7/18/2019 Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes

http://slidepdf.com/reader/full/standard-guide-for-conducting-and-evaluating-galvanic-corrosion-tests-in-electrolytes 1/5

Standard Guide for Conducting and Evaluating GalvanicCorrosion Tests in Electrolytes

G71 − 81 (Reapproved 201!

Scope1.1 This guide covers conducting and evaluating galvanic corrosion

tests to characterize the behaviour of two dissimilar metals in electricalcontact in an electrolyte under low-ow conditions. It can be adaptedto wrought or cast metals and alloys.

3.2 This standard is presented as a guide for conducting galvanic corrosiontests in liuid electrolyte solutions! both in the laboratory and in serviceenvironments. "dherence to this guide will aid in avoiding some of theinherent di#culties in such testing.

Test Speci"ensMaterial  Test specimens should be manufactured from the same material as thoseused in the service application being modelled. $inor compositional orprocessing di%erences between materials or between di%erent heats cangreatly a%ect the results in some cases.

Size and Shape: The size and shape of the test specimens are dependent on restrictionsimposed by the test location. &hen determining material behaviour in thelaboratory! it is advisable to use the largest specimens permissible within

the constraints of the test euipment. In general! the ratio of surface areato metal volume should be large in order to favour ma'imum corrosionloss per weight. (u#cient thic)ness should be employed! however! tominimize the possibility of perforation of the specimens during the teste'posure.

*.3 Specimen Preparation: The edges of the test specimens should be prepared so as to eliminate allsheared or cold-wor)ed metal e'cept that cold-wor)ing introduced bystamping for identi+cation. (hearing will! in some cases! causeconsiderable attac). Therefore! specimens having sheared edges shouldnot be used. The edges should be +nished by machining or polishing. Theslight amount of cold wor)ing resulting from machining will not introduceany serious error.

*.3.2 (pecimens should be cleaned in accordance with ,ractice 1! or elsethe specimen surface condition should be similar to the application beingmodeled. The metallurgical condition of the specimens should be similarto the application being modeled. In all cases surface contamination! suchas dirt! grease! oil! and thic) o'ides! should be removed prior to weighingand e'posure to the test environment.

7/18/2019 Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes

http://slidepdf.com/reader/full/standard-guide-for-conducting-and-evaluating-galvanic-corrosion-tests-in-electrolytes 2/5

*.3.3 The specimen identi+cation system must be one that will endurethroughout the test period. dge notches! drilled holes! stamped numbers!and tags are some of the methods used for identi+cation. Theidenti+cation system must not induce corrosion attac) in any way.

*.* Number of Specimens: The number of galvanic couples to be tested will be determined bywhether or not one or more periodic specimen removals are scheduledduring the course of the test. "s minimum! duplicate and preferablytriplicate specimens! should be tested for any given test period todetermine the variability in the galvanic corrosion behaviour. The e%ect of the number of replications on the application of the results is set forth inuide 1/.

*.*.2 0ontrol specimens should also be tested to provide corrosion ratesof the individual metals and alloys without coupling for comparisons. These specimens should be of the same alloys! shapes! sizes! andmetallurgical conditions as the materials in the couple.

Test Environ"ent.1 Laboratory Tests:

 In the laboratory! the test solution should closely appro'imate the serviceenvironment. The amount of test solution used depends on the size of thetest specimens. " good rule of thumb is to use * cm3 of test solution forevery 1 cm2 of e'posed surface area of both members of the couple. Thevolume of test solution may be varied to closely appro'imate the serviceapplication.

alvanic corrosion tests conducted for an e'tensive period of time maye'haust important constituents of the original solution. (ome accumulatedcorrosion products may act as corrosion accelerators or inhibitors. Thesevariables may greatly change the end results! and replenishment of thesolution should be chosen to be representative of the service application." test system using continuously replenished test electrolytes is often theonly solution to this problem.

.1.3 ,eriodic measurements of the test environment should be made

when the test duration in a +'ed volume solution is for periods of severaldays or longer. These observations may include temperature! p! 42! 2(!042! 53! conductivity! and pertinent metal ion content.

#rocedure/.1 Laboratory versus Field Testing:alvanic corrosion tests are conducted in the laboratory for severalpurposes6718 Ine'pensive screening to reduce e'pensive +eld testing! 728 (tudy of the e%ects of environmental variables! and738 (tudy of the corrosion accelerating or protective e%ects of various

anode9cathode surface area ratios.

7/18/2019 Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes

http://slidepdf.com/reader/full/standard-guide-for-conducting-and-evaluating-galvanic-corrosion-tests-in-electrolytes 3/5

/.2 Test Procedure:(pecimens should be electrically :oined before e'posure. There are anumber of methods for :oining the specimens. ;aboratory testinggenerally employs e'ternal electrical connection through wires such as toallow current measurement 7see <ig. 18.

,rime considerations are that the electrical bond to the specimen will notcorrode! which could result in decoupling! that the method of :oining willnot in itself be a galvanic couple or introduce other corrosion mechanisms7crevice! and so forth8! and that the resistance of the electrical path besmall compared to the polarization resistance of the couple materials.

