ASTM B825-02(2008)
(Test Method)Standard Test Method for Coulometric Reduction of Surface Films on Metallic Test Samples
Standard Test Method for Coulometric Reduction of Surface Films on Metallic Test Samples
SIGNIFICANCE AND USE
The present trend in environmental testing of materials with electrically conductive surfaces is to produce, under accelerated laboratory conditions, corrosion and film-forming reactions that are similar to those that cause failures in service environments. In many of these procedures the parts under test are exposed for days or weeks to controlled quantities of both water vapor and pollutant gases, which may be present in extremely dilute concentrations.
Note 2—Descriptions of such tests can be found in Practice B 827.
Many of these environmental test methods require monitoring of the conditions within the chamber during the test in order to confirm that the intended environmentally related reactions are actually taking place. The most common type of monitor consists of copper, silver, or other metallic coupons that are placed within the test chamber and that react with the corrosive environment in much the same way as the significant surfaces of the parts under test.
In practice, a minimum number of control coupons are placed in each specified location (see Test Method B 810) within the chamber for a specified exposure time, depending upon the severity of the test environment. At the end of this time interval, the metal samples are removed and analyzed by the coulometric reduction procedure.
Other corrosion film evaluation techniques for metallic coupons are also available. The most common of these is mass gain, which is nondestructive to the surface films, but is limited to the determination of the total amount of additional mass acquired by the metal as a result of the environmental attack.
Note 3—Detailed instructions for conducting such weighings, as well as coupon cleaning and surface preparation procedures, are included as part of Test Method B 810.
Note 4—Some surface analytical techniques (such as X-ray methods) can provide nondestructive identification of some compounds in the films, but such methods, for example, X-ray diffraction, can mis...
SCOPE
1.1 This test method covers procedures and equipment for determining the relative buildup of corrosion and tarnish films (including oxides) on metal surfaces by the constant-current coulometric technique, also known as the cathodic reduction method.
1.2 This test method is designed primarily to determine the relative quantities of tarnish films on control coupons that result from gaseous environmental tests, particularly when the latter are used for testing components or systems containing electrical contacts.
1.3 This test method may also be used to evaluate test samples that have been exposed to indoor industrial locations or other specific application environments. (See 4.6 for limitations.)
1.4 This test method has been demonstrated to be applicable particularly to copper and silver test samples (see (1)). Other metals require further study to prove their applicability within the scope of this test method.
1.5 The values stated in SI units are the preferred units. The values provided in parentheses are for information only.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to become familiar with all hazards including those identified in the appropriate Material Safety Data Sheet (MSDS) for this product/material as provided by the manufacturer, to establish appropriate safety and health practices, and determine the applicability of regulatory limitations prior to use.
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Designation:B825 −02(Reapproved2008)
Standard Test Method for
Coulometric Reduction of Surface Films on Metallic Test
Samples
This standard is issued under the fixed designation B825; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
1.1 This test method covers procedures and equipment for 2.1 ASTM Standards:
determining the relative buildup of corrosion and tarnish films B809Test Method for Porosity in Metallic Coatings by
(including oxides) on metal surfaces by the constant-current Humid Sulfur Vapor (“Flowers-of-Sulfur”)
coulometric technique, also known as the cathodic reduction B810TestMethodforCalibrationofAtmosphericCorrosion
method. Test Chambers by Change in Mass of Copper Coupons
B827Practice for Conducting Mixed Flowing Gas (MFG)
1.2 This test method is designed primarily to determine the
Environmental Tests
relative quantities of tarnish films on control coupons that
D1193Specification for Reagent Water
result from gaseous environmental tests, particularly when the
latter are used for testing components or systems containing
3. Summary of Test Method
electrical contacts.
