In the proximity of a protected structure (a pipeline or tank), the soil will receive a negative potential swing. A pipeline passing through this zone will suffer corrosion by current that enters the pipe remote from the protected structure and that leaves the pipeline in a region close to the protected structure. This kind of stray current corrosion is called cathode interference.
In Figure 6, such a situation has been calculated for a foreign pipe at a distance of 10 m from the protected tank. All characteristics of the foreign pipe (geometry, polarisation, ...) are as before. Corrosion of this kind is more serious than its anodic fellow, since the current leaving the pipe close to the protected structure has a high current density, due to the localised anodic zone on the foreign pipe. This foreign body only needs to be within the area of negative potential over a relatively short distance, for a severe concentrated corrosion to occur.
This can be seen from the overpotential distribution along the foreign pipe, presented in Figure 7. Note that the whole pipe has an overvoltage substantially below the protection level of -.85 Volt, indicating corrosion danger. The fact that the pipe resides at a potential below the protection level, is explained by the shape of the used polarisation curve (see figure 3), that needs a higher cathodic current density for local protection. The entering current has such a low current density that the protection level is not obtained.
The best method to reduce or eliminate cathode interference is to coat the protected pipeline. Coating the anodic zone of the foreign pipeline is not advised, since this would in-crease, in case of a coating failure, the current density and the danger of pitting corrosion. For large and bulky structures, the application of a coating might become difficult, therefore some alternative techniques exist.
First of all, the foreign pipeline can be bonded to the protected structure using a current drainage technique. Consequently, it will receive cathodic protection too. This amount of protection can be limited by introducing a resistance in the connection circuit. Figure 8 shows the impact of the connection of the foreign pipeline to the protected tank. The pipeline now receives protection too and no longer suffers from anodic currents. Of course, this is an expensive solution since the protection current is distributed between the tank and the pipe. This means that for the same amount of input power, the tank will receive less protection current.
A second method of preventing cathode interference is to place a piece of scrap metal, anodic to the foreign pipeline, between the two structures and connect it electrically to the foreign body. This metallic piece will now act as a sacrificial anode, since it provides an easier path for the stray current that returns to the protected structure through the soil. The closer the scrap metal approaches the protected tank, the less anodic current leaves directly the pipeline.
See also: Anode interference, Cathode Interference, Combined Interference, DC Traction Interference, FPSO ICCP , Induced Interference
Study and Evaluation of Stray Current Influences on Cathodic Protection Systems of Buried Pipelines, L. Bortels, ELSYCA - Kranenberg 6 - 1731 - BELGIUM, ELSYCA