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)

Anodic, Active (Read down)
Magnesium
Zinc
Aluminum
Mild Steel
Wrought Iron
Gray Iron and Ductile Iron
Lead
Tin
Brass
Copper
Stainless Steel, Type 304
Stainless Steel, Type 316
Titanium
Silver
Gold
Platinum (Read up)
Cathodic, Noble

Corrosion Solution

In the complexity of a metropolitan district, the path of stray currents can be difficult to follow. To reiterate the important point, anodes corrode; cathodes do not. As such, it is only necessary to make a pipeline sufficiently cathodic to prevent its corrosion. If a current is passed from the earth to a pipeline, the incoming currents will nullify any outgoing currents from the anodes of local corrosion cells. Further, the pipe, receiving current over its entire area, will be immune from corrosion. A current through the earth can be easily produced by galvanic action from the energy of corrosion of a magnesium anode. In this case, a piece of magnesium is connected to the pipe with wire and buried, away from the pipe. Because magnesium is much more active than the iron or steel in the pipe (as demonstrated in Table 1), a considerable voltage is established between the magnesium and the pipe. Current will flow from the magnesium (anode), through the earth, to the pipe (cathode). The electrical circuit is completed when the current flows from the pipe to the anode through the wire (see Figure 3).

Today, the vast majority of buried pipelines are coated with some kind of organic coating. Corrosion can be prevented completely by either maintaining a perfect coating or impressing a protective cathodic current density on a bare line. Maintaining a perfect coating is impossible as a perfect coating does not exist. Impressing a protective cathodic current density is overly expensive due to the high current demand. Somewhere between these two extremes lies the economically optimal combination of a good coating supplemented by cathodic protection to protect the inevitable imperfections in the coating