-
Standard
Test Method
Measurement Techniques Related to Criteria for
Cathodic Protection of Underground Storage Tank
Systems
Revised 2012-03-10 Approved 2001-11-07
NACE International 1440 South Creek Drive
Houston, TX 77084-4906 +1 281-228-6200
ISBN 1-57590-137-4
2012, NACE International
NACE Standard TM0101-2012 Item No. 21240
This NACE Standard is being made available to you at no charge
because it is incorporated by reference in the New York State
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pipeline integrity issues, please visit
www.nace.org/Pipelines-Tanks-Underground-Systems/. NACE members are
entitled to unlimited downloads of NACE standards, reports and
conference papers for free as part of their member benefits.
___________________________________________________________________
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Learn more about NACE at www.nace.org.
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TM0101-2012
NACE International i
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
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of this standard in specific instances. NACE International assumes
no responsibility for the interpretation or use of this standard by
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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
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this standard prior to its use. This NACE International standard
may not necessarily address all potential health and safety
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within this standard. Users of this NACE International standard are
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_________________________________________________________________________
Foreword
This NACE International standard test method provides
descriptions of the measurement techniques most commonly used on
underground storage tank (UST) systems to determine whether a
specific cathodic protection (CP) criterion has been complied with
at a test site. This standard includes only those measurement
techniques that relate to the criteria or special conditions
contained in NACE SP0285.
1
The measurement techniques described in this standard require
that measurements be made in the field. Because these measurements
are obtained under widely varying circumstances of field conditions
and tank 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 is
intended for use by corrosion control personnel concerned with the
external corrosion of UST systems or similar structures, including
those used to contain oil, gas, and water. This standard was
prepared by Task Group (TG) 209 (formerly Work Group T-10A-14b),
and was revised by TG 364, Testing of Cathodic Protection Systems
of Underground Storage Tanks, in 2012. TG 364 is administered by
Specific Technology Group (STG) 35, Pipelines, Tanks, and Well
Casings, and is sponsored by STG 05, Cathodic/Anodic Protection.
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. 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. The terms shall and must are used to
state a requirement, and are considered mandatory. The term should
is used to state something good and is recommended, but is not
considered mandatory. The term may is used to state
something considered optional.
_________________________________________________________________________
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_________________________________________________________________________
NACE International
Standard Test Method
Measurement Techniques Related to Criteria for Cathodic
Protection
of Underground Storage Tank Systems
Contents
1. General
..........................................................................................................................................1
2. Definitions
......................................................................................................................................1
3. Safety Considerations
....................................................................................................................3
4. Instrumentation and Measurement Guidelines
..............................................................................3
5. Structure-to-Electrolyte Potential Measurements
...........................................................................5
6. Causes of Measurement Errors
...........................................................................................
9 7. Voltage Drops Other Than Across the Structure-to-Electrolyte
Interface ..................................... 10 8. Test Method
1Negative 850 mV Structure-to-Electrolyte Potential of Steel
Underground
Storage Tank Systems with Cathodic Protection Applied
......................................................... 11 9.
Test Method 2Negative 850 mV Polarized Structure-to-Electrolyte
Potential of Steel
Underground Storage Tank Systems
........................................................................................
13 10. Test Method 3100 mV Cathodic Polarization of Steel
Underground Storage Tank Systems . 15 11. Test Methods for
Continuity Testing of Steel Underground Storage Tank Systems
.................. 20 12. Piping and Appurtenances
.........................................................................................................
22 13. Records
.....................................................................................................................................
23 References
......................................................................................................................................
23 Bibliography
.....................................................................................................................................
23 Appendix A: Using CP Coupons to Determine the Adequacy of
Cathodic Protection
(Nonmandatory)
.........................................................................................................................
24 Appendix B: Checklists for CP Systems (Nonmandatory)
................................................................ 28
FIGURES: Figure 1: Instrument Connections
......................................................................................................5
Figure 2: Cathodic Polarization
Curves............................................................................................
15 TABLE: Table 1: Conversion of Other Potential Measurements to
CSE Equivalents......................................6
_________________________________________________________________________
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_________________________________________________________________________
Section 1: General 1.1 This standard provides procedures to test
compliance with the CP criteria presented in NACE SP0285
1 on UST systems.
Included are instrumentation and general measurement guidelines,
procedures for three commonly used test methods, practices for
taking voltage drops into consideration and preventing incorrect
data from being collected and used, and procedures for testing for
electrical continuity between USTs and other metallic equipment.
The use of CP coupons to determine the adequacy of CP is described
in Appendix A (Nonmandatory). 1.2 The provisions of this test
method shall be applied by personnel who have acquired, by
education and related practical experience, knowledge of the
principles of CP of UST systems. Such individuals, at a minimum,
must either be NACE International certified CP Testers, NACE
International CP Specialists, or individuals qualified by
professional education and related practical experience.
1.3 A given test technique may be ineffective or only partially
effective. Conditions that may cause this to occur include elevated
temperatures, disbonded dielectric or thermally insulating
coatings, shielding, bacterial attack, and unusual contaminants in
the electrolyte.
1.4 Deviation from this test method may be warranted in specific
situations if corrosion control personnel can demonstrate that
adequate CP 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. It is usually the electrode where corrosion occurs and
metal ions enter solution.)
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 Polarization: (1) The change of electrode potential
caused by a cathodic current flowing across the
electrode/electrolyte interface; (2) a forced active (negative)
shift in electrode potential. (See Polarization.) Cathodic
Protection: A technique to reduce the corrosion rate of a metal
surface by making that surface the cathode of an
electrochemical cell.
Coating: (1) A liquid, liquefiable, or mastic composition that,
after application to a surface, is converted into a solid
protective,
decorative, or functional adherent film; (2) (in a more general
sense) a thin layer of solid material on a surface that provides
improved protective, decorative, or functional properties.
Conductor: A bare or insulated material suitable for carrying
electric current.
Contact Resistance: The resistance in the measurement circuit
present in the interface between a reference electrode and an
electrolyte.
Corrosion: The deterioration of a material, usually a metal,
that results from a chemical or electrochemical reaction with
its
environment. Corrosion Potential (Ecorr): The potential of a
corroding surface in an electrolyte measured under open-circuit
conditions relative to a reference electrode. (Also known as
Electrochemical Corrosion Potential, Free Corrosion Potential, and
Open-Circuit Potential.)
(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 accordance with those in NACE/ASTM G 193 (latest
revision).
2
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CP Coupon: A metal sample representing the UST at the test site,
used for CP testing, and having a chemical composition
approximating that of the UST. The CP coupon size should be
small to avoid excessive current drain on the CP system. CP
Criterion: A standard for assessment of the effectiveness of a CP
system.
Electrical Isolation: The condition of being electrically
separated from other metallic structures or the environment.
Electrode: A material that conducts electrons, is 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 of 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 purposes of this standard,
electrolyte
refers to the soil or liquid adjacent to and in contact with a
buried or submerged metallic UST system, including the moisture and
other chemicals contained therein.
Foreign Structure: Any metallic structure that is not intended
as a part of a system under cathodic protection.
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 electron source in one
type of cathodic protection. Groundbed: One or more anodes
installed below the earths surface for the purpose of supplying
cathodic protection current.
