ASTM G150-18

Standard Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels and Related Alloys

SIGNIFICANCE AND USE
5.1 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.  
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 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 method 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).2 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 30-Apr-2018
• ICS: 77.140.20 - Stainless steels
• Technical Committee: G01 - Corrosion of Metals
• Drafting Committee: G01.11 - Electrochemical Measurements in Corrosion Testing

Relations

• Replaces: ASTM G150-13 - Standard Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels
• Effective Date: 01-May-2018
• Refers: ASTM G3-14(2019) - Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing
• Effective Date: 01-May-2019
• Refers: ASTM G3-14 - Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing
• Effective Date: 15-Dec-2014
• Refers: ASTM G5-14 - Standard Reference Test Method for Making Potentiodynamic Anodic Polarization Measurements
• Effective Date: 01-Nov-2014
• Refers: ASTM E177-14 - Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods
• Effective Date: 01-May-2014
• Refers: ASTM G3-13 - Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing
• Effective Date: 01-Dec-2013
• Refers: ASTM G46-94(2013) - Standard Guide for Examination and Evaluation of Pitting Corrosion
• Effective Date: 01-May-2013
• Refers: ASTM E177-13 - Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods
• Effective Date: 01-May-2013
• Refers: ASTM E691-13 - Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
• Effective Date: 01-May-2013
• Refers: ASTM G5-13 - Standard Reference Test Method for Making Potentiodynamic Anodic Polarization Measurements
• Effective Date: 01-Feb-2013
• Refers: ASTM G5-13e1 - Standard Reference Test Method for Making Potentiodynamic Anodic Polarization Measurements
• Effective Date: 01-Feb-2013
• Refers: ASTM G5-13e2 - Standard Reference Test Method for Making Potentiodynamic Anodic Polarization Measurements
• Effective Date: 01-Feb-2013
• Refers: ASTM G5-12 - Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements
• Effective Date: 15-Nov-2012
• Refers: ASTM D1293-12 - Standard Test Methods for pH of Water
• Effective Date: 01-Jan-2012
• Refers: ASTM G1-03(2011) - Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens
• Effective Date: 01-Dec-2011

Overview

ASTM G150-18 is the internationally recognized standard test method for electrochemical critical pitting temperature (CPT) testing of stainless steels and related alloys. Developed by ASTM, this standard specifies a procedure to evaluate the resistance of stainless steel and related alloys to pitting corrosion by determining the potential-independent critical pitting temperature. The test is designed for wrought and cast products including plate, sheet, tubing, bar, forgings, and welds.

Used primarily for product acceptance, alloy development, and manufacturing process control, ASTM G150-18 provides a quantitative measure for comparing pitting corrosion resistance in a controlled laboratory setting. It is not intended for direct design purposes, as the test conditions accelerate corrosion in a manner that does not simulate actual service environments.

Key Topics

• Critical Pitting Temperature (CPT): The minimum temperature at which stable, propagating pitting occurs on the test surface under specified electrochemical conditions.
• Electrochemical Techniques: Utilizes a potentiostatic method with a temperature scan and precise specimen handling to eliminate crevice corrosion and ensure testing accuracy.
• Applicability: Suitable for austenitic stainless steels and related alloys, typically within the corrosion resistance range of solution-annealed UNS S31600 (Type 316) to UNS S31254 (6% Mo stainless steel).
• Test Parameters: Involves exposure to a standard chloride solution under controlled temperature increases, monitoring of anodic current density, and post-test visual inspection for pitting.
• Surface Finish Agreement: For product acceptance testing, agreement on specimen surface preparation between supplier and user is required due to the surface's influence on results.
• Safety and Units: Test procedures utilize SI units and require users to address any safety or environmental concerns related to the test.

Applications

ASTM G150-18 is widely used in industries that rely on stainless steels and high-performance alloys, including:

• Quality Assurance and Product Acceptance: Provides objective data for material specification compliance between suppliers and users.
• Material Selection: Assists in comparing different stainless steel grades and related alloys based on their resistance to pitting in chloride environments.
• Alloy Development: Facilitates research and development by enabling the comparison of new alloy compositions and processing techniques for pitting resistance.
• Process Control: Useful in manufacturing settings to monitor and maintain material quality by detecting variations in corrosion resistance due to process changes.
• Academic and Industrial Research: Offers a standardized procedure for generating reproducible data for scientific studies related to corrosion mechanisms, alloy performance, and surface treatments.

