Nace CP 1, Course Manual

Page: 501 / 535

TM0497-2002
NACE International 21
placed over the pipe centerline in clean, moist
earth a selected distance from the first reference
electrode and is connected to the positive side of
the instrument.
B3.11.3 The potential between the two reference
electrodes
is then measured and recorded.
Special attention shall be given to the polarity of
the measurement between the two reference
electrodes.
B3.11.4 The measured value is then algebraically
added
to the pipe-to-electrolyte potential
measured in the first step of this procedure. The
sum obtained from the algebraic addition is the
pipe-to-electrolyte potential at the location of the
second reference electrode.
B3.11.5 The rear reference electrode (connected
to the instrument negative terminal) is moved
forward and placed in the same spot previously
occupied by the front reference electrode.
B3.11.6 The front reference electrode is moved
ahead over the line to the previously selected
distance.
B3.11.7 The potential between the two reference
electrodes
is again measured with special
attention to reference electrode polarity. This
value is algebraically added to the calculated value
for the previous test.
This calculated pipe-to-
electrolyte potential is the pipe-to-electrolyte
potential at the location of the front reference
electrode.
B3.11.8 This process is repeated until the next test
station is met. At this time the last calculated pipe-
to-electrolyte potential is compared with the pipe-
to-electrolyte potential measured using the test
station. If the survey is carefully performed, upon
comparison these two values should be nearly
identical.
B3.11.9 These potential data can then be plotted
as a typical pipe-to-electrolyte potential curve.
B3.12 Errors in observing instrument polarities,
incorrect algebraic calculations, unbalanced reference
electrodes,
and poor earth/reference electrode
contacts cause the calculated values to be incorrect.
B3.13 To use the data collected effectively, a form
having a suitable format should be developed. The
specific needs of each user should be considered
when a data form is being developed. The form should
have space for each measured numerical value, the
polarity
of each value, calculated values, and
comments. It is also useful to provide space for a
sketch of the area surveyed.
B4 Data Interpretation:
B4.1 Interpretation of survey data is complex but
should consider the following:
(a) Polarity change of a measured value;
(b) Magnitude of the value measured;
(c) Magnitude of the lateral two-reference-electrode
value;
(d)
Soil resistivity;
(e) Unknown pipe resistances;
(f) Physical location of the pipe with respect to other
structures; and
(g)
Known corrosion leak history.
B5 Pipe-to-Electrolyte Potential Survey
B5.1 Pipe-to-electrolyte potential measurements
measure the potential difference between a CSE in
contact with the earth and a connection to the pipeline.
When taken and recorded at measurement intervals of
3
m (10 ft) directly over a pipeline, these
measurements are useful in locating suspected anodic
conditions of an unprotected pipeline. The interval of
measurement
may be shortened when anodic
conditions are indicated or other unusual conditions
occur (see Figures B2a and B2b).
B5.2 Individual users may find it appropriate to modify
the above suggested spacing based on the following
conditions.
(a) Pipeline length;
(b) Availability of test leads to the pipe;
(c) Terrain characteristics;
(d) Accessibility;
(e) Presence of foreign pipelines and cathodic
protection systems;
(f)
Coating condition or lack of coating;
(g) Corrosion history of the pipeline;
(h) Results of previous surveys; and
(i) Pipe depth.
B5.3 The survey consists of measuring and recording
voltages along an unprotected
pipeline at specific
intervals as shown in Figures B2a and B2b. To
interpret the survey data correctly and to ensure
meaningful results, the pipeline must be electrically
continuous, or the location of insulating or high-
resistance joints must be known. The “peaks,” or areas
of highest negative potential, usually indicate anodic
conditions. Pipe-to-electrolyte potential measurements
should be plotted or tabulated (see Figure B2c).
B5.4 The presence of an unknown galvanic anode
affects measurements, causing a location to appear to
be an anodic condition. If records or measurements do
not indicate that a galvanic anode has been installed,
all “peaks” shall be considered as anodic conditions. If
records regarding galvanic anodes in the area are not
available or are believed to be inaccurate, a few
additional measurements can help to determine the
source of the peaks. Pipe-to-electrolyte (or electrode-
to-electrode) potential measurements should be made