1、Designation: F 2129 06Standard Test Method forConducting Cyclic Potentiodynamic PolarizationMeasurements to Determine the Corrosion Susceptibility ofSmall Implant Devices1This standard is issued under the fixed designation F 2129; the number immediately following the designation indicates the year o
2、foriginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method assesses the corrosion susceptibility of
3、small, metallic, implant medical devices, or componentsthereof, using cyclic (forward and reverse) potentiodynamicpolarization. Examples of device types that may be evaluatedby this test method include, but are not limited to, vascularstents, ureteral stents (Specification F 1828), filters, supports
4、egments of endovascular grafts, cardiac occluders, aneurysmor ligation clips, staples, and so forth.1.2 This test method is used to assess a device in its finalform and finish, as it would be implanted. These small devicesshould be tested in their entirety. The upper limit on device sizeis dictated
5、by the electrical current delivery capability of thetest apparatus (see Section 6). It is assumed that test methods,such as Reference Test Method G5and Test Method G61havebeen used for material screening.1.3 Because of the variety of configurations and sizes ofimplants, this test method provides a v
6、ariety of specimenholder configurations.1.4 This test method is intended for use on implantabledevices made from metals with a relatively high resistance tocorrosion.1.5 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of t
7、he 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:2D 1193 Specification for Reagent WaterF 1828 Specification for Ureteral StentsG3 Practice for ConventionsA
8、pplicable to ElectrochemicalMeasurements in Corrosion TestingG5 Reference Test Method for Making Potentiostatic andPotentiodynamic Anodic Polarization MeasurementsG15 Terminology Relating to Corrosion and CorrosionTestingG61 Test Method for Conducting Cyclic PotentiodynamicPolarization Measurements
9、for Localized Corrosion Sus-ceptibility of Iron-, Nickel-, or Cobalt-Based Alloys3. Terminology3.1 Definitions:3.1.1 potentiostat, nan instrument for automatically main-taining an electrode in an electrolyte at a constant potential orcontrolled potentials with respect to a suitable referenceelectrod
10、e (see Terminology G15).3.1.2 potentiodynamic cyclic polarization (forward and re-verse polarization), na technique in which the potential ofthe test specimen is controlled and the corrosion currentmeasured by a potentiostat. The potential is scanned in thepositive or noble (forward) direction as de
11、fined in Practice G3.The potential scan is continued until a predetermined potentialor current density is reached. Typically, the scan is run until thetranspassive region is reached, and the specimen no longerdemonstrates passivity, as defined in Practice G3. The poten-tial scan direction then is re
12、versed until the specimen repassi-vates or the potential reaches a preset value.3.1.3 scan rate, nthe rate at which the controlling voltageis changed.3.2 Symbols:3.2.1 Eb= Breakdown or Critical Pitting Potentialtheleast noble potential at which pitting or crevice corrosion orboth will initiate and p
13、ropagate as defined in TerminologyG15. An increase in the resistance to pitting corrosion isassociated with an increase in Eb.3.2.2 Er= Rest Potentialthe potential of the workingelectrode relative to the reference electrode measured undervirtual open-circuit conditions (working electrode is not pola
14、r-ized).3.2.3 Ezc= Zero Current Potentialthe potential at whichthe current reaches a minimum during the forward scan.1This test method is under the jurisdiction of ASTM Committee F04 on Medicaland Surgical Materials and Devices and is the direct responsibility of SubcommitteeF04.15 on Material Test
15、Methods.Current edition approved May 15, 2006. Published May 2006. Originallyapproved in 2001. Last previous edition approved in 2004 as F 2129 04.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandard
16、s volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.2.4 Ef= Final Potentiala preset potential at which thescan is stopped.3.2.5 Ei= Initial Potential
17、the potential at which thepotentiostat begins the controlled potentiodynamic scan.3.2.6 Ep= Protection Potentialthe potential at which thereverse scan intersects the forward scan at a value that is lessnoble than Eb. Epcannot be determined if there is nobreakdown. Whereas, pitting will occur on a pi
18、t-free surfaceabove Eb, it will occur only in the range of potentials betweenEpand Ebif the surface is already pitted. The severity ofcrevice corrosion susceptibility increases with increasing hys-teresis of the polarization curve, the difference between EbandEp.3.2.7 Ev= Vertex Potentiala preset po
19、tential, at which thescan direction is reversed.3.2.8 it= Threshold Current Density (mA/cm2)a presetcurrent density, at which the scan direction is reversed.Typically, the scan is reversed when a current density twodecades higher than the current density at the breakdownpotential (Eb) is reached.4.
