Nace CP 1, Course Manual

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TM0497-2002
NACE International 13
________________________________________________________________________
Section 10: Test Method 3—100 mV Cathodic Polarization
of Steel, Cast Iron, Aluminum, and Copper Piping
10.1 Test Method 3 describes the use of either pipeline
polarization decay or pipeline polarization formation to
determine whether cathodic protection is adequate at the
test site according to the criterion. Consequently, this test
method consists of two mutually independent parts, Test
Methods 3a and 3b, that describe the procedures for
testing. Cathodic polarization curves for Test Methods 3a
and 3b are shown in Figure 3.
These are schematic
drawings of generic polarization decay and formation.
10.2 Test Method 3a — Use of Pipeline Polarization Decay
(Figure 3a)
10.2.1 Scope
This method uses pipeline polarization decay to assess
the adequacy of cathodic protection on a steel, cast
iron, aluminum, or copper pipeline according to the
criterion stated in NACE Standard RP0169,
1
Paragraph
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.
10.2.2 General
10.2.2.1 Interrupting the known cathodic
protection source(s) eliminates voltage drops
associated with the protective current(s) being
interrupted.
10.2.2.2 Other current sources that can affect the
accuracy of this test method include the following:
(a) Unknown, inaccessible, or direct-connected
galvanic anodes;
(b)
Cathodic protection systems on associated
piping or foreign structures;
(c)
Electric railway systems;
(d) HVDC electric power systems;
(e) Telluric currents;
(f) Galvanic, or bimetallic, cells;
(g) DC mining equipment;
(h) Parallel coated pipelines, electrically
connected and polarized to different potentials;
(i)
Uninterrupted current sources;
(j) Unintentional connections to other structures
or bonds to mitigate interference; and
(k)
Long-line currents.
10.2.2.3 The magnitude and duration of a voltage
spike caused by current interruption can vary, but
the duration is typically within 0.5 second. After
the current is interrupted, the time elapsed until the
measurement is recorded should be long enough
to avoid errors caused by voltage spiking. On-site
measurements with appropriate instruments may
be necessary to determine the duration and
magnitude of the spiking.
10.2.3 Comparison with Other Methods
10.2.3.1 Advantages
(a) This method is especially useful for bare or
ineffectively coated pipe; and
(b)
This method is advantageous when
corrosion potentials may be low (for example, 500
mV or less negative) and/or the current required to
meet a negative 850 mV polarized potential
criterion would be considered excessive.
10.2.3.2 Disadvantages
(a) Additional equipment is required;
(b) Additional time, personnel, and vehicles may
be required to set up equipment and to make pipe-
to-electrolyte potential measurements; and
(c)
Test results are difficult or impossible to
analyze when direct-connected galvanic anodes or
foreign impressed current devices are present and
cannot be interrupted, or when stray currents are
present.
10.2.4 Basic Test Equipment
10.2.4.1 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.
10.2.4.1.1 Recording voltmeters can be
useful to record polarization decay.
10.2.4.2 Two color-coded meter leads with clips
for connection to the pipeline and reference
electrode.
10.2.4.3 Sufficient current interrupters to interrupt
influential cathodic protection current sources
simultaneously.