1. Introduction
Owing to their high capacity and velocity, urban railway tracks are the main type of transportation in many cities worldwide. Currently, urban track systems mostly use the DC traction power supply mode, and rails are normally used as return conductors from vehicles to substation [
1,
2,
3,
4]. The electrical resistance of the rail results from creating a potential drop in the rail, and because it is impossible to completely insulate the rail from its surroundings, part of the current leaks into the track bed and soil [
1,
5,
6,
7]. This current is called stray current. Stray current enters nearby metal objects that have a lower resistance than soil, such as buried metal pipelines or steel in reinforced concrete, and flows back to the source [
8,
9,
10].
The current path through a rail or other metal object can be very long and not harmful; however, stray current corrosion starts at the point where the current leaves the metal and enters the electrolyte [
11,
12]. Stray current corrosion became a source of concern for transit authorities and utility companies immediately after the first electrified tracks began to operate. Initially, it was thought that the corrosion problem was caused by a chemical mixture of the soil; however, after some research, it became clear that soil alone could not cause the severe corrosion found in rail bases and nearby utilities. It was concluded that the cause of this corrosion was a stray current [
13,
14]. Stray current corrosion is an electrochemical process that involves two simultaneous reactions. A cathodic reaction occurs in the cathodic zone, where the current enters the metal structure or rail, whereas an anodic reaction occurs where the current leaves the rail or metal structure and enters the soil or other electrolyte, called the anodic zone [
15,
16,
17,
18,
19]. Corrosion and metal loss occur in the anodic zone.
Currently, most studies [
20,
21,
22,
23,
24,
25] focus on stray current corrosion in pipelines or on stray current simulations and modeling [
7,
16,
26,
27]. The main objectives of some conducted studies are listed in
Table 1. Even though minor corrosion damage is sufficient to compromise the integrity of pipelines, it is not surprising that many researchers have focused on analyzing the detrimental impact of stray current corrosion on pipelines; however, stray current can also have detrimental effects on urban tracks and lead to the degradation of rails and rail fastening systems. Corrosion itself does not cause a significant mass loss in the track and is generally hidden and difficult to detect by visual inspection [
28]. This could be one of the main reasons why rail corrosion has not yet attracted sufficient attention and has become a major concern for operators. However, a small loss of rail material can lead to a significant change in the material properties or even act as a “stress concentrator” for the development of cracks [
28,
29,
30,
31,
32]. Conditions such as sunlight, relative humidity, temperature, and atmosphere have major impacts on corrosion. In addition, salt deposition is an important factor that increases the corrosion rate [
33].
According to Faraday’s law of electrolysis, a current of 1 A results in a loss of 9.1 kg of steel per year [
26]. The EN 50122-2:2022 standard [
41] states that experience indicates that no damage occurs in tracks if the average stray current per length does not exceed 2.5 mA/m. The methods used to reduce the stray current at the source can be divided into two groups [
19,
20,
42,
43].
- -
Decreasing the longitudinal electrical resistance of the rails, and
- -
Increasing the rail-to-ground resistance.
The shape and material properties of rails are mainly selected according to mechanical requirements, whereas the electrical properties are related to the background [
44,
45,
46]. However, the longitudinal electrical resistance of the rail can be reduced by methods such as using rails with a larger cross section and reducing the distance between substations [
47]. Because the electrical resistance of the rail depends on the rail cross section, the resistance increases during exploitation owing to rail head wear and cross-sectional thinning due to the corrosion process. According to [
34], rail wear can be caused by increasing the rail electrical resistance by 19%.
According to [
35], the rail-to-ground resistance is required to control the stray current. The resistance depends on the type of track and rail insulation [
17]. For tracks with continuously supported and fastened rails, where the rails are fully insulated with an elastic material, a high rail-to-ground resistance is achieved and stray current is prevented. For tracks with discretely supported and fastened rails, it is very difficult to ensure sufficient resistance between the rail and ground; therefore, stray currents can greatly affect the track and metal objects near the track. Because the type and properties of fastening systems are selected depending on the required elasticity of the track, planned traffic load, and type of rail [
48], the electrical resistance of the fastening system is not defined in the specifications. According to [
36,
42], even perfect rail insulation lasts only a few years if the tracks are not properly maintained; therefore, rail-to-ground resistance depends on the electrical resistance of the concrete base layer and the soil, which can vary greatly depending on the location of the tracks. Consequently, the rail-to-ground resistance values provided in the guidelines vary across countries for the same type of track construction, as shown in [
49,
50]. The corrosion rate and stray current are higher in closed urban tracks because it is very difficult to provide adequate drainage, and the tracks dry very slowly. Thus, the remaining water accelerates the corrosion and stray current. Urban tracks embedded in lanes shared by road vehicles (
Figure 1a,b) are also exposed to chlorides that are used in winter to prevent icing of the pavement [
29]. These chlorides dissolve in water and enter the tracks [
30], which results in a decrease in the electrical resistance between the rail and ground.
