1、Designation: G108 94 (Reapproved 2010)Standard Test Method forElectrochemical Reactivation (EPR) for DetectingSensitization of AISI Type 304 and 304L Stainless Steels1This standard is issued under the fixed designation G108; the number immediately following the designation indicates the year oforigi
2、nal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers a laboratory procedure forconducting an
3、electrochemical reactivation (EPR) test on AISIType 304 and 304L (UNS No. S30400 and S30403, respec-tively) stainless steels. This test method can provide a nonde-structive means of quantifying the degree of sensitization inthese steels (1, 2, 3).2This test method has found wideacceptance in studies
4、 of the effects of sensitization on inter-granular corrosion and intergranular stress corrosion crackingbehavior (see Terminology G15). The EPR technique has beensuccessfully used to evaluate other stainless steels and nickelbase alloys (4), but the test conditions and evaluation criteriaused were m
5、odified in each case from those cited in this testmethod.1.2 The values stated in SI units are to be regarded as thestandard. The inch-pound units given in parentheses are forinformation only.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It
6、 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.2. Referenced Documents2.1 ASTM Standards:3A262 Practices for Detecting Susceptibility to IntergranularAttack in Austenitic
7、 Stainless SteelsD1193 Specification for Reagent WaterE3 Guide for Preparation of Metallographic SpecimensE7 Terminology Relating to MetallographyE112 Test Methods for Determining Average Grain SizeG1 Practice for Preparing, Cleaning, and Evaluating Corro-sion Test SpecimensG3 Practice for Conventio
8、ns Applicable to ElectrochemicalMeasurements in Corrosion TestingG5 Reference Test Method for Making Potentiostatic andPotentiodynamic Anodic Polarization MeasurementsG15 Terminology Relating to Corrosion and CorrosionTesting4G28 Test Methods for Detecting Susceptibility to Inter-granular Corrosion
9、in Wrought, Nickel-Rich, Chromium-Bearing AlloysG61 Test Method for Conducting Cyclic PotentiodynamicPolarization Measurements for Localized Corrosion Sus-ceptibility of Iron-, Nickel-, or Cobalt-Based Alloys3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 integrated charge (Q)
10、the charge measured, in cou-loumbs, during reactivation as given by the time integral ofcurrent density below the reactivation peak of the curve.3.1.2 maximum anodic current density (Ir)the currentdensity measured at the peak of the anodic curve duringreactivation.3.1.3 normalized charge (Pa)the int
11、egrated current nor-malized to the specimen size and grain size. Parepresents thecharge (in coulombs/cm2) of the grain-boundary area. Themethod for calculating Pais given in 9.2.3.1.4 reactivationin the electrochemical reactivation(EPR) test, the potential sweep from the passivation potentialreturni
12、ng to the corrosion potential.3.1.5 scan ratethe rate at which the electrical potentialapplied to a specimen in a polarization test is changed.4. Summary of Test Method4.1 The EPR test is accomplished by a potentiodynamicsweep from the passive to the active regions of electrochemicalpotentials in a
13、process referred to as reactivation. The EPR testmeasures the amount of charge associated with the corrosion ofthe chromium-depleted regions surrounding chromium carbide1This test method is under the jurisdiction of ASTM Committee G01 onCorrosion of Metals and is the direct responsibility of Subcomm
14、ittee G01.11 onElectrochemical Measurements in Corrosion Testing.Current edition approved May 1, 2010. Published May 2010. Originallyapproved in 1992. Last previous edition approved in 2004 as G10894(2004)1.DOI: 10.1520/G0108-94R10.2The boldface numbers in parentheses refer to a list of references a
15、t the end ofthis standard.3For referenced 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.4Withdrawn. The last approved versi
16、on of this historical standard is referencedon www.astm.org.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.precipitated particles. Most of these particles in a sensitizedmicrostructure are located at grain boundaries (see Terminol-o
17、gy E7). Discrete particles located within the grain (referred toas intragranular precipitates) will also contribute to the totalmeasured charge. Therefore, it is important to examine thealloy microstructure following an EPR test, to determine therelative proportion of corrosion site associated with
18、intergranu-lar versus intragranular precipitates.4.2 The chromium-depleted zones around carbide precipi-tates in sensitized steels are particularly susceptible to corro-sion in oxidizing acid solutions. Corrosion at chromium-depleted grain boundary sites causes a rapid rise in the currentdensity whe
19、n the electrochemical potential is changed from thepassive to the active region.4.3 A sensitized steel produces a curve similar to the activeportion of the polarization curve during the reactivation fromthe passive region back to the rest potential (Ecorr) as shown inFig. 1. A nonsensitized (solutio
20、n annealed) steel polarizedunder the conditions given in this test method will produce acurve with lower current densities than a sensitized steel.4.4 The EPR test results are readily reproducible, as long asthe electrolyte temperature, electrolyte composition, and scanrate are carefully controlled.
