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Figure 9 presents a combined interference situation that often occurs in reality. A long pipe (B1-B2) passes close to the anode (GB1,GB2) and, more remote along its route, crosses the protected (and cathodic) structure. Close to the anode, the pipe will pick up current that leaves the foreign pipe close to the cathode. These effects amplify each other and this type of corrosion is called combined interference.
In Figure 10, a simulation of a combined interference situation is presented. Note the dip in the current density pattern at the crossing of both pipes. There, the pipe receives an additional protection current that comes from the foreign pipe. This means that at that location, the foreign pipe can suffer from severe corrosion, since there exists a localised spot where a concentrated current density leaves the foreign pipe and enters the protected pipe.
The techniques for preventing this type of corrosion are similar to those of the anode and cathode interference, since both mechanisms are present here. Indeed, the pipe under interference suffers anodic interference in the proximity of the groundbed and cathodic interference at the crossing with the protected structure.
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