Related Standards

The following standards are referenced or associated with the application of ASTM G150-18 and are relevant for organizations seeking comprehensive corrosion testing protocols:

• ASTM D1193: Specification for Reagent Water
• ASTM G1: Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens
• ASTM G3: Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing
• ASTM G5: Reference Test Method for Making Potentiodynamic Anodic Polarization Measurements
• ASTM G46: Guide for Examination and Evaluation of Pitting Corrosion
• ASTM E691: Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method

These related ASTM standards support the rigorous and reproducible application of the CPT test method, ensuring consistency across laboratories and applications.

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Keywords: ASTM G150-18, critical pitting temperature, CPT testing, stainless steel corrosion, pitting resistance, electrochemical corrosion testing, quality assurance, alloy development, corrosion standards.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: G150 − 18
Standard Test Method for
Electrochemical Critical Pitting Temperature Testing of
Stainless Steels and Related Alloys
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 Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
1.1 This test method covers a procedure for the evaluation
Barriers to Trade (TBT) Committee.
of the resistance of stainless steel and related alloys to pitting
corrosion based on the concept of the determination of a
2. Referenced Documents
potential independent critical pitting temperature (CPT).
2.1 ASTM Standards:
1.2 This test method applies to wrought and cast products
D1193Specification for Reagent Water
includingbutnotrestrictedtoplate,sheet,tubing,bar,forgings,
D1293Test Methods for pH of Water
and welds, (see Note 1).
E177Practice for Use of the Terms Precision and Bias in
NOTE 1—Examples of CPT measurements on sheet, plate, tubing, and
ASTM Test Methods
weldedspecimensforvariousstainlesssteelscanbefoundinRef (1). See
E691Practice for Conducting an Interlaboratory Study to
the research reports (Section 14).
Determine the Precision of a Test Method
1.3 The standard parameters recommended in this test
G1Practice for Preparing, Cleaning, and Evaluating Corro-
method are suitable for characterizing the CPT of austenitic
sion Test Specimens
stainless steels and other related alloys with a corrosion
G3Practice for Conventions Applicable to Electrochemical
resistance ranging from that corresponding to solution an-
Measurements in Corrosion Testing
nealed UNS S31600 (Type 316 stainless steel) to solution
G5Reference Test Method for Making Potentiodynamic
annealed UNS S31254 (6 % Mo stainless steel).
Anodic Polarization Measurements
1.4 Thistestmethodmaybeextendedtostainlesssteelsand
G46Guide for Examination and Evaluation of Pitting Cor-
other alloys related to stainless steel that have a CPT outside rosion
the measurement range given by the standard parameters
G107Guide for Formats for Collection and Compilation of
described in this test method. Appropriate test potential and
Corrosion Data for Metals for Computerized Database
solution must then be determined.
Input
G193Terminology and Acronyms Relating to Corrosion
1.5 The values stated in SI units are to be regarded as
standard. No other units of measurement are included in this
3. Terminology
standard.
3.1 Definitions:
1.6 This standard does not purport to address all of the
3.1.1 critical pitting temperature (CPT)—the lowest tem-
safety concerns, if any, associated with its use. It is the
perature on the test surface at which stable propagating pitting
responsibility of the user of this standard to establish appro-
occurs under specified test conditions indicated by a rapid
priate safety, health, and environmental practices and deter-
increase beyond a set limit of the measured anodic current
mine the applicability of regulatory limitations prior to use.
density of the specimen.
1.7 This international standard was developed in accor-
dance with internationally recognized principles on standard-
3.1.2 pitting potential range—the range of measured poten-
ization established in the Decision on Principles for the
tials where pitting is initiated. This potential range only exists
above the minimum critical pitting temperature; see also
Appendix X1.
ThistestmethodisunderthejurisdictionofG01onCorrosionofMetalsandis
thedirectresponsibilityofSubcommitteeG01.11onElectrochemicalMeasurements
in Corrosion Testing.
Current edition approved May 1, 2018. Published May 2018. Originally
approved in 1997. Last previous edition approved in 2013 as G150 – 13. DOI: For referenced ASTM standards, visit the ASTM website, www.astm.org, or
