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Procedia Engineering 165 ( 2016 ) 583 – 592

15th International scientific conference “Underground Urbanisation as a Prerequisite for Sustainable Development” Measures against electrolytic rail in Tokyo metro subway tunnels

a, a a a Hikaru Isozaki *, Junichirou Oosawa , Yousuke Kawano , Ryo Hirasawa , Souichi Kubota a, Shinji Konishi a

a Tokyo Metro Co., Ltd., Tokyo, Japan

Abstract

Tokyo Metro Co., Ltd. is operating nine railroad lines (195.1 km) forming a network mainly consisting of subways in Tokyo, the capital of Japan. Good maintenance of tracks is essential to the safety of passengers. However, rail breakage due to electrolytic rail corrosion is occurring in some underground spaces. Rail corrosion occur frequently in subway tunnel due to underground water leakage. For some place, we have to change rails every a half year. So it is big problem for underground space. We have been examining electrolytic rail corrosion for three years. By producing maps of places prone to electrolytic rail corrosion, divided into 25-m sections, we analyzed track conditions and identified the causative factors of electrolytic corrosion depending on track conditions. In addition, we focused our attention on chemical approaches to the prevention of electrolytic corrosion. First, we conducted chemical analyses of samples of electrolytic corrosion products and samples of water collected from the environment where electrolytic corrosion occurred. The results suggested that akaganeite, which forms in the high Cl– concentration in underground spaces. promoted the loss of rail strength. Next, we examined the effectiveness of the sacrificial metal method in delaying electrolytic corrosion. The use of , a metal with higher ionizing tendency than , in the form of adhesive tape attached to steel surfaces, was effective in delaying electrolytic rail corrosion. This report discusses the details of the conventional physical approach relying on rail replacement planning and the chemical approach to the prevention of electrolytic corrosion with some insight into future developments. © 20162016 Published The Authors. by Elsevier Published Ltd. Thisby Elsevier is an open Ltd access. article under the CC BY-NC-ND license (Peerhttp://creativecommons.org/licenses/by-nc-nd/4.0/-review under responsibility of the scientific). committee of the 15th International scientific conference “Underground Peer-reviewUrbanisation under as a responsibility Prerequisite offor the Sustainable scientific committee Development of the. 15th International scientific conference “Underground Urbanisation as a Prerequisite for Sustainable Development

* Corresponding author. Tel.: +03-3837-7092. E-mail address: [email protected]

1877-7058 © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 15th International scientific conference “Underground Urbanisation as a Prerequisite for Sustainable Development doi: 10.1016/j.proeng.2016.11.754 584 Hikaru Isozaki et al. / Procedia Engineering 165 ( 2016 ) 583 – 592

Keywords: Subway tunnel, Electrolytic rail corrosion, Rail track structures, Sacrificial metal method.

1. Introduction

Tokyo Metro Co., Ltd. (“Tokyo Metro”) is operating nine railroad lines with the total operating length of 195.1 km used by about 6.84 million passengers a day. About 85% of Tokyo Metro lines are in underground structures. They are often exposed to a moist environment due to water leaks in soft grounds with high aquifer levels. Although measures to prevent water leaks are taken, it is difficult at present to seal all leaks. In a wet environment, the electric current sent back from the train to the substation via the rails leaks into the ground, and this causes electrochemical corrosion (Figure 1). Electrolytic rail corrosion of this type is occurring in several sections, sometimes causing rail breakage (Photo 1, Photo 2). Although our company has been taking various actions, including track bed cleaning and the repair of water leaks in tunnels, there is no complete solution to this problem. This report, therefore, discusses the physical approaches to control electrolytic corrosion such as the optimization of rail replacement cycles using detailed inspection of electrolytic corrosion and chemical approaches such as zinc tape.

Substation

Electric power line

Rail

Electrolytic corrosion Ground

Fig. 1. Mechanism of electrolytic corrosion.

Ph. 1, Ph. 2. Examples of electrolytic corrosion.

2. Physical Approaches to the Analysis and Control of Electrolytic Corrosion

2.1. Survey Features and Methods

We routinely conduct management and observation of electrolytic corrosion by rail inspection, rail flaw detection, patrolling, etc. However, it is difficult to find electrolytic corrosion at the base of rails by visual inspection and flaw detection. We, therefore, selected the sections with high occurrence of electrolytic corrosion, and conducted detailed inspections of electrolytic corrosion for three years. Visual inspection was used as the basic Hikaru Isozaki et al. / Procedia Engineering 165 ( 2016 ) 583 – 592 585 method of survey, and places that cannot be directly observed (base of rails) were inspected touch after the removal of fastening devices. The survey sections were selected based on the abundance of electrolytic corrosion found during inspections by rail-defect inspection cars and on-foot inspection (Table 1).

