Coating Condition
Current concentrates at holidays and exposed metal. Poor or damaged coating increases total demand and changes the potential profile.
Current Distribution in Cathodic Protection
Current distribution describes where CP current actually goes after it leaves the anode system. It is the difference between simply producing current and delivering useful protection to the structure locations that need it.
Factors That Control Current Distribution
Anode Location
Remote, close, deep, distributed, and linear anodes all produce different gradients and current paths.
Electrolyte Resistivity
Soil resistivity differences can redirect current and create uneven protection along the structure.
Structure Geometry
Branches, tanks, casings, bonds, risers, and congested piping affect current paths.
Electrical Continuity
Current cannot distribute correctly across unintended isolations, poor bonds, or discontinuous sections.
Time and Polarization
Potential response changes as the structure polarizes and as environmental conditions change.
Why It Matters
A rectifier can have sufficient output and a groundbed can have acceptable resistance while parts of the structure remain underprotected. The reverse can also occur: high current may be flowing because of a short, foreign structure, or large coating defect, not because the system is protecting efficiently.
Current distribution is therefore central to CP 3 and CP 4 judgment. It connects rectifier output, groundbed design, coating quality, interference, attenuation, and survey interpretation.
Field Clues
- Large potential changes near an anode bed but little change farther away.
- Protected readings at test stations but inadequate readings in a CIS between stations.
- High rectifier current with limited polarization response.
- Localized overprotection close to distributed anodes.
- Potential dips near casings, bonds, crossings, or shielded areas.