5. Cathodic Protection

5.7 Cathodic Protection Interference

Every cathodic protection scheme operates by injection of direct current into the mass of soil in the proximity of the protected pipeline in metallic structure. This  current is likely to be picked up by other underground structures such as water pipes and power cables and returned to the protected pipe via the soil at some other potential. In a way, the secondary pipe would act as a secondary ground bed that discharges current and corrode the point of discharge. Corrosion of the unprotected pipe in the vicinity of the cathodically protected pipe would take place at all points of discharge of current. The corrosion damage caused to underground structures by cathodic protection systems in another structure is called "interference".


Figure shows two pipes. At point 1, the secondary pipeline (foreign pipeline) crosses the cathodically protected pipeline passing through the zone of positive earth potential and picks up the current. The current must complete the circuit and return to the negative terminal of rectifier; the current is discharged from the earth, picked up by by the protected pipeline and returned to negative terminal. Corrosion would therefore occur at the sites of the secondary pipeline where the current leaves the structure. The greater the magnitude of the current, the greater is the damage.

Consider now the location 2. The unprotected pipeline picks up the current resulting in protection at the pick up area. The current leaves the pipe at the other point which becomes anodic and corrodes. The pipe-to-soil potential measured with the Cu-CuSO4 electrode becomes more positive when the rectifier is switched off. A positive charge of potential when the rectifier is switched off and on indicates an uncontrolled (stray) current. Current would be picked up in the area of positive earth potential. The current would leave the secondary pipeline to flow to the protected pipeline and back to the rectifier to complete the current.

The situation with a pipe parallel to the cathodically protected pipe is shown below.

Correction of Interference:

  1. Use of drainage band: A drainage band could be installed to correct  interference. The band provides the amount of current drain required to restore potential with the "rectifier-ON" to the same value as "rectifier-OFF".

  2. Coating of the secondary pipeline in the area of influence (positive soil potentials).

  3. Installation of galvanic anodes such as Magnesium on the secondary structure.

  4. Relocation of the groundbed.

  5. Reduction of the current output of the rectifier.

  6. By electrically bonding the pipeline (protected) and the secondary pipeline.