3.1 In constant-current coulometry, a fixed reduction-
1.3 This test method may also be used to evaluate test
current density is applied to the sample in an electrolytically
samples that have been exposed to indoor industrial locations
conductive solution, and the resulting variations in potential—
or other specific application environments. (See 4.6 for limi-
measured against a standard reference electrode in the same
tations.)
solution—are followed as a function of time. Typically, with
well-behaved surface films, the voltage-time plot should show
1.4 Thistestmethodhasbeendemonstratedtobeapplicable
a number of horizontal portions, or steps, each corresponding
particularly to copper and silver test samples (see (1)). Other
to a specific reduction potential or voltage (Fig. 1). The final
metals require further study to prove their applicability within
potential step, which is always present with all substances,
the scope of this test method.
corresponds to the reduction of hydrogen ions in the solution
1.5 ThevaluesstatedinSIunitsarethepreferredunits.The
(toformhydrogengas),andrepresentsalimitbeyondwhichno
values provided in parentheses are for information only.
higher potential reduction process can occur.
1.6 This standard does not purport to address all of the
NOTE 1—As shown in Figs. 1 and 2, a differential circuit is recom-
safety concerns, if any, associated with its use. It is the
mendedtohelpinresolvingtheindividualvoltagestepsbypinpointingthe
responsibility of the user of this standard to become familiar
corresponding inflection points on the main reduction curve (see 6.2.3).
with all hazards including those identified in the appropriate
3.2 Fromtheelapsedtimesatthevarioussteps,conclusions
Material Safety Data Sheet (MSDS) for this product/material
canoftenbedrawnregardingthecorrosionprocessesthathave
as provided by the manufacturer, to establish appropriate
takenplacetoproducethesurfacefilms.Also,calculationscan
safety and health practices, and determine the applicability of
bemadefromthetimeateachvoltagestepinordertocalculate
regulatory limitations prior to use.
the number of coulombs of electrical charge required to
complete the reduction process at that particular voltage.
This test method is under the jurisdiction of ASTM Committee B02 on Furthermore, since the reduction of any particular chemical
Nonferrous Metals and Alloys and is the direct responsibility of Subcommittee
compound takes place at a characteristic reduction potential or
B02.11 on Electrical Contact Test Methods.
Current edition approved March 1, 2008. Published March 2008. Originally
approved in 1997. Last previous edition approved in 2002 as B825-02. DOI: For referenced ASTM standards, visit the ASTM website, www.astm.org, or
10.1520/B0825-02R08. contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof Standards volume information, refer to the standard’s Document Summary page on
this standard. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B825−02(2008)
4.2 Many of these environmental test methods require
monitoringoftheconditionswithinthechamberduringthetest
in order to confirm that the intended environmentally related
reactions are actually taking place. The most common type of
monitor consists of copper, silver, or other metallic coupons
that are placed within the test chamber and that react with the
corrosiveenvironmentinmuchthesamewayasthesignificant
surfaces of the parts under test.
4.3 In practice, a minimum number of control coupons are
FIG. 1Ideal Reduction Behavior of Oxide and Sulfide Films on
Copper (from Ref 1) placed in each specified location (see Test Method B810)
within the chamber for a specified exposure time, depending
upon the severity of the test environment. At the end of this
time interval, the metal samples are removed and analyzed by
the coulometric reduction procedure.
4.4 Other corrosion film evaluation techniques for metallic
couponsarealsoavailable.Themostcommonoftheseismass
gain,whichisnondestructivetothesurfacefilms,butislimited
to the determination of the total amount of additional mass
acquired by the metal as a result of the environmental attack.
NOTE 3—Detailed instructions for conducting such weighings, as well
NOTE 1—No chlorine is present in test environment.
as coupon cleaning and surface preparation procedures, are included as
FIG. 2Typical Reduction Behavior of Films on Copper from 72-h
part of Test Method B810.
