Nace CP 1, Course Manual

Page: 534 / 535

TM0101-2001
NACE International 23
A.7.3.2 Determine the location of coupon
placement for potential measurements. Selection
of a site may be based on:
(a) Location accessible for future monitoring;
(b) Other protection systems, structures, and
anodes that may influence the tank-to-electrolyte
and coupon-to-electrolyte potentials;
(c) Electrical
midpoints between protection
devices;
(d) Known location of an ineffective coating if
the tank is coated; and
(e) Location of a known or suspected corrosive
environment.
A.7.3.3 Make electrical contact between the
reference electrode and the electrolyte at the test
site as close to the coupon as is practicable.
A.7.3.3.1 Record the location of the
electrode to allow it to be returned to the
same location for subsequent tests.
A.7.3.4 Connect the voltmeter to the coupon test
lead and reference electrode as described in
Paragraph 5.6.
A.7.3.5 Measure and record the tank and coupon
“on” potentials.
A.7.3.6 Disconnect the coupon test lead from the
tank test lead and immediately measure the
coupon-to-electrolyte potential.
A.7.3.6.1 The coupon-to-electrolyte potential
becomes the “base line value” from which
polarization decay is measured.
A.7.3.7 Record the coupon-to-electrolyte instant-
off potential and its polarity with respect to the
reference electrode.
A.7.3.8 Leave the coupon test lead disconnected
to allow the coupon to depolarize.
A.7.3.9 Measure and record the coupon-to-
electrolyte potential periodically. The difference
between it and the instant-off potential is the
amount of polarization decay.
Continue to
measure and record the coupon-to-electrolyte
potential until it either:
(a) Has become at least 100 mV less negative
than the instant-off potential; or
(b) Has reached a stable depolarized level.
A.7.3.10 Reconnect the coupon test lead to the
tank test lead for normal operations.
A.7.3.11 The use of polarization formation, as
stated in the criterion, can also be used to assess
the
adequacy of cathodic protection. The
procedure is different in that a “base line value” for
the coupon is measured prior to connection to the
tank. That potential is then compared to the
instant-off potential of the coupon after it has been
allowed to polarize.
A.7.4 Evaluation of Data
A.7.4.1 Cathodic protection may be judged
adequate at the test site if 100 mV or more of
polarization formation or decay is measured with
respect to a standard reference electrode.
A.7.4.2 The depolarized potential of the coupon
depends on the coupon surface condition, the soil
in which the coupon is placed, its level of
polarization, and its time polarized. Therefore, the
depolarized potential of the coupon may not be the
same as that of the tank and may not accurately
reflect the polarization on the tank at the coupon
location.
A.7.4.3 The polarization measured on the tank is
a “resultant” of the variations of polarization on the
tank at the test site. These variations are caused
by the tank surface condition, soil strata variations,
oxygen differentials, and time the tank has been
polarized. Making precise comparisons between
the polarization of the tank and the polarization of
the coupon may not be possible.
A.7.5 Monitoring
A.7.5.1 When at least 100 mV or more of
polarization
formation or decay has been
measured, the tank “on” potential at the test site
may be used for monitoring unless significant
environmental, structural, coating integrity, or
cathodic
protection system parameters have
changed.