21、 The EPR test is significantlyaffected by the composition, thermomechanical condition andsurface finish of the specimen as well as the presence ofnon-metallic inclusions, that result in pitting of the etchedmicrostructure.NOTE 1Various cutting and grinding operations can promote sensiti-zation of Ty
22、pe 304 (5). Superficial carbide precipitation can occur duringcutting and grinding or during subsequent low temperature heat treat-ments, such as 24 h at 500C.4.5 The criteria used to distinguish between sensitized andsolution annealed samples are the activation charge density, Q(given by the time i
23、ntegral of current density below thereactivation peak of the curve), or the maximum anodic currentdensity, Ir, in the active state. Sensitized steels are easilyactivated and show higher Q and Irvalues than solutionannealed steels, that are not susceptible to intergranular corro-sion. The value Q is
24、normalized for both specimen size andgrain size.The value normalized in this fashion is called Paandrepresents the charge (in units of coulombs) per unit grain-boundary area. This normalization permits direct comparisonsof different heats of material that exhibit different Q valuessolely as a result
25、 of differences in grain size.5. Significance and Use5.1 This test method describes an EPR test method forquantitatively determining the relative degree of sensitizationin AISI Type 304 and 304L stainless steels. The EPR test hasfound wide use as a means to provide a numerical level ofsensitization
26、in studies of the effects of sensitization onintergranular corrosion and intergranular stress corrosioncracking behavior. The results of this test method correlatewith other test methods (for example, Practices A262 and TestMethods G28) that are commonly used to assess sensitizationin stainless stee
27、ls.5.2 The EPR test can also be used for product acceptance,service evaluation, regulatory statutes, and manufacturingcontrols providing that both the supplier and user have agreedupon appropriate acceptance criteria and a sensitizing treat-ment. The test is not intended for design purposes since th
28、e testconditions accelerate corrosion in a manner that does notsimulate any actual service environment.5.3 The EPR test involves the measurement of the amountof charge resulting from the corrosion of the chromium-depleted regions surrounding the precipitated chromium car-bide particles. Most of thes
29、e particles in a sensitized micro-structure are located at the grain boundaries. However, discreteparticles located within grains (referred to as intragranularprecipitates) will also contribute to the total measured charge.(See Fig. 2.) Therefore, it is important to examine the alloymicrostructure f
30、ollowing an EPR test to determine the relativeproportion of corrosion sites associated with intergranularversus intragranular precipitates. Sites of intergranular attackwill appear similar to grain boundary ditching as defined inPractice A of Practices A262.6. Apparatus6.1 The apparatus necessary fo
31、r obtaining EPR data consistsof electronic instruments and a test cell.These instruments mayFIG. 1 Schematic EPR Curves for Sensitized and SolutionizedAISI Type 304 Stainless SteelNOTEThe calculation of Pais based on the assumptions illustrated atleft. Mild cases of sensitization usually result in a
32、 combination ofintergranular attack and pitting as illustrated at right (7).FIG. 2 Schematic Microstructures After EPR TestingG108 94 (2010)2be integrated into one instrument package or may be individualcomponents. Either form of instrumentation can provide ac-ceptable data.6.2 Typical apparatus, as
33、 illustrated in Fig. 3, shall consist ofthe following: scanning potentiostat (or potentiostat/voltageramp generator combination), potential measuring instrument,current and current integration measuring instruments, and testcell and specimen holder.6.2.1 Scanning PotentiostatRequirements shall be in
34、 ac-cordance with 4.2 of Test Method G5 with the followingrefinements: the potentiostat shall control the potential within65 mV accuracy over the range of potential and currentdensity encountered in the EPR measurements. The poten-tiostat shall be operable in a potential range of 600 to +500mV (SCE)
35、 and a current density range of 1 Ato 100 mA/cm2.The applied potential is changed either automatically ormanually in the following manners:6.2.1.1 Shifting the potential from the open circuit potentialto a potential in the passive range, and6.2.1.2 Scanning back to the open circuit potential (reacti
