1、Designation: G102 89 (Reapproved 2010)Standard 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
2、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.1. Scope1.1 This practice covers the providing of guidance inconverting the results of electroch
3、emical 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 ra
4、tes 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 Test Methods for Corrosivity of Water in the Ab-sence of Heat Transfer (Electrical Methods)3G1 Practice for Pr
5、eparing, Cleaning, and Evaluating Corro-sion Test SpecimensG5 Reference Test Method for Making Potentiostatic andPotentiodynamic Anodic Polarization MeasurementsG59 Test Method for Conducting Potentiodynamic Polar-ization Resistance Measurements3. Significance and Use3.1 Electrochemical corrosion ra
6、te 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 prac
7、tice 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 conv
8、ersion 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 res
9、ults 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
10、-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
11、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 calcul
12、ation may beexpressed as follows:icor5IcorA(1)where:icor= corrosion current density, A/cm2,Icor= total anodic current, A, andA = exposed specimen area, cm2.Other units may be used in this calculation. In somecomputerized polarization equipment, this calculation is madeautomatically after the specime
13、n area is programmed into thecomputer. A sample calculation is given in Appendix X1.1This 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 appro
14、ved May 1, 2010. Published May 2010. Originallyapproved in 1989. Last previous edition approved in 2004 as G10289(2004)1.DOI: 10.1520/G0102-89R10.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards
15、 volume information, refer to the standards Document Summary page onthe ASTM website.3Withdrawn. The last approved version of this historical standard is referencedon www.astm.org.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.4.2 E
16、quivalent 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.NOTE 1The value of EW is not dependent on the unit system chosenand so may be considered dimensionless.For pure elements,
17、 the equivalent weight is given by:EW 5Wn(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, thevalence of the element.4.3 For alloys, the equivalent weight is more complex. It isusually assumed th
18、at the process of oxidation is uniform anddoes not occur selectively to any component of the alloy. If thisis not true, then the calculation approach will need to beadjusted to reflect the observed mechanism. In addition, somerationale must be adopted for assigning values of n to theelements in the
19、alloy because many elements exhibit more thanone valence value.4.4 To calculate the alloy equivalent weight, the followingapproach may be used. Consider a unit mass of alloy oxidized.The electron equivalent for1gofanalloy, Q is then:Q 5 (nifiWi(3)where:fi = the mass fraction of the ithelement in the
20、 alloy,Wi = the atomic weight of the ithelement in the alloy, andni = the valence of the ithelement of the alloy.Therefore, the alloy equivalent weight, EW, is the reciprocalof this quantity:EW 51(nifiWi(4)Normally only elements above 1 mass percent in the alloyare included in the calculation. In ca
21、ses where the actualanalysis of an alloy is not available, it is conventional to use themid-range of the composition specification for each element,unless a better basis is available. A sample calculation is givenin Appendix X2 (1).44.5 Valence assignments for elements that exhibit multiplevalences
22、can create uncertainty. It is best if an independenttechnique can be used to establish the proper valence for eachalloying element. Sometimes it is possible to analyze thecorrosion products and use those results to establish the propervalence. Another approach is to measure or estimate theelectrode
23、potential of the corroding surface. Equilibrium dia-grams showing regions of stability of various phases as afunction of potential and pH may be created from thermody-namic data. These diagrams are known as Potential-pH (Pour-baix) diagrams and have been published by several authors (2,3). The appro
