1、Designation:G389(Reapproved 2004)Standard Practice forConventions Applicable to Electrochemical Measurementsin Corrosion Testing1This standard is issued under the fixed designation G 3; the number immediately following the designation indicates the year of originaladoption or, in the case of revisio
2、n, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscriptepsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers conventions for reporting anddisplaying electrochemical corrosion data. Conventio
3、ns forpotential, current density, electrochemical impedance and ad-mittance, as well as conventions for graphical presentation ofsuch data are included.1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of thi
4、s 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
5、Use3.1 This practice provides guidance for reporting, display-ing, and plotting electrochemical corrosion data and includesrecommendations on signs and conventions. Use of this prac-tice will result in the reporting of electrochemical corrosiondata in a standard format, facilitating comparison betwe
6、en datadeveloped at different laboratories or at different times. Therecommendations outlined in this standard may be utilizedwhen recording and reporting corrosion data obtained fromelectrochemical tests such as potentiostatic and potentiody-namic polarization, polarization resistance, electrochemi
7、calimpedance 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
8、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 nonpassiv
9、e 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
10、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
11、 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 pote
12、ntial will be positive if the meter is on scale when thepolarity switch is negative, and vice versa.NOTE 1In 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 o
13、n 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
14、 magnesium or zinc should be negativeina1N NaCl solution if measured against a saturated standard calomelelectrode (SCE).5. Sign Convention for Electrode Potential TemperatureCoefficients5.1 There are two types of temperature coefficients ofelectrode potential: isothermal temperature coefficients an
15、d thethermal coefficients. The sign convention recommended forboth types of temperature coefficients is that the temperature1This 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
16、 Corrosion Testing.Current edition approved Nov 1, 2004. Published November 2004. Originallyapproved in 1968. Last previous edition approved in 1999 as G389(1999).2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book
17、 of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3The boldface numbers in parentheses refer to the list of references at the end ofthis practice.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, U
18、nited States.coefficient is positive when an increase in temperature pro-duces an increase (that is, it becomes more positive) in theelectrode potential. Likewise, the second temperature coeffi-cient is positive when an increase in temperature produces anincrease (that is, it becomes more positive)
19、in the first tem-perature coefficient.6. Sign Convention for Current and Current Density6.1 The sign convention in which anodic currents andcurrent densities are considered positive and cathodic currentsand current densities are negative is recommended. When thepotential is plotted against the logar
20、ithm of the current density,only the absolute values of the current density can be plotted.In such plots, the values which are cathodic should be clearlydifferentiated from the anodic values if both are present.7. Conventions for Displaying Polarization Data7.1 Sign ConventionsThe standard mathemati
21、cal practicefor plotting graphs is recommended for displaying electro-chemical corrosion data. In this practice, positive values areplotted above the origin on the ordinate axis and to the right ofthe origin on the abscissa axis. In logarithmic plots, theabscissa value increases from left to right a
22、nd the ordinatevalue increases from bottom to top.7.2 Current Density-Potential PlotsA uniform conventionis recommended for plotting current density-potential data,namely, plot current density along the abscissa and potentialalong the ordinate. In current density potential plots, thecurrent density
23、may be plotted on linear or logarithmic axes. Ingeneral, logarithmic plots are better suited to incorporation ofwide ranges of current density data and for demonstrating Tafelrelationships. Linear plots are recommended for studies inwhich the current density or potential range is small, or in casesw
24、here the region in which the current density changes fromanodic to cathodic is important. Linear plots are also used forthe determination of the polarization resistance Rp, which isdefined as the slope of a potential-current density plot at thecorrosion potential Ecorr. The relationship between the
25、polar-ization resistance Rpand the corrosion current density icorris asfollows (2, 3):FdDE!diGDE 5 05 Rp5babc2.303ba1 bc!icorr(1)where:ba= anodic Tafel slope,bc= cathodic Tafel slope, andDE = the difference E Ecorr, where E is the specimenpotential.Fig. 2 is a plot of polarization, E Ecorr, versus c
26、urrent densityi (solid line) from which the polarization resistance Rphas beendetermined as the slope of the curve at the corrosion potentialEcorr.7.3 Potential Reference PointsIn plots where electrodepotentials are displayed, some indication of the conversion ofthe values displayed to both the stan
27、dard hydrogen electrodescale (SHE) and the saturated calomel electrode scale (SCE) isrecommended if they are known. For example, when electrodepotential is plotted as the ordinate, then the SCE scale could beshown at the extreme left of the plot and the SHE scale shownat the extreme right. An altern
28、ative, in cases where thereference electrode was not either SCE or SHE, would be toshow on the potential axis the potentials of these electrodesagainst the reference used. In cases where these points are notshown on the plot, an algebraic conversion could be indicated.For example, in the case of a s
29、ilver-silver chloride referenceelectrode (1 M KCl), the conversion could be shown in the titlebox as:SCE 5 E 2 0.006 V (2)SHE 5 E 1 0.235 Vwhere E represents electrode potential measured against thesilver-silver chloride standard (1 M KCl).NOTE 2A table of potentials for various common reference ele
