Guide 5 of 5: Understanding why protection levels vary along a structure.
Current Distribution and Attenuation Explained: Why Protection Is Never Uniform
1. Current Does Not Spread Evenly
When protective current is applied, it does not distribute uniformly along a structure. Current follows paths of least resistance through both the metallic path and the electrolyte.
Areas closer to the anode or power source typically receive higher current density than distant areas.
2. Resistance Causes Attenuation
As current flows through the metallic and electrolyte paths, it encounters circuit resistance. That resistance causes the current magnitude to decrease with distance. This gradual reduction is attenuation.
Attenuation is governed by Ohm’s Law and depends on structure resistance, soil resistivity, coating condition, and geometry.
3. Impact on Current Density
Because current decreases with distance, current density at the metal–electrolyte interface also decreases.
Lower current density produces lower polarization, increasing risk of underprotection at remote sections.
4. Geometry and Coating Effects
Changes in pipe diameter, branching lines, coating defects, and soil resistivity variations all alter current distribution.
Large bare areas draw more current, further increasing non-uniformity across the system.
5. Design Implications
Proper anode spacing, rectifier output, and bonding strategies are required to compensate for attenuation.
CP design aims to achieve sufficient polarization everywhere, not just near the power source.
6. The Complete Logic Chain
Driving Voltage → Current Flow → Resistance → Attenuation → Reduced Current Density → Reduced Polarization → Potential Underprotection
Understanding attenuation explains why protection criteria must be verified along the entire structure.
- From Driving Voltage to Polarization
- Why Instant-Off Matters
- Understanding Corrosion Using Mixed Potential Theory
- Why AC Can Still Cause Corrosion
- Current Distribution and Attenuation Explained (Current Guide)