10.1520/G0150-18. contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof Standards volume information, refer to the standard’s Document Summary page on
this standard. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G150 − 18
3.1.3 potential dependent CPT—the CPT determined at a the specimen with regard to surface finish. The test is not
potential within the pitting potential range of the tested intended for design purposes since the test conditions acceler-
material; see also Appendix X1. ate corrosion in a manner that does not simulate any actual
service environment.
3.1.4 potential independent CPT— the CPTdetermined at a
potential above the pitting potential range, but below the
5.2 Another method to determine the potential independent
transpassive potential; see also Appendix X1.
CPT with an electrochemical technique has been discussed in
the literature (1-4). This test method involves a potentiody-
3.1.5 temperature ramp—the rate (°C/min) at which the test
namic (potential sweep) procedure performed on specimens at
temperature is increased during the test.
different temperatures. A comparison (2) of the test method
3.2 sign conventions—the sign conventions used in this
described in this test method and the potentiodynamic tech-
procedure are in agreement with Practice G3.
nique has indicated no difference in the test result obtained.
3.3 Unless otherwise stated, this test method uses the
general terminology relating to corrosion and corrosion testing
6. Apparatus
as defined in Terminology G193.
6.1 The apparatus necessary for determining the CPT con-
sists of instruments for measuring electronic signals, a tem-
4. Summary of Test Method
perature controlling apparatus, a specimen holder, and a test
4.1 The test method determines the potential independent
cell. The instruments for measuring electronic signals may be
critical pitting temperature (CPT) by way of a potentiostatic
integrated into one instrument package or may be individual
technique using a temperature scan and a specimen holder that
components. Either form of instrumentation can provide ac-
is designed to eliminate the occurrence of crevice corrosion
ceptabledata.Typicaltestequipmentconsistsofthefollowing:
(seeFig.1).Thespecimenisexposed,eitherentirelyorinpart,
(1) potentiostat (2) potential measuring instrument (3) current
depending on test cell configuration to a 1M NaCl solution,
measuring instrument (4) temperature controller (5) tempera-
initially at 0°C. After an initial temperature stabilization
turemeasuringinstrument(6)testcell(7)specimenholder,and
period, the solution is heated at a rate of 1°C/min.About 60 s
(8) electrodes.
before the temperature scan is commenced, the specimen is
6.2 Potentiostat—The potentiostat shall be able to apply the
anodically polarized to a potential above the pitting potential
constant potential to within 1 mV at a current density of 10
range. This potential is held constant during the whole tem-
mA/cm .The applied potential is changed either automatically
perature scan. A potential of 700 mV versus SCE (25°C) has
or manually by shifting the potential from the open circuit
been found suitable for most stainless steels. The current is
potential to another more noble potential.
monitored during the temperature scan, and the CPTis defined
asthetemperatureatwhichthecurrentincreasesrapidly,which
6.3 PotentialMeasuringInstrument—Requirementsshallbe
for practical reasons is defined as the temperature at which the
in accordance with the section on Potential Measuring Instru-
current density exceeds 100 µA/cm for 60 s. Pitting on the
ments in Test Method G5.
specimen is confirmed visually after the test.
6.4 Current Measuring Instruments—An instrument that is
capable of measuring a current accurately to within 5% of the
5. Significance and Use
actual value. The typical current densities encountered during
5.1 This test method provides a prediction of the resistance 2 2
the CPT test are in the range of 1 µA/cm to 10 mA/cm .
to stable propagating pitting corrosion of stainless steels and
6.5 Temperature Controller:
relatedalloysinastandardmedium(seeNote1).TheCPTtest
6.5.1 Thermostat equipment is required that can provide
canbeusedforproductacceptance,alloydevelopmentstudies,
cooling and heating of the test solution in the temperature
and manufacturing control. In the case of product acceptance,
range from 0°C to approximately 100°C. Further, the tempera-
the supplier and user must agree upon the preconditioning of
ture controller is used to provide controlled heating, which
gives the test solution temperature a temperature increase rate
of 1°C/min in the range from 0°C to approximately 100°C.
6.5.2 Above 10°C, the average rate of temperature change
