6.5 Native, On, Polarized, & Off Potentials Comparison

Native, On, Polarized, & Off Potentials Comparison

The Four States of Potential

Throughout the lifecycle of a pipeline and its cathodic protection system, a CP tester will encounter four distinct types of potential measurements. Understanding the differences between these readings, what they represent, and how they relate to one another is the key to advanced CP system diagnostics and troubleshooting.

1. Native Potential (Static Potential)

The Native Potential is the baseline voltage of the structure in its environment before any cathodic protection current has ever been applied. It represents the natural, freely corroding state of the metal in that specific soil chemistry.

• Typical Values: For carbon steel in aerated soil, native potentials generally range from -0.500 V to -0.650 V CSE. In highly active, anaerobic soils, it might be more negative (e.g., -0.750 V).
• Purpose: Native potentials are critical during the design phase of a CP system and are the ultimate baseline for calculating the 100 mV polarization shift.

2. 'On' Potential

The 'On' Potential is measured while the cathodic protection system is actively operating and current is flowing to the structure.

• Composition: 'On' Potential = Polarized Potential + IR Drop Error.
• Typical Values: Can range anywhere from -0.850 V to -2.000 V CSE or more, depending heavily on soil resistivity and coating condition.
• Purpose: While the 'On' potential cannot be used to prove compliance with the -850 mV polarized criterion, it is highly useful for verifying that the rectifiers are on and that current is reaching the test point. A sudden drop in 'On' potentials along a pipeline often indicates a coating failure or a short to a foreign structure.

3. Polarized Potential (Instant-Off Potential)

The Polarized Potential, typically measured using the Instant-Off technique, is the true electrochemical potential across the structure-to-electrolyte interface, entirely free of IR drop.

• Typical Values: To meet NACE criteria, this must be at least -0.850 V CSE. Well-protected pipelines usually sit between -0.850 V and -1.150 V CSE.
• Purpose: This is the definitive measurement used to prove compliance with the NACE SP0169 -850 mV criterion. It confirms that the thermodynamics of corrosion have been halted.

4. Depolarized Potential (Off Potential)

The Depolarized Potential (or simply 'Off' potential, though distinct from instant-off) is measured after the CP system has been turned off and the structure has been allowed to rest for an extended period, allowing the protective chemical polarization to dissipate.

• Typical Values: It will drift from the Instant-Off potential towards the original Native potential. It may never fully reach the original Native value due to permanent conditioning of the environment around the pipe.
• Purpose: This measurement is explicitly used to calculate compliance with the 100 mV polarization decay criterion (Decay = | Instant-Off - Depolarized |).

Diagnostic Comparisons and Troubleshooting

By comparing these four potentials, a CP technician can diagnose the health of both the pipeline coating and the surrounding environment.

Evaluating IR Drop (On vs. Instant-Off)

The difference between the 'On' potential and the Instant-Off potential represents the total IR drop at that location.

Large IR Drop (e.g., On = -1.800 V, Instant-Off = -0.900 V; IR Drop = 900 mV): This indicates a high resistance in the circuit. This is typically caused by two things: either the pipeline has an excellent, high-dielectric coating with very few holidays (meaning all current is squeezed through tiny pinholes, creating high resistance), or the soil is exceptionally dry and highly resistive (like sand or rock). A large IR drop is generally a sign of a well-coated pipe.

Small IR Drop (e.g., On = -0.950 V, Instant-Off = -0.900 V; IR Drop = 50 mV): This indicates very low resistance in the circuit. This usually points to poor or degraded pipeline coating, exposing massive amounts of bare steel to the soil. It can also indicate extremely low resistivity soil (like swamps or saltwater marshes). If a pipeline historically had a large IR drop that suddenly becomes small, it strongly suggests a massive coating failure or a direct short to a bare foreign structure (like a grounding grid).

Evaluating Polarization Capacity

If a rectifier is turned up significantly, but the Instant-Off potential fails to shift more negative (while the 'On' potential skyrockets), the structure has reached its polarization limit. Additional current is simply being wasted generating hydrogen gas and creating massive IR drop, rather than providing better protection. In this case, applying the 100 mV decay criterion might be the only viable path to compliance.