تبلیغات
انسانم آرزوست... - NACE2
انسانم آرزوست...
*من همچنان عینکی زده ام که شیشه ندارد*مدام به آن ، ها میکنم تا پاک شود * غافل از آنکه چشمانم غبار آلود است نه عینکم*
گروه طراحی قالب من
درباره وبلاگ


مبهم وشاید کمی پیچیده
.
.
.

این همه آن چیزیست که در سرزمین جاوید انتظارش را میکشیم...


مدیر وبلاگ : pc7a
نویسندگان
10.3.2 General
Ferrous, aluminum, and copper pipelines may be
adequately cathodically protected if applying cathodic
protection causes a polarization change of 100 mV or
more with respect to a reference potential.
10.3.2.1 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.
10.3.3 Comparison with Other Methods
10.3.3.1 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 potential criterion would
be considered excessive.
10.3.3.2 Disadvantages
(a) Additional equipment is required;
(b) Additional time, personnel, and vehicles may
be required to set up equipment and to make the
pipe-to-electrolyte potential measurements; and
(c) Test results are difficult or impossible to
analyze when stray currents are present or when
direct-connected galvanic anodes or foreign
impressed currents are present and cannot be
interrupted.
10.3.4 Basic Test Equipment
10.3.4.1 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.
10.3.4.2 Two color-coded meter leads with clips
for connection to the pipeline and reference
electrode.
10.3.4.3 Sufficient current interrupters to interrupt
influential cathodic protection current sources
simultaneously.
10.3.4.4 Reference electrode
10.3.4.4.1 CSE.
10.3.4.4.2 Other standard reference
electrodes may be substituted for the CSE.
These reference electrodes are described in
Appendix A, Paragraph A2.
10.3.5 Procedure
10.3.5.1 Before the test, verify that cathodic
protection equipment has been installed but is not
operating.
10.3.5.2 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 protection
devices;
(d) Known location of an ineffective coating if
the line is coated; and
(e) Location of a known or suspected corrosive
environment.
10.3.5.3 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.
10.3.5.3.1 Identify the location of the
electrode to allow it to be returned to the
same location for subsequent tests.
10.3.5.4 Connect the voltmeter to the pipeline and
reference electrode as described in Paragraph
5.6.
10.3.5.5 Measure and record the pipe-toelectrolyte
corrosion potential and its polarity with
respect to the reference electrode.
10.3.5.5.1 This potential is the value from
which the polarization formation is calculated.
10.3.5.6 Apply the cathodic protection current.
Time should be allowed for the pipeline potentials
to reach polarized values.
19
10.3.5.7 Install and place in operation necessary
interrupter equipment in all significant DC sources
protecting the pipe at the test site, and place in
operation with a synchronized and/or known “off”
and “on” cycle. The “off” cycle should be kept as
short as possible but still long enough to read a
polarized pipe-to-electrolyte potential after any
“spike” as shown in Figure 3a has collapsed.
10.3.5.8 Measure and record the pipe-toelectrolyte
“on” and “off” potentials and their
polarities with respect to the reference electrode.
The difference between the “off” potential and the
corrosion potential is the amount of polarization
formation.
10.3.5.8.1 If spiking may be present, use an
appropriate instrument, such as an
oscilloscope or high-speed recording device,
to verify that the measured values are not
influenced by a voltage spike.
10.3.6 Evaluation of Data
Cathodic protection shall be judged adequate if
100 mV or more of polarization formation is
measured with respect to a standard reference
electrode.
10.3.7 Monitoring
When at least 100 mV or more of polarization
formation has been measured, the pipeline “on”
potential may be used for monitoring unless
significant environmental, structural, coating
integrity, or cathodic protection system parameters
have changed.
________________________________________________________________________
References
1. NACE Standard RP0169 (latest revision), “Control of
External Corrosion on Underground or Submerged Metallic
Piping Systems” (Houston, TX: NACE).
2. NACE Standard RP0177 (latest revision), “Mitigation of
Alternating Current and Lightning Effects on Metallic
Structures and Corrosion Control Systems” (Houston, TX:
NACE).
3. F.J. Ansuini, J.R. Dimond, “Factors Affecting the
Accuracy of Reference Electrodes,” MP 33, 11 (1994), p.
