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Standard
Test Method
Measurement Techniques Related to Criteria for
Cathodic Protection on Underground or
Submerged Metallic Piping Systems

This NACE International standard represents a consensus of those individual members who have
reviewed this document, its scope, and provisions. Its acceptance does not in any respect
preclude anyone, whether he has adopted the standard or not, from manufacturing, marketing,
purchasing, or using products, processes, or procedures not in conformance with this standard.
Nothing contained in this NACE International standard is to be construed as granting any right, by
implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or
product covered by Letters Patent, or as indemnifying or protecting anyone against liability for
infringement of Letters Patent. This standard represents minimum requirements and should in no
way be interpreted as a restriction on the use of better procedures or materials. Neither is this
standard intended to apply in all cases relating to the subject. Unpredictable circumstances may
negate the usefulness of this standard in specific instances. NACE International assumes no
responsibility for the interpretation or use of this standard by other parties and accepts
responsibility for only those official NACE International interpretations issued by NACE
International in accordance with its governing procedures and policies which preclude the
issuance of interpretations by individual volunteers.
Users of this NACE International standard are responsible for reviewing appropriate health, safety,
environmental, and regulatory documents and for determining their applicability in relation to this
standard prior to its use. This NACE International standard may not necessarily address all
potential health and safety problems or environmental hazards associated with the use of
materials, equipment, and/or operations detailed or referred to within this standard. Users of this
NACE International standard are also responsible for establishing appropriate health, safety, and
environmental protection practices, in consultation with appropriate regulatory authorities if
necessary, to achieve compliance with any existing applicable regulatory requirements prior to the
use of this standard.
CAUTIONARY NOTICE: NACE International standards are subject to periodic review, and may be
revised or withdrawn at any time without prior notice. NACE International requires that action be
taken to reaffirm, revise, or withdraw this standard no later than five years from the date of initial
publication. The user is cautioned to obtain the latest edition. Purchasers of NACE International
standards may receive current information on all standards and other NACE International
publications by contacting the NACE International Membership Services Department, 1440 South
Creek Drive, Houston, Texas 77084-4906 (telephone +1 281/228-6200).

-3

Foreword
This NACE International standard test method provides descriptions of the measurement
techniques and cautionary measures most commonly used on underground piping to determine
whether a specific criterion has been complied with at a test site. This test method includes only
those measurement techniques that relate to the criteria or special conditions, such as a net
protective current, contained in NACE Standard RP0169.1 This test method is intended for use by
corrosion control personnel concerned with the corrosion of buried underground or submerged
piping systems, including oil, gas, water, and similar structures.
The measurement techniques described require that the measurements be made in the field.
Because the measurements are obtained under widely varying circumstances of field conditions
and pipeline design, this standard is not as prescriptive as those NACE standard test methods that
use laboratory measurements. Instead, this standard gives the user latitude to make testing
decisions in the field based on the technical facts available.
This standard contains instrumentation and general measurement guidelines. It includes methods
for voltage drop considerations when making pipe-to-electrolyte potential measurements and
provides guidance to prevent incorrect data from being collected and used.
The measurement techniques provided in this standard were compiled from information submitted
by committee members and others with expertise on the subject. Variations or other techniques
not included may be equally effective. The complexity and diversity of environmental conditions
may require the use of other techniques.
Appendix A contains information on the common types, use, and maintenance of reference
electrodes. Appendix B contains information for the net protective current technique, which, while
not a criterion, is a useful technique to reduce corrosion. Appendix C contains information
regarding the use of coupons to evaluate cathodic protection. While some engineers use these
techniques, they are not universally accepted practices. However, there is ongoing research into
their use.
The test methods in this standard were originally prepared by NACE Task Group T-10A-3 on Test
Methods and Measurement Techniques Related to Cathodic Protection Criteria, a component of
Unit Committee T-10A on Cathodic Protection. It was reviewed by Task Group 020 and reaffirmed
in 2002 by Specific Technology Group (STG) 35 on Pipelines, Tanks, and Well Casings. This
standard is issued by NACE under the auspices of STG 35.

In NACE standards, the terms shall, must, should, and may are used in accordance with the
definitions of these terms in the NACE Publications Style Manual, 4th ed., Paragraph 7.4.1.9. Shall
and must are used to state mandatory requirements. Should is used to state that which is considered
good and is recommended but is not absolutely mandatory. May is used to state that which is
considered optional.

-2

NACE International
Standard
Test Method
Measurement Techniques Related to Criteria
for Cathodic Protection on Underground
or Submerged Metallic Piping Systems
Contents