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: The polarized half-cell potential of an
electrode taken immediately after the cathodic protection current
is
stopped, which closely approximates the potential without IR
drop (i.e., the polarized potential) when the current was on.
Interference: Any electrical disturbance on a metallic structure
as a result of stray current.
Off or On: A condition whereby cathodic protection current is
either turned off or on.
Polarization: The change from the corrosion potential as a
result of current flow across the electrode/electrolyte
interface.
Polarized Potential: (1) (general use) the potential across the
electrode/electrolyte interface that is the sum of the
corrosion
potential and the applied polarization; (2) (cathodic protection
use) 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.
Potentiometer: A device for the measurement of an electromotive
force by comparison with a known potential difference.
Protection Potential: The most noble potential at which pitting
or crevice corrosion, or both, will not propagate in a specific
environment.
Reference Electrode: An electrode having a stable and
reproducible potential, which is used in the measurement of
other
electrode potentials.
Remote Earth: A location on the earth far enough from the
affected structure that the soil potential gradients associated
with
currents entering the earth from the affected structure are
insignificant.
Shielding: (1) Protecting; protective cover against mechanical
damage. (2) Preventing or diverting cathodic protection current
from its natural path. Stray Current: Current flowing through
paths other than the intended circuit.
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Structure: Tanks, piping, and associated equipment that may/may
not be under the influence of a cathodic protection system.
Structure-to-Electrolyte Potential: The potential difference
between the surface of a buried or submerged metallic structure
and
the electrolyte that is measured with reference to an electrode
in contact with the electrolyte. Test Lead: A wire or cable
attached to a structure for connection of a test instrument to make
cathodic protection potential or
current measurements.
Underground Storage Tank (UST) System: The equipment and
facility constructed, maintained, or used for underground
storage of products including tanks, piping, pumps, and
appurtenances associated with filling, storage, and dispensing of
the stored products.
Voltage: An electromotive force or a difference in electrode
potentials expressed in volts (V).
Voltage Drop: The voltage across a resistance when current is
applied in accordance with Ohms law. [also sometimes referred to as
IR drop]
Wire: A slender rod or filament of drawn metal. In practice, the
term is also used for smaller-gauge conductors (6 mm2
[0.009 in2]
[No. 10 American Wire Gauge {AWG}] or smaller).
_________________________________________________________________________
Section 3: Safety Considerations
3.1 Personnel who install, adjust, repair, remove, or test
impressed current CP equipment shall be knowledgeable and qualified
in electrical safety precautions before work is begun. The
following procedures shall be implemented when electrical
measurements are made:
3.1.1 Use properly insulated test lead clips and terminals to
avoid contact with an unanticipated high voltage (HV). Attach test
clips one at a time using a single-hand technique for each
connection.
3.1.2 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. Refer to NACE
SP0177
3 for additional information about electrical
safety.
3.1.3 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 exist. 3.1.4 Avoid
testing when thunderstorms are in the area.
3.1.5 Use caution when opening manways and when stringing test
leads across streets, roads, and other locations subject to
vehicular and pedestrian traffic. When conditions warrant, use
appropriate barricades, flagging, and flag persons.
3.1.6 Before entering excavations and confined spaces, inspect
these areas to determine whether they are safe. Inspections may
include shoring requirements for excavations and testing for
hazardous atmospheres in confined spaces. 3.1.7 Observe appropriate
electrical codes and applicable safety regulations.
_________________________________________________________________________
Section 4: Instrumentation and Measurement Guidelines 4.1
Electrical measurements of CP systems require the proper selection
and use of instruments. Instruments used to determine
structure-to-electrolyte potential, voltage drop, potential
difference, and similar measurements shall have appropriate voltage
ranges. The user should know the capabilities and limitations of
the equipment, follow the manufacturers instruction manual, and be
skilled in the use of electrical instruments. Failure to select and
use instruments correctly causes errors in CP measurements.
4.1.1 Analog instruments are usually specified in terms of input
resistance or internal resistance. This is usually expressed as
ohms/volt (/V) of full-scale meter deflection.
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4 NACE International
4.1.2 Digital instruments are usually specified in terms of
input impedance expressed as megohms.
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 climate
limitations.
4.2.1 Digital instruments are capable of measuring and
processing voltage readings many times per second. Evaluation of
the input waveform processing may be required if an instrument does
not give consistent results.
4.2.2 Measurement of structure-to-electrolyte potentials on UST
systems 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
and mining equipment.
4.3 Instrument Effects on Voltage Measurements
4.3.1 To measure structure-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.
4.3.1.1 A digital meter used to measure structure-to-electrolyte
potentials should have an input impedance of 10 megohms or more.
However, 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 because of
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 shall 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 effect of measurement circuit resistance errors may be
evaluated using an instrument with two or more input impedances
(internal resistance for analog instruments) and comparing the
values measured using different input impedances. If the measured
values are essentially identical, measurement circuit resistance
errors are negligible. Corrections must be made if measured values
are not essentially identical. Digital voltmeters that have a
single input impedance shall not be used for indicating measurement
circuit resistance errors. Alternative measurements may be made
with a potentiometer or by using two digital voltmeters with
different input impedance values. 4.3.5 Specialized equipment that
uses various techniques to measure the impressed current waveform
and calculate a structure-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 direct current (DC) sources.
4.4 Instrument Accuracy
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4.4.1 Instruments must be scheduled for periodic calibration to
a certified standard. 4.4.2 Instruments shall be checked for
accuracy before use by comparing measurements to a standard voltage
cell, to another acceptable voltage source, or to another
appropriate instrument that has been appropriately calibrated for
accuracy.
_________________________________________________________________________
Section 5: Structure-to-Electrolyte Potential Measurements 5.1
Voltmeters used to measure AC voltage, DC voltage, or other
electrical functions usually have one terminal designated common
(COM). This terminal may be black in color or have a negative
symbol (). The positive terminal may be red in color or have a
positive symbol (+).
5.1.1 The positive and negative symbols in the voltmeter display
indicate the direction of the current flow through the instrument.
For example, a positive value in the voltmeter display indicates
current flowing from the positive terminal through the voltmeter to
the negative terminal (see Figure 1[a]).
5.1.2 Usually, one voltmeter test lead is black and the other is
red. The black test lead should be connected to the negative
terminal of the voltmeter, and the red lead should be connected to
the positive terminal.
5.2 The usual technique to determine the DC voltage across
battery terminals, across a structure-to-electrolyte interface, or
from another 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 respect to the negative terminal.
A digital instrument connected in the same manner displays a
digital value, preceded by a positive symbol or no symbol at all.
In each situation, the measured voltage is positive with respect to
the instruments negative terminal. (see Figure 1[a].) 5.3 Voltage
measurements should be made using the lowest range on the
instrument. For an analog instrument, the voltage measurement is
more accurate when it is measured in the upper two-thirds of a
range selected for a particular instrument. 5.4 The voltage
difference between a reference electrode and the structure being
tested shall be measured with a voltmeter. The reference electrode
potential is normally positive with respect to the structure;
conversely, the structure is negative with respect to the reference
electrode.
A structure-to-electrolyte potential shall be measured using a
DC voltmeter having an appropriate input impedance (internal
resistance for an analog instrument), voltage range(s), test leads,
and a stable reference electrode, such as a saturated copper/copper
sulfate electrode (CSE), silver/silver chloride (Ag/AgCl)
electrode, or saturated potassium chloride (KCl) calomel reference
electrode. The reference electrode can may be portable or one
designed for permanent installation. Potential measurements taken
using reference electrodes other than a CSE shall be converted to
the CSE equivalents as shown in Table 1.
