Field Data Collection and Testing
The criteria
Common criteria are a structure-to-electrolyte potential of -850 mV or more negative with the reference electrode contacting the electrolyte (with CP applied), a polarized potential of -850 mV measured instant-off, and a 100 mV polarization or depolarization shift from the native potential. Each criterion requires a different measurement sequence, and the report must state which was used.
Sequence matters
Measure structure-to-soil potentials at test stations and by close-interval survey; use synchronized interruption to obtain instant-off readings that exclude IR drop; allow full depolarization before measuring native potential, which can take hours or days. Use coupons where the structure is inaccessible. For the 100 mV shift, confirm the direction of change and that the current was genuinely interrupted.
Traps: reporting an on-potential as compliance; measuring native potential while the system is still polarized; assuming interruption worked without confirming a current change; comparing readings taken with different electrode types; ignoring stray-current fluctuation by taking only one reading.
Worked example: on = -1.20 V, instant off = -0.98 V, native = -0.62 V, all versus CSE. The polarized potential is -0.98 V, which satisfies the -850 mV polarized criterion. The polarization shift is 0.98 - 0.62 = 0.36 V, comfortably above 100 mV, so the structure also meets that criterion.
Rule of thumb: the number that counts is the one taken the instant the current stops, not the one with the current on.