ASTM F746-04(2009)e1
(Test Method)Standard Test Method for Pitting or Crevice Corrosion of Metallic Surgical Implant Materials
Standard Test Method for Pitting or Crevice Corrosion of Metallic Surgical Implant Materials
SIGNIFICANCE AND USE
This test method is designed solely for determining comparative laboratory indices of performance. The results may be used for ranking alloys in order of increasing resistance to pitting and crevice corrosion under the specific conditions of this method. It should be noted that the method is intentionally designed to reach conditions that are sufficiently severe to cause breakdown of at least one alloy (Type 316 L stainless steel) currently considered acceptable for surgical implant use, and that those alloys which suffer pitting or crevice corrosion during the more severe portions of the test do not necessarily suffer localized corrosion when placed within the human body as a surgical implant.
SCOPE
1.1 This test method covers the determination of resistance to either pitting or crevice corrosion of metals and alloys from which surgical implants will be produced. It is a modified version of an established test and is used as a screening test to rank surgical implant alloys in order of their resistance to localized corrosion.
1.2 This test method applies only to passive metals and alloys. Nonpassive alloys (other than noble alloys) are susceptible to general corrosion and are not normally suitable for implant use.
1.3 This test method is intended for use as a laboratory screening test for metals and alloys which undergo pitting or crevice corrosion, or both.
1.4 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard.
1.5 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: F746 − 04(Reapproved 2009)
Standard Test Method for
Pitting or Crevice Corrosion of Metallic Surgical Implant
Materials
ThisstandardisissuedunderthefixeddesignationF746;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoptionor,inthecaseofrevision,theyearoflastrevision.Anumberinparenthesesindicatestheyearoflastreapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
´ NOTE—Units information was editorially corrected in January 2010.
1. Scope F86Practice for Surface Preparation and Marking of Metal-
lic Surgical Implants
1.1 This test method covers the determination of resistance
F2129Test Method for Conducting Cyclic Potentiodynamic
to either pitting or crevice corrosion of metals and alloys from
Polarization Measurements to Determine the Corrosion
which surgical implants will be produced. It is a modified
2 Susceptibility of Small Implant Devices
versionofanestablishedtest andisusedasascreeningtestto
G3Practice for Conventions Applicable to Electrochemical
rank surgical implant alloys in order of their resistance to
Measurements in Corrosion Testing
localized corrosion.
G5Reference Test Method for Making Potentiostatic and
1.2 This test method applies only to passive metals and
Potentiodynamic Anodic Polarization Measurements
alloys. Nonpassive alloys (other than noble alloys) are suscep-
G15TerminologyRelatingtoCorrosionandCorrosionTest-
tible to general corrosion and are not normally suitable for
ing (Withdrawn 2010)
implant use.
3. Summary of Test Method
1.3 This test method is intended for use as a laboratory
3.1 Acylindricalspecimenfittedwithaninerttaperedcollar
screening test for metals and alloys which undergo pitting or
is immersed in a phosphate buffered saline electrolyte at 37°C
crevice corrosion, or both.
for1hto establish a corrosion potential. Pitting (or crevice
1.4 The values stated in either SI units or inch-pound units
corrosion) is then stimulated by potentiostatically polarizing
are to be regarded separately as standard. The values stated in
thespecimentoapotentialmuchmorenoblethanthecorrosion
each system may not be exact equivalents; therefore, each
potential. Stimulation of pitting (or crevice corrosion) will be
system shall be used independently of the other. Combining
marked by a large and generally increasing polarizing current.
values from the two systems may result in non-conformance
3.2 Immediately after the stimulation step, the potential is
with the standard.
decreased as rapidly as possible to one of several preselected
1.5 This standard does not purport to address all of the
potentials at, or more noble than, the corrosion potential. If the
safety concerns, if any, associated with its use. It is the
alloy is susceptible to pitting (or crevice corrosion) at the
responsibility of the user of this standard to establish appro-
preselected potential, the polarizing current will remain at
priate safety and health practices and determine the applica-
relatively high values and will fluctuate or increase with time.
bility of regulatory limitations prior to use.
