Rail Base Corrosion Detection and Prevention (2007)

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27 CHAPTER 5: CORROSION PREVENTION METHODS 5.1 Presence of Salts on the Tracks The presence of salt on rails creates a very detrimental effect on the integrity of the rails because the salts form electrolytes promoting oxygen to react with the rails, thereby accelerating corrosion. During site visits, salt deposits on top of the tie plates were observed at several locations. Most of the locations are usually humid so there is consistent contact between the rail/tie/clip and ground. Figure 16 shows some of the deposited salts on top of the tie plates and rail found at several different sites. Figure 17 displays the differences between a clean and well insulated tie and tie plate and a tie plate with deposited salts and corrosion. The chemical analysis as reported by the Edmonton Transit System includes mainly alkaline salts, chorines, and sulfates (7). This was further confirmed by other transit authorities (13-15). Appendix E shows the chemical analysis conducted by the Edmonton Transit System. (a) Figure 16. (a) Salts deposited along the tracks at Port Authority Trans- Hudson.

28 (b) Figure 16. (b) Salts deposited along the tracks at Southeastern Pennsylvania Transportation Authority. Figure 17. Examples of Pandrol clips showing (a) good condition and (b) salts deposited around the Pandrol clip. Both pictures were taken at the Toronto Transit Commission subway during the site visit. a b

29 5.2 Analysis of Ballast An analysis was performed on ballast specimens from the research team to determine the effect of salt. Figure 18 shows the ballast specimens used. Figure 18. Ballast specimens used for the resistivity test: ( a) all tested ballast and slag based ballast (b) green section, and (c) black/glassy section. Two types of ballast were used for the test: mineral and slag based. The resistivity of the ballast was measured under dry and wet conditions. To simulate wet conditions, ballast types were immersed in water for 5 seconds and then excess water was removed. This was completed in order to measure the effects of absorbed water (humidity), rather than the surface deposited water. Table 3 shows the average resistivity results of at least 10 measurements of all ballast types under dry and wet conditions. TABLE 3. Resistivity results on dry and wet ballast. The >> symbol is used to report the average resistivity measurements of specimens 3 and 6 and respective 7 and 9 specimens. The other measurements were OL = Overload. Ballast 1 2 3 4 5 6 7 8 9 Dry OL OL OL OL OL OL OL OL OL 5 seconds in water 10 MΩ 15 MΩ 5 MΩ 2 MΩ 4 MΩ 5 MΩ >> 20 MΩ 8 MΩ >> 11 MΩ 9 8 7 6 5 4 3 2 1 a b c

30 The results of the test indicate that both ballast types have high resistivity after excess water is removed. Observations showed that the resistivity for all dry ballast overloaded the meter showing that under dry conditions ballast is a good insulator. Comparison of the results found in Table 3 concludes that the ballast types tested have negligible effect on corrosion. Furthermore, an evaluation of the parameters extrapolated to rail base corrosion (Table 2) indicates that the severity of corrosion is 15+, of which 5 points are from resistivity, 0 points for pH, 5 points for REDOX, 3.5 points for the presence of sulfates, and 2 points from moisture. The 5 points from resistivity is a conservative number and varies from tunnel to tunnel and environmental conditions, particularly salt deposits. Therefore, the environmental conditions at which the rail is exposed can be substantially detrimental to the rail’s integrity. Some of the literature reviewed by the research team recommended avoiding slag based ballast because of its relatively high conductivity (probably due to its high metallic content) (5). However, in the laboratory test, slag based ballast was found to have the highest resistivity under dry conditions (see Figure 18 and Table 3 ballast specimens 7-9). Ballast specimens 3 and 9 in Table 3 were selected to undergo a second test, immersing the ballast for 5 minutes in water to see if there would be any changes in the resistivity. There was no change in specimen 6 between a 5-second and 5-minute immersion, but the resistivity of specimen 9 was reduced from 8 MΩ to 2.5 MΩ. That is still a very high resistance, indication of a good insulator, except for when the ballast is thoroughly wet. 5.3 Rail Steels Rail steels are mainly made of iron and carbon. Iron is usually found in the form of α-Fe or ferrite (dilute solid solution of Fe and C) forming carbides, intermetallics, and inclusions (oxides and other nonmetallic compounds). However, the presence of inclusions is considerably low. Most of the iron in steel is found as pearlite, where the α-Fe lamellas are considerably more vulnerable to corrosion than any other steel component (iron carbide and Fe3C). In fact, elements like carbon in steel have very little, if any, effect on corrosion (1). Rail steel is usually a low alloy steel with some C, Cr, Mn, Cu, Ni, V, Mo, Nb, etc., with minor additions of other elements. Elements such as C, Cr, V, Mo, Mn are added for hardening, corrosion resistance, and strengthening. Larger additions of strategic elements (Cr and Ni) can considerably increase the resistance to atmospheric corrosion and/or corrosion in aqueous systems (1). However, this would considerably increase the cost of rail and probably have limited to no corrosion reduction benefits because the main factor that accelerates the rail base corrosion is the return current. Corrosion of carbon steel in water is controlled by the availability of oxygen to the metal surface. In rail structures, the water or humidity deposited on the rail usually has high amounts of dissolved oxygen, and the water layer is thin enough to permit an easy flow of oxygen. Under static conditions, carbon steel corrodes at rates between 100 and 200 μm/year, depending upon the oxygen level and temperature variations at different locations. As velocity causes a mass flow of oxygen to the surface, corrosion is very dependent on flow rate and can increase by a factor of 100 (2). This factor of 100 does not consider the presence of stray currents, a major concern for transit systems. Additionally, when the deposited salts on top of the rails become dry, very aggressive corrosion conditions are formed. This is due to the relatively good conductivity and the ability to dissolve oxygen, resulting in an increase of the rate at which corrosion erodes the rail.