Table 1. Total elongation of survey segments by line. Line Year 2011 2012 2013 A Line 18.608m 18.608m 18.620m B Line 16.070m 16.070m 20.850m C Line 12.890m 12.890m 16.000m Electrolytic corrosion was evaluated according to the three-point color-coded scale (red- mild corrosion, yellow- medium corrosion, white- severe corrosion) describing the extent of electrolytic corrosion (Figure 2). The result of evaluations was recorded and also marked by painting directly on the rail (Photo 3).

Fig. 2. Evaluation criteria and color codes.

Ph. 3. Painting of evaluation result.

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2.2. Survey Results and Discussion

2.2.1. Trends in the Number of Electrolytic Corrosion Spots

The trends in the number of electrolytic corrosion spots were as shown in Figure 3 for each line. Because the length of survey sections varied from year to year, the data are shown as the total number of electrolytic corrosion spots divided by the total length of survey sections. It is clear that the number of electrolytic corrosion spots decreased gradually from the first year of survey. This was the result of rail replacement and other measures conducted every year. Continuation of these efforts can reduce the possibility of rail breakage due to electrolytic corrosion.

0.4 Red 0.35 Yellow 0.3 White

0.25

0.2

0.15

0.1

0.05

0 2011 2012 2013 2011 2012 2013 2011 2012 2013 Number of electrolytic corrosion spots per 1 m per (spots/m) spots corrosion of electrolytic Number A Line B Line C Line

Fig. 3. Trends in the number of electrolytic corrosion spots.

2.2.2. Progression of Electrolytic Corrosion

The electrolytic corrosion spots identified in the 2013 survey were checked for the progression of corrosion based on the comparison with past survey records. Progression was defined as a change in the evaluation color code, irrespective of the extent of progression, and the appearance of new spots. The results, shown in Figure 4, were broadly classified into three categories: no progression; progression in two years; and, progression in one year (including appearance of new corrosion). In each line, there were many spots where progression of electrolytic corrosion was confirmed in one year. Because the spots evaluated as white (severe) do not change color code. If there is further progression of electrolytic corrosion, no progression is recorded. There may be more spots where electrolytic corrosion progressed but are not shown in these data.

Hikaru Isozaki et al. / Procedia Engineering 165 ( 2016 ) 583 – 592 587

100% No progression 90% Progression in 2 years 80% 44.84% Progression in 1 year 53.84% 70% 60.71% 60%

50% 4.54% 1.03% 40% 0.77% 30% 50.62% 20% 45.12% 38.51% 10% Rateof progression of electrolytic corrosion 0% 㸿 Line 㹀 Line 㹁 Line

Fig. 4. Progression of electrolytic corrosion.

2.2.3. Identification of Sections with Electrolytic Corrosion and the Visualization of Data for Assessing the Level of Progression

To identify the sections with electrolytic corrosion, the survey results were divided into sections at a 25-m pitch, and electrolytic corrosion maps to visualize the results of electrolytic corrosion surveys and real replacement information were produced (Figure 5). These maps, visualizing the sections with frequent electrolytic corrosion, are expected to be useful for the efficient management of rail replacement and repair.

Fig. 5. Electrolytic corrosion map.

Next, to assess the level of progression in the sections with electrolytic corrosion, each 25-m segment was evaluated by assigning scores to the electrolytic corrosion in the segment and then adding up these scores. The scores were 0 for no electrolytic corrosion, 1 for red, 2 for yellow, and 3 for white. Using the score for each segment, the effects of track shape characteristics (inside rail, outside rail, and straight line) and the separation of rail from the track bed (Figure 6) were analyzed. The separation from the track bed was classified into large and small depending on the type of sleepers (RC short sleeper or vibration-reducing PC sleeper).

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Fig. 6. Separation from track bed.

