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17–
19 May 2018, Zadar, Croatia
5
th
International Conference on Road and Rail Infrastructure
Katarina Vranešić, Stjepan Lakušić, Marijana Serdar
University of Zagreb, Faculty for Civil Engineering, Zagreb, Croatia
Abstract
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 eect can be notice. Dierent 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
Introduction
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 dierent 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, [].
DOI:?https://doi.org/10.5592/CO/.2018.936
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 dierence 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
accelerated.
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 insuciently 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 eective, 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 dierent 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 dierent trac 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 trac 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 dierent values of potential as it is shown in Table .
Table 1 Stray current activity considering dierent 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 aect the train trac 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 eect 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
dierent 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 eective 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 trac 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, [].
Conclusion
Stray current corrosion issue is recognized among the world. DC transit system operators
are investing lot of money for implementing dierent 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 dierent 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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