Polarization
Polarization is the change in potential of a metal surface caused by electrochemical reactions, including the cathodic shift caused by cathodic protection current.
Quick Definition
In cathodic protection, polarization is the shift in structure potential caused by protective current entering the metal surface.
Why This Term Matters
CP does not merely make a meter reading more negative. The protective effect is related to electrochemical polarization of the structure surface.
Polarization is central to instant-off readings, depolarization testing, the 100 mV polarization criterion, and interpretation of whether CP is actually affecting the structure.
Core Concept
Cathodic shift
When CP current enters a structure, the structure potential typically shifts in the cathodic direction. This is cathodic polarization.
Polarized potential
A polarized potential is a potential that represents the structure while polarization from CP remains present, commonly evaluated using instant-off methods when applicable.
Depolarization
Depolarization is the loss of polarization after CP current is removed. The potential shifts back toward a less protected condition.
Polarization criteria
Some CP criteria evaluate whether a sufficient amount of polarization or depolarization has been demonstrated.
Common Mistakes
-
Confusing voltage drop with polarization.
Why it is wrong: Voltage drop is an electrical measurement effect; polarization is an electrochemical change at the structure surface. -
Using unrelated readings to calculate polarization.
Why it is wrong: The compared readings must represent a valid test condition for the same structure or test point. -
Assuming polarization is instant everywhere.
Why it is wrong: Polarization depends on current distribution, coating, electrolyte, and time.
Field Example
A structure has an instant-off potential of −780 mVCSE and a depolarized potential of −660 mVCSE. The 120 mV shift indicates polarization change if the test conditions are valid.