ASTM A262-15(2021)
(Practice)Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels
Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels
ABSTRACT
This specification covers the standard practices for detecting susceptibility to intergranular attack in austenitic stainless steels. These practices include five intergranular corrosion tests, namely: (1) oxalic acid etch test for classification of etch structures of austenitic stainless steels; (2) ferric sulfate-sulfuric acid test, (3) nitric acid test and (4) copper-copper sulfate-sulfuric acid test for detecting susceptibility to intergranular attack in austenitic stainless steels; and (5) copper-copper sulfate-50% sulfuric acid test for detecting susceptibility to intergranular attack in molybdenum-bearing cast austenitic stainless steels. Methods for preparing the test specimens, rapid screening tests, apparatus setup and testing procedures, and calculations and report contents are described for each testing practice. The etch structure types used to classify the specimens are: step structure, dual structure, ditch structure, isolated ferrite, interdendritic ditches, end-grain pitting I, and end-grain pitting II.
SIGNIFICANCE AND USE
6.1 Use of the etch test allows rapid acceptance of specific lots of material without the need to perform time-consuming and costly hot acid immersion tests on those lots.
SCOPE
1.1 These practices cover the following five tests:
1.1.1 Practice A—Oxalic Acid Etch Test for Classification of Etch Structures of Austenitic Stainless Steels (Sections 4 to 13, inclusive),
1.1.2 Practice B—Ferric Sulfate-Sulfuric Acid Test for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels (Sections 14 to 25, inclusive),
1.1.3 Practice C—Nitric Acid Test for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels (Sections 26 to 36, inclusive),
1.1.4 Practice E—Copper–Copper Sulfate–Sulfuric Acid Test for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels (Sections 37 to 46, inclusive), and
1.1.5 Practice F—Copper–Copper Sulfate–50 % Sulfuric Acid Test for Detecting Susceptibility to Intergranular Attack in Molybdenum-Bearing Austenitic Stainless Steels (Sections 47 to 58, inclusive).
1.2 The Oxalic Acid Etch Test is a rapid method of identifying, by simple etching, those specimens of certain stainless steel grades that are essentially free of susceptibility to intergranular attack associated with chromium carbide precipitates. These specimens will have low corrosion rates in certain corrosion tests and therefore can be eliminated (screened) from testing as “acceptable.” The etch test is applicable only to those grades listed in the individual hot acid tests and classifies the specimens either as “acceptable” or as “suspect.”
1.3 The ferric sulfate-sulfuric acid test, the copper–copper sulfate–50 % sulfuric acid test, and the nitric acid test are based on weight loss determinations and, thus, provide a quantitative measure of the relative performance of specimens evaluated. In contrast, the copper–copper sulfate–16 % sulfuric acid test is based on visual examination of bend specimens and, therefore, classifies the specimens only as acceptable or nonacceptable.
1.4 The presence or absence of intergranular attack in these tests is not necessarily a measure of the performance of the material in other corrosive environments. These tests do not provide a basis for predicting resistance to forms of corrosion other than intergranular, such as general corrosion, pitting, or stress-corrosion cracking.
Note 1: See Appendix X1 for information regarding test selection.
1.5 The values stated in SI units are to be regarded as standard. The inch-pound equivalents are in parentheses and may be approximate.
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 limitatio...
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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: A262 − 15 (Reapproved 2021)
Standard Practices for
Detecting Susceptibility to Intergranular Attack in Austenitic
Stainless Steels
This standard is issued under the fixed designation A262; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope based on visual examination of bend specimens and, therefore,
classifies the specimens only as acceptable or nonacceptable.
1.1 These practices cover the following five tests:
1.4 The presence or absence of intergranular attack in these
1.1.1 Practice A—Oxalic Acid Etch Test for Classification
of Etch Structures ofAustenitic Stainless Steels (Sections 4 to tests is not necessarily a measure of the performance of the
material in other corrosive environments. These tests do not
13, inclusive),
provide a basis for predicting resistance to forms of corrosion
1.1.2 Practice B—Ferric Sulfate-Sulfuric Acid Test for De-
other than intergranular, such as general corrosion, pitting, or
tecting Susceptibility to Intergranular Attack in Austenitic
Stainless Steels (Sections 14 to 25, inclusive), stress-corrosion cracking.