1. General........................................................................................................................ 1
2. Definitions.................................................................................................................... 1
3. Safety Considerations .................................................................................................. 3
4. Instrumentation and Measurement Guidelines............................................................. 3
5. Pipe-to-Electrolyte Potential Measurements ................................................................ 4
6. Causes of Measurement Errors ................................................................................... 7
7. Voltage Drops Other Than Across the Pipe Metal/Electrolyte Interface....................... 8
8. Test Method 1—Negative 850 mV Pipe-to-Electrolyte Potential
of Steel and Cast Iron Piping With Cathodic Protection Applied ................................ 10
9. Test Method 2—Negative 850 mV Polarized Pipe-to-Electrolyte
Potential of Steel and Cast Iron Piping....................................................................... 11
10. Test Method 3—100 mV Cathodic Polarization
of Steel, Cast Iron, Aluminum, and Copper Piping..................................................... 13
References....................................................................................................................... 17
Bibliography ...................................................................................................................... 17
Appendix A: Reference Electrodes.................................................................................. 18
Appendix B: Net Protective Current ................................................................................. 19
Appendix C: Using Coupons to Determine Adequacy of Cathodic Protection................. 25
Figures
Figure 1: Instrument Connections...................................................................................... 6
Figure 2: Pipe-to-Electrolyte Potential Corrections for Pipeline Current Flow.................... 9
Figure 3: Cathodic Polarization Curves............................................................................ 14
Figure B1: Surface Potential Survey................................................................................ 23
Figure B2: Pipe-to-Electrolyte Potential Survey of a
Noncathodically Protected Pipeline............................................................................. 24

-1

Section 1: General
1.1 This standard provides testing procedures to comply
with the requirements of a criterion at a test site on a buried
or submerged steel, cast iron, copper, or aluminum pipeline.
1.2 The provisions of this standard shall be applied by
personnel who have acquired by education and related
practical experience the principles of cathodic protection of
buried and submerged metallic piping systems.
1.3 Special conditions in which a given test technique is
ineffective or only partially effective sometimes exist. Such
conditions may include elevated temperatures, disbonded
dielectric or thermally insulating coatings, shielding,
bacterial attack, and unusual contaminants in the
electrolyte. Deviation from this standard may be warranted
in specific situations. In such situations corrosion control
personnel should be able to demonstrate that adequate
cathodic protection has been achieved.

Section 2: Definitions(1)
Anode: The electrode of an electrochemical cell at which
oxidation occurs. Electrons flow away from the anode in the
external circuit. Corrosion usually occurs and metal ions
enter the solution at the anode.
Cable: A bound or sheathed group of insulated conductors.
Cathode: The electrode of an electrochemical cell at which
reduction is the principal reaction. Electrons flow toward the
cathode in the external circuit.
Cathodic Disbondment: The destruction of adhesion
between a coating and the coated surface caused by
products of a cathodic reaction.
Cathodic Polarization: The change of electrode potential
in the active (negative) direction caused by current across
the electrode/electrolyte interface. See also Polarization.
Cathodic Protection: A technique to reduce the corrosion
of a metal surface by making that surface the cathode of an
electrochemical cell.
Cathodic Protection Coupon: A metal sample
representing the pipeline at the test site, used for cathodic
protection testing, and having a chemical composition
approximating that of the pipe. The coupon size should be
small to avoid excessive current drain on the cathodic
protection system.
Coating: A liquid, liquefiable, or mastic composition that,
after application to a surface, is converted into a solid
protective, decorative, or functional adherent film.
Conductor: A bare or insulated material suitable for
carrying electric current.
Corrosion: The deterioration of a material, usually a metal,
that results from a reaction with its environment.
Corrosion Potential (Ecorr): The potential of a corroding
surface in an electrolyte relative to a reference electrode
under open-circuit conditions (also known as rest potential,
open-circuit potential, or freely corroding potential).
Criterion: A standard for assessment of the effectiveness
of a cathodic protection system.
Current Density: The current to or from a unit area of an
electrode surface.
Electrical Isolation: The condition of being electrically
separated from other metallic structures or the environment.
Electrode: A conductor used to establish contact with an
electrolyte and through which current is transferred to or
from an electrolyte.
Electrode Potential: The potential of an electrode in an
electrolyte as measured against a reference electrode.
(The electrode potential does not include any resistance
losses in potential in either the electrolyte or the external
circuit. It represents the reversible work to move a unit
charge from the electrode surface through the electrolyte to
the reference electrode.)
Electrolyte: A chemical substance containing ions that
migrate in an electric field. (For the purpose of this
standard, electrolyte refers to the soil or liquid, including
contained moisture and other chemicals, next to and in
contact with a buried or submerged metallic piping system.)
Foreign Structure: Any metallic structure that is not
intended as part of a system under cathodic protection.

(1) Definitions in this section reflect common usage among practicing corrosion control personnel and apply specifically to how terms are used
in this standard. As much as possible, these definitions are in accord with those in the “NACE Glossary of Corrosion-Related Terms”
(Houston, TX: NACE).

