Standard test method for conducting cyclic potentiodynamic polarization measurements to determine the corrosion susceptibility of small lmplant devices
1 Scope
1.1 This test method assesses the corrosion susceptibility of small, metallic, implant medical devices, or components thereof, using cyclic (forward and reverse) potentiodynamic polarization. Examples of device types that may be evaluated by this test method include, but are not limited to, vascular stents, ureteral stents, filters, support segments of endovascular grafts, cardiac occluders, aneurysm or ligation clips, staples, and so forth.
1.2 This test method is used to assess a device in its final form and finish, as it would be implanted. These small devices should be tested in their entirety. The upper limit on device size is dictated by the electrical current delivery capability of the test apparatus (see Clause 6). It is assumed that test methods, such as ASTM G 5 and ASTM G 61have been used for material screening.
1.3 Because of the variety of configurations and sizes of implants, this test method provides a variety of specimen holder configurations.
1.4 This test method is intended for use on implantable devices made from metals with a relatively high resistance to corrosion.
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.
2 Normative References
The following normative documents contain provisions which, through reference in this text, constitute provisions of this standard. For dated references, subsequent amendments (excluding corrections), or revisions, of any of these publications do not apply to this standard. However, parties to agreements based on this standard are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. For undated references, the latest edition of the normative document referred to applies.
GB 6682 Water for Analytical Laboratory Use — Specification and Test Methods
ASTM F 1828 Specification for Ureteral Stents
ASTM G 3 Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing
ASTM G 5 Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements
ASTM G 15 Terminology Relating to Corrosion and Corrosion Testing
ASTM G 61 Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys
3 Terms and Definitions
For the purposes of this standard, the following terms and definitions apply.
3.1
potentiostat
an instrument for automatically maintaining an electrode in an electrolyte at a constant potential or controlled potentials with respect to a suitable reference electrode (see ASTM G 15)
3.2
potentiodynamic cyclic polarization (forward and reverse polarization)
a technique in which the potential of the test specimen is controlled and the corrosion current measured by a potentiostat. The potential is scanned in the positive or noble (forward) direction as defined in ASTM G 3. The potential scan is continued until a predetermined potential or current density is reached. Typically, the scan is run until the transpassive region is reached, and the specimen no longer demonstrates passivity, as defined in ASTM G 3. The potential scan direction then is reversed until the specimen repassivates or the potential reaches a preset value
3.3
scan rate
the rate at which the controlling voltage is changed
3.4
breakdown or critical pitting potential
Eb
the least noble potential at which pitting or crevice corrosion or both will initiate and propagate as defined in ASTM G 15. An increase in the resistance to pitting corrosion is associated with an increase in Eb
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3.5
rest potential
Er=Ec
the potential of the working electrode relative to the reference electrode measured under virtual open-circuit conditions (working electrode is not polarized)
3.6
zero current potential
Epc
the potential at which the current reaches a minimum during the forward scan
3.7
final potential
Ef(x)
a preset potential at which the scan is stopped
3.8
initial potential
Ei
the potential at which the potentiostat begins the controlled potentiodynamic scan
3.9
cathodic protection potential
Ep
the potential at which the reverse scan intersects the forward scan at a value that is less noble than Eb. Ep cannot be determined if there is no breakdown. Whereas, pitting will occur on a pit-free surface above Eb, it will occur only in the range of potentials between Ep and Eb if the surface is already pitted. The severity of crevice corrosion susceptibility increases with increasing hysteresis of the polarization curve, the difference between Eb and Ep
3.10
vertex potential
Ev
a preset potential, at which the scan direction is reversed
3.11
threshold current density
it
mA/cm2
a preset current density, at which the scan direction is reversed. Typically, the scan is reversed when a current density two decades higher than the current density at the breakdown potential is reached
Foreword II
1 Scope
2 Normative References
3 Terms and Definitions
4 Summary of Test Method
5 Significance and Use
6 Apparatus
7 Specimen Holders
8 Reagents
9 Test Specimen
10 Procedure
11 Report
12 Precision and Bias
Annex A (Informative) Rationale
Annex B (Informative) Composition of Different Physiological Environments
Annex B (Informative) Method for Mounting Stents or Cylindrical Devices
