1、Designation: G 71 81 (Reapproved 2009)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 () 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 dissi
3、milarmetals 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 be
4、havior 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 The values stated in SI units are to be regarded asstandard. No other unit
5、s of measurement are included in thisstandard.1.4 This standard does not purport 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 regul
6、atory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3G1 Practice for Preparing, Cleaning, and Evaluating Cor-rosion Test SpecimensG3 Practice for ConventionsApplicable to ElectrochemicalMeasurements in Corrosion TestingG4 Guide for Conducting Corrosion Tests in Field Appli-catio
7、nsG16 Guide for Applying Statistics to Analysis of CorrosionDataG31 Practice for Laboratory Immersion Corrosion Testingof MetalsG46 Guide for Examination and Evaluation of PittingCorrosion3. Significance and Use3.1 Use of this guide is intended to provide information onthe galvanic corrosion of meta
8、ls in electrical contact in anelectrolyte that does not have a flow velocity sufficient to causeerosion-corrosion 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. Adheren
9、ce to thisguide will aid in avoiding some of the inherent difficulties insuch testing.4. Test Specimens4.1 MaterialTest specimens should be manufactured fromthe same material as those used in the service application beingmodeled. Minor compositional or processing differences be-tween materials or be
10、tween different heats can greatly affectthe results in some cases.4.2 Size and Shape:4.2.1 The size and shape of 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
11、within the constraints of the testequipment. In general, the ratio of surface area to metal volumeshould be large 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.
12、When modeling large components, the sizeof the specimens should be as large as practical. Whenmodeling smaller components, 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 rat
13、io is defined as the surfacearea of one member of the couple divided by the surface areaof the other member of the couple. Only the area in contactwith the electrolyte (wetted area) is used in this calculation. In1This guide is under the jurisdiction of ASTM Committee G01 on Corrosion ofMetals and i
14、s the direct responsibility of Subcommittee G01.11 on ElectrochemicalMeasurements in Corrosion Testing.Current edition approved May 1, 2009. Published May 2009. Originallyapproved in 1981. Last previous edition approved in 2003 as G 7181(2003).2The boldface numbers in parentheses refer to a list of
15、references at 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.1Copyright ASTM Interna
16、tional, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.low-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 aluminum,
17、 there may be effects of corrosionproducts washing from one electrode to another which mayhave to be considered in determining specimen placement.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
18、should 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 co
19、uple 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 som
20、e 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 withPr
21、actice 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
22、toweighing 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 at
23、tack 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 te
24、st 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
25、 comparisons. 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 d
26、epends 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 ex
27、tensiveperiod 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 servi
28、ceapplication. 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 mayi
29、nclude 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
30、O2, 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
31、 corrosion 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
32、Procedure: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
33、contactphysical 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 m
34、echanisms(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 connection
35、s 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 situationb
36、eing 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-
37、sion of the mounting of specimens is included in Guide G4.G 71 81 (2009)2The 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
38、allow prediction 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 bedeveloped that can be extrapolated to the service duration,FIG. 1 Laboratory Galvanic Corrosion Test Setup With Facility for Measurin
39、g Galvanic CurrentNOTEThe length of the plastic insulation rod should approximate the distance between the anode and the cathode of the final product.FIG. 2 Specimen Configuration for Galvanic Corrosion Tests of Bar Stock MaterialG 71 81 (2009)3provided that steady-state conditions have been reached
40、 andthat no transient environmental conditions are expected inservice to affect this steady state.6.2.4 Specimen removal should be based on a preplannedremoval schedule.7. Evaluation of Test Specimens7.1 Measurements During ExposureData recorded duringexposures may include galvanic current measureme
41、nts 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 m
42、ay 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. Col
43、orphotographs 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.Additiona
44、l 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
45、case, 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 Guide G46, and those sufferingcrevice corrosion should have the depth of att
46、ack measured anddescribed in detail, with attention to changes at the edges aswell as the surfaces. In addition, changes in physical propertiessuch as breaking strength can also be measured. Metallo-graphic examination of specimen cross sections may be nec-essary to determine parting corrosion depth
47、.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
48、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
49、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, metallurgical 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 cond