ofthetestsolutionshallbe1.0 60.3°C/min,wheretheaverage
is calculated over a temperature range of 10°C.
6.6 Temperature Measurement Instrumentation, shall be
capable of measuring the temperature of the test solution with
an accuracy of 60.4°C.
6.7 Test Cell:
6.7.1 Option 1, G5 Type—The test cell should be similar to
the one described in Test Method G5. Other similar polariza-
tion cells may be equally suitable. The gas purger should
distribute the gas in numerous small bubbles.
6.7.2 Option 2, Flushed-port Cell—Thiscelldesignisbased
FIG. 1 Determination of CPT onthatpublishedbyR.Qvarfort (3)andincludesthespecimen
G150 − 18
holderinthedesign.Theadvantagesofthiscelldesignarethat 7.4 Specimensremovedfromaworkpieceorcomponentby
the specimen edges and back do not need to be machined, the shearing, cutting, burning, and so forth shall have the affected
specimen does not have to be mounted inside the cell, and edgesremovedbygrindingormachining,unlessitisexplicitly
crevice corrosion at the contact area of the cell port is intended to study the effects of these edge factors.
completely eliminated, even at elevated test temperatures. See
8. Reagents and Materials
Appendix X2 for a description of this cell. The gas purger
should distribute the gas in numerous small bubbles.
8.1 Purity of Reagents—Reagent grade chemicals shall be
6.7.3 The test cell shall be able to contain a test solution used in all tests. Unless otherwise indicated, it is intended that
volumeofminimum100mLpersquarecentimetretestarea.A all reagents conform to the specifications of the Committee on
maximum dilution of 15% of the test solution during the test Analytical Reagents of theAmerican Chemical Society where
period is allowed in case a flushed port cell or similar such specifications are available. Other grades may be used,
arrangement is used. provided it is first ascertained that the reagent is of sufficiently
high purity to permit its use without lessening the accuracy of
6.8 Specimen Holder:
the determination.
6.8.1 Any part of the specimen holder coming in contact
8.2 Purity of Water—Unless otherwise indicated, references
with the test solution during testing shall be made of an inert
to purified water shall be understood to mean reagent water as
material, and any seal shall not allow leakage of electrolyte.
defined by Type IV of Specification D1193.
6.8.2 The specimen holder shall have a design that ensures
no occurrence of crevice corrosion at the contact area between 8.3 Standard Test Solution—To prepare 1 L of 1 M sodium
specimen holder and specimen. chloride (NaCl) solution, dissolve 58.45 g sodium chloride
(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
holderdesignsliesintheallowablespecimengeometryandthe
8.4 Purging Gas—Nitrogengasofminimum99.99%purity
number of surfaces on the specimen that are being tested
should be used.
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
isnecessaryforCPTtesting.Theelectrodematerialshallbeof
9.2 Alternative Potential:
a type which can be considered inert under the test conditions.
9.2.1 If uncertainty exists concerning whether the standard
6.9.2 Reference Electrode—Thereferenceelectrodeshallbe potentialissufficientlyhightoobtainthepotentialindependent
kept at room temperature outside the actual test cell. The CPT, a test at 800 mV versus SCE (25°C) may be performed.
A significant deviation between the CPT obtained at 700 mV
reference electrode shall be capable of ensuring a constant
reference potential within 65 mV during the entire test and 800 mV will indicate a need for a reevaluation and new
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
test solution. Requirements shall otherwise be in accordance
versus SCE (25°C) is fully acceptable, provided the determined CPT still
with the section on Reference Electrode in Test Method G5.
is potential independent. To change the measurement range provided by
the standard test conditions, a new test solution composition will have to
NOTE 2—It may be difficult to ensure a fully constant reference
be chosen. Following the choice of test solution, a test potential that
potential due to the large variations in temperature of the test solution;
ensures the determination of a potential independent CPT will have to be
therefore, the allowable is 65 mV. This does, however, not affect the
determined.
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
location of storage between the final mechanical or chemical surface
10.1 Sample Mounting, Cleaning and Placement:
treatment and testing. The time and location of storage may, therefore, in
some situations be considered an integral part of the surface finish.
Reagent Chemicals, American Chemical Society Specifications, American
