1、Designation: A 262 02a (Reapproved 2008)Standard Practices forDetecting Susceptibility to Intergranular Attack in AusteniticStainless Steels1This standard is issued under the fixed designation A 262; the number immediately following the designation indicates the year oforiginal adoption or, in the c
2、ase of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1. Scope1.1 These p
3、ractices cover the following five tests:1.1.1 Practice AOxalic Acid Etch Test for Classificationof Etch Structures of Austenitic Stainless Steels (Sections 3 to7, inclusive),1.1.2 Practice BFerric SulfateSulfuric Acid Test forDetecting Susceptibility to Intergranular Attack in AusteniticStainless St
4、eels (Sections 8 to 14, inclusive),1.1.3 Practice CNitric Acid Test for Detecting Suscepti-bility to Intergranular Attack in Austenitic Stainless Steels(Sections 15 to 21, inclusive),1.1.4 Practice ECopperCopper SulfateSulfuric AcidTest for Detecting Susceptibility to Intergranular Attack inAustenit
5、ic Stainless Steels (Sections 22 to 31, inclusive), and1.1.5 Practice FCopperCopper Sulfate50 % SulfuricAcid Test for Detecting Susceptibility to Intergranular Attackin Molybdenum-Bearing Cast Austenitic Stainless Steels (Sec-tions 32 to 38, inclusive).1.2 The following factors govern the applicatio
6、n of thesepractices:1.2.1 Susceptibility to intergranular attack associated withthe precipitation of chromium carbides is readily detected in allsix tests.1.2.2 Sigma phase in wrought chromium-nickel-molybdenum steels, which may or may not be visible in themicrostructure, can result in high corrosio
7、n rates only in nitricacid.1.2.3 Sigma phase in titanium or columbium stabilizedalloys and cast molybdenum-bearing stainless alloys, whichmay or may not be visible in the microstructure, can result inhigh corrosion rates in both the nitric acid and ferric sulfate-sulfuric acid solutions.1.3 The oxal
8、ic acid etch test is a rapid method of identify-ing, by simple etching, those specimens of certain stainlesssteel grades that are essentially free of susceptibility tointergranular attack associated with chromium carbide precipi-tates. These specimens will have low corrosion rates in certaincorrosio
9、n tests and therefore can be eliminated (screened) fromtesting as “acceptable.”1.4 The ferric sulfatesulfuric acid test, the coppercoppersulfate50 % sulfuric acid test, and the nitric acid test are basedon weight loss determinations and, thus, provide a quantitativemeasure of the relative performanc
10、e of specimens evaluated. Incontrast, the coppercopper sulfate16 % sulfuric acid test isbased on visual examination of bend specimens and, therefore,classifies the specimens only as acceptable or nonacceptable.1.5 In most cases either the 15-h coppercopper sul-fate16 % sulfuric acid test or the 120-
11、h ferric sulfatesulfuricacid test, combined with the oxalic acid etch test, will providethe required information in the shortest time. All stainlessgrades listed in the accompanying table may be evaluated inthese combinations of screening and corrosion tests, exceptthose specimens of molybdenum-bear
12、ing grades (for example316, 316L, 317, and 317L), which represent steel intended foruse in nitric acid environments.1.6 The 240-h nitric acid test must be applied to stabilizedand molybdenum-bearing grades intended for service in nitricacid and to all stainless steel grades that might be subject to
13、endgrain corrosion in nitric acid service.1.7 Only those stainless steel grades are listed in Table 1 forwhich data on the application of the oxalic acid etch test and ontheir performance in various quantitative evaluation tests areavailable.1.8 Extensive test results on various types of stainless s
14、teelsevaluated by these practices have been published in Ref (1).21.9 The values stated in SI units are to be regarded asstandard. The inch-pound equivalents are in parentheses andmay be approximate.1.10 This standard does not purport to address all of thesafety problems, if any, associated with its
15、 use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use. (Specific precau-tionary statements are given in 5.6, 11.1.1, 11.1.9, and 35.1.)1These practices are under the jur
16、isdiction of ASTM Committee A01 on Steel,Stainless Steel and Related Alloys and are the direct responsibility of SubcommitteeA01.14 on Methods of Corrosion Testing.Current edition approved March 1, 2008. Published March 2008. Originallyapproved in 1943. Last previous edition approved in 2002 as A 26
17、2 02ae3.2The boldface numbers in parentheses refer to the list of references found at theend of these practices.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.2. Referenced Documents2.1 ASTM Standards:3A 370 Test Methods and Definit