" coating may be applied to the electrical connections to preventelectrolyte access as in <ig. 2! provided the coating does not result inother corrosion phenomena! such as crevice attac)! and is su#cientlyresistant to the environment.

 The physical relationship between the members of each couple shouldappro'imate that of the service situation being modelled. This isparticularly important in electrolytes with low conductivity! since thee%ect of I drops will be more noticeable. The specimens may bepositioned by the use of nonconductive holders! provided that these do

not result in other corrosion phenomena 7crevice! and so forth8. "discussion of the mounting of specimens is included in uide *.

7/18/2019 Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes

http://slidepdf.com/reader/full/standard-guide-for-conducting-and-evaluating-galvanic-corrosion-tests-in-electrolytes 4/5

 The supporting device should not be a%ected by or cause contaminationof the test solution.

 The coupled assembly is ne't immersed in the test electrolyte for theperiod of e'posure. 'posure duration should be su#cient to allow

prediction of the behaviour for the entire service duration. If the serviceduration is long! corrosion data can be ta)en as a function of time until acurve can be developed that can be e'trapolated to the service duration!provided that steady-state conditions have been reached and that notransient environmental conditions are e'pected in service to a%ect thissteady state.

7$ Evaluation of Test Speci"ens!easurements during "#posure=ata recorded during e'posures may include galvanic currentmeasurements and couple and control specimen potentials measuredrelative to a suitable reference half-cell as recommended in ,ractice3.0urrent data can then be converted into a theoretical corrosion rate basedon <araday>s law.

!easurements after emoval:"fter removal! samples of corrosion products may be obtained forchemical and physical analysis. The specimens should then be cleaned of deposits 7such as biofouling from fresh or seawater8 by scraping orbrushing with a wooden scraper or soft bristle brush. ?isual observationsshould be recorded before and after this initial cleaning operation. 0olour

photographs may be ta)en of each specimen before and after cleaning.<inal cleaning of specimens should be in accordance with ,ractice 1 afterwhich the specimens should be weighed to determine galvanic corrosionmass loss which can be converted to corrosion rate as set forth in ,ractice31.

"dditional recommendations for specimen cleaning may be found in uide* and ,ractice 31.@.2.2 in some cases! mass loss measurements willnot be possible or meaningful. <or e'ample! soldered assemblies cannotbe separated into their components without introducing e'tra mass due tothe remaining solder. In this case! corrosion evaluation of the end product

con+guration must be based on visual assessments! thic)ness lossmeasurements! or on other techniues. $aterials su%ering localizedcorrosion such as pitting may be analysed using uide */! and thosesu%ering crevice corrosion should have the depth of attac) measured anddescribed in detail! with attention to changes at the edges as well as thesurfaces. In addition! changes in physical properties such as brea)ingstrength can also be measured. $etallographic e'amination of specimencross sections may be necessary to determine parting corrosion depth.

Aegardless of the method of assessment! the behaviour of the coupled

materials should be compared to that of the uncoupled controls.(ubtracting control values from values of (pecimen removal should be

7/18/2019 Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes

http://slidepdf.com/reader/full/standard-guide-for-conducting-and-evaluating-galvanic-corrosion-tests-in-electrolytes 5/5

based on a pre-planned removal schedule. 0oupled specimen>s yields theincrease in corrosion rate due to coupling. " ratio of couple data to theuncoupled data has been used to determine a percentage change incorrosion due to the couple 7acceleration factor8.

&here replicate couples are e'posed! statistical analysis of the data! asset forth in uide 1/! may be applied to generate con+dence intervalsfor predictive purposes.

Report The report should include detailed descriptions of the e'posed specimensincluding wetted areas! pertinent data on e'posure conditions includingthe geometry used! the deposits formed! and results of the corrosionevaluation.  =ata for the e'posed specimens should include physical dimensions!chemical composition! metallurgical history! surface preparation! andafter-e'posure cleaning methods. =etails of e'posure conditions should include location! dates! and periodsof e'posure and description of the environmental conditions prevailingduring the e'posure period! including electrolyte conductivity. The results of the tests may be e'pressed as corrosion rate in penetrationper unit time 7for e'ample! millimetres per year8 or loss in thic)ness ormass during the e'posure period.Aates for both control 7uncoupled8 and coupled samples should bereported! with the change in rate due to the coupling reported as eitherthe di%erence between control and coupled samples or as the coupled

rate divided by the control rate7acceleration factor8.In cases where corrosion is in the form of pitting or crevice corrosion! apitting factor should be reported in accordance with uide */. =epths of pitting should be recorded. ,itting factors or depths can then be used todetermine the change in corrosion due to coupling.If any physical property of the specimens is measured after e'posure! thechange in that property should be reported and these values can becompared between control and coupled samples in the same manner ascorrosion rates or pitting factors.0hanges in the physical appearance of the specimens during the e'posureperiod should be reported