Table 1
Conversion of Other Potential Measurements to CSE
Equivalents
Reference Electrode Equivalent to 850 mV CSE Correction Calomel
780 mV Add 70 mV Silver/silver chloride 800 mV Add 50 mV Zinc +250
mV Add 1,100 mV
5.5.1 CSEs are usually used for measurements if the electrolyte
is soil or fresh water; they are sometimes used for measurements in
salt water. If a CSE is used in a high chloride environment, the
stability (lack of contamination) of the CSE must be determined
before the measurements may be considered valid. 5.5.2 The Ag/AgCl
reference electrode is usually used in seawater environments.
5.5.3 The saturated KCl calomel reference electrode is used
mainly for laboratory work. However, more rugged, polymer body, gel
filled, saturated KCl calomel reference electrodes that are
suitable for field work are available, though modifications may be
necessary to increase their contact area with the environment.
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5.6 Meter Polarity
5.6.1 Structure-to-electrolyte potentials are often measured by
connecting the negative terminal of the measuring instrument to the
structure and the positive terminal to the reference electrode
placed in the electrolyte, which is in contact with the structure.
With this connection, the instrument indicates that the reference
electrode is positive with respect to the structure. Therefore, the
structure potential is negative with respect to the reference
electrode (see Figure 1[a]).
5.6.2 Structure-to-electrolyte potentials are sometimes measured
with the reference electrode connected to the instrument negative
terminal and the structure connected to the positive terminal. This
produces a negative voltage display on digital meters (see Figure
1[b]).
-
DC
VOLT
COM
+
-
0.850
+
Pipe Test Lead
Voltmeters
+-
Direction of meter current
+
ReferenceElectrode
CL
Electrode potentialdoes not vary
Pipe potentialis the variable
Pipe
0 1
Figure 1(a): Conventional instrument connection.
Tank Test Lead
Tank potential is the variable.
Tank
does not vary.
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Pipe Test Lead
Voltmeter
+-
Direction of meter current
ReferenceElectrode
CL
Electrode potentialdoes not vary
Pipe potentialis the variable
Pipe
DC
VOLT
COM
-
-
0.850
+
Figure 1(b): Alternative instrument connection.
Figure 1: Instrument Connections
5.7 The structure-to-electrolyte potential measurement of a
structure should be made with the reference electrode placed close
to the structure-to-electrolyte interface. The most common practice
on a UST, for example, is to place the reference electrode as close
to the UST as practicable, which is usually at the surface of the
earth above the centerline of the UST (see Figure 1). 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) Structure-to-electrolyte interface; and
(g) Reference electrode-to-electrolyte interface.
NOTE: A high input impedance (>10 megohm) voltmeter or
potentiometer voltmeter should be used to eliminate the effects of
Paragraph 5.7(a), (b), (c), (f), and (g) on the potential
measurement. 5.8 The structure-to-electrolyte potential measurement
is a result of the voltage drop created by current flowing through
the electrical resistances of the items listed in Paragraph 5.7.
For a coated UST, coating deterioration should be considered. 5.9
All measurements shall be taken with reference electrodes that are
in contact with the electrolyte. Measurements shall not be taken
through concrete or asphalt. Soil contact may be established
through at-grade openings, by drilling a small hole through the
concrete or asphalt, or by contacting a seam of soil between
concrete and asphalt.
Tank Test Lead
Tank potential is the variable.
Tank
does not vary.
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5.10 The following conditions should be taken into consideration
when structure-to-electrolyte potential measurements are made to
determine the level of CP at the test site:
(a) Effectiveness of coatings, particularly those known or
suspected to be deteriorated or damaged; (b) Uncoated sections of
the UST system being cathodically protected; (c) Bonds to mitigate
interference; (d) Parallel coated USTs, 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 and depth of anodes; (i) Existence
of electrical isolation devices, including high-resistance pipe
connections and compression couplings; (j) Chemical composition of
electrolytes, such as unusual corrosives, chemical spills, presence
of hydrocarbons in soil, extreme soil resistivity changes, acidic
waters, and contamination from sewer spills; (k) Possible sources
of DC interference currents, such as welding equipment, foreign
rectifiers, mining equipment, and electric railway or transit
systems; (l) Contacts with other metals or structures; (m) Areas of
construction activity during the UST system history; (n)
Underground metallic structures close to or crossing the UST
system; (o) Other appurtenances; and (p) Electrolyte pH.
5.11 The effect of voltage drops other than those across the
structure-to-electrolyte interface shall be considered for valid
interpretation of structure-to-electrolyte potential measurements
made to satisfy a criterion. Measurement errors should be minimized
to ensure reliable structure-to-electrolyte potential
measurements.
5.12 The primary method to determine the effect of voltage drops
on a structure-to-electrolyte potential measurement is by
interrupting all significant current sources before taking the
potential measurement. This measurement must be taken without delay
after the interruption of current to avoid loss of polarization.
This measurement is referred to as an instant-off potential, and is
considered to be the polarized potential of the structure at that
location. This measurement does not account for voltage drops
across the structure-to-electrolyte interface, which is part of the
protection potential. NOTE: The current interruption may cause a
voltage spike. This spike shall not be recorded as the instant-off
potential. The magnitude and duration of the voltage spike can
vary; however, the duration is usually within 0.5 second. 5.13
Examples of situations in which it may not be practical to
interrupt all current sources to make the instant-off potential
measurement include:
5.13.1 Galvanic anodes are connected directly to the structure
without benefit of aboveground connections. Interruption of this
kind of system requires excavation of the connections. 5.13.2
Interference from CP devices on foreign structure or electrical
continuity with foreign structure. 5.13.3 Manmade sources of DC
stray currents, such as other CP systems, mass transit, DC welding,
or mining operations, are nearby.
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5.14 Significant environmental, structural, or CP system
parameter changes may include:
(a) Replacement or addition of UST components and systems; (b)
Addition, relocation, or deterioration of CP systems; (c) Failure
of electrical isolation or bonding devices; (d) Changes in the
effectiveness of coatings; (e) Influence of foreign structures; and
(f) Modification of the environment.
5.15 After a CP system is operating, time may be required for
the UST system to polarize. This should be taken into consideration
when the potential at a test site on a newly protected UST system
is measured or after a CP device is re-energized.
_________________________________________________________________________
Section 6: Causes of Measurement Errors 6.1 The following
factors may contribute to faulty potential measurements:
6.1.1 UST system and instrument test leads
(a) Broken or frayed wire strands (may not be visible inside
insulation of the wire);
(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 UST system 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, or
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 without consideration of the voltage drop caused by the
anode, including reference electrode placement over the top of the
structure(s) protected by close anodes;
(f) Reference electrode positioned in the potential gradient of
a metallic structure other than the one whose potential is being
measured without consideration of the voltage drop caused by the
potential gradient of the metallic structure; (g) Electrolyte
between structure and disbonded coating causing error because of
electrode placement in electrolyte on opposite side of coating;
(h) Defective permanently installed reference electrode;
(i) Temperature correction not applied when needed;
(j) Photosensitive measurement error (in CSE with a clear-view
window) caused by light striking the electrode electrolyte solution
(photovoltaic effect); and
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(k) Remote reference electrode placement in which voltage drops
are not considered as part of the measurement. 6.1.3 Unknown
electrical isolation devices, such as disbonded tubing or pipe
systems, may cause the UST system to be electrically discontinuous
between the test connection and the reference electrode location.