1

Free Corrosion Potential: SeeCorrosion Potential.
Galvanic Anode: A metal that provides sacrificial
protection to another metal that is more noble when
electrically coupled in an electrolyte. This type of anode is
the current source in one type of cathodic protection.
Holiday: A discontinuity in a protective coating that
exposes unprotected surface to the environment.
Impressed Current: An electric current supplied by a
device employing a power source that is external to the
electrode system. (An example is direct current for cathodic
protection.)
“Instant Off” Potential: A measurement of a pipe-toelectrolyte
potential made without perceptible delay
following the interruption of cathodic protection.
Interference: Any electrical disturbance on a metallic
structure as a result of stray current.
Isolation: SeeElectrical Isolation.
Long-Line Current: Current through the earth between an
anodic and a cathodic area that returns along an
underground metallic structure.
Long-Line Current Voltage Drop Error: That voltage drop
error in the “off” potential that is caused by current flow in
the soil due to potential gradients along the pipe surface.
“Off” or “On”: A condition whereby cathodic protection
current is either turned off or on.
Pipe-to-Electrolyte Potential: The potential difference
between the pipe metallic surface and electrolyte that is
measured with reference to an electrode in contact with the
electrolyte. This measurement is commonly termed pipe-tosoil
(P/S).
Pipe-to-Soil: SeePipe-to-Electrolyte Potential.
Polarization: The change from the open-circuit potential as
a result of current across the electrode/electrolyte interface.
Polarized Potential: The potential across the
structure/electrolyte interface that is the sum of the
corrosion potential and the cathodic polarization.
Potential Gradient: A change in the potential with respect
to distance, expressed in millivolts per unit of distance.
Protection Potential: A measured potential meeting the
requirements of a cathodic protection criterion.
Reference Electrode: An electrode whose open-circuit
potential is constant under similar conditions of
measurement, which is used for measuring the relative
potentials of other electrodes.
Resistance to Electrolyte: The resistance of a structure to
the surrounding electrolyte.
Reverse-Current Switch: A device that prevents the
reversal of direct current through a metallic conductor.
Shielding: Preventing or diverting the cathodic protection
current from its intended path to the structure to be
protected.
Shorted Pipeline Casing: A casing that is in metallic
contact with the carrier pipe.
Side Drain Potential: A potential gradient measured
between two reference electrodes, one located over the
pipeline and the other located a specified distance lateral to
the direction of the pipe.
Sound Engineering Practices: Reasoning exhibited or
based on thorough knowledge and experience, logically
valid, and having true premises showing good judgment or
sense in the application of science.
Stray Current: Current through paths other than the
intended circuit.
Telluric Current: Current in the earth that results from
geomagnetic fluctuations.
Test Lead: A wire or cable attached to a structure for
connection of a test instrument to make cathodic protection
potential or current measurements.
Voltage: An electromotive force or a difference in electrode
potentials expressed in volts.
Voltage Drop: The voltage across a resistance according
to Ohm’s Law.
Voltage Spiking: A momentary surging of potential that
occurs on a pipeline when the protective current flow from
an operating cathodic protection device is interrupted or
applied. This phenomenon is the result of inductive and
capacitive electrical characteristics of the system and may
be incorrectly recorded as an “off” or “on” pipe-to-electrolyte
potential measurement. This effect may last for several
hundred milliseconds and is usually larger in magnitude
near the connection of the cathodic protection device to the
pipeline. An oscilloscope or similar instrument may be
necessary to identify the magnitude and duration of the
spiking.
Wire: A slender rod or filament of drawn metal. In practice,
the term is also used for smaller gauge conductors (size 6
mm2 [No. 10 AWG(2)] or smaller).

2

Section 3: Safety Considerations
3.1 Appropriate safety precautions, including the following,
shall be observed when making electrical measurements.
3.1.1 Be knowledgeable and qualified in electrical
safety precautions before installing, adjusting,
repairing, removing, or testing impressed current
cathodic protection equipment.
3.1.2 Use properly insulated test lead clips and
terminals to avoid contact with unanticipated high
voltage (HV). Attach test clips one at a time using a
single-hand technique for each connection.
3.1.3 Use caution when long test leads are extended
near overhead high-voltage alternating current (HVAC)
power lines, which can induce hazardous voltages onto
the test leads. High-voltage direct current (HVDC)
power lines do not induce voltages under normal
operation, but transient conditions may cause
hazardous voltages.
3.1.3.1 Refer to NACE Standard RP01772 for
additional information about electrical safety.
3.1.4 Use caution when making tests at electrical
isolation devices. Before proceeding with further tests,
use appropriate voltage detection instruments or
voltmeters with insulated test leads to determine
whether hazardous voltages may exist.
3.1.5 Avoid testing when thunderstorms are in the
area. Remote lightning strikes can create hazardous
voltage surges that travel along the pipe under test.
3.1.6 Use caution when stringing test leads across
streets, roads, and other locations subject to vehicular
and pedestrian traffic. When conditions warrant, use
appropriate barricades, flagging, and/or flag persons.
3.1.7 Before entering, inspect excavations and
confined spaces to determine that they are safe.
Inspections may include shoring requirements for
excavations and testing for hazardous atmospheres in
confined spaces.
3.1.8 Observe appropriate electrical codes and
applicable safety regulations.

