ASTM G3-89(2004)
(Practice)Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing
Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing
SIGNIFICANCE AND USE
This practice provides guidance for reporting, displaying, and plotting electrochemical corrosion data and includes recommendations on signs and conventions. Use of this practice will result in the reporting of electrochemical corrosion data in a standard format, facilitating comparison between data developed at different laboratories or at different times. The recommendations outlined in this standard may be utilized when recording and reporting corrosion data obtained from electrochemical tests such as potentiostatic and potentiodynamic polarization, polarization resistance, electrochemical impedance and admittance measurements, galvanic corrosion, and open circuit potential measurements.
SCOPE
1.1 This practice covers conventions for reporting and displaying electrochemical corrosion data. Conventions for potential, current density, electrochemical impedance and admittance, as well as conventions for graphical presentation of such data are included.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:G3–89(Reapproved 2004)
Standard Practice for
Conventions Applicable to Electrochemical Measurements
in Corrosion Testing
This standard is issued under the fixed designation G3; the number immediately following the designation indicates the year of original
adoptionor,inthecaseofrevision,theyearoflastrevision.Anumberinparenthesesindicatestheyearoflastreapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 4. Sign Convention for Electrode Potential
1.1 This practice covers conventions for reporting and 4.1 The Stockholm sign invariant convention is recom-
displaying electrochemical corrosion data. Conventions for mended for use in reporting the results of specimen potential
potential, current density, electrochemical impedance and ad- measurements in corrosion testing. In this convention, the
mittance, as well as conventions for graphical presentation of positivedirectionofelectrodepotentialimpliesanincreasingly
such data are included. oxidizing condition at the electrode in question. The positive
1.2 This standard does not purport to address all of the direction has also been denoted as the noble direction because
safety concerns, if any, associated with its use. It is the thecorrosionpotentialsofmostnoblemetals,suchasgold,are
responsibility of the user of this standard to establish appro- more positive than the nonpassive base metals. On the other
priate safety and health practices and determine the applica- hand,thenegativedirection,oftencalledtheactivedirection,is
bility of regulatory limitations prior to use. associated with reduction and consequently the corrosion
potentials of active metals, such as magnesium. This conven-
2. Referenced Documents
tionwasadoptedunanimouslybythe1953InternationalUnion
2.1 ASTM Standards: of Pure and Applied Chemistry as the standard for electrode
IEEE/ASTM SI 10 Standard for Use of the International
potential (1).
System of Units (SI) (the Modern Metric System) 4.2 In the context of a specimen electrode of unknown
potential in an aqueous electrolyte, consider the circuit shown
3. Significance and Use
in Fig. 1 with a reference electrode connected to the ground
3.1 This practice provides guidance for reporting, display-
terminal of an electrometer. If the electrometer reads on scale
ing, and plotting electrochemical corrosion data and includes
when the polarity switch is negative, the specimen electrode
recommendations on signs and conventions. Use of this prac-
potential is negative (relative to the reference electrode).
tice will result in the reporting of electrochemical corrosion
Conversely, if the electrometer reads on scale when polarity is
datainastandardformat,facilitatingcomparisonbetweendata
positive, the specimen potential is positive. On the other hand,
developed at different laboratories or at different times. The
if the specimen electrode is connected to the ground terminal,
recommendations outlined in this standard may be utilized
the potential will be positive if the meter is on scale when the
when recording and reporting corrosion data obtained from
polarity switch is negative, and vice versa.
electrochemical tests such as potentiostatic and potentiody-
NOTE 1—In cases where the polarity of a measuring instrument is in
namic polarization, polarization resistance, electrochemical
doubt, a simple verification test can be performed as follows: connect the
impedance and admittance measurements, galvanic corrosion,
measuring instrument to a dry cell with the lead previously on the
and open circuit potential measurements.
referenceelectrodetothenegativebatteryterminalandtheleadpreviously
on the specimen electrode to the positive battery terminal. Set the range
switchtoaccommodatethedrycellvoltage.Themeterdeflectionwillnow
This practice is under the jurisdiction ofASTM Committee G01 on Corrosion
show the direction of positive potential.
ofMetalsandisthedirectresponsibilityofSubcommitteeG01.11onElectrochemi-
Also, the corrosion potential of magnesium or zinc should be negative
cal Measurements in Corrosion Testing.
ina1 N NaCl solution if measured against a saturated standard calomel
Current edition approved Nov 1, 2004. Published November 2004. Originally
electrode (SCE).
approved in 1968. Last previous edition approved in 1999 as G3–89 (1999). DOI:
10.1520/G0003-89R04.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
the ASTM website. this practice.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
G3–89 (2004)
where the region in which the current density changes from
anodic to cathodic is important. Linear plots are also used for
the determination of the polarization resistance R , which is
p
defined as the slope of a potential-current density plot at the
corrosion potential E . The relationship between the polar-
corr
izationresistanceR andthecorrosioncurrentdensityi isas
p corr
follows (2, 3):
d~DE! b b
a c
5 R 5 (1)
F G p
di 2.303~b 1 b !i
a c corr
DE 50
where:
b = anodic Tafel slope,
a
b = cathodic Tafel slope, and
c
DE = the difference E− E , where E is the specimen
corr
potential.
