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Table of Contents
                            MAIN MENU
NACE Standard RP0104-2004
	Section 1
	Section 2
	Section 3
	Section 4
	Section 5
	Section 6
	Section 7
	Section 8
	Section 9
	Appendix A
	Appendix B
	Appendix C
	Appendix D
Document Text Contents
Page 1


Recommended Practice

The Use of Coupons for
Cathodic Protection Monitoring Applications

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 or she 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
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Creek Drive, Houston, Texas 77084-4906 (telephone +1 [281] 228-6200).

Approved 2004-12-03
NACE International

1440 South Creek Drive
Houston, Texas 77084-4906

+1 (281) 228-6200

ISBN 1-5790-196-X
©2004 NACE International

NACE Standard RP0104-2004
Item No. 21105

Page 2

NACE Standard RP0104-2004, Foreword


NACE International i



Coupons are used to determine the level of corrosion protection provided by a cathodic protection
(CP) system to a variety of structures, such as buried or submerged pipelines, underground
storage tanks (USTs), aboveground (on-grade) storage tank bottoms, and steel in reinforced
concrete structures. Structure-to-electrolyte potential measurements have long been used as the
basis for assessing CP levels and compliance with CP criteria. It is well known that a voltage (IR)
drop exists in the soil and across the coating, and that this IR drop produces an error in the
structure-to-electrolyte potential measurement. This IR drop can be a function of reference
electrode placement, soil resistivity, burial depth of the structure, coating condition, stray currents,
local or long-line corrosion cells, and the amount of CP current applied.

CP coupons have been used since the 1930s by several pioneers of the corrosion-control industry,
both in North America and in Europe. CP coupons have been shown to be a practical tool for
determining the level of polarization of a structure and to confirm the IR drop in a potential
measurement. Research sponsored by the pipeline industry has explored the use of CP coupons
and has helped validate the use of this technology. The purpose of this standard recommended
practice is to provide a method for evaluating the effectiveness of a CP system using coupons. It is
intended for use by people who design and maintain CP systems for buried or submerged
pipelines, USTs, on-grade storage tank bottoms, reinforcing steel in concrete, water storage tanks,
and various other structures in buried or aqueous environments.

The body of the standard primarily addresses applications for coupons attached to buried pipelines.
Appendices cover the use of coupons for other applications, including USTs, aboveground storage
tanks (ASTs), internal surfaces of water tanks, and reinforced concrete structures.

This standard was prepared by Task Group (TG) 210 on Coupon Technology for Cathodic
Protection Applications. TG 210 is administered by Specific Technology Group (STG) 35 on
Pipelines, Tanks, and Well Casings and is sponsored by STG 05 on Cathodic/Anodic Protection.
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 Shall
and must are used to state mandatory requirements. The term should is used to state something
good and is recommended but is not mandatory. The term may is used to state something
considered optional.


Page 14

NACE Standard RP0104-2004, Bibliography



continuously applied to the structure by briefly
disconnecting the coupon from the structure.

8.3.3 When the coupon on, instant-off, and instant-
disconnect potentials are all similar, the IR drop in the
electrolyte is small for each measurement. Either the
coupon on or instant-disconnect potential may be used
as long as the current density, soil resistivity, and other
operating conditions do not change substantially and
the reference electrode placement is the same as in
the test condition.

8.3.4 When the coupon instant-off and instant-
disconnect potentials are similar, but the on potential is
substantially different, the IR drop is significant for the
on-potential measurements, but not for the others. In
this case, only the instant-disconnect potential may be
used as long as the current density, soil resistivity, and
other operating conditions do not change substantially
and the reference electrode placement is the same as
in the test condition.

8.3.5 When the coupon instant-off, instant-disconnect,
and on potential are not similar there may be a
significant IR drop incorporated in the measurements.
Additional testing should be done to determine the
amount of the IR drop in each case using the
procedure in Appendix D. When this additional testing
proves unsuccessful, there may be something wrong
with the test station, such as incorrect arrangement or
geometry of the coupon and reference electrodes. The
test station should not be used until the problem is
identified and corrected.

.4 The IR-drop free value of the CP coupon-to-electrolyte

nstant-disconnect potential represents the polarized
otential of an area on the structure near the coupon that is

n the same electrolyte conditions and has the same
esistance-to-earth as the coupon.

