1、Designation: G3 89 (Reapproved 2010)G3 13Standard 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
2、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 and displaying electrochemical corrosion data. Conven
3、tions for potential,current density, electrochemical impedance and admittance, as well as conventions for graphical presentation of such data areincluded.1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.See also 7.4.1.3 Thi
4、s standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatorylimitations prior to use.2. Referenced Documents2
5、.1 ASTM Standards:2IEEE/ASTM SI 10 Standard for Use of the International System of Units (SI) (the Modern Metric System)3. Significance and Use3.1 This practice provides guidance for reporting, displaying, and plotting electrochemical corrosion data and includesrecommendations on signs and conventio
6、ns. Use of this practice will result in the reporting of electrochemical corrosion data ina standard format, facilitating comparison between data developed at different laboratories or at different times. Therecommendations outlined in this standard may be utilized when recording and reporting corro
7、sion data obtained fromelectrochemical tests such as potentiostatic and potentiodynamic polarization, polarization resistance, electrochemical impedanceand admittance measurements, galvanic corrosion, and open circuit potential measurements.4. Sign Convention for Electrode Potential4.1 The Stockholm
8、 sign invariant convention is recommended for use in reporting the results of specimen potentialmeasurements in corrosion testing. In this convention, the positive direction of electrode potential implies an increasingly oxidizingcondition at the electrode in question. The positive direction has als
9、o been denoted as the noble direction because the corrosionpotentials of most noble metals, such as gold, are more positive than the nonpassive base metals. On the other hand, the negativedirection, often called the active direction, is associated with reduction and consequently the corrosion potent
10、ials of active metals,such as magnesium. This convention was adopted unanimously by the 1953 International Union of Pure and Applied Chemistryas the standard for electrode potential (1).34.2 In the context of a specimen electrode of unknown potential in an aqueous electrolyte, consider the circuit s
11、hown in Fig.1 with a reference electrode connected to the ground terminal of an electrometer. If the electrometer reads on scale when thepolarity switch is negative, the specimen electrode potential is negative (relative to the reference electrode). Conversely, if the1 This practice is under the jur
12、isdiction of ASTM Committee G01 on Corrosion of Metals and is the direct responsibility of Subcommittee G01.11 on ElectrochemicalMeasurements in Corrosion Testing.Current edition approved May 1, 2010Dec. 1, 2013. Published May 2010December 2013. Originally approved in 1968. Last previous edition app
13、roved in 20042010 asG389(2004).G389 (2010). DOI: 10.1520/G0003-89R10.10.1520/G0003-13.2 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary
14、page on the ASTM website.3 The boldface numbers in parentheses refer to a list of references at the end of this standard.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Becauseit
15、may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the official document.Copyright ASTM International, 100 Barr Ha
16、rbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1electrometer reads on scale when polarity is positive, the specimen potential is positive. On the other hand, if the specimenelectrode is connected to the ground terminal, the potential will be positive if the meter is on scale
17、 when the polarity switch isnegative, and vice versa.NOTE 1In cases where the polarity of a measuring instrument is in doubt, a simple verification test can be performed as follows: connect themeasuring instrument to a dry cell with the lead previously on the reference electrode to the negative batt
18、ery terminal and the lead previously on thespecimen electrode to the positive battery terminal. Set the range switch to accommodate the dry cell voltage. The meter deflection will now show thedirection of positive potential.Also, the corrosion potential of magnesium or zinc should be negative in a 1
19、 N NaCl solution if measured against a saturated standard calomel electrode(SCE).NOTE 1The electrode potential of specimen is negative as shown.FIG. 1 Schematic Diagram of an Apparatus to Measure Electrode Potential of a SpecimenG3 1325. Sign Convention for Electrode Potential Temperature Coefficien