A post-test examination of the metal specimen establishes
whether localized corrosion has occurred by pitting of the
2. Referenced Documents
exposed surface or by preferential attack at the crevice formed
2.1 ASTM Standards:
by the tapered collar, or both.
D1193Specification for Reagent Water
3.3 If the pit (or crevice) surface repassivates at the pre-
selected potential and localized corrosion is halted, the polar-
ThistestmethodisunderthejurisdictionofASTMCommitteeF04onMedical
izing current will drop to values typical for passive surfaces
andSurgicalMaterialsandDevicesandisthedirectresponsibilityofSubcommittee
and the current will decrease continuously. The parameter of
F04.15 on Material Test Methods.
interest, the critical potential for pitting (or crevice corrosion),
Current edition approved Dec. 1, 2009. Published January 2010. Originally
is defined as the highest (most noble) pre-selected potential at
approved in 1981. Last previous edition approved in 2004 as F746–04. DOI:
10.1520/F0746-04R09E01.
whichpit(orcrevice)surfacesrepassivateafterthestimulation
Syrett, B. C., Corrosion, Vol 33, 1977, p. 221.
step.
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 The last approved version of this historical standard is referenced on
the ASTM website. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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F746 − 04 (2009)
4. Significance and Use thickness of 3.18 6 0.20 mm [0.125 6 0.008 in.]. The inside
diameter of the tapered collar should range from 0.38 mm
4.1 This test method is designed solely for determining
[0.015 in.] smaller than the diameter of the specimen to 0.38
comparative laboratory indices of performance. The results
mm [0.015 in.] larger. To be consistent with the dimensions
maybeusedforrankingalloysinorderofincreasingresistance
suggested in 5.2, the inside diameter should taper from 5.97 6
topittingandcrevicecorrosionunderthespecificconditionsof
0.05 mm [0.235 6 0.002 in.] to 6.73 6 0.05 mm [0.265 6
this method. It should be noted that the method is intentionally
0.002 in.]. See Fig. 1 for drawing of the tapered collar. The
designed to reach conditions that are sufficiently severe to
relativelyfinetolerancesareneededtoensureareproduciblefit
cause breakdown of at least one alloy (Type 316L stainless
and crevice.
steel) currently considered acceptable for surgical implant use,
and that those alloys which suffer pitting or crevice corrosion 5.4 In Reference Test Method G5, the method of specimen
during the more severe portions of the test do not necessarily
attachmentistodrillandtapthespecimentoreceiveathreaded
suffer localized corrosion when placed within the human body stainless steel connection rod. A4-40 thread is used, typically.
as a surgical implant.
However, because many surgical implant alloys are not easily
drilled,externalthreadsmayalsobemachined,ground,orcast,
5. Apparatus
as illustrated in Fig. 1. A small stainless steel adapter is fitted
5.1 The following required equipment is described in Ref- onto these threads and the adapter then accepts the connection
erence Test Method G5:
rod.
5.1.1 Standard Polarization Cell, of 1000 cm .
5.5 Determine the total exposed surface area of the speci-
5.1.2 Electrode Holders, for auxiliary and working elec-
men before placement of the PTFE collar, A ; determine the
T
trodes.
areaontheinternalsurfaceofthecollar(thecrevicedarea),A ;
C
5.1.3 Potentiostat, calibrated in accordance with Reference
and determine the exposed surface area of the specimen after
Test Method G5.
placement of the collar, A (where: A = A − A ). Dimensions
S S T C
5.1.4 Potential-Measuring Instrument.
should be measured to the nearest 0.1 mm.
