ASTM B825-97

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Designation: B 825 – 97
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 Environmental Tests
D 1193 Specification for Reagent Water
1.1 This test method covers procedures and equipment for
determining the relative buildup of corrosion and tarnish films
3. Summary of Test Method
(including oxides) on metal surfaces by the constant-current
3.1 In constant-current coulometry, a fixed reduction-
coulometric technique, also known as the cathodic reduction
current density is applied to the sample in an electrolytically
method.
conductive solution, and the resulting variations in potential—
1.2 This test method is designed primarily to determine the
measured against a standard reference electrode in the same
relative quantities of tarnish films on control coupons that
solution—are followed as a function of time. Typically, with
result from gaseous environmental tests, particularly when the
well-behaved surface films, the voltage-time plot should show
latter are used for testing components or systems containing
a number of horizontal portions, or steps, each corresponding
electrical contacts.
to a specific reduction potential or voltage (Fig. 1). The final
1.3 This test method may also be used to evaluate test
potential step, which is always present with all substances,
samples that have been exposed to indoor industrial locations
corresponds to the reduction of hydrogen ions in the solution
or other specific application environments. (See 4.6 for limi-
(to form hydrogen gas), and represents a limit beyond which no
tations.)
higher potential reduction process can occur.
1.4 This test method has been demonstrated to be applicable
particularly to copper and silver test samples (see (1)). Other
NOTE 1—As shown in Figs. 1 and 2, a differential circuit is recom-
metals require further study to prove their applicability within mended to help in resolving the individual voltage steps by pinpointing the
corresponding inflection points on the main reduction curve (see 6.2.3).
the scope of this test method.
1.5 The values stated in SI units are the preferred units. The
3.2 From the elapsed times at the various steps, conclusions
values provided in parentheses are for information only.
can often be drawn regarding the corrosion processes that have
1.6 This standard does not purport to address all of the
taken place to produce the surface films. Also, calculations can
safety concerns, if any, associated with its use. It is the
be made from the time at each voltage step in order to calculate
responsibility of the user of this standard to establish appro-
the number of coulombs of electrical charge required to
priate safety and health practices and determine the applica-
complete the reduction process at that particular voltage.
bility of regulatory limitations prior to use.
Furthermore, since the reduction of any particular chemical
compound takes place at a characteristic reduction potential or
2. Referenced Documents
voltage range, this voltage can be used to indicate the presence
2.1 ASTM Standards:
of a compound or compounds whose characteristic reduction
B 809 Test Method for Porosity in Metallic Coatings by
potential has already been established under the conditions of
Humid Sulfur Vapor (“Flowers-of-Sulfur”)
the test. Under ideal conditions it may also be possible to
B 810 Test Method for Calibration of Atmospheric Corro-
determine the number of reducible compounds present in the
sion Test Chambers by Change in Mass of Copper Cou-
tarnish film.
pons
3.3 For the purpose of this test method, tarnish films shall be
B 827 Practice for Conducting Mixed Flowing Gas (MFG)
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 signifi-
This test method is under the jurisdiction of ASTM Committee B-2 on
cantly from it.
Nonferrous Metals and Alloys and is the direct responsibility of Subcommittee
3.4 The basic techniques for the reduction of films on
B02.11 on Electrical Contact Test Methods.
copper and silver were described as early as the late 1930s by
Current edition approved May 10, 1997. Published December 1997.
The boldface numbers in parentheses refer to the list of references at the end of
this standard.
Annual Book of ASTM Standards, Vol 02.05.
4 5
Annual Book of ASTM Standards, Vol 03.04. Annual Book of ASTM Standards, Vol 11.01.
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NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
B 825
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)
FIG. 1 Ideal Reduction Behavior of Oxide and Sulfide Films on
can provide nondestructive identification of some compounds in the films,
Copper (from Ref 1)
but such methods can miss amorphous compounds and compounds
present in quantities less than 5 % of the tarnish film volume.
4.5 With the coulometric technique, it is possible to resolve
the complex total film into a number of individual components
(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 cham-
bers and of different test runs with 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
NOTE 1—No chlorine is present in test environment.
(6). However, for outdoor exposures, some constraints may
FIG. 2 Typical Reduction Behavior of Films on Copper from 72-h
Exposure to the Humid Sulfur Vapor Test (see Test Method have to be put on the amount and type of corrosion products
B 809)
allowed, particularly those involving moisture condensation
and the possible loss of films due to flaking (also see 4.9 and
8.3.2).
Miley (2) and by Campbell and Thomas (3). Important
4.7 In laboratory environmental testing, the coulometric-
observations of the effects of changing experimental variables
reduction procedure is of greatest utility after repeated charac-
were later reported by Albano (4) and by Lambert and Trevoy
terizations of a given corrosive environment have been made to
(5) in the 1950s. The details and recommendations in this test
establish a characteristic reduction curve for that environment.
method are primarily from a recently published paper (1) on
These multiple runs should come from both the use of multiple
the practical development over the past fifteen years of
specimens within a given test exposure as well as from several
coulometric reduction for monitoring environmental tests.
consecutive test runs with the same test conditions.