7.2 Sampling—Whenusingthistestmethodtomeetproduct
Chemical Society, Washington, DC. For suggestions on the testing of reagents not
acceptance criteria, the means of sampling of a test specimen
listed by the American Chemical Society, see Analar Standards for Laboratory
Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia
shall be decided by agreement between the parties involved.
and National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville,
7.3 Test Area—Aminimum test area of 1 cm shall be used. MD.
G150 − 18
NOTE6—Pitsmaybedifficulttolocateifthetestisstoppedshortlyafter
10.1.1 The recommendations given in Practice G1 are to be
pitting initiation. Pits may be located based on leaking rust or by using a
followed, where applicable, unless otherwise stated in this
needle to uncover pits hidden below a thin metal or oxide film. The lack
procedure.
of visible pitting may indicate that general corrosion has occurred, for
10.1.2 Clean the specimen just before immersion in the
example, transpassive corrosion. However, only a more thorough exami-
electrolyte by degreasing with a suitable detergent, rinsing in
nation of the tested specimen can give a possible explanation.
purified water, followed by ethanol or similar solvent, and air
11. Visual Examination of Test Electrode
drying. After degreasing, handle the specimen with clean
gloves, soft clean tongs, or equivalent preventive measures, to 11.1 Confirm the existence of pits and the absence of
avoid surface contamination.
crevice corrosion using a microscope at 20× magnification.
Note the location of pits relative to the test geometry (in the
10.2 Test Solution Preparation:
center, on edges, at the bottom center, at the bottom edge, and
10.2.1 Prepare the test solution according to Section 8.
so forth).
10.2.2 Bring the solution to an initial temperature at or
below 3°C.
11.2 Any crevice corrosion observed on the specimen after
testing means that the test results are invalid and must be
10.3 Mount the specimen in the specimen holder.
discarded.
10.4 Place the specimen, counter electrode, salt bridge
11.3 If required, a more thorough examination of the elec-
probe, and other components in the empty test cell.
trodecaninvolvemeasurementofpitdensityandpitdepthson
10.5 Fill the cell with cold (≤3°C) solution.
specimen electrodes, according to Guide G46, using a micro-
scope at 20× magnification.
10.6 Ensurethatthesaltbridgeisfilledwiththetestsolution
and is free of air bubbles, particularly in the restricted space at
12. Data Analysis
the tip. A lugging probe should contain a wick, or equivalent
device, to ensure electric contact even when small gas bubbles
12.1 Measured current as a function of time shall be
are formed during the test. convertedtocurrentdensityvalues.Thedatamaybepresented
as in Fig. 1, which shows an example of a current density
10.7 Commencebubblingnitrogengasthroughthesolution.
versus temperature plot.
NOTE5—Thepurposeofthepurginggasistoenhancethestirringofthe
12.2 Evaluation of the CPT:
test solution. No reduction in the oxygen level of the test solution is
12.2.1 Standard Evaluation—The critical pitting tempera-
intended. Presence or absence of oxygen has been shown not to affect the
test result (1).
ture is taken as the temperature at which the current increases
above 100 µA/cm (see Note 7) and stays above this critical
10.8 Connect the electrodes to the potentiostat and data
current density for a minimum of 60 s; see Fig. 1.
recordingdeviceandtheconnectionsfortemperaturemeasure-
ment and control. Let the system cool and stabilize at 0 6 1°C
NOTE 7—The CPT is defined as the lowest temperature at which stable
for a
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: G150 − 13 G150 − 18
Standard Test Method for
Electrochemical Critical Pitting Temperature Testing of
Stainless Steels and Related Alloys
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. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. 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 methodsmethod 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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.7 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D1193 Specification for Reagent Water
D1293 Test Methods for pH of Water
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
G1 Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens
G3 Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing
G5 Reference Test Method for Making Potentiodynamic Anodic Polarization Measurements
G46 Guide for Examination and Evaluation of Pitting Corrosion
G107 Guide for Formats for Collection and Compilation of Corrosion Data for Metals for Computerized Database Input
G193 Terminology and Acronyms Relating to Corrosion
