100 mV Polarization Criterion: Standards Context

Why Standards Context Matters

The 100 mV criterion is often misunderstood as a simple fallback when a fixed potential criterion is not satisfied. That is incorrect. The criterion must be permitted by the governing requirement and supported by valid test data.

A measured 100 mV difference is not automatically acceptable. The comparison must represent the same structure or test point, the same reference electrode scale, appropriate timing, and a valid protected-to-native or protected-to-depolarized condition.

The applicable standard or project requirement determines whether the criterion may be used, how it is documented, and what limitations or exceptions apply.

Standards-Related Evaluation Points

Applicability

The first question is whether the criterion is allowed for the structure, environment, material, and governing requirement. It should not be applied automatically across pipelines, tank bottoms, UST systems, marine structures, and reinforced concrete.

Valid comparison

The readings compared must represent a valid polarization or depolarization relationship. Unrelated readings, readings from different locations, unstable reference electrodes, or mixed test conditions do not support a defensible conclusion.

Current source control

Remaining CP current sources can invalidate or weaken a depolarization test. The report should identify which sources were interrupted, disconnected, or left active, and how that affects the conclusion.

Timing

Polarization and depolarization are time-dependent. The timing of readings must be consistent with the selected method and documented clearly.

Reference electrode scale

The reference electrode type must be identified. Readings on different reference scales are not directly interchangeable without proper interpretation.

Documentation Expectations

A defensible 100 mV polarization statement should identify the structure, test location, reference electrode, test condition, timing, interrupted or disconnected current sources, measured values, calculated shift, and applicable requirement.

Reports should avoid vague statements such as "passed CP" without explaining which criterion was evaluated and what data supports that conclusion.

Common Standards Mistakes

1. Using the 100 mV criterion as an automatic fallback.
   Why it is wrong: The criterion must be allowed by the governing requirement and supported by valid data.
2. Comparing unrelated readings.
   Why it is wrong: The measured shift must represent the same structure condition or valid test point.
3. Ignoring remaining current sources.
   Why it is wrong: Active current sources can prevent valid depolarization or distort the measured shift.
4. Failing to document timing.
   Why it is wrong: Polarization and depolarization are time-dependent measurements.
5. Citing the criterion without the applicable standard or requirement.
   Why it is wrong: Acceptance depends on the governing document, structure type, environment, and test method.

Field Example

A tank-bottom fixed reference cell records an instant-off potential of −760 mV_CSE. After depolarization, the reading shifts to −640 mV_CSE. The measured shift is 120 mV.

That calculation alone is not the full conclusion. The report must also establish that the reference cell is reliable, the current sources were properly controlled, the timing is valid, and the applicable requirement permits the 100 mV polarization criterion for the condition being evaluated.

Practice Questions

1. Why is the 100 mV criterion not an automatic fallback when a fixed potential criterion is not satisfied?
2. What must be documented when reporting a 100 mV polarization result?
3. Why can remaining current sources invalidate a depolarization test?
4. Why must the compared readings use the same reference electrode scale?
5. Why should the official standard or project requirement be consulted before using this criterion?

Related Pages

• 100 mV Polarization Criterion
• Polarization Change
• Instant-Off Potential
• Cathodic Protection Criteria
• CP Criteria Comparison