4. Significance and Use
4.8 The coulometric-reduction procedure is destructive in
that the tarnish films are removed during the electrochemical
4.1 The present trend in environmental testing of materials
reduction process. Nondestructive evaluation methods, such as
with electrically conductive surfaces is to produce, under
mass gain, can be carried out with the same samples that are to
accelerated laboratory conditions, corrosion and film-forming
be tested coulometrically. However, such procedures must
reactions that are similar to those that cause failures in service
precede coulometric reduction.
environments. In many of these procedures the parts under test
4.9 The conditions specified in this test method are intended
are exposed for days or weeks to controlled quantities of both
primarily for tarnish films whose total nominal thickness is of
water vapor and pollutant gases, which may be present in
2 3 3 4
the order of 10 to 10 nm (10 to 10 Å). Environmentally
extremely dilute concentrations.
produced films that are much thicker than 10 nm are often
NOTE 2—Descriptions of such tests can be found in Practice B 827.
poorly adherent and are more likely to undergo loosening or
4.2 Many of these environmental test methods require
flaking upon placement in the electrolyte solution.
monitoring of the conditions within the chamber during the test
5. Interferences
in order to confirm that the intended environmentally related
reactions are actually taking place. The most common type of 5.1 For reproducible results the following precautions shall
monitor consists of copper, silver, or other metallic coupons
be taken in order to avoid interferences.
that are placed within the test chamber and that react with the 5.1.1 Remove dissolved oxygen gas from the electrolyte
corrosive environment in much the same way as the significant solution (see 8.1.3), and prevent it from reentering the solution
surfaces of the parts under test. by keeping the cell closed, with an inert gas flowing over the
4.3 In practice, a minimum number of control coupons are solution during the reduction (see 8.3.2 and 8.3.3).
placed in each specified location (see Test Method B 810) 5.1.2 Use fresh electrolyte solution for each new coupon in
within the chamber for a specified exposure time, depending order to avoid contamination from the reduction of previous
upon the severity of the test environment. At the end of this coupons (see 8.3.5).
time interval, the metal samples are removed and analyzed by 5.1.3 Do not apply masking finishes or other nonmetallic
the coulometric reduction procedure. coatings to the coupons, prior to environmental exposure,
NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
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B 825
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 )
unless they have first been shown to have no contaminating
effect (see 8.2.4).
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).
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).
FIG. 4 Schematic of Reduction Cell with Storage Reservoir, for
6.1.2 Reference Electrode—A silver/silver-chloride refer-
Procedure A (8.1.3.1)
ence is preferred since much of the data in the technical
literature have been obtained with this type of electrode. It can The area of the counter-electrodes preferably should be equal
be obtained commercially or made in-house from pure silver to or greater than the sample area.
strip or wire (see Appendix X1). 6.1.5 Wire Hook or Clip for Holding the Sample—The
6.1.2.1 In-house electrodes must be checked periodically by upper part of the hook or clip shall be attached to a wire
testing them against a standard reference electrode (for ex- (inserted into a glass or plastic tube) for ultimate connection to
ample, saturated calomel electrode) using a potentiometer or 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
pH meter. The potential exhibited when measuring these
silver/silver-chloride electrodes in 0.1-M potassium chloride as the sample. If a clip is used, it shall be heavily gold plated
solution against a saturated calomel reference should be 0.05 V (3 μm or more in thickness) and attached to a platinum wire
(60.01 V) (7). hook for electrical contact.
6.1.3 Inert-Gas Purging Tube—The end that is in the 6.2 Electronic Equipment—For producing the constant ca-
electrolyte should be fitted with fritted glass or drawn to a fine thodic current and measuring the resulting voltages as a
tip (for example, 0.5-mm inner diameter or less). function of time comprises three basic functional modules
6.1.4 Counter-Electrodes—Pure platinum foil or wire shall whose recommended characteristics (for routine tarnish-film
be used. The number of counter-electrodes may vary from 2 to analysis) are listed as follows:
4 and shall be positioned symmetrically around the sample. 6.2.1 Constant Current Power Supply, such as, a
NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
B 825
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
distilled or deionized water (Type IV or better as specified in
Specification D 1193).
8. Procedure
8.1 Cell Preparation:
8.1.1 Assemble the reduction cell in accordance with either
Fig. 4 or Fig. 5, making sure that all components are chemi-
cally clean. For each sample size or geometry, determine in
advance the level of liquid that is required to cover the
specified sample surface. Mark this level on the outside of the
cell. A minimum volume of 300-mL solution is recommended
for each an
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