ASTM G108-94(2004)e1
(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).² 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 (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: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 (´) indicates an editorial change since the last revision or reapproval.
´ NOTE—The equation in Item X in 9.2 was corrected editorially in July 2005.
1. Scope E112 Test Methods for Determining Average Grain Size
G1 Practice for Preparing, Cleaning, and Evaluating Corro-
1.1 This test method covers a laboratory procedure for
sion Test Specimens
conducting an electrochemical reactivation (EPR) test onAISI
G3 Practice for ConventionsApplicable to Electrochemical
Type 304 and 304L (UNS No. S30400 and S30403, respec-
Measurements in Corrosion Testing
tively) stainless steels. This test method can provide a nonde-
G5 Reference Test Method for Making Potentiostatic and
structive means of quantifying the degree of sensitization in
Potentiodynamic Anodic Polarization Measurements
these steels (1, 2, 3). This test method has found wide
G15 Terminology Relating to Corrosion and Corrosion
acceptance in studies of the effects of sensitization on inter-
Testing
granular corrosion and intergranular stress corrosion cracking
G28 Test Methods for Detecting Susceptibility to Inter-
behavior (seeTerminology G15).The EPR technique has been
granular Corrosion in Wrought, Nickel-Rich, Chromium-
successfully used to evaluate other stainless steels and nickel
Bearing Alloys
base alloys (4), but the test conditions and evaluation criteria
G61 Test Method for Conducting Cyclic Potentiodynamic
used were modified in each case from those cited in this test
Polarization Measurements for Localized Corrosion Sus-
method.
ceptibility of Iron-, Nickel-, or Cobalt-Based Alloys
1.2 The values stated in SI units are to be regarded as the
standard. The inch-pound units given in parentheses are for
3. Terminology
information only.
3.1 Definitions of Terms Specific to This Standard:
1.3 This standard does not purport to address all of the
3.1.1 integrated charge (Q)—the charge measured, in cou-
safety concerns, if any, associated with its use. It is the
loumbs, during reactivation as given by the time integral of
responsibility of the user of this standard to establish appro-
current density below the reactivation peak of the curve.
priate safety and health practices and determine the applica-
3.1.2 maximum anodic current density (I )—the current
r
bility of regulatory limitations prior to use.
density measured at the peak of the anodic curve during
2. Referenced Documents reactivation.
3.1.3 normalized charge (P )—the integrated current nor-
a
2.1 ASTM Standards:
malized to the specimen size and grain size. P represents the
a
A262 Practices for Detecting Susceptibility to Intergranular
charge (in coulombs/cm ) of the grain-boundary area. The
Attack in Austenitic Stainless Steels
method for calculating P is given in 9.2.
D1193 Specification for Reagent Water a
3.1.4 reactivation—in the electrochemical reactivation
E3 Guide for Preparation of Metallographic Specimens
(EPR) test, the potential sweep from the passivation potential
E7 Terminology Relating to Metallography
returning to the corrosion potential.
3.1.5 scan rate—the rate at which the electrical potential
This test method is under the jurisdiction of ASTM Committee G01 on
applied to a specimen in a polarization test is changed.
Corrosion of Metals and is the direct responsibility of Subcommittee G01.11 on
Electrochemical Measurement in Corrosion Testing.
4. Summary of Test Method
Current edition approved Nov 1, 2004. Published November 2004. Originally
approved in 1992. Last previous edition approved in 1999 as G108–94 (1999).
4.1 The EPR test is accomplished by a potentiodynamic
DOI: 10.1520/G0108-94R04E01.
sweepfromthepassivetotheactiveregionsofelectrochemical
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
potentialsinaprocessreferredtoasreactivation.TheEPRtest
the text.
measurestheamountofchargeassociatedwiththecorrosionof
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
the chromium-depleted regions surrounding chromium carbide
Standards 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.
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G108–94 (2004)
precipitated particles. Most of these particles in a sensitized grainsize.Thevaluenormalizedinthisfashioniscalled P and
a
microstructure are located at grain boundaries (see Terminol- represents the charge (in units of coulombs) per unit grain-
ogyE7).Discreteparticleslocatedwithinthegrain(referredto boundary area. This normalization permits direct comparisons
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
relativeproportionofcorrosionsiteassociatedwithintergranu-
5. Significance and Use
lar versus intragranular precipitates.
5.1 This test method describes an EPR test method for
4.2 The chromium-depleted zones around carbide precipi-
quantitatively determining the relative degree of sensitization
tates in sensitized steels are particularly susceptible to corro-
in AISI Type 304 and 304L stainless steels. The EPR test has
sion in oxidizing acid solutions. Corrosion at chromium-
found wide use as a means to provide a numerical level of
depleted grain boundary sites causes a rapid rise in the current
sensitization in studies of the effects of sensitization on
densitywhentheelectrochemicalpotentialischangedfromthe
intergranular corrosion and intergranular stress corrosion
passive to the active region.
cracking behavior. The results of this test method correlate
4.3 Asensitized steel produces a curve similar to the active
with other test methods (for example, Practice A262 and Test
portion of the polarization curve during the reactivation from
MethodG28)thatarecommonlyusedtoassesssensitizationin
thepassiveregionbacktotherestpotential(E )asshownin
corr
stainless steels.
