Cathodic Protection for Pipelines

Why Pipeline CP Matters

Buried pipelines are exposed to soil electrolytes, coating damage, disbonded coating, moisture variation, stray current, electrical continuity issues, and other external corrosion risks.

Coatings are the primary external corrosion barrier, but coatings are not perfect. Cathodic protection is used to protect exposed steel at holidays, damaged coating, field joints, and other defects.

Pipeline CP must be evaluated over distance. A reading at one test station does not prove that the pipeline is protected between test stations.

Core Concept

Coating and CP work together

Pipeline coatings reduce the exposed steel area and lower current demand. CP supplies current to exposed metal at coating defects.

Galvanic pipeline CP

Galvanic anodes may be used for short isolated pipelines, well-coated segments, casings, temporary protection, or localized protection where current demand is low.

Impressed current pipeline CP

Impressed current systems are commonly used for long pipelines or systems with higher current demand. Rectifiers, anode groundbeds, bonds, and test stations are used to control and monitor protection.

Electrical continuity

CP current must be able to return through the metallic pipeline. Isolation devices, bonds, casings, foreign structures, and facility grounding can all affect continuity and current distribution.

Current distribution

Pipeline CP performance varies along the route. Soil resistivity, coating condition, groundbed location, shielding, interference, and pipeline geometry all affect current distribution.

Field Application

Pipeline CP is commonly evaluated using test station surveys, close interval surveys, rectifier inspections, current interruption, bond current measurements, casing tests, isolation tests, and interference surveys.

Annual surveys provide routine monitoring at selected points. Close interval surveys provide more detailed data along the pipeline route and are better suited for identifying localized underprotection.

Potential data must be interpreted with the applicable criterion, reference electrode, test condition, and survey method.

Common Mistakes

1. Assuming test station readings prove the entire pipeline is protected.
   Why it is wrong: Localized underprotection may exist between test stations.
2. Ignoring coating condition.
   Why it is wrong: Coating breakdown increases current demand and affects current distribution.
3. Ignoring isolation and bonds.
   Why it is wrong: Electrical continuity to unintended structures can change CP current demand.
4. Using ON potentials without considering voltage drop.
   Why it is wrong: ON potentials can include IR drop and may overstate protection.
5. Increasing rectifier output without evaluating interference.
   Why it is wrong: Higher output can create stray-current effects on nearby structures.

Field Example

A pipeline satisfies CP criteria at annual test stations, but a close interval survey identifies a short area with less negative potentials between two stations.

The annual survey did not provide enough spatial coverage to identify the localized issue. Additional evaluation may include coating assessment, interference testing, current distribution review, and rectifier adjustment.

Practice Questions

1. Why are coatings and CP commonly used together on pipelines?
2. Why can annual test station readings miss localized underprotection?
3. What is the purpose of a close interval survey?
4. Why can failed electrical isolation affect pipeline CP?
5. Why should interference be considered before increasing rectifier output?

Related Pages

• CP Survey Methods
• Structure-to-Electrolyte Potential
• Electrical Isolation
• Bonds
• AMPP NACE SP0169 Overview