Cathodic Protection Theory and Protection Criteria
Why CP works
Corrosion is an electrochemical cell in which metal dissolves at the anode. Cathodic protection forces the entire exposed structure to behave as a cathode by supplying electrons from an external source. Two things can happen at the surface: the potential is shifted into the region where the metal is thermodynamically immune (for steel, more negative than about −0.85 V CSE), or the cathodic reaction generates alkalinity that passivates the steel. In practice both mechanisms operate together.
The criteria you must know
Most specifications are built from three criteria:
- −850 mV versus copper-copper sulfate with protection applied.
- −850 mV polarized (IR-free) - the potential measured immediately after interrupting all current sources, so that no voltage drop from current flowing through soil or coating is included.
- 100 mV of polarization - either a formation shift of at least 100 mV from the native potential, or a decay of at least 100 mV when the current is switched off. This criterion is valuable where structure-to-soil potentials are unreliable, such as in anaerobic soil or where stray currents exist.
Other recognized approaches use net protective current or E-log I curves from a stepped-current test. Always check which standard the specification cites; criteria are structure- and environment-specific.
Traps
Reporting a reading taken with current on as an IR-free potential; measuring decay before the structure has repolarized; ignoring that native potentials of steel in soil typically sit near −0.50 to −0.65 V CSE; overprotecting coated pipe to the point of coating disbondment and hydrogen evolution.
Worked example
Native potential −0.55 V CSE. Current on: −0.92 V. Instant off: −0.86 V. IR drop = 0.06 V. Polarized potential (−0.86 V) meets −850 mV, and the polarization shift is 0.31 V, far beyond the 100 mV criterion. The reading satisfies both criteria.