2.2.4. Analysis of Visualized Data for the Causative Factors of Electrolytic Corrosion and the Result of Analysis

Figure 7 shows the mean score of survey sections classified according to track shape characteristics and the separation from the track bed. A small separation was more likely to occur electrolytic corrosion than a large separation by a factor of about 1.3. Scores were similar between straight rails and curved inside rails. This may be explained by the effect of water pooling. Figure 8 shows the situation of water pooling. The situation of water pooling leads to the following idea: (i) Straight sections tend to have water pooling at the base of rails, if the track bed surface directly beneath the rails does not have a draining gradient; (ii) Curved sections with poor drainage are likely to have water pooling around the inside rail. Our results indicated that straight rails and the inside rails in curves are likely to be affected by water, while the outside rails in curves are relatively less susceptible to the effects of water. Because the places with a small rail-to-bed separation in an environment prone to water pooling are liable to suffer from electrolytic corrosion and its rapid progression, such places need to be monitored with special attention and scheduled for periodic rail replacement.

Fig. 7. Relationship between separation and electrolytic corrosion (A Line). Hikaru Isozaki et al. / Procedia Engineering 165 ( 2016 ) 583 – 592 589

Fig.8. Water pooling.

2.2.5. Future Prospects

At Tokyo Metro, we are making efforts to expand the use of structures with a large separation (use of vibration- reducing PC sleepers), and this is expected to be effective in the prevention of electrolytic corrosion.

3. Chemical Approaches to the Analysis and Control of Electrolytic Corrosion

3.1. Survey Features and Methods

Apart from the conventional physical measures such as insulation and isolation, we began studying chemical approaches. As the first step, we collected electrolytic corrosion products in electrolytic corrosion-prone sections (four samples) and water from the environment where electrolytic corrosion occurred (two samples), and performed chemical analysis of their compositions (Table 2). The water samples were taken directly from a water pool near the rail with electrolytic corrosion and tunnel leakage water in an electrolytic corrosion-prone segment, and analyzed for pH and Cl–.

3.2. Survey Results and Discussion

3.2.1. Chemical Analysis of Electrolytic Corrosion

The results of analysis of electrolytic product samples and water samples are shown in Tables 3 and 4. According to past studies and literature,1) iron oxyhydroxide FeOOH is forms on the rail surface, and there are several types of iron oxyhydroxide with different crystalline structure, including α-, β-, and γ-FeOOH. The analysis in this study detected α-FeOOH (goethite) and β-FeOOH (akaganeite) in all four samples. Akaganeite is a low-density crystalline substance formed in the presence of salt. The total chloride ion content of electrolytic corrosion products was as high as 3~8 mass%. In addition, the chemical analysis of water samples showed that water sample No. 1 collected near the rail had a high Cl– concentration of 3.57% as compared with sample No. 2 taken from water leakage, indicating a process of evaporative concentration to a high level. The above analytical results suggested the process in which the stepwise ionization of iron in the rails through corrosion reactions led to the progression of oxidation. In a past study, Uno et al.2) have clarified that iron is formed through different processes under different environmental conditions, and their results have suggested that the precipitating components of electrolytic corrosion products are formed through the processes shown in Figure 9. 590 Hikaru Isozaki et al. / Procedia Engineering 165 ( 2016 ) 583 – 592

Electrolytic corrosion reactions are known to make iron atoms to take up OH– from the of water and oxygen O2 in air bind to form iron oxyhydroxide FeOOH.

Table 2. Types of samples. Type of sample Sampling place Analytical test items A Electrolytic corrosion products Near the rail Powder X-ray diffraction B analysis, total chloridw ion analysis C D 1 Water Near the rail pH measurement, Cl analysis 2 Tunnel leakage water

Table 3. Results of analysis of electrolytic corrosion products. Type of sample Go ethite Akaganeite Lepidocrocite Magnetite Cl content (α-FeOOH) (β-FeOOH) (β-FeOOH) (Fe3O4) (mass %) A Electrolytic corrosion + + ++ 3.09 products B + ++ + ++++ 7.86 C + ++++ +++ 4.54 D + ++ 8.36

Table 4. Results of analysis of water samples. Type of sample pH Cl 1 Water 2.1 3.57% 2 7.1 0.26%

The formation of lepidocrocite, goethite, and magnetite through common rusting processes, combined with the concentration of chloride ions Cl– in leaked water, is considered to be promoting the formation of akaganeite in the second stage of corrosion reactions. Because akaganeite forms low-density crystals, this substance inferred to be the main culprit for the loss of rail strength.