(Section 11 provides guidance on methods of troubleshooting that
identify continuity or discontinuity.)
6.1.4 Parallel paths may be 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 The
use of defective or inappropriate instruments, incorrect voltage
range selection, instruments not calibrated or zeroed, or damp
instruments sitting on wet earth may cause measurement errors.
6.1.6 Instruments that have an analog-to-digital converter may
operate at such fast speeds that the voltage spikes produced by
current interruption are indicated as the potential measurement
instead of the actual on and off values. 6.1.7 The polarity of the
measured value may be incorrectly observed.
6.1.8 Measurement errors may occur if the conductor carrying the
CP current is used as a test lead for a UST system potential
measurement. 6.1.9 Electromagnetic interference or induction
resulting from AC power lines or RF transmitters can may induce
test lead and 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. For this
reason, DC voltmeters must have sufficient AC rejection capability,
which may be determined by referring to the manufacturers
specification. 6.1.10 The use of an internal UST connection via the
fill pipe in the absence of a CP test wire when the UST has been
lined may cause faulty potential measurements.
6.2 Several methods may be used to reduce contact resistance
caused by the following factors:
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
Structure-to-Electrolyte Interface Voltage drops present when
structure-to-electrolyte potential measurements are made may occur
in the following:
7.1 Measurement Circuit
7.1.1 The voltage drop other than across the
structure-to-electrolyte interface in the measurement circuit is
the sum of the individual voltage drops caused by the meter current
flow through:
(a) Instrument test lead and connection resistances;
(b) Reference electrode internal resistance;
(c) Reference electrode-to-electrolyte contact resistance;
(d) Coating resistance;
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(e) Structure metallic resistance;
(f) Electrolyte resistance;
(g) Analog meter internal resistance; and
(h) Digital meter internal impedance.
7.1.2 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.
7.2 Electrolyte
7.2.1 When a structure-to-electrolyte potential is measured with
CP current applied, the voltage drop in the electrolyte between the
reference electrode and the structure-to-electrolyte interface
shall be considered. Measurements taken close to sacrificial or
impressed current anodes may contain a large voltage drop.
7.2.2 Such a voltage drop may consist of, but is not limited to,
the following:
(a) A voltage drop caused by current flowing to coating holidays
when the UST system is coated; and (b) A voltage drop caused by
large voltage gradients in the electrolyte that occur near
operating anodes (sometimes called raised earth effect).
7.2.3 Testing to locate anodes by moving the reference electrode
along the UST may be necessary when the locations are not
known.
7.3 Coatings
7.3.1 Most coatings provide protection to the UST system by
reducing contact between the UST system surface and the
environment. While the insulation provided by a coating reduces the
current required for CP of a coated UST system versus that required
for an uncoated UST system, coatings are not impervious to current
flow. 7.3.2 Coatings resist current flow because of their relative
ionic impermeability. Current flow through the resistance of the
coating causes the voltage drop to be greater than that occurring
when the UST system is bare, under the same environmental
conditions.
_________________________________________________________________________
Section 8: Test Method 1Negative 850 mV Structure-to-Electrolyte
Potential of Steel Underground Storage Tank Systems with Cathodic
Protection Applied
8.1 This section describes a test method to satisfy the 850 mV
with CP applied criterion stated in NACE SP0285. This section
should be used for factory-installed galvanic anode CP systems in
which the anodes cannot be disconnected.
8.1.1 Voltage drop must be considered when using this CP
criterion.
8.1.2 The primary method for considering voltage drop(s) shall
be by taking instant-off potential measurements in accordance with
Sections 9 and 10. When anodes cannot be disconnected, other
methods such as the following may be used:
(a) Measuring or calculating the voltage drop(s); (b) Reviewing
the historical performance of the CP system; (c) Evaluating the
physical and electrical characteristics of the UST system and its
environment; (d) Determining whether there is physical evidence of
corrosion; and
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(e) Recording measurements with reference electrode placed
electrically remote from the UST system (i.e., remote earth) in
conjunction with local measurements.
8.2 General
8.2.1 Test Method 1 measures the structure-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 structure coating from current sources such as
impressed current and galvanic anodes.
8.2.2 CP current shall remain on during the measurement process.
This potential is commonly referred to as the on potential.
8.2.3 Because voltage drops other than those across the
structure-to-electrolyte interface may be included in this
measurement, these voltage drops shall be considered, as discussed
in Paragraph 8.6.
8.3 Comparison with Other Methods
8.3.1 Advantages
(a) Requires minimal equipment, and (b) Less time is required to
make measurements.
8.3.2 Disadvantages
(a) The potential measured includes voltage drops other than
those across the structure-to-electrolyte interface; (b) Meeting
the requirements for considering the significance of voltage drops
(see Paragraph 8.6) may result in a need for additional time to
assess adequacy of CP at the test site; and (c) Test results are
difficult or impossible to analyze if stray currents are present or
foreign impressed current devices are present and cannot be
interrupted.
8.4 Basic Test Equipment
8.4.1 A voltmeter with adequate input impedance. Commonly used
digital instruments have a nominal impedance of 10 megohms. 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 Meter leads with insulated wire and terminal connections
suitable for making reliable electrical contact with the structure
and reference electrode. Color-coded meter leads should be used to
avoid confusion of polarity for the measured value.
8.4.3 A CSE or other standard reference electrode may be used.
Reference electrodes that may be used in place of a CSE are
described in Paragraph 5.5.
8.5 The following procedure shall be followed when this test is
performed:
8.5.1 Before the test, verify that CP equipment has been
installed and is operating properly. Sufficient time should be
allowed for the structure potentials to reach polarized values.
8.5.2 Determine the location of reference electrode placement for
potential measurements. Selection of a site may be based on:
(a) Accessibility for future monitoring; (b) Other protection
systems, structures, and anodes that may influence the
structure-to-electrolyte potential; (c) Electrical midpoints
between protective devices; (d) Known location of an ineffective
coating if the structure is coated; and
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(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, in a location that minimizes
the voltage gradient from anodes, other structures, and coating
defects (if the structure is coated).
8.5.3.1 For factory-installed galvanic anode CP systems in which
the anodes cannot be disconnected, a minimum of one local potential
measurement near the UST center and away from the anodes and one
remote potential measurement should be taken. Alternatively, a
minimum of three potential measurements, one at each of the UST
ends and one near the center of the UST, may be taken. 8.5.3.2 In
general, the reference electrode should be placed as close to the
UST surface as possible and as far from anodes as possible.
8.5.4 Record the location of the reference electrode to allow it
to be returned to the same location for subsequent tests. 8.5.5
Connect the voltmeter to the structure and reference electrode as
described in Paragraph 5.6.
8.5.6 Evaluate the effect of measurement circuit resistance on
the structure-to-electrolyte potential as indicated in Paragraph
5.7. 8.5.7 Record the structure-to-electrolyte potential and its
polarity with respect to the reference electrode. 8.5.8 Record a
sufficient number of potential measurements to determine the level
of CP over the entire structure.
