Nace CP 1, Course Manual

Page: 499 / 535

TM0497-2002
NACE International 19
A3 It is good practice to verify the accuracy of reference
electrodes used in the field by comparing them with a
carefully prepared master reference electrode that, to avoid
contamination, is never used for field measurements. The
accuracy of a field reference electrode can be verified by
placing it along with the master reference electrode in a
common solution, such as fresh water, and measuring the voltage difference between the two electrodes. A maximum
voltage difference of 5 mV between a master reference
electrode and another reference electrode of the same type
is usually satisfactory for pipeline potential measurements.
When reference electrode-to-reference electrode potential
measurements are made in the field, it is necessary that
electrodes with matching potentials be used.
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Appendix B: Net Protective Current
B1 NACE Standard RP0169,
1
Paragraph 6.2.2.2.1, states
that measuring the net protective current from the
electrolyte to the pipe surface by an earth current technique
at predetermined current discharge points may be sufficient
on bare or ineffectively coated pipelines when long-line
corrosion activity is of primary concern.
B1.1 This technique is a measure of the net protective
current from the electrolyte onto the pipe surface and is
most practicable for use on bare pipelines.
B1.2 The electrolyte current measurements often are
not meaningful in multiple pipe rights-of-way, high-
resistivity
electrolyte, deeply buried pipe, large-
diameter pipe, stray current areas, and pipe that is not
electrically isolated from other underground structures.
Using this technique does not confirm elimination of
local corrosion cell action.
B2 Measurement Techniques for Net Protective Current
B2.1 The principal anodic areas along the pipeline
should be located.
Sufficient cathodic protection
current should be applied to cause a net protective
current from the electrolyte to the pipe surface. The
pipe-to-electrolyte potential measurements for these
techniques are performed on piping that is not
cathodically protected.
B2.2 The two-reference-electrode potential survey or
a pipe-to-electrolyte potential survey method is used to
detect the probable current discharge (anodic) areas
along a pipeline.
B2.2.1 The two-reference-electrode method
measures the direction of the potential gradient
along the earth’s surface. Measurements should
be made at 3-m (10-ft) intervals directly over the
centerline of the pipe.
The instrument positive
terminal is connected to the lead (front) reference
electrode in the direction of survey travel.
A
suspected anodic condition is indicated by a
change of the instrument polarity indication.
Suspected anodic conditions and their magnitudes
can be confirmed by making two-reference-
electrode tests laterally to the pipeline.
One
reference electrode is placed over the line and the
other spaced laterally the same distance as for the
transverse measurements over the line.
These
tests should be made on both sides of the pipe to
verify that current is leaving the line.
B2.3 The pipe-to-electrolyte potential survey, when
used as a tool for locating probable anodic conditions
on unprotected pipe, should be conducted by making
individual readings at 3-m (10-ft) intervals along the
route of the pipe.
Probable anodic conditions are
indicated at survey points where the most negative
readings are determined.
It may be desirable to
confirm these suspected anodic conditions by making
the two-reference-electrode test lateral to the pipe as
described for the two-reference-electrode method.
B3 Two-Reference-Electrode Surface Survey
B3.1 Two-reference-electrode surface measurements
consist of measuring the potential difference between
two matched CSEs in contact with the earth. This type
of test, when made directly over the route of the pipe, is
useful in locating suspected anodic conditions on the
pipe. The two-reference-electrode survey is particularly
suited for bare pipe surveys to locate anodic areas for
applying a “hot spot” type of protection. The technique
is not usually used on coated pipe.
B3.2 For this survey technique to be effective, special
attention shall be given to the reference electrodes
used. Because potential values to be measured can
be expected to be as low as 1 mV, the reference
electrodes shall be balanced to within 3 mV of each
other.
The potential difference between reference
electrodes can be measured by:
(a) Placing about 2.5 cm (1 in.) depth of tap water in
a small plastic or glass container;
(b)
Placing the two reference electrodes in the water;
and
(c)
Measuring the potential difference between
them.
B3.2.1 If the potential difference between the two
reference electrodes is not satisfactory, they can
be
corrected by servicing both reference
electrodes. This may be accomplished by
thoroughly cleaning the inside of the plastic body,
rinsing it with distilled water, soaking the porous
plug in distilled water or simply replacing the old
plug with a new one, cleaning the copper rod