ASTM_G_3_-_14.pdf

Designation:G314Standard Practice forConventions Applicable to Electrochemical Measurementsin Corrosion Testing1This standard is issued under the fixed designation G3;the number immediately following the designation indicates the year of originaladoption or,in the case of revision,the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon()indicates an editorial change since the last revision or reapproval.1.Scope1.1 This practice covers conventions for reporting anddisplaying electrochemical corrosion data.Conventions forpotential,current density,electrochemical impedance andadmittance,as well as conventions for graphical presentationof such data are included.1.2 The values stated in SI units are to be regarded asstandard.No other units of measurement are included in thisstandard.See also 7.4.1.3 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 regulatory limitations prior to use.2.Referenced Documents2.1 ASTM Standards:2IEEE/ASTM SI 10 Standard for Use of the InternationalSystem of Units(SI)(the Modern Metric System)3.Significance and Use3.1 Thispracticeprovidesguidanceforreporting,displaying,and plotting electrochemical corrosion data andincludes recommendations on signs and conventions.Use ofthis practice will result in the reporting of electrochemicalcorrosion data in a standard format,facilitating comparisonbetween data developed at different laboratories or at differenttimes.The recommendations outlined in this standard may beutilized when recording and reporting corrosion data obtainedfrom electrochemical tests such as potentiostatic and potentio-dynamic polarization,polarization resistance,electrochemicalimpedance and admittance measurements,galvanic corrosion,and open circuit potential measurements.4.Sign Convention for Electrode Potential4.1 The Stockholm sign invariant convention is recom-mended for use in reporting the results of specimen potentialmeasurements in corrosion testing.In this convention,thepositive direction of electrode potential implies an increasinglyoxidizing condition at the electrode in question.The positivedirection has also been denoted as the noble direction becausethe corrosion potentials of most noble metals,such as gold,aremore positive than the nonpassive base metals.On the otherhand,the negative direction,often called the active direction,isassociated with reduction and consequently the corrosionpotentials of active metals,such as magnesium.This conven-tion was adopted unanimously by the 1953 International Unionof Pure and Applied Chemistry as the standard for electrodepotential(1).34.2 In the context of a specimen electrode of unknownpotential in an aqueous electrolyte,consider the circuit shownin Fig.1 with a reference electrode connected to the groundterminal of an electrometer.If the electrometer reads on scalewhen the polarity switch is negative,the specimen electrodepotential is negative(relative to the reference electrode).Conversely,if the electrometer reads on scale when polarity ispositive,the specimen potential is positive.On the other hand,if the specimen electrode is connected to the ground terminal,the potential will be positive if the meter is on scale when thepolarity switch is negative,and vice versa.NOTE1In cases where the polarity of a measuring instrument is indoubt,a simple verification test can be performed as follows:connect themeasuring instrument to a dry cell with the lead previously on thereference electrode to the negative battery terminal and the lead previouslyon the specimen electrode to the positive battery terminal.Set the rangeswitch to accommodate the dry cell voltage.The meter deflection will nowshow the direction of positive potential.Also,the corrosion potential of magnesium or zinc should be negativein a 1 N NaCl solution if measured against a saturated standard calomelelectrode(SCE).1This practice is under the jurisdiction of ASTM Committee G01 on Corrosionof Metals and is the direct responsibility of Subcommittee G01.11 on Electrochemi-cal Measurements in Corrosion Testing.Current edition approved Dec.15,2014.Published December 2014.Originallyapproved in 1968.Last previous edition approved in 2013 as G313.DOI:10.1520/G0003-14.2For 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.3The boldface numbers in parentheses refer to a list of references at the end ofthis standard.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 5.Sign Convention for Electrode Potential TemperatureCoefficients5.1 There are two types of temperature coefficients ofelectrode potential