1.1.3 Practice C—Nitric Acid Test for Detecting Suscepti-
NOTE 1—See Appendix X1 for information regarding test selection.
bility to Intergranular Attack in Austenitic Stainless Steels
1.5 The values stated in SI units are to be regarded as
(Sections 26 to 36, inclusive),
standard. The inch-pound equivalents are in parentheses and
1.1.4 Practice E—Copper–Copper Sulfate–Sulfuric Acid
may be approximate.
Test for Detecting Susceptibility to Intergranular Attack in
1.6 This standard does not purport to address all of the
Austenitic Stainless Steels (Sections 37 to 46, inclusive), and
safety concerns, if any, associated with its use. It is the
1.1.5 Practice F—Copper–Copper Sulfate–50% Sulfuric
responsibility of the user of this standard to establish appro-
Acid Test for Detecting Susceptibility to Intergranular Attack
priate safety, health, and environmental practices and deter-
in Molybdenum-Bearing Austenitic Stainless Steels (Sections
mine the applicability of regulatory limitations prior to use.
47 to 58, inclusive).
Some specific hazards statements are given in 10.1, 20.1.1,
1.2 The Oxalic Acid Etch Test is a rapid method of
20.1.9, 31.3, 34.4, 53.1.1, and 53.1.10.
identifying, by simple etching, those specimens of certain
1.7 This international standard was developed in accor-
stainless steel grades that are essentially free of susceptibility
dance with internationally recognized principles on standard-
to intergranular attack associated with chromium carbide
ization established in the Decision on Principles for the
precipitates. These specimens will have low corrosion rates in
Development of International Standards, Guides and Recom-
certain corrosion tests and therefore can be eliminated
mendations issued by the World Trade Organization Technical
(screened) from testing as “acceptable.” The etch test is
Barriers to Trade (TBT) Committee.
applicable only to those grades listed in the individual hot acid
tests and classifies the specimens either as “acceptable” or as
2. Referenced Documents
“suspect.”
2.1 ASTM Standards:
1.3 The ferric sulfate-sulfuric acid test, the copper–copper
A370Test Methods and Definitions for Mechanical Testing
sulfate–50%sulfuricacidtest,andthenitricacidtestarebased
of Steel Products
on weight loss determinations and, thus, provide a quantitative
A380/A380MPractice for Cleaning, Descaling, and Passi-
measureoftherelativeperformanceofspecimensevaluated.In
vation of Stainless Steel Parts, Equipment, and Systems
contrast, the copper–copper sulfate–16% sulfuric acid test is
D1193Specification for Reagent Water
E3Guide for Preparation of Metallographic Specimens
These practices are under the jurisdiction of ASTM Committee A01 on Steel,
StainlessSteelandRelatedAlloysandarethedirectresponsibilityofSubcommittee
A01.14 on Methods of Corrosion Testing. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Sept. 1, 2021. Published September 2021. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1943. Last previous edition approved in 2015 as A262–15. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/A0262-15R21. the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
A262 − 15 (2021)
2.2 ASME Code: E—Copper-CopperSulfate–16%SulfuricAcidTest,andPrac-
BPVCASME Boiler and Pressure Vessel Code, Section IX tice F—Copper-Copper Sulfate–50% Sulfuric Acid Test.
2.3 ACS Specifications:
4.4 Each of these other practices contains a table showing
ACS Reagent ChemicalsSpecifications and Procedures
which classifications of etch structures on a given stainless
2.4 ISO Standard:
steelgradeareequivalenttoacceptableorsuspectperformance
ISO 3651-2Determination of resistance to intergranular
in that particular test. Specimens having acceptable etch
corrosion of stainless steels — Part 2: Ferritic, austenitic
structuresneednotbesubjectedtothehotacidtest.Specimens
and ferritic-austenitic (duplex) stainless steels — Corro-
having suspect etch structures must be tested in the specified
sion test in media containing sulfuric acid
hot acid solution.
4.5 There are two classes of specimens to be considered:
3. Purity of Reagents
base metal, and process-affected metal.
