6.3 The 100 mV Polarization Decay Criterion

The 100 mV Polarization Decay Criterion

Overview of the Criterion

While the -850 mV CSE polarized criterion is the most widely used standard for cathodic protection, it is not always feasible to achieve. For these situations, NACE SP0169 provides an alternative: the 100 mV polarization criterion. This criterion dictates that a structure is adequately protected if there is a minimum of 100 millivolts of cathodic polarization between the structure surface and a stable reference electrode contacting the electrolyte.

This polarization can be measured in two ways: as polarization formation (the shift in potential from a native state after CP is applied) or as polarization decay (the shift in potential from a polarized state after CP is removed). In field practice, polarization decay is the most common and practical method of evaluating this criterion.

Calculating the 100 mV Decay

The calculation for the 100 mV decay is straightforward but requires precise field measurements. It is defined as the absolute difference between the Instant-Off Potential (the polarized potential immediately after CP current is interrupted) and the Depolarized Potential (the potential of the structure after the CP current has been off long enough for the electrochemical polarization to dissipate).

Equation: Polarization Decay (mV) = | Instant-Off Potential - Depolarized Potential |

For example, if an instant-off reading is -0.750 V (-750 mV) and the structure depolarizes over several hours to a resting potential of -0.620 V (-620 mV), the decay is 130 mV. Since 130 mV is greater than 100 mV, the structure meets the criterion for adequate cathodic protection, even though it never reached the -850 mV threshold.

Measurement Procedures and Timelines

Executing a 100 mV polarization decay test requires patience and coordination:

1. Establish the Instant-Off: All sources of CP current affecting the structure must be interrupted simultaneously. The instant-off potential is recorded immediately (usually within 100-500 milliseconds) to capture the true polarized potential free of IR drop.
2. Leave Current Off: The rectifiers must remain turned off.
3. Monitor Depolarization: The potential of the pipeline will slowly drift in the positive (less negative) direction as the structure depolarizes. This decay is not instantaneous; it is an electrochemical relaxation process.
4. Final Reading: The time required for full depolarization varies wildly based on soil chemistry, moisture, and coating quality. It can take anywhere from a few hours to several days or even weeks. However, for the sake of the criterion, you only need to wait until the potential has decayed by at least 100 mV from the instant-off reading. Once that 100 mV shift is confirmed, the criterion is met, and the CP system can be turned back on.

When to Use the 100 mV Criterion

The 100 mV criterion is incredibly valuable in specific scenarios where the -850 mV criterion is impractical or potentially harmful:

1. Bare or Poorly Coated Structures

Older pipelines with degraded coatings, or entirely bare steel structures, have massive surface areas exposed to the electrolyte. Attempting to force the entire surface of a bare pipeline to a polarized potential of -850 mV CSE would require a tremendous amount of CP current. This might be economically unfeasible due to power costs and the need for massive anode groundbeds. In these cases, achieving a 100 mV polarization shift provides adequate corrosion mitigation with vastly reduced current requirements.

2. Preventing Coating Disbondment

On well-coated pipelines, applying too much CP current to overcome local depolarizing effects can drive the potential at the drainage point to extremely negative values (e.g., -1.200 V or more). These highly negative potentials cause the generation of hydrogen gas at the steel surface and an increase in local alkalinity, both of which can destroy the adhesion of the pipeline coating (cathodic disbondment). The 100 mV criterion allows the operator to lower the rectifier output, protecting the coating while still meeting a recognized NACE protection standard.

Exclusions and Risks

The 100 mV criterion cannot be used blindly. NACE SP0169 explicitly warns against its use in certain environments:

1. Mixed-Metal Structures

When steel is electrically coupled to a more noble metal like copper, they form a galvanic cell. The native (depolarized) potential of this coupled system will be a mixed potential, not the true native potential of the steel alone. Because the baseline is skewed by the copper, a 100 mV shift from this mixed baseline does not guarantee that the steel has reached a protective potential. Therefore, the 100 mV criterion is generally invalid for mixed-metal systems.

2. Stray Currents and Telluric Interference

In areas subjected to severe dynamic stray currents (such as near electrified DC transit systems) or telluric currents (geomagnetic storms), the potential of the pipeline is constantly fluctuating. These external currents make it impossible to accurately measure a stable instant-off or depolarized potential, rendering the 100 mV decay calculation meaningless.

3. Elevated Temperatures and SRB

As discussed in the previous section, environments with Sulfate-Reducing Bacteria (SRB) or elevated temperatures represent highly aggressive corrosion conditions. The 100 mV shift may not provide sufficient polarization to overcome these aggressive depolarizing forces. In these cases, the -850 mV (or -950 mV) criterion is heavily preferred.