30、ctrodesis presented in Appendix X2.7.4 UnitsThe recommended unit of potential is the volt.In cases where only small potential ranges are covered,millivolts or microvolts may be used. The SI units for currentdensity are ampere per square metre or milliampere per squarecentimetre (IEEE/ASTM SI-10). St
31、ill in use are units ex-pressed in amperes per square centimetre, and microamperesper square centimetre.7.5 Sample Polarization CurvesSample polarization plotsemploying these recommended practices are shown in Figs.2-6. Fig. 3 and Fig. 4 are hypothetical curves showing activeand active-passive anode
32、 behavior, respectively. Fig. 5 and Fig.6 are actual polarization data for Type 430 stainless steel (UNS43000) (4) and two aluminum samples (5). Fig. 3 and Fig. 4 areexhibited to illustrate graphically the location of various pointsused in discussion of electrochemical methods of corrosiontesting. T
33、he purpose of Fig. 5 and Fig. 6 is to show how varioustypes of electrode behavior can be plotted in accordance withthe proposed conventions.8. Conventions for Displaying ElectrochemicalImpedance Data8.1 Three graphical formats in common use for reportingelectrochemical impedance data are the Nyquist
34、, Bode, andAdmittance formats. These formats are discussed for a simpleNOTE 1The electrode potential of specimen is negative as shown.FIG. 1 Schematic Diagram of an Apparatus to Measure ElectrodePotential of a SpecimenG389 (2004)2electrode system modelled by an equivalent electrical circuit asshown
35、in Fig. 7. In the convention utilized the impedance isdefined as:Z 5 Z8 1 jZ9 (3)where:Z = real or in-phase component of impedance,Z9 = the imaginary or out-of-phase component of imped-ance, andj2= 1.The impedance magnitude or modulus is defined as|Z|2=(Z8)2+(Z9). For the equivalent electrical circu
36、it shown inFig. 7, the imaginary component of impedanceZ9 5212pfC(4)where:f = frequency in cycles per second (or hertz, Hz, where oneHz is equal to 2p radians/s, and w =2pf, where theunits for w are radians/s), andC = capacitance in farads.The phase angle, u is defined as:u5arctan Z9/Z8!. (5)The adm
37、ittance, Y, is defined as1/Z 5 Y 5 Y8 1 jY9 (6)where:Y8 = real or in-phase component of admittance, andY9 = the imaginary of out-of-phase component of admit-tance.8.2 Nyquist Format (Complex Plane, or Cole-Cole):8.2.1 The real component of impedance is plotted on theabscissa and the negative of the
38、imaginary component isplotted on the ordinate. In this practice positive values of thereal component of impedance are plotted to the right of theorigin parallel to the x axis (abscissa). Negative values of theimaginary component of impedance are plotted vertically fromthe origin parallel to the y ax
39、is (ordinate).8.2.2 Fig. 8 shows a Nyquist plot for the equivalent circuitof Fig. 7. The frequency dependence of the data is not shownexplicitly on this type of plot. However, the frequency corre-sponding to selected data points may be directly annotated onthe Nyquist plot. The magnitude of the appr
40、opriate impedancecomponents increases when moving away from the origin ofthe corresponding axes. Higher frequency data points aretypically located towards the origin of the plot while lowerfrequency points correspond to the increasing magnitude of theimpedance components.8.2.3 Recommended units for
41、both axes are ohmcm2. Theunits ohmcm2are obtained by multiplying the measuredresistance or impedance by the exposed specimen area. For aresistor and capacitor, or dummy cell equivalent circuit, theassumed area is 1 cm2. Regarding the impedance data shown inFig. 8 for the circuit of Fig. 7, the dista
42、nce from the origin tothe first (high frequency) intercept with the abscissa corre-sponds to Rs. The distance between the first intercept and thesecond (low frequency) intercept with the abscissa correspondsto Rp.8.3 Bode Format:FIG. 2 Hypothetical Linear Polarization PlotG389 (2004)38.3.1 Electroch
43、emical impedance data may be reported astwo types of Bode plots. In the first case, the base ten logarithmof the impedance magnitude or Modulus, |Z|, is plotted on theordinate and the base ten logarithm of the frequency is plottedon the abscissa. In this practice increasing frequency values areplott
44、ed to the right of the origin parallel to the x axis (abscissa)and increasing values of impedance magnitude are plottedvertically from the origin parallel to the y axis (ordinate). Theorigin itself is chosen at appropriate nonzero values of imped-ance magnitude and frequency.FIG. 3 Hypothetical Cath
45、odic and Anodic Polarization DiagramFIG. 4 Hypothetical Cathodic and Anodic Polarization Plots for a Passive AnodeG389 (2004)48.3.2 Fig. 9 shows a typical plot for the simple electricalcircuit model of Fig. 7. The magnitude of the high frequencyimpedance where the impedance magnitude is independent
46、offrequency corresponds to Rs. The difference in magnitudebetween the low frequency and the high frequency frequency-independent regions of impedance magnitude corresponds toRp. These resistances are identical to those on the Nyquistformat plot shown in Fig. 8.8.3.3 In the second type of Bode plot,
47、the negative of thephase angle, u, is plotted on the ordinate and the base tenlogarithm of the frequency is plotted on the abscissa. In thispractice increasing values of the negative of the phase angle areplotted in the vertical direction from the origin along the y axis(ordinate). In this format, a
48、 pure capacitive behavior is plottedas a positive value of 90. Fig. 10 shows a typical plot for thesimple electrode model shown in Fig. 7.8.3.4 The units for the frequency on both plots are eitherhertz (cycles per second) or radians per second (radians persecond = 2p radians per cycle multiplied by
49、the number ofcycles per second). The units of the impedance magnitude areFIG. 5 Typical Potentiostatic Anodic Polarization Plot for Type 430 Stainless Steel in 1.0 N H2SO4FIG. 6 Typical Polarization Plots for Aluminum Materials in 0.2 N NaCl SolutionG389 (2004)5ohmcm2. The units ohmcm2are obtained by multiplying themeasured resistance or impedance by the exposed specimenarea. The units of the phase angle are degrees.8.4 Admittance Format (Complex Plane)The real compo-nent of admittance is plotted on the abscissa and the imaginarycomponent of admittance is plott
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