5.1.5 Current-Measuring Instrument.
5.5.1 Example—Usingthedimensionssuggestedpreviously
5.1.6 Anodic Polarization Circuit.
forthespecimendiameter(d =6.35mm),thespecimenlength
5.1.7 Platinum Auxiliary Electrodes.
(l =20.00 mm), and the collar thickness (t =3.18 mm),
5.1.8 Saturated Calomel Electrode (SCE).
πd
5.1.9 Salt Bridge Probe.
A 5 πdl1 5 431 mm (1)
T
5.2 A cylindrical working electrode is fabricated from the
test material by machining, grinding, and suggested final
A 5 πdt 5 63mm (2)
C
polishing with 600-grit metallographic paper. It is suggested
A 5 A 2 A 5 386mm (3)
that the part of the cylindrical specimen that is exposed to the
S T C
testsolutionhavealengthof20.00 61.00mm[0.787 60.039
in.]andadiameterof6.35 60.03mm[0.250 60.001in.](see
6. Reagents
Fig. 1).
5.3 A crevice is created by fitting the cylindrical specimen 6.1 Electrolyte—Unless otherwise specified, phosphate
with a tapered collar, machined from commercial purity buffered saline (PBS) should be used as the standard test
polytetrafluoroethylene (PTFE). The collar should have an solution. A standard PBS formulation (see TableX2.3 of Test
outer diameter of 12.70 6 0.05 mm [0.500 6 0.002 in.] and a Method F2129) is the following: NaCl 8.0 g/L, KCl 0.2 g/L,
NOTE 1—Unless shown, dimensional tolerances are given in text.
FIG. 1 Dimensions of Specimen and Collar
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F746 − 04 (2009)
Na HPO ·12H O 1.15 g/L, KH PO 0.2 g/L, and bring to 1 L 7.7 Transferthespecimenelectrodeassemblytothetestcell
2 4 2 2 4
volumetrically using distilled water. and adjust the submerged salt bridge probe tip so it is about 2
6.1.1 The water shall be distilled conforming to the purity mm [0.08 in.] from the center of the bottom portion of the
requirements of Specification D1193, Type IV reagent water. specimen (below the collar).
6.1.2 After transferring the appropriate amount of electro-
lyte to the test cell (7.5), the pH is measured both before and 8. Procedure
after the test.
8.1 Continuouslyrecordthecorrosionpotentialofthework-
ing electrode (specimen) with respect to the saturated calomel
7. Preparation of Specimens and Conditioning
electrode for 1 h, starting immediately after immersing the
7.1 Prepare the test specimen surface within1hofthe start
specimen. The potential observed upon immersion in the
of the experiment by the method described in Reference Test
electrolyte shall be called the initial corrosion potential. The
Method G5.
potential at the end of the 1 h shall be known as the final
7.2 Using a suitable mechanical jig, force-fit the PTFE corrosion potential, E .
collar onto the cylindrical specimen so that the base of the
8.2 After the 1-h period, the potential should be potentio-
collarisup10 62mm[0.393 60.079in.]fromthebottomof
staticallyshiftedto+0.8V(saturatedcalomelelectrode(SCE))
the specimen (see Fig. 2). Care should be taken to avoid
to stimulate pitting (or crevice corrosion).
scratching the metal surface.
NOTE 3—In the stimulation step, the change in potential either from E
NOTE 1—Once the collar is removed from the specimen, it should not
or from one of the preselected potentials to+0.8 V (SCE) should be
be reused.
essentially instantaneous. Such instantaneous changes are facilitated by
use of a two-channel potentiostat in which the new control voltage can be
7.3 Mount the specimen on the holder and on the electrode
selected on the channel not in use. However, if a single channel
rod as described in Reference Test Method G5.
potentiostatisused,itshouldbeswitchedtemporarilytothestandbymode
7.4 Ultrasonically degrease the electrode assembly in either
(no impressed current) while the set-potential control is being adjusted to
a setting of+0.8 V (SCE); after the adjustment is made, the potentiostat
acetone, toluene, or boiling benzene (with caution, under
should be switched from the standby mode to the operate mode to allow
hood), rinse in distilled water, and dry.
