ASTM G5-94(1999)

Designation:G5–94 (Reapproved 1999)
Standard Reference Test Method for
Making Potentiostatic and Potentiodynamic Anodic
Polarization Measurements
This standard is issued under the fixed designation G 5; the number immediately following the designation indicates the year of original
adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript
epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope available for those who wish to check their own test procedure
and equipment.
1.1 This test method describes an experimental procedure
3.3 Standard potentiostatic and potentiodynamic polariza-
for checking experimental technique and instrumentation. If
tion plots are supplied with the purchase of the reference
followed, this test method will provide repeatable potentio-
material. These reference data are based on the results from
static and potentiodynamic anodic polarization measurements
different laboratories that followed the standard procedure,
that will reproduce data determined by others at other times
using that material in 1.0 N H SO . Maximum and minimum
2 4
and in other laboratories provided all laboratories are testing
current values are shown at each potential to indicate the
reference samples from the same lot of Type 430 stainless steel.
acceptable range of values.
1.2 Values stated in SI units are to be regarded as the
3.4 This test method may not be appropriate for polarization
standard. Inch-pound units given in parentheses are for infor-
testing of all materials or in all environments.
mation only.
3.5 This test method is intended for use in evaluating the
1.3 This standard does not purport to address all of the
accuracy of a given electrochemical test apparatus, not for use
safety concerns, if any, associated with its use. It is the
in evaluating materials performance. Therefore, the use of the
responsibility of the user of this standard to establish appro-
plots in Figs. 1 and 2 or Appendix X2 is not recommended to
priate safety and health practices and determine the applica-
evaluate alloys other than Type 430, or lots of Type 430 other
bility of regulatory limitations prior to use.
than those available through ASTM. The use of the data in this
2. Referenced Documents test method in this manner is beyond the scope and intended
use of this test method. Users of this test method are advised to
2.1 ASTM Standards:
evaluate test results relative to the scatter bands corresponding
E 1338 Guide for the Identification of Metals and Alloys in
to the particular lot of Type 430 stainless steel that was tested.
Computerized Material Property Databases
G 3 Practice for Conventions Applicable to Electrochemical
4. Apparatus
Measurements in Corrosion Testing
4.1 The test cell should be constructed to allow the follow-
G 107 Guide for Formats for Collection and Compilation of
ing items to be inserted into the solution chamber: the test
Corrosion Data for Metals for Computerized Database
electrode, two auxiliary electrodes, a Luggin capillary with
Input
salt-bridge connection to the reference electrode, inlet and
3. Significance and Use
outlet for an inert gas, and a thermometer. The test cell shall be
constructed of materials that will not corrode, deteriorate, or
3.1 The availability of a standard procedure, standard ma-
otherwise contaminate the test solution.
terial, and a standard plot should make it easy for an investi-
gator to check his techniques. This should lead to polarization
NOTE 1—Borosilicate glass and TFE-fluorocarbon have been found
curves in the literature which can be compared with confi-
suitable.
dence.
4.1.1 A suitable cell is shown in Fig. 3 (1). A 1-L,
3.2 Samples of a standard ferritic Type 430 stainless steel
roundbottom flask has been modified by the addition of various
(UNS S43000) used in obtaining standard reference plot are
necks to permit the introduction of electrodes, gas inlet and
outlet tubes, and a thermometer. The Luggin probe-salt bridge
separates the bulk solution from the saturated calomel refer-
ence electrode, and the probe tip can be easily adjusted to bring
This test method is under the jurisdiction of ASTM Committee G-1 on
Corrosion of Metals and is the direct responsibility of G01.11 on Electrochemical
Measurements in Corrosion Testing.
Current edition approved March 15, 1994. Published May 1994. Originally
These standard samples are available from ASTM Headquarters. Generally, one
published as G 5 – 69. Last previous edition G5–87.
sample can be repolished and reused for many runs. This procedure is suggested to
Annual Book of ASTM Standards, Vol 14.01.
conserve the available material. Order PCN 12-700050-00.
Annual Book of ASTM Standards, Vol 03.02.