Fig. 9. Mechanism of electrolytic rail corrosion. Hikaru Isozaki et al. / Procedia Engineering 165 ( 2016 ) 583 – 592 591

3.2.2. Corrosion Control Using the Sacrificial Metal Method

Unlike the conventional approach to stop the development of electrolytic corrosion, the new method called sacrificial corrosion protection aims to retard the progression of electrolytic corrosion making use of the ionizing property of metals. The methods for preventing rail corrosion can be divided into “environmental insulation type” depending on the application of anticorrosive agents and electrical insulating coatings, and “sacrificial protection type”, discussed here. Corrosion prevention via the Āenvironmental insulation type” has the problem that the coatings applied on the junctions between the rail and the fastening device peal off due to the vibration from the passage of trains, and that this method by itself can hardly provide complete protection against electrolytic corrosion. In the “sacrificial corrosion protection” studied here, a substance that is more easily ionized (dissolved and corroded) than iron is attached to the steel surface, so that it would be corroded in substitute for iron to extend the life of steel members. This method has already been used in ships and the structures such as the bridges near the sea. Based on this principle, we tested the use of zinc, which is more easily corroded than iron (rails). Zinc was used in the form of adhesive tape (Photo 4), so that it could be easily applied on-site without requiring any special training (Photos 5 and 6).

3.2.3. Test Application of Sacrificial Corrosion Protection and Results

Zinc tape were used in electrolytic corrosion-prone segment on a trial basis. Corrosion with wrinkled surfaces appeared on the tape within two months (Photos 7 and 8). While the tested railroad segment had been showing the signs of electrolytic corrosion in one month, this treatment delayed the development of electrolytic corrosion by about six months. The result confirmed, albeit qualitatively, that the use of zinc tape being corroded and dissolved in place of rails was effective in reducing rail corrosion.

3.2.4. Future Prospects

We are currently conducting the quantitative analysis of the effectiveness of zinc tape and assessing the range of effectiveness of zinc tape. In addition, we plan to test the effects of neutralizing the highly acidic pH of water around the places with electrolytic corrosion as an attempt to retard the progression of electrolytic corrosion.

4. Conclusion

Graphical mapping and analysis of the places found to have electrolytic rail corrosion was performed based on the results of the three year survey aiming to identify the places requiring periodic rail replacement and helping replacement planning. In addition, as a chemical approach to corrosion control, we analyzed the components of electrolytic corrosion products and the water from the environment where electrolytic corrosion occurred.

Ph. 4, Ph. 5, Ph. 6. Attachment of zinc tape. 592 Hikaru Isozaki et al. / Procedia Engineering 165 ( 2016 ) 583 – 592

The results indicated that water pooling was likely to occur around straight rails and the inside rails in curves, and where there is a small rail-to-bed separation which is more prone to water pooling are therefore significantly more prone to electrolytic corrosion. Measures to prevent rail breakage should be taken, focusing on sections where the small rail separation encourages electrolytic corrosion, and special attention should be paid during inspection patrols to sections with pooling water and a small rail separation, including those located outside of the coverage of this study. In addition, the electrolytic corrosion maps prepared in this study is expected to help appropriate planning of rail replacement considering the difference in the speed of progression of electrolytic corrosion. While we are currently expanding the use of track designs with a large rail-to-bed separation (vibration-reducing PC sleepers), the results of these analyses confirmed the effectiveness of this measure in controlling electrolytic corrosion. Our study also clarified the processes of the ionization of iron in the rails and the stepwise progression of corrosion. Because akaganeite formed in these processes is a low-density crystalline substance, it is likely that akaganeite is a major culprit for the loss of rail strength. Based on this examination, we performed sacrificial corrosion protection using zinc tape, and confirmed the effectiveness of zinc tape in retarding electrolytic corrosion. We continue efforts to control electrolytic corrosion both from physical approaches such as the optimization of rail replacement cycles based on the detailed inspection of electrolytic corrosion, and from chemical approaches such as the use of zinc tape. We also study other measures including the neutralization of water in the environment causing electrolytic corrosion, aiming to further improve rail maintenance in underground spaces.

References

[1] Y. Ishikawa, R. Hoshiko, and J. Osawa, Consideration on the Factors Promoting Electrolytic Corrosion Related to Railroad Tracks, The 68th Annual Scientific Lecture Meeting of Japan Society of Civil Engineers, September ,2013. [2] J.Uno, S. Nakamura, T. Yamaoto, and T. Miyagawa: Consideration on the Effect of Difference in Environment on Iron Rust Formation Processes, The 67th Annual Scientific Lecture Meeting of Japan Society of Civil Engineers.102 (2012).