This test method is under the jurisdiction of G01 on Corrosion of Metals and is the direct responsibility of Subcommittee G01.11 on Electrochemical Measurements in
Corrosion Testing.
Current edition approved May 1, 2013May 1, 2018. Published May 2013May 2018. Originally approved in 1997. Last previous edition approved in 20102013 as G150
– 99 (2010). 13. DOI: 10.1520/G0150-13.10.1520/G0150-18.
The boldface numbers in parenthesisparentheses refer to the list of references at the end of this standard.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM 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
G150 − 18
3. Terminology
3.1 Definitions:
3.1.1 critical pitting temperature (CPT)—the lowest temperature on the test surface at which stable propagating pitting occurs
under specified test conditions indicated by a rapid increase beyond a set limit of the measured anodic current density of the
specimen.
3.1.2 pitting potential range—the range of measured potentials where pitting is initiated. This potential range only exists above
the minimum critical pitting temperature; see also Appendix X1.
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.
3.1.4 potential independent CPT— the CPT determined at a potential above the pitting potential range, but below the
transpassive potential; see also Appendix X1.
3.1.5 temperature ramp—the rate (°C/min) at which the test temperature is increased during the test.
3.2 sign conventions—the sign conventions used in this procedure are in agreement with Practice G3.
3.3 Unless otherwise stated, this test method uses the general terminology relating to corrosion and corrosion testing as defined
in Terminology G193.
4. Summary of Test Method
4.1 The test method determines the potential independent critical pitting temperature (CPT) by way of a potentiostatic technique
using a temperature scan and a specimen holder that is designed to eliminate the occurrence of crevice corrosion (see Fig. 1). The
specimen is exposed, either entirely or in part, depending on test cell configuration to a 1M NaCl solution, initially at 0°C. After
an initial temperature stabilization period, the solution is heated at a rate of 1°C/min. About 60 s before the temperature scan is
commenced, the specimen is anodically polarized to a potential above the pitting potential range. This potential is held constant
during the whole temperature scan. A potential of 700 mV versus SCE (25°C) has been found suitable for most stainless steels.
The current is monitored during the temperature scan, and the CPT is defined as the temperature at which the current increases
rapidly, which for practical reasons is defined as the temperature at which the current density exceeds 100 μA/cm for 60 s. Pitting
on the specimen is confirmed visually after the test.
5. Significance and Use
5.1 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.
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 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.
FIG. 1 Determination of CPT
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6. Apparatus
6.1 The apparatus necessary for determining the CPT consists of instruments for measuring electronic signals, a temperature
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 acceptable data. Typical test
equipment consists of the following: (1) potentiostat (2) potential measuring instrument (3) current measuring instrument (4)
temperature controller (5) temperature measuring instrument (6) test cell (7) specimen holder, and (8) electrodes.
6.2 Potentiostat—The potentiostat shall be able to apply the constant potential to within 1 mV at a current density of 10
mA/cm . The applied potential is changed either automatically or manually by shifting the potential from the open circuit potential
to another more noble potential.
6.3 Potential Measuring Instrument—Requirements shall be in accordance with the section on Potential Measuring Instruments
in Test Method G5.
6.4 Current Measuring Instruments—An instrument that is capable of measuring a current accurately to within 5 % of the actual
2 2
value. The typical current densities encountered during the CPT test are in the range of 1 μA/cm to 10 mA/cm .
6.5 Temperature Controller:
6.5.1 Thermostat equipment is required that can provide cooling and heating of the test solution in the temperature range from
0°C to approximately 100°C. Further, the temperature controller is used to provide controlled heating, which gives the test solution
temperature a temperature increase rate of 1°C/min in the range from 0°C to approximately 100°C.
6.5.2 Above 10°C, the average rate of temperature change of the test solution shall be 1.0 6 0.3°C/min, where the average is
calculated over a temperature range of 10°C.
