6.2 The -850 mV CSE Polarized Criterion (NACE SP0169)

The -850 mV CSE Polarized Criterion (NACE SP0169)

Overview of the Criterion

The most widely recognized and utilized criterion for the cathodic protection of steel and cast iron structures is defined in NACE International (now AMPP) Standard Practice SP0169. This criterion states that adequate protection is achieved when a negative (cathodic) polarized potential of at least -850 millivolts (-0.850 V) is achieved relative to a saturated copper-copper sulfate (CSE) reference electrode.

Key components of the -850 mV criterion include:

• Polarized Potential: The potential measured at the structure-to-electrolyte interface with all voltage drops (IR drops) across the soil and coating removed (typically via instant-off testing).
• Reference Electrode Standard: All measurements are referenced against a Saturated Copper-Copper Sulfate (CSE) half-cell in soil/freshwater.
• Target Value: Minimum negative potential of -850 mV (-0.850 V) CSE.

While the number -850 mV is easy to memorize, the critical component of this criterion is the word polarized. A polarized potential represents the true electrochemical voltage across the structure-to-electrolyte interface. It strictly excludes any extraneous voltage drops (IR drops) that occur in the soil or the pipeline coating when cathodic protection current is actively flowing. Therefore, a simple 'On' potential reading (taken while the CP current is running) cannot be directly compared to the -850 mV criterion unless the IR drop is definitively known and subtracted, which is practically impossible in most field scenarios. Instead, the polarized potential is typically measured using the instant-off technique.

Theoretical and Thermodynamic Basis

The -850 mV CSE criterion is not an arbitrary number; it is firmly rooted in the thermodynamics of corrosion, specifically utilizing Pourbaix diagrams (potential-pH diagrams) for iron.

Corrosion of steel in soil is essentially the dissolution of iron atoms into iron ions ($Fe \rightarrow Fe^{2+} + 2e^-$). By supplying direct current to the pipeline (making it a cathode), we shift its potential in the negative direction. The most active native potential of steel observed in typical aerated soils is around -0.800 V CSE. By polarizing the structure to a potential that is more negative than its most active natural state—specifically, to -850 mV CSE—we thermodynamically halt the anodic dissolution reaction. At this potential, the surface of the steel becomes fully cathodic, and the tendency for iron atoms to leave the metal matrix and enter the electrolyte is effectively reduced to zero.

Environmental Adjustments for CP Criteria

Considerations and Limitations

While the -850 mV polarized criterion is robust, it is not universally applicable to all situations. Several critical environmental and structural factors must be considered:

1. Anaerobic Environments and Sulfate-Reducing Bacteria (SRB)

In environments devoid of oxygen (anaerobic conditions), specialized microorganisms known as Sulfate-Reducing Bacteria (SRB) can thrive. SRB metabolize sulfates in the soil and produce hydrogen sulfide ($H_2S$) as a byproduct. This metabolic process depolarizes the cathode and highly accelerates the corrosion of steel.

Because of this aggressive microbial action, the standard -850 mV criterion is insufficient to halt corrosion. NACE SP0169 explicitly states that when active SRB are present, the protection criterion must be shifted to a more negative value, typically -950 mV CSE polarized. This extra 100 millivolts of polarization is required to overcome the depolarizing effects of the bacteria and ensure the steel remains protected.

2. Elevated Temperatures

The thermodynamics of corrosion are temperature-dependent. The -850 mV criterion is based on ambient soil temperatures. When pipelines operate at elevated temperatures (e.g., compressor station discharge piping, heated product lines), the rate of corrosion increases, and the required protective potential shifts. Additionally, the potential of the CSE reference electrode itself changes with temperature (approximately 0.5 mV per degree Fahrenheit).

For structures operating at elevated temperatures, the polarized potential criterion must often be adjusted to more negative values to maintain adequate protection, and temperature corrections must be applied to the field readings.

3. Mixed Metal Structures

When a steel pipeline is electrically continuous with a more noble metal (such as a copper grounding grid), a galvanic couple is formed. The native potential of this mixed-metal system will be significantly less negative (more noble) than steel alone.

Attempting to polarize a massive copper grounding grid to -850 mV CSE to protect the connected steel pipeline can require an astronomically high amount of CP current. In these scenarios, the 100 mV polarization decay criterion (discussed in the next section) is often a more practical alternative. Furthermore, pushing potentials too negative can be detrimental if amphoteric metals like aluminum are part of the structure, as high alkaline conditions created by excessive cathodic protection can corrode aluminum.

Conclusion on the -850 mV Criterion

The -850 mV CSE polarized criterion remains the gold standard for verifying cathodic protection on steel pipelines. However, CP testers must always remember that this criterion applies to the polarized (IR drop free) potential, not the 'On' potential. Furthermore, a thorough understanding of the specific soil environment—including the presence of SRB and elevated temperatures—is necessary to know when the standard criterion must be adjusted to ensure complete structural integrity.