24、priate diagrams for the various alloying elementscan be consulted to estimate the stable valence of each elementat the temperature, potential, and pH of the contacting electro-lyte that existed during the test.NOTE 2Some of the older publications used inaccurate thermody-namic data to construct the
25、diagrams and consequently they are in error.4.6 Some typical values of EW for a variety of metals andalloys are given in Table 1.4.7 Calculation of Corrosion RateFaradays Law can beused to calculate the corrosion rate, either in terms of penetra-tion rate (CR) or mass loss rate (MR) (4):CR 5 K1icorr
26、EW (5)MR 5 K2icorEW (6)where:CR is given in mm/yr, icorin A/cm2,K1= 3.27 3 103, mm g/A cm yr (Note 3),r = density in g/cm3, (see Practice G1 for density valuesfor many metals and alloys used in corrosion test-ing),MR = g/m2d, andK2= 8.954 3 103,gcm2/A m2d(Note 3).NOTE 3EW is considered dimensionless
27、 in these calculations.Other values for K1and K2for different unit systems aregiven in Table 2.4.8 Errors that may arise from this procedure are discussedbelow.4.8.1 Assignment of incorrect valence values may causeserious errors (5).4.8.2 The calculation of penetration or mass loss fromelectrochemic
28、al measurements, as described in this standard,assumes that uniform corrosion is occurring. In cases wherenon-uniform corrosion processes are occurring, the use of thesemethods may result in a substantial underestimation of the truevalues.4.8.3 Alloys that include large quantities of metalloids orox
29、idized materials may not be able to be treated by the aboveprocedure.4.8.4 Corrosion rates calculated by the method above whereabrasion or erosion is a significant contributor to the metal lossprocess may yield significant underestimation of the metal lossrate.5. Polarization Resistance5.1 Polarizat
30、ion resistance values may be approximatedfrom either potentiodynamic measurements near the corrosionpotential (see Practice G59) or stepwise potentiostatic polar-ization using a single small potential step, DE, usually either10 mV or 10 mV, (see Test Method D2776). Values of 65 and620 mV are also co
31、mmonly used. In this case, the specimencurrent, DI, is measured after steady state occurs, and DE/DI iscalculated. Potentiodynamic measurements yield curves of Iversus E and the reciprocal of the slope of the curve (dE/dI) atthe corrosion potential is measured. In most programmable4The boldface numb
32、ers in parentheses refer to the list of references at the end ofthis standard.G102 89 (2010)2TABLE 1 Equivalent Weight Values for a Variety of Metals and AlloysNOTE 1Alloying elements at concentrations below 1 % by mass were not included in the calculation, for example, they were considered part of
33、thebasis metal.NOTE 2Mid-range values were assumed for concentrations of alloying elements.NOTE 3Only consistent valence groupings were used.NOTE 4Eq 4 was used to make these calculations.CommonDesignationUNSElementsw/ConstantValenceLowest Second Third FourthVariableValenceEquivalentWeightVariableVa
34、lenceEquivalentWeightElement/ValenceEquivalentWeightElement/ValenceEquivalentWeightAluminum Alloys:AA1100AA91100 Al/3 8.99AA2024 A92024 Al/3, Mg/2 Cu/1 9.38 Cu/2 9.32AA2219 A92219 Al/3 Cu/1 9.51 Cu/2 9.42AA3003 A93003 Al/3 Mn/2 9.07 Mn/4 9.03 Mn 7 8.98AA3004 A93004 Al/3, Mg/2 Mn/2 9.09 Mn/4 9.06 Mn
35、7 9.00AA5005 A95005 Al/3, Mg/2 9.01AA5050 A95050 Al/3, Mg/2 9.03AA5052 A95052 Al/3, Mg/2 9.05AA5083 A95083 Al/3, Mg/2 9.09AA5086 A95086 Al/3, Mg/2 9.09AA5154 A95154 Al/3, Mg/2 9.08AA5454 A95454 Al/3, Mg/2 9.06AA5456 A95456 Al/3, Mg/2 9.11AA6061 A96061 Al/3, Mg/2 9.01AA6070 A96070Al/3, Mg/2,Si/48.98A
36、A6101 A96161 Al/3 8.99AA7072 A97072 Al/3, Zn/2 9.06AA7075 A97075Al/3, Zn/2,Mg/2Cu/1 9.58 Cu/2 9.55AA7079 A97079Al/3, Zn/2,Mg/29.37AA7178 A97178Al/3, Zn/2,Mg/2Cu/1 9.71 Cu/2 9.68Copper Alloys:CDA110 C11000 Cu/1 63.55 Cu/2 31.77CDA220 C22000 Zn/2 Cu/1 58.07 Cu/2 31.86CDA230 C23000 Zn/2 Cu/1 55.65 Cu/2
37、 31.91CDA260 C26000 Zn/2 Cu/1 49.51 Cu/2 32.04CDA280 C28000 Zn/2 Cu/1 46.44 Cu/2 32.11CDA444 C44300 Zn/2 Cu/1, Sn/2 50.42 Cu/1, Sn/4 50.00 Cu/2, Sn/4 32.00CDA687 C68700 Zn/2, Al/3 Cu/1 48.03 Cu/2 30.29CDA608 C60800 Al/3 Cu/1 47.114 Cu/2 27.76CDA510 C51000 Cu/1, Sn/2 63.32 Cu/1, Sn/4 60.11 Cu/2, Sn/4
38、 31.66CDA524 C52400 Cu/1, Sn/2 63.10 Cu/1, Sn/4 57.04 Cu/2, Sn/4 31.55CDA655 C65500 Si/4 Cu/1 50.21 Cu/2 28.51CDA706 C70600 Ni/2 Cu/1 56.92 Cu/2 31.51CDA715 C71500 Ni/2 Cu/1 46.69 Cu/2 30.98CDA752 C75200 Ni/2, Zn/2 Cu/1 46.38 Cu/2 31.46Stainless Steels:304 S30400 Ni/2 Fe/2, Cr/3 25.12 Fe/3, Cr/3 18.
