Nace CP 1, Course Manual

Page: 127 / 535

Underground Corrosion Control 3:29
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
Po
te
nt
ia
l
(-
m
V
)
(+)
(  )
ON Potential
OFF Potential
IR IR
Native (Free Corroding, Static) Potential
100 mV
Polarization
100 mV Depolarization
“ON-IR” −850 mV
CSE
“OFF” −850 mV
CSE
Po
te
nt
ia
l
(-
m
V
)
(+)
(  )
ON Potential
OFF Potential
IR IR
Native (Free Corroding, Static) Potential
100 mV
Polarization
100 mV Depolarization
“ON-IR” −850 mV
CSE
“OFF” −850 mV
CSE
Figure 3.10 Structure-to-electrolyte Potentials
APPLICATION OF THESE CRITERIA
The first criterion, a potential of –850 mV obtained with current applied, is
based on negligible IR drop or eliminating the IR drop from the
measurement. The IR drop is usually negligible when the current density
and/or the resistivity are low.
The second criterion, a polarized potential of –850 mV, is based on
eliminating IR drop during the measurement. IR drop can be eliminated by
either removing the resistance of the electrolyte or by shutting off the
current. On a bare structure, if the potential is measured at the structure-to-
electrolyte boundary, then the electrolyte is no longer in the measuring
circuit and its resistance is nearly zero. If cathodic protection current is
interrupted, then current is zero. Remember that polarization will dissipate
when current is interrupted, and polarization is the measurement of interest.
Therefore, when current is interrupted, the potential should be measured at
“instant off,” which refers to the potential after IR drop is eliminated but
before polarization begins to dissipate.
The 100-mV polarization criterion can be applied by either starting with the
known corrosion potential (native or original “off” potential) or the
polarized structure potential. Begin by momentarily interrupting the current
flow to determine the “instant off” (polarized) potential. With current
interrupted, watch the decay of the polarized potential. If the potential drops
(becomes more positive) at least 100 mV, the criterion has been met.
Likewise, if the “instant off” reading is at least 100mV more negative than
the native reading, the criterion has been met.