1、Designation: G 71 81 (Reapproved 2003)Standard Guide forConducting and Evaluating Galvanic Corrosion Tests inElectrolytes1This standard is issued under the fixed designation G 71; the number immediately following the designation indicates the year of originaladoption or, in the case of revision, the
2、 year of last revision. A number in parentheses indicates the year of last reapproval. A superscriptepsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide covers conducting and evaluating galvaniccorrosion tests to characterize the behavior of two diss
3、imilarmetals in electrical contact in an electrolyte under low-flowconditions. It can be adapted to wrought or cast metals andalloys.1.2 This guide covers the selection of materials, specimenpreparation, test environment, method of exposure, and methodfor evaluating the results to characterize the b
4、ehavior ofgalvanic couples in an electrolyte.NOTE 1Additional information on galvanic corrosion testing andexamples of the conduct and evaluation of galvanic corrosion tests inelectrolytes are given in Refs (1)2through (7).1.3 This standard does not purport to address all of thesafety concerns, if a
5、ny, 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:G1 Practice for Preparing, Cleaning, and Evaluatin
6、g Cor-rosion Test Specimens3G3 Practice for ConventionsApplicable to ElectrochemicalMeasurements in Corrosion Testing3G4 Guide for Conducting Corrosion Coupon Tests in FieldApplications3G16 Guide for Applying Statistics to Analysis of CorrosionData3G31 Practice for Laboratory Immersion Corrosion Tes
7、tingof Metals3G46 Guide for Examination and Evaluation of PittingCorrosion33. Significance and Use3.1 Use of this guide is intended to provide information onthe galvanic corrosion of metals in electrical contact in anelectrolyte that does not have a flow velocity sufficient to causeerosion-corrosion
8、 or cavitation.3.2 This standard is presented as a guide for conductinggalvanic corrosion tests in liquid electrolyte solutions, both inthe laboratory and in service environments. Adherence to thisguide will aid in avoiding some of the inherent difficulties insuch testing.4. Test Specimens4.1 Materi
9、alTest specimens should be manufactured fromthe same material as those used in the service application beingmodeled. Minor compositional or processing differences be-tween materials or between different heats can greatly affectthe results in some cases.4.2 Size and Shape:4.2.1 The size and shape of
10、the test specimens are dependenton restrictions imposed by the test location. When determiningmaterial behavior in the laboratory, it is advisable to use thelargest specimens permissible within the constraints of the testequipment. In general, the ratio of surface area to metal volumeshould be large
11、 in order to favor maximum corrosion loss perweight. Sufficient thickness should be employed, however, tominimize the possibility of perforation of the specimens duringthe test exposure. When modeling large components, the sizeof the specimens should be as large as practical. Whenmodeling smaller co
12、mponents, specimen size should be asclose as possible to that of the application being modeled.Surface area ratio in the test should be identical to theapplication being modeled. This ratio is defined as the surfacearea of one member of the couple divided by the surface areaof the other member of th
13、e couple. Only the area in contactwith the electrolyte (wetted area) is used in this calculation. Inlow-resistivity electrolytes, maintaining proximity between thematerials being coupled may be more important than maintain-ing the exact area ratio. Also, with some couples, such ascopper coupled to a
14、luminum, there may be effects of corrosionproducts washing from one electrode to another which mayhave to be considered in determining specimen placement.1This guide is under the jurisdiction of ASTM Committee G01 on Corrosion ofMetals and is the direct responsibility of Subcommittee G01.11 on Elect
15、rochemicalMeasurements in Corrosion Testing.Current edition approved Oct. 1, 2003. Published October 2003. Originallyapproved in 1981. Last previous edition approved in 1998 as G 71 81 (1998)e1.2The boldface numbers in parentheses refer to the list of references appended tothe practice.3Annual Book
16、of ASTM Standards, Vol 03.02.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.4.2.2 Laboratory tests are normally performed on rectangu-lar plates or on cylinders. When modeling service applications,the shapes of the couple members sh
17、ould approximate theshapes in the application. Frequently complex shapes aresimplified for testing purposes. The shape of the specimen ismore important in electrolytes of low conductivity, wherevoltage drop in the electrolyte is significant. In highly conduc-tive electrolytes, the shapes of the coup