Exposure to the Humid Sulfur Vapor Test (see Test Method B809)
NOTE 4—Some surface analytical techniques (such as X-ray methods)
canprovidenondestructiveidentificationofsomecompoundsinthefilms,
but such methods, for example, X-ray diffraction, can miss amorphous
compounds and compounds present in quantities less than 5% of the
tarnish film volume.
voltagerange,thisvoltagecanbeusedtoindicatethepresence
of a compound or compounds whose characteristic reduction 4.5 With the coulometric technique, it is possible to resolve
thecomplextotalfilmintoanumberofindividualcomponents
potential has already been established under the conditions of
the test. Under ideal conditions it may also be possible to (Fig.1)sothatcomparisonscanbemade.Thisresolvingpower
provides a fingerprint capability for identifying significant
determine the number of reducible compounds present in the
tarnish film. deviations from intended test conditions, and a comparison of
the corrosive characteristics of different environmental cham-
3.3 Forthepurposeofthistestmethod,tarnishfilmsshallbe
bers and of different test runs within the same chamber.
defined as the corrosion products of the reactions of oxygen or
4.6 Thecoulometricreductionprocedurecanalsobeusedin
sulfur (or of other reactive gases or vapors) with the metallic
surface that adhere to the surface and do not protrude signifi- testdevelopmentandintheevaluationoftestsamplesthathave
been exposed at industrial or other application environments
cantly from it.
(6). However, for outdoor exposures, some constraints may
3.4 The basic techniques for the reduction of films on
have to be put on the amount and type of corrosion products
copper and silver were described as early as the late 1930s by
allowed, particularly those involving moisture condensation
Miley (2) and by Campbell and Thomas (3). Important
and the possible loss of films due to flaking (also see 4.9 and
observations of the effects of changing experimental variables
8.3.2).
were later reported byAlbano (4) and by Lambert and Trevoy
(5) in the 1950s. The details and recommendations in this test
4.7 In laboratory environmental testing, the coulometric-
method are primarily from a recently published paper (1) on reduction procedure is of greatest utility after repeated charac-
the practical development over the past fifteen years of
terizationsofagivencorrosiveenvironmenthavebeenmadeto
coulometric reduction for monitoring environmental tests. establish a characteristic reduction curve for that environment.
Thesemultiplerunsshouldcomefromboththeuseofmultiple
4. Significance and Use specimens within a given test exposure as well as from several
consecutive test runs with the same test conditions.
4.1 The present trend in environmental testing of materials
with electrically conductive surfaces is to produce, under 4.8 The coulometric-reduction procedure is destructive in
accelerated laboratory conditions, corrosion and film-forming
that the tarnish films are transformed during the electrochemi-
reactions that are similar to those that cause failures in service cal reduction process. Nondestructive evaluation methods,
environments.Inmanyoftheseproceduresthepartsundertest
such as mass gain, can be carried out with the same samples
are exposed for days or weeks to controlled quantities of both thataretobetestedcoulometrically.However,suchprocedures
water vapor and pollutant gases, which may be present in
must precede coulometric reduction.
extremely dilute concentrations.
4.9 Theconditionsspecifiedinthistestmethodareintended
NOTE 2—Descriptions of such tests can be found in Practice B827. primarily for tarnish films whose total nominal thickness is of
B825−02(2008)
NOTE 1—The vertical lines correspond to major peaks in the differential curve (not shown) and delineate the main reducible film types from this
environment.
FIG. 3Typical Reduction Curve of Copper from 48-h Exposure to High Sulfide (100 ppb H S) Mixed Flowing Gas (with 20 ppb Cl and
2 2
200 ppb NO )
2 3 3 4
the order of 10 to 10 nm (10 to 10 Å). Environmentally
produced films that are much thicker than 10 nm are often
poorly adherent and are more likely to undergo loosening or
flaking upon placement in the electrolyte solution.
5. Interferences
5.1 For reproducible results the following precautions shall
be taken in order to avoid interferences.
5.1.1 Remove dissolved oxygen gas from the electrolyte
solution(see8.1.3),andpreventitfromreenteringthesolution
by keeping the cell closed, with an inert gas flowing over the
solution during the reduction (see 8.3.2 and 8.3.3).
5.1.2 Use fresh electrolyte solution for each new coupon in
order to avoid contamination from the reduction of previous
coupons (see 8.3.5).
