ASTM G150-99(2010)
(Test Method)Standard Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels
Standard Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels
SIGNIFICANCE AND USE
This test method provides a prediction of the resistance to stable propagating pitting corrosion of stainless steels and related alloys in a standard medium (see Note 1). The CPT test can be used for product acceptance, alloy development studies, and manufacturing control. In the case of product acceptance, the supplier and user must agree upon the preconditioning of the specimen with regard to surface finish. The test is not intended for design purposes since the test conditions accelerate corrosion in a manner that does not simulate any actual service environment.
Another method to determine the potential independent CPT with an electrochemical technique has been discussed in the literature (1-4). This test method involves a potentiodynamic (potential sweep) procedure performed on specimens at different temperatures. A comparison (2) of the test method described in this test method and the potentiodynamic technique has indicated no difference in the test result obtained.
SCOPE
1.1 This test method covers a procedure for the evaluation of the resistance of stainless steel and related alloys to pitting corrosion based on the concept of the determination of a potential independent critical pitting temperature (CPT).
1.2 This test methods applies to wrought and cast products including but not restricted to plate, sheet, tubing, bar, forgings, and welds, (see Note 1).
Note 1—Examples of CPT measurements on sheet, plate, tubing, and welded specimens for various stainless steels can be found in Ref (1). See the research reports (Section 14).
1.3 The standard parameters recommended in this test method are suitable for characterizing the CPT of austenitic stainless steels and other related alloys with a corrosion resistance ranging from that corresponding to solution annealed UNS S31600 (Type 316 stainless steel) to solution annealed UNS S31254 (6 % Mo stainless steel).
1.4 This test method may be extended to stainless steels and other alloys related to stainless steel that have a CPT outside the measurement range given by the standard parameters described in this test method. Appropriate test potential and solution must then be determined.
1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
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.
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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: G150 − 99(Reapproved 2010)
Standard Test Method for
Electrochemical Critical Pitting Temperature Testing of
Stainless Steels
This standard is issued under the fixed designation G150; 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
2.1 ASTM Standards:
1.1 This test method covers a procedure for the evaluation
D1193Specification for Reagent Water
of the resistance of stainless steel and related alloys to pitting
E177Practice for Use of the Terms Precision and Bias in
corrosion based on the concept of the determination of a
ASTM Test Methods
potential independent critical pitting temperature (CPT).
E691Practice for Conducting an Interlaboratory Study to
1.2 This test methods applies to wrought and cast products
Determine the Precision of a Test Method
includingbutnotrestrictedtoplate,sheet,tubing,bar,forgings,
G1Practice for Preparing, Cleaning, and Evaluating Corro-
and welds, (see Note 1).
sion Test Specimens
G3Practice for Conventions Applicable to Electrochemical
NOTE 1—Examples of CPT measurements on sheet, plate, tubing, and
Measurements in Corrosion Testing
weldedspecimensforvariousstainlesssteelscanbefoundinRef (1). See
G5Reference Test Method for Making Potentiostatic and
the research reports (Section 14).
Potentiodynamic Anodic Polarization Measurements
1.3 The standard parameters recommended in this test
G15TerminologyRelatingtoCorrosionandCorrosionTest-
method are suitable for characterizing the CPT of austenitic
ing (Withdrawn 2010)
stainless steels and other related alloys with a corrosion
G46Guide for Examination and Evaluation of Pitting Cor-
resistance ranging from that corresponding to solution an-
rosion
nealed UNS S31600 (Type 316 stainless steel) to solution
G107Guide for Formats for Collection and Compilation of
annealed UNS S31254 (6 % Mo stainless steel).
Corrosion Data for Metals for Computerized Database
Input
1.4 Thistestmethodmaybeextendedtostainlesssteelsand
other alloys related to stainless steel that have a CPT outside
3. Terminology
the measurement range given by the standard parameters
3.1 Definitions:
described in this test method. Appropriate test potential and
3.1.1 critical pitting temperature (CPT)—the lowest tem-
solution must then be determined.
perature on the test surface at which stable propagating pitting
1.5 The values stated in SI units are to be regarded as
occurs under specified test conditions indicated by a rapid
standard. No other units of measurement are included in this
increase beyond a set limit of the measured anodic current
standard.
density of the specimen.
3.1.2 pitting potential range—the range of measured poten-
1.6 This standard does not purport to address all of the
tials where pitting is initiated. This potential range only exists
safety concerns, if any, associated with its use. It is the
above the minimum critical pitting temperature; see also
responsibility of the user of this standard to establish appro-
Appendix X1.
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
3.1.3 potential dependent CPT—the CPT determined at a
potential within the pitting potential range of the tested
material; see also Appendix X1.
ThistestmethodisunderthejurisdictionofG01onCorrosionofMetalsandis
thedirectresponsibilityofSubcommitteeG01.11onElectrochemicalMeasurements
in Corrosion Testing. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved May 1, 2010. Published May 2010. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approvedin1997.Lastpreviouseditionapprovedin2004asG150–99(2004).DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/G0150-99R10. the ASTM website.