18、le members maytherefore deviate somewhat from the shapes in the application.4.3 Specimen Preparation:4.3.1 The edges of the test specimens should be prepared soas to eliminate all sheared or cold-worked metal except thatcold-working introduced by stamping for identification. Shear-ing will, in some
19、cases, cause considerable attack. Therefore,specimens having sheared edges should not be used. The edgesshould be finished by machining or polishing. The slightamount of cold working resulting from machining will notintroduce any serious error.4.3.2 Specimens should be cleaned in accordance withPrac
20、tice G1, or else the specimen surface condition should besimilar to the application being modeled. The metallurgicalcondition of the specimens should be similar to the applicationbeing modeled. In all cases surface contamination, such as dirt,grease, oil, and thick oxides, should be removed prior to
21、weighing and exposure to the test environment.4.3.3 The specimen identification system must be one thatwill endure throughout the test period. Edge notches, drilledholes, stamped numbers, and tags are some of the methodsused for identification. The identification system must notinduce corrosion atta
22、ck in any way.4.4 Number of Specimens:4.4.1 The number of galvanic couples to be tested will bedetermined by whether or not one or more periodic specimenremovals are scheduled during the course of the test. As aminimum, duplicate and preferably triplicate specimens shouldbe tested for any given test
23、 period to determine the variabilityin the galvanic corrosion behavior. The effect of the number ofreplications on the application of the results is set forth inGuide G16.4.4.2 Control specimens should also be tested to providecorrosion rates of the individual metals and alloys withoutcoupling for c
24、omparisons. These specimens should be of thesame alloys, shapes, sizes, and metallurgical conditions as thematerials in the couple.5. Test Environment5.1 Laboratory Tests:5.1.1 In the laboratory, the test solution should closelyapproximate the service environment. The amount of testsolution used dep
25、ends on the size of the test specimens.Agoodrule of thumb is to use 40 cm3of test solution for every 1 cm2of exposed surface area of both members of the couple. Thevolume of test solution may be varied to closely approximatethe service application.5.1.2 Galvanic corrosion tests conducted for an exte
26、nsiveperiod of time may exhaust important constituents of theoriginal solution. Some accumulated corrosion products mayact as corrosion accelerators or inhibitors. These variables maygreatly change the end results, and replenishment of thesolution should be chosen to be representative of the service
27、application. A test system using continuously replenished testelectrolytes is often the only solution to this problem.5.1.3 Periodic measurements of the test environment shouldbe made when the test duration in a fixed volume solution is forperiods of several days or longer. These observations mayinc
28、lude temperature, pH, O2,H2S, CO2,NH3, conductivity, andpertinent metal ion content.5.2 Field TestsField testing should be performed in anenvironment similar to the service environment. Periodicmeasurements of those environmental variables which couldvary with time, such as temperature, dissolved O2
29、, and so forth,should be made.6. Procedure6.1 Laboratory Versus Field Testing:6.1.1 Galvanic corrosion tests are conducted in the labora-tory for several purposes: (1) inexpensive screening to reduceexpensive field testing, (2) study of the effects of environmen-tal variables, and (3) study of the c
30、orrosion accelerating orprotective effects of various anode/cathode surface area ratios.6.1.2 The materials proven in the laboratory to be the mostpromising should also be tested in the field, since it isfrequently impossible to duplicate the actual service environ-ment in the laboratory.6.2 Test Pr
31、ocedure:6.2.1 Specimens should be electrically joined before expo-sure. There are a number of methods for joining the specimens.Laboratory testing generally employs external electrical con-nection through wires such as to allow current measurement(see Fig. 1). Field tests frequently employ direct co
32、ntactphysical bonding by threaded rods as in Fig. 2, soldering,brazing, and so forth. Prime considerations are that theelectrical bond to the specimen will not corrode, which couldresult in decoupling, that the method of joining will not in itselfbe a galvanic couple or introduce other corrosion mec
33、hanisms(crevice, and so forth), and that the resistance of the electricalpath be small compared to the polarization resistance of thecouple materials. Soldering or brazing will prevent the use ofmass measurements for calculating corrosion rates. A coatingmay be applied to the electrical connections
34、to prevent elec-trolyte access as in Fig. 2, provided the coating does not resultin other corrosion phenomena, such as crevice attack, and issufficiently resistant to the environment.6.2.2 The physical relationship between the members ofeach couple should approximate that of the service situationbei
