Nace CP 1, Course Manual

Page: 506 / 535

TM0497-2002
26 NACE International
C2.5.1 Before the test, verify that:
(a) Cathodic protection equipment has been
installed and is operating properly; and
(b)
Coupon is in place and connected to a
pipeline test lead.
Time should be allowed for the pipeline and
coupon potentials to reach polarized values.
C2.5.2 Determine the location of the site to be
tested. 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 pipe-to-electrolyte
and coupon-to-electrolyte potentials;
(c)
Electrical midpoints between protection
devices;
(d)
Known location of an ineffective coating
when the line is coated; and
(e)
Location of a known or suspected corrosive
environment.
C2.5.3 Make electrical contact between the
reference electrode and the electrolyte at the test
site as close to the coupon as is practicable.
C2.5.4 Connect the voltmeter to the coupon test
lead and reference electrode as described in
Paragraph 5.6.
C2.5.5 Measure and record the pipeline and
coupon “on” potentials.
C2.5.6 Momentarily disconnect the coupon test
lead from the pipeline test lead and immediately
measure and record the coupon-to-electrolyte “off”
potential and its polarity with respect to the
reference electrode.
This should be performed
quickly to avoid depolarization of the coupon.
C2.5.7 Reconnect the coupon test lead to the
pipeline test lead for normal operations.
C2.6 Evaluation of Data
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. 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 its time
polarized.
Therefore, the polarized potential of the
coupon may not be the same as that of the pipe and
may not accurately reflect the polarization on the pipe
at the coupon location. It must also be understood that
the polarization measured on the pipeline is a
“resultant” of the variations of polarization on the pipe
at the test site. The causes of these variations include
the pipe surface condition, soil strata variations, oxygen
differentials, and length of time the pipe has been
polarized. Making precise comparisons may not be
possible.
C2.7 Monitoring
When the polarized coupon-to-electrolyte potential has
been determined to equal or to exceed a negative 850
mV, the pipeline “on” potential may be used for
monitoring unless significant environmental, structural,
coating
integrity, or cathodic protection system
parameters have changed.
C3 Cathodic Protection Coupon Test Method 2—for 100
mV Cathodic Polarization of Steel, Cast Iron, Aluminum,
and Copper Piping
C3.1 Scope
This method uses cathodic protection coupon
polarization decay to assess the adequacy of cathodic
protection on a steel, cast iron, aluminum, or copper
pipeline according to the criteria stated in NACE
Standard RP0169,
1
Paragraphs 6.2.2.1.3, 6.2.3.1, or
6.2.4.1 (depending on the pipe metal). The paragraph
below states Paragraph 6.2.2.1.3:
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.
C3.2 General
Ferrous, aluminum, and copper pipelines may be
adequately
cathodically protected when applying
cathodic protection causes a polarization change of
100 mV or more with respect to a reference potential.
C3.2.1 This method uses a coupon to assess the
adequacy of cathodic protection applied at a test
site.
A cathodic protection coupon is a metal
sample representing the pipeline at the test site
and used for cathodic protection testing. The
coupon should be:
(a) Nominally of the same metal and surface
condition as the pipe;
(b)
Small to avoid excessive current drain on
the cathodic protection system;
(c)
Placed at pipe depth in the same backfill as
the pipe;
(d)
Prepared with all mill scale and foreign
materials removed from the surface; and
(e)
Placed at a known location of an ineffective
coating when the line is coated.
C3.2.2 The significance of voltage drops due to
currents from other sources may be accounted for
when a coupon is used to represent the pipeline.
The magnitude of these voltage drops can be