3.1 Purity of Reagents—Reagent grade chemicals shall be
4.5.1 Process-affected metal contains any condition that
used in all tests. Unless otherwise indicated, it is intended that
affectsthecorrosionpropertiesofthematerialinanon-uniform
all reagents conform to the specifications of the Committee on
way, such as (but not limited to) welds; carburized. nitrided, or
AnalyticalReagentsoftheAmericanChemicalSociety where
oxidized surfaces; mechanical deformation; and areas affected
such specifications are available. Other grades may be used,
by heat. Base metal has none of these conditions.
provided it is first ascertained that the reagent is of sufficiently
4.5.2 Because Practices B, C, and F involve immersing the
high purity to permit its use without lessening the accuracy of
entire specimen and averaging the mass loss over the total
the test result.
specimenarea,andbecausewelding,carburization,mechanical
3.2 Purity of Water—Unless otherwise indicated, references
deformation, and the like affect only part of a specimen, the
to water shall be understood to mean reagent water as defined
presence of process-affected metal in a specimen can affect the
by Type IV of Specification D1193.
test result in an unpredictable way depending on the propor-
tions of the area affected.
PRACTICE A—OXALIC ACID ETCH TEST FOR
4.5.3 Ifthepresenceoftheseorotherlocalizedconditionsis
CLASSIFICATION OF ETCH STRUCTURES OF
a concern to the purchaser, then tests that do not average the
AUSTENITIC STAINLESS STEELS (1)
masslossoverthetotalspecimensurfacearea,suchasPractice
A,theOxalicAcidEtchTest,orPracticeE,theCopper–Copper
4. Scope
Sulfate–Sulfuric Acid Test for Detecting Susceptibility to
4.1 The Oxalic Acid Etch Test is used for acceptance of
Intergranular Attack in Austenitic Stainless Steels, should be
wrought or cast austenitic stainless steel material but not for
considered.
rejection of material. Use of A262 PracticeAas a stand-alone
test may reject material that the applicable hot acid test would 5. Summary of Practice
find acceptable; such use is outside the scope of this practice.
5.1 A specimen representative of the material to be evalu-
ated is polished to a specified finish and over-etched using
4.2 This test is intended to be used in connection with other
evaluation tests described in these practices to provide a rapid oxalic acid electrolytically. The etched specimen is then
examined using a metallurgical microscope. The etched struc-
method for identifying qualitatively those specimens that are
certain to be free of susceptibility to rapid intergranular attack ture is compared with reference photographs to determine
whether the material is acceptable or suspect. Suspect material
intheseothertests.Suchspecimenshavelowcorrosionratesin
the various hot acid tests which require from 15 to 240 h of is then subjected to the specified hot acid immersion test.
exposure. These specimens are identified by means of their
6. Significance and Use
etch structures, which are classified according to the criteria
given in Section 11.
6.1 Use of the etch test allows rapid acceptance of specific
lots of material without the need to perform time-consuming
4.3 The Oxalic Acid Etch Test may be used to screen
and costly hot acid immersion tests on those lots.
specimens intended for testing in Practice B—Ferric Sulfate-
Sulfuric Acid Test, Practice C—Nitric Acid Test, Practice
7. Apparatus
7.1 Etching Cell:
Available from American Society of Mechanical Engineers (ASME), ASME
7.1.1 An etching cell may be assembled using components
International Headquarters, Two Park Ave., New York, NY 10016-5990, http://
as described in this section. Alternatively, a commercial
www.asme.org.
4 electropolisher/etcher (as used for metallographic sample
Available from American Chemical Society (ACS), 1155 Sixteenth St., NW,
Washington, DC 20036, http://www.acs.org.
preparation) may be used for small specimens provided the
Available from International Organization for Standardization (ISO), ISO
current density requirement of 10.2 is met.
Central Secretariat, Chemin de Blandonnet 8, CP 401, 1214 Vernier, Geneva,
7.1.2 Source of Direct Current—Battery, generator, or rec-
Switzerland, https://www.iso.org.
tifier capable of supplying about 15 V and 20 A.
For suggestions on the testing of reagents not listed by theAmerican Chemical
Society, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset,
7.1.3 Ammeter—For direct current; used to measure the
U.K., and the United States Pharmacopeia and National Formulary, U.S. Pharma-
current on the specimen to be etched.
copeial Convention, Inc. (USPC), Rockville, MD.