5.1.3 Do not apply masking finishes or other nonmetallic
FIG. 4Schematic of Reduction Cell with Storage Reservoir, for
coatings to the coupons, prior to environmental exposure.
Procedure A (8.1.3.1)
5.1.4 Donotusethistestmethodtoanalyzepoorlyadherent
films (see 4.9).
5.1.5 If the sample had been exposed to environments that
were likely to deposit soluble particulates (in addition to the be obtained commercially or made in-house from pure silver
underlying insoluble overall films), care must be taken to
strip or wire (see Appendix X1).
remove most of the particulates prior to coulometric reduction 6.1.2.1 In-houseelectrodesmustbecheckedperiodicallyby
(see 8.3.2 for procedure).
testing them against a standard reference electrode (for
example,saturatedcalomelelectrode)usingapotentiometeror
6. Apparatus
pH meter. The potential exhibited when measuring these
6.1 Electrolytic Reduction Cell and Ancillary Equipment: silver/silver-chloride electrodes in 0.1-M potassium chloride
6.1.1 Reduction Cell, preferably of glass, with a total solutionagainstasaturatedcalomelreferenceshouldbe0.05V
internal volume of at least 600 mL.The cell shall be enclosed, (60.01 V) (7).
but should have a sufficient number of entry ports or tubes to 6.1.3 Inert-Gas Purging Tube—The end that is in the
accommodatetherequiredancillaryequipment(seeFigs.4and electrolyte should be fitted with fritted glass or drawn to a fine
5 for examples of typical cell systems). tip (for example, 0.5-mm inner diameter or less).
6.1.2 Reference Electrode—A silver/silver-chloride refer- 6.1.4 Counter-Electrodes—Pure platinum foil or wire shall
ence is preferred since much of the data in the technical be used.The number of counter-electrodes may vary from 2 to
literature have been obtained with this type of electrode. It can 4 and shall be positioned symmetrically around the sample.
B825−02(2008)
6.2.3 Differential Circuit, or Commercial Differential Volt-
age Output Apparatus—If a digital recording system is used in
conjunctionwith,ortoreplace,ananalogrecordingsystem,the
following method can be used to create a differential curve.
After the reduction is recorded completely, each data point,
except for the first and last, must be analyzed. For a given
point, X, determine the slope to the previous point, X , and the
p
subsequent point, X . Knowing the time interval, T, between
s
each reading, the required slopes are as follows:
S 5 X 2 X /TS 5 X 2 X /T (1)
~ ! ~ !
p p s s
An approximation of the slope at X is then found by taking
the average of the slopes S and S as follows:
p s
S 5 S 1S /2 (2)
~ !
p s
Each value of S is recorded with the concurrent value of X
for later analysis. Slopes at the first and last data points can be
assumed to be zero. A method for enhancing these digitally
produced differential curves can be found in Appendix X2.
7. Reagents
7.1 The only reagents required for routine procedures are
ACS reagent-grade potassium chloride (for the electrolyte),
Pre-Purified-grade nitrogen or other inert gas, and a source of
FIG. 5Schematic of Reduction Cell for Procedure B (8.1.3.2)
distilled or deionized water (Type IV or better as specified in
Specification D1193).
The area of the counter-electrodes preferably should be equal
to or greater than the sample area.
8. Procedure
6.1.5 WireHookorClipforHoldingtheSample—Theupper
8.1 Cell Preparation:
partofthehookorclipshallbeattachedtoawire(insertedinto
8.1.1 Assemble the reduction cell in accordance with either
a glass or plastic tube) for ultimate connection to the negative
Fig. 4 or Fig. 5, making sure that all components are chemi-
output of the power supply. If the wire hook is to be immersed
cally clean. For each sample size or geometry, determine in
in the solution, it shall be made of the same metal as the
advance the level of liquid that is required to cover the
sample. If a clip is used, it shall be heavily gold plated (3 µm
specified sample surface. Mark this level on the outside of the
or more in thickness) and a
...