8.6 Evaluation of Data
8.6.1 The significance of voltage drops may be considered by
comparing historical levels of CP with physical evidence from the
UST system to determine whether corrosion has occurred. 8.6.2
Physical evidence of corrosion may be determined by evaluating
items such as leak history data or UST system inspection report
data regarding locations of coating failures, localized conditions
of more corrosive electrolyte, or whether substandard CP levels
have been experienced.
8.6.3 The cathodic protection shall meet the 850 mV polarized
criterion stated in NACE SP0285.
_________________________________________________________________________
Section 9: Test Method 2Negative 850 mV Polarized
Structure-to-Electrolyte Potential of Steel Underground Storage
Tank Systems
9.1 This section describes a method that uses an interrupter(s)
to eliminate the CP system voltage drop from the
structure-to-electrolyte potential measurement for comparison with
the CP criterion stated in NACE SP0285.
9.1.1 If directly connected galvanic anodes that cannot be
interrupted are present, this test method shall not be used. 9.1.2
For impressed current CP systems, the tester must take instant-off
potential measurements, unless a NACE Corrosion Specialist or NACE
CP Specialist has determined that instant-off potential
measurements are not required.
9.2 General
9.2.1 Interrupting the known CP current source(s) eliminates
voltage drops associated with the protective currents. However,
significant voltage drops may also occur because of currents from
other sources. 9.2.2 Current sources that may affect the accuracy
of this test method include the following:
(a) Unknown, inaccessible, or directly connected galvanic
anodes; (b) Other CP systems on associated piping or foreign
structures; (c) Electric railway systems;
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(d) Galvanic or bimetallic cells; (e) DC mining equipment; (f)
Adjacent tanks, electrically connected and polarized to different
potentials; and (g) Unintentional connections to other structures
or bonds to mitigate interference.
9.2.3 To avoid significant depolarization of the UST system, the
off period should be limited to the time necessary to make an
accurate potential measurement. The off period is typically less
than three seconds.
9.3 Comparison with Other Methods
9.3.1 Advantages
Voltage drops associated with the protective currents being
interrupted are eliminated.
9.3.2 Disadvantages
(a) Additional equipment is required;
(b) Additional time may be required to set up equipment and to
make structure-to-electrolyte potential measurements; and (c) Test
results are difficult or impossible to analyze with regard to
whether stray currents are present or foreign impressed current
devices are present and cannot be interrupted.
9.4 Basic Test Equipment
9.4.1 A voltmeter with adequate input impedance. Commonly used
digital instruments have a nominal impedance of 10 megohms. 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. 9.4.2 Meter leads with insulated wire and terminal
connections suitable for making reliable electrical contact with
the structure and reference electrode. Color-coded meter leads
should be used to avoid confusion of polarity of the measured
value. 9.4.3 A CSE or other standard reference electrode may be
used. Reference electrodes that may be substituted for the CSE are
described in Paragraph 5.5.
9.4.4 Sufficient and adequate means to interrupt CP current
sources (such as sacrificial anodes, rectifiers, and electrical
bonds) that are influencing the structure simultaneously.
9.5 The following procedure shall be followed when this test is
performed.
9.5.1 Before the test, verify that CP equipment has been
installed and is operating properly. Sufficient time should be
allowed for the UST system potentials to reach polarized values.
9.5.2 Install and place in operation necessary interrupter
equipment in all DC sources influencing the UST system at the test
site. The off interval should be kept as short as possible but
still long enough to measure a polarized structure-to-electrolyte
potential after any spike (see Figure 2[a]) has collapsed. 9.5.3
Determine the location of reference electrode placement for
potential measurements. Selection of a site may be based on:
(a) Accessibility for future monitoring; (b) Other protection
systems, structures, and anodes that may influence the
structure-to-electrolyte potential; (c) Electrical midpoints
between protective devices;
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(d) Known location of an ineffective coating if the structure is
coated; and (e) Location of a known or suspected corrosive
environment.
9.5.4 Make electrical contact between the reference electrode
and the electrolyte at the test site, in a location that minimizes
the voltage gradient from other structures and coating defects (if
the structure is coated). 9.5.5 Record the location of the
reference electrode to allow it to be returned to the same location
for subsequent tests. 9.5.6 Connect the voltmeter to the tank and
reference electrode as described in Paragraph 5.6.
9.5.6.1 If spiking could be present, delay measurement of the
tank-to-electrolyte potential to eliminate the voltage spike from
the measured value. Spiking usually occurs within 0.5 second of the
interruption of the CP currents. 9.5.6.2 Appropriate
instrumentation such as an oscilloscope or high-speed recording
device may be used to verify the presence and duration of the
spiking.
9.5.7 Evaluate the effect of measurement circuit resistance on
the structure-to-electrolyte potential as indicated in Paragraph
5.7. 9.5.8 Record the structure-to-electrolyte on and instant-off
potentials and their polarities with respect to the reference
electrode. 9.5.9 Record a sufficient number of potential
measurements to determine the level of CP over the entire UST
system.
9.6 Evaluation of Data CP shall be judged adequate at the test
site if the polarized (instant-off) structure-to-electrolyte
potential meets the CP criterion stated in NACE SP0285.
_________________________________________________________________________
Section 10: Test Method 3100 mV Cathodic Polarization of Steel
Underground Storage Tank Systems
10.1 This section describes the use of either polarization decay
or polarization formation to determine whether the UST system CP is
adequate at the test site in accordance with the 100 mV cathodic
polarization criterion. Consequently, this test method consists of
two mutually independent parts, Test Methods 3a and 3b, which
describe the procedures for polarization decay and polarization
formation testing, respectively. Generic cathodic polarization
curves for Test Methods 3a and 3b are shown in Figure 2. Figure 2
contains schematic drawings of generic polarization decay and
polarization formation. If directly connected galvanic anodes that
cannot be interrupted are present, these test methods shall not be
used.
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Figure 2(a): Polarization decay
Figure 2(b): Polarization Formation
Figure 2: Cathodic polarization curves.
Spike
"On" Potential
"Instant-Off" Potential (Polarized Potential)
Depolarizing Line
Voltage Drop (IR Drop)
Polarization Decay
Figure 3b
Polarization Formation
Normal Operation
"On" Potential
Current Interruption
"Instant-Off"
Potential
Polarization
Cathodic Protection Applied
Corrosion Potential
Polarizing Line
Time Period
(May be seconds, minutes, hours, or days)
Pip
e-t
o-E
lec
tro
lyte
Po
ten
tia
l (-
mV
) P
ipe
-to
-Ele
ctr
oly
te P
ote
nti
al (-
mV
)
Time Period
(May be seconds, minutes, hours, or days)
1,200
1,100
1,000
1,100
1,0000
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10.2 Test Method 3aUse of Polarization Decay (See Figure 2[a])
10.2.1 This test method uses polarization decay to assess the
adequacy of CP of a steel UST system in accordance with the
cathodic polarization criterion stated in NACE SP0285.
