Galvanic vs Impressed Current CP

Galvanic and impressed current CP systems both reduce corrosion by supplying protective current, but they differ in current source, output capacity, control, maintenance, and typical applications.

Quick Definition

Galvanic CP uses sacrificial anodes as the current source, while impressed current CP uses an external DC power source to drive current from anodes to the protected structure.

Why the Difference Matters

Selecting the wrong CP system type can result in inadequate protection, excessive maintenance, wasted cost, or interference problems. Galvanic systems are simpler but limited. Impressed current systems are more powerful and adjustable but require more control.

The correct choice depends on current demand, coating condition, electrolyte resistivity, structure size, electrical isolation, design life, monitoring requirements, and nearby structures.

Core Concept

Galvanic CP

Galvanic CP uses an anode material that is more active than the protected structure. When electrically connected in a common electrolyte, the anode corrodes preferentially and supplies current to the structure.

Galvanic systems are normally simple, passive, and self-powered. Their current output depends on driving voltage, circuit resistance, anode condition, electrolyte resistivity, and structure current demand.

Impressed current CP

Impressed current CP uses an external DC power source, commonly a rectifier, to drive current from anodes to the protected structure. The output is adjustable and can support larger or higher-current-demand structures.

Impressed current systems require more monitoring and control. Poor adjustment can cause underprotection, overprotection, or interference with nearby structures.

Comparison Table

Feature Galvanic CP Impressed Current CP
Current source Sacrificial anode material External DC power source
Adjustability Limited Adjustable
Typical current output Lower Higher
Power required No external power Requires power source
Common use Small or well-coated structures Large or high-current-demand structures
Main risk Insufficient output Improper adjustment or interference

Field Application

Galvanic systems are often used for underground storage tanks, short isolated piping, small coated structures, and marine attachments. Impressed current systems are often used for long pipelines, large tank bottoms, facility piping networks, and structures with larger current demand.

In the field, galvanic systems are commonly checked for anode condition, current output, lead continuity, and structure potentials. Impressed current systems are checked for rectifier output, anode circuit performance, structure circuit continuity, current distribution, and interference.

Common Mistakes

  1. Choosing galvanic CP for a structure with high current demand.
    Why it is wrong: Galvanic systems may not have enough driving voltage or current capacity.
  2. Choosing impressed current CP when a simple galvanic system would work.
    Why it is wrong: Impressed current systems add power, monitoring, adjustment, and interference concerns.
  3. Assuming galvanic systems require no inspection.
    Why it is wrong: Galvanic anodes are consumed and connections can fail.
  4. Assuming impressed current systems are better in every case.
    Why it is wrong: More output and adjustability do not automatically mean better design.
  5. Ignoring electrical isolation.
    Why it is wrong: Unintended continuity can increase current demand and change system behavior.

Standards Relevance

This page is educational and does not replace the applicable AMPP, NACE, ISO, DOT, API, or project-specific requirements.

Standards generally do not treat galvanic and impressed current systems as interchangeable in all applications. The system type must be evaluated against the structure, environment, current demand, and required monitoring method.

Field Example

A short isolated coated line in low-resistivity soil may be a reasonable candidate for galvanic CP. A long pipeline with many coating defects and high current demand would usually require impressed current CP.

The decision should not be based on preference. It should be based on current requirement, design life, electrolyte conditions, operating constraints, and survey results.

Practice Questions

  1. What is the current source in a galvanic CP system?
  2. What is the current source in an impressed current CP system?
  3. Which system type is generally more adjustable?
  4. Why might a galvanic system be inadequate for a large bare structure?
  5. What is one major risk of excessive impressed current output?

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