18、ions for Mechanical Testingof Steel Products2.2 ISO Standard:4ISO 3651-2 Determination of Resistance to IntergranularCorrosion of Stainless SteelsPart 2: Ferritic, Austenitic,and Ferritic-Austenitic (Duplex) Stainless SteelsCorrosion Test in Media Containing Sulfuric AcidPRACTICE AOXALIC ACID ETCH T
19、EST FORCLASSIFICATION OF ETCH STRUCTURES OFAUSTENITIC STAINLESS STEELS 23. Scope3.1 The oxalic acid etch test is used for acceptance ofmaterial but not for rejection of material. This may be used inconnection with other evaluation tests to provide a rapidmethod for identifying those specimens that a
20、re certain to befree of susceptibility to rapid intergranular attack in these othertests. Such specimens have low corrosion rates in the varioushot acid tests, requiring from 4 to 240 h of exposure. Thesespecimens are identified by means of their etch structures,which are classified according to the
21、 following criteria:3.2 The oxalic acid etch test may be used to screenspecimens intended for testing in Practice BFerric Sulfate-Sulfuric Acid Test, Practice CNitric Acid Test, PracticeECopperCopper Sulfate16 % Sulfuric Acid Test, andPractice FCopperCopper Sulfate50 % Sulfuric Acid Test.3.2.1 Each
22、practice contains a table showing which classi-fications of etch structures on a given stainless steel grade areequivalent to acceptable, or possibly nonacceptable perfor-mance in that particular test. Specimens having acceptable etchstructures need not be subjected to the hot acid test. Specimensha
23、ving nonacceptable etch structures must be tested in thespecified hot acid solution.3.3 The grades of stainless steels and the hot acid tests forwhich the oxalic acid etch test is applicable are listed in Table2.3.4 Extra-lowcarbon grades, and stabilized grades, such as304L, 316L, 317L, 321, and 347
24、, are tested after sensitizingheat treatments at 650 to 675C (1200 to 1250F), which is therange of maximum carbide precipitation. These sensitizingtreatments must be applied before the specimens are submittedto the oxalic acid etch test. The most commonly used sensitiz-ing treatment is1hat675C (1250
25、F).4. Apparatus4.1 Source of Direct CurrentBattery, generator, or recti-fier capable of supplying about 15 V and 20 A.4.2 AmmeterRange 0 to 30 A (Note 1).4.3 Variable Resistance (Note 1).4.4 CathodeA cylindrical piece of stainless steel or,preferably, a 1-qt (0.946-L) stainless steel beaker.3For ref
26、erenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4Available from International Organization for Standardization (ISO),
27、 1 rue deVaremb, Case postale 56, CH-1211, Geneva 20, Switzerland, http:/www.iso.ch.TABLE 1 Application of Evaluation Tests for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless SteelsNOTE 1For each corrosion test, the types of susceptibility to intergranular attack detected a
28、re given along with the grades of stainless steels in whichthey may be found. These lists may contain grades of steels in addition to those given in the rectangles. In such cases, the acid corrosion test is applicable,but not the oxalic acid etch test.NOTE 2The oxalic acid etch test may be applied t
29、o the grades of stainless steels listed in the rectangles when used in connection with the test indicatedby the arrow.OXALIC ACID ETCH TESTAISIA: 304, 304L AISI: 304, 304L, 316, 316L,317, 317LAISI: 201, 202, 301, 304,304L, 304H, 316, 316L,316H, 317, 317L, 321, 347ACI: CF-3M, CF-8M,ACIB: CF-3, CF-8 A
30、CI: CF-3, CF-8, CF-3M,CF-8MNitric Acid TestC(240hinboiling solution)Ferric SulfateSulfuric Acid Test(120 h in boiling solution)CopperCopper SulfateSulfuric Acid Test (15 hin boiling solution)CopperCopper Sulfate50 % Sulfuric AcidTesting Boiling SolutionChromium carbide in: 304,304L, CF-3, CF-8Chromi
31、um carbide and sigmaphase in:D316, 316L, 317,317L, 321, 347, CF-3M, CF-8MEnd-grain in: all gradesChromium carbide in: 304,304L, 316, 316L, 317, 317L,CF-3, CF-8Chromium carbide and sigmaphase in: 321, CF-3M, CF-8MEChromium carbide in: 201,202, 301, 304, 304L, 316,316L, 317, 317L, 321, 347Chromium car
32、bide in: CF-3M,CF-8MAAISI: American Iron and Steel Institute designations for austenitic stainless steels.BACI: Alloy Casting Institute designations.CThe nitric acid test may be also applied to AISI 309, 310, 348, and AISI 410, 430, 446, and ACI CN-7M.DMust be tested in nitric acid test when destine
33、d for service in nitric acid.ETo date, no data have been published on the effect of sigma phase on corrosion of AISI 347 in this test.A 262 02a (2008)24.5 Large Electric ClampTo hold specimen to be etched.4.6 Metallurgical MicroscopeFor examination of etchedmicrostructures at 250 to 500 diameters.4.
