1. Criteria for Cathodic Protection
CHAPTER 4
Criteria for Cathodic Protection

2. Criteria for Cathodic Protection
This chapter describes the industry accepted criteria, and Chapter 5 describes the survey methods and techniques used to assess whether the criteria are met.
The discussion below is a review of the NACE criteria presented in NACE Standard RP (1996 Revision) Control of External Corrosion of Underground or Submerged Metallic Piping Systems.

3. CRITERIA FOR STEEL AND CAST IRON PIPING
Three primary criteria for CP of underground or submerged steel or cast iron piping are listed in Section 6 of NACE Standard RP (1996 Revision):
-850 mV (CSE)a with the CP applied,
A polarized potential of -850 mV (CSE)
100 mV of polarization.
A fourth criterion, the net protective current criterion, is also listed in Section 6 under a special conditions section, for bare or poorly coated pipelines where long-line corrosion activity is the primary concern.

4. -850 mV with Cathodic Protection Applied Criterion
The full criterion states that adequate protection is achieved with a negative (cathodic) potential of at least 850mV with the CP applied.
This potential is measured with respect to a saturated copper/copper sulfate reference electrode contacting the electrolyte.
Voltage drops other than those across the structure-to- electrolyte boundary must be considered for valid interpretation of this voltage measurement.

5. -850 mV with Cathodic Protection Applied Criterion
Applications:
The -850 mV criterion with CP applied, is the most widely used for determining if a buried or submerged steel or cast iron structure has attained an acceptable level of CP.
In the case of a buried steel or cast iron structure, an acceptable level of protection is achieved, based on this criterion, if the potential difference between the structure and a CSE contacting the soil directly above and as close as possible to the structure is equal to or more negative than (larger in absolute value) -850 mV.
As described above, voltage drops other than those across the structure-to-electrolyte boundary must be considered for valid interpretation of this voltage measurement.

6. -850 mV with Cathodic Protection Applied Criterion
These voltage drops are a result of current flow in the electrolyte (soil) and are generally referred to as ohmic or IR voltage drops.
IR voltage drops are more prevalent in the vicinity of an anode bed or in areas where stray currents are present and generally increase with increasing soil resistivity.
For bare or very poorly coated structures, IR voltage drops can be reduced by placing the reference electrode as close as possible to the structure.
For the majority of coated structures, most of the IR voltage drop is across the coating, and the measurement is less affected by reference electrode placement.
The IR voltage drop can also be minimized or eliminated by interrupting all of the direct current sources of the CP system and measuring the instantaneous off-potential.

7. -850 mV with Cathodic Protection Applied Criterion
Limitations:
The potential reading should be taken with the reference electrode contacting the electrolyte directly over the structure, to minimize ohmic voltage drop errors in the measurement and to minimize the extent of averaging over large areas of the structure.
Alternative criteria may be required where the reference electrode cannot be properly placed, such as at river crossings or road crossings.
The criterion also is most commonly used for well-coated structures, where it can be economically met.
For poorly coated or bare structures, the high CP currents required to meet this criterion can be prohibitive, such that alternative criteria are typically used.

8. -850 mV with Cathodic Protection Applied Criterion
Limitations:
Potentials can vary significantly from one area of an underground structure to another as a result of variations in soil conditions, coating damage, interference effects, etc.
This creates the possibility that potentials less negative than -850 mV (CSE) exist between the measurement points.
This problem can be addressed for pipelines by means of close interval surveys, as described in Chapter 5.
If the close-interval survey establishes that this problem exists, one should maintain more-negative potentials at the test stations to ensure that adequate protection is achieved on the entire structure.
However, the more negative potentials required will result in increased power consumption.

