ASTM G108-94(1999)
(Test Method)Standard Test Method for Electrochemical Reactivation (EPR) for Detecting Sensitization of AISI Type 304 and 304L Stainless Steels
Standard Test Method for Electrochemical Reactivation (EPR) for Detecting Sensitization of AISI Type 304 and 304L Stainless Steels
SCOPE
1.1 This test method describes a laboratory procedure for conducting an electrochemical reactivation (EPR) test on AISI Type 304 and 304L (UNS No. S30400 and S30403, respectively) stainless steels. This test method can provide a nondestructive means of quantifying the degree of sensitization in these steels (1, 2, 3). This test method has found wide acceptance in studies of the effects of sensitization on intergranular corrosion and intergranular stress corrosion cracking behavior (see Terminology G15). The EPR technique has been successfully used to evaluate other stainless steels and nickel base alloys (4), but the test conditions and evaluation criteria used were modified in each case from those cited in this test method.
1.2 The values stated in SI units are to be regarded as the standard. The inch-pound units given in parentheses are for information only.
1.3 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.
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Designation: G 108 – 94 (Reapproved 1999)
Standard Test Method for
Electrochemical Reactivation (EPR) for Detecting
Sensitization of AISI Type 304 and 304L Stainless Steels
This standard is issued under the fixed designation G 108; 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 rosion Test Specimens
G 3 Practice for Conventions Applicable to Electrochemical
1.1 This test method describes a laboratory procedure for
Measurements in Corrosion Testing
conducting an electrochemical reactivation (EPR) test on AISI
G 5 Reference Test Method for Making Potentiostatic and
Type 304 and 304L (UNS No. S30400 and S30403, respec-
Potentiodynamic Anodic Polarization Measurements
tively) stainless steels. This test method can provide a nonde-
G 15 Terminology Relating to Corrosion and Corrosion
structive means of quantifying the degree of sensitization in
Testing
these steels (1, 2, 3). This test method has found wide
G 28 Test Methods for Detecting Susceptibility to Inter-
acceptance in studies of the effects of sensitization on inter-
granular Attack in Wrought Nickel-Rich, Chromium-
granular corrosion and intergranular stress corrosion cracking
Bearing Alloys
behavior (see Terminology G 15). The EPR technique has been
G 61 Test Method for Conducting Cyclic Potentiodynamic
successfully used to evaluate other stainless steels and nickel
Polarization Measurements for Localized Corrosion Sus-
base alloys (4), but the test conditions and evaluation criteria
ceptibility of Iron-, Nickel-, or Cobalt-Based Alloys
used were modified in each case from those cited in this test
method.
3. Terminology
1.2 The values stated in SI units are to be regarded as the
3.1 Definitions of Terms Specific to This Standard:
standard. The inch-pound units given in parentheses are for
3.1.1 integrated charge (Q)—the charge measured, in cou-
information only.
loumbs, during reactivation as given by the time integral of
1.3 This standard does not purport to address all of the
current density below the reactivation peak of the curve.
safety concerns, if any, associated with its use. It is the
3.1.2 maximum anodic current density (I )—the current
r
responsibility of the user of this standard to establish appro-
density measured at the peak of the anodic curve during
priate safety and health practices and determine the applica-
reactivation.
bility of regulatory limitations prior to use.
3.1.3 normalized charge (P )—the integrated current nor-
a
2. Referenced Documents malized to the specimen size and grain size. P represents the
a
charge (in coulombs/cm ) of the grain-boundary area. The
2.1 ASTM Standards:
method for calculating P is given in 9.2.
A 262 Practices for Detecting Susceptibility to Intergranu- a
3.1.4 reactivation—in the electrochemical reactivation
lar Attack in Austenitic Stainless Steels
4 (EPR) test, the potential sweep from the passivation potential
D 1193 Specification for Reagent Water
returning to the corrosion potential.
E 3 Methods of Preparation of Metallographic Specimens
5 3.1.5 scan rate—the rate at which the electrical potential
E 7 Terminology Relating to Metallography
applied to a specimen in a polarization test is changed.
E 112 Test Methods for Determining Average Grain Size
G 1 Practice for Preparing, Cleaning, and Evaluating Cor-
4. Summary of Test Method
4.1 The EPR test is accomplished by a potentiodynamic
sweep from the passive to the active regions of electrochemical
This test method is under the jurisdiction of ASTM Committee G-1 on
potentials in a process referred to as reactivation. The EPR test
Corrosion of Metals and is the direct responsibility of Subcommittee G01.11 on
measures the amount of charge associated with the corrosion of
Electrochemical Measurement in Corrosion Testing.
the chromium-depleted regions surrounding chromium carbide
Current edition published Feb. 15, 1994. Approved April 1994. Originally
published as G 108 – 92. Last previous edition G 108 – 92. precipitated particles. Most of these particles in a sensitized
The boldface numbers in parentheses refer to the list of references at the end of
microstructure are located at grain boundaries (see Terminol-
the text.
ogy E 7). Discrete particles located within the grain (referred to
Annual Book of ASTM Standards, Vol 01.03.
