CP Criteria Comparison

Cathodic protection criteria are not interchangeable. The correct criterion depends on structure type, reference electrode, survey method, environment, and governing requirements.

Use Warning

This page is educational and does not replace applicable AMPP, NACE, ISO, API, DOT, regulatory, owner, or project-specific requirements. Official standards must be consulted for exact requirements.

Quick Definition

A CP criteria comparison explains how different protection criteria are used and why they must be matched to the correct structure, reference electrode, and test method.

Why Criteria Comparison Matters

A common CP error is treating all criteria as if they mean the same thing. A fixed potential criterion, an instant-off criterion, an applied potential criterion, and a polarization criterion evaluate different conditions.

The measurement method matters as much as the number. A reading collected under the wrong condition may not support the intended conclusion.

Comparison Table

Criterion Type What It Evaluates Main Limitation
Fixed potential criterion Whether a structure reaches a specified potential threshold Can be affected by voltage drop and applicability limits
Instant-off polarized potential Structure potential immediately after current interruption Requires proper interruption and timing
100 mV polarization Whether CP caused sufficient cathodic shift Requires valid comparison between protected and depolarized or native states
Coupon-based evaluation CP response at a controlled exposed metal surface Represents the coupon location, not every structure location

Core Concept

Fixed potential criteria

Fixed potential criteria compare a measured potential to a specified value. The reading must use the correct reference electrode and test condition.

Instant-off criteria

Instant-off criteria reduce voltage drop effects by measuring immediately after CP current is interrupted. The interruption method must be valid.

100 mV polarization

The 100 mV polarization criterion evaluates the magnitude of polarization or depolarization caused by CP. It is not the same as reaching a fixed potential value.

Application-specific criteria

Pipelines, tank bottoms, underground storage tanks, marine structures, and reinforced concrete may use different accepted criteria or test methods.

Field Application

Before collecting CP data, the technician should know which criterion will be evaluated. This determines whether readings should be ON, instant-off, native, depolarized, coupon-based, or another test condition.

Reports should avoid vague statements such as “the structure passed CP” unless they identify the criterion, reference electrode, test condition, and standard or requirement being applied.

Common Mistakes

  1. Using ON readings to support an instant-off criterion.
    Why it is wrong: ON readings can include voltage drop.
  2. Using 100 mV polarization as a fallback without valid data.
    Why it is wrong: The criterion requires a valid measured shift caused by CP.
  3. Comparing readings from different reference electrode types.
    Why it is wrong: Different reference scales are not directly interchangeable.
  4. Applying pipeline criteria to all structures.
    Why it is wrong: Different assets may require different standards and criteria.
  5. Ignoring survey limitations.
    Why it is wrong: Poor access, shielding, failed reference cells, or unsynchronized interruption can affect conclusions.

Field Example

A structure has an instant-off potential of −810 mVCSE and a depolarized potential of −690 mVCSE. It does not satisfy a fixed −850 mVCSE polarized potential criterion, but the measured depolarization is 120 mV.

Depending on the applicable standard and test validity, the structure may satisfy a 100 mV polarization criterion even though it does not satisfy the fixed potential criterion.

Practice Questions

  1. Why are CP criteria not interchangeable?
  2. Why can ON readings be unsuitable for some criteria?
  3. What does the 100 mV polarization criterion evaluate?
  4. Why must the reference electrode type be identified?
  5. What information should a CP report include when stating that a criterion was satisfied?

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