35、ng modeled. This is particularly important in electrolyteswith low conductivity, since the effect of IR drops will be morenoticeable. The specimens may be positioned by the use ofnonconductive holders, provided that these do not result inother corrosion phenomena (crevice, and so forth). A discus-si
36、on of the mounting of specimens is included in Method G4.The supporting device should not be affected by or causecontamination of the test solution.6.2.3 The coupled assembly is next immersed in the testelectrolyte for the period of exposure. Exposure durationshould be sufficient to allow prediction
37、 of the behavior for theentire service duration. If the service duration is long, corrosiondata can be taken as a function of time until a curve can beG 71 81 (2003)2developed that can be extrapolated to the service duration,provided that steady-state conditions have been reached andthat no transien
38、t environmental conditions are expected inservice to affect this steady state.FIG. 1 Laboratory Galvanic Corrosion Test Setup With Facility for Measuring Galvanic CurrentNOTEThe length of the plastic insulation rod should approximate the distance between the anode and the cathode of the final produc
39、t.FIG. 2 Specimen Configuration for Galvanic Corrosion Tests of Bar Stock MaterialG 71 81 (2003)36.2.4 Specimen removal should be based on a preplannedremoval schedule.7. Evaluation of Test Specimens7.1 Measurements During ExposureData recorded duringexposures may include galvanic current measuremen
40、ts andcouple and control specimen potentials measured relative to asuitable reference half-cell as recommended in Practice G3.Current data can then be converted into a theoretical corrosionrate based on Faradays law.7.2 Measurements After Removal:7.2.1 After removal, samples of corrosion products ma
41、y beobtained for chemical and physical analysis. The specimensshould then be cleaned of deposits (such as biofouling fromfresh or seawater) by scraping or brushing with a woodenscraper or soft bristle brush. Visual observations should berecorded before and after this initial cleaning operation. Colo
42、rphotographs may be taken of each specimen before and aftercleaning. Final cleaning of specimens should be in accordancewith Practice G1after which the specimens should be weighedto determine galvanic corrosion mass loss which can beconverted to corrosion rate as set forth in Practice G31.Additional
43、 recommendations for specimen cleaning may befound in Guide G4and Practice G31.7.2.2 In some cases, mass loss measurements will not bepossible or meaningful. For example, soldered assembliescannot be separated into their components without introducingextra mass due to the remaining solder. In this c
44、ase, corrosionevaluation of the end product configuration must be based onvisual assessments, thickness loss measurements, or on othertechniques. Materials suffering localized corrosion such aspitting may be analyzed using Practice G46, and thosesuffering crevice corrosion should have the depth of a
45、ttackmeasured and described in detail, with attention to changes atthe edges as well as the surfaces. In addition, changes inphysical properties such as breaking strength can also bemeasured. Metallographic examination of specimen cross sec-tions may be necessary to determine parting corrosion depth
46、.7.2.3 Regardless of the method of assessment, the behaviorof the coupled materials should be compared to that of theuncoupled controls. Subtracting control values from values ofcoupled specimens yields the increase in corrosion rate due tocoupling. A ratio of couple data to the uncoupled data has b
47、eenused to determine a percentage change in corrosion due to thecouple (acceleration factor).7.2.4 Where replicate couples are exposed, statistical analy-sis of the data, as set forth in Guide G16, may be applied togenerate confidence intervals for predictive purposes.8. Report8.1 The report should
48、include detailed descriptions of theexposed specimens including wetted areas, pertinent data onexposure conditions including the geometry used, the depositsformed, and results of the corrosion evaluation.8.2 Data for the exposed specimens should include physicaldimensions, chemical composition, meta
49、llurgical history, sur-face preparation, and after-exposure cleaning methods.8.3 Details of exposure conditions should include location,dates, and periods of exposure and description of the environ-mental conditions prevailing during the exposure period, in-cluding electrolyte conductivity.8.4 The results of the tests may be expressed as corrosionrate in penetration per unit time (for example, millimetres peryear) or loss in thickness or mass during the exposure period.Rates for both control (uncoupled) and coupled samples shouldbe reported, with the change in rate due to the c
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