Annual Book of ASTM Standards, Vol 11.01.
Annual Book of ASTM Standards, Vol 03.01.
Annual Book of ASTM Standards, Vol 03.02.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
G 108 – 94 (1999)
as intragranular precipitates) will also contribute to the total of different heats of material that exhibit different Q values
measured charge. Therefore, it is important to examine the solely as a result of differences in grain size.
alloy microstructure following an EPR test, to determine the
5. Significance and Use
relative proportion of corrosion site associated with intergranu-
5.1 This test method describes an EPR test method for
lar versus intragranular precipitates.
quantitatively determining the relative degree of sensitization
4.2 The chromium-depleted zones around carbide precipi-
in AISI Type 304 and 304L stainless steels. The EPR test has
tates in sensitized steels are particularly susceptible to corro-
found wide use as a means to provide a numerical level of
sion in oxidizing acid solutions. Corrosion at chromium-
sensitization in studies of the effects of sensitization on
depleted grain boundary sites causes a rapid rise in the current
intergranular corrosion and intergranular stress corrosion
density when the electrochemical potential is changed from the
cracking behavior. The results of this test method correlate
passive to the active region.
with other test methods (for example, Practice A 262 and Test
4.3 A sensitized steel produces a curve similar to the active
Method G 28) that are commonly used to assess sensitization
portion of the polarization curve during the reactivation from
in stainless steels.
the passive region back to the rest potential (E ) as shown in
corr
5.2 The EPR test can also be used for product acceptance,
Fig. 1. A nonsensitized (solution annealed) steel polarized
under the conditions given in this test method will produce a service evaluation, regulatory statutes, and manufacturing
controls providing that both the supplier and user have agreed
curve with lower current densities than a sensitized steel.
4.4 The EPR test results are readily reproducible, as long as upon appropriate acceptance criteria and a sensitizing treat-
ment. The test is not intended for design purposes since the test
the electrolyte temperature, electrolyte composition, and scan
rate are carefully controlled. The EPR test is significantly conditions accelerate corrosion in a manner that does not
simulate any actual service environment.
affected by the composition, thermomechanical condition and
surface finish of the specimen as well as the presence of 5.3 The EPR test involves the measurement of the amount
of charge resulting from the corrosion of the chromium-
non-metallic inclusions, that result in pitting of the etched
depleted regions surrounding the precipitated chromium car-
microstructure.
bide particles. Most of these particles in a sensitized micro-
NOTE 1—Various cutting and grinding operations can promote sensiti-
structure are located at the grain boundaries. However, discrete
zation of Type 304 (5). Superficial carbide precipitation can occur during
particles located within grains (referred to as intragranular
cutting and grinding or during subsequent low temperature heat treat-
precipitates) will also contribute to the total measured charge.
ments, such as 24 h at 500°C.
(See Fig. 2.) Therefore, it is important to examine the alloy
4.5 The criteria used to distinguish between sensitized and
microstructure following an EPR test to determine the relative
solution annealed samples are the activation charge density, Q
proportion of corrosion sites associated with intergranular
(given by the time integral of current density below the
versus intragranular precipitates. Sites of intergranular attack
reactivation peak of the curve), or the maximum anodic current
will appear similar to grain boundary ditching as defined in
density, I , in the active state. Sensitized steels are easily
r
Practice A of Practices A 262.
activated and show higher Q and I values than solution
r
annealed steels, that are not susceptible to intergranular corro-
6. Apparatus
sion. The value Q is normalized for both specimen size and
6.1 The apparatus necessary for obtaining EPR data consists
grain size. The value normalized in this fashion is called P and
a
of electronic instruments and a test cell. These instruments may
represents the charge (in units of coulombs) per unit grain-
be integrated into one instrument package or may be individual
boundary area. This normalization permits direct comparisons
components. Either form of instrumentation can provide ac-
ceptable data.
6.2 Typical apparatus, as illustrated in Fig. 3, shall consist of
the following: scanning potentiostat (or potentiostat/voltage
NOTE 1—The calculation of P is based on the assumptions illustrated
a
at left. Mild cases of sensitization usually result in a combination of
intergranular attack and pitting as illustrated at right (7).