This document is not anASTM standard and is intended only to provide the user of anASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation:B 825–97 Designation: B 825 – 02 (Reapproved 2008)
Standard Test Method for
Coulometric Reduction of Surface Films on Metallic Test
Samples
This standard is issued under the fixed designation B 825; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers procedures and equipment for determining the relative buildup of corrosion and tarnish films
(including oxides) on metal surfaces by the constant-current coulometric technique, also known as the cathodic reduction method.
1.2 This test method is designed primarily to determine the relative quantities of tarnish films on control coupons that result
from gaseous environmental tests, particularly when the latter are used for testing components or systems containing electrical
contacts.
1.3 This test method may also be used to evaluate test samples that have been exposed to indoor industrial locations or other
specific application environments. (See 4.6 for limitations.)
1.4 This test method has been demonstrated to be applicable particularly to copper and silver test samples (see (1)). Other
metals require further study to prove their applicability within the scope of this test method.
1.5 The values stated in SI units are the preferred units. The values provided in parentheses are for information only.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use. This standard does not purport to address all of the safety concerns, if any, associated with its use. It is
the responsibility of the user of this standard to become familiar with all hazards including those identified in the appropriate
Material Safety Data Sheet (MSDS) for this product/material as provided by the manufacturer, to establish appropriate safety and
health practices, and determine the applicability of regulatory limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
B 809 Test Method for Porosity in Metallic Coatings by Humid Sulfur Vapor (“Flowers-of-Sulfur”) (Flowers-of-Sulfur)
B 810 Test Method for Calibration of Atmospheric Corrosion Test Chambers by Change in Mass of Copper Coupons
B 827 Practice for Conducting Mixed Flowing Gas (MFG) Environmental Tests
D 1193 Specification for Reagent Water
3. Summary of Test Method
3.1 In constant-current coulometry, a fixed reduction-current density is applied to the sample in an electrolytically conductive
solution, and the resulting variations in potential—measured against a standard reference electrode in the same solution—are
followed as a function of time. Typically, with well-behaved surface films, the voltage-time plot should show a number of
horizontalportions,orsteps,eachcorrespondingtoaspecificreductionpotentialorvoltage(Fig.1).Thefinalpotentialstep,which
is always present with all substances, corresponds to the reduction of hydrogen ions in the solution (to form hydrogen gas), and
represents a limit beyond which no higher potential reduction process can occur.
NOTE 1—As shown in Figs. 1 and 2, a differential circuit is recommended to help in resolving the individual voltage steps by pinpointing the
corresponding inflection points on the main reduction curve (see 6.2.3).
3.2 From the elapsed times at the various steps, conclusions can often be drawn regarding the corrosion processes that have
taken place to produce the surface films. Also, calculations can be made from the time at each voltage step in order to calculate
ThistestmethodisunderthejurisdictionofASTMCommitteeB-2onNonferrousMetalsandAlloysandisthedirectresponsibilityofSubcommitteeB02.11onElectrical
Contact Test Methods.
Current edition approved May 10, 1997. Published December 1997.
This test method is under the jurisdiction of ASTM Committee B02 on Nonferrous Metals and Alloys and is the direct responsibility of Subcommittee B02.11 on
Electrical Contact Test Methods.
Current edition approved March 1, 2008. Published March 2008. Originally approved in 1997. Last previous edition approved in 2002 as B 825 - 02.
The boldface numbers in parentheses refer to the list of references at the end of this standard.
For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
, Vol 02.05.volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
B 825 – 02 (2008)
FIG. 1 Ideal Reduction Behavior of Oxide and Sulfide Films on
Copper (from Ref 1)
NOTE 1—No chlorine is present in test environment.
FIG. 2 Typical Reduction Behavior of Films on Copper from 72-h
Exposure to the Humid Sulfur Vapor Test (see Test Method
B 809)
NOTE 1—The vertical lines correspond to major peaks in the differential curve (not shown) and delineate the main reducible film types from this
environment.