9. -850 mV with Cathodic Protection Applied Criterion
Limitations:
Potentials more negative than -850 mV (CSE) also are required in the presence of bacteria or with a hot pipeline.
In the latter case, the current required for CP can increase by a factor of two for every 10±C (18±F) increase in temperature of the pipe.
Typically, a potential of -950 mV (CSE) is used for hot pipelines.
In the case of microbes, the kinetics of the corrosion reaction and the environment at the pipe surface are altered such that a more-negative potential is typically required to mitigate corrosion. Where the presence of microbes is confirmed or suspected, a minimum potential criterion of mV (CSE) is typically used.
Care should be exercised to avoid overprotection, which can result in coating damage and may promote hydrogen damage of susceptible steels.

10. Polarized Potential of -850 mV Criterion
This criterion states that adequate protection is achieved with “a negative polarized potential of at least 850 mV relative to a saturated copper/copper sulfate reference electrode.”
The polarized potential is defined as the “potential across the structure/electrolyte interface that is the sum of the corrosion potential and the cathodic polarization.”
The polarized potential is measured directly after the interruption of all current sources and is often referred to as the off- or instant off-potential.
The difference in potential between the native potential and the off or polarized potential is the amount of polarization that has occurred as a result of the application of the CP.
The difference in potential between the on-potential and the off-potential is the error in the on-potential introduced as a result of voltage drops in the electrolyte (soil) and the metallic return path in the measuring circuit.

11. Polarized Potential of -850 mV Criterion

12. Polarized Potential of -850 mV Criterion
Applications:
This criterion is more direct than the -850mV criterion with CP applied by clearly defining the method by which voltage drops errors in the on-potential are considered.
These errors are minimized or eliminated.
The voltage drop errors, which are often referred to as ohmic potential drop or IR drop errors, occur as a result of the flow of CP or stray current in the electrolyte (soil) or in the structure.
They are measurement errors because the cathodic polarization at the structure-to- electrolyte interface is the only part of the on-potential measurement that contributes to a reduction in the rate of corrosion of the structure.
This criterion is most commonly applied to coated structures where the sources of DC current can be readily interrupted.

13. Polarized Potential of -850 mV Criterion
Limitations:
An important limitation of this criterion is the requirement that all sources of DC current be interrupted.
For standard survey techniques, the interruption must be performed simultaneously on all current sources.
On gas transmission pipelines, interrupting all current sources may require the use of a large number of synchronous interrupters for all rectifiers, sacrificial anodes, and bonds affecting the section of pipeline that is being evaluated.
In some cases, the number of rectifiers affecting a test section is not known without experimental verification.
On gas distribution systems, sacrificial anodes are more commonly used for CP and the electrical leads for the anodes are usually bonded directly to the pipe with no means available to interrupt the current.
For those situations, this criterion cannot be used.

14. Polarized Potential of -850 mV Criterion
Limitations:
Achieving the criterion also may require the application of high CP currents, resulting in overprotection of some portions of the structure and related problems such as cathodic disbondment of coatings and hydrogen embrittlement of susceptible steels.
Many of the difficulties of accurately measuring pipe-to-soil potentials that were described under limitations to the -850m V criterion with CP applied apply to the polarized potential of -850 mV criterion as well.
These include access to the structure, seasonal fluctuations in the potential between testing times, spatial fluctuations in potential between test stations, the presence of multiple pipelines with different levels of coating quality in a right-of-way, telluric current effects, and shielding of the structure surface by disbonded coatings, rocks, and thermal insulation.

15. 100 mV of Polarization Criterion
This criterion states that adequate protection is achieved with “a minimum of 100 mV of cathodic polarization between the structure surface and a stable reference electrode contacting the electrolyte.
As described before, the corrosion rate decreases and the rate of the reduction reaction on the metal surface increases as the underground structure is polarized in the negative direction from the native potential.
The difference between the corrosion rate (expressed as a current) and the rate of the reduction reaction is equal to the applied CP current.
These processes can be shown graphically in a diagram of E versus log I, referred to as an Evans diagram.