2 4
The boldface numbers in parenthesis refer to the list of references at the end of The last approved version of this historical standard is referenced on
this standard. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G150 − 99 (2010)
intended for design purposes since the test conditions acceler-
ate corrosion in a manner that does not simulate any actual
service environment.
5.2 Another method to determine the potential independent
CPT with an electrochemical technique has been discussed in
the literature (1-4). This test method involves a potentiody-
namic (potential sweep) procedure performed on specimens at
different temperatures. A comparison (2) of the test method
described in this test method and the potentiodynamic tech-
nique has indicated no difference in the test result obtained.
6. Apparatus
6.1 The apparatus necessary for determining the CPT con-
sists of instruments for measuring electronic signals, a tem-
FIG. 1 Determination of CPT
perature controlling apparatus, a specimen holder, and a test
cell. The instruments for measuring electronic signals may be
integrated into one instrument package or may be individual
components. Either form of instrumentation can provide ac-
3.1.4 potential independent CPT— the CPTdetermined at a
ceptabledata.Typicaltestequipmentconsistsofthefollowing:
potential above the pitting potential range, but below the
(1) potentiostat (2) potential measuring instrument (3) current
transpassive potential; see also Appendix X1.
measuring instrument (4) temperature controller (5) tempera-
3.1.5 temperature ramp—the rate (°C/min) at whichthetest
turemeasuringinstrument(6)testcell(7)specimenholder,and
temperature is increased during the test.
(8) electrodes.
3.2 sign conventions—the sign conventions used in this
6.2 Potentiostat—Thepotentiostatshallbeabletoapplythe
procedure are in agreement with Practice G3.
constant potential to within 1 mV at a current density of 10
3.3 Unless otherwise stated, this test method uses the mA/cm .The applied potential is changed either automatically
general terminology relating to corrosion and corrosion testing or manually by shifting the potential from the open circuit
as defined in Terminology G15. potential to another more noble potential.
6.3 PotentialMeasuringInstrument—Requirementsshallbe
4. Summary of Test Method
in accordance with the section on Potential Measuring Instru-
4.1 The test method determines the potential independent ments in Test Method G5.
critical pitting temperature (CPT) by way of a potentiostatic
6.4 Current Measuring Instruments—An instrument that is
technique using a temperature scan and a specimen holder that
capable of measuring a current accurately to within 5% of the
is designed to eliminate the occurrence of crevice corrosion
actual value. The typical current densities encountered during
(seeFig.1).Thespecimenisexposed,eitherentirelyorinpart, 2 2
the CPT test are in the range of 1 µA/cm to 10 mA/cm .
depending on test cell configuration to a 1M NaCl solution,
6.5 Temperature Controller:
initially at 0°C. After an initial temperature stabilization
6.5.1 Thermostat equipment is required that can provide
period, the solution is heated at a rate of 1°C/min.About 60 s
cooling and heating of the test solution in the temperature
before the temperature scan is commenced, the specimen is
range from 0°C to approximately 100°C. Further, the tempera-
anodically polarized to a potential above the pitting potential
ture controller is used to provide controlled heating, which
range. This potential is held constant during the whole tem-
gives the test solution temperature a temperature increase rate
perature scan. A potential of 700 mV versus SCE (25°C) has
of 1°C/min in the range from 0°C to approximately 100°C.
been found suitable for most stainless steels. The current is
6.5.2 Above 10°C, the average rate of temperature change
monitoredduringthe temperature scan, and the CPTisdefined
ofthetestsolutionshallbe1.0 60.3°C/min,wheretheaverage
asthetemperatureatwhichthecurrentincreasesrapidly,which
is calculated over a temperature range of 10°C.
for practical reasons is defined as the temperature at which the
current density exceeds 100 µA/cm for 60 s. Pitting on the 6.6 Temperature Measurement Instrumentation, shall be
specimen is confirmed visually after the test.
capable of measuring the temperature of the test solution with
an accuracy of 60.4°C.
5. Significance and Use
6.7 Test Cell:
5.1 This test method provides a prediction of the resistance 6.7.1 Option 1, G5 Type—The test cell should be similar to
to stable propagating pitting corrosion of stainless steels and the one described in Test Method G5. Other similar polariza-
relatedalloysinastandardmedium(seeNote1).TheCPTtest tion cells may be equally suitable. The gas purger should
canbeusedforproductacceptance,alloydevelopmentstudies, distribute the gas in numerous small bubbles.
and manufacturing control. In the case of product acceptance, 6.7.2 Option 2, Flushed-port Cell—Thiscelldesignisbased
the supplier and user must agree upon the preconditioning of onthatpublishedbyR.Qvarfort (3)andincludesthespecimen
the specimen with regard to surface finish. The test is not holderinthedesign.Theadvantagesofthiscelldesignarethat
G150 − 99 (2010)
the specimen edges and back do not need to be machined, the edgesremovedbygrindingormachining,unlessitisexplicitly
specimen does not have to be mounted inside the cell, and intended to study the effects of these edge factors.