31 5.4 Cathodic Protection Cathodic protection is one process used to prevent steel corrosion. A zinc (Zn) coating is most commonly used. Zn is used under normal atmospheric conditions, not because it is inert to corrosion, but rather because it corrodes considerably faster than steel, resulting in a coupled system (Figure 19). Zn coatings show increased corrosion rates under nonstatic conditions, so it would only provide a limited benefit to the transit system because rails are a nonstatic system. Galvanizing is more appropriate for static systems (1,16). Figure 19. Examples of galvanic corrosion using zinc and tin on steels. Note that while zinc makes a protective layer preventing the corrosion of steel, tin is protected by the corrosion of steel. 5.5 Sacrifice Anodes Figure 20 shows another typical cathodic Protection method used in the pipeline industry. This type of protection is very useful for static systems (i.e., pipelines), but in particular systems that have no introduction of external currents. This method closes an electrical circuit by introducing a more active element (Mg) that corrodes faster than the material under protection. For instance, a steel pipe in a corrosive environment with Mg cathodic protection will force the Mg to become more vulnerable to corrosion than the steel that creates a corrosion protection shield for the steel. This type of system is widely used by several industries and is a reliable method for corrosion protection. However, one of the conditions of this type of protection is that no current should be passing thorough the material under protection; otherwise, the current will alter the effectiveness of the anode. Therefore, cathodic protection will be ineffective for transit systems because there is a return current along the rails (1,16). Steel Zinc Steel Tin

32 Figure 20. Protection of an underground pipeline with a magnesium anode (1). The return current for transit systems is the byproduct of the train that closes the circuit of the overhead catenary or third rail. Ideally, the return current on well insulated rails will not have detrimental effects. However, in most cases there are stray currents caused by leaks where the current breaks the circuit through the path with less resistance (usually wet soil with high concentrations of salts, drain systems, electrical city circuits, etc). The electrical current always travels along the path of least resistance, or the current is divided along several paths in proportional amounts of current. For example, when the electrical continuity of the track structure is poor or the circuit is broken, more stray current will return through another path. The corrosion rate is directly proportional to the stray currents, limiting or eliminating the stray current occurrence will considerably reduce and probably eliminate rail base corrosion. 5.6 Recommendations for Rail Corrosion Protection Cathodic protection has great potential to prevent corrosion; however, in most cases, it is only applicable for static systems in the absence of dynamic currents. Rails are subjected to high dynamic stresses and are a good path for return current to the ground. In addition, the presence of deposited salts on the rails and the corroded rail itself increases the corrosion rate. The presence of moisture, salts, and iron-based powder(s) amplify the corrosion effects on the rails because the salts form an electrolyte when combined with DC that promotes electrolytic reactions increasing the corrosion rate. Furthermore, salt and iron powders have a large surface Backfill Steel Pipe Coated Copper Wire Current Ground Level Earth Environment Mg Anode

33 area promoting the formation of stray current locations that result in increasing the detrimental effects of oxygen, thus increasing corrosion rate, as Figure 1 shows. Therefore, the best way to prevent the corrosion of the rails is by properly insulating the rails (see Chapter 7), avoiding any DC leaks from the rail to the ground, forcing the current to return properly and closing the circuit. References 1, 4, 6, 16-35 contain information on corrosion and corrosion prevention with particular interest to the transit industry.