Polarization Change Formula for Cathodic Protection

Polarization change calculations compare related structure-to-electrolyte potentials to determine how much a structure has polarized or depolarized. These calculations are commonly used when evaluating the 100 mV polarization criterion.

The calculation is based on the magnitude of the potential shift. The direction of the shift matters technically, but the comparison is usually reported as a positive millivolt change.

Formula

Polarization Change = absolute value of (Potential 1 - Potential 2)

Term Meaning Common Unit
Potential 1 Native, polarized, instant-off, or depolarized potential, depending on the test method mVCSE
Potential 2 Comparable potential measured at the same structure location and reference scale mVCSE
Polarization Change Magnitude of the potential shift mV

Important Setup Requirements

The two readings must be comparable. They should represent the same structure, same test location, same reference electrode scale, and valid test conditions.

  • Do not compare unrelated test points.
  • Do not mix CSE, silver-silver chloride, and zinc readings without proper scale-aware interpretation.
  • Do not calculate polarization from a questionable or failed reference cell reading.
  • Document whether the calculation is based on polarization development or depolarization decay.

Worked Example 1: Native to Polarized Shift

A structure has a native potential of −650 mVCSE. After CP is applied, the polarized potential is −780 mVCSE. Determine the polarization shift.

Polarization Change = absolute value of [-780 - (-650)]

Polarization Change = absolute value of [-130]

Polarization Change = 130 mV

The structure polarized by 130 mV at that location. If the applicable requirement allows the 100 mV polarization criterion and the data are valid, this shift would satisfy a 100 mV polarization requirement at that test location.

Worked Example 2: Instant-Off to Depolarized Shift

A structure has an instant-off potential of −820 mVCSE. After depolarization, the potential is −700 mVCSE. Determine the depolarization.

Depolarization = absolute value of [-820 - (-700)]

Depolarization = absolute value of [-120]

Depolarization = 120 mV

The measured depolarization is 120 mV. This supports a 100 mV depolarization result if the current sources were properly interrupted or removed and the timing and measurement conditions were valid.

Worked Example 3: Does Not Satisfy 100 mV

A structure has an instant-off potential of −760 mVCSE. After depolarization, the potential is −690 mVCSE. Determine whether the measured depolarization satisfies a 100 mV polarization criterion.

Depolarization = absolute value of [-760 - (-690)]

Depolarization = absolute value of [-70]

Depolarization = 70 mV

The measured depolarization is 70 mV. Based on these values alone, the 100 mV polarization criterion is not satisfied at that test location.

CP Interpretation Notes

  • Polarization change is a magnitude calculation, usually reported as a positive mV value.
  • The readings must be comparable and tied to the same structure location.
  • The reference electrode scale must be the same unless a valid conversion or scale-aware interpretation is used.
  • Remaining current sources can invalidate depolarization data.
  • A 100 mV result should be tied to the applicable standard, specification, or owner requirement.

Common Mistakes

Comparing Unrelated Readings

Do not calculate polarization change from different structures, different test points, or readings taken under incompatible conditions.

Ignoring Current Sources

Depolarization data may be invalid if another CP current source remains active and continues to influence the structure.

Mixing Reference Scales

Do not compare mVCSE, zinc, and silver-silver chloride readings as though they are the same reference scale.

Practice Problems

  1. A structure shifts from −640 mVCSE native to −770 mVCSE polarized. What is the polarization change?
  2. A structure shifts from −815 mVCSE instant-off to −705 mVCSE depolarized. What is the depolarization?
  3. A structure shifts from −780 mVCSE instant-off to −710 mVCSE depolarized. Does this satisfy 100 mV based on these values alone?

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