Section 4: Instrumentation and Measurement Guidelines
4.1 Cathodic protection electrical measurements require
proper selection and use of instruments. Pipe-to-electrolyte
potential, voltage drop, potential difference, and similar
measurements require instruments that have appropriate
voltage ranges. The user should know the capabilities and
limitations of the equipment, follow the manufacturer’s
instruction manual, and be skilled in the use of electrical
instruments. Failure to select and use instruments correctly
causes errors in cathodic protection measurements.
4.1.1 Analog instruments are usually specified in
terms of input resistance or internal resistance. This is
usually expressed as ohms per volt of full meter scale
deflection.
4.1.2 Digital instruments are usually specified in terms
of input impedance expressed as megaohms.
4.2 Factors that may influence instrument selection for field
testing include:
(a) Input impedance (digital instruments);
(b) Input resistance or internal resistance (analog
instruments);
(c) Sensitivity;
(d) Conversion speed of analog-to-digital converters used
in digital or data logging instruments;
(e) Accuracy;
(f) Instrument resolution;
(g) Ruggedness;
(h) Alternating current (AC) and radio frequency (RF)
signal rejection; and
(i) Temperature and/or climate limitations.
4.2.1 Some instruments are capable of measuring and
processing voltage readings many times per second.
Evaluation of the input wave-form processing may be
required if an instrument does not give consistent
results.
4.2.2 Measurement of pipe-to-electrolyte potentials on
pipelines affected by dynamic stray currents may
require the use of recording or analog instruments to
improve measurement accuracy. Dynamic stray
currents include those from electric railway systems,
HVDC transmission systems, mining equipment, and
telluric currents.
4.3 Instrument Effects on Voltage Measurements
4.3.1 To measure pipe-to-electrolyte potentials
accurately, a digital voltmeter must have a high input
impedance (high internal resistance, for an analog
instrument) compared with the total resistance of the
measurement circuit.

3

4.3.1.1 An input impedance of 10 megaohms or
more should be sufficient for a digital meter. An
instrument with a lower input impedance may
produce valid data if circuit contact errors are
considered. One means of making accurate
measurements is to use a potentiometer circuit in
an analog meter.
4.3.1.2 A voltmeter measures the potential across
its terminals within its design accuracy. However,
current flowing through the instrument creates
measurement errors due to voltage drops that
occur in all resistive components of a
measurement circuit.
4.3.2 Some analog-to-digital converters used in digital
and data logging instruments operate so fast that the
instrument may indicate only a portion of the input
waveform and thus provide incorrect voltage
indications.
4.3.3 Parallax errors on an analog instrument can be
minimized by viewing the needle perpendicular to the
face of the instrument on the centerline projected from
the needle point.
4.3.4 The accuracy of potential measurements should
be verified by using an instrument having two or more
input impedances (internal resistance, for analog
instruments) and comparing potential values measured
using different input impedances. If the measured
values are virtually the same, the accuracy is
acceptable. Corrections need to be made if measured
values are not virtually identical. Digital voltmeters that
have a constant input impedance do not indicate a
measurement error by changing voltage ranges. An
alternative is to use a meter with a potentiometer
circuit.
4.4 Instrument Accuracy
4.4.1 Instruments shall be checked for accuracy
before use by comparing readings to a standard
voltage cell, to another acceptable voltage source, or to
another appropriate instrument known to be accurate.

Section 5: Pipe-to-Electrolyte Potential Measurements
5.1 Instruments used to measure AC voltage, direct current
(DC) voltage, or other electrical functions usually have one
terminal designated “Common” (COM). This terminal either
is black in color or has a negative (-) symbol. The positive
terminal either is red in color or has a positive (+) symbol.
The positive and negative symbols in the meter display
indicate the current flow direction through the instrument
(Figure 1a). For example, a positive symbol in the meter
display indicates current flowing from the positive terminal
through the meter to the negative terminal. One instrument
test lead is usually black in color and the other red. The
black test lead is connected to the negative terminal of the
instrument and the red lead to the positive terminal.
5.2 Voltage measurements should be made using the
lowest practicable range on the instrument. A voltage
measurement is more accurate when it is measured in the
upper two-thirds of a range selected for a particular
instrument. Errors can occur, for example, when an
instrument with a 2-V range is used to measure a voltage of
15 mV. Such a value might be a voltage drop caused by
current flowing in a metal pipeline or through a calibrated
shunt. A much more accurate measurement would be
made using an instrument having a 20-mV range.
5.3 The usual technique to determine the DC voltage
across battery terminals, pipeline metal/electrolyte interface,
or other DC system is to connect the black test lead to the
negative side of the circuit and the red test lead to the
positive side of the circuit. When connected in this manner,
an analog instrument needle moves in an upscale
(clockwise) direction indicating a positive value with relation
to the negative terminal. A digital instrument connected in
the same manner displays a digital value, usually preceded
by a positive symbol. In each situation the measured
voltage is positive with respect to the instrument’s negative
terminal. (See instrument connections in Figure 1a.)
5.4 The voltage present between a reference electrode and
a metal pipe can be measured with a voltmeter. The
reference electrode potential is normally positive with
respect to ferrous pipe; conversely the ferrous pipe is
negative with respect to the reference electrode.
5.5 A pipe-to-electrolyte potential is measured using a DC
voltmeter having an appropriate input impedance (or
internal resistance, for an analog instrument), voltage
range(s), test leads, and a stable reference electrode, such
as a saturated copper/copper sulfate (CSE), silver/silver
chloride (Ag/AgCl), or saturated potassium chloride (KCl)
calomel reference electrode. The CSE is 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 (lack of
contamination) of the CSE must be determined before the
readings may be considered valid. The Ag/AgCl reference
electrode is usually used in seawater environments. The
saturated KCl calomel electrode is used more often for
laboratory work. However, more-rugged, polymer-body,
gel-filled saturated KCl calomel electrodes are available,
though modifications may be necessary to increase contact
area with the environment.

