ASTM G150-99
(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
SCOPE
1.1 This test method describes 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).
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Designation: G 150 – 99
Standard Test Method for
Electrochemical Critical Pitting Temperature Testing of
Stainless Steels
This standard is issued under the fixed designation G 150; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope E 177 Practice for Use of the Terms Precision and Bias in
ASTM Test Methods
1.1 This test method covers a procedure for the evaluation
E 691 Practice for Conducting an Interlaboratory Study to
of the resistance of stainless steel and related alloys to pitting
Determine the Precision of a Test Method
corrosion based on the concept of the determination of a
G 1 Practice for Preparing, Cleaning, and Evaluating Cor-
potential independent critical pitting temperature (CPT).
rosion Test Specimens
1.2 This test methods applies to wrought and cast products
G 3 Practice for Conventions Applicable to Electrochemical
including but not restricted to plate, sheet, tubing, bar, forgings,
Measurements in Corrosion Testing
and welds, (see Note 1).
G 5 Reference Test Method for Making Potentiostatic and
NOTE 1—Examples of CPT measurements on sheet, plate, tubing, and 6
Potentiodynamic Anodic Polarization Measurements
welded specimens for various stainless steels can be found in Ref. (1).
G 15 Terminology Relating to Corrosion and Corrosion
See the research report.
Testing
1.3 The standard parameters recommended in this test
G 46 Guide for Examination and Evaluation of Pitting
method are suitable for characterizing the CPT of austenitic
Corrosion
stainless steels and other related alloys with a corrosion
G 107 Guide for Formats for Collection and Compilation of
resistance ranging from that corresponding to solution an-
Corrosion Data for Metals for Computerized Database
nealed UNS S31600 (Type 316 stainless steel) to solution
Input
annealed UNS S31254 (6 % Mo stainless steel).
3. Terminology
1.4 This test method may be extended to stainless steels and
other alloys related to stainless steel that have a CPT outside
3.1 Definitions:
the measurement range given by the standard parameters
3.1.1 critical pitting temperature (CPT)—the lowest tem-
described in this test method. Appropriate test potential and
perature on the test surface at which stable propagating pitting
solution must then be determined.
occurs under specified test conditions indicated by a rapid
1.5 The values stated in SI units are to be regarded as
increase beyond a set limit of the measured anodic current
standard.
density of the specimen.
1.6 This standard does not purport to address all of the
3.1.2 pitting potential range—the range of measured poten-
safety concerns, if any, associated with its use. It is the
tials where pitting is initiated. This potential range only exists
responsibility of the user of this standard to establish appro-
above the minimum critical pitting temperature; see also
priate safety and health practices and determine the applica-
Appendix X1.
bility of regulatory limitations prior to use.
3.1.3 potential independent CPT— the CPT determined at a
potential above the pitting potential range, but below the
2. Referenced Documents
transpassive potential; see also Appendix X1.
2.1 ASTM Standards:
3.1.4 potential dependent CPT—the CPT determined at a
D 1193 Specification for Reagent Water
potential within the pitting potential range of the tested
material; see also Appendix X1.
3.1.5 temperature ramp—the rate (°C/min) at which the test
This test method is under the jurisdiction of G-1 on Corrosion of Metals and is
temperature is increased during the test.
the direct responsibility of Subcommittee G01.11 on Electrochemical Measurements
in Corrosion Testing. 3.2 sign conventions—the sign conventions used in this
Current edition approved Jan. 10, 1999. Published March 1999. Originally
procedure are in agreement with Practice G 3.
published as G 150 – 97. Last previous edition G 150 – 97.
The boldface numbers in parenthesis refer to the list of references at the end of
this standard.
3 5
See ASTM Research Report G01-1017. Available from ASTM Headquarters. Annual Book of ASTM Standards, Vol 14.02.
