Shielding

Why This Term Matters

Shielding is dangerous because normal CP readings may look acceptable while a shielded area remains underprotected. The CP system may be operating, but current may not reach the metal surface where corrosion is occurring.

Shielding can make survey data difficult to interpret and may require additional inspection methods beyond standard potential measurements.

Core Concept

Disbonded coating

Disbonded coating can trap electrolyte against the structure while preventing CP current from reaching the steel surface.

Casings and sleeves

Casings, sleeves, or conduits can create shielded environments where CP current distribution is limited.

Concrete or grout

Concrete encasement, grout, or other coverings may restrict current access depending on conductivity, continuity, cracking, and moisture.

Deposits and debris

Marine growth, scale, sediment, deposits, or debris can restrict current access to the metal surface.

Measurement limitation

A nearby potential reading may not represent the shielded surface condition.

Common Mistakes

1. Assuming acceptable nearby potentials prove shielded areas are protected.
   Why it is wrong: The reading may not represent the shielded metal surface.
2. Ignoring disbonded coatings.
   Why it is wrong: Disbonded coating can block CP current while allowing electrolyte contact.
3. Assuming more rectifier output solves shielding.
   Why it is wrong: If current cannot reach the surface, higher output may not correct the shielded condition and may create interference or overprotection elsewhere.

Field Example

A pipeline has acceptable potentials at nearby test stations, but corrosion is found under disbonded coating during excavation. The CP current did not effectively reach the steel beneath the disbonded coating even though nearby readings appeared acceptable.

Related Pages

• Cathodic Protection Troubleshooting
• CP for Pipelines
• CP for Facility Piping
• CP Survey Methods
• IR Drop