4

5.6 Meter Polarity
5.6.1 Pipe-to-electrolyte potentials are usually
measured by connecting the instrument negative
terminal to the pipe and the positive terminal to the
reference electrode, which is in contact with the pipe
electrolyte. With this connection the instrument
indicates that the reference electrode is positive with
respect to the pipe. Because the reference electrode
has a positive value with respect to the pipe, the pipe
voltage is negative with respect to the reference
electrode (see Figure 1a). This negative pipe-toelectrolyte
potential is the value used for NACE criteria.
5.6.2 Pipe-to-electrolyte potential measurements are
sometimes made with the reference electrode
connected to the instrument negative terminal and the
pipeline to the positive terminal. Figure 1b illustrates
this connection.
5.6.2.1 If the instrument is a data logging device,
the recorded data may be printed out with a
negative symbol unless a polarity reversal occurs.
5.7 The pipe-to-electrolyte potential measurement of a
buried pipe should be made with the reference electrode
placed close to the metal/electrolyte interface of the pipe.
The common practice, however, is to place the reference
electrode as close to the pipe as practicable, which is
usually at the surface of the earth above the centerline of
the pipe. (See Figure 1a.) This measurement includes a
combination of the voltage drops associated with the:
(a) Voltmeter;
(b) Test leads;
(c) Reference electrode;
(d) Electrolyte;
(e) Coating, if applied;
(f) Pipe; and
(g) Pipe metal/electrolyte interface.
5.8 The pipe-to-electrolyte potential measurement as
described above is a resultant of the:
(a) Voltage drop created by current flowing through the
electrical resistances of the items listed in Paragraph 5.7;
and
(b) For coated pipe, the influence of coating holidays,
depending on their location, number, and size.
5.9 Pipe-to-electrolyte potential measurements made to
determine the level of cathodic protection at the test site
should consider the following:
(a) Effectiveness of coatings, particularly those known or
suspected to be deteriorated or damaged;
(b) Bare sections of pipe;
(c) Bonds to mitigate interference;
(d) Parallel coated pipelines, electrically connected and
polarized to different potentials;
(e) Shielding;
(f) Effects of other structures on the measurements;
(g) History of corrosion leaks and repairs;
(h) Location of impressed current anodes;
(i) Unknown, inaccessible, or direct-connected galvanic
anodes;
(j) Location of isolation devices, including high-resistance
pipe connections and compression couplings;
(k) Presence of electrolytes, such as unusual corrosives,
chemical spills, extreme soil resistivity changes, acidic
waters, and contamination from sewer spills;
(l) Location of shorted or isolated casings;
(m) DC interference currents, such as HVDC, telluric,
welding equipment, foreign rectifier, mining equipment, and
electric railway or transit systems;
(n) Contacts with other metals or structures;
(o) Locations where the pipe enters and leaves the
electrolyte;
(p) Areas of construction activity during the pipeline
history;
(q) Underground metallic structures close to or crossing
the pipeline;
(r) Valves and other appurtenances; and
(s) HVAC overhead power lines.
5.10 Voltage drops other than those across the pipe
metal/electrolyte interface shall be considered for valid
interpretation of pipe-to-electrolyte voltage measurements
made to satisfy a criterion. Measurement errors should be
minimized to ensure reliable pipe-to-electrolyte potential
measurements.
5.11 The effect of voltage drops on a pipe-to-electrolyte
potential measurement can be determined by interrupting
all significant current sources and then making the
measurement. This measurement is referred to as an
“instant-off” potential. The measurement must be made
without perceptible delay after current interruption to avoid
loss of polarization. The voltage value measured is
considered to be the “polarized potential” of the pipe at that
location. Because the current interruption may cause a
voltage spike, recording the spike as the “instant-off
potential” must be avoided. The magnitude and duration of
the voltage spike can vary; however, the duration is usually
within 0.5 second. The following are examples of when it
may not be practical to interrupt all current sources to make
the “instant-off potential” measurement.
5.11.1 Galvanic Anodes
5.11.1.1 Galvanic anodes connected directly to
the pipe without benefit of aboveground test
stations or connections. Interruption requires
excavation of the connections.
5.11.2 Impressed Current Systems
5.11.2.1 Galvanic anodes directly connected to
piping protected using an impressed current
system;
5.11.2.2 Multiple impressed current sources;

5

5.11.2.3 Impressed current devices on foreign
piping; and
5.11.2.4 Numerous cross bonds to parallel
pipelines.
5.11.3 Natural and Manmade Stray Currents
5.11.3.1 Telluric currents; and
5.11.3.2 Manmade DC stray currents, such as
those from mass transit and mining operations.
5.12 When voltage drops have been evaluated at a test
location and the pipe-to-electrolyte potential found to be
satisfactory, the “on” pipe-to-electrolyte potential value may
be used for monitoring until significant environmental,
structural, or cathodic protection system parameters
change.
5.12.1 Significant environmental, structural, or
cathodic protection system parameter changes may
include:
(a) Replacement or addition of piping;
(b) Addition, relocation, or deterioration of cathodic
protection systems;
(c) Failure of electrical isolating devices;
(d) Effectiveness of coatings; and
(e) Influence of foreign structures.
5.13 After a cathodic protection system is operating, time
may be required for the pipe to polarize. This should be
considered when measuring the potential at a test site on a
newly protected pipe or after reenergizing a cathodic
protection device.