The boldface numbers in parentheses refer to the list of references at the end of
this test method.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
G5
CURRENT DENSITY (μA/cm )
FIG. 1 Typical Standard Potentiostatic Anodic Polarization Plot
CURRENT DENSITY (μA/cm )
FIG. 2 Typical Standard Potentiodynamic Anodic Polarization Plot
it in close proximity with the working electrode. circuits are provided with most potentiostats. Instruments
4.2 Potentiostat (Note 2): should have sufficient sensitivity and accuracy to detect a
4.2.1 A potentiostat that will maintain an electrode potential change of 1.0 mV over a potential range between −0.6 and 1.6
within 1 mV of a preset value over a wide range of applied V.
currents should be used. For the type and size of standard 4.4 Current-Measuring Instruments (Note 2):
specimen supplied, the potentiostat should have a potential 4.4.1 An instrument that is capable of measuring a current
range from −0.6 to 1.6 V and an anodic current output range accurately to within 1 % of the absolute value over a current
5 5
from 1.0 to 10 μA. range between 1.0 and 10 μA for a Type 430 stainless steel
4.3 Potential-Measuring Instruments (Note 2): (UNS S43000) specimen with a surface area of approximately
4.3.1 The potential-measuring circuit should have a high 5cm .
11 14
input impedance on the order of 10 to 10 V to minimize 4.5 Anodic Polarization Circuit:
current drawn from the system during measurements. Such 4.5.1 A schematic potentiostatic anodic polarization wiring
G5
FIG. 3 Schematic Diagram of Polarization Cell (1)
diagram (2) is illustrated in Fig. 4.
4.5.2 A scanning potentiostat is used for potentiodynamic
measurements. For such measurements the potentiostat shall be
capable of automatically varying the potential at a constant rate
between two preset potentials. A record of the potential and
current is plotted continuously using such instruments as an
X-Y recorder and a logarithmic converter incorporated into the
FIG. 5 Specimen Mounted on Electrode Holder
circuit shown in Fig. 4. Some potentiostats have an output of
the logarithm of the current as a voltage, which allows direct
much pressure may cause shielding of the electrode or break-
plotting of the potential log current curve using an X-Y
age of the glass holder, and too little pressure may cause
recorder.
leakage and subsequently crevice corrosion which may affect
the test results.)
NOTE 2—The instrumental requirements are based upon values typical
4.7 Electrodes:
of the instruments in 15 laboratories.
4.7.1 Working Electrode, prepared from a 12.7-mm ( ⁄2-in.)
4.6 Electrode Holder (1):
length of 9.5-mm ( ⁄8-in.) diameter rod stock. Each electrode is
4.6.1 The auxiliary and working electrodes are mounted in
drilled, tapped, and mounted in the manner discussed in 4.6.1.
the type of holder shown in Fig. 5. A longer holder is required
NOTE 3—If specimen forms are used other than those called for by this
for the working electrode than for the auxiliary electrode. A
test method, for example, flat sheet specimen, care should be taken since
leak-proof assembly is obtained by the proper compression fit
it was shown that crevices may be introduced which can lead to erroneous
between the electrode and a TFE-fluorocarbon gasket. (Too
results (see Fig. X1.1).
4.7.1.1 The standard AISI Type 430 stainless steel (UNS
S43000) should be used if one wishes to reproduce a standard
reference plot. This material is prepared from a single heat of
metal that is mill-annealed for ⁄2 h at 815°C (1500°F) and air
cooled. The chemical composition of the standard stainless
steel is supplied with the purchase of reference material.
4.7.2 Auxiliary Electrodes:
4.7.2.1 Two platinum auxiliary electrodes are prepared from
high-purity rod stock. Each electrode is drilled, tapped, and
mounted with a TFE-fluorocarbon gasket in the same manner
as the working electrode. A large platinum sheet sealed into a
glass holder is also acceptable.
4.7.2.2 A platinized surface may be utilized because of the
increased surface area. This may be accomplished by cleaning
the surface in hot aqua regia (3 parts concentrated HCl and 1
part concentrated HNO ), washing, and then drying. Both
electrodes are platinized by immersing them in a solution of
3 % platinic chloride and 0.02 % lead acetate and electrolyzing
at a current density of 40 to 50 mA/cm for4or5min (1, 3).
FIG. 4 Schematic Potentiostatic Anodic Polarization
Wiring Diagram (2) The polarity is reversed every minute. Occluded chloride is
G5
removed by electrolyzing in a dilute (10 %) sulfuric acid mounting nut until the gasket is properly compressed.
solution for several minutes with a reversal in polarity every
5.8 Degrease the specimen just prior to immersion and then
minute. Electrodes are rinsed thoroughly and stored in distilled
rinse in distilled water.
water until ready for use. Since certain ions can poison these
5.9 Transfer the specimen to the test cell and adjust the
electrodes, periodic checks of platinized platinum potentials
salt-bridge probe tip so it is about 2 mm or 2 times the tip
against a known reference electrode should be made.
diameter, whichever is larger from the specimen electrode.
