TM0101-2001
22 NACE International
Sufficient time should be allowed for the tank and
coupon potentials to reach polarized values.
A.6.3.2 Determine the location of coupon
placement. 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 when
the tank is coated; and
(e) Location of a known or suspected corrosive
environment.
A.6.3.3 Make electrical contact between the
reference electrode and the electrolyte at the test
site as close to the coupon as is practicable.
A.6.3.3.1 Record the location of the electrode
to allow it to be returned to the same location
for subsequent tests.
A.6.3.4 Connect the voltmeter to the coupon test
lead and reference electrode as described in
Paragraph 5.6.
A.6.3.5 Measure and record the tank and coupon
“on” potentials.
A.6.3.6 Momentarily disconnect the coupon test
lead from the tank test lead and immediately
measure and record the coupon-to-electrolyte
instant-off potential and its polarity with respect to
the reference electrode. This should be performed
quickly to avoid depolarization of the coupon.
A.6.3.7 Reconnect the coupon test lead to the
tank test lead for normal operations.
A.6.4 Evaluation of Data
A.6.4.1 Cathodic protection may be judged
adequate at the test site if the polarized coupon-to-
electrolyte potential is negative 850 mV, or more
negative, with respect to a CSE.
A.6.4.2 The polarized potential of the coupon
depends on the coupon surface condition, the soil
in which the coupon is placed, its level of
polarization, and the amount of time it has been
polarized. Therefore, the polarized 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.6.4.3 The polarization measured on the tank is
a “resultant” of the variations of polarization on the
tank at the test site. The causes of these
variations include the tank surface condition, soil
strata variations, oxygen differentials, and length of
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.6.5 Monitoring
A.6.5.1 When the polarized coupon-to-electrolyte
potential has been determined to equal or to
exceed a negative 850 mV, the tank “on” potential
may be used for monitoring unless significant
environmental, structural, coating integrity, or
cathodic protection system parameters have
changed.
A.7 Cathodic Protection Coupon Test Method B—for 100-
mV Cathodic Polarization of a Steel Tank
A.7.1 This method uses cathodic protection coupon
polarization decay to assess the adequacy of cathodic
protection on a steel tank according to the criterion
stated in NACE Standard RP0285,
1
as follows:
“The following criterion shall apply: A
minimum of 100 mV of cathodic
polarization between the structure
surface and a stable reference
electrode contacting the electrolyte. The
formation or decay of polarization can
be measured to satisfy this criterion.”
A.7.2 Comparison with Other Methods
A.7.2.1 Advantages
(a) Can measure coupon-to-electrolyte
polarization formation or decay with a minimum of
specialized equipment, personnel, and vehicles;
and
(b) Can provide an indication of the amount of
polarization present at the test site without
interrupting the cathodic protection current
supplied to the tank.
A.7.2.2 Disadvantages
(a) Initial installation of coupons can be
expensive, especially for an existing tank.
A.7.3 Procedure
A.7.3.1 Before the test, verify that:
(a) Cathodic protection equipment is installed
and operating properly; and
(b) Coupon is in place and connected to a tank
test lead.
Sufficient time shall be allowed for the tank and
coupon potentials to reach polarized values.
To Next Page
Loading...