Nace CP 1, Course Manual

Page: 490 / 535

TM0497-2002
10 NACE International
________________________________________________________________________
Section 8: Test Method 1—Negative 850 mV Pipe-to-Electrolyte Potential
of Steel and Cast Iron Piping with Cathodic Protection Applied
8.1 Scope
Test Method 1 describes a procedure to assess the
adequacy of cathodic protection on a steel or cast iron
pipeline according to the criterion stated in NACE Standard
RP0169,
1
Paragraph 6.2.2.1.1:
A negative (cathodic) potential of at least 850 mV with
the cathodic protection applied.
This potential is
measured with respect to a saturated copper/copper
sulfate reference electrode (CSE) contacting the
electrolyte. Voltage drops other than those across the
structure-to-electrolyte boundary must be considered
for valid interpretation of this voltage measurement.
NOTE: Consideration is understood to mean the
application of sound engineering practice in
determining the significance of voltage drops by
methods such as:
(a) Measuring or calculating the voltage drop(s);
(b) Reviewing the historical performance of the
cathodic protection system;
(c) Evaluating
the physical and electrical
characteristics of the pipe and its environment; and
(d) Determining whether there is physical evidence of
corrosion
.
8.2 General
8.2.1 Cathodic protection current shall remain “on”
during the measurement process.
This potential is
commonly referred to as the “on” potential.
8.2.2 Test Method 1 measures the pipe-to-electrolyte
potential as the sum of the polarized potential and any
voltage drops in the circuit.
These voltage drops
include those through the electrolyte and pipeline
coating from current sources such as impressed
current, galvanic anodes, and telluric effects.
8.2.3 Because voltage drops other than those across
the pipe metal/electrolyte interface may be included in
this measurement, these drops shall be considered, as
discussed in Paragraph 8.6
.
8.3 Comparison with Other Methods
8.3.1 Advantages
(a) Minimal equipment, personnel, and vehicles are
required; and
(b) Less time is required to make measurements
.
8.3.2 Disadvantages
(a) Potential measured includes voltage drops other
than those across the pipe metal/electrolyte interface;
and
(b) Meeting the requirements for considering the
significance of voltage drops (see Paragraph 8.6) can
result in added time to assess adequacy of cathodic
protection at the test site
.
8.4 Basic Test Equipment
8.4.1 Voltmeter with adequate input impedance.
Commonly used digital instruments have a nominal
impedance of 10 megaohms.
An analog instrument
with an internal resistance of 100,000 ohms per volt
may be adequate in certain circumstances in which the
circuit resistance is low. A potentiometer circuit may be
necessary in other instances.
8.4.2 Two color-coded meter leads with clips for
connection to the pipeline and reference electrode.
8.4.3 Reference Electrode
8.4.3.1 CSE.
8.4.3.2 Other standard reference electrodes may
be substituted for the CSE.
These reference
electrodes are described in Appendix A,
Paragraph A2.
8.5 Procedure
8.5.1 Before the test, verify that cathodic protection
equipment
has been installed and is operating
properly. Time should be allowed for the pipeline
potentials to reach polarized values.
8.5.2 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 potential;
(c) Electrical midpoints between protective devices;
(d) Known location of an ineffective coating if the line
is coated; and
(e) Location of a known or suspected corrosive
environment
.
8.5.3 Make electrical contact between the reference
electrode and the electrolyte at the test site, directly
over the centerline of the pipeline or as close to it as is
practicable.