The most widely used cathodic protection criterion for steel and cast iron: a structure-to-electrolyte potential of -850 mV or more negative versus CSE. It is applied to the polarized, instant-off potential, or to the on-potential when IR drop has been accounted for. Where sulfate-reducing bacteria are active, many operators work to -950 mV instead, and readings against silver chloride in seawater use about -800 mV for the same protection.
Half the posts on the north lateral don't make -850 mV off, we need another rectifier or a bigger groundbed.
An on-reading of -900 with no IR consideration is the classic rookie report. If the off reading is not past -850, the pipe has not met this criterion.
Also heard as
- -0.85 V criterion
- eight-fifty
- 850 off
Do not mix it up with
- 100 mV polarization criterion: The -850 mV criterion sets an absolute potential to reach; the 100 mV criterion only asks for a 100 mV shift from the native potential.
Related terms
100 mV polarization criterion
CP criterion met when a structure is polarized at least 100 mV more negative than its native potential.
Instant-off potential
Structure potential read in the split second after CP current is switched off, free of IR drop.
CSE
Copper-copper sulfate electrode, the standard reference for pipe-to-soil potential readings.
Overprotection
So much cathodic current that potentials get too negative, risking coating damage and hydrogen problems.