16. 100 mV of Polarization Criterion
To determine the magnitude of the shift as a result of the formation of polarization, one must first determine the native potential of the underground structure at test locations before applying CP.
The potential is then re-measured after the CP system is energized and the structure has had sufficient time to polarize.
Typically, the on-potential is continuously monitored at one test location directly after energization the CP system, and an off-potential reading is made when there is no measurable shift in the on-potential reading for several minutes.
The off potential is then compared with the native potential; if the difference exceeds 100 mV, then the 100 mV criterion has been satisfied at that location.
These measurements are shown graphically below;

17. 100 mV of Polarization Criterion

18. 100 mV of Polarization Criterion
Applications:
The 100 mV polarization criterion is most commonly used on poorly coated or bare structures where it is difficult or costly to achieve either of the -850mV criteria.
In many cases, 100mV of polarization can be achieved where the off-potential is less negative than -850mV (CSE).
The application of the 100mV polarization criterion has the advantage of minimizing coating degradation and hydrogen embrittlement, both of which can occur as a result of overprotection.
In piping networks, the 100 mV polarization criterion can be used for the older, poorly coated pipes; whereas, a -850mV (CSE) polarized potential criterion can be used for the newer piping in the network.
Because of its fundamental underpinnings, the 100 mV polarization criterion also can be used on metals other than steel, for which no specific potential required for protection has yet been established.

19. 100 mV of Polarization Criterion
Limitations:
The time required for full depolarization of a poorly coated or bare structure can be several days to several weeks, making the method very time-consuming and leaving the structure unprotected for an extended period of time.
The 100 mV polarization criterion is frequently used to minimize the costs for upgrading CP systems, and the associated increase in power costs, in areas with degrading coatings.

20. 100 mV of Polarization Criterion
Limitations:
Because of the complicated nature of the measurements, the cost of conducting surveys for the assessment of the 100 mV polarization criterion is considerably higher than for the -850mV criteria. Thus, an economical analysis maybe required to determine whether an actual cost savings is associated with application of the 100 mV polarization criterion.
The 100mV polarization criterion should not be used in areas subject to stray currents because 100mV of polarization may not be sufficient to mitigate corrosion in these areas.

21. 100 mV of Polarization Criterion
Limitations:
The 100 mV polarization criterion should not be used on structures that contain dissimilar metal couples because 100 mV of polarization may not be adequate to protect the active metal in the couple.
This criterion also should not be used in areas where the intergranular form of external stress corrosion cracking (SCC), also referred to as high-pH or classical SCC, is suspected.

22. Net Protective Current Criterion
This criterion was originally based on the criteria for Steel and Cast Iron concept that if the net current at any point on a structure is flowing from the electrolyte to the structure, there cannot be any corrosion current discharging from that point on the structure.
Typically, the criterion is applied by first performing, with the CP system deenergized, a close-interval pipe-to-soil potential survey, or a cell-to-cell potential survey to locate the anodic discharge points along the pipeline.

23. Net Protective Current Criterion
Applications:
The net protective current criterion is normally used on poorly coated or uncoated pipelines, where the primary concern is long-line corrosion activity.
The technique also is normally only used in situations where other criteria cannot be easily or economically met.
With these exceptions, this criterion is not a standard criterion for establishing the effectiveness of a CP system.

24. Net Protective Current Criterion
Limitations:
This criterion essentially states that any magnitude of net current flow to the structure is adequate to mitigate corrosion. In general, that is not the case and therefore, the criterion should be considered for use only as a last resort.
Application of the criterion should be avoided in areas of stray current activity or in common pipeline corridors because of the possibility of misinterpretation of the potential readings.
The criterion also may not be effective in areas with high-resistivity soils, for deeply buried pipelines, or where the separation distance of the corrosion cells is small.

25. Net Protective Current Criterion
Limitations:
This criterion essentially states that any magnitude of net current flow to the structure is adequate to mitigate corrosion. In general, that is not the case and therefore, the criterion should be considered for use only as a last resort.
Application of the criterion should be avoided in areas of stray current activity or in common pipeline corridors because of the possibility of misinterpretation of the potential readings.
The criterion also may not be effective in areas with high-resistivity soils, for deeply buried pipelines, or where the separation distance of the corrosion cells is small.