# ASTM G106-89 (Reapproved 2015)

Designation: G106 − 89 (Reapproved 2015)Standard Practice forVerification of Algorithm and Equipment for ElectrochemicalImpedance Measurements1This standard is issued under the fixed designation G106; 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.1. Scope1.1 This practice covers an experimental procedure whichcan be used to check one’s instrumentation and technique forcollecting and presenting electrochemical impedance data. Iffollowed, this practice provides a standard material,electrolyte, and procedure for collecting electrochemical im-pedance data at the open circuit or corrosion potential thatshould reproduce data determined by others at different timesand in different laboratories. This practice may not be appro-priate for collecting impedance information for all materials orin all environments.1.2 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.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:2D1193 Specification for Reagent WaterG3 Practice for Conventions Applicable to ElectrochemicalMeasurements in Corrosion TestingG5 Reference Test Method for Making PotentiodynamicAnodic Polarization MeasurementsG15 Terminology Relating to Corrosion and Corrosion Test-ing (Withdrawn 2010)3G59 Test Method for Conducting Potentiodynamic Polariza-tion Resistance Measurements3. Terminology3.1 Definitions—For definitions of corrosion related terms,see Terminology G15.3.2 Symbols:C = capacitance (farad-cm−2)Eʹ = real component of voltage (volts)E“ = imaginary component of voltage (volts)E = complex voltage (volts)f = frequency (s−1)Iʹ = real component of current (amp-cm−2)I“ = imaginary component of current (amp-cm−2)I = complex current (amp-cm−2)j ==21L = inductance (henry − cm2)Rs= solution resistance (ohm-cm2)Rp= polarization resistance (ohm-cm2)Rt= charge transfer resistance (ohm-cm2)Zʹ = real component of impedance (ohm-cm2)Z“ = imaginary component of impedance (ohm-cm2)Z = complex impedance (ohm-cm2)α = phenomenological coefficients caused by depressionof the Nyquist plot below the real axis, α is theexponent and τ is the time constant(s).θ = phase angle (deg)ω = frequency (radians-s−1)3.3 Subscripts:x = in-phase componenty = out-of-phase component4. Summary of Practice4.1 Reference impedance plots in both Nyquist and Bodeformat are included. These reference plots are derived from theresults from nine different laboratories that used a standarddummy cell and followed the standard procedure using a1This 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 Nov. 1, 2015. Published December 2015. Originallyapproved in 1989. Last previous edition approved in 2010 as G106–89(2010). DOI:10.1520/G0106-89R15.2For 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 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 States1specific ferritic type alloy UNS-S430004in 0.005 M H2SO4and 0.495 M Na2SO4. The plots for the reference material arepresented as an envelope that surrounds all of the data with andwithout inclusion of the uncompensated resistance. Plots forone data set from one laboratory are presented as well. Sincethe results from the dummy cell are independent of laboratory,only one set of results is presented.4.2 A discussion of the electrochemical impedancetechnique, the physics that underlies it, and some methods ofinterpreting the data are given in the Appendix X1 – AppendixX6. These sections are included to aid the individual inunderstanding the electrochemical impedance technique andsome of its capabilities. The information is not intended to beall inclusive.5. Significance and Use5.1 The availability of a standard procedure, standardmaterial, and standard plots should allow the investigator tocheck his laboratory technique. This practice should lead toelectrochemical impedance curves in the literature which canbe compared easily and with confidence.5.2 Samples of a standard ferritic type 430 stainless steel(UNS 430000) used to obtain the reference plots are availablefor those who wish to check their equipment. Suitable resistorsand capacitors can be obtained from electronics supply houses.5.3 This test method may not be appropriate for electro-chemical impedance measurements of all materials or in allenvironments.6. Apparatus6.1 Dummy Cell—The dummy cell used to check theequipment and method for generating electrochemical imped-ance data is composed of a 10 Ω precision resistor placed inseries with a circuit element composed of a 100 Ω precisionresistor in parallel with a 100 µF capacitor. The resistors shouldhave a stated precision of 60.1 %. The capacitor can have aprecision of 620 %. The cell can be constructed