20、ts5.1 There are two types of temperature coefficients of electrode potential: isothermal temperature coefficients and the thermalcoefficients. The sign convention recommended for both types of temperature coefficients is that the temperature coefficient ispositive when an increase in temperature pro
21、duces an increase (that is, it becomes more positive) in the electrode potential.Likewise, the second temperature coefficient is positive when an increase in temperature produces an increase (that is, it becomesmore positive) in the first temperature coefficient.6. Sign Convention for Current and Cu
22、rrent Density6.1 The sign convention in which anodic currents and current densities are considered positive and cathodic currents and currentdensities are negative is recommended. When the potential is plotted against the logarithm of the current density, only the absolutevalues of the current densi
23、ty can be plotted. In such plots, the values which are cathodic should be clearly differentiated from theanodic values if both are present.7. Conventions for Displaying Polarization Data7.1 Sign ConventionsThe standard mathematical practice for plotting graphs is recommended for displaying electroch
24、emicalcorrosion data. In this practice, positive values are plotted above the origin on the ordinate axis and to the right of the origin onthe abscissa axis. In logarithmic plots, the abscissa value increases from left to right and the ordinate value increases from bottomto top.7.2 Current Density-P
25、otential PlotsAuniform convention is recommended for plotting current density-potential data, namely,plot current density along the abscissa and potential along the ordinate. In current density potential plots, the current density maybe plotted on linear or logarithmic axes. In general, logarithmic
26、plots are better suited to incorporation of wide ranges of currentdensity data and for demonstrating Tafel relationships. Linear plots are recommended for studies in which the current density orpotential range is small, or in cases where the region in which the current density changes from anodic to
27、 cathodic is important.Linear plots are also used for the determination of the polarization resistance Rp, which is defined as the slope of a potential-currentdensity plot at the corrosion potential Ecorr. The relationship between the polarization resistance Rp and the corrosion currentdensity icorr
28、 is as follows (2, 3):FdE!di GE505Rp 5 babc2.303ba1bc!icorr(1)where:ba = anodic Tafel slope,bc = cathodic Tafel slope, andE = the difference E Ecorr, where E is the specimen potential.Fig. 2 is a plot of polarization, E Ecorr, versus current density i (solid line) from which the polarization resista
29、nce Rp has beendetermined as the slope of the curve at the corrosion potential Ecorr.Fig. 2 is a plot of polarization, E Ecorr, versus current density i (solid line) from which the polarization resistance Rp has beendetermined as the slope of the curve at the corrosion potential Ecorr.7.3 Potential
30、Reference PointsIn plots where electrode potentials are displayed, some indication of the conversion of thevalues displayed to both the standard hydrogen electrode scale (SHE) and the saturated calomel electrode scale (SCE) isrecommended if they are known. For example, when electrode potential is pl
31、otted as the ordinate, then the SCE scale could beshown at the extreme left of the plot and the SHE scale shown at the extreme right. An alternative, in cases where the referenceelectrode was not either SCE or SHE, would be to show on the potential axis the potentials of these electrodes against the
32、 referenceused. In cases where these points are not shown on the plot, an algebraic conversion could be indicated. For example, in the caseof a silver-silver chloride reference electrode (1 M KCl), the conversion could be shown in the title box as:SCE5E 20.006 V (2)SHE5E10.235 Vwhere E represents el
33、ectrode potential measured against the silver-silver chloride standard (1 M KCl).NOTE 2A table of potentials for various common reference electrodes is presented in Appendix X2.7.4 UnitsThe recommended unit of potential is the volt. In cases where only small potential ranges are covered, millivolts
34、ormicrovolts may be used. The SI units for current density are ampere per square metre or milliampere per square centimetre(IEEE/ASTM SI 10). Still in use are units expressed in amperes per square centimetre, and microamperes per square centimetre.7.5 Sample Polarization CurvesSample polarization pl
