ASTM_G_102_-_89_2015e1.pdf

Designation:G10289(Reapproved 2015)1Standard Practice forCalculation of Corrosion Rates and Related Informationfrom Electrochemical Measurements1This standard is issued under the fixed designation G102;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon()indicates an editorial change since the last revision or reapproval.1NOTEEditorially corrected the legend below Eq 1 in 4.1 in November 2015.1.Scope1.1 This practice covers the providing of guidance inconverting the results of electrochemical measurements to ratesof uniform corrosion.Calculation methods for convertingcorrosion current density values to either mass loss rates oraverage penetration rates are given for most engineering alloys.In addition,some guidelines for converting polarization resis-tance values to corrosion rates are provided.1.2 The values stated in SI units are to be regarded asstandard.No other units of measurement are included in thisstandard.2.Referenced Documents2.1 ASTM Standards:2D2776 Methods of Test for Corrosivity of Water in theAbsence of Heat Transfer(Electrical Methods)(With-drawn 1991)3G1 Practice for Preparing,Cleaning,and Evaluating Corro-sion Test SpecimensG5 Reference Test Method for Making PotentiodynamicAnodic Polarization MeasurementsG59 Test Method for Conducting Potentiodynamic Polariza-tion Resistance Measurements3.Significance and Use3.1 Electrochemical corrosion rate measurements often pro-vide results in terms of electrical current.Although the con-version of these current values into mass loss rates or penetra-tion rates is based on Faradays Law,the calculations can becomplicated for alloys and metals with elements havingmultiple valence values.This practice is intended to provideguidance in calculating mass loss and penetration rates for suchalloys.Some typical values of equivalent weights for a varietyof metals and alloys are provided.3.2 Electrochemical corrosion rate measurements may pro-vide results in terms of electrical resistance.The conversion ofthese results to either mass loss or penetration rates requiresadditional electrochemical information.Some approaches forestimating this information are given.3.3 Use of this practice will aid in producing more consis-tent corrosion rate data from electrochemical results.This willmake results from different studies more comparable andminimize calculation errors that may occur in transformingelectrochemical results to corrosion rate values.4.Corrosion Current Density4.1 Corrosion current values may be obtained from galvaniccells and polarization measurements,including Tafel extrapo-lations or polarization resistance measurements.(See Refer-ence Test Method G5 and Practice G59 for examples.)The firststep is to convert the measured or estimated current value tocurrent density.This is accomplished by dividing the totalcurrent by the geometric area of the electrode exposed to thesolution.The surface roughness is generally not taken intoaccount when calculating the current density.It is assumed thatthe current distributes uniformly across the area used in thiscalculation.In the case of galvanic couples,the exposed area ofthe anodic specimen should be used.This calculation may beexpressed as follows:icor5IcorA(1)where:icor=corrosion current density,A/cm2,Icor=total anodic current,A,and1This practice is under the jurisdiction of ASTM Committee G01 on Corrosionof Metalsand is the direct responsibility of Subcommittee G01.11 on Electrochemi-cal Measurements in Corrosion Testing.Current edition approved Nov.1,2015.Published December 2015.Originallyapproved in 1989.Last previous edition approved in 2010 as G10289(2010).DOI:10.1520/G0102-89R15E01.2For 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 last approved version of this historical standard is referenced onwww.astm.org.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 A=exposed specimen area,cm2.Other units may be used in this calculation.In somecomputerized polarization equipment,this calculation is madeautomatically after the specimen area is programmed into thecomputer.A sample calculation is given in Appendix X1.4.2 Equivalent WeightEquivalent weight,EW,may bethought of as the mass of metal in grams that will be oxidizedby the passage of one Faraday(96 489 6 2 C(amp-sec)ofelectric charge.NOTE1The value of EW is not dependent on the unit system chosenand so may be considered dimensionless.For pure elements,the equivalent weight is given by:EW5Wn(2)where:W=the atomic weight of the element,andn=the number of electrons required to oxidize an atom ofthe element in the corrosion process,that is,the valenceof