pipeline, cathodic zone at the rail), [, ]. By this analyses, area of stray current
   
 2018 – th International Conference on Road and Rail Infrastructure
activity on rails and metal objects can be detected. Stray current activity can be categorized
considering dierent values of potential as it is shown in Table .
Table 1 Stray current activity considering dierent values of potential change, [19]
Potential shift [∆V] Stray current influence category and remedy
<  Negligible
–  Low – no further evaluation recommended
–  Moderate – further evaluation recommended based
on the structure and protection levels
>  High – further evaluation recommended
According to the norm EN -: [] continuous monitoring of electrical potential at
DC traction system is necessary, []. To make adequate calculation of average potential,
period of  hour is recommended. If it is noticed bigger change in average rail potential, rail
to earth resistance might become weaker, which mean that insulation material of the rail has
loosen his performance. At cases like this the assumption is that current started to leak at
the area of changed potential, []. This method does not aect the train trac and the rail
potential is registered at dedicated locations along the line, like in substations or passenger
stations, []. Passenger stations are dedicated location because at these places vehicle is
accelerating and decelerating. During the acceleration the amount of stray current is increa-
sing (Figure ), []. This negative eect can be reduced by placing substation near points of
maximum acceleration, [].
Change in voltage and current during the acceleration period of the vehicle, [21]
Stray current reduction at the source
DC transit system operators recognized stray current corrosion problem and they suggested
dierent types of measures for reducing stray current leakage at the source. Today the most
often measures can be sorted at two groups, []:
Increasing rail to earth resistance,
Decreasing electrical resistance of the negative return (rail).
    
 2018 – th International Conference on Road and Rail Infrastructure
Since stray current jeopardize buried communal infrastructure in the vicinity of the rails, me-
asures for reducing stray current at the source often aren’t enough so metal pipelines have to
be protected from stray current corrosion by covering, coating or today mostly used method
– cathodic protection, [, ].
. Decreasing electrical resistance of the negative return (rail)
By welding rails are continuously connected. In this way less resistance of negative return
(rail) is achieved, []. To split the return current proportional to both parallel rails, they have
to be mutually connected at least every  meters, [].
Electrical resistance of the negative return is decreasing by using rails with a bigger cross sec-
tion, []. In this system rails represent electrical conductor and resistance of conductor beco-
mes smaller with the bigger cross section and vice versa, []. Resistance also depends on a
length of the conductor. Shorter conductors are resulting with the smaller electrical resistance
and because that it is recommended to reduce distance between electrical substations. But
with less distance between two substations the construction costs become bigger. This me-
asure can be installed only at the time when new traction system is under construction, [].
. Railway structure
Conductance per unit length of the rail is the most important parameter in defining stray
current leaking. If almost perfect insulation of rail is ensured and electrical conductivity of
rail to the ground is reduced, amount of stray current will be at the minimum, []. Rail to
ground resistance depends on the type of railway structure, quality of the railway fastening
insulation material, elastic pads under the rails (if the rail is placed on the sleepers) and
electrical resistance of the ground, [, ]. In the classic railway structure (especially with
the wooden sleepers and broken stone as ballast material) rail to ground resistance can be up
to  times bigger compared with the embedded railway structure, which is the most often
construction in urban rails, []. The most eective way for reducing stray current leakage is
improving electrical resistance at the metal/electrolyte interface.
Adequate insulation of the rails can be ensure by installing rail boot on the whole length of the
structure or by ensuring adequate insulation of fastening system, []. Among the operators
rail rubber boots are used as one of the most often method for reducing stray current (figure
). This solution, except stray current, also reduces vibration of the rails, []. But by total
insulation of the rails from the soil high voltage at the rail/soil interface is creating, which is
representing huge danger for the users of railway infrastructure, [].
Rail boot placed on whole length of the rails, [24]
   
 2018 – th International Conference on Road and Rail Infrastructure
During the exploitation time and under the influence of trac load, weather condition and
inadequate drainage in the railway structure, degradation of rail boot is occurring, []. Accor-
ding to the experience of DC transit system operators, measures that have to be implemented
to extend lifetime of rail boot at classic railway structures are, []:
Maintain dray and clean tracks (ensure adequate drainage of the railway gauge),
Regular visual inspection of the track,
Regular checking for the voids or loose connections at the boot sleeves (on the places where
boot overlaps).
When the new railway tracks is building it is necessary to conduct the analysis of danger
from corrosion in the cooperation with the corrosion engineers responsible for underground
structures. Analyses like this should contain information about electrical resistance of the
ground, values of stray current at the underground structures, stray current source, duration
and magnitude of stray current and existing methods of stray current protection that are used
in underground structures, [].