4 6
Annual Book of ASTM Standards, Vol 11.01. Annual Book of ASTM Standards, Vol 03.02.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
G 150–99
3.3 Unless otherwise stated, this test method uses the 6. Apparatus
general terminology relating to corrosion and corrosion testing
6.1 The apparatus necessary for determining the CPT con-
as defined in Terminology G 15.
sists of instruments for measuring electronic signals, a tem-
perature controlling apparatus, a specimen holder, and a test
4. Summary of Test Method
cell. The instruments for measuring electronic signals may be
4.1 The test method determines the potential independent
integrated into one instrument package or may be individual
critical pitting temperature (CPT) by way of a potentiostatic
components. Either form of instrumentation can provide ac-
technique using a temperature scan and a specimen holder that
ceptable data. Typical test equipment consists of the following:
is designed to eliminate the occurrence of crevice corrosion
( 1) potentiostat (2) potential measuring instrument (3) current
(see Fig. 1). The specimen is exposed, either entirely or in part,
measuring instrument ( 4) temperature controller (5) tempera-
depending on test cell configuration to a 1M NaCl solution,
ture measuring instrument (6) test cell ( 7) specimen holder,
initially at 0°C. After an initial temperature stabilization
and (8) electrodes.
period, the solution is heated at a rate of 1°C/min. About 60 s
6.2 Potentiostat—The potentiostat shall be able to apply the
before the temperature scan is commenced, the specimen is
constant potential to within 1 mV at a current density of 10
anodically polarized to a potential above the pitting potential
mA/cm . The applied potential is changed either automatically
range. This potential is held constant during the whole tem-
or manually by shifting the potential from the open circuit
perature scan. A potential of 700 mV versus SCE (25°C) has
potential to another more noble potential.
been found suitable for most stainless steels. The current is
6.3 Potential Measuring Instrument—Requirements shall
monitored during the temperature scan, and the CPT is defined
be in accordance with the section on Potential Measuring
as the temperature at which the current increases rapidly, which
Instruments in Test Method G 5.
for practical reasons is defined as the temperature at which the
2 6.4 Current Measuring Instruments—An instrument that is
current density exceeds 100 μA/cm for 60 s. Pitting on the
capable of measuring a current accurately to within 5 % of the
specimen is confirmed visually after the test.
actual value. The typical current densities encountered during
2 2
the CPT test are in the range of 1 μA/cm to 10 mA/cm .
5. Significance and Use
6.5 Temperature Controller:
5.1 This test method provides a prediction of the resistance
6.5.1 Thermostat equipment is required that can provide
to stable propagating pitting corrosion of stainless steels and
cooling and heating of the test solution in the temperature
related alloys in a standard medium (see Note 1). The CPT test
range from 0°C to approximately 100°C. Further, the tempera-
can be used for product acceptance, alloy development studies,
ture controller is used to provide controlled heating, which
and manufacturing control. In the case of product acceptance,
gives the test solution temperature a temperature increase rate
the supplier and user must agree upon the preconditioning of
of 1°C/min in the range from 0°C to approximately 100°C.
the specimen with regard to surface finish. The test is not
6.5.2 Above 10°C, the average rate of temperature change
intended for design purposes since the test conditions acceler-
of the test solution shall be 1.0 6 0.3°C/min, where the average
ate corrosion in a manner that does not simulate any actual
is calculated over a temperature range of 10°C.
service environment.
5.2 Another method to determine the potential independent 6.6 Temperature Measurement Instrumentation, shall be
CPT with an electrochemical technique has been discussed in capable of measuring the temperature of the test solution with
the literature (1-4). This test method involves a potentiody- an accuracy of 6 0.4°C.
namic (potential sweep) procedure performed on specimens at
6.7 Test Cell:
different temperatures. A comparison (2) of the test method
6.7.1 Option 1, G5 Type—The test cell should be similar to
described in this test method and the potentiodynamic tech-
the one described in Test Method G 5. Other similar polariza-
nique has indicated no difference in the test result obtained.
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—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.