Fig. 1. A nonsensitized (solution annealed) steel polarized
5.2 The EPR test can also be used for product acceptance,
under the conditions given in this test method will produce a
service evaluation, regulatory statutes, and manufacturing
curve with lower current densities than a sensitized steel.
controls providing that both the supplier and user have agreed
4.4 The EPR test results are readily reproducible, as long as
upon appropriate acceptance criteria and a sensitizing treat-
the electrolyte temperature, electrolyte composition, and scan
ment.Thetestisnotintendedfordesignpurposessincethetest
rate are carefully controlled. The EPR test is significantly
conditions accelerate corrosion in a manner that does not
affected by the composition, thermomechanical condition and
simulate any actual service environment.
surface finish of the specimen as well as the presence of
5.3 The EPR test involves the measurement of the amount
non-metallic inclusions, that result in pitting of the etched
of charge resulting from the corrosion of the chromium-
microstructure.
depleted regions surrounding the precipitated chromium car-
NOTE 1—Various cutting and grinding operations can promote sensiti- bide particles. Most of these particles in a sensitized micro-
zation of Type 304 (5). Superficial carbide precipitation can occur during
structurearelocatedatthegrainboundaries.However,discrete
cutting and grinding or during subsequent low temperature heat treat-
particles located within grains (referred to as intragranular
ments, such as 24 h at 500°C.
precipitates) will also contribute to the total measured charge.
4.5 The criteria used to distinguish between sensitized and
(See Fig. 2.) Therefore, it is important to examine the alloy
solution annealed samples are the activation charge density, Q microstructure following an EPR test to determine the relative
(given by the time integral of current density below the proportion of corrosion sites associated with intergranular
reactivationpeakofthecurve),orthemaximumanodiccurrent
versus intragranular precipitates. Sites of intergranular attack
density, I , in the active state. Sensitized steels are easily will appear similar to grain boundary ditching as defined in
r
activated and show higher Q and I values than solution
Practice A of Practices A262.
r
annealed steels, that are not susceptible to intergranular corro-
sion. The value Q is normalized for both specimen size and
6. Apparatus
6.1 TheapparatusnecessaryforobtainingEPRdataconsists
ofelectronicinstrumentsandatestcell.Theseinstrumentsmay
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
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G108–94 (2004)
beintegratedintooneinstrumentpackageormaybeindividual time trace and then, subsequently, integrating it by various
components. Either form of instrumentation can provide ac- methods. When potentiostat measurements are available in a
ceptable data. digitized format, an appropriate computer integration routine
6.2 Typicalapparatus,asillustratedinFig.3,shallconsistof can also be used to obtain a value for charge.
the following: scanning potentiostat (or potentiostat/voltage 6.2.5 EPR Test Cell—Requirements shall be in accordance
ramp generator combination), potential measuring instrument, with 4.1 of Practice G5.Adeareation tube is not required and
current and current integration measuring instruments, and test only one counter electrode is required for EPR testing. A
cell and specimen holder. suitable cell and electrode arrangement is shown in Fig. 4.
6.2.1 Scanning Potentiostat—Requirements shall be in ac- 6.2.6 Electrode Holder—Requirements shall be in accor-
cordance with 4.2 of Practice G5 with the following refine- dancewith4.6ofPracticeG5or4.2.1ofTestMethodG61.The
ments: the potentiostat shall control the potential within 65 requirementsfortheworkingelectrode(specimen)andcounter
mV accuracy over the range of potential and current density electrode holders are that the holders be made of an inert
encountered in the EPR measurements. The potentiostat shall material and any seals must not allow leakage of the electro-
beoperableinapotentialrangeof−600to+500mV(SCE)and lyte. When using the Practice G5-type holder the working
a current density range of 1 µA to 100 mA/cm . The applied electrode can be mounted as shown in Fig. 5 and described in
potential is changed either automatically or manually in the Appendix X1.
following manners: 6.2.7 Auxiliary (Counter) Electrodes—Requirements are in
6.2.1.1 Shifting the potential from the open circuit potential accordance with 4.7.2 of Practice G5 except that only one
to a potential in the passive range, and counter electrode is necessary for EPR testing. However, two
6.2.1.2 Scanning back to the open circuit potential (reacti- auxiliary electrodes can provide for a more uniform distribu-
vation) at a voltage scan rate of 1.67 mV/s (6 V/h). tion of current.Titanium or high-purity carbon may be used in
6.2.2 Potential Measuring Instruments—Requirements place of platinum for the counter electrode since it is always
shall be in accordance with 4.3 of Practice G5 except that the the cathode.
potential range is as stated above. 6.2.8 Calomel Reference Electrode—Requirements are in
6.2.3 Current Measuring Instruments—Requirements shall accordance or equivalent to 4.7.3 of Practice G5.
be in accordance with 4.4 of Practice G5. However, current
7. Sampling, Test Specimens, and Test Units
measurements are essential for passivation assessment and
7.1 Sampling:
other intermediate checks of system stability. The currents
7.1.1 When using this test method to meet product accep-
encountered in EPR for a specimen with the dimensions given
tancecriteria,themeansofsamplingofatestspecimenshallbe
in 7.3 are in the range of 1 µAto 100 mA/cm . For samples of
2 2
decided by agreement between the parties involved; for in-
less than 100 mm test area, currents above about 20 mA/cm
stance, but not limited to, a user and a supplier.
rarely have been reported.
7.1.2 Specimens removed form a piece ofAISIType 304 or
6.2.4 Current Integration Measurement Instruments
304Lsteelbyshearing,cutting,burning,andsoforthshallhave
(Optional)—Current integration, or charge, can be measured
the affected edges removed by grinding or machining.
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
NOTE—The sample face is completely immersed but the connection to
the electrode holder is not immersed.
FIG. 4 Schematic Diagram of an Electrochemical Cell for EPR
FIG. 3 Schematic Diagram of an EPR Test Apparatus Testing
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G108–94 (2004)
7.3.3 The front surface of the specimen will be evaluated in
the EPR test. The back surface of the test specimen is used to
establish electrical contact with the specimen (see Note 2).
NOTE 2—A convenient way to make this attachment may be either by
spot welding or by using a conducting cement to fasten a stainless steel
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
occasionallyusefultomaskthear
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