Section 6: Causes of Measurement Errors
6.1 Factors that contribute to faulty potential
measurements include:
6.1.1 Pipe and instrument test leads
(a) Broken or frayed wire strands (may not be visible
inside the insulation);
(b) Damaged or defective test lead insulation that
allows the conductor to contact wet vegetation, the
electrolyte, or other objects;
(c) Loose, broken, or faulty pipe or instrument
connections; and
(d) Dirty or corroded connection points.
6.1.2 Reference electrode condition and placement
(a) Contaminated reference electrode solution or
rod, and solutions of insufficient quantity or saturation
(only laboratory-grade chemicals and distilled water, if
water is required, should be used in a reference
electrode);
(b) Reference electrode plug not sufficiently porous
to provide a conductive contact to the electrolyte;
(c) Porous plug contaminated by asphalt, oil, or
other foreign materials;
(d) High-resistance contact between reference
electrode and dry or frozen soil, rock, gravel,
vegetation, or paving material;
(e) Reference electrode placed in the potential
gradient of an anode;
(f) Reference electrode positioned in the potential
gradient of a metallic structure other than the one with
the potential being measured;
(g) Electrolyte between pipe and disbonded coating
causing error due to electrode placement in electrolyte
on opposite side of coating;
(h) Defective permanently installed reference
electrode;
(i) Temperature correction not applied when
needed; and
(j) Photo-sensitive measurement error (in CSE with
a clear-view window) due to light striking the electrode
electrolyte solution (photovoltaic effect).
6.1.3 Unknown isolating devices, such as unbonded
tubing or pipe compression fittings, causing the pipe to
be electrically discontinuous between the test
connection and the reference electrode location.
6.1.4 Parallel path inadvertently established by test
personnel contacting instrument terminals or metallic
parts of the test lead circuit, such as test lead clips and
reference electrodes, while a potential measurement is
being made.
6.1.5 Defective or inappropriate instrument, incorrect
voltage range selection, instrument not calibrated or
zeroed, or a damp instrument sitting on wet earth.
6.1.6 Instrument having an analog-to-digital converter
operating at such a fast speed that the voltage spikes
produced by current interruption are indicated instead
of the actual “on” and “off” values.
6.1.7 Polarity of the measured value incorrectly
observed.
6.1.8 Cathodic protection current-carrying conductor
used as a test lead for a pipe potential measurement.

7

6.1.9 Interference
6.1.9.1 Electromagnetic interference or induction
resulting from AC power lines or radio frequency
transmitters inducing test lead and/or instrument
errors. This condition is often indicated by a fuzzy,
fluctuating, or blurred pointer movement on an
analog instrument or erratic displays on digital
voltmeters. A DC voltmeter must have sufficient
AC rejection capability, which can be determined
by referring to the manufacturer’s specification.
6.1.9.2 Telluric or stray DC currents flowing
through the earth and piping.
6.2 Reference electrode contact resistance is reduced by:
6.2.1 Soil moisture—If the surface soil is so dry that
the electrical contact of the reference electrode with the
electrolyte is impaired, the soil around the electrode
may be moistened with water until the contact is
adequate.
6.2.2 Contact surface area—Contact resistance may
be reduced by using a reference electrode with a larger
contact surface area.
6.2.3 Frozen soil—Contact resistance may be reduced
by removing the frozen soil to permit electrode contact
with unfrozen soil.
6.2.4 Concrete or asphalt-paved areas—Contact
resistance may be reduced by drilling through the
paving to permit electrode contact with the soil.

Section 7: Voltage Drops Other Than Across the Pipe Metal/Electrolyte Interface
7.1 Voltage drops that are present when pipe-to-electrolyte
potential measurements are made occur in the following:
7.1.1 Measurement Circuit—The voltage drop other
than across the pipe metal/electrolyte interface in the
measurement circuit is the sum of the individual
voltage drops caused by the meter current flow through
individual resistances that include:
(a) Instrument test lead and connection resistances;
(b) Reference electrode internal resistance;
(c) Reference electrode-to-electrolyte contact
resistance;
(d) Coating resistance;
(e) Pipe metallic resistance;
(f) Electrolyte resistance;
(g) Analog meter internal resistance; and
(h) Digital meter internal impedance.
A measurement error occurs if the analog meter
internal resistance or the digital meter internal
impedance is not several orders of magnitude higher
than the sum of the other resistances in the
measurement circuit.
7.1.2 Pipe—Current flowing within the pipe wall
creates a voltage drop. This voltage drop and the
direction of the current shall be considered when the
reference electrode is not near the pipe connection and
significant current is conducted by the pipe.
Consideration is needed because an error in the pipeto-
electrolyte potential measurement will occur if the
pipe current causes a significant voltage drop. Current
directed to the pipe connection from the reference
electrode causes the measured potential to be more
negative by the amount of the pipe current voltage drop
(see Figure 2a). Conversely, the potential is less
negative by that amount if the pipe current direction is
from the pipe connection to the reference electrode
(see Figure 2b).
7.1.3 Electrolyte—When a pipe-to-electrolyte potential
is measured with cathodic protection current applied,
the voltage drop in the electrolyte between the
reference electrode and the metal/electrolyte interface
shall be considered. Measurements taken close to
sacrificial or impressed current anodes can contain a
large voltage drop. Such a voltage drop can consist of,
but is not limited to, the following:
(a) A voltage drop caused by current flowing to
coating holidays when the line is coated; and
(b) A voltage drop caused by large voltage gradients
in the electrolyte that occur near operating anodes
(sometimes termed “raised earth effect”).
7.1.3.1 Testing to locate galvanic anodes by
moving the reference electrode along the
centerline of the line may be necessary when the
locations are not known.
7.1.4 Coatings—Most coatings provide protection to
the pipe by reducing the pipe surface contact with the
environment. Due to the relative ionic impermeability
of coatings, they resist current flow. While the
insulating ability of coatings reduces the current
required for cathodic protection, coatings are not
impervious to current flowing through them. Current
flow through the coating causes a voltage drop that is
greater than when the pipe is bare, under the same
environmental conditions.
7.2 Specialized equipment that uses various techniques to
measure the impressed current wave form and to calculate
a pipe-to-electrolyte potential free of voltage drop is available. This equipment may minimize problems resulting
from spiking effects, drifting of interrupters, and current from
other DC sources.

