Native Potential: Standards Context

Native potential is the structure-to-electrolyte potential measured before cathodic protection is applied or after the structure has sufficiently depolarized.

Quick Definition

Native potential is the natural corrosion potential of a structure in an electrolyte before CP influence or after CP influence has been removed long enough for depolarization.

Why Native Potential Matters

Native potential is used as a baseline for understanding how much a structure shifts when cathodic protection is applied. It can also help evaluate polarization or depolarization criteria where applicable.

Native potential is not a fixed universal value. It depends on material, electrolyte, oxygen concentration, coating condition, temperature, moisture, corrosion activity, and other environmental conditions.

A native reading must be interpreted carefully. If CP current, stray current, bonds, or nearby systems still influence the structure, the reading may not represent a true native condition.

Core Concept

Unprotected condition

A native potential is intended to represent the structure without cathodic protection influence. This may be measured before CP installation or after CP current has been removed and the structure has depolarized.

Use in polarization evaluation

Native potential may be compared with a polarized potential to determine the amount of cathodic shift caused by CP.

Depolarized condition

In existing CP systems, a fully native condition may not be practical to obtain. A depolarized potential may be used as part of a defined test method when allowed by the governing requirement.

Measurement limitations

Native potential can be affected by residual polarization, foreign CP systems, stray current, coating condition, moisture, and reference electrode placement.

Field Application

Native or depolarized readings are used during current requirement testing, commissioning, depolarization surveys, troubleshooting, and 100 mV polarization evaluations.

Field personnel must document how the native or depolarized condition was obtained, how long CP current was removed, which sources were interrupted or disconnected, and whether other current sources may still affect the structure.

Common Mistakes

  1. Calling a reading native while CP current is still influencing the structure.
    Why it is wrong: The reading does not represent the unprotected or depolarized condition.
  2. Assuming native potential is the same everywhere on a structure.
    Why it is wrong: Electrolyte and surface conditions vary by location.
  3. Using native readings from one location to calculate polarization at another.
    Why it is wrong: The readings must represent a valid comparison for the same structure condition.
  4. Ignoring residual polarization.
    Why it is wrong: Structures may remain polarized for some time after CP is removed.
  5. Ignoring foreign current sources.
    Why it is wrong: Nearby CP systems or stray current can shift the measured potential.

Standards Relevance

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

Native and depolarized potential use depends on the applicable criterion, test method, structure type, and governing requirement.

Field Example

A buried steel structure has a native potential of −610 mVCSE before CP is energized. After CP is applied and stabilized, the instant-off potential is −760 mVCSE.

The cathodic shift is 150 mV. If the method is valid and the applicable requirement permits the criterion, this may support a 100 mV polarization evaluation.

Practice Questions

  1. What is native potential?
  2. Why can residual polarization affect a native or depolarized reading?
  3. Why should current sources be documented when obtaining native readings?
  4. Why is native potential not the same everywhere on a structure?
  5. How can native potential be used in a polarization calculation?

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