Driving a structure's polarized potential far beyond what protection needs, typically more negative than about -1100 to -1200 mV CSE instant-off for coated steel. At those levels water is reduced to hydrogen at the surface, which blisters and disbonds coatings, generates alkali that attacks aluminum and lead, and can embrittle high-strength steel. It also wastes rectifier power and anode life.
Instant-off is -1350 next to the groundbed, that's overprotection, tap it down before we blow the coating off.
Also heard as
- over-protection
- excessive polarization
Related terms
Cathodic disbondment
Loss of coating adhesion around a holiday caused by the alkali and hydrogen that cathodic protection generates.
Hydrogen embrittlement
Loss of ductility when atomic hydrogen gets into steel, leading to sudden cracking under stress.
-850 mV criterion
Steel counts as protected when its potential is -850 mV or more negative against a copper sulfate electrode.
Rectifier
Transformer-rectifier unit that turns AC power into adjustable DC current for an impressed current CP system.