Corrosion Basics: Stray Current Effects

The sources of dynamic stray currents include direct current (DC) railway systems.

Before preparing a cathodic protection (CP) design, the possible presence of stray currents must be considered. Stray currents are defined as those that follow a path other than the one intended. Where stray currents discharge from a structure into the electrolyte environment in order to return to the source, corrosion will occur. If the corrosion is concentrated over a limited surface area, the integrity of the structure may be threatened in a relatively short period. The most severe stray current influences are often those that continually vary in quantity. The sources of dynamic stray currents include direct current (DC) railway systems, DC-powered mining operations, and DC welding operations.

When a train passes a specific location, particularly during acceleration, the rails of the transit system tend to discharge a portion of the load current into the earth as stray current. When no train is nearby, the rails tend to behave as extensions of the traction power substation’s negative bus and draw current from the adjacent earth and buried metallic structures. Typically, these driving voltages are smaller than those associated with discharges from the rails; however, they represent a majority of the transit system’s operating schedule.

The magnitudes of anodic potentials at areas of stray current discharge may be so great that they cannot be counteracted readily with the usual CP system. In some cases, the large operating voltages of the stray current sources can make an affected structure several volts positive with respect to its environment. Mitigation of such conditions may require extraordinary approaches such as limited recoating in the pickup or discharge area or relocation of the structure. Metallic bonds from the affected structure to the stray current source, to limit the amount of current discharging through the environment, are generally undesirable because they establish interdependent operation with the source system; however, there may be no practical alternative.

Another type of varying stray current is “magnetic storm” or sunspot activity. Long structures such as pipelines can be affected when the intensity of the earth’s magnetic field varies. Fluctuating potentials can be introduced in a pipeline in much the same manner as potentials are induced in an electric generator.

Manmade variable stray currents can often be identified by characteristic patterns, such as those associated with acceleration/deceleration or rush-hour peaks of transit systems. While they may be correlated with solar activity, magnetic disturbances rarely, if ever, exhibit any pattern. Because stray telluric currents are usually of relatively short duration and are seldom concentrated in any specific area, they are not expected to cause as much corrosion as uncontrolled manmade stray currents.

In addition to dynamic influences, steady-state stray currents also may be encountered. These may be caused by an impressed current CP system on an adjacent but electrically separate structure if the groundbed is too close to the foreign structure, especially where the foreign structure is positioned between the groundbed and the structure to be protected. This is an example of a critical design flaw that should be avoided. If a foreign pipeline passes through the potential gradient field around the groundbed, the soil is typically positive with respect to the pipeline and causes stray current to accumulate on the pipeline. Because there is no metallic path by which this current can flow back to its source, the stray current must complete its circuit by discharging into the earth. This current discharge may be concentrated at a crossing with the protected pipeline. Possible solutions include limited recoating, properly designed metallic bonds, or relocation of the influencing groundbed.

CP system design must consider possible stray current effects on nearby structures. This includes careful placement of groundbeds, which can be especially important for impressed current systems because of the greater driving voltages involved.

This article was adapted by MP Technical Editor Norm Moriber, Mears Group, from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 501-502.

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