14.
4. NACE Publication 35201 (latest revision), “Technical
Report on the Application and Interpretation of Data from
External Coupons Used in the Evaluation of Cathodically
Protected Metallic Structures” (Houston, TX: NACE).
________________________________________________________________________
Bibliography
Ansuini, F.L., and J.R. Dimond. “Factors Affecting the
Accuracy of Reference Electrodes.” MP 33, 11 (1994):
pp. 14-17.
Applegate, L.M. Cathodic Protection. New York, NY:
McGraw-Hill, 1960.
Bushman, J.B., and F.E. Rizzo. “IR Drop in Cathodic
Protection Measurements.” MP 17, 7 (1978): pp. 9-13.
Cathodic Protection Criteria — A Literature Survey. Ed.
coord. R.A. Gummow. Houston, TX: NACE, 1989.
Corrosion Control/System Protection, Book TS-1, Gas
Engineering and Operating Practices Series. Arlington,
VA: American Gas Association, 1986.
Dabkowski, J., and T. Hamilton. “A Review of Instant-Off
Polarized Potential Measurement Errors.”
CORROSION/93, paper no. 561. Houston, TX: NACE,
1993.
Dearing, B.M. “The 100-mV Polarization Criterion.” MP 33,
9 (1994): pp. 23-27.
DeBethune, A.J. “Fundamental Concepts of Electrode
Potentials.” Corrosion 9, 10 (1953): pp. 336-344.
Escalante, E., ed. Underground Corrosion, ASTM STP 741.
Philadelphia, PA: ASTM, 1981.
Ewing, S.P. “Potential Measurements for Determining
Cathodic Protection Requirements.” Corrosion 7, 12
(1951): pp. 410-418.
Gummow, R.A. “Cathodic Protection Potential Criterion for
Underground Steel Structures.” MP 32, 11 (1993): pp.
21-30.
Jones, D.A. “Analysis of Cathodic Protection Criteria.”
Corrosion 28, 11 (1972): pp. 421-423.
NACE Publication 2C154. “Some Observations on
Cathodic Protection Potential Criteria in Localized
Pitting.” Houston, TX: NACE, 1954.
NACE Publication 2C157. “Some Observations on
Cathodic Protection Criteria.” Houston, TX: NACE,
1957.
20
NACE Publication 35201 (latest revision). “Technical Report
on the Application and Interpretation of Data from
External Coupons Used in the Evaluation of
Cathodically Protected Metallic Structures.” Houston,
TX: NACE, 2001.
NACE Publication 54276. “Cathodic Protection Monitoring
for Buried Pipelines.” Houston, TX: NACE, 1990.
Peabody’s Control of Pipeline Corrosion. 2nd ed. R.
Bianchetti, ed. Houston, TX: NACE, 2001.
Parker, M.E. Pipeline Corrosion and Cathodic Protection.
2nd ed. Houston, TX: Gulf Publishing, 1962.
Stephens, R.W. “Surface Potential Survey Procedure and
Interpretation of Data,” in Proceedings of the
Appalachian Corrosion Short Course, held May 1980.
Morgantown, WV: University of West Virginia, 1980.
West, L.H. “Fundamental Field Practices Associated with
Electrical Measurements,” in Proceedings of the
Appalachian Corrosion Short Course, held May 1980.
Morgantown, WV: University of West Virginia, 1980.
________________________________________________________________________
Appendix A: Reference Electrodes
A1 Pipeline metals have unstable electrical potentials
when placed in an electrolyte such as soil or water.
However, a half-cell that has a stable, electrochemically
reversible potential characterized by a single, identifiable
half-cell reaction is a reference electrode. The stability of a
reference electrode makes it useful as an electrical
reference point or benchmark for measuring the potential of
another metal in soil or water. When connected by a
voltmeter to another metal in soil or water, the reference
electrode becomes one half of a corrosion cell. The
reference electrodes used for measuring potentials on
buried or submerged pipelines have voltage values that are
normally positive with respect to steel.