10.2.2 General
10.2.2.1 Interrupting the known CP source(s) eliminates voltage
drops associated with the protective current(s). 10.2.2.2 Other
current sources that may affect the accuracy of this test method
include the following:
(a) Unknown, inaccessible, or directly connected galvanic
anodes; (b) CP systems on associated tank systems or foreign
structures;
(c) Electric railway systems;
(d) Galvanic, or bimetallic, cells;
(e) DC mining equipment;
(f) Adjacent tanks, electrically connected and polarized to
different potentials;
(g) Unintentional connections to other structures or bonds to
mitigate interference; and
(h) C welding equipment.
10.2.3 Comparison with Other Methods
10.2.3.1 Advantages
(a) This method is especially useful for bare or ineffectively
coated tanks; and
(b) This method is advantageous in places where corrosion
potentials may be low (for example, 500 mV or less negative) 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 may be required to set up equipment and to
make tank-to-electrolyte potential measurements; and
(c) Test results are difficult or impossible to analyze if
foreign impressed current devices are present and cannot be
interrupted or if stray currents are present.
10.2.4 Basic Test Equipment
10.2.4.1 A voltmeter with adequate input impedance. Commonly
used digital instruments have a nominal impedance of 10 megohms. An
analog instrument with an internal resistance of 100,000 ohms/volt
may be adequate in certain circumstances in which the circuit
resistance is low. A potentiometer circuit may be necessary in
other instances. Recording voltmeters may be useful for recording
polarization decay. 10.2.4.2 Meter leads with insulated wire and
terminal connections suitable for making reliable electrical
contact with the structure and reference electrode. Color-coded
meter leads should be used to avoid confusion of polarity of the
measured value. 10.2.4.3 Sufficient and adequate means to interrupt
CP current sources (such as sacrificial anodes, rectifiers, and
electrical bonds) that are influencing the UST system
simultaneously.
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10.2.4.4 A CSE or other standard reference electrode may be
used. Reference electrodes that may be substituted for the CSE are
described in Paragraph 5.5.
10.2.5 The following procedure shall be used when this test is
performed:
10.2.5.1 Before the test, verify that CP equipment has been
installed and is operating properly. Sufficient time should be
allowed for the UST system potentials to reach polarized values.
10.2.5.2 Provide means for current interruption in all DC sources
influencing the UST system at the test site. The off interval
should be kept as short as possible but still long enough to
measure a polarized structure-to-electrolyte potential after any
spike (see Figure 2a) has collapsed. 10.2.5.3 Determine the
location of reference electrode placement for potential
measurements. 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 structure-to-electrolyte potential;
(c) Electrical midpoints between protective devices;
(d) Known location of an ineffective coating if the UST system
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 in a location that minimizes
the voltage gradient from other structures and coating defects (if
the UST system is coated). 10.2.5.5 Record the location of the
reference electrode to allow it to be returned to the same location
for subsequent tests. 10.2.5.6 Connect the voltmeter to the UST
system and reference electrode as described in Paragraph 5.6. If
spiking could 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. 10.2.5.7
Measure and record the structure-to-electrolyte on and instant-off
potentials and their polarities with respect to the reference
electrode. The instant-off structure-to-electrolyte potential is
the baseline potential from which the polarization decay is
calculated. 10.2.5.8 Turn off the CP current sources that influence
the UST system (i.e., those interrupted in Paragraph 10.2.5.2) at
the test site. Continue to measure and record the
structure-to-electrolyte potential until it either:
(a) Becomes at least 100 mV less negative than the instant-off
potential, or
(b) Reaches a stable depolarized level.
10.2.5.8.1 Measurements shall be made at sufficiently frequent
intervals to avoid attaining and remaining at a corrosion potential
for an unnecessarily extended period. 10.2.5.8.2 If 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.
10.2.5.9 Record a sufficient number of potential measurements to
determine the level of CP over the entire UST system.
10.2.6 Evaluation of Data The CP shall meet the 100 mV cathodic
polarization criterion stated in NACE SP0285.
10.2.7 Monitoring
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When at least 100 mV or more of polarization decay has been
measured, the UST system on potential at the test site may be used
for monitoring unless significant environmental, structural,
coating integrity, or CP system parameters have changed.
10.3 Test Method 3bUse of Polarization Formation (See Figure
2[b])
10.3.1 This test method provides a procedure using the formation
of polarization to assess the adequacy of CP at a test site on a
steel UST system in accordance with the cathodic polarization
criterion stated in NACE SP0285. 10.3.2 General
10.3.2.1 Steel UST systems may be adequately protected if, from
the off potential, applying CP causes a polarized potential that is
at least 100 mV more negative.
10.3.2.2 Current sources that may affect the accuracy of this
test method include the following:
(a) Unknown, inaccessible, or directly connected galvanic
anodes; (b) CP systems on associated tank systems or foreign
structures;
(c) Electric railway systems;
(d) Galvanic, or bimetallic, cells;
(e) DC mining equipment;
(f) Adjacent tanks, electrically connected and polarized to
different potentials;
(g) Unintentional connections to other structures or bonds to
mitigate interference; and
(h) DC welding equipment.
10.3.3 Comparison with Other Methods
10.3.3.1 Advantages
(a) This method is especially useful for a bare or ineffectively
coated UST system; and
(b) This method is advantageous if corrosion potentials could be
low (for example, 500 mV or less negative) 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 may be required to set up equipment and to
make the structure-to-electrolyte potential measurements; and
(c) Test results are difficult or impossible to analyze if
foreign impressed currents are present and cannot be interrupted or
if stray currents are present.
10.3.4 Basic Test Equipment
10.3.4.1 A voltmeter with adequate input impedance. Commonly
used digital instruments have a nominal impedance of 10 megohms. An
analog instrument with an internal resistance of 100,000 ohms/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 Meter leads with insulated wire and
terminal connections suitable for making reliable electrical
contact with the structure and reference electrode. Color-coded
meter leads should be used to avoid confusion of polarity of the
measured value.
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10.3.4.3 Sufficient and adequate means to interrupt CP current
sources (such as sacrificial anodes, rectifiers, and electrical
bonds) that are influencing the tank simultaneously. 10.3.4.4 A CSE
or other standard reference electrode may be used. Reference
electrodes that can be substituted for the CSE are described in
Paragraph 5.5.
10.3.5 The following procedure shall be used when this test is
performed: 10.3.5.1 Before the test, verify that CP equipment has
been installed but is not operating. 10.3.5.2 Determine the
location of reference electrode placement for potential
measurements. 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 structure-to-electrolyte potential; (c) Electrical
midpoints between protective devices; (d) Known location of an
ineffective coating if the UST system 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 in a location that minimizes
the voltage gradient from other structures and coating defects (if
the UST system is coated). 10.3.5.4 Record the location of the
reference electrode to allow it to be returned to the same location
for subsequent tests. 10.3.5.5 Connect the voltmeter to the
structure and reference electrode as described in Paragraph 5.6.
10.3.5.6 Measure and record the structure-to-electrolyte potential
and its polarity with respect to the reference electrode. This
potential shall be the value from which the polarization formation
is calculated. 10.3.5.7 Apply the CP current. Sufficient time
should be allowed for the UST system potentials to reach polarized
values. 10.3.5.8 Install and place in operation necessary
interrupter equipment in the DC sources influencing the UST system
at the test site. The off interval should be kept as short as
possible but still long enough to measure a polarized
structure-to-electrolyte potential after any spike (see Figure
2[a]) has collapsed. 10.3.5.9 Measure and record the
structure-to-electrolyte on and instant-off potentials and their
polarities with respect to the reference electrode. The difference
between the instant-off potential and the original potential is the
amount of polarization formation.