39、99 Fe/3, Cr/6 15.72321 S32100 Ni/2 Fe/2, Cr/3 25.13 Fe/3, Cr/3 19.08 Fe/3, Cr/6 15.78309 S30900 Ni/2 Fe/2, Cr/3 24.62 Fe/3, Cr/3 19.24 Fe/3, Cr/6 15.33310 S31000 Ni/2 Fe/2, Cr/3 24.44 Fe/3, Cr/3 19.73 Fe/3, Cr/6 15.36316 S31600 Ni/2 Fe/2, Cr/3, Mo/3 25.50 Fe/2, Cr/3, Mo/4 25.33 Fe/3, Cr/6, Mo/6 19.1
40、4 Fe/3, Cr/6, Mo/6 16.111317 S31700 Ni/2 Fe/2, Cr/3, Mo/3 25.26 Fe/2, Cr/3, Mo/4 25.03 Fe/3, Cr/3, Mo/6 19.15 Fe/3, Cr/6, Mo/6 15.82410 S41000 Fe/2, Cr/3 25.94 Fe/3, Cr/3 18.45 Fe/3, Cr/6 16.28430 S43000 Fe/2, Cr/3 25.30 Fe/3, Cr/3 18.38 Fe/3, Cr/6 15.58446 S44600 Fe/2, Cr/3 24.22 Fe/3, Cr/3 18.28 F
41、e/3, Cr/6 14.4620CB3AN08020 Ni/2Fe/2, Cr/3, Mo/3,Cu/123.98Fe/2, Cr/3, Mo/4, Cu/123.83Fe/3, Cr/3, Mo/6, Cu/218.88Fe/3, Cr/6, Mo/6,Cu/215.50Nickel Alloys:200 N02200 NI/2 29.36 Ni/3 19.57400 N04400 Ni/2 Cu/1 35.82 Cu/2 30.12600 N06600 Ni/2 Fe/2, Cr/3 26.41 Fe/3, Cr/3 25.44 Fe/3, Cr/6 20.73800 N08800 Ni
42、/2 Fe/2, Cr/3 25.10 Fe/3, Cr/3 20.76 Fe/3, Cr/6 16.59825 N08825 Ni/2Fe/2, Cr/3, Mo/3,Cu/125.52Fe/2, Cr/3, Mo/4, Cu/125.32Fe/3, Cr/3, Mo/6, Cu/221.70Fe/3, Cr/6, Mo/6,Cu/217.10B N10001 Ni/2 Mo/3, Fe/2 30.05 Mo/4, Fe/2 27.50 Mo/6, Fe/2 23.52 Mo/6, Fe/3 23.23G102 89 (2010)3potentiodynamic polarization e
43、quipment, the current is con-verted to current density automatically and the resulting plot isof i versus E. In this case, the polarization resistance is givenby dE/di at the corrosion potential and 5.2 is not applicable.5.2 It is necessary to multiply the dE/dI or DE/DI valuecalculated above by the
44、 exposed specimen geometric area toobtain the polarization resistance. This is equivalent to thecalculation shown in 4.1 for current density.5.3 The Stern-Geary constant B must be estimated orcalculated to convert polarization resistance values to corrosioncurrent density (6, 8).5.3.1 Calculate Ster
45、n-Geary constants from known Tafelslopes where both cathodic and anodic reactions are activationcontrolled, that is, there are distinct linear regions near thecorrosion potential on an E log i plot:B 5ba bc2.303 ba 1 bc!(7)where:ba = slope of the anodic Tafel reaction, when plotted onbase 10 logarit
46、hmic paper in V/decade,bc = slope of the cathodic Tafel reaction when plotted onbase 10 logarithmic paper in V/decade, andB = Stern-Geary constant, V.5.3.2 In cases where one of the reactions is purely diffusioncontrolled, the Stern-Geary constant may be calculated:B 5b2.303(8)where:b = the activati
47、on controlled Tafel slope in V/decade.5.3.3 It should be noted in this case that the corrosioncurrent density will be equal to the diffusion limited currentdensity. A sample calculation is given in Appendix X4.5.3.4 Cases where both activation and diffusion effects aresimilar in magnitude are known
48、as mixed control. The reactionunder mixed control will have an apparently larger b value thanpredicted for an activation control, and a plot of E versus logI will tend to curve to an asymptote parallel to the potentialaxis.The estimation of a B value for situations involving mixedcontrol requires mo
49、re information in general and is beyond thescope of this standard. In general, Eq 7 and Eq 8 may be used,and the corrosion rate calculated by these two approximationsmay be used as lower and upper limits of the true rate.NOTE 4Electrodes exhibiting stable passivity will behave as if theanodic reaction were diffusion limited, except that the passive currentdensity is not affected by agitation.5.3.5 It is possible to estimate baand bcfrom the deviationfrom linearity of polarization curves in the 2050 mV regionaround the corrosion potential. Several approaches