.5 A variety of instruments are used to monitor CP
oupons. Some of the equipment used to monitor CP
oupons is commonly used for other CP readings, while
ther equipment is more specialized. There are
dvantages, disadvantages, and varying degrees of
ccuracy for the different options. Table 1 lists the
quipment used for the seven most common
easurements. An appropriate instrument must be

elected for the intended measurement and the operator of
he selected test equipment must be experienced in its
roper use. When properly used, each of these alternatives
an obtain satisfactory data. The operator must use
xperience and judgment when selecting the appropriate
quipment for the circumstances in order to acquire the
ata accurately.

8.5.1 The limitations of these instruments in the
accurate measurement of each parameter must be
recognized. For example, a digital voltmeter may be
satisfactory for measuring on-potential readings, but
may require special procedures for determining instant-
off and instant-disconnect potentials because the
refresh rate of the meter may not allow it to display the
precise value repeatedly.

NACE International 11

Page 15

NACE Standard RP0104-2004, Section 8


Table 1: Equipment Commonly Used to Measure Coupons

Measurement Equipment Used

On potential
• High-impedance voltmeter and reference electrode(A)
• Automated CP coupon reader and reference electrode(A)
• Data logger and/or chart recorder and reference electrode(A)

Instant-off and

• High-impedance voltmeter and reference electrode(A)
• Automated CP coupon reader and reference electrode(A)
• Oscilloscope/chart recorder or wave-form-capable high-impedance voltmeter and

reference electrode(A)
• Data logger and/or chart recorder and reference electrode(A)

Depolarized potential • High-impedance voltmeter and reference electrode(A)
• Automated CP coupon reader and reference electrode(A)
• Data logger and/or chart recorder and reference electrode(A)


• High-impedance voltmeter and reference electrode(A)
• Automated CP coupon reader and reference electrode(A)
• Data logger and/or chart recorder and reference electrode(A)

CP coupon current • Zero-resistance ammeter
• Multimeter with in-line current-measuring capability in the µA range
• Automated CP coupon reader
• High-impedance voltmeter and shunt

Current direction • Zero-resistance ammeter
• Multimeter with in-line current-measuring capability in the µA range
• Automated CP coupon reader
• High-impedance, high-resolution voltmeter and shunt

Corrosion rate • Electrochemical impedance spectroscopy (EIS) equipment
• Linear polarization-resistance (LPR) equipment
• Electrical-resistance (ER) equipment

(A) Reference electrode is either a stationary reference electrode or a portable reference electrode.

8.6 Minimizing the IR drop in a coupon-to-electrolyte
potential depends on the placement of the reference
electrode when CP current sources are operating. The
reference electrode may be placed in different locations to
confirm the accuracy of the measurements in order to
reduce the error to acceptable levels. Reference electrode
placement varies depending on specific site conditions. As
shown in Figure 3, an on-potential reading with the
reference electrode in the soil-access tube reduces IR-drop
error. Typical reference electrode placements are:

• Portable reference electrode inside a soil-access tube
(Location A),

• Portable reference electrode at grade next to the soil-
access tube (Location B),

• Stationary reference electrode buried near the CP
coupon (Location C),

• Stationary reference electrode buried inside the soil-
access tube (Location D).

12 NACE International

Page 28

NACE Standard RP0104-2004, Section 2


The probe shall be centered in the rebar-net
square. The reference shall be located above or
below the probe and about 25 mm (1.0 in.)
away. All cables should be strapped to the
rebar net.

If either the rebar probe or the
reference electrode has a rebar lead,
it should be bonded to the reinforcing
bar net. The bond should be at least
300 mm (1 ft) away from the
reference electrode.

reference cable


rebar probe

center between reinforcing bars

25 mm (1.0 in.) nominal

rebar probe cable
FIGURE C1: Rebar Probe Installation

When rebar probes are retrofitted to an existing structure,
the corrosion potential, corrosion rate, and current demand
of the probes are partly determined by the patching cement
used for installation. This material is not necessarily the
same as that in contact with the reinforcing bars embedded
in the balance of the structure. This factor shall be
considered when data from these probes are interpreted.
Note: Patching cements with high electrical resistance,
such as polymer-based grouts, must not be used for
installing rebar probes.

Potential Measurements.

When a probe is being used to make accurate potential
measurements, the probe’s lead wire must be bonded to a
lead wire coming from a rebar with the same depth of cover
as the probe. The bonding must be done at an accessible
test station and the lead wire from the reference electrode
must also come to the same test station. The rebar lead is
usually joined to the rebar by brazing and this joint should
be located at least 300 mm (1 ft) away from the probe
location so as not to affect the potential measurements (see
Figure C1).