Stray current corrosion issue is recognized among the world. DC transit system operators
are investing lot of money for implementing dierent types of monitoring system in order to
detect spots of stray current leakage. Stray current corrosion is a specific type of electroche-
mical corrosion that jeopardize railway infrastructure and buried metal object in the vicinity of
railway. Although dierent measures for reducing stray current at the source are used today,
adequate solution for completely stopping the leakage of current at the source still doesn’t
exist. Because of that, except reducing stray current at the source, it is also necessary to pro-
tect buried metal object. Due to the interdisciplinary of this area by measuring stray current at
the buried pipelines, it is possible to detect place of entering current at the pipelines which is
corresponding to the place of leaving current from the rails and vice versa. On that way opera-
tors can discovered potentially endangered places (anodic zones) on their infrastructure and
implement adequate protection to reduce harmful consequence of stray current corrosion.
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... Methods that are currently used for reducing stray currents can be separated in two groups: i) increasing rail to earth resistance and ii) decreasing electrical resistance of the rail (negative return conductor) [5]. Based on the standard EN 50122-2 [6] rails and return conductors need to be completely insulated from the ground, but level of rail to earth insulation has to satisfy balance between stray currents and dangerous electrical potential at the rails that can occur due the high rail to earth resistance [4]. ...
Conference Paper
Full-text available
Stray current corrosion is a consequence of current leaking from the rail. To stop this negative effect, operators among the world are using different type of measures that are described in standards. In the present work, a theoretical model was used to analyses the potential distribution at rail infrastructure. Based on the rail potential it is possible to detect sections of the railway infrastructure that are jeopardize by current leakage.
Full-text available
Subway system has been widely applied to the urban rail transit system. However, many negative impacts such as the electrochemical corrosion on the subway track and third-party buried metallic infrastructure caused by the stray current, has gradually appeared. This kind of electrochemical corrosion will cause a great threat on buried metallic structure and personal safety. Numerous scholars studied this from various aspects, in which the stray current model is extensively used as an effective means of analysis. In this paper, the existing stray current distributing model is firstly reviewed and analyzed. Secondly, rail potential problem analyzed by means of the stray current distributing model is reviewed. Thirdly, stray current simulation methods are also presented and discussed based on the stray current distributing model. At last, the developing direction and trend of stray current distributing model in the future study is point out based on the content reviewed above.
This article is directed towards furnishing stray current modelling on DC traction systems to cope with the variability of a number of influencing parameters. To this end, the archival value of this work is gained by virtue of two new modelling techniques. These new techniques firstly, include a Monte Carlo based approach to take into account the variability of the dominant factors influencing the conductance per unit length between the track and the earth. Secondly, a simulation technique that can provide a more robust representation of the conductance per unit length between the track and the earth coupled with uniform and non-uniform soil models is presented in an attempt to comprehensively assess the levels of stray currents leaving a floating DC traction system
A system to emulate railway DC systems is presented in this paper. The emulator is designed to simulate the behavior of railway systems in terms of stray currents and touch voltages. Power electronics circuits are used as variable resistors to simulate the train motion. The emulator provides a test-bed to not only study and analyze the effects of stray currents and touch voltages, but to also evaluate techniques that mitigate these effects before they are physically implemented. The proposed emulator system is verified through simulation studies and hardware experimentations.
Conference Paper
Stray current leakage and the corrosion caused by direct current (dc) traction power systems has been found to be a concern in slab/embedded tracks. Embedded tracks typically run through dense traffic areas, urban commercial centers, inner city areas, tunnels, and tread between utility lines that require the rail to be continuously isolated. This isolation of the embedded tracks is necessary to provide adequate track-to-earth resistance. Compared to embedded tracks, ballasted tracks have lower stray current leakage since the entire rail does not require continuous isolation from earth and separation is only needed at the contact points which are generally insulated. Stray currents can cause safety risks, thus stray current mitigation is an important element of the overall design of a rail transit system. This paper presents and evaluates the isolation and mitigation method(s), their effectiveness, and the existing testing and maintenance plan for the Houston Metropolitan Transit Authority of Harris County (METRO). The stray current isolation methods adopted by METRO, supplemented by an effective maintenance plan, have been successful in containing stray current, have eliminated loss of public infrastructure, and minimized recurring cost of repairs.
Conference Paper
All direct current traction power systems using rails for return of traction current have a level of current leakage. This leakage of current is dependent on both design and operating factors affecting the efficiency of the rail return path and is referred to as stray current. Stray currents have been detected since the first electric railways were placed into operation during the latter half of the nineteenth century and have serious effects on utility structures and the neighboring infrastructure at large. Stray currents can create safety hazards thereby rendering the design of stray current mitigation an important element of the overall design of a rail transit system. Like any other design/construction project, a baseline survey is an important and significant step in the data collection and fact finding process for a light rail system. Such a survey would aid in finding the soil resistivity data and the results of the stray current levels on existing buried metal utilities. Similarly defining the design criteria for stray current mitigation, monitoring, and testing for a new light rail design project is also important. Most of the design criteria for the older rail transit systems have been developed as an aftermath of the corrosion problem and/or after the design of new extension to the system. Some older transit systems still do not have a specified design or mitigation criteria for stray current, and corrosion issues are handled as they surface and are prioritized based on severity. In the absence of guidelines, it is hard to understand the reasoning behind the limiting criteria suggested in the transit agency manuals particularly when there is no record of testing or soil resistivity investigation. For these older transit systems the limiting criterion was developed based on the information from other transit services. Having applicable design criteria for stray current control and mitigation will help standardize the process for the transit and will lower the cost of mitigation. This paper has been written by a Civil Engineer with an effort to understand the source and the scientific reasoning behind the limiting values suggested by the transit agencies associated with stray current testing procedures and its control. In order to understand the limited stray current corrosion criteria and the respective testing, various transit agencies were interviewed. These interviews were supplemented by a thorough review of the respective transit agency criteria manual/guidelines (where such information was available and accessible). Critical evaluations of the testing procedures were conducted to analyze if these tests and mitigation methods were effective.
