Cathodic Protection Criteria

Why CP Criteria Matter

CP criteria convert field measurements into technical conclusions. Without criteria, a potential reading is only a number. With the correct criterion, reference electrode, measurement condition, and survey method, the reading can support a defensible conclusion about protection.

Criteria are not universal shortcuts. A criterion that is appropriate for a buried steel pipeline may not be appropriate for an aboveground storage tank bottom, underground storage tank, marine structure, reinforced concrete structure, or mixed-metal system.

Misusing criteria is one of the most serious CP interpretation errors. It can create false passing conclusions, unnecessary repairs, missed corrosion risk, invalid reports, or confusing recommendations.

Main Types of CP Criteria

Fixed-potential criteria

Fixed-potential criteria evaluate whether the structure reaches a specified potential relative to a defined reference electrode. The common −850 mV_CSE concept is an example for certain buried or submerged steel applications, when used under the proper test condition.

Polarization-shift criteria

Polarization-shift criteria evaluate whether CP has caused a specified change in structure potential. The 100 mV polarization criterion is the most familiar example. It focuses on the structure response to CP rather than only an absolute potential value.

Application-specific criteria

Some structures require application-specific evaluation methods. UST systems, AST bottoms, reinforced concrete, marine structures, internal surfaces, and high-temperature or unusual environments may require criteria and test procedures that differ from common pipeline assumptions.

Core Interpretation Principles

Criteria are tied to measurement method

A CP criterion must be matched to the way the data were collected. ON potentials, instant-off potentials, native potentials, depolarized potentials, and coupon measurements do not represent the same test condition.

Reference electrode type must be known

A potential value is incomplete without the reference electrode. A reading of −850 mV means something different if it was measured versus CSE, silver-silver chloride, zinc, or another reference scale.

Voltage drop must be considered

ON potentials may include voltage drop through the electrolyte. That voltage drop can make a reading appear more electro-negative than the polarized structure surface condition. This is why instant-off measurements are important in many polarized-potential evaluations.

Criteria support judgment; they do not replace it

Field readings may be affected by shielding, coating condition, current distribution, stray current, electrical isolation problems, reference electrode placement, continuity issues, and unstable electrolyte conditions. Criteria must be interpreted with those limitations in mind.

−850 mV_CSE vs 100 mV Polarization

The −850 mV_CSE criterion and the 100 mV polarization criterion answer different questions.

The −850 mV_CSE criterion asks whether the structure reached a specified potential level relative to a copper-copper sulfate reference electrode under the required measurement condition. The 100 mV criterion asks whether CP caused a sufficient change in structure potential.

Because they evaluate different things, a structure can fail one criterion and satisfy the other, depending on the applicable standard and the validity of the test method. A technically sound report should identify which criterion was evaluated and why the conclusion follows from the data.

What CP Criteria Do Not Prove

A passing criterion result at one location does not prove the entire structure is protected. It does not prove there is no coating damage, no shielding, no interference, no isolated metallic area, and no current distribution problem.

Criteria must be applied with adequate survey coverage and supporting field observations. For example, a test station reading may not represent a remote coating holiday, a shielded casing section, an isolated branch line, or a poorly bonded appurtenance.

Field Application Workflow

1. Identify the structure. Determine material, coating, electrolyte exposure, continuity, and appurtenances.
2. Identify the governing requirement. Confirm the applicable standard, regulation, owner specification, or project criterion.
3. Select the measurement method. Decide whether ON, instant-off, native, depolarized, or coupon data are needed.
4. Control current sources. Interrupt, disconnect, or document CP sources as required by the method.
5. Collect representative data. Use appropriate reference electrode placement and survey coverage.
6. Interpret with limitations. Consider IR drop, shielding, interference, continuity, and data quality.
7. Document the conclusion. State the criterion, readings, reference electrode, test condition, and basis for pass/fail interpretation.

Field Example

A pipeline test point has an ON potential of −1,120 mV_CSE and an instant-off potential of −790 mV_CSE. The ON value appears more negative than a commonly used −850 mV_CSE threshold, but the instant-off value does not satisfy a −850 mV_CSE polarized-potential criterion.

The correct conclusion depends on the applicable standard and criterion. The technician should not claim adequate protection based only on the ON reading if the criterion requires an instant-off or polarized potential. If allowed, the location may need evaluation by a valid 100 mV polarization method or further troubleshooting.

Common Mistakes

1. Applying one criterion to every structure type.
   Why it is wrong: Different structures and environments may require different criteria, standards, and test methods.
2. Using ON potentials as if they were polarized potentials.
   Why it is wrong: ON readings may include voltage drop and may not satisfy an instant-off or polarized-potential requirement.
3. Ignoring the reference electrode.
   Why it is wrong: Criteria are tied to a reference electrode scale.
4. Assuming a pass at one test point proves the whole asset is protected.
   Why it is wrong: Current distribution, shielding, coating condition, and continuity can vary by location.
5. Failing to document the test condition.
   Why it is wrong: A potential value is incomplete without knowing whether it was ON, instant-off, native, depolarized, or coupon-based.

Study Checklist

• Can you define what a CP criterion is?
• Can you explain why a reading needs a reference electrode and test condition?
• Can you distinguish fixed-potential criteria from polarization-shift criteria?
• Can you explain why ON potentials may include IR drop?
• Can you identify when instant-off or depolarization data may be needed?
• Can you explain why passing at one location does not prove full-system protection?

Related Pages

• −850 mV_CSE Criterion
• 100 mV Polarization Criterion
• CP Criteria Comparison
• Structure-to-Electrolyte Potential
• Instant-Off Potential
• Reference Electrodes
• CP 3 Certification Study Guide
• CP Criteria Interactive Quiz