Potential measurements in concrete must be made with a
high-impedance voltmeter in order to minimize
measurement circuit IR-drop errors. An input impedance of
100 MΩ is considered the minimum for concrete
measurements with 200 MΩ being preferred.

Reinforced concrete structures can have an IR-drop field
pattern created by macro cells. Therefore, it is advisable to
take some instant-off potential measurements, using
techniques described throughout this standard, in order to
determine whether IR-drop error is significant at the probe
location. Significant IR-drop errors must be considered
when potential measurements made at the location of the
error are interpreted.

Corrosion-Rate Measurements

By using rebar probes periodically to monitor the corrosion
rate by LPR measurements, it is possible to determine
when the corrosiveness of the concrete at the depth of the
rebar probe has increased. Possible causes of an increase
in corrosiveness of concrete include ingress of chlorides,
failure of surface sealant, or loss of efficacy of inhibitive
additives. When a rebar probe is used to measure
corrosion rate, it must not be connected to the rebar net.
This is to ensure that the corrosion rate is determined solely
by the surrounding concrete environment and is not
influenced by macro cells. During LPR measurements, the
rebar probe is the working electrode and the rebar net is the
counter electrode. The reference electrode is positioned as
shown in Figure C1.

Corrosion rates measured by this technique may not be
numerically accurate because of uncertainty in the value of
the polarization constant used for data reduction. Because
the purpose of these measurements is to detect a change in
corrosion rate over an extended time interval, the change is

NACE International 25

Page 29

NACE Standard RP0104-2004, Section 6


apparent as long as the same values are used for the
constants during all measurements.

Current-Demand Measurements

Rebar probes may be used to estimate the amount of
current required to protect a reinforced concrete structure
cathodically. In this application, the same relative
placement of rebar probe, reference electrode, and rebar
net used in potential measurements is used (Figure C1).
When a rebar probe is retrofitted to an existing structure, it

should be located in the most anodic site as determined by
surface potential mapping. Provision must be made to
measure the current flowing through one of the probe lead
wires. The other probe wire is used to measure the
potential between the probe and the reference electrode.

Current is applied to the probe to shift its potential to the
desired protected potential and the current required to do so
is measured. This measured current is converted to current
density on the probe, which may then be used to calculate
the amount of current required for the entire structure.

Appendix D: Coupon IR-Drop Calculation Procedure

The following test procedure may be used to determine the
specific amount of IR-drop in a coupon instant-disconnect
potential reading. This is useful in cases in which the
coupon instant-off and instant-disconnect potentials are not
similar or for determining the IR-drop value that should be
considered in future coupon instant-disconnect potential-to-
electrolyte measurements.

The following potential-to-earth measurements should be
taken in the soil near the coupon(11):

1. Structure-to- electrolyte with current applied and
coupon connected (Vs-a-c)

2. Structure-to-electrolyte with current applied and coupon
disconnected (Vs-a-d)

3. Structure-to-electrolyte with current interrupted and
coupon connected (Vs-i-c)

4. Structure-to-electrolyte with current interrupted and
coupon disconnected (Vs-i-d)

5. Coupon-to-electrolyte with current applied and coupon
connected (Vc-a-c)

6. Coupon-to-electrolyte with current applied and coupon
disconnected (Vc-a-d)

7. Coupon-to-electrolyte with current interrupted and
coupon connected (Vc-i-c)

8. Coupon-to-electrolyte with current interrupted and
coupon disconnected (Vc-i-d)

Vs-i-d represents the instant-off potential of the protected
structure. Vc-i-d represents the coupon-to-electrolyte
potential that simulates Vs-i-d (it is not always convenient to
interrupt the current to the structure). The measurement
that is most convenient is Vc-a-d. The difference between Vc-
a-d and Vc-i-d is the IR drop in the coupon-to-electrolyte
potential when current is applied to the structure. This is
called the “coupon IR drop” (VC-IR).

The coupon IR-drop (VC-IR) is added to the coupon-to-
electrolyte potential with current applied and coupon
disconnected (Vc-a-d) to determine the IR-drop-free value of
the coupon-to-electrolyte potential (VC-IR FREE). VC-IR should
remain stable as long as soil conditions and current remain
stable and may be used for future measurements until
conditions change. Such changes could be the result of
changes in seasons, rainfall, current from known or
unknown sources, adjustments by potential-controlled
rectifiers, etc.

26 NACE International

(11) The subscripts used in Steps 1 through 8 represent, in order:
1. structure or coupon
2. applied or interrupted
3. connected or disconnected

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