Interference
Interference occurs when electrical current from one system affects another metallic structure, potentially causing corrosion where current leaves the affected structure.
Quick Definition
CP interference is an unintended current interaction between structures or systems that can shift potentials, transfer current, or create corrosion risk.
Why This Term Matters
Interference can cause corrosion even when a CP system appears to be operating correctly. A protected structure may discharge current onto another structure, or a foreign structure may be affected by a nearby CP groundbed.
Increasing rectifier output without checking interference can improve protection at one location while creating corrosion risk somewhere else.
Core Concept
Current pickup
Current pickup occurs where current enters a structure from the electrolyte. This usually shifts potentials in the cathodic direction.
Current discharge
Current discharge occurs where current leaves a structure and enters the electrolyte. Discharge areas can be corrosion risk locations.
Foreign structures
Nearby pipelines, tanks, casings, grounding systems, utility structures, and other metallic systems may be affected by a CP system.
Bonds and mitigation
Bonds, resistors, diodes, drainage systems, coating repair, anode relocation, or output adjustment may be used to control interference depending on the condition.
Testing
Interference testing may include synchronized current interruption, potential shift measurements, bond current measurements, current direction testing, and coordination with other structure owners.
Common Mistakes
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Assuming more CP output is always better.
Why it is wrong: Higher output can increase interference on nearby structures. -
Ignoring current discharge locations.
Why it is wrong: Corrosion risk is often associated with current leaving the affected structure. -
Changing bonds without checking system-wide effects.
Why it is wrong: Bond changes can redistribute current and create new problems.
Field Example
A pipeline rectifier is adjusted upward. The protected pipeline potentials improve, but a nearby foreign pipeline shows a positive shift at a crossing and current discharge is suspected. The adjustment may have created or worsened interference.