Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
38 CHAPTER 7: GUIDELINES FOR CONTROLLING AND DETECTING RAIL BASE CORROSION One of the most effective ways to control corrosion is by insulating the contact between materials or components with different chemistries that have a tendency to form galvanic pairs. However, this is close to impossible because perfect insulators do not exist. Therefore, the solution is to prevent DC leaks or stray currents from the rail to the ground. Refer to references 4 and 16 for additional information on this subject. Eliminating water leaks to the tunnels is significantly difficult; nonetheless, water should be re-directed out of the tracks in order to avoid humidity that can promote stray currents from the rail to the ground. In the following subsections recommended guidelines for corrosion prevention and corrosion detection are discussed. 7.1 Corrosion Prevention Preventing stray currents (current leaks) and reducing humidity (particularly the salts in water leaks) minimizes corrosion. Therefore, the following guidelines are recommended to eliminate or to reduce conditions causing or contributing to corrosion (see also reference 16). ⢠Maintain good maintenance and good insulation. Clean and keep track roadbed water free. ⢠Maintain a stray current control program by conducting rail-to-earth resistance and sub- station-to-earth tests (16). ⢠Identify locations where stray currents are occurring or have a tendency to occur and create proper insulating conditions. Stray currents should be avoided when possible. ⢠Install welded rail in place of jointed rail because welded rail has significant traction current return. Make sure the rail is electrically bonded if jointed rail is installed. Otherwise electric arcs or leaks can be formed and produce stray currents. ⢠Insulate rail from fastening systems (Figure 23). For embedded tracks, it is crucial to coat and/or encase the rail with a good insulator. â A good insulation system can be made using as a base the insulators from the Pandrol. In addition, Figure 23 and reference 16 provide the details for insulators as well as for the encasing materials that can potentially prevent rail corrosion. ⢠Maintain clean and dry ballast or slabs. Any direct contact between the rail and ballast must be avoided. It is recommended to have at least 1 in. (25 mm) of clearance between the rail and the ballast (16). ⢠Include an extra line of welded conductors along the railâs web to provide an alternative low resistant path to prevent stray currents. ⢠Consider using plastic ties to better insulate the track from the ground (see Chapter 8). These guidelines are of crucial importance for the design and construction of new tracks in order to prevent and/or reduce costly corrosion prevention methods in the future.
39 Figure 23. Examples of tracks insulating (a) insulation at the rail base and (b) isolation of the fastening or fastener base. For more information consult reference 16, Chapter 5. (a) (b)
40 7.1.1 Effect of Direct Current on Corrosion A constant leak of current of one ampere (A) can corrode up to 20 pounds of iron per year (16). Therefore, an electric system where the return current can be as high as 750 A or higher, can result in damages of the rail of up to 15,000 lb or 7.5 tons of iron per year. Thus, it is important to prevent or eliminate stray currents by properly insulating the rails from the ground. (Proper insulation can be applied to new track when it is being built.) In contrast, the use of sacrificial anodes can be useful, but most likely insufficient, when the return current from a train is present. Nonetheless, sacrificial anodes can help prevent natural corrosion (due to the environment) along the rails. 7.1.2 Effect of Improper Drainage on Corrosion Maintaining good drainage is important in preventing corrosion. Most of the transit systems visited showed water leaks along their tunnels, and, in all cases, the water was pumped or directed to a channel located in the middle of the tracks. This resulted in an increase in humidity of the surroundings and in some cases caused wet tracks (including the concrete slab, ballast, ties, etc.). It is recommended to re-direct this water to an alternative path as far as possible from the electrified tracks. A good example was observed at the TTC-Toronto facilities where all ceiling leaks were re-directed by a drain-like system to a channel out of the tracks. The re- direction of the water will probably not solve the problem, but can potentially reduce the moisture on the track. Examples of wet track are shown in Figure 24. Deposited salts on top of tracks are carried by the water leaks and the deposits are presumably formed due to water evaporation at locations where stray currents occur. This occurs because stray currents create an arc that probably evaporates the present water leaving dissolved salt deposits. This can compromise the trackâs insulation and lead inevitably a perfect location that sponsors corrosion due to the formation of a highly concentrated electrolyte. It is for this reason that eliminating the deposited salts from track is important; in fact, it is better to avoid them by installing welded jumper cables that can considerably reduce electrical resistance at this particular location.
41 Figure 24. Wet tracks and detrimental corrosion effects on rails, fastening system, and tie plates. Notice the rust shells peeling out of the rail tracks. (a) (b)
42 7.2 Corrosion Detection The best way to detect corrosion is by visual inspection. Expert track walkers can detect corrosion on exposed rail easier than on rail that is embedded. For instance, locations where there is homogeneous erosion, as Figure 8 shows, visual detection is easy. Locations where corrosion is hidden between the tie plate and the base of the rail make its detection more difficult until erosion forms on the rail flanges. However, these locations can be visually detected where salts are deposited on the track, particularly at the rails section seated above the tie plate and where iron like oxide powder is observed at the tie plate locations. Once the presence of rail base, corrosion, salts, and/or iron like powder are detected along a track, it is suggested that the rail be scanned using non-destructive techniques. The research team scanned rails showing severe rail base corrosion using the samples presented in Figures 8 and 9. As a result, it was found that the practices to monitor rails with corrosion could be quite complicated. In contrast, it is suggested that each transit system should develop its own corrosion detection practice, which can be more precise and adequate for its own needs in accordance to the location, environment, and severity. In an effort to reliably scan rails, it was found that grease instead of couplant has some advantages. Chapter 6 provides potential techniques that can be considered to monitor rail base corrosion, particularly with a rail flaw detection vehicle.