from readilyavailable circuit elements by following the circuit diagramshown in Fig. 1.6.2 Test Cell—The test cell should be constructed to allowthe following items to be inserted into the solution chamber:the test electrode, two counter electrodes or a symmetricallyarranged counter electrode around the working electrode, aLuggin-Haber capillary with salt bridge connection to thereference electrode, an inlet and an outlet for an inert gas, anda thermometer or thermocouple holder. The test cell must beconstructed of materials that will not corrode, deteriorate, orotherwise contaminate the solution.6.2.1 One type of suitable cell is described in ReferenceTestMethod G5. Cells are not limited to that design. For example,a 1-L round-bottom flask can be modified for the addition ofvarious necks to permit the introduction of electrodes, gas inletand outlet tubes, and the thermometer holder. A Luggin-Habercapillary probe could be used to separate the bulk solution fromthe saturated calomel electrode. The capillary tip can be easilyadjusted to bring it into close proximity to the workingelectrode. The minimum distance should be no less than twocapillary diameters from the working electrode.6.3 Electrode Holder—The auxiliary and working elec-trodes can be mounted in the manner shown in Reference TestMethod G5. Precautions described in Reference Test MethodG5 about assembly should be followed.6.4 Potentiostat—The potentiostat must be of the kind thatallows for the application of a potential sweep as described inReference Test Method G5 and Reference Practice G59. Thepotentiostat must have outputs in the form of voltage versusground for both potential and current. The potentiostat musthave sufficient bandwidth for minimal phase shift up to at least1000 Hz and preferably to 10 000 Hz. The potentiostat must becapable of accepting an external excitation signal. Manycommercial potentiostats meet the specification requirementsfor these types of measurements.6.5 Collection and Analysis of Current-Voltage Response—The potential and current measuring circuits must have thecharacteristics described in Reference Test Method G5 alongwith sufficient band-width as described above. The impedancecan be calculated in several ways, for example, by means of atransfer function analyzer, Lissajous figures on an oscilloscope,or transient analysis of a white noise input using a Fast FourierTransform algorithm. Other methods of analysis exist.6.6 Electrodes:4These standard samples are available fromASTM Headquarters. Generally, onesample can be repolished and reused for many runs. This procedure is suggested toconserve the available material.FIG. 1 Circuit Diagram for Dummy Cell Showing Positions for Hook-Up to PotentiostatG106 − 89 (2015)26.6.1 Working electrode preparation should follow Refer-ence Test Method G5, which involves drilling and tapping thespecimen and mounting it on the electrode holder.6.6.2 Auxillary electrode preparation should follow Refer-ence Test Method G5. The auxillary electrode arrangementshould be symmetrical around the working electrode.6.6.3 Reference electrode type and usage should followReference Test Method G5. The reference electrode is to be asaturated calomel electrode.7. Experimental Procedure7.1 Test of Algorithm and Electronic Equipment (DummyCell):7.1.1 Measure the impedance of a dummy cell consisting ofa10Ω resistor in series with a parallel combination of a 100 Ωresistor and a 100 µF capacitor. The circuit diagram is shownin Fig. 1.7.1.2 Typical connections from the potentiostat are shown inFig. 1. Connect the auxiliary electrode and reference electrodeleads to the series resistor side of the circuit. Connect theworking electrode lead to the opposite side of the circuitbeyond the resistor-capacitor parallel combination.7.1.3 Set the potential at 0.0 V. Collect the electrochemicalimpedance data between 10 000 Hz (10 kHz) and 0.1 Hz(100 mHz) at 8 to 10 steps per frequency decade. The ampli-tude must be the same as that used to check the electrochemicalcell, 10 mV. The resulting frequency response when plotted inNyquist format (the negative of the imaginary impedanceversus the real impedance) must agree with that shown in Figs.2-4. Testing with the electrochemical cell should not beattempted until that agreement is established. Results using thedummy circuit were found to be independent of laboratory.7.2 Test of Electrochemical Cell:7.2.1 Test specimens of the reference material should beprepared following the procedure described in Reference TestMethod G5. This procedure involves polishing the specimenwith wet SiC paper with a final wet polish using 600 grit SiCpaper prior to the experiment. There should be a maximumdelay of 1 h between final polishing and immersion in the testsolution.7.2.2 Prepare a 0.495 M Na2SO4solution containing 0.005MH2SO4from reagent grade sulfuric acid and sodium sulfateFIG. 