FIG. 1 Schematic EPR Curves for Sensitized and Solutionized
AISI Type 304 Stainless Steel FIG. 2 Schematic Microstructures After EPR Testing
G 108 – 94 (1999)
6.2.5 EPR Test Cell—Requirements shall be in accordance
with 4.1 of Practice G 5. A deareation tube is not required and
only one counter electrode is required for EPR testing. A
suitable cell and electrode arrangement is shown in Fig. 4.
6.2.6 Electrode Holder—Requirements shall be in accor-
dance with 4.6 of Practice G 5 or 4.2.1 of Test Method G 61.
The requirements for the working electrode (specimen) and
counter electrode holders are that the holders be made of an
inert material and any seals must not allow leakage of the
electrolyte. When using the Practice G 5-type holder the
working electrode can be mounted as shown in Fig. 5 and
described in Appendix X1.
6.2.7 Auxiliary (Counter) Electrodes—Requirements are in
accordance with 4.7.2 of Practice G 5 except that only one
counter electrode is necessary for EPR testing. However, two
auxiliary electrodes can provide for a more uniform distribu-
tion of current. Titanium or high-purity carbon may be used in
FIG. 3 Schematic Diagram of an EPR Test Apparatus
place of platinum for the counter electrode since it is always
the cathode.
6.2.8 Calomel Reference Electrode—Requirements are in
ramp generator combination), potential measuring instrument,
accordance or equivalent to 4.7.3 of Practice G 5.
current and current integration measuring instruments, and test
7. Sampling, Test Specimens, and Test Units
cell and specimen holder.
6.2.1 Scanning Potentiostat—Requirements shall be in ac-
7.1 Sampling:
cordance with 4.2 of Practice G 5 with the following refine-
7.1.1 When using this test method to meet product accep-
ments: the potentiostat shall control the potential within 6 5
tance criteria, the means of sampling of a test specimen shall be
mV accuracy over the range of potential and current density
decided by agreement between the parties involved; for in-
encountered in the EPR measurements. The potentiostat shall
stance, but not limited to, a user and a supplier.
be operable in a potential range of −600 to +500 mV (SCE) and
7.1.2 Specimens removed form a piece of AISI Type 304 or
a current density range of 1 μA to 100 mA/cm . The applied
304L steel by shearing, cutting, burningtc, and so forth shall
potential is changed either automatically or manually in the
have the affected edges removed by grinding or machining.
following manners:
7.2 Sensitization of Test Specimens—Specimens can be
6.2.1.1 Shifting the potential from the open circuit potential
given a sensitizing treatment when it is desired to assess the
to a potential in the passive range, and
influence of a thermal exposure during fabrication on corrosion
6.2.1.2 Scanning back to the open circuit potential (reacti-
resistance.
vation) at a voltage scan rate of 1.67 mV/s (6 V/h).
6.2.2 Potential Measuring Instruments—Requirements
shall be in accordance with 4.3 of Practice G 5 except that the
potential range is as stated above.
6.2.3 Current Measuring Instruments—Requirements shall
be in accordance with 4.4 of Practice G 5. However, current
measurements are essential for passivation assessment and
other intermediate checks of system stability. The currents
encountered in EPR for a specimen with the dimensions given
in 7.3 are in the range of 1 μA to 100 mA/cm . For samples of
2 2
less than 100 mm test area, currents above about 20 mA/cm
rarely have been reported.
6.2.4 Current Integration Measurement Instruments
(Optional)—Current integration, or charge, can be measured
by an electronic device incorporated into the potentiostat, or by
a separate electronic device, such as a coulometer. If a
coulometer is used, it shall be capable of measuring charges
from 0.001 to 2 coulombs. The use of a coulometer shall be
considered optional. Charge can also be measured by using a
chart recorder, as illustrated in Fig. 3, to record a current versus
time trace and then, subsequently, integrating it by various
NOTE 1—The sample face is completely immersed but the connection
methods. When potentiostat measurements are available in a
to the electrode holder is not immersed.
digitized format, an appropriate computer integration routine
FIG. 4 Schematic Diagram of an Electrochemical Cell for EPR
can also be used to obtain a value for charge. Testing
G 108 – 94 (1999)
machine screw (for example, NC4-40 3 0.3 cm (0.75 in.) long) to the
back surface of the specimen. This assembly is mounted in a suitable
compound that is inert in the EPR electrolyte (see Appendix X1) such that
the front surface upon immersion in the EPR electrolyte is fully in contact
with the electrolyte.
7.3.4 Measure the surface area of the front surface of the test
specimen within 0.1 mm precision and record on the EPR data
record sheet (see Appendix X2).
7.3.5 Specimens can be in any shape that will not be
susceptible to crevice corrosion in the solution. Test surface
2 2
area shall be at least 10 6 0.1 mm (0.016 in. ). It is
occasionally usef
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