8

Section 8: Test Method 1—Negative 850 mV Pipe-to-Electrolyte Potential
of Steel and Cast Iron Piping with Cathodic Protection Applied
8.1 Scope
Test Method 1 describes a procedure 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 6.2.2.1.1:
A negative (cathodic) potential of at least 850 mV with
the cathodic protection applied. This potential is
measured with respect to a saturated copper/copper
sulfate reference electrode (CSE) contacting the
electrolyte. Voltage drops other than those across the
structure-to-electrolyte boundary must be considered
for valid interpretation of this voltage measurement.
NOTE: Consideration is understood to mean the
application of sound engineering practice in
determining the significance of voltage drops by
methods such as:
(a) Measuring or calculating the voltage drop(s);
(b) Reviewing the historical performance of the
cathodic protection system;
(c) Evaluating the physical and electrical
characteristics of the pipe and its environment; and
(d) Determining whether there is physical evidence of
corrosion.
8.2 General
8.2.1 Cathodic protection current shall remain “on”
during the measurement process. This potential is
commonly referred to as the “on” potential.
8.2.2 Test Method 1 measures the pipe-to-electrolyte
potential as the sum of the polarized potential and any
voltage drops in the circuit. These voltage drops
include those through the electrolyte and pipeline
coating from current sources such as impressed
current, galvanic anodes, and telluric effects.
8.2.3 Because voltage drops other than those across
the pipe metal/electrolyte interface may be included in
this measurement, these drops shall be considered, as
discussed in Paragraph 8.6.
8.3 Comparison with Other Methods
8.3.1 Advantages
(a) Minimal equipment, personnel, and vehicles are
required; and
(b) Less time is required to make measurements.
8.3.2 Disadvantages
(a) Potential measured includes voltage drops other
than those across the pipe metal/electrolyte interface;
and
(b) Meeting the requirements for considering the
significance of voltage drops (see Paragraph 8.6) can
result in added time to assess adequacy of cathodic
protection at the test site.
8.4 Basic Test Equipment
8.4.1 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.
8.4.2 Two color-coded meter leads with clips for
connection to the pipeline and reference electrode.
8.4.3 Reference Electrode
8.4.3.1 CSE.
8.4.3.2 Other standard reference electrodes may
be substituted for the CSE. These reference
electrodes are described in Appendix A,
Paragraph A2.
8.5 Procedure
8.5.1 Before the test, verify that cathodic protection
equipment has been installed and is operating
properly. Time should be allowed for the pipeline
potentials to reach polarized values.
8.5.2 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 protective devices;
(d) Known location of an ineffective coating if the line
is coated; and
(e) Location of a known or suspected corrosive
environment.
8.5.3 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

8.5.4 Connect the voltmeter to the pipeline and
reference electrode as described in Paragraph 5.6.
8.5.5 Record the pipe-to-electrolyte potential and its
polarity with respect to the reference electrode.
8.6 Considering the Significance of Voltage Drops for Valid
Interpretation of the Criterion
8.6.1 The significance of voltage drops can be
considered by:
8.6.1.1 Comparing historical levels of cathodic
protection with physical evidence from the pipeline
to determine whether corrosion has occurred.
8.6.1.2 Comparing soil corrosiveness with
physical evidence from the pipeline to determine
whether corrosion has occurred.
8.6.2 Physical evidence of corrosion is determined by
evaluating items such as:
(a) Leak history data;
(b) Buried pipeline inspection report data regarding
locations of coating failures, localized conditions of
more-corrosive electrolyte, or substandard cathodic
protection levels have been experienced; and/or
(c) Verification of in-line inspection-tool metal loss
indications by follow-up excavation of anomalies and
inspection of the pipe external surface.
8.6.3 Cathodic protection shall be judged adequate at
the test site if:
(a) The pipe-to-electrolyte potential measurement is
negative 850 mV, or more negative, with respect to a
CSE; and
(b) The significance of voltage drops has been
considered by applying the principles described in
Paragraphs 8.6.1 or 8.6.2.
8.7 Monitoring
When the significance of a voltage drop has been
considered at the test site, the measured potentials may be
used for monitoring unless significant environmental,
structural, coating integrity, or cathodic protection system
parameters have changed.