7.1.4 Variable Resistance—Used to control the current on
The boldface numbers in parentheses refer to a list of references at the end of
this standard. the specimen to be etched.
A262 − 15 (2021)
7.1.5 Cathode—A stainless steel container, for example, a 10. Procedure
1L (1-qt) stainless steel beaker.
10.1 (Warning—Etching should be carried out under a
7.1.5.1 Alternate Cathode—Apiece of flat stainless steel at
ventilatedhood.Gas,whichisrapidlyevolvedattheelectrodes
least as large as the specimen surface.
with some entrainment of oxalic acid, is poisonous and
7.1.6 Electrical Clamp—To hold the specimen to be etched
irritating to mucous membranes.)
andtocompletetheelectricalcircuitbetweenthespecimenand
10.2 Etch the polished specimen at 1 A/cm for 1.5 min.
the power source such that the specimen is the anode of the
10.2.1 To obtain the correct specified current density:
cell.
10.2.1.1 Measurethetotalimmersedareaofthespecimento
7.1.7 Thepowersource,resistor,andammetermustbesized
be etched in square centimetres.
appropriately for providing and controlling the current as
10.2.1.2 Adjust the variable resistance until the ammeter
specified in 10.2 of this practice.
reading in amperes is equal to the total immersed area of the
7.1.8 As described, the electrolyte container is the cathode;
specimen in square centimetres.
it may be a stainless steel beaker or fabricated from stainless
steel such as by welding a section of tube or pipe to a flat plate 10.3 A yellow-green film is gradually formed on the cath-
ode.Thisincreasestheresistanceoftheetchingcell.Whenthis
or sheet. Alternatively, the electrolyte container may be glass
(or other non-conducting, corrosion resisting material) in lieu occurs, remove the film by rinsing the inside of the stainless
steelbeaker(orthesteelusedasthecathode)withanacidsuch
ofastainlesssteelcontainer,andthecathodemaybeaflatplate
orsheetofacorrosionresistingalloy.Inthislattercase,theflat as 30% HNO .
surface of the cathode must be at least as large as, facing, and
10.4 The temperature of the etching solution gradually
approximately centered on, the prepared surface of the speci-
increases during etching. Keep the temperature below 50°C.
men. Other configurations of the electrodes might not provide
This may be done by alternating two containers. One may be
uniform etching over the specimen surface. In any case, the
cooled in tap water while the other is used for etching.
sizeandshapeofthespecimendictatethesizeandconstruction
10.4.1 The rate of heating depends on the total current
of the etching cell and of the power source and controls. The
(ammeter reading) passing through the cell. Therefore, keep
overriding principle is that the etch needs to be uniform over
theareatobeetchedassmallaspossiblewhileatthesametime
the surface to be examined.
meeting the requirements of desirable minimum area to be
7.2 Metallurgical Microscope—For examination of etched etched.
microstructures at 250 to 500 diameters.
10.5 Avoid immersing the clamp holding the specimen in
the etching solution.
8. Reagents and Materials
10.6 Rinsing—Following etching, rinse the specimen thor-
8.1 Etching Solution (10 %)—Dissolve 100 g of reagent
oughly in hot water and then in acetone or alcohol to avoid
grade oxalic acid crystals (H C O ·2H O) in 900 mL of
2 2 4 2
crystallization of oxalic acid on the etched surface during
reagent water. Stir until all crystals are dissolved.
drying.
8.1.1 Alternate Etching Solution(See10.7)—Dissolve100g
10.7 It may be difficult to reveal the presence of step
of reagent grade ammonium persulfate ((NH ) S O)in
4 2 2 8
structures on some specimens containing molybdenum (AISI
900mL of reagent water. Stir until dissolved.
316, 316L, 317, 317L), which are free of chromium carbide
sensitization, by electrolytic etching with oxalic acid. In such
9. Sampling and Test Specimens
cases, an alternate electrolyte of ammonium persulfate may be
9.1 The specified hot acid test provides instructions for
used in place of oxalic acid. (See 8.1.1.) An etch for 5 or 10
sampling and for specimen preparation such as a se
...
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