6.6 Temperature Measurement Instrumentation, shall be capable of measuring the temperature of the test solution with an
accuracy of 60.4°C.
6.7 Test Cell:
6.7.1 Option 1, G5 Type—The test cell should be similar to the one described in Test Method G5. Other similar polarization cells
may be equally suitable. The gas purger should distribute the gas in numerous small bubbles.
6.7.2 Option 2, Flushed-port Cell—This cell design is based on that published by R. Qvarfort (3) and includes the specimen
holder in the design. The advantages of this cell design are that the specimen edges and back do not need to be machined, the
specimen does not have to be mounted inside the cell, and crevice corrosion at the contact area of the cell port is completely
eliminated, even at elevated test temperatures. See Appendix X2 for a description of this cell. The gas purger should distribute the
gas in numerous small bubbles.
6.7.3 The test cell shall be able to contain a test solution volume of minimum 100 mL per square centimetre test area. A
maximum dilution of 15 % of the test solution during the test period is allowed in case a flushed port cell or similar arrangement
is used.
6.8 Specimen Holder:
6.8.1 Any part of the specimen holder coming in contact with the test solution during testing shall be made of an inert material,
and any seal shall not allow leakage of electrolyte.
6.8.2 The specimen holder shall have a design that ensures no occurrence of crevice corrosion at the contact area between
specimen holder and specimen.
6.8.3 Two examples of specimen holder designs in accordance with this standard are shown in Appendix X2 and Appendix X3.
The major difference between the specimen holder designs lies in the allowable specimen geometry and the number of surfaces
on the specimen that are being tested simultaneously.
6.9 Electrodes:
6.9.1 Auxiliary (Counter) Electrode—Requirements shall be in accordance with the section Auxiliary Electrodes in Test Method
G5 with the exception that only one counter electrode is necessary for CPT testing. The electrode material shall be of a type which
can be considered inert under the test conditions.
6.9.2 Reference Electrode—The reference electrode shall be kept at room temperature outside the actual test cell. The reference
electrode shall be capable of ensuring a constant reference potential within 65 mV during the entire test procedure (see Note 2).
Electrical contact to the test solution shall be provided by the use of a luggin capillary placed in the test solution. Requirements
shall otherwise be in accordance with the section on Reference Electrode in Test Method G5.
NOTE 2—It may be difficult to ensure a fully constant reference potential due to the large variations in temperature of the test solution; therefore, the
allowable is 65 mV. This does, however, not affect the measured potential independent CPT (1).
7. Test Specimens
7.1 Finish—Any geometry and surface finish (see Note 3) compatible with the chosen specimen holder as specified in 6.8 may
be used.
NOTE 3—The state of the surface may be dependent on the time and location of storage between the final mechanical or chemical surface treatment
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and testing. The time and location of storage may, therefore, in some situations be considered an integral part of the surface finish.
7.2 Sampling—When using this test method to meet product acceptance criteria, the means of sampling of a test specimen shall
be decided by agreement between the parties involved.
7.3 Test Area—A minimum test area of 1 cm shall be used.
7.4 Specimens removed from a work piece or component by shearing, cutting, burning, and so forth shall have the affected
edges removed by grinding or machining, unless it is explicitly intended to study the effects of these edge factors.
8. Reagents and Materials
8.1 Purity of Reagents—Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all
reagents conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society where such
specifications are available. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity
to permit its use without lessening the accuracy of the determination.
8.2 Purity of Water—Unless otherwise indicated, references to purified water shall be understood to mean reagent water as
defined by Type IV of Specification D1193.
8.3 Standard Test Solution—To prepare 1 L of 1 M sodium chloride (NaCl) solution, dissolve 58.45 g sodium chloride (NaCl)
in purified water to a total solution volume of 1 L. The solution can be made up in bulk and stored for one month at room
temperature.
8.4 Purging Gas—Nitrogen gas of minimum 99.99 % purity should be used.
9. Applied Potential