Section 9: Test Method 2—Negative 850 mV Polarized Pipe-to-Electrolyte
Potential of Steel and Cast Iron Piping
9.1 Test Method 2 describes the most commonly used test
method to satisfy this criterion (see Paragraph 9.2). This
method uses current interruption to determine whether
cathodic protection is adequate at the test site according to
the criterion.
9.2 Scope
This method uses an interrupter(s) to eliminate the cathodic
protection system voltage drop from the pipe-to-electrolyte
potential measurement for comparison with 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).
9.3 General
9.3.1 Interrupting the known cathodic protection
current source(s) eliminates voltage drops associated
with the protective currents being interrupted.
However, significant voltage drops may also occur
because of currents from other sources, as discussed
in Section 7.
9.3.2 To avoid significant depolarization of the pipe,
the “off” period should be limited to the time necessary
to make an accurate potential measurement. The “off”
period is typically less than 3 seconds.
9.3.3 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.
9.3.4 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.

11

9.4 Comparison with Other Methods
9.4.1 Advantages
(a) Voltage drops associated with the protective
currents being interrupted are eliminated.
9.4.2 Disadvantages
(a) Additional equipment is required;
(b) Additional time, personnel, and vehicles may be
required to set up equipment and to make pipe-toelectrolyte
potential measurements; and
(c) Test results are difficult or impossible to analyze
when stray currents are present or direct-connected
galvanic anodes or foreign impressed current devices
are present and cannot be interrupted.
9.5 Basic Test Equipment
9.5.1 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.
9.5.2 Two color-coded meter leads with clips for
connection to the pipeline and reference electrode.
9.5.3 Sufficient current interrupters to interrupt
influential cathodic protection current sources
simultaneously.
9.5.4 Reference electrode
9.5.4.1 CSE.
9.5.4.2 Other standard reference electrodes may
be substituted for the CSE. These reference
electrodes are described in Appendix A,
Paragraph A2.
9.6 Procedure
9.6.1 Before the test, verify that cathodic protection
equipment has been installed and is operating
properly. Time should be allowed for the pipeline
potentials to reach polarized values.
9.6.2 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 pipeto-
electrolyte potential after any “spike” as shown in
Figure 3a has collapsed.
9.6.3 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 when
the pipeline is coated; and
(e) Location of a known or suspected corrosive
environment.
9.6.4 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.
9.6.5 Connect voltmeter to the pipeline and reference
electrode as described in Paragraph 5.6.
9.6.5.1 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.
9.6.6 Record the pipe-to-electrolyte “on” and “off”
potentials and their polarities with respect to the
reference electrode.
9.7 Evaluation of Data
Cathodic protection shall be judged adequate at the test site
if the polarized pipe-to-electrolyte potential is negative 850
mV, or more negative, with respect to a CSE.
9.8 Monitoring
When the polarized pipe-to-electrolyte potential has been
determined to equal or 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.

12

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 pipeto-
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.

13

10.2.4.4 Reference electrode
10.2.4.4.1 CSE.
10.2.4.4.2 Other standard reference
electrodes may be substituted for the CSE.
These reference electrodes are described in
Appendix A, Paragraph A2.
10.2.5 Procedure
10.2.5.1 Before the test, verify that cathodic
protection equipment has been installed and is
operating properly. Time should be allowed for the
pipeline potentials to reach polarized values.
10.2.5.2 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.
10.2.5.3 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 pipeline is coated; and
(e) Location of a known or suspected corrosive
environment.
10.2.5.4 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.
10.2.5.4.1 Identify the location of the
electrode to allow it to be returned to the
same location for subsequent tests.
10.2.5.5 Connect the voltmeter to the pipeline and
reference electrode as described in Paragraph
5.6.
10.2.5.5.1 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.
10.2.5.6 Measure and record the pipe-toelectrolyte
“on” and “instant off” potentials and their
polarities with respect to the reference electrode.
10.2.5.6.1 The “instant off” pipe-to-electrolyte
potential is the “baseline” potential from which
the polarization decay is calculated.
10.2.5.7 Turn off sufficient cathodic protection
current sources that influence the pipe at the test
site until at least 100 mV cathodic polarization
decay has been attained.
10.2.5.7.1 Continue to measure and record
the pipe-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.
10.2.5.7.2 Measurements shall be made at
sufficiently frequent intervals to avoid attaining
and remaining at a corrosion potential for an
unnecessarily extended period.
10.2.5.7.3 When extended polarization
decay time periods are anticipated, it may be
desirable to use recording voltmeters to
determine when adequate polarization decay
or a corrosion potential has been attained.
10.2.6 Evaluation of Data
Cathodic protection shall be judged adequate at the
test site if 100 mV or more of polarization decay is
measured with respect to a standard reference
electrode.
10.2.7 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.
10.3 Test Method 3b—Use of Pipeline Polarization
Formation (Figure 3b)
10.3.1 Scope
This method provides a procedure using the formation
of polarization to assess the adequacy of cathodic
protection at a test site on steel, cast iron, aluminum, or
copper piping 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.

15





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