2 Nyquist Plot of Electrochemical Impedance Response forDummy CellFIG. 3 Bode Plot, Impedance Magnitude Versus Frequency, ofElectrochemical Impedance Response for Dummy CellFIG. 4 Bode Plot, Phase Angle Versus Frequency, of Electro-chemical Impedance Response for Dummy CellG106 − 89 (2015)3and Type IV reagent water described in Specification D1193.The test is to be carried out at 30 6 1°C.7.2.3 At least 1 h before specimen immersion, start purgingthe solution with oxygen-free argon, hydrogen, or nitrogen gasat a flow rate of about 100 to 150 cm3/min. Continue the purgethroughout the test.7.2.4 Transfer the specimen to the test cell. Adjust theLuggin-Haber probe tip so that it is no less than two capillarydiameters from the sample. However, since this distance willaffect the uncompensated solution resistance, the greater thedistance, the larger the resistance. Therefore, close placementis important.7.2.5 Connect the potentiostat leads to the appropriateelectrodes, for example, working electrode lead to workingelectrode, counter electrode lead to counter electrode, andreference electrode lead to reference electrode. Hook-up in-structions provided with the potentiostat must be followed.7.2.6 Record the open circuit potential, that is, the corrosionpotential, for 1 h. The potential should be about −645 6 10 mVrelative to the saturated calomel electrode. If the potential ismore positive than −600 mV (SCE) then the specimen mayhave passivated. If so, remove the specimen and repolish with600 grit wet silicon carbide paper. Then reimmerse the sampleand monitor the corrosion potential for 1 h. If the potentialagain becomes more positive than −600 mV (SCE) check foroxygen contamination of the solution.7.2.7 Record the frequency response between 10 000 Hz(10 kHz) and 0.1 Hz (100 mHz) at the corrosion potentialrecorded after 1 h of exposure using 8 to 10 steps per frequencydecade. The amplitude must be the same as that used in 7.1.3,10 mV.7.2.8 Plot the frequency response in both Nyquist format(real response versus the negative of the imaginary response)and Bode format (impedance modulus and phase angle versusfrequency). Frequency can be reported in units of radians/second or hertz (cycles/s).7.2.9 There was no attempt to estimate circuit analogues forthe electrochemical impedance curves since there is no univer-sally recognized, standard method for making such estimates.8. Standard Reference Results and Plots8.1 Dummy Cell:8.1.1 The results from nine different laboratories werevirtually identical and overlaid each other almost perfectly.Typical plots of the raw data are shown in Figs. 2-4. No attempthas been made to estimate the variance and standard deviationof the results from the nine laboratories. The measured valuesof Rs,Rp, and the frequency at which the phase angle is amaximum must agree with these curves within the specifica-tions of the instrumentation, resistors, and capacitors beforetesting of the electrochemical cell commences. See 9.1.1.8.2 Electrochemical Cell:8.2.1 Standard electrochemical impedance plots in bothNyquist format and Bode format are shown in Figs. 5-7. Theseare actual results from one laboratory. Figs. 8-10 show plots inboth Nyquist and Bode formats which envelop all of the resultsfrom the nine laboratories. The solution resistance from eachlaboratory was not subtracted out prior to making this plot.8.2.2 The average solution resistance from the nine labora-tories in 3.3 Ω-cm26 1.8 Ω-cm2(one standard deviation). Thesolution resistance of the user’s test cell as measured by thehigh frequency intercept on the Nyquist plot must lie in thisrange to use agreement with Figs. 8-10 for verification of theelectrochemical test cell. If the uncompensated resistance liesoutside of this range, it should be subtracted from the results(see 7.2.4). Then, results from the electrochemical test cell canbe compared with the results in Figs. 11-13 to verify the testcell. Figs. 11-13 envelop all of the results from the ninelaboratories with the uncompensated resistance subtracted out.9. Precision and Bias9.1 Dummy Cell:9.1.1 Reproducibility of the results for the dummy cell isdependent on the precision of the resistors and capacitor usedFIG. 5 Nyquist Plot of Typical Frequency Response for UNS-S43000 From One LaboratoryFIG. 6 Bode Plot, Impedance Magnitude Versus Frequency, forUNS-S43000 From One LaboratoryG106 − 89 (2015)4to construct the d