Fig.2isaplotofpolarization, E− E ,versuscurrentdensity
corr
NOTE 1—The electrode potential of specimen is negative as shown.
i(solidline)fromwhichthepolarizationresistanceR hasbeen
FIG. 1 Schematic Diagram of an Apparatus to Measure Electrode p
Potential of a Specimen determined as the slope of the curve at the corrosion potential
E .
corr
7.3 Potential Reference Points—In plots where electrode
5. Sign Convention for Electrode Potential Temperature
potentials are displayed, some indication of the conversion of
Coefficients
the values displayed to both the standard hydrogen electrode
5.1 There are two types of temperature coefficients of
scale (SHE) and the saturated calomel electrode scale (SCE) is
electrode potential: isothermal temperature coefficients and the
recommended if they are known. For example, when electrode
thermal coefficients. The sign convention recommended for
potentialisplottedastheordinate,thentheSCEscalecouldbe
both types of temperature coefficients is that the temperature
shown at the extreme left of the plot and the SHE scale shown
coefficient is positive when an increase in temperature pro-
at the extreme right. An alternative, in cases where the
duces an increase (that is, it becomes more positive) in the
reference electrode was not either SCE or SHE, would be to
electrode potential. Likewise, the second temperature coeffi-
show on the potential axis the potentials of these electrodes
cient is positive when an increase in temperature produces an
against the reference used. In cases where these points are not
increase (that is, it becomes more positive) in the first tem-
shown on the plot, an algebraic conversion could be indicated.
perature coefficient.
For example, in the case of a silver-silver chloride reference
electrode(1MKCl),theconversioncouldbeshowninthetitle
6. Sign Convention for Current and Current Density
box as:
6.1 The sign convention in which anodic currents and
SCE 5 E 20.006V (2)
current densities are considered positive and cathodic currents
SHE 5 E 10.235V
and current densities are negative is recommended. When the
where E represents electrode potential measured against the
potential is plotted against the logarithm of the current density,
silver-silver chloride standard (1 M KCl).
only the absolute values of the current density can be plotted.
In such plots, the values which are cathodic should be clearly NOTE 2—Atableofpotentialsforvariouscommonreferenceelectrodes
is presented in Appendix X2.
differentiated from the anodic values if both are present.
7.4 Units—The recommended unit of potential is the volt.
7. Conventions for Displaying Polarization Data
In cases where only small potential ranges are covered,
7.1 Sign Conventions—The standard mathematical practice millivolts or microvolts may be used. The SI units for current
for plotting graphs is recommended for displaying electro-
density are ampere per square metre or milliampere per square
chemical corrosion data. In this practice, positive values are centimetre(IEEE/ASTMSI-10IEEE/ASTMSI10).Stillinuse
plotted above the origin on the ordinate axis and to the right of are units expressed in amperes per square centimetre, and
the origin on the abscissa axis. In logarithmic plots, the microamperes per square centimetre.
abscissa value increases from left to right and the ordinate 7.5 Sample Polarization Curves—Samplepolarizationplots
value increases from bottom to top. employing these recommended practices are shown in Figs.
7.2 CurrentDensity-PotentialPlots—Auniformconvention 2-6. Fig. 3 and Fig. 4 are hypothetical curves showing active
is recommended for plotting current density-potential data, andactive-passiveanodebehavior,respectively.Fig.5andFig.
namely, plot current density along the abscissa and potential 6areactualpolarizationdataforType430stainlesssteel(UNS
along the ordinate. In current density potential plots, the 43000)(4)andtwoaluminumsamples(5).Fig.3andFig.4are
currentdensitymaybeplottedonlinearorlogarithmicaxes.In exhibitedtoillustrategraphicallythelocationofvariouspoints
general, logarithmic plots are better suited to incorporation of used in discussion of electrochemical methods of corrosion
widerangesofcurrentdensitydataandfordemonstratingTafel testing.ThepurposeofFig.5andFig.6istoshowhowvarious
relationships. Linear plots are recommended for studies in types of electrode behavior can be plotted in accordance with
whichthecurrentdensityorpotentialrangeissmall,orincases the proposed conventions.