4.7.2.3 Alternatively, graphite auxiliary electrodes can be
5.10 Record the open-circuit specimen potential, that is, the
used, but material retained by the graphite may contaminate
corrosion potential, after 55 min immersion. If platinum
subsequent experiments. This contamination can be minimized
counter electrodes and hydrogen gas are used, record the
by using high-density graphite or avoided by routinely replac-
platinum potential 50 min after immersion of the specimen.
ing the graphite electrode.
5.11 Potential Scan:
4.7.3 Reference Electrode (4):
5.11.1 Start the potential scan or step 1 h after specimen
4.7.3.1 A saturated calomel electrode with a controlled rate
immersion, beginning at the corrosion potential (E ) for
corr
of leakage (about 3 μL/h) is recommended. This type of
potentiodynamic measurements and the nearest 50-mV incre-
electrode is durable, reliable, and commercially available.
ment above E for the potentiostatic measurements. Proceed
corr
Precautions shall be taken to ensure that it is maintained in the
through + 1.60 V versus saturated calomel electrode (SCE)
proper condition. The potential of the calomel electrode should
(active to noble).
be checked at periodic intervals to ensure the accuracy of the
5.11.2 In the potentiostatic method, use a potentiostatic
electrode. For other alloy-electrolyte combinations a different
potential step rate of 50 mV every 5 min, recording the current
reference electrode may be preferred in order to avoid con-
at the end of each 5-min period at potential. These steps are
tamination of the reference electrode or the electrolyte.
repeated until a potential of + 1.6 V SCE is reached.
4.7.3.2 Alternatively, a saturated calomel electrode utilizing
5.11.3 In the potentiodynamic method, use a potentiody-
a semi-permeable membrane or porous plug tip may be used.
namic potential sweep rate of 0.6 V/h (65 %) recording the
These may require special care.
current continuously with change in potential from the corro-
5. Experimental Procedure
sion potential to + 1.6 V SCE.
5.1 Prepare 1 L of 1.0 N H SO from A.C.S. reagent grade
2 4 5.12 Plot anodic polarization data on semilogarithmic paper
acid and distilled water, for example, by using 27.8 mL of 98 %
in accordance with Practice G 3, (potential-ordinate, current
H SO /L of solution. Transfer 900 mL of solution to the clean
2 4 density-abscissa). If a potentiostat with a logarithmic converter
polarization cell.
is used, this plot can be produced directly during the measure-
5.2 Place the platinized auxiliary electrodes, salt-bridge
ment.
probe, and other components in the test cell and temporarily
close the center opening with a glass stopper. Fill the salt
6. Standard Reference Plots
bridge with test solution.
6.1 Standard polarization plots prepared from data obtained
NOTE 4—When using a controlled leakage salt bridge, the levels of the
by following the standard procedure discussed in this test
solution in the reference and polarization cells should be the same to avoid
method are supplied with the purchase of reference material.
siphoning. If this is impossible, a closed solution-wet (not greased)
Typical data are shown in Fig. 1 and Fig. 2 (5). The plots show
stopcock can be used in the salt bridge to eliminate siphoning, or a
a range of acceptable current density values at each potential.
semi-permeable membrane or porous plug tip may be used on the salt
The average corrosion potential is − 0.52 V, and the average
bridge.
platinized platinum potential is − 0.26 V.
5.3 Bring the temperature of the solution to 30 6 1°C by
immersing the test cell in a controlled-temperature water bath NOTE 5—The plots in Fig. 1 and Fig. 2 correspond to a lot of Type 430
stainless steel that is no longer available from ASTM (after July 1992).
or by other convenient means.
Figs. 1 and 2 are presented primarily for the discussion of precision and
5.4 Reduce oxygen levels in solution prior to immersion of
bias in Sections 6, 7, and Appendix X1. The scatter bands presented in
the test specimen. This may be accomplished by bubbling an
Appendix X2 were developed from a round robin testing program on the
oxygen-free gas such as hydrogen, argon, or nitrogen at a rate
lot of Type 430 stainless steel that is currently available from ASTM.
of 150 cm /min for a minimum of ⁄2 h.
6.2 Typical deviations from the standard potentiostatic plot
5.5 Prepare the working electrode surface within1hofthe
are shown and discussed in Appendix X1. Reference to this
experiment. Wet grind with 240-grit SiC paper, wet polish with
discussion may be helpful in determining the reasons for
600-grit SiC paper until previous coarse scratches are removed,
differences between an experimental curve and the standard
rinse, a
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