ASTM G108-94(2004)
(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 covers 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 G 15). The EPR technique has been successfully used to evaluate other stainless steels and nickel base alloys (), 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:G108–94 (Reapproved 2004)
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 G108; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope G1 Practice for Preparing, Cleaning, and Evaluating Cor-
rosion Test Specimens
1.1 This test method covers a laboratory procedure for
G3 PracticeforConventionsApplicabletoElectrochemical
conducting an electrochemical reactivation (EPR) test onAISI
Measurements in Corrosion Testing
Type 304 and 304L (UNS No. S30400 and S30403, respec-
G5 Reference Test Method for Making Potentiostatic and
tively) stainless steels. This test method can provide a nonde-
Potentiodynamic Anodic Polarization Measurements
structive means of quantifying the degree of sensitization in
G15 Terminology Relating to Corrosion and Corrosion
these steels (1, 2, 3). This test method has found wide
Testing
acceptance in studies of the effects of sensitization on inter-
G28 Test Methods for Detecting Susceptibility to Inter-
granular corrosion and intergranular stress corrosion cracking
granular Attack in Wrought Nickel-Rich, Chromium-
behavior(seeTerminologyG15).TheEPRtechniquehasbeen
Bearing Alloys
successfully used to evaluate other stainless steels and nickel
G61 Test Method for Conducting Cyclic Potentiodynamic
base alloys (4), but the test conditions and evaluation criteria
Polarization Measurements for Localized Corrosion Sus-
used were modified in each case from those cited in this test
ceptibility of Iron-, Nickel-, or Cobalt-Based Alloys
method.
1.2 The values stated in SI units are to be regarded as the
3. Terminology
standard. The inch-pound units given in parentheses are for
3.1 Definitions of Terms Specific to This Standard:
information only.
3.1.1 integrated charge (Q)—the charge measured, in cou-
1.3 This standard does not purport to address all of the
loumbs, during reactivation as given by the time integral of
safety concerns, if any, associated with its use. It is the
current density below the reactivation peak of the curve.
responsibility of the user of this standard to establish appro-
3.1.2 maximum anodic current density (I )—the current
r
priate safety and health practices and determine the applica-
density measured at the peak of the anodic curve during
bility of regulatory limitations prior to use.
reactivation.
2. Referenced Documents 3.1.3 normalized charge (P )—the integrated current nor-
a
malized to the specimen size and grain size. P represents the
a
2.1 ASTM Standards:
charge (in coulombs/cm ) of the grain-boundary area. The
A262 Practices for Detecting Susceptibility to Intergranu-
method for calculating P is given in 9.2.
a
lar Attack in Austenitic Stainless Steels
3.1.4 reactivation—in the electrochemical reactivation
D1193 Specification for Reagent Water
(EPR) test, the potential sweep from the passivation potential
E3 Methods of Preparation of Metallographic Specimens
returning to the corrosion potential.
E7 Terminology Relating to Metallography
3.1.5 scan rate—the rate at which the electrical potential
E112 Test Methods for Determining Average Grain Size
applied to a specimen in a polarization test is changed.
4. Summary of Test Method
This test method is under the jurisdiction of ASTM Committee G01 on
Corrosion of Metals and is the direct responsibility of Subcommittee G01.11 on 4.1 The EPR test is accomplished by a potentiodynamic
Electrochemical Measurement in Corrosion Testing.
sweepfromthepassivetotheactiveregionsofelectrochemical
Current edition approved Nov 1, 2004. Published November 2004. Originally
potentialsinaprocessreferredtoasreactivation.TheEPRtest
approved in 1992. Last previous edition approved in 1999 as G108–94 (1999).
measurestheamountofchargeassociatedwiththecorrosionof
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
the text.
the chromium-depleted regions surrounding chromium carbide
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
precipitated particles. Most of these particles in a sensitized
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
microstructure are located at grain boundaries (see Terminol-
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. ogyE7).Discreteparticleslocatedwithinthegrain(referredto
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
G108–94 (2004)
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
relativeproportionofcorrosionsiteassociatedwithintergranu-
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
densitywhentheelectrochemicalpotentialischangedfromthe
cracking behavior. The results of this test method correlate
passive to the active region.
with other test methods (for example, PracticeA262 and Test
4.3 Asensitized steel produces a curve similar to the active
Method G28) that are commonly used to assess sensitization
portion of the polarization curve during the reactivation from
in stainless steels.
thepassiveregionbacktotherestpotential(E )asshownin
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.Thetestisnotintendedfordesignpurposessincethetest
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-
structurearelocatedatthegrainboundaries.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
reactivationpeakofthecurve),orthemaximumanodiccurrent
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 A262.
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 TheapparatusnecessaryforobtainingEPRdataconsists
grainsize.Thevaluenormalizedinthisfashioniscalled P and
a
ofelectronicinstrumentsandatestcell.Theseinstrumentsmay
represents the charge (in units of coulombs) per unit grain-
beintegratedintooneinstrumentpackageormaybeindividual
boundary area. This normalization permits direct comparisons
components. Either form of instrumentation can provide ac-
ceptable data.
6.2 Typicalapparatus,asillustratedinFig.3,shallconsistof
the following: scanning potentiostat (or potentiostat/voltage
NOTE—The calculation of P is based on the assumptions illustrated at
a
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
G108–94 (2004)
6.2.5 EPR Test Cell—Requirements shall be in accordance
with 4.1 of Practice G5.Adeareation 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 G5 or 4.2.1 of Test Method G61.
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 G5-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 G5 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 G5.
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 G5 with the following refine-
7.1.1 When using this test method to meet product accep-
ments: the potentiostat shall control the potential within 65
tancecriteria,themeansofsamplingofatestspecimenshallbe
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.
beoperableinapotentialrangeof−600to+500mV(SCE)and
7.1.2 Specimens removed form a piece ofAISIType 304 or
a current density range of 1 µA to 100 mA/cm . The applied
304Lsteelbyshearing,cutting,burning,andsoforthshallhave
potential is changed either automatically or manually in the
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
influenceofathermalexposureduringfabricationoncorrosion
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 G5 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 G5. 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 µAto 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
byanelectronicdeviceincorporatedintothepotentiostat,orby
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
chartrecorder,asillustratedinFig.3,torecordacurrentversus
time trace and then, subsequently, integrating it by various
NOTE—The sample face is completely immersed but the connection to
methods. When potentiostat measurements are available in a
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
G108–94 (2004)
machine screw (for example, NC4-40 30.3 cm (0.75 in.) long) to the
back surface of the specimen. This assembly is mounted in a suitable
compoundthatisinertintheEPRelectrolyte(seeAppendixX1)suchthat
thefrontsurfaceuponimmersionintheEPRelectrolyteisfullyincontact
with the electrolyte.
7.3.4 Measurethesurfaceareaofthefrontsurfaceofthetest
specimenwithin0.1mm precisionandrecordontheEPRdata
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
occasionallyusefultomasktheareatobemeasuredleavingan
opening for exposure to the electrolyte. One suitable masking
method uses precut pieces of an acid resistant tape. Care must
be taken not to introduce undercutting of the tape during the
EPR measurement because it will caus
...
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