FIG. 1 Determination of CPT 6.8 Specimen Holder:
G 150–99
6.8.1 Any part of the specimen holder coming in contact such specifications are available. Other grades may be used,
with the test solution during testing shall be made of an inert provided it is first ascertained that the reagent is of sufficiently
material, and any seal shall not allow leakage of electrolyte. high purity to permit its use without lessening the accuracy of
the determination.
6.8.2 The specimen holder shall have a design that ensures
8.2 Purity of Water—Unless otherwise indicated, references
no occurrence of crevice corrosion at the contact area between
to purified water shall be understood to mean reagent water as
specimen holder and specimen.
defined by Type IV of Specification D 1193.
6.8.3 Two examples of specimen holder designs in accor-
8.3 Standard Test Solution—To prepare 1 L of 1 M sodium
dance with this standard are shown in Appendix X2 and
chloride (NaCl) solution, dissolve 58.45 g sodium chloride
Appendix X3. The major difference between the specimen
(NaCl) in purified water to a total solution volume of 1 L. The
holder designs lies in the allowable specimen geometry and the
solution can be made up in bulk and stored for one month at
number of surfaces on the specimen that are being tested
room temperature.
simultaneously.
8.4 Purging Gas—Nitrogen gas of minimum 99.99 % purity
6.9 Electrodes:
should be used.
6.9.1 Auxiliary (Counter) Electrode—Requirements shall
be in accordance with the section Auxiliary Electrodes in Test
9. Applied Potential
Method G 5 with the exception that only one counter electrode
9.1 Standard Potential—An anodic potential of 700 mV
is necessary for CPT testing. The electrode material shall be of
versus SCE (25°C) is used. This has been found appropriate for
a type which can be considered inert under the test conditions.
most stainless steels (1).
6.9.2 Reference Electrode—The reference electrode shall be
9.2 Alternative Potential:
kept at room temperature outside the actual test cell. The
9.2.1 If uncertainty exists concerning whether the standard
reference electrode shall be capable of ensuring a constant
potential is sufficiently high to obtain the potential independent
reference potential within 65 mV during the entire test
CPT, a test at 800 mV versus SCE (25°C) may be performed.
procedure (see Note 2). Electrical contact to the test solution
A significant deviation between the CPT obtained at 700 mV
shall be provided by the use of a luggin capillary placed in the
and 800 mV will indicate a need for a reevaluation and new
test solution. Requirements shall otherwise be in accordance
choice of potential.
with the section on Reference Electrode in Test Method G 5.
NOTE 4—Using a lower potential than the standard potential of 700 mV
NOTE 2—It may be difficult to ensure a fully constant reference versus SCE (25°C) is fully acceptable, provided the determined CPT still
potential due to the large variations in temperature of the test solution; is potential independent. To change the measurement range provided by
therefore, the allowable is 65 mV. This does, however, not affect the the standard test conditions, a new test solution composition will have to
measured potential independent CPT (1). 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.
7. Test Specimens
9.2.2 Evaluation of differences in obtained CPT at the two
7.1 Finish—Any geometry and surface finish (see Note 3)
potentials should take into account the repeatability of the test
compatible with the chosen specimen holder as specified in 6.8
method. The homogeneity of the material used for the two
may be used.
different potentials shall also be considered before an alterna-
NOTE 3—The state of the surface may be dependent on the time and
tive potential is used.
location of storage between the final mechanical or chemical surface
treatment and testing. The time and location of storage may, therefore, in
10. Procedure
some situations be considered an integral part of the surface finish.
10.1 Sample Mounting, Cleaning and Placement:
7.2 Sampling—When using this test method to meet product
10.1.1 The recommendations given in Practice G 1 are to be
acceptance criteria, the means of sampling of a test specimen
followed, where applicable, unless otherwise stated in this
shall be decided by agreement between the parties involved.
procedure.
7.3 Test Area—A minimum test area of 1 cm shall be used.
10.1.2 Clean the specimen just before immersion in the
7.4 Specimens removed from a work piece or component by
electrolyte by degreasing with a suitable detergent, rinsing in
shearing, cutting, burning, and so forth shall have the affected purified water, followed by
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