9.1 Standard Potential—An anodic potential of 700 mV versus SCE (25°C) is used. This has been found appropriate for most
stainless steels (1).
9.2 Alternative Potential:
9.2.1 If uncertainty exists concerning whether the standard potential is sufficiently high to obtain the potential independent CPT,
a test at 800 mV versus SCE (25°C) may be performed. A significant deviation between the CPT obtained at 700 mV and 800 mV
will indicate a need for a reevaluation and new choice of potential.
NOTE 4—Using a lower potential than the standard potential of 700 mV versus SCE (25°C) is fully acceptable, provided the determined CPT still is
potential independent. To change the measurement range provided by the standard test conditions, a new test solution composition will have to be chosen.
Following the choice of test solution, a test potential that ensures the determination of a potential independent CPT will have to be determined.
9.2.2 Evaluation of differences in obtained CPT at the two potentials should take into account the repeatability of the test
method. The homogeneity of the material used for the two different potentials shall also be considered before an alternative
potential is used.
10. Procedure
10.1 Sample Mounting, Cleaning and Placement:
10.1.1 The recommendations given in Practice G1 are to be followed, where applicable, unless otherwise stated in this
procedure.
10.1.2 Clean the specimen just before immersion in the electrolyte by degreasing with a suitable detergent, rinsing in purified
water, followed by ethanol or similar solvent, and air drying. After degreasing, handle the specimen with clean gloves, soft clean
tongs, or equivalent preventive measures, to avoid surface contamination.
10.2 Test Solution Preparation:
10.2.1 Prepare the test solution according to Section 8.
10.2.2 Bring the solution to an initial temperature at or below 3°C.
10.3 Mount the specimen in the specimen holder.
10.4 Place the specimen, counter electrode, salt bridge probe, and other components in the empty test cell.
10.5 Fill the cell with cold (≤3°C) solution.
10.6 Ensure that the salt bridge is filled with the test solution and is free of air bubbles, particularly in the restricted space at
the tip. A lugging probe should contain a wick, or equivalent device, to ensure electric contact even when small gas bubbles are
formed during the test.
10.7 Commence bubbling nitrogen gas through the solution.
Reagent Chemicals, American Chemical Society Specifications, , American Chemical Society, Washington, DC. For suggestions on the testing of reagents not listed by
the American Chemical Society, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia and National
Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville, MD.
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NOTE 5—The purpose of the purging gas is to enhance the stirring of the test solution. No reduction in the oxygen level of the test solution is intended.
Presence or absence of oxygen has been shown not to affect the test result (1).
10.8 Connect the electrodes to the potentiostat and data recording device and the connections for temperature measurement and
control. Let the system cool and stabilize at 0 6 1°C for a minimum of 10 min.
10.9 Record the open circuit potential (OCP) of the test specimen shortly before the test is begun.
10.10 Set the data acquisition for electrode current and solution temperature. The sampling rate shall correspond to a minimum
of two measurements every minute to follow solution temperature and the variation of the current on the working electrode.
10.11 Apply the constant anodic potential to the working electrode.
10.12 The potential shall be applied for 60 6 5 s before the temperature increase.
10.13 The temperature is then ramped at 1°C/min.
10.14 Continue measuring the temperature until the CPT has been determined or the maximum required temperature is reached.
The CPT is determined when the current density reaches 100 μA/cm and remains above this level for a minimum of 60 s.
Terminate the test and data acquisition after either of these limits has been reached.
10.15 Completion of Test—Dismount the specimen as soon as possible after test completion. Inspect the specimen to locate pits
(see Note 6). Rinse the specimen in water, clean with ethanol (95 % is suggested) or detergent, rinse again with water, and then
air dry.
NOTE 6—Pits may be difficult to locate if the test is stopped shortly after pitting initiation. Pits may be located based on leaking rust or by using a needle
to uncover pits hidden below a thin metal or oxide film. The lack of visible pitting may indicate that general corrosion has occurred, for example,
transpassive corrosion. However, o
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