35、ots employing these recommended practices are shown in Figs. 2-6.Fig. 3 and Fig. 4 are hypothetical curves showing active and active-passive anode behavior, respectively. Fig. 5 and Fig. 6 areactual polarization data for Type 430 stainless steel (UNS 43000) (4) and two aluminum samples (5).Fig. 3 an
36、d Fig. 4 are exhibitedG3 133to illustrate graphically the location of various points used in discussion of electrochemical methods of corrosion testing. Thepurpose of Fig. 5 and Fig. 6 is to show how various types of electrode behavior can be plotted in accordance with the proposedconventions.8. Con
37、ventions for Displaying Electrochemical Impedance Data8.1 Three graphical formats in common use for reporting electrochemical impedance data are the Nyquist, Bode, andAdmittance formats. These formats are discussed for a simple electrode system modelled by an equivalent electrical circuit asshown in
38、 Fig. 7. In the convention utilized the impedance is defined as:Z5Z1j Z“ (3)where:Z = real or in-phase component of impedance,Z“ = the imaginary or out-of-phase component of impedance, andj2 = 1.The impedance magnitude or modulus is defined as |Z|2 = (Z)2 + (Z“). For the equivalent electrical circui
39、t shown in Fig. 7, theimaginary component of impedanceZ“5 212pifC (4)FIG. 2 Hypothetical Linear Polarization PlotG3 134where:FIG. 3 Hypothetical Cathodic and Anodic Polarization DiagramFIG. 4 Hypothetical Cathodic and Anodic Polarization Plots for a Passive AnodeG3 135f = frequency in cycles per sec
40、ond (or hertz, Hz, where one Hz is equal to 2pi radians/s, and w = 2pif, where the units for w areradians/s), andC = capacitance in farads.The phase angle, is defined as:5arctanZ“/Z! (5)The admittance, Y, is defined as1/Z 5Y 5Y1jY“ (6)FIG. 5 Typical Potentiostatic Anodic Polarization Plot for Type 4
41、30 Stainless Steel in 1.0 N H2SO4FIG. 6 Typical Polarization Plots for Aluminum Materials in 0.2 N NaCl SolutionG3 136where:Y = real or in-phase component of admittance, andY“ = the imaginary of out-of-phase component of admittance.8.2 Nyquist Format (Complex Plane, or Cole-Cole):8.2.1 The real comp
42、onent of impedance is plotted on the abscissa and the negative of the imaginary component is plotted onthe ordinate. In this practice positive values of the real component of impedance are plotted to the right of the origin parallel tothe x axis (abscissa). Negative values of the imaginary component
43、 of impedance are plotted vertically from the origin parallel tothe y axis (ordinate).8.2.2 Fig. 8 shows a Nyquist plot for the equivalent circuit of Fig. 7. The frequency dependence of the data is not shownexplicitly on this type of plot. However, the frequency corresponding to selected data points
44、 may be directly annotated on theNyquist plot. The magnitude of the appropriate impedance components increases when moving away from the origin of thecorresponding axes. Higher frequency data points are typically located towards the origin of the plot while lower frequency pointscorrespond to the in
45、creasing magnitude of the impedance components.8.2.3 Recommended units for both axes are ohmcm2. The units ohmcm2 are obtained by multiplying the measured resistanceor impedance by the exposed specimen area. For a resistor and capacitor, or dummy cell equivalent circuit, the assumed area is1 cm2. Re
46、garding the impedance data shown in Fig. 8 for the circuit of Fig. 7, the distance from the origin to the first (highfrequency) intercept with the abscissa corresponds to Rs. The distance between the first intercept and the second (low frequency)intercept with the abscissa corresponds to Rp.8.3 Bode
47、 Format:8.3.1 Electrochemical impedance data may be reported as two types of Bode plots. In the first case, the base ten logarithm ofthe impedance magnitude or Modulus, |Z|, is plotted on the ordinate and the base ten logarithm of the frequency is plotted on theabscissa. In this practice increasing
48、frequency values are plotted to the right of the origin parallel to the x axis (abscissa) andincreasing values of impedance magnitude are plotted vertically from the origin parallel to the y axis (ordinate). The origin itselfis chosen at appropriate nonzero values of impedance magnitude and frequenc
49、y.FIG. 7 Equivalent Electrical Circuit Model for a Simple Corroding ElectrodeFIG. 8 Nyquist Plot for Equivalent Circuit of Fig. 7G3 1378.3.2 Fig. 9 shows a typical plot for the simple electrical circuit model of Fig. 7. The magnitude of the high frequencyimpedance where the impedance magnitude is independent of frequency corresponds to Rs. The difference in magnitude betweenthe low frequency and the high frequency frequency-independent regions of impedance magnitude corresponds to Rp. Theseresistances are identical to those on the Nyquist f