10.3.5.9.1 If spiking may be present, delay measurement of the
structure-to-electrolyte potential to eliminate the spike voltages
from the measured value. Spiking usually occurs within 0.5 second
of the interruption of the CP currents. 10.3.5.9.2 Appropriate
instrumentation such as an oscilloscope or high-speed recording
device may be used to verify the presence and duration of the
spiking.
10.3.5.10 Record a sufficient number of potential measurements
to determine the level of CP over the entire UST system.
10.3.6 Evaluation of Data
The CP shall meet the 100 mV cathodic polarization criterion
stated in NACE SP0285.
10.3.7 Monitoring
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When at least 100 mV or more of polarization formation has been
measured, the structure on potential may be used for monitoring
unless significant environmental, structural, coating integrity, or
CP system parameters have changed.
_________________________________________________________________________
Section 11: Test Methods for Continuity Testing of Steel
Underground Storage Tank Systems 11.1 The following test methods
may be used to determine whether a tank is electrically continuous
with piping, electrical equipment, conduit, and other appurtenances
or structures in the immediate area. This list of tests is not
all-inclusive. Other methods or equipment may be used to test for
electrical continuity or discontinuity. 11.2 UST systems may have
been designed to be electrically isolated from other metallic
structures such as piping, conduit, grounded electrical equipment,
and hold-down devices. A lack of electrical isolation from these
structures can result in lowered levels of CP or a reduction in the
life of the CP system.
11.2.1 Some UST systems are designed to have electrical
continuity between the UST and the related piping, electrical
equipment, and other appurtenances or accessories. Discontinuity
between the UST and another structure or appurtenance can result in
a lack of protection, and in some cases, damage to the UST or
appurtenances that are isolated. 11.2.2 One method to verify the
continuity of the negative bond wire to the intended structure(s)
being protected is to disconnect the negative lead wire from the
rectifier and connecting one lead of the voltmeter to this negative
lead wire and measuring the potential difference between the
negative lead wire and other structures.
11.3 Fixed Cell/Moving Ground Technique
11.3.1 This technique uses basic CP test equipment to test for
an indication of possible electrical continuity through the use of
structure-to-electrolyte potential comparison measurements. 11.3.2
The following procedure shall be followed when testing for
continuity using the fixed cell/moving ground technique:
11.3.2.1 Make electrical contact between the reference electrode
and the electrolyte at a location remote from the UST system to be
tested.
11.3.2.1.1 The location should not be within the potential
gradient of an anode or any other structure. Placement of the
reference electrode in a location that is shielded by another tank
or structure may result in erroneous data concerning the continuity
of the shielded tank(s) or structures. Alternate reference
electrode placements may be necessary to determine the continuity
of all of the structures at a test site. 11.3.2.1.2 Once placed,
the reference electrode shall not be moved for the duration of this
test procedure.
11.3.2.2 Connect the voltmeter to the structure. 11.3.2.3
Measure and record the structure-to-electrolyte potential with
respect to the reference electrode. 11.3.2.4 Disconnect the test
lead from the structure and continue to test other structures by
connecting that lead to the structure in question. 11.3.2.5 Measure
and record structure-to-electrolyte potentials for structures that
are to be evaluated.
11.3.3 Evaluation of Data Electrical continuity or discontinuity
may be indicated for structures that meet the CP criterion stated
in NACE SP0285.
11.4 Point-to-Point Technique
11.4.1 This technique is used to test for an indication of
possible electrical continuity using a voltmeter to measure the
difference in electrical potential between two underground
structures. 11.4.2 To perform this test, one test lead from a
voltmeter shall be connected to the structure to be tested. The
second test lead from the voltmeter shall be connected to a
separate structure or appurtenance that is suspected to be
electrically continuous.
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11.4.3 With the rectifier deenergized, one technique to verify
the continuity of the negative bond wire to the intended
structure(s) being protected is to disconnect the negative
structural lead wire from the rectifier and connect one lead of the
voltmeter to this negative lead and measure the potential
difference between the negative lead wire and other structures.
11.4.4 Evaluation of Data Electrical continuity or discontinuity
shall be indicated for structures that meet the CP criterion stated
in NACE SP0285.
NOTE: Structures and appurtenances found in a UST system may
misrepresent the results of this test. Galvanized pipe or conduit
and coated or uncoated structures are some examples of different
alloys or conditions that could cause misrepresentation of the
results of this test.
11.5 Applied Current Technique
11.5.1 This technique uses either a temporary DC source or an
existing interruptible CP system and basic CP test equipment to
determine electrical continuity. This technique may also be used to
confirm the fixed cell/moving ground and potential difference
technique test results. 11.5.2 The following procedure shall be
followed when testing using the applied current technique:
11.5.2.1 Make electrical contact between the reference electrode
and the electrolyte at a location remote from the UST system to be
tested.
11.5.2.1.1 The location should not be within the potential
gradient of anodes or other structures. Placement of the reference
electrode in a location that is shielded by another tank or
structure may result in erroneous data concerning the continuity of
the shielded tanks or structures. Alternate reference electrode
placements may be necessary to determine the continuity of all
structures at the test site. 11.5.2.1.2 Once placed, the reference
electrode shall not be moved for the duration of this test
procedure. 11.5.2.1.3 Prior to testing, existing impressed current
CP systems should be turned off. If a galvanic anode CP system is
being used, the anodes should be disconnected from the structure,
if practical.
11.5.2.2 Measure and record the structure-to-electrolyte
potential with respect to the reference electrode. 11.5.2.3 Measure
and record a structure-to-electrolyte potential for other
structures under test. 11.5.2.4 Use either of the following for the
applied current procedure:
(a) Energize the existing impressed current CP system or
galvanic anode CP system; or
(b) Energize a temporary groundbed. The temporary groundbed must
be electrically isolated from the structures under test.
NOTE: When using this technique, it is not safe to create a
connection to a structure in a vapor-rich environment. Connections
should not be made inside the UST above the fuel level.
11.5.2.5 Remeasure and record the structure-to-electrolyte
potentials with respect to the reference electrode. 11.5.2.6
Remeasure and record the structure-to-electrolyte potentials for
structures that are designed to be continuous or isolated.
11.5.3 Evaluation of Data
The potentials taken before the CP system or groundbed was
energized shall be compared with those taken afterward. Electrical
continuity or discontinuity should be indicated for structures that
meet the CP criterion stated in NACE SP0285.
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11.6 Invalid Techniques
Techniques that use continuity testers common to the electrical
trades are not valid for continuity testing on UST systems in a
common electrolyte. Some equipment in this category includes the
following:
(a) DC ohmmeter
(b) Diode tester
(c) Continuity test light
11.7 Corrective Action
11.7.1 Further investigation may be required to confirm the
presence or lack of continuity, depending on the original UST
system design. 11.7.2 For some UST systems, correction of the
defect may preclude the installation of additional CP measures.
11.7.3 In other instances, correction of the defect may be
necessary for the original UST system or supplemental CP to be
totally effective.