Compared to the mass of the rail and the metal parts of the fastening system, stray current corrosion does not cause a significant loss of material on the track, but the deterioration caused by stray current is localized. If it occurs in the places where the rail is fastened to the concrete base using a discrete fastening system, the corrosion combined with the traffic load may lead to breakage of the rail foot and the elements of the fastening system [
51]. In this case, the rail is no longer fastened to the base, and traffic safety may be compromised. According to [
48], in most urban track systems the main task of the rail fastening system is to position and fasten the rails and transfer the vehicle load from the rails to the track substructure. The type and characteristics of the fastening system are selected depending on the required elasticity of the track, planned load, and rail type. The research carried out in this study shows that the type of fastening system also has an influence on the stray current and that the prevention of stray current must be one of the tasks that the fastening system must fulfill.
This paper is divided into five sections. State-of-the-art methods are presented in the introduction (
Section 1).
Section 2 explains the motivation for conducting this study and research objectives. The materials and methods are described in
Section 3.
Section 3 also describes the test samples and the differences between them. The results are presented in
Section 4. In
Section 5, a tabular overview of the influence of stray currents on the observed test samples is provided, and the results of the research are summarized. Finally,
Section 6 presents the main conclusions.
2. Research Objectives
Based on the state of the art, it can be concluded that previous studies are mostly focused on the following:
- -
modeling of stray currents in track construction, especially in metro systems
- -
analysis of rail potential
- -
the effects of stray currents on endangered structures near the tracks, particularly pipelines
- -
analysis of methods for protecting buried pipelines from the effects of stray currents.
However, previous research has not analyzed the effects of stray currents on rails and fastening systems in urban railway tracks, which are the main sources of stray currents. After identifying the damage caused by the stray currents in the track, effective measures to protect the track can be determined. This reduces the stray current and prevents damage to structures near the track.
To define the negative effects of stray currents on urban railway tracks, the authors performed a laboratory simulation of stray currents on four real-scale samples of the entire rail with all the fastening components. The difference among these four samples was the type of fastening system used. The fastening systems used for this test are characteristic of the tramway infrastructure in the city of Zagreb, Croatia, which has one of the highest traffic loads in Europe and where the problem of stray currents has been recognized. The samples prepared for testing were placed in plastic tubs to allow the tests to be performed under dry conditions and at different water levels. Different water levels simulate different conditions that can occur in tracks when drainage is inadequate and water is retained in the track.
3. Materials and Methods
3.1. Sample Description and Preparation
Laboratory tests were performed on four different types of fastening systems that are characteristic of the tram track infrastructure in Zagreb: ZG 3/2, PPE, DEPP, and ZG 21-CTT. ZG 3/2 and PPE are direct fastening systems, where the rails are laid on elastic rail pads and steel plates and fastened using clips and anchor bolts (
Figure 2 and
Figure 3). Anchor bolts were used to anchor the fastening system to a concrete base. In the PPE fastening system, an elastomer pad was placed between the steel plate and the leveling layer, whereas in the ZG 3/2 fastening system, the steel plate was in direct contact with the leveling layer. In the ZG 3/2 fastening system, the SKL-2 clip was placed on a steel rib, whereas in the PPE fastening system, the SKL-1 clip was placed on a plastic rib.
In the DEPP and Zagreb 21-CTT fastening systems, the rails are laid on the rail pad and steel plate but are fastened to the steel plate using clips and T-bolts (
Figure 4 and
Figure 5). The anchor bolts are dislocated and used only to anchor the fastening system to the concrete base. In the Zagreb 21-CTT fastener, the underside of the steel plate is vulcanized. The bores for the anchor bolts on the steel plate are also vulcanized such that the anchor bolts do not directly contact the steel plate (
Figure 5a).
First, concrete bases with dimensions of 60 × 30 × 25 cm were fabricated for the samples. The leveling layers were created as in situ, with the rails adjusted primarily by direction and height. For the test samples, the steel base plates were raised approximately 5 cm above the concrete block, corresponding to the height of the leveling layer on the tracks. For the PPE, DEPP, and Zagreb 21-CTT fastening systems, the leveling layers of the concrete reinforced with synthetic microfibers were cast at the middle of the height of the elastomeric pads, whereas for the ZG 3/2 fastening system, the leveling layer was cast at half the height of the steel plate because there was no elastomer pad under the steel plate.
3.2. Methodology
The samples were placed in plastic tubes and treated with a DC current from a laboratory power supply. Each sample was connected to a separate DC source. A voltage of 26 V was applied to the samples, and the current value depended on their electrical resistance. The rail was connected to the positive terminal of the power supply, and the circuit was closed using a steel rib built into the concrete (
Figure 6a). In this circuit, current flows into the rail and fastening system and through the electrolyte (in these cases, concrete or water) back to the negative terminal of the power supply.
The experiment was performed in three different states of the samples:
- -
- -
Water on the top of the concrete base (
Figure 7b)
- -
In the immersed condition: the water level was half the height of the neck of the rail (
Figure 7c).
