# ASTM G3-14

Designation: G3 − 14Standard 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 This practice provides guidance for reporting,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.NOTE 1—In 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 negativeina1N 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 G3–13. DOI:10.1520/G0003-14.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at service@astm.org. For Annual Book of ASTMStandards volume information, refer to the standard’s 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 States15. Sign Convention for Electrode Potential TemperatureCoefficients5.1 There are two types of temperature coefficients ofelectrode potential: isothermal temperature coefficients and thethermal coefficients. The sign convention recommended forboth types of temperature coefficients is that the temperaturecoefficient 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) 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 logarithm 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 Conventions—The standard mathematical 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 and the ordinatevalue increases from bottom to top.7.2 Current Density-Potential Plots—A 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 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 caseswhere 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 polar-ization resistance Rpand the corrosion current density icorris asfollows (2, 3):Fd~∆E!diG∆E505 Rp5babc2.303~ba1bc!icorr(1)where:ba= anodic Tafel slope,bc= cathodic Tafel slope, and∆E = the difference E − Ecorr, where E is the specimenpotential.Fig. 2 is a plot of polarization, E − Ecorr, versus currentdensity i (solid line) from which the polarization resistance Rphas been determined as the slope of the curve at the corrosionpotential Ecorr.7.3 Potential Reference Points—In plots where electrodepotentials are displayed, some indication of the conversion ofthe values displayed to both the standard 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 alternative, 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 silver-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 E10.235 Vwhere E represents electrode potential measured against thesilver-silver chloride standard (1 M KCl).NOTE 2—Atable of potentials for various common reference electrodesis presented in Appendix X2.7.4 Units—The recommended unit of potential is the volt. Incases where only small potential ranges are covered, millivoltsor microvolts may be used. The SI units for current density areampere per square metre or milliampere per square centimetre(IEEE/ASTM SI 10). Still in use are units expressed inamperes per square centimetre, and microamperes per squarecentimetre.7.5 Sample Polarization Curves—Sample polarization plotsemploying these recommended practices are shown in Figs.2-6. Fig. 3 and Fig. 4 are hypothetical curves showing activeand active-passive anode 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. The purpose of Fig. 5 and Fig. 6 is to show how variousNOTE 1—The electrode potential of specimen is negative as shown.FIG. 1 Schematic Diagram of an Apparatus to Measure ElectrodePotential of a SpecimenG3−142types 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, Bode, andAdmittance formats. These formats are discussed for a simpleelectrode system modelled by an equivalent electrical circuit asshown in Fig. 7. In the convention utilized the impedance isdefined as:Z 5 Z 1jZ“ (3)where:Z = real or in-phase component of impedance,Z“ = the imaginary or out-of-phase component ofimpedance, andj2= −1.The impedance magnitude or modulus is defined as|Z|2=(Z )2+(Z“). For the equivalent electrical circuit shown inFig. 7, the imaginary component of impedanceZ“ 5212πfC(4)where:f = frequency in cycles per second (or hertz, Hz, where oneHz is equal to 2π radians/s, and w =2πf, where the unitsfor w are radians/s), andC = capacitance in farads.The phase angle, θ is defined as:θ 5 arctan~Z“/Z ! (5)The admittance, Y, is defined as1/Z 5 Y 5 Y 1jY“ (6)where:FIG. 2 Hypothetical Linear Polarization PlotG3−143FIG. 3 Hypothetical Cathodic and Anodic Polarization DiagramFIG. 4 Hypothetical Cathodic and Anodic Polarization Plots for a Passive AnodeG3−144Y = 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 component of impedance is plotted on theabscissa and the negative of the 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 axis (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 appropriate impedancecomponents increases when moving away from the origin ofFIG. 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 SolutionG3−145the 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 both axes are ohm·cm2. Theunits ohm·cm2are 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 distance 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:8.3.1 Electrochemical 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 areplotted 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.8.3.2 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 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, the negative of thephase angle, −θ, 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 angleare plotted in the vertical direction from the origin along the yaxis (ordinate). In this format, a pure capacitive behavior isplotted as a positive value of 90°. Fig. 10 shows a typical plotfor the simple 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 = 2π radians per cycle multiplied by the number ofcycles per second). The units of the impedance magnitude areohm·cm2. The units ohm·cm2are 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