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Consider the anode interference situation of Figure 2. A coated pipeline is protected by an impressed current groundbed. In the vicinity of the groundbed, the electrolyte will obtain a higher potential level than more remote areas. If a foreign, i.e. a non connected, pipeline passes through this area, it will suffer from earth current corrosion. Current will enter the pipeline close to the anode and will leave the pipeline at places remote from it. Indeed, the total current balance for a non-connected pipeline should be zero. This kind of interference is called anode interference.
A simulation of such a situation has been performed for a soil with a uniform resistivity of 30 W m. The protected pipeline (outer diameter of 0.15 m, length 2000 m) receives 8 A from the groundbed and is well coated, having a coating with resistance of 3800 W m2. The foreign pipe, having similar geometrical characteristics, is considered bare. Polarisation data for the bare steel can be entered in the computer program as a set of spline data points, as presented in Figure 3. In this and all the following simulations, the characteristics of the foreign pipeline remain the same, except for the cases where it is coated.
The predicted phenomena are observed in Figure 4, representing the overpotential along the foreign pipe under interference when it is at a distance of 5 m from the ground-bed. Current enters the pipe close to the groundbed (negative current density) and locally protects the pipe (overvoltage < -0.85 V). At distant areas, this gathered current will leak away, with corrosion danger as a logic consequence.
The value and distribution of the positive potential close to the groundbed depends on the groundbed shape, its current/voltage operation mode, and the ground resistivity. The simplest manner for avoiding anode interference is to keep a sufficient distance between the groundbed and the foreign structure. This can be seen from Figure 5 where the current density along the foreign pipe is plotted as a function of the pipe-to-groundbed distance. It is obvious that from a certain distance, the danger of corrosion by virtue of anodic interference becomes negligible. Another remedy consists of coating the foreign pipe in the vicinity of the groundbed. This reduced the amount of current that is picked up and consequently leaves at the distant anodic zones.
Finally, sacrificial anodes can be attached at the remote anodic zones. The stray current will then prefer these sacrificial metals as output to earth zones. CatPro allows to model such situations, but no results have been presented here.
As indicated, close to the groundbed, the foreign structure has a cathodic aspect and receives a protection current over well located areas. This current returns to earth at zones remote from the groundbed. As a result, the picked up current leaves the pipes over vast areas, generally by virtue of small current densities. Therefore, this type of interference seldom causes severe and localised corrosion, except when the pipeline is coated at the anodic zones.
On the object of stray current corrosion see also: DC traction, Cathodic protection, Coating, Contour plots, Definition, Detection, Examples, External currents, Historical perspective, Impressed current, Interference, Mechanisms, Modeling. Pipeline, Potential distribution, Prevention, Stray fields and leakage, Transit systems
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