Solving Stray Current Mitigation In Portland’s Rail System

By Aaron Eder, P.E. | August 2014, Vol. 69, No. 8
Rendering of the Portland-Milwaukie light rail system. (photo courtesy of TriMet)

The basic fundamental cause of corrosion can be explained in terms of energy. It is much more natural for a metal to exist in the form of a compound, since compounds such as oxides contain less energy than metals and are therefore much more stable. When metallic pipes are made, iron is separated from its associated oxygen in a blast furnace. This produces a lot of energy in the form of heat. As long as they remain metallic, steel and iron pipes retain this energy, bound up within itself, with a natural tendency to corrode back to the ore from which it was derived. It is this energy that drives the corrosion process.

In order for corrosion to occur, there must be a complete electrical circuit, including an anode and a cathode. The anode is the location where the current leaves the metal, and the cathode is the location that the current flows to. In the case of electric light rail systems, the current goes out from the substation along the overhead wire until it reaches the train, passing through the motors and theoretically returning along the rails to the substation. However, if a reasonably low-resistance parallel path exists, whereby the current may follow pipelines and cables to return to the negative ground of the substation, a portion of the current will take this route. In these cases, the current follows a parallel metallic path, such as a water main. Portions of water mains can be electrically continuous due to typical construction methods such as leaded joints on cast iron pipes and mechanically restrained joints on ductile iron pipes.

Where the current jumps from the rails to the water main, the rail is serving as the anode and the water main is serving as the cathode. In these cases, the water main is protected, because electrons are not being removed from the pipe. Where the current leaves the water main to enter the lead cable, the water main is serving as the anode and the cable is serving as the cathode. In these cases, electrons are leaving the pipe with the current and the pipe is corroding. Table 1 presents a galvanic series of selected metals common in construction. In a galvanic cell of two dissimilar metals, the more active metal will act as the anode and be corroded. The more noble metal will act as the cathode and be protected.

Galvanic Series of Selected Metals Common in Construction