G3–89 (2004)
FIG. 2 Hypothetical Linear Polarization Plot
FIG. 3 Hypothetical Cathodic and Anodic Polarization Diagram
G3–89 (2004)
FIG. 4 Hypothetical Cathodic and Anodic Polarization Plots for a Passive Anode
FIG. 5 Typical Potentiostatic Anodic Polarization Plot for Type 430 Stainless Steel in 1.0N H SO
2 4
8. Conventions for Displaying Electrochemical electrodesystemmodelledbyanequivalentelectricalcircuitas
Impedance Data
shown in Fig. 7. In the convention utilized the impedance is
defined as:
8.1 Three graphical formats in common use for reporting
electrochemical impedance data are the Nyquist, Bode, and
Z 5 Z8 1jZ9 (3)
Admittance formats. These formats are discussed for a simple
G3–89 (2004)
FIG. 6 Typical Polarization Plots for Aluminum Materials in 0.2N NaCl Solution
Y9 = 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 the
abscissa and the negative of the imaginary component is
plotted on the ordinate. In this practice positive values of the
real component of impedance are plotted to the right of the
origin parallel to the x axis (abscissa). Negative values of the
imaginary component of impedance are plotted vertically from
the origin parallel to the y axis (ordinate).
FIG. 7 Equivalent Electrical Circuit Model for a Simple Corroding
8.2.2 Fig. 8 shows a Nyquist plot for the equivalent circuit
Electrode
of Fig. 7. The frequency dependence of the data is not shown
explicitly on this type of plot. However, the frequency corre-
where:
sponding to selected data points may be directly annotated on
Z = real or in-phase component of impedance,
the Nyquist plot. The magnitude of the appropriate impedance
Z9 = the imaginary or out-of-phase component of imped-
ance, and
j = −1.
The impedance magnitude or modulus is defined as
2 2
|Z| =(Z8) +(Z9).Fortheequivalentelectricalcircuitshownin
Fig. 7, the imaginary component of impedance
Z9 5 (4)
2pfC
where:
f = frequencyincyclespersecond(orhertz,Hz,whereone
Hz is equal to 2p radians/s, and w=2pf, where the
units for w are radians/s), and
C = capacitance in farads.
The phase angle, u is defined as:
u5arctan ~Z9/Z8!. (5)
The admittance, Y, is defined as
1/Z 5 Y 5 Y8 1 jY9 (6)
where:
Y8 = real or in-phase component of admittance, and
FIG. 8 Nyquist Plot for Equivalent Circuit of Fig. 7
G3–89 (2004)
components increases when moving away from the origin of 8.3.3 In the second type of Bode plot, the negative of the
the corresponding axes. Higher frequency data points are phase angle,−u, is plotted on the ordinate and the base ten
typically located towards the origin of the plot while lower logarithm of the frequency is plotted on the abscissa. In this
frequencypointscorrespondtotheincreasingmagnitudeofthe practiceincreasingvaluesofthenegativeofthephaseangleare
impedance components. plottedintheverticaldirectionfromtheoriginalongthe yaxis
8.2.3 Recommended units for both axes are ohm·cm . The (ordinate). In this format, a pure capacitive behavior is plotted
units ohm·cm are obtained by multiplying the measured as a positive value of 90°. Fig. 10 shows a typical plot for the
resistance or impedance by the exposed specimen area. For a simple electrode model shown in Fig. 7.
resistor and capacitor, or dummy cell equivalent circuit, the 8.3.4 The units for the frequency on both plots are either
assumedareais1cm .Regardingtheimpedancedatashownin hertz (cycles per second) or radians per second (radians per
Fig. 8 for the circuit of Fig. 7, the distance from the origin to second=2p radians per cycle multiplied by the number of
the first (high frequency) intercept with the abscissa corre- cycles per second). The units of the impedance magnitude are
2 2
sponds to R . The distance between the first intercept and the ohm·cm . The units ohm·cm are obtained by multiplying the
s
second(lowfrequency)interceptwiththeabscissacorresponds measured resistance or impedance by the exposed specimen
to R . area. The units of the phase angle are degrees.
p
8.3 Bode Format: 8.4 Admittance Format (Complex Plane)—Therealcompo-
8.3.1 Electrochemical impedance data may be reported as nent of admittance is plotted on the abscissa and the imaginary
twotypesofBodeplots.Inthefirstcase,thebasetenlogarithm component of admittance is plotted on the ordinate. In this
of the impedance magnitude or Modulus, |Z|, is plotted on the practice positive values of the real component of admittance
ordinate and the base ten logarithm of the frequency is plotted are plotted to the right of the origin parallel to the x axis
ontheabscissa.Inthispr
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