Conference Paper
Stray current leakage and the corrosion caused by direct current (DC) traction power systems have been an ongoing issue especially in slab/embedded tracks. These tracks typically run through urban traffic areas, city centers, tunnels, and between the utility lines that require the rail to be continuously isolated to provide adequate track-to-earth resistance. Ballasted and direct fixation tracks provide much better track-to-earth resistance when equipped with isolation pads under the rail and exhibit higher stray current protection. Various isolation techniques have been implemented by the DC powered rail transit agencies for the control of track-to-earth resistance in embedded tracks including the use of rail boot. In the last two decades, the practice of rail boot usage has seen a significant increase by transit agencies in the United States for controlling the leakage of current in embedded track sections. However, experience has shown that the rail boot alone cannot always control the stray current leakage and that it is important to supplement the rail boot with additional stray current collection and mitigation techniques. These methods thereby reduce the stray current corrosion by using various combinations of mitigation and collection techniques including, but not limited to, the use of elastomeric grout, insulated rail fasteners, embedding rail in troughs, providing current collection mats, and collector cables. This paper presents and reviews different isolation and/or current collection methods that are presently in use to supplement the rail boot. These current isolation and/or collection methods when implemented together with the rail boot have significantly reduced stray current related problems, including: signal failures, controlling rail-to-earth voltages, minimizing recurring cost of repairs, and the damage to the public infrastructure. Additionally this paper provides general recommendations on the maintenance of the embedded tracks to avoid associated system problems.
Conference Paper
The Los Angeles Metropolitan Transportation Authority (LACMTA), like many other transit agencies throughout the country, is currently addressing the stray current corrosion problems on its rail system. Numerous capital projects have been released by the authority for the rehabilitation of their corroded infrastructures along their right-of-way. In addition, new maintenance procedures have been implemented to minimize corrosion problems to the rail and utility infrastructures located adjacent to the electrified railroad. The corrosion effect on rail infrastructures is often overlooked by most electrified railroad authorities because an immediate corrective response is not necessary for train movement during rail operations. The corrosion process is a natural process that occurs slowly and continuously throughout the life span of all materials, and may be accelerated due to stray currents from the railroad electrification system. Several key locations along the right-of-way have greater impact from stray current corrosion issues: at street crossing due to poor rail insulated boots, switch machines and accumulation of brake dust near passenger platforms. Other significant locations that suffer from stray current effects are overhead and under grade bridges, tunnel structures, rail spikes, fire suppression pipes, sewage pipes and underground feeder cable connections. Moreover, stray current can also cause other vital systems such as signaling systems and communication systems to malfunction. The other aspect of stray current is intentional discharging of stray current into earth ground through Negative Grounding Device (NGD) to maintain negative rail over voltage level for safe operation and reliability of rail service to the public. Typical negative rail voltage with respect to earth ground in operation is over 100VDC which is over the recommended voltage of 50VDC. Corrosion problems can be controlled by the implementation of a cathodic protection system, proper inspection of the running rail, impedance bond connections and proper maintenance of the cathodic protection system and negative grounding devices. The purpose of this paper is to address corrosion issues associated with DC electrified railways, recommend maintenance practices to control stray current sources, recommended maintenance practices for cathodic protection systems and discuss the balancing act to control negative rail over voltage versus intentional discharge of stray current to earth ground.
Teorijske osnove korozijskih procesa i metode zaštita, Faculty of mechanical engineering and naval architecture
  • V Alar
  • V Šimunović
  • I Juraga
Alar, V., Šimunović, V., Juraga, I.: Teorijske osnove korozijskih procesa i metode zaštita, Faculty of mechanical engineering and naval architecture, University of Zagreb,.
Gepotech izolacija od lutajuće struje, elastični podupirači tračnica
  • Sustav Schomburg
Schomburg, sustav Gepotech izolacija od lutajuće struje, elastični podupirači tračnica
Modeling stray current and its influence on corrosion of steel sheet piling
  • A Fagot
  • A Schmitt
Fagot, A., Schmitt, A.: Modeling stray current and its influence on corrosion of steel sheet piling, Port Infrastructure Seminar, Delft, Netherlands, June