Pipelines and light rail transit: a delicate balance: keeping stray-current corrosion at bay.

In many American cities, especially those with widely dispersed, fast-growing populations, light rail transit (LRT) has become an increasingly popular transportation alternative, offering a way to relieve congestion, prepare for future growth, and in some cases, spur downtown revitalization.

Twenty U.S. municipalities, including Boston, Philadelphia, Baltimore, Washington DC, San Jose (California), Minneapolis, Denver, Dallas, Houston, Portland (Oregon), Sacramento and Los Angeles, have put new LRT systems into operation since 1960. Another 37 cities have LRT systems planned or in the works, according to the American Public Transportation Association (APTA). For pipeline operators in those areas, this means new operational and safety concerns, including the risk of stray-current corrosion.

Most LRT systems resemble the streetcars of a century ago, with lightweight passenger units operating singly or in short, usually two-car, trains on fixed rails in a right-of-way that may be partly or entirely shared with other traffic. Most are powered electrically. Substations with DC (direct-current) rectifiers supply electricity to the train through an overhead cable via a trolley or a pantograph. DC power is preferred for rail transit because of its superior torque characteristics. The overhead cable is connected to the positive side of the rectifier. The rails on which the train travels serve as the negative (return) conductors connected to the negative side of the rectifier.

The DC current will use any conductive medium to return to the substation. This means the entire LRT system, including trackwork, power system and other equipment, including train yards, shops, etc., can potentially send stray electric currents, sometimes called "leakage currents"--i.e., unwanted, non-designed currents--into the surrounding soil and onto nearby buried structures. Stray-current corrosion occurs at the point where the current leaves the pipeline, carrying iron ions that became positively charged when they lose one or more electrons.

The damage is unseen but the effect can be major. A 1-ampere current discharging continuously from a steel pipeline will remove approximately 20 pounds of steel in a year. Since many LRTs are located in crowded utility corridors and in city streets with gas and oil pipelines and other underground utilities and structures, uncontrolled stray current can cause extensive damage to rail and concrete reinforcing steel, cables, and pipelines. By one estimate, stray-current corrosion damage in the U.S. may total over $500 billion annually.