1、Designation: G 108 94 (Reapproved 2004)e1Standard Test Method forElectrochemical Reactivation (EPR) for DetectingSensitization of AISI Type 304 and 304L Stainless Steels1This standard is issued under the fixed designation G 108; the number immediately following the designation indicates the year ofo
2、riginal adoption or, in the case 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.e1NOTEThe equation in Item X in 9.2 was corrected editorially in July 2
3、005.1. Scope1.1 This test method covers a laboratory procedure forconducting an 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 sensitizatio
4、n inthese steels (1, 2, 3).2This test method has found wideacceptance in studies 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 n
5、ickelbase alloys (4), but the test conditions and evaluation criteriaused were modified 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 p
6、urport to address all of thesafety concerns, if any, associated with its 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.2. Referenced Documents2.1 ASTM Standards:3
7、A 262 Practices for Detecting Susceptibility to Intergranu-lar Attack in Austenitic Stainless SteelsD 1193 Specification for Reagent WaterE3 Methods of Preparation of Metallographic SpecimensE7 Terminology Relating to MetallographyE112 Test Methods for Determining Average Grain SizeG1 Practice for P
8、reparing, Cleaning, and Evaluating Cor-rosion Test SpecimensG3 Practice for ConventionsApplicable to ElectrochemicalMeasurements in Corrosion TestingG5 Reference Test Method for Making Potentiostatic andPotentiodynamic Anodic Polarization MeasurementsG15 Terminology Relating to Corrosion and Corrosi
9、onTestingG28 Test Methods for Detecting Susceptibility to Inter-granular Attack 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. Terminolog
10、y3.1 Definitions of Terms Specific to This Standard:3.1.1 integrated charge (Q)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
11、peak of the anodic curve duringreactivation.3.1.3 normalized charge (Pa)the integrated 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 re
12、activation(EPR) test, the potential sweep from the passivation potentialreturning 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 potentiodynamic
13、sweep from the passive to the active regions of electrochemicalpotentials in a 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
14、Committee G01 onCorrosion of Metals and is the direct responsibility of Subcommittee G01.11 onElectrochemical Measurement in Corrosion Testing.Current edition approved Nov 1, 2004. Published November 2004. Originallyapproved in 1992. Last previous edition approved in 1999 as G 108 94 (1999).2The bol
15、dface numbers in parentheses refer to the list of references at the end ofthe text.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 pa
16、ge onthe ASTM website.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-ogy E7). Discrete particles located wit
17、hin 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 intergranu-lar versus intragranular pr
18、ecipitates.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 when the electrochemical potential is cha
19、nged 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 (solution annealed) steel polarizedunder the c
20、onditions 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. The EPR test is significantlyaffected
21、 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 Type 304 (5). Superficial carbide precip
22、itation 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 integral of current density below there
23、activation 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 normalized for both specimen size andg
24、rain 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 of differences in grain size.5. Signi
25、ficance 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 in studies of the effects of sensitiza
26、tion onintergranular corrosion and intergranular stress corrosioncracking behavior. The results of this test method correlatewith other test methods (for example, Practice A 262 and TestMethod G28) that are commonly used to assess sensitizationin stainless steels.5.2 The EPR test can also be used fo
27、r 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 the testconditions accelerate corrosion i
28、n 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 these particles in a sensitized micro-struc
29、ture 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 following an EPR test to determine the r
30、elativeproportion of corrosion sites associated with intergranularversus intragranular precipitates. Sites of intergranular attackwill appear similar to grain boundary ditching as defined inPractice A of Practices A 262.6. Apparatus6.1 The apparatus necessary for obtaining EPR data consistsof electr
31、onic 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 combination ofintergranular attack an
32、d pitting as illustrated at right (7).FIG. 2 Schematic Microstructures After EPR TestingG 108 94 (2004)e12be integrated into one instrument package or may be individualcomponents. Either form of instrumentation can provide ac-ceptable data.6.2 Typical apparatus, as illustrated in Fig. 3, shall consi
33、st 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 ac-cordance with 4.2 of Practice G
34、5with the following refine-ments: the potentiostat shall control the potential within 65mV accuracy over the range of potential and current densityencountered in the EPR measurements. The potentiostat shallbe operable in a potential range of 600 to +500 mV(SCE) anda current density range of 1 A to 1
35、00 mA/cm2. The appliedpotential is changed either automatically or manually in thefollowing 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-vation) at a voltage scan rate of 1.67
36、 mV/s (6 V/h).6.2.2 Potential Measuring InstrumentsRequirementsshall be in accordance with 4.3 of Practice G5except that thepotential range is as stated above.6.2.3 Current Measuring InstrumentsRequirements shallbe in accordance with 4.4 of Practice G5. However, currentmeasurements are essential for
37、 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 20 mA/cm2rarely have been reported.6.2.4 Curre
38、nt 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 of measuring chargesfrom 0.001 to 2 coulombs
39、. 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 measurements are available in adigitized format, an
40、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 Practice G5. A deareation tube is not required andonly one counter electrode is required for EPR testing. Asuitable cell and electrode arrangement i
41、s shown in Fig. 4.6.2.6 Electrode HolderRequirements shall be in accor-dance with 4.6 of Practice G5or 4.2.1 of Test Method G61.The requirements for the working electrode (specimen) andcounter electrode holders are that the holders be made of aninert material and any seals must not allow leakage of
42、theelectrolyte. When using the Practice 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 Practice G5except that only onecounter electrode is necessary for EPR testing. How
43、ever, 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 ElectrodeRequirements are inaccordance or equivalent to 4.7.3 of Pract
44、ice 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 in-stance, but not limited to, a user and a supplier.7.1.2 Sp
45、ecimens 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 completely immersed but the connection tothe electrode holder is
46、 not immersed.FIG. 4 Schematic Diagram of an Electrochemical Cell for EPRTestingG 108 94 (2004)e137.2 Sensitization of Test SpecimensSpecimens can begiven a sensitizing treatment when it is desired to assess theinfluence of a thermal exposure during fabrication on corrosionresistance.7.2.1 Specimens
47、 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 to the use and conditions of a sensitizationtreatment. The
48、 most common sensitizing treatment is1hat675C (1250F) according to 15.3 of Practices A 262.7.2.2 Heat treatment, particularly carburization, may alterthe surface to be tested and may invalidate the EPR test results.Precautions shall be taken to ensure that the specimen surfaceis representative of th
49、e product form in service. Refer toSection 6 of Practice G1for descaling procedures and 7.3 aswell as Section 5 of Practices A 262 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 thermocouplesneeded to obtain a reliable reading for all specimens