stimulation of localized corrosion. After stimulation, the single-channel
7.5 Transfer 500 mL of electrolyte solution to a clean
potentiostat must remain in the operate mode during the shift to the
preselectedpotential,andthelattershiftshouldbeperformedmanuallyas
polarizationcell.Bringthetemperatureofthesolutionto37 6
rapidlyaspossible.Manualshiftingofthepotentialmayalsobenecessary
1°C by immersing the test cell in a controlled temperature
after the stimulation step when using a two-channel potentiostat if the
water bath or by other suitable means.
switch from+0.8 V (SCE) to the preselected potential would result in a
potential transient to values more active than the preselected potential.
7.6 Place the platinum auxiliary electrodes, salt bridge
Suchtransientscouldleadtorepassivationandtotheincorrectassumption
probe and other components in the test cell and temporarily
that the repassivation occurred at the preselected potential.
closethecenteropeningwithastopper.Fillthesalt-bridgewith
8.3 Thecurrentshallberecordedusingastripchartrecorder
the electrolyte.
with a minimum chart speed of 60 mm/min and a maximum
NOTE2—Thelevelsofthesolutioninthereferenceandthepolarization
currentscaleof0to3mA.Thecurrentwillberecordedat+0.8
cells should be the same to avoid siphoning. If this is not possible, a
V (SCE) for a period that depends upon the reaction (see Fig.
solution-wet (not greased) stopcock can be used in the salt-bridge to
eliminate siphoning. 3).
FIG. 2 Assembly into G5 Electrode Holder
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F746 − 04 (2009)
Note a—Current density instantly exceeds 500 µA/cm . Return immediately to pre-selected potential.
Note b—Current generally increases with time but does not ever exceed 500 µA/cm . Return to the pre-selected potential after 20 s.
Note c—Localized corrosion is not stimulated within the initial 20 s. Continue for an additional 15 min.
Note d—If localized corrosion is eventually stimulated, return to the pre-selected potential.
Note e—If localized corrosion cannot be stimulated even after the 15 min, the test is terminated.
FIG. 3 Stimulation of Localized Corrosion
8.3.1 Iflocalizedcorrosionisnotstimulatedintheinitial20
s, the polarizing currents will remain very small or decrease
rapidly with time. Proceed to 8.4.
8.3.2 Stimulation of localized corrosion will be marked
eitherbypolarizationcurrentsthatgenerallyincreasewithtime
or by current densities that exceed 500 µA/cm (for the
suggested specimen size this would be equivalent to a current
of approximately 2 mA).
8.3.2.1 If the current increases with time, after 20 s proceed
to 8.5.
8.3.2.2 If at any time a current density of 500 µA/cm is
exceeded,proceedimmediatelyto8.5.Insomeinstances,upon
shifting to+0.8 V (SCE), the current density will almost
instantaneously exceed 500 µA/cm . In such cases, proceed
directly to 8.5 without pause.
8.4 If localized corrosion is not stimulated within the initial
20 s, continue at+0.8 V (SCE) for an additional 15 min; the
chart speed may be reduced to a minimum of 5 mm/min after
the initial 20 s. If localized corrosion is eventually stimulated,
proceedto8.5.Iflocalizedcorrosioncannotbestimulatedeven
in 15 min, the test is terminated, and the material is considered
to have a very high resistance to localized corrosion in the test
environment. Report the critical potential as >+0.8 V (SCE).
8.5 If localized corrosion is stimulated at+0.8V(SCE), the
potential is then returned as rapidly as possible (see Note 3)to
E (which is the first preselected potential) to determine if the
1 FIG. 4 Examples of Typical Current – Time Curves at a Prese-
specimen will repassivate or if localized corrosion will con- lected Potential
tinue to propagate at the preselected potential.
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