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.
1 TCRP D-7/Task 14 Rail Base Corrosion Study SUMMARY Under Transit Cooperative Research Program (TCRP) Project D-7, the Transportation Technology Center, Inc., (TTCI) studied the effects and prevention of rail base corrosion. The following tasks were accomplished: ⢠Distributed questionnaire to various transit agencies in order to identify the major problems associated with rail base corrosion and actions taken by them. ⢠Completed a metallurgical examination of rails with corrosion present, including an electrochemical study of the rails under different corrosive environments (i.e., chlorines (KCl), sulfates (NaSO4)) and direct current (DC) conditions. ⢠Created a Finite Element Analysis (FEA) model using ANSYS® to determine the state of stresses created by the localized corrosion and assess the risk of failure. ⢠Developed a draft recommended industry practice for rail base corrosion detection and prevention. A 7-page survey was developed and sent to 28 rail transit agencies and commuter rail systems in North America. The survey concentrated on rail base corrosion and several environments under which corrosion occurs. Of the 28 agencies, 16 responded to the survey. The remaining agencies had minimal rail base corrosion in their systems. Therefore, they believed they had little to contribute to the project. In addition to the survey, site visits to 7 of the 16 responding transit systems were conducted. The survey responses showed that 12 of the 16 respondents incurred serious rail base corrosion problems. Of the 12 respondents with corrosion problems, the research team determined that the most severe indication of corrosion was seen in systems with underground (tunnel) sections, in locations with significant history of humidity. Conversely, transit systems located above ground in the open air exhibited only minimal corrosion. There are two main reasons for humidity in transit systems: (1) leakage from drainage systems and (2) water sources such as a river, lake, or sea. Underground systems present a situation where the corrosion rate is increased by the presence of chlorines and sulfates from chemicals being disposed into the drain systems and from underground organic matter. More important, during the winter season, salts used to melt ice and snow within a city dissolve and are carried underground leaking into the tunnels providing a major contribution to corrosion.
2 The following transit systems were visited and provided several examples of rail base corrosion: ⢠Philadelphia: Southeastern Pennsylvania Transportation Authority (SEPTA) ⢠New York: Metropolitan Transportation Authority (MTA) New York City Transit ⢠New Jersey: Port Authority Transit-Hudson (PATH) ⢠New York: Amtrak - South tunnel ⢠Baltimore: Amtrak - Baltimore station ⢠Toronto, Canada: Toronto Transit Commission (TTC) ⢠Mexico City: Systema de Transporte Collectivo and Tren Ligero All of the transit systems visited use nondestructive evaluation (NDE) techniques to detect the flaws and cracks that can compromise the integrity of the rails and overall safety. Some of the NDE practices include visual inspection. In several cases, visual inspection is enough to identify rail base corrosion because corrosion occurs mainly in the tie area and can be easily observed. Unfortunately, some of the rail base corrosion is hidden between the base of the rail and the tie plate, making visual inspection vulnerable to error. In addition to the site visits, the research team performed a metallurgical evaluation on the corroded rail samples. The evaluation included NDE methods (dye penetrant and magnetic particles inspection methods) and destructive methods (metallographic analysis) of the rail. The results of the NDE showed that there is no further damage to the parent rail materials by the corroded area. The NDE findings were confirmed by means of light optical stereoscopy. The light optical microscopy detected two regions: (1) the corroded section and (2) the parent rail. These findings are significant because it indicates that the parent rail microstructure did not present traceable changes, which means the microstructure is fully pearlitic, free of detectable crack or micro-cracks and clearly different from the oxide layer. Two typical corrosion conditions were used as cases for the FEA model: ⢠Significant erosion at the rail base, which removes the majority of the rail base on top of the tie plate ⢠Inward corrosion growth into the base of the rail The first case is easy to detect visually because it extends along the entire tie plate. This type of corrosion has a major contribution to stress concentrations in the vicinity of the ties because sharp corners form along the base of the rail and behave as stress risers. To decrease the risk of a derailment, a standard practice followed by some transit authorities is to remove the rail when 1/8 in. to 1/4 in. of the rail base is removed due to corrosion. The second case is usually more severe and unpredictable. It is more difficult to detect inward growth by visual inspection methods. In addition to the difficulties of visual inspection, this type of corrosion is more severe because it has a tendency to grow internally in both vertical and horizontal directions forming irregular cavities that act as stress concentrators. In the majority of the cases, the internal cavities are not exposed to the exterior, and if they are not detected in a timely manner, they can represent a major risk for catastrophic failure that can result in rail breaks and potential derailment(s).
3 The FEA showed that areas of high stress concentration occur more often near sharp edges. In addition, sharp edges are sometimes hidden at the base of the rail making detection difficult. This can result in an unexpected rail failure. The research team conducted a laboratory corrosion test as supporting evidence of a hypothesis that arose over the course of this project. In this hypothesis, it is proposed that insulation of the track helps to avoid any leak of direct current from the tracks to the ground via clips, spikes, bolts, and tie plates. The research team also proposes a remedy to reduce and ideally eliminate rail corrosion. Chapter 7 details some recommended practices that could be used to reduce or minimize corrosion. These practices can be reduced to the following points: ⢠Conduct proper track maintenance and cleaning practices ⢠Install suitable insulation to prevent stray currents ⢠Redirect water present along the tracks ⢠Identify potential locations for corrosion and prevent the presence of stray currents ⢠Avoid direct contact between rail and track components (fastening system, ballast, ties) ⢠Recognize that timber ties are marginal insulators particularly when chemically treated ⢠Use continuously welded rail where possible; where joint bars are present, use bonding wires for proper connections ⢠Use coated and/or encased insulator materials for embedded tracks