36、-vation) at a voltage scan rate of 1.67 mV/s (6 V/h).6.2.2 Potential Measuring InstrumentsRequirementsshall be in accordance with 4.3 of Test Method G5 except thatthe potential range is as stated above.6.2.3 Current Measuring InstrumentsRequirements shallbe in accordance with 4.4 ofTest Method G5. H
37、owever, currentmeasurements are essential for passivation assessment andother intermediate checks of system stability. The currentsencountered in EPR for a specimen with the dimensions givenin 7.3 are in the range of 1 A to 100 mA/cm2. For samples ofless than 100 mm2test area, currents above about 2
38、0 mA/cm2rarely have been reported.6.2.4 Current Integration Measurement Instruments(Optional)Current integration, or charge, can be measuredby an electronic device incorporated into the potentiostat, or bya separate electronic device, such as a coulometer. If acoulometer is used, it shall be capable
39、 of measuring chargesfrom 0.001 to 2 coulombs. The use of a coulometer shall beconsidered optional. Charge can also be measured by using achart recorder, as illustrated in Fig. 3, to record a current versustime trace and then, subsequently, integrating it by variousmethods. When potentiostat measure
40、ments are available in adigitized format, an appropriate computer integration routinecan also be used to obtain a value for charge.6.2.5 EPR Test CellRequirements shall be in accordancewith 4.1 of Test Method G5. A deareation tube is not requiredand only one counter electrode is required for EPR tes
41、ting. Asuitable cell and electrode arrangement is shown in Fig. 4.6.2.6 Electrode HolderRequirements shall be in accor-dance with 4.6 ofTest Method G5 or 4.2.1 ofTest Method G61.The requirements for the working electrode (specimen) andcounter electrode holders are that the holders be made of aninert
42、 material and any seals must not allow leakage of theelectrolyte. When using the Test Method G5-type holder theworking electrode can be mounted as shown in Fig. 5 anddescribed in Appendix X1.6.2.7 Auxiliary (Counter) ElectrodesRequirements are inaccordance with 4.7.2 of Test Method G5 except that on
43、ly onecounter electrode is necessary for EPR testing. However, twoauxiliary electrodes can provide for a more uniform distribu-tion of current. Titanium or high-purity carbon may be used inplace of platinum for the counter electrode since it is alwaysthe cathode.6.2.8 Calomel Reference ElectrodeRequ
44、irements are inaccordance or equivalent to 4.7.3 of Test Method G5.7. Sampling, Test Specimens, and Test Units7.1 Sampling:7.1.1 When using this test method to meet product accep-tance criteria, the means of sampling of a test specimen shall bedecided by agreement between the parties involved; for i
45、n-stance, but not limited to, a user and a supplier.7.1.2 Specimens removed form a piece of AISI Type 304 or304Lsteel by shearing, cutting, burning, and so forth shall havethe affected edges removed by grinding or machining.FIG. 3 Schematic Diagram of an EPR Test ApparatusNOTEThe sample face is comp
46、letely immersed but the connection tothe electrode holder is not immersed.FIG. 4 Schematic Diagram of an Electrochemical Cell for EPRTestingG108 94 (2010)37.2 Sensitization of Test SpecimensSpecimens can begiven a sensitizing treatment when it is desired to assess theinfluence of a thermal exposure
47、during fabrication on corrosionresistance.7.2.1 Specimens may be tested in a condition simulatingthat of the product as installed in service. Specimens may bewelded or heat treated in as nearly the same manner as theproduct will experience in fabrication or service. The user andsupplier must agree t
48、o the use and conditions of a sensitizationtreatment. The most common sensitizing treatment is1hat675C (1250F) according to 15.3 of Practices A262.7.2.2 Heat treatment, particularly carburization, may alterthe surface to be tested and may invalidate the EPR test results.Precautions shall be taken to
49、 ensure that the specimen surfaceis representative of the product form in service. Refer toSection 6 of Practice G1 for descaling procedures and 7.3,aswell as Section 5 of Practices A262 for guidance in preparingspecimens.7.2.3 Expose specimens to be given a sensitization treat-ment prior to EPR testing in a furnace at the requiredtemperature and for the required time and then water-quench.Use a thermocouple and a timer to ensure that the entirespecimen cross sections are at the specified temperature for thespecified amount of time. The number of thermocoupl