A2 Pipeline potentials are usually measured using either a
saturated copper/copper sulfate (CSE), a silver/silver
chloride (Ag/AgCl), or a saturated potassium chloride (KCl)
calomel reference electrode. CSEs are usually used for
measurements when the electrolyte is soil or fresh water,
and less often for salt water. When a CSE is used in a
high-chloride environment, the stability (i.e., lack of
contamination) of the electrode must be determined before
the readings may be considered valid. Ag/AgCl reference
electrodes are usually used for seawater environments.
The KCl calomel electrodes are more often used for
laboratory work because they are generally less rugged,
unless specially constructed, than the other two reference
electrodes.
A2.1 The voltage equivalents (at 25°C [77°F]) to
negative 850 mV referred to a CSE are:
A2.1.1 Ag/AgCl seawater reference electrode
used in 25 ohm-cm seawater: -800 mV,3 and
A2.1.2 Saturated KCl calomel reference
electrode: -780 mV.
A2.2 A CSE is composed of a pure copper rod
immersed in a saturated solution of distilled water and
copper sulfate (CuSO4). The pure copper rod extends
from one end of the reference electrode, providing a
means of connection to a voltmeter. The other end of
the reference electrode has a porous plug that is used
to make an electrical contact with the pipeline
electrolyte. Undissolved CuSO4 crystals in the
reference electrode should always be visible to ensure
the solution is saturated. The reference is reasonably
accurate (within 5 mV when measured against a
reference electrode known to be free of contamination).
The advantages of this reference electrode are low
cost and ruggedness.
A2.3 Ag/AgCl reference electrodes are used in marine
and soil environments. The construction and the
electrode potential vary with the application and with
relation to the potential of a CSE reference electrode.
The electrolytes involved may be natural seawater,
saturated KCl, or other concentrations of KCl. The
user shall utilize the manufacturer’s recommendations
and potential values for the type of Ag/AgCl cell used.
The Ag/AgCl reference electrode has a high accuracy
(typically less than 2 mV when handled and maintained
correctly) and is very durable.
A2.4 A saturated KCl calomel reference electrode for
laboratory use is composed of a platinum wire in
contact with a mercury/mercurous chloride mixture
contacting a saturated KCl solution enclosed in a glass
container, a voltmeter connection on one end, and a
porous plug on the other end for contact with the
pipeline electrolyte. For field use a more-rugged,
polymer-body, gel-filled KCl calomel electrode is
available, though modifications may be necessary to
increase contact area with the environment. The
presence of mercury in this electrode makes it
environmentally less desirable for field use.
A2.5 In addition to these standard reference
electrodes, an alternative metallic material or structure
may be used in place of the saturated CSE if the
stability of its electrode potential is ensured and if its
voltage equivalent referred to a CSE is established.
A2.6 A permanently installed reference electrode may
be used; however, whether it is still accurate should be
determined.
21
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.
________________________________________________________________________
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, highresistivity
electrolyte, deeply buried pipe, largediameter
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-referenceelectrode
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
22
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-referenceelectrode
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-referenceelectrode
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 tworeference-
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-toelectrolyte
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
23
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-toelectrolyte
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 pipeto-
electrolyte potential is compared with the pipeto-
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 highresistance
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 electrodeto-
electrode) potential measurements should be made
24
in 0.3-m (1-ft) increments for about 1.5 to 3.0 m (5 to
10 ft) laterally to the pipe and through the “peak.” The
maximum potential will occur a few feet to the side of
the pipe if the peak is due to a galvanic anode.
Moreover, if the pipe location is known with certainty
and a galvanic anode is present, the potentials will be a
minimum over or to the side of the pipe opposite where
the maximum occurs. The closer the transverse
measurements are to the anode, the more the location
of the minimum will be shifted away from the side of the
pipe opposite the location of the maximum.
B5.5 Stray current flowing to a pipe from sources such
as foreign rectifiers and electrified railroads cause the
pipe at that location to have more-negative potential
and may be misinterpreted as an anodic condition.
Stray current discharging from a pipe can cause a lessnegative
potential and be misinterpreted as a cathodic
condition.
B6 Cathodic Protection Using the Net Protective Current
Technique
B6.1 Cathodic protection should be applied to the
anodic area(s).
B6.2 It is necessary to wait until polarization has
stabilized before making a detailed evaluation of the
net current protective level. Polarization of bare pipe
may require a relatively long time ranging up to several
months.
