Nace CP 1, Course Manual

Page: 525 / 535

TM0101-2001
14 NACE International
10.2.5.6.1 If spiking could be present, use an
appropriate
instrument, such as an
oscilloscope or high-speed recording device,
to verify that the measured values are not
influenced by a voltage spike.
10.2.5.7 Measure and record the tank-to-
electrolyte “on” and instant-off potentials and their
polarities with respect to the reference electrode.
10.2.5.7.1 The instant-off tank-to-electrolyte
potential is the “baseline” potential from which
the polarization decay is calculated.
10.2.5.8 Turn off the cathodic protection current
sources that influence the tank (i.e., those
interrupted in Paragraph 10.2.5.2) at the test site.
Continue to measure and record the tank-to-
electrolyte potential until it either:
(a) Becomes at least 100 mV less negative than
the instant-off potential, or
(b) Reaches a stable depolarized level.
10.2.5.8.1 Measurements shall be made at
sufficiently frequent intervals to avoid attaining
and remaining at a corrosion potential for an
unnecessarily extended period.
10.2.5.8.2 If extended polarization decay time
periods are anticipated, it may be desirable to
use recording voltmeters to determine when
adequate polarization decay or a corrosion
potential has been attained.
10.2.5.9 Record a sufficient number of
measurements to determine the level of cathodic
protection over the entire structure.
10.2.6 Evaluation of Data
10.2.6.1 Cathodic protection shall be judged
adequate at the test site if 100 mV or more of
polarization decay is measured with respect to a
standard reference electrode.
10.2.7 Monitoring
10.2.7.1 When at least 100 mV or more of
polarization 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.
10.3 Test Method 3b—Use of Tank Polarization Formation
(Figure 2b)
10.3.1 This method provides a procedure using the
formation of polarization to assess the adequacy of
cathodic protection at a test site on a steel tank
according to the criterion stated in NACE Standard
RP0285:
1
“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.”