Different water levels simulate different conditions that can occur on tram tracks. In tracks embedded in lanes shared by road vehicles, the drying process is very slow; therefore, the concrete base is wet for most of the year. These conditions were simulated using the water level at the top of the concrete base. If track drainage is inadequate, water will remain in the track, resulting in direct contact between the rail and the fastening system with the electrolyte. These conditions were simulated by immersing the samples in water to half the height of the neck of the rail. Before immersing the samples in water, the electrical conductivity of water was measured using an SI Analytics Lab 945 conductometer. The electrical conductivity of water was 0.0775 S/m. The electrical conductivity of water affects the electrical resistance of the entire sample. Therefore, it is important that the conductivity of water be the same in all samples. Notably, the electrical resistance of the water retained in the track structure depends on the substances dissolved in the water, mainly salt. This experiment can also be performed using water containing dissolved salts to accelerate the corrosion process. However, because the aim of this study was to determine the influence of stray currents on rails and fasteners, the experiment was carried out with tap water with a defined electrical conductivity.
The experiment lasted 191 days, during which, for 92 days, the water level was at half the height of the rail neck, and for 74 days, the water level was on the concrete base.
During testing, the current and voltage were measured, and the samples were periodically visually inspected. The current was recorded using a multimeter GWInstek GDM9061, and the voltage was measured for all the components of the fastening systems. Measurements were taken at the beginning of the test, when the samples were dry, when the water level was at the top of the concrete base (case I), in the immersed condition (case II), and after the test was finished and the water level was lowered by the top of the concrete base (case III). The voltage was measured between the analyzed elements (rail, clip, anchor bolt, steel plate, T-bolt, and steel rib) and the negative terminal of the current source. Before starting the measurements, the resistance of the testing equipment was measured to exclude its influence on the measurement results.
Because the accumulation of corrosion products under immersion conditions increased the electrical resistance, the water in the tubs was changed every week to maintain a constant current.
5. Discussion
When the samples were dried, the current leaked through the fastening system into the lower part of the track structure, and stray current corrosion occurred on the elements that were in contact with the concrete—i.e., anchor bolts and steel plate in the ZG 3/2 fastening system. In the ZG 3/2 and PPE fastening systems, the rails are in direct contact with the clip and anchor bolts, and the stray current depends only on the electrical resistance of the concrete layer. Under the immersed condition, corrosion started at the locations where the current allowed the metal elements to enter the electrolyte. The components of the fastening system in which corrosion occurred under different conditions are listed in
Table 8.
Although all the samples were immersed in water to half the height of the rail neck and exposed to the same DC current, no uniform deterioration of the fasteners was observed. Thinning of the rail foot cross section was observed in all fastening systems, but the highest level of deterioration was observed for ZG 3/2 and PPE. No clip deterioration was observed in the ZG 3/2 fastening system; however, severe corrosion was observed on the steel rib with which the clip was in direct contact. The most severe clip deterioration was observed in the PPE and DEPP fastening systems, whereas in the Zagreb 21-CTT fastening system, clip deterioration was not as pronounced. The steel base plate corroded in all the tested samples. Owing to their larger dimensions, the corrosion of the plate was not as pronounced in the DEPP and Zagreb 21-CTT fastening systems. From the performed corrosion classification, it can be concluded that in the tested samples, when the samples were immersed in water up to the middle of the height of the rail neck, a high level of corrosion was observed in the following components:
- -
ZG 3/2 fastening system: rail, steel ribs, anchor bolts,
- -
PPE fastening system: rail, clip SKL-1 (1), steel plate, and anchor bolts.
- -
DEPP fastening system: clip SKL-1 (2), anchor bolt,
- -
Zagreb 21-CTT fastening system: only medium level of corrosion on the clip SKL-12 (2).
This deterioration is a result of the current leaving the components of the fastening system and entering water or concrete.
6. Conclusions
For tracks with discretely fastened rails, the fastening points are “discharge points” for the current from the rail to the ground. Therefore, by better insulating the fastening system and ensuring adequate track drainage in a track asphalted into a car-running surface, stray current can be prevented. Despite the significant damage caused by stray currents to the rail and the components of the fastening system, insufficient attention has been paid to the prevention of stray currents. Moreover, the prevention of stray currents is not one of the characteristics that a fastening system must satisfy. The experiments described in this paper have shown that, to prevent stray current corrosion in the track, it is essential to provide adequate drainage so that water does not remain in the track. Under dry conditions, indirect fastening systems with insulated anchor bolts should be used to prevent stray currents. In addition, the elastomeric components of fastening systems must have high electrical resistivity, which was not the case for the fastening systems tested in this study. When direct fastening systems are used, an elastomeric pad should be installed between the rail foot and clip to prevent direct contact between them.
Stray currents can be significantly reduced by modifying the existing fastening systems in tracks with discretely fastened rails and by improving track maintenance. In addition to reducing stray currents and preventing harmful corrosion degradation of rails and fastening system components, modifying existing fastening systems can also improve ride comfort and reduce high levels of rail traffic noise and vibration.