FIG. 3 Typical Reduction Curve of Copper from 48-h Exposure to High Sulfide (100 ppb H S) Mixed Flowing Gas (with 20 ppb Cl and
2 2
200 ppb NO )
the number of coulombs of electrical charge required to complete the reduction process at that particular voltage. Furthermore,
since the reduction of any particular chemical compound takes place at a characteristic reduction potential or voltage range, this
voltage can be used to indicate the presence of a compound or compounds whose characteristic reduction potential has already
been established under the conditions of the test. Under ideal conditions it may also be possible to determine the number of
reducible compounds present in the tarnish film.
3.3 For the purpose of this test method, tarnish films shall be defined as the corrosion products of the reactions of oxygen or
sulfur (or of other reactive gases or vapors) with the metallic surface that adhere to the surface and do not protrude significantly
from it.
3.4 The basic techniques for the reduction of films on copper and silver were described as early as the late 1930s by Miley (2)
B 825 – 02 (2008)
and by Campbell and Thomas (3). Important observations of the effects of changing experimental variables were later reported by
Albano (4) and by Lambert and Trevoy (5) in the 1950s. The details and recommendations in this test method are primarily from
a recently published paper (1) on the practical development over the past fifteen years of coulometric reduction for monitoring
environmental tests.
4. Significance and Use
4.1 Thepresenttrendinenvironmentaltestingofmaterialswithelectricallyconductivesurfacesistoproduce,underaccelerated
laboratory conditions, corrosion and film-forming reactions that are similar to those that cause failures in service environments.
In many of these procedures the parts under test are exposed for days or weeks to controlled quantities of both water vapor and
pollutant gases, which may be present in extremely dilute concentrations.
NOTE 2—Descriptions of such tests can be found in Practice B 827.
4.2 Many of these environmental test methods require monitoring of the conditions within the chamber during the test in order
toconfirmthattheintendedenvironmentallyrelatedreactionsareactuallytakingplace.Themostcommontypeofmonitorconsists
of copper, silver, or other metallic coupons that are placed within the test chamber and that react with the corrosive environment
in much the same way as the significant surfaces of the parts under test.
4.3 In practice, a minimum number of control coupons are placed in each specified location (see Test Method B 810) within
the chamber for a specified exposure time, depending upon the severity of the test environment. At the end of this time interval,
the metal samples are removed and analyzed by the coulometric reduction procedure.
4.4 Other corrosion film evaluation techniques for metallic coupons are also available.The most common of these is mass gain,
which is nondestructive to the surface films, but is limited to the determination of the total amount of additional mass acquired
by the metal as a result of the environmental attack.
NOTE 3—Detailed instructions for conducting such weighings, as well as coupon cleaning and surface preparation procedures, are included as part of
Test Method B 810.
NOTE 4—Some surface analytical techniques (such as X-ray methods) can provide nondestructive identification of some compounds in the films, but
such methods, for example, X-ray diffraction, can miss amorphous compounds and compounds present in quantities less than 5 % of the tarnish film
volume.
4.5 Withthecoulometrictechnique,itispossibletoresolvethecomplextotalfilmintoanumberofindividualcomponents(Fig.
1) so that comparisons can be made. This resolving power provides a fingerprint capability for identifying significant deviations
from intended test conditions, and a comparison of the corrosive characteristics of different environmental chambers and of
different test runs within the same chamber.
4.6 The coulometric reduction procedure can also be used in test development and in the evaluation of test samples that have
been exposed at industrial or other application environments (6). However, for outdoor exposures, some constraints may have to
be put on the amount and type of corrosion products allowed, particularly those involving moisture condensation and the possible
loss of films due to flaking (also see 4.9 and 8.3.2).
4.7 In laboratory environmental testing, the coulometric-reduction procedure is of greatest utility after repeated characteriza-
tions of a given corrosive environment have been made to establish a characteristic reduction curve for that environment. These
multiple runs should come from both the use of multiple specimens within a given test exposure as well as from several
consecutive test runs with the same test conditions.