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Section 12: Piping and Appurtenances 12.1 Any other cathodically
protected structures associated with the UST system, such as
metallic product piping and metallic flexible connectors that
routinely contain product, must be tested in accordance with this
test method, NACE SP0169,
4 and NACE
Standard TM0497.5
12.2 A minimum of two potential measurements shall be taken for
each piping run less than 30 m (100 ft). Three or more potential
measurements shall be taken for piping runs that are 30 m (100 ft)
or greater in length. When three or more potential measurements are
taken, one potential measurement shall be taken near the center of
the run or away from the anodes. The other two potential
measurements shall be taken near the ends of the piping run.
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Section 13: Records
13.1 Persons performing CP testing shall provide the owner of
the UST system with a record of the CP survey. 13.2 The record
should contain the information identified in one of the following
checklists, as appropriate (see Appendix B [Nonmandatory] for the
checklists):
(a) Checklist for CP Systems When the Current Cannot Be
Interrupted
(b) Checklist for CP Systems When the Current Can Be
Interrupted
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References 1. NACE SP0285 (formerly RP0285) (latest revision),
Corrosion Control of Underground Storage Tank Systems by Cathodic
Protection (Houston, TX: NACE). 2. NACE/ASTM G193 (latest
revision), Standard Terminology and Acronyms Relating to Corrosion
(Houston, TX: NACE and West Conshohocken, PA: ASTM). 3. NACE SP0177
(formerly RP0177) (latest revision), Mitigation of Alternating
Current and Lightning Effects on Metallic Structures and Corrosion
Control Systems (Houston, TX: NACE).
4. NACE SP0169 (formerly RP0169) (latest revision), Control of
External Corrosion on Underground or Submerged Metallic Piping
Systems (Houston, TX: NACE).
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5. NACE Standard TM0497 (latest revision), Measurement
Techniques Related to Criteria for Cathodic Protection on
Underground or Submerged Metallic Piping Systems (Houston, TX:
NACE).
6. 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).
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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. 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. West Conshohocken, 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. Gummow, R.A., ed. Cathodic Protection
Criteria a Literature Survey. Houston, TX: NACE, 1989. Jones, D.A.
Analysis of Cathodic Protection Criteria. Corrosion 28, 11 (1972):
pp. 421-423. Parker, M.E. Pipeline Corrosion and Cathodic
Protection. 2nd ed. Houston, TX: Gulf Publishing, 1962
Peabody, A.W. Control of Pipeline Corrosion. Houston, TX: NACE,
2001.
Stephens, R.W. Surface Potential Survey Procedure and
Interpretation of Data 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 Proceedings of the Appalachian
Corrosion
Short Course, held May 1980. Morgantown, WV: University of West
Virginia, 1980.
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Appendix A Using CP Coupons to Determine the Adequacy of
Cathodic Protection
(Nonmandatory)
This appendix is considered nonmandatory, although it may
contain mandatory language. It is intended only to provide
supplementary information or guidance. The user of this standard is
not required to follow, but may choose to follow, any or all of the
provisions herein.
A1 CP coupons, particularly when accompanied by other
engineering tools and data, have been used to evaluate whether CP
at a test site complies with a given criterion.
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A2 The small size of a CP coupon may reduce the effect of
voltage drops caused by currents from other sources. The magnitude
of these voltage drops can be determined by interrupting CP current
sources while the CP coupon is disconnected and noting whether
there is a shift in the CP coupon-to-electrolyte potential. A3
These methods use CP coupons to assess the adequacy of CP applied
to a selected test site. A CP coupon is a metal sample representing
the UST at the test site and used for CP testing. The CP coupons
used for these tests should be:
(a) Of the same material and with the same or nearly the same
properties as the UST; (b) Known not to interfere with determining
the adequacy of the CP system; (c) On a coated UST, the size of the
CP coupon should represent the largest anticipated coating defect;
(d) Placed at UST depth in the same backfill as the UST; (e)
Prepared with all mill scale and foreign materials removed from the
surface; and (f) Placed at a location of an ineffective coating, if
known.
A4 During normal operations, a CP coupon has an insulated test
lead brought above ground and connected to a UST test lead. The CP
coupon receives CP current and represents the UST at the test site.
For testing purposes, this connection is opened, and the polarized
potential of the CP coupon is measured.
A4.1 The time the connection is open to measure the CP coupons
instant-off potential should be minimized to avoid significant
depolarization of the CP coupon. The CP coupon is then allowed to
depolarize. A4.2 If possible, CP coupon current direction and
magnitude should be verified using a current clip gauge or resistor
permanently placed in series with the CP coupon lead. Measurements
showing discharge of current from the CP coupon should be reason to
question the validity of using a CP coupon at the test site.
A5 The basic test equipment for both of these tests is the
same:
A5.1 Voltmeter with adequate input impedance. Commonly used
digital instruments have a nominal impedance of 10 megohms. An
analog instrument with an internal circuit resistance of 100,000
ohms per volt may be adequate in certain circumstances if the
circuit resistance is low. A potentiometer circuit may be necessary
in other circumstances.
A5.2 Meter leads with insulated wire and terminal connections
suitable for making reliable electrical contact with the UST and
reference electrode. Color-coded meter leads should be used to
avoid confusion of polarity of the measured value. A5.3 A CSE or
other standard reference electrode may be used. Reference
electrodes that may be substituted for the CSE are described in
Paragraph 5.5.
A6 CP Coupon Test Method Afor Negative 850 mV Polarized
Structure-to-Electrolyte Potential of a Steel UST System
A6.1 This test method uses a CP coupon to assess the adequacy of
CP on a steel UST system in accordance with the CP criterion stated
in NACE SP0285.
A6.2 Comparison with Other Methods
A6.2.1 Advantages
(a) Provides a polarized CP coupon-to-electrolyte potential,
free of voltage drop, with a minimum of specialized equipment,
personnel, and vehicles; and (b) Provides a more comprehensive
evaluation of the polarization at the test site than conventional
structure-to-electrolyte potential measurements that may be
influenced by the location, size, and number of coating holidays,
if the UST system is coated.
A6.2.2 Disadvantages May have high initial costs to install CP
coupons, especially for existing UST system.
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A6.3 Procedure
A6.3.1 Before the test, verify that:
(a) CP equipment has been installed and is operating properly;
and (b) CP coupon is in place and connected to a structure test
lead.
NOTE: Sufficient time should be allowed for the structure and CP
coupon potentials to reach polarized values.
A6.3.2 Determine the location of CP coupon placement. 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
structure-to-electrolyte and CP coupon-to-electrolyte
potentials;
(c) Electrical midpoints between protection devices; (d) Known
location of an ineffective coating when the UST system structure is
coated; and (e) Location of a known or suspected corrosive
environment.
A6.3.3 Make electrical contact between the reference electrode
and the electrolyte at the test site as close to the CP coupon as
is practicable. A6.3.4 Record the location of the reference
electrode to allow it to be returned to the same location for
subsequent tests. A6.3.5 Connect the voltmeter to the CP coupon
test lead and reference electrode as described in Paragraph 5.6.
A6.3.6 Measure and record the structure and CP coupon on
potentials. A6.3.7 Momentarily disconnect the CP coupon test lead
from the structure test lead and immediately measure and record the
CP coupon-to-electrolyte instant-off potential and its polarity
with respect to the reference electrode. This should be performed
quickly to avoid depolarization of the CP coupon. A6.3.8 Reconnect
the CP