Nace CP 1, Course Manual

Page: 500 / 535

TM0497-2002
20 NACE International
inside the reference electrode, and replacing the
solution with new, clean saturated copper sulfate
solution. If the first cleaning does not achieve the
desired results, the process should be repeated.
The copper rod should never be cleaned with
emery cloth or any other material with metallic
abrasive. Only nonmetallic sandpaper should be
used.
NOTE: Reference electrode potential values may
change during the survey.
Therefore, it is
desirable to check reference electrodes
periodically for balance and to have matched or
balanced spares available for replacement if
needed.
B3.3 A voltmeter with sufficiently high input
impedance, at least 10 megaohms, and sufficiently low
ranges should be used to make the two-reference-
electrode surface survey. Measured values are usually
less than 50 mV.
The required equipment for this
survey includes an appropriate voltmeter, two balanced
CSEs, and related test leads.
The front reference
electrode in the direction of travel shall be connected to
the positive terminal of the instrument.
(See Figure
B1.)
B3.4 Careful placement of reference electrodes is
essential
when using the two-reference-electrode
surface survey. Minor measurement errors due to
incorrect placement of the reference electrodes can
result in misinterpretation of the data.
Before the
survey is conducted, the pipe should be accurately
located and marked, using a dependable locating
device. Special care shall be exercised in situations in
which multiple pipelines are on the same right-of-way.
B3.5 Reference electrode spacing should be uniform.
A spacing of 3 m (10 ft) is acceptable. When a ground
gradient reversal (anodic condition) has been located,
the spacing may be reduced by one half and the area
reexamined to locate the anodic area more closely.
B3.6 The survey is made by placing two reference
electrodes in the earth at the selected spacing directly
over the pipeline. The front test lead in the direction of
travel is connected to the positive terminal of the
instrument. Because the voltage values between the
reference electrodes arel normally low, it is desirable
that the reference electrode contact with the earth be
free of leaves, grass, rocks, and other debris.
B3.7 Results of the measurement are recorded on an
appropriate form. Special attention shall be given to
recording the polarity of each voltage measurement
correctly. With the reference electrodes placed and the
instrument connected as described, a possibly anodic
condition is indicated when a polarity change occurs.
(When the polarity of the measured value changes
again, a possibly cathodic condition is indicated.) (See
Figure B1.)
B3.8 The severity and extent of an anodic condition
may be further determined by making two-reference-
electrode surface measurements lateral to the direction
of the pipe. This is accomplished by relocating the rear
reference electrode to the side of the pipe. A positive
value measured from this side reference electrode
indicates current flowing from the pipe into the
electrolyte, which is an anodic condition. A negative
value measured from this side reference electrode
toward the reference electrode over the pipe indicates
current flowing from the electrolyte toward the pipe,
which is a cathodic condition. Measurements should
be taken on both sides of the pipe.
Enough
measurements along the pipe and on both sides of the
pipe should be taken to define the limits of the anodic
condition.
B3.9 The presence of a galvanic anode connected to
the
pipe affects two-reference-electrode surface
measurements and generally appears as an anodic
condition. Close observation of measured values quite
often suggests the presence of galvanic anodes. As
an anode is approached, its presence is usually
indicated by earth gradients that are somewhat higher
than normal for the area being surveyed.
The two-
reference-electrode lateral test may provide higher
measured values on the side of the pipe where the
anode is buried and lower values on the side of the
pipe
opposite the anode. Service taps, side
connections, other components of the pipe (such as
mechanical couplings or screw collars with a higher
metallic resistance than the pipe), or other close buried
metallic structures may provide measured values that
indicate an anodic condition. The lateral test is useful
to evaluate the data. Any situation not determined to
be caused by some other factor shall be considered as
an anodic condition.
Adequate marking of anodic
conditions is necessary so they can be located for
future attention.
B3.10 Soil resistivity tests should be made at anodic
areas discovered by using the two-reference-electrode
surface survey. These tests are helpful in evaluating
the severity of ongoing corrosion, anode current, and
anode life.
B3.11 The two-reference-electrode surface potential
survey data may be used to generate a pipe-to-
electrolyte potential gradient curve using closely
spaced measurements.
This curve appears as any
other pipe-to-electrolyte potential curve and is
generated by the following procedure:
B3.11.1 The pipe-to-electrolyte potential is
measured at a test point, such as a test station.
This value is recorded and becomes the reference
value to which all other two-reference-electrode
measurements are referenced.
B3.11.2 The reference electrode is left in the same
location and is connected to the negative terminal
of the voltmeter. A second reference electrode is