4.8 The coulometric-reduction procedure is destructive in that the tarnish films are removedtransformed during the
electrochemical reduction process. Nondestructive evaluation methods, such as mass gain, can be carried out with the same
samples that are to be tested coulometrically. However, such procedures must precede coulometric reduction.
4.9 The conditions specified in this test method are intended primarily for tarnish films whose total nominal thickness is of the
2 3 3 4 3
orderof10 to10 nm(10 to10 Å).Environmentallyproducedfilmsthataremuchthickerthan10 nmareoftenpoorlyadherent
and are more likely to undergo loosening or flaking upon placement in the electrolyte solution.
5. Interferences
5.1 For reproducible results the following precautions shall be taken in order to avoid interferences.
5.1.1 Remove dissolved oxygen gas from the electrolyte solution (see 8.1.3), and prevent it from reentering the solution by
keeping the cell closed, with an inert gas flowing over the solution during the reduction (see 8.3.2 and 8.3.3).
5.1.2 Usefreshelectrolytesolutionforeachnewcouponinordertoavoidcontaminationfromthereductionofpreviouscoupons
(see 8.3.5).
5.1.3Do not apply masking finishes or other nonmetallic coatings to the coupons, prior to environmental exposure, unless they
have first been shown to have no contaminating effect (see 8.2.4).
5.1.3 Do not apply masking finishes or other nonmetallic coatings to the coupons, prior to environmental exposure.
5.1.4 Do not use this test method to analyze poorly adherent films (see 4.9).
5.1.5 If the sample had been exposed to environments that were likely to deposit soluble particulates (in addition to the
underlying insoluble overall films), care must be taken to remove most of the particulates prior to coulometric reduction (see 8.3.2
for procedure).
B 825 – 02 (2008)
6. Apparatus
6.1 Electrolytic Reduction Cell and Ancillary Equipment:
6.1.1 Reduction Cell, preferably of glass, with a total internal volume of at least 600 mL. The cell shall be enclosed, but should
have a sufficient number of entry ports or tubes to accommodate the required ancillary equipment (see Figs. 4 and 5 for examples
of typical cell systems).
6.1.2 Reference Electrode—Asilver/silver-chloride reference is preferred since much of the data in the technical literature have
been obtained with this type of electrode. It can be obtained commercially or made in-house from pure silver strip or wire (see
Appendix X1).
6.1.2.1 In-house electrodes must be checked periodically by testing them against a standard reference electrode (for example,
saturated calomel electrode) using a potentiometer or pH meter. The potential exhibited when measuring these silver/silver-
chloride electrodes in 0.1-M potassium chloride solution against a saturated calomel reference should be 0.05 V (60.01 V) (7).
6.1.3 Inert-Gas Purging Tube—The end that is in the electrolyte should be fitted with fritted glass or drawn to a fine tip (for
example, 0.5-mm inner diameter or less).
6.1.4 Counter-Electrodes—Pure platinum foil or wire shall be used. The number of counter-electrodes may vary from 2 to 4
and shall be positioned symmetrically around the sample. The area of the counter-electrodes preferably should be equal to or
greater than the sample area.
6.1.5 Wire Hook or Clip for Holding the Sample—The upper part of the hook or clip shall be attached to a wire (inserted into
a glass or plastic tube) for ultimate connection to the negative output of the power supply. If the wire hook is to be immersed in
the solution, it shall be made of the same metal as the sample. If a clip is used, it shall be heavily gold plated (3 µm or more in
thickness) and attached to a platinum wire hook for electrical contact.
6.2 Electronic Equipment—For producing the constant cathodic current and measuring the resulting voltages as a function of
time comprises three basic functional modules whose recommended characteristics (for routine tarnish-film analysis) are listed as
follows:
6.2.1 Constant Current Power Supply, such as, a potentiostat/galvanostat, capable of supplying a constant direct current, and
adjustable from 0.02 to 2 mA with
...
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