crevice corrosion at the contact area of the cell port is
8. Reagents and Materials
completely eliminated, even at elevated test temperatures. See
8.1 Purity of Reagents—Reagent grade chemicals shall be
Appendix X2 for a description of this cell. The gas purger
used in all tests. Unless otherwise indicated, it is intended that
should distribute the gas in numerous small bubbles.
all reagents conform to the specifications of the Committee on
6.7.3 The test cell shall be able to contain a test solution
Analytical Reagents of theAmerican Chemical Society where
volumeofminimum100mLpersquarecentimetretestarea.A
such specifications are available. Other grades may be used,
maximum dilution of 15% of the test solution during the test
provided it is first ascertained that the reagent is of sufficiently
period is allowed in case a flushed port cell or similar
high purity to permit its use without lessening the accuracy of
arrangement is used.
the determination.
6.8 Specimen Holder:
8.2 Purity of Water—Unless otherwise indicated, references
6.8.1 Any part of the specimen holder coming in contact
to purified water shall be understood to mean reagent water as
with the test solution during testing shall be made of an inert
defined by Type IV of Specification D1193.
material, and any seal shall not allow leakage of electrolyte.
6.8.2 The specimen holder shall have a design that ensures
8.3 Standard Test Solution—To prepare 1 L of 1 M sodium
no occurrence of crevice corrosion at the contact area between
chloride (NaCl) solution, dissolve 58.45 g sodium chloride
specimen holder and specimen.
(NaCl) in purified water to a total solution volume of 1 L.The
6.8.3 Two examples of specimen holder designs in accor-
solution can be made up in bulk and stored for one month at
dance with this standard are shown in Appendix X2 and
room temperature.
Appendix X3. The major difference between the specimen
8.4 Purging Gas—Nitrogengasofminimum99.99%purity
holderdesignsliesintheallowablespecimengeometryandthe
should be used.
number of surfaces on the specimen that are being tested
simultaneously.
9. Applied Potential
6.9 Electrodes:
9.1 Standard Potential—An anodic potential of 700 mV
6.9.1 Auxiliary (Counter) Electrode—Requirementsshallbe
versusSCE(25°C)isused.Thishasbeenfoundappropriatefor
in accordance with the section Auxiliary Electrodes in Test
most stainless steels (1).
Method G5 with the exception that only one counter electrode
9.2 Alternative Potential:
isnecessaryforCPTtesting.Theelectrodematerialshallbeof
9.2.1 If uncertainty exists concerning whether the standard
a type which can be considered inert under the test conditions.
potentialissufficientlyhightoobtainthepotentialindependent
6.9.2 Reference Electrode—Thereferenceelectrodeshallbe
CPT, a test at 800 mV versus SCE (25°C) may be performed.
kept at room temperature outside the actual test cell. The
A significant deviation between the CPT obtained at 700 mV
reference electrode shall be capable of ensuring a constant
and 800 mV will indicate a need for a reevaluation and new
reference potential within 65 mV during the entire test
choice of potential.
procedure (see Note 2). Electrical contact to the test solution
shall be provided by the use of a luggin capillary placed in the NOTE4—Usingalowerpotentialthanthestandardpotentialof700mV
versus SCE (25°C) is fully acceptable, provided the determined CPT still
test solution. Requirements shall otherwise be in accordance
is potential independent. To change the measurement range provided by
with the section on Reference Electrode in Test Method G5.
the standard test conditions, a new test solution composition will have to
be chosen. Following the choice of test solution, a test potential that
NOTE 2—It may be difficult to ensure a fully constant reference
ensures the determination of a potential independent CPT will have to be
potential due to the large variations in temperature of the test solution;
determined.
therefore, the allowable is 65 mV. This does, however, not affect the
measured potential independent CPT (1).
9.2.2 Evaluation of differences in obtained CPT at the two
potentials should take into account the repeatability of the test
7. Test Specimens
method. The homogeneity of the material used for the two
7.1 Finish—Any geometry and surface finish (see Note 3) different potentials shall also be considered before an alterna-
compatiblewiththechosenspecimenholderasspecifiedin6.8 tive potential is used.
may be used.
10. Procedure
NOTE 3—The state of the surface may be dependent on the time and
10.1 Sample Mounting, Cleaning and Placement:
location of storage between the final mechanical or chemical surface
10.1.1 The recommendations given in Practice G1 are to be
treatment and testing. The time and location of storage may, therefore, in
some situations be considered an integral part of the surface finish. followed, where applicable, unless otherwise stated in this
procedure.
7.2 Sampling—Whenusingthistestmethodtomeetproduct
acceptance criteria, the means of sampling of a test specimen
Reagent Chemicals, America
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