B6.3 When an impressed current source is used, the
side drain potential (potential gradient lateral to the
pipe longitudinal direction) should be measured at the
predetermined anodic condition with the protective
current applied. Relative to the reading directly over
the pipe, a higher (more-negative) reading with the
reference electrode lateral to the pipe indicates that
current is being conducted to the pipe at this point.
The amount of current flow indicated by this method
may not be enough to control small local corrosion
cells.
B6.4 Galvanic anodes are usually installed at or near
the location of the anodic areas. Caution shall be used
when interpreting the results of pipe-to-electrolyte
potential measurements made close to an anode.
B6.5 Monitoring of cathodic protection can be
simplified by establishing test points and recording the
pipe-to-electrolyte potential exhibited when the side
drain measurements indicate a net current flow to the
pipe. These potentials may then be used to monitor
the level of cathodic protection.
25
Appendix C: Using Coupons to Determine Adequacy of Cathodic Protection
C1 Coupons have been used judiciously, particularly when
accompanied by other engineering tools and data, to
evaluate whether cathodic protection at a test site complies
with a given criterion. See NACE Publication 352014 for
more information on coupon use. The following test
procedures are suggested as guides.
C2 Cathodic Protection Coupon Test Method 1—for
Negative 850 mV Polarized Pipe-to-Electrolyte Potential of
Steel and Cast Iron Piping
C2.1 Scope
This method uses a cathodic protection coupon 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 6.2.2.1.2:
A negative polarized potential of at least 850 mV
relative to a saturated copper/copper sulfate
reference electrode (CSE).
C2.2 General
C2.2.1 This method uses a coupon to assess the
adequacy of cathodic protection applied to a
selected test site. A cathodic protection coupon is
a metal sample representing the pipeline at the
test site and used for cathodic protection testing.
The coupon should be:
(a) Nominally of the same metal and surface
condition as the pipe;
(b) Small to avoid excessive current drain on
the cathodic protection system;
(c) Placed at pipe depth in the same backfill as
the pipe;
(d) Prepared with all mill scale and foreign
materials removed from the surface; and
(e) Placed at a known location of an ineffective
coating when the line is coated.
C2.2.2 A coupon has an insulated test lead
brought above ground and, during normal
operations, connected to a pipeline test lead. The
coupon receives cathodic protection current and
represents the pipeline at the test site. For testing
purposes, this connection is opened, and the
polarized potential of the coupon is measured.
The time the connection is open to measure the
coupon’s “off” potential should be minimized to
avoid significant depolarization of the coupon. The
“off” period is typically less than 3 seconds. When
possible, coupon current direction and magnitude
should be verified, using a current clip gauge or
resistor permanently placed in series with the
coupon lead. Measurements showing discharge
of current from the coupon should be reason to
question the validity of using a coupon at the test
site.
C2.2.3 The significance of voltage drops due to
currents from other sources may not be a problem
when a coupon is used to represent the pipeline.
The coupon’s small size may reduce the effect of
these voltage drops. The magnitude of these
voltage drops can be quantified by interrupting
cathodic protection current sources while the
coupon is disconnected and noting whether there
is a shift in the coupon-to-electrolyte potential.
C2.3 Comparison with Other Methods
C2.3.1 Advantages
(a) Can provide a polarized coupon-toelectrolyte
potential, free of voltage drop, with a
minimum of specialized equipment, personnel,
and vehicles; and
(b) Can provide a more comprehensive
evaluation of the polarization at the test site than
conventional pipe-to-electrolyte potential
measurements that may be influenced by the
location, size, and number of coating holidays
when the pipeline is coated.
C2.3.2 Disadvantage—Can have high initial costs
to install coupons, especially for existing pipelines.
C2.4 Basic Test Equipment
C2.4.1 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.
C2.4.2 Two color-coded meter leads with clips for
connection to the coupon and reference electrode.
C2.4.3 Reference electrode
C2.4.3.1 CSE
C2.4.3.2 Other standard reference
electrodes may be substituted for the CSE.
These reference electrodes are described in
Appendix A, Paragraph A2.
C2.5 Procedure
28
C2.5.1 Before the test, verify that:
(a) Cathodic protection equipment has been
installed and is operating properly; and
(b) Coupon is in place and connected to a
pipeline test lead.
