1、Designation: G71 81 (Reapproved 2014)Standard Guide forConducting and Evaluating Galvanic Corrosion Tests inElectrolytes1This standard is issued under the fixed designation G71; the number immediately following the designation indicates the year of originaladoption or, in the case of revision, the y
2、ear 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 dissimi
3、larmetals 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 beha
4、vior 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 (2).1.3 The values stated in SI units are to be regarded asstandard. No other units
5、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 regulat
6、ory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3G1 Practice for Preparing, Cleaning, and Evaluating Corro-sion Test SpecimensG3 Practice for Conventions Applicable to ElectrochemicalMeasurements in Corrosion TestingG4 Guide for Conducting Corrosion Tests in Field Applica-tion
7、sG16 Guide for Applying Statistics to Analysis of CorrosionDataG31 Guide for Laboratory Immersion Corrosion Testing ofMetalsG46 Guide for Examination and Evaluation of Pitting Cor-rosion3. Significance and Use3.1 Use of this guide is intended to provide information onthe galvanic corrosion of metals
8、 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. Adherence
9、 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 betw
10、een 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 wi
11、thin 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. Wh
12、en 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 ratio
13、 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 is
14、the direct responsibility of Subcommittee G01.11 on ElectrochemicalMeasurements in Corrosion Testing.Current edition approved May 1, 2014. Published May 2014. Originallyapproved in 1981. Last previous edition approved in 2009 as G7181(2009). DOI:10.1520/G0071-81R14.2The boldface numbers in parenthes
15、es refer to a list of 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.C
16、opyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1low-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
17、 coupled to aluminum, 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 o
18、f the couple members 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,
19、 the shapes of the couple 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. S
20、hear-ing will, in some 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
21、 in accordance withPractice 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, sho
22、uld be removed prior 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 n
23、otinduce corrosion attack 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 te
24、sted for any given test 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 alloy
25、s withoutcoupling for 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 o
26、f testsolution used depends 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 test
27、s conducted for an extensiveperiod 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 repres
28、entative of the serviceapplication. 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. Th
29、ese observations mayinclude 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 te
30、mperature, dissolved 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,
31、 and (3) study of the 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
32、 laboratory.6.2 Test 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 freq
33、uently employ direct 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 introd
34、uce other corrosion mechanisms(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
35、electrical connections 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 t
36、he service situationbeing 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 inG71 81 (2014)2other corrosion phenomena
37、 (crevice, and so forth). A discus-sion of the mounting of specimens is included in Guide 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 durationshou
38、ld be sufficient to allow prediction of the behavior for theentire service duration. If the service duration is long, corrosionFIG. 1 Laboratory Galvanic Corrosion Test Setup With Facility for Measuring Galvanic CurrentNOTE 1The length of the plastic insulation rod should approximate the distance be
39、tween the anode and the cathode of the final product.FIG. 2 Specimen Configuration for Galvanic Corrosion Tests of Bar Stock MaterialG71 81 (2014)3data can be taken as a function of time until a curve can bedeveloped that can be extrapolated to the service duration,provided that steady-state conditi
40、ons have been reached 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 galva
41、nic current measurements 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
42、 corrosion products may 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 cl
43、eaning operation. Colorphotographs may be taken of each specimen before and aftercleaning. Final cleaning of specimens should be in accordancewith Practice G1 after which the specimens should be weighedto determine galvanic corrosion mass loss which can beconverted to corrosion rate as set forth in
44、Practice G31.Additional recommendations for specimen cleaning may befound in Guide G4 and 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 rem
45、aining solder. In this 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 shoul
46、d have the depth of attack 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
47、parting corrosion depth.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 t
48、he uncoupled data has beenused 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. Repor
49、t8.1 The report should 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-c