34、7 Electrodes of the Etching CellThe specimen to beetched is made the anode, and a stainless steel beaker or a pieceof stainless steel as large as the specimen to be etched is madethe cathode.4.8 ElectrolyteOxalic acid, (H2C2O42H2O), reagentgrade, 10 weight % solution.NOTE 1The variable resistance an
35、d the ammeter are placed in thecircuit to measure and control the current on the specimen to be etched.5. Preparation of Test Specimens5.1 CuttingSawing is preferred to shearing, especially onthe extra-lowcarbon grades. Shearing cold works adjacentmetal and affects the response to subsequent sensiti
36、zation.Microscopical examination of an etch made on a specimencontaining sheared edges, should be made on metal unaffectedby shearing. A convenient specimen size is 25 by 25 mm (1 by1 in.).5.2 The intent is to test a specimen representing as nearly aspossible the surface of the material as it will b
37、e used in service.Therefore, the preferred sample is a cross section including thesurface to be exposed in service. Only such surface finishingshould be performed as is required to remove foreign materialand obtain a standard, uniform finish as described in 5.3. Forvery heavy sections, specimens sho
38、uld be machined to repre-sent the appropriate surface while maintaining reasonablespecimen size for convenient testing. Ordinarily, removal ofmore material than necessary will have little influence on thetest results. However, in the special case of surface carburiza-tion (sometimes encountered, for
39、 instance, in tubing or castingswhen lubricants or binders containing carbonaceous materialsare employed) it may be possible by heavy grinding ormachining to completely remove the carburized surface. Suchtreatment of test specimens is not permissible, except in testsundertaken to demonstrate such ef
40、fects.5.3 PolishingOn all types of materials, cross sectionalsurfaces should be polished for etching and microscopicalexamination. Specimens containing welds should include baseplate, weld heat-affected zone, and weld metal. Scale should beremoved from the area to be etched by grinding to an 80- or1
41、20-grit finish on a grinding belt or wheel without excessiveheating and then polishing on successively finer emery papers,No. 1,12 ,10 ,20 , and30 , or finer. This polishing operationcan be carried out in a relatively short time since all largescratches need not be removed. Whenever practical, a pol
42、ishedarea of 1 cm2or more is desirable. If any cross-sectionaldimension is less than 1 cm, a minimum length of 1 cm shouldbe polished. When the available length is less than 1 cm, a fullcross section should be used.5.4 Etching SolutionThe solution used for etching isprepared by adding 100 g of reage
43、nt grade oxalic acid crystals(H2C2O42H2O) to 900 mL of distilled water and stirring untilall crystals are dissolved.5.5 Etching ConditionsThe polished specimen should beetched at 1 A/cm2for 1.5 min. To obtain the correct currentdensity:5.5.1 The total immersed area of the specimen to be etchedshould
44、 be measured in square centimetres, and5.5.2 The variable resistance should be adjusted until theammeter reading in amperes is equal to the total immersed areaof the specimen in square centimetres.5.6 Etching Precautions:5.6.1 CautionEtching should be carried out under aventilated hood. Gas, which i
45、s rapidly evolved at the electrodeswith some entrainment of oxalic acid, is poisonous andirritating to mucous membranes.5.6.2 A yellow-green film is gradually formed on thecathode. This increases the resistance of the etching cell. Whenthis occurs, the film should be removed by rinsing the inside of
46、the stainless steel beaker (or the steel used as the cathode) withan acid such as 30 % HNO3.5.6.3 The temperature of the etching solution graduallyincreases during etching. The temperature should be keptbelow 50C by alternating two beakers. One may be cooled intap water while the other is used for e
47、tching. The rate ofheating depends on the total current (ammeter reading) passingthrough the cell. Therefore, the area etched should be kept assmall as possible while at the same time meeting the require-ments of desirable minimum area to be etched.5.6.4 Immersion of the clamp holding the specimen i
48、n theetching solution should be avoided.5.7 RinsingFollowing etching, the specimen should bethoroughly rinsed in hot water and in acetone or alcohol toavoid crystallization of oxalic acid on the etched surface duringdrying.5.8 On some specimens containing molybdenum (AISI 316,316L, 317, 317L), which
49、 are free of chromium carbidesensitization, it may be difficult to reveal the presence of stepstructures by electrolytic etching with oxalic acid. In suchcases, an electrolyte of a 10 % solution of ammonium persul-fate, (NH4)2S2O8, may be used in place of oxalic acid. An etchof 5 or 10 min at 1 A/cm2in a solution at room temperaturereadily develops step structures on such specimens.TABLE 2 Applicability of Etch TestAISI Grade No. ACI Grade No.Practice BFerric SulfateSulfuric Acid Test 304, 304L, 316, 316L, 317