Time should be allowed for the pipeline and
coupon potentials to reach polarized values.
C2.5.2 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
and coupon-to-electrolyte potentials;
(c) Electrical midpoints between protection
devices;
(d) Known location of an ineffective coating
when the line is coated; and
(e) Location of a known or suspected corrosive
environment.
C2.5.3 Make electrical contact between the
reference electrode and the electrolyte at the test
site as close to the coupon as is practicable.
C2.5.4 Connect the voltmeter to the coupon test
lead and reference electrode as described in
Paragraph 5.6.
C2.5.5 Measure and record the pipeline and
coupon “on” potentials.
C2.5.6 Momentarily disconnect the coupon test
lead from the pipeline test lead and immediately
measure and record the coupon-to-electrolyte “off”
potential and its polarity with respect to the
reference electrode. This should be performed
quickly to avoid depolarization of the coupon.
C2.5.7 Reconnect the coupon test lead to the
pipeline test lead for normal operations.
C2.6 Evaluation of Data
Cathodic protection may be judged adequate at the
test site if the polarized coupon-to-electrolyte potential
is negative 850 mV, or more negative, with respect to a
CSE. The polarized potential of the coupon depends
on the coupon surface condition, the soil in which the
coupon is placed, its level of polarization, and its time
polarized. Therefore, the polarized potential of the
coupon may not be the same as that of the pipe and
may not accurately reflect the polarization on the pipe
at the coupon location. It must also be understood that
the polarization measured on the pipeline is a
“resultant” of the variations of polarization on the pipe
at the test site. The causes of these variations include
the pipe surface condition, soil strata variations, oxygen
differentials, and length of time the pipe has been
polarized. Making precise comparisons may not be
possible.
C2.7 Monitoring
When the polarized coupon-to-electrolyte potential has
been determined to equal or to exceed a negative 850
mV, the pipeline “on” potential may be used for
monitoring unless significant environmental, structural,
coating integrity, or cathodic protection system
parameters have changed.
C3 Cathodic Protection Coupon Test Method 2—for 100
mV Cathodic Polarization of Steel, Cast Iron, Aluminum,
and Copper Piping
C3.1 Scope
This method uses cathodic protection coupon
polarization decay to assess the adequacy of cathodic
protection on a steel, cast iron, aluminum, or copper
pipeline according to the criteria stated in NACE
Standard RP0169,1 Paragraphs 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.
C3.2 General
Ferrous, aluminum, and copper pipelines may be
adequately cathodically protected when applying
cathodic protection causes a polarization change of
100 mV or more with respect to a reference potential.
C3.2.1 This method uses a coupon to assess the
adequacy of cathodic protection applied at a test
site. A cathodic protection coupon is a metal
sample representing the pipeline at the test site
and used for cathodic protection testing. The
coupon should be:
(a) Nominally of the same metal and surface
condition as the pipe;
(b) Small to avoid excessive current drain on
the cathodic protection system;
(c) Placed at pipe depth in the same backfill as
the pipe;
(d) Prepared with all mill scale and foreign
materials removed from the surface; and
(e) Placed at a known location of an ineffective
coating when the line is coated.
C3.2.2 The significance of voltage drops due to
currents from other sources may be accounted for
when a coupon is used to represent the pipeline.
The magnitude of these voltage drops can be
29
quantified by interrupting cathodic protection
sources while the coupon is disconnected and
noting whether there is a shift in the coupon-toelectrolyte
potential.
C3.2.3 A coupon has an insulated test lead
brought above ground and, during normal
operations, connected to a pipeline test lead. The
coupon receives cathodic protection current and
represents the pipeline at the test site. For testing
purposes, this connection is opened, and the
polarized “off” potential of the coupon is measured.
The time the connection is open to measure the
coupon’s “off” potential should be minimized to
avoid significant depolarization of the coupon. The
“off” period is typically less than 3 seconds. The
coupon is then allowed to depolarize. When
possible, coupon current direction and magnitude
should be verified, using a current clip gauge or
resistor permanently placed in series with the
coupon lead. Measurements showing discharge
of current from the coupon should be reason to
question the validity of using a coupon at the test
site.
