Nace CP 1, Course Manual

Page: 523 / 535

TM0101-2001
12 NACE International
9.5 The following procedure shall be followed when this
test is conducted.
9.5.1 Before the test, verify that cathodic protection
equipment
has been installed and is operating
properly. Sufficient time should be allowed for the tank
potentials to reach polarized values.
9.5.2 Install and place in operation necessary
interrupter equipment in all DC current sources
influencing the tank at the test site. The “off” interval
should be kept as short as possible but still long
enough to read a polarized tank-to-electrolyte potential
after any “spike” (see Figure 2a) has collapsed.
9.5.3 Determine the location of reference electrode
placement for potential measurements. Selection of a
site may be based on:
(a) Accessibility for future monitoring;
(b) Other protection systems, structures, and anodes
that may influence the structure-to-electrolyte potential;
(c) Electrical midpoints between protective devices;
(d) Known location of an ineffective coating if the tank
structure is coated; and
(e) Location of a known or suspected corrosive
environment.
9.5.4 Make electrical contact between the reference
electrode and the electrolyte at the test site, in a
location that minimizes the voltage gradient from other
structures, and coating defects (if the tank is coated).
9.5.4.1 Record the location of the electrode to
allow it to be returned to the same location for
subsequent tests.
9.5.5 Connect the voltmeter to the tank and reference
electrode as described in Paragraph 5.6.
9.5.5.1 If spiking could be present, delay
measurement of the tank-to-electrolyte potential to
eliminate the voltage spike from the measured
value. Spiking usually occurs within 0.5 second of
the interruption of the cathodic protection currents.
Appropriate
instrumentation such as an
oscilloscope or high-speed recording device may
be used to verify the presence and duration of the
spiking.
9.5.5.2 Evaluate the effect of measurement circuit
resistance on the tank-to-electrolyte potential as
indicated in Paragraph 5.7.
9.5.6 Record the tank-to-electrolyte “on” and instant-
off potentials and their polarities with respect to the
reference electrode.
9.5.7 Record a sufficient number of measurements to
determine the level of cathodic protection over the
entire structure.
9.6 Evaluation of Data
9.6.1 Cathodic protection shall be judged adequate at
the test site if the polarized (instant-off) tank-to-
electrolyte potential is negative 850 mV, or more
negative, with respect to a CSE.
9.7 Monitoring
9.7.1 When the polarized tank-to-electrolyte potential
has been determined to be equal to or greater than
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.
________________________________________________________________________
Section 10: Test Method 3—100-mV Cathodic Polarization of Steel Tanks
10.1 This section describes the use of either tank
polarization
decay or tank polarization formation to
determine whether cathodic protection is adequate at the
test site according to the 100-mV criterion. Consequently,
this test method consists of two mutually independent parts,
Test Methods 3a and 3b, which describe the procedures for
testing. Generic cathodic polarization curves for Test
Methods 3a and 3b are shown in Figure 2.
Figure 2
contains schematic drawings of generic polarization decay
and formation. If direct-connected galvanic anodes that
cannot be interrupted are present, these methods are not
applicable.
10.2 Test Method 3a—Use of Tank Polarization Decay
(Figure 2a)
10.2.1 This method uses tank 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. The
formation or decay of
polarization can be measured
to satisfy this criterion.”
10.2.2 General
10.2.2.1 Cathodically protected steel tanks may be
adequately
protected if, from the polarized