C3.3 Comparison with Other Methods
C3.3.1 Advantages
(a) Can measure coupon-to-electrolyte polarization
decay with a minimum of specialized
equipment, personnel, and vehicles;
(b) Can provide an indication of the amount of
polarization present at the test site without
interrupting the cathodic protection current
supplied to the pipeline;
(c) Can provide a better indication of cathodic
protection levels due to eliminating the effects of
“long-line” current flow when the pipeline “off”
potentials are measured.
C3.3.2 Disadvantage—Can have high initial costs
to install a coupon, especially for existing
pipelines.
C3.4 Basic Test Equipment
C3.4.1 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 where the circuit resistance is low.
A potentiometer circuit may be necessary in other
instances.
C3.4.2 Two color-coded meter leads with clips for
connection to the coupon and reference electrode.
C3.4.3 Reference electrode
C3.4.3.1 CSE.
C3.4.3.2 Other standard reference
electrodes may be substituted for the CSE.
These reference electrodes are described in
Appendix A, Paragraph A2.
C3.5 Procedure
C3.5.1 Before the test, verify that:
(a) Cathodic protection equipment is installed
(b) and operating properly; and
(c) Coupon is in place and connected to a
pipeline test lead.
Time shall be allowed for the pipeline and coupon
potentials to reach polarized values.
C3.5.2 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
and coupon-to-electrolyte potentials;
(c) Electrical midpoints between protection
devices;
(d) Known location of an ineffective coating
when the line is coated; and
(e) Location of a known or suspected corrosive
environment.
C3.5.3 Make electrical contact between the
reference electrode and the electrolyte at the test
site as close to the coupon as is practicable.
C3.5.3.1 Identify the location of the
electrode to allow it to be returned to the
same location for subsequent tests.
C3.5.4 Connect the voltmeter to the coupon test
lead and reference electrode as described in
Paragraph 5.6.
C3.5.5 Measure and record the pipeline and
coupon “on” potentials.
C3.5.6 Disconnect the coupon test lead from the
pipeline test lead and immediately measure the
coupon-to-electrolyte potential.
C3.5.6.1 The coupon-to-electrolyte
potential becomes the “base line value” from
which polarization decay is measured.
C3.5.7 Record the coupon-to-electrolyte “off”
potential and its polarity with respect to the
reference electrode.
C3.5.8 Leave the coupon test lead disconnected
to allow the coupon to depolarize.
30
C3.5.9 Measure and record the coupon-toelectrolyte
potential periodically. The difference
between it and the “off” potential is the amount of
polarization decay. Continue to measure and
record the coupon-to-electrolyte potential until it
either:
(a) Has become at least 100 mV less negative
than the “off” potential; or
(b) Has reached a stable depolarized level.
C3.5.10Reconnect the coupon test lead to the
pipeline test lead for normal operations.
C3.6 Evaluation of Data
Cathodic protection may be judged adequate at the
test site when 100 mV or more of polarization decay is
measured with respect to a standard reference
electrode. The depolarized potential of the coupon
depends on the coupon surface condition, the soil in
which the coupon is placed, its level of polarization,
and its time polarized. Therefore, the depolarized
potential of the coupon may not be the same as that of
the pipe and may not accurately reflect the polarization
on the pipe at the coupon location. It must also be
understood that the polarization measured on the
pipeline is a “resultant” of the variations of polarization
on the pipe at the test site. These variations are
caused by the pipe surface condition, soil strata
variations, oxygen differentials, and time the pipe has
been polarized. Making precise comparisons may not
be possible.
C3.7 Monitoring
When at least 100 mV or more of polarization decay
has been measured, the pipeline “on” potential at the
test site may be used for monitoring unless significant
environmental, structural, coating integrity, or cathodic
protection system parameters have changed.
31




آمار وبلاگ
  • کل بازدید :
  • بازدید امروز :
  • بازدید دیروز :
  • بازدید این ماه :
  • بازدید ماه قبل :
  • تعداد نویسندگان :
  • تعداد کل پست ها :
  • آخرین بازدید :
  • آخرین بروز رسانی :
پی کو باکس کسب درآمد
امکانات جانبی
به سایت ما خوش آمدید
     
کلیه حقوق این وبلاگ برای انسانم آرزوست... محفوظ است