20、Summary of Test Method4.1 The device is placed in an appropriate deaerated simu-lated physiological solution, and the rest potential (Er)isrecorded for1hor,alternatively, until the rest potentialstabilizes to a rate of change less than 3 mV/min. Thepotentiodynamic scan is then started at Erand scann
21、ed in thepositive or noble (forward) direction. The scan is reversed aftereither the vertex potential is reached or the current density hasreached a value approximately two decades greater than thecurrent density measured at the breakdown potential. Thereverse scan is stopped after the current has b
22、ecome less thanthat in the forward direction or the potential reaches Er. Thedata is plotted with the current density in mA/cm2on the x axis(logarithmic axis) versus the potential in mV on the y axis(linear axis).5. Significance and Use5.1 Corrosion of implantable medical devices can havedeleterious
23、 effects on the device performance or may result inthe release of corrosion products with harmful biologicalconsequences; therefore, it is important to determine thegeneral corrosion behavior as well as the susceptibility of thedevices to localized corrosion.5.2 The forming and finishing steps used
24、to create animplantable device may have significant effects on the corro-sion resistance of the material out of which the device isfabricated. During the selection process of a material for use asan implantable device, testing the corrosion resistance of thematerial is an essential step; however, it
25、 does not necessarilyprovide critical data regarding device performance.5.3 To accommodate the wide variety of device shapes andsizes encountered, a variety of holding devices can be used.5.4 Note that the method is intentionally designed to reachconditions that are sufficiently severe to cause brea
26、kdown anddeterioration of the medical devices and that these conditionsmay not be necessarily encountered in vivo. The results of thiscorrosion test conducted in artificial physiological electrolytescan provide useful data for comparison of different devicematerials, designs, or manufacturing proces
27、ses. However, notethat this test method does not take into account the effects ofcells, proteins, and so forth on the corrosion behavior in vivo.6. Apparatus6.1 Potentiostat, calibrated in accordance with ReferenceTest Method G5.6.2 Working Electrode, to be used as the test specimen, asdescribed in
28、Section 9. Its configuration and holder will dependon the type of specimen being tested, as described in Section 7.In all cases, the metallurgical and surface condition of aspecimen simulating a device must be in the same condition asthe device.6.3 Reference ElectrodeA saturated calomel electrode(SC
29、E), as described in Reference Test Method G 5, shall beused as a reference electrode.6.4 Salt Bridge, such as a Luggin probe, shall be usedbetween the working and reference electrode, such as the typeshown in Reference Test Method G5.6.5 Auxiliary Electrodes:6.5.1 Two platinum auxiliary electrodes m
30、ay be preparedfrom high-purity rod stock. The surfaces may be platinized, asper Reference Test Method G5.6.5.2 Alternatively, high-purity graphite auxiliary electrodesmay be used in accordance with Reference Test Method G5.Care should be taken to ensure that they do not get contami-nated during a te
31、st.6.5.3 The auxiliary electrode surface area should be at leastfour times greater than the sample surface area. Use ofwire-mesh platinum might be more cost-effective than plati-num cylinders when testing larger specimens or whole devices.6.6 Suitable Polarization Cell, with a volume of about 1000cm
32、3, equivalent to or similar to that recommended in ReferenceTest Method G5. Furthermore, the cell needs to be appropri-ately sealed to avoid oxygen access and include a secondarybubbler for the release of exhaust gas without the backdiffusion of oxygen.6.7 Water Bath, or other heating appliance capa
33、ble ofmaintaining the test solution temperature at 37 6 1C (seeX1.5).6.8 Purge Gas Delivery System, capable of delivering nitro-gen gas at 150 cm3/min.7. Specimen Holders7.1 There are a variety of holders that may be used in thistest method. Each is designed for a specific type or class ofdevice.7.2
34、 Short wire or coil specimens:7.2.1 Specimens can be held suspended from a clampingdevice. For example, the threaded end of a Reference TestMethod G5holder can be used to hold two stainless steel nuts.The wire test specimen is clamped between these nuts and bentso as to enter the test solution.7.2.2
35、 The surface area of the test specimen shall be calcu-lated based on the length of wire or coil immersed in the testsolution.7.2.3 This type of holder exposes the specimen to theair-liquid interface, which is subject to localized crevicecorrosion. Test specimens should be examined carefully afterF21
36、29062testing to ensure that there is no localized corrosion at or justbelow the interface. If specimens show evidence of localizedcorrosion at the air-liquid interface, then the portion of thespecimen passing across this interface shall be sealed with animpervious coating.7.2.4 Alternatively, one ma
37、y choose to coat the portion ofthe specimen out of the solution and the connection to thespecimen holder with a suitable coating. The surfaces out ofsolution will tend to have test solution condensed on them andthis may lead to undesirable results.7.3 One method for holding stents or cylindrical dev
38、ices isshown in Appendix X3.8. Reagents8.1 Reagent grade chemicals shall be used for this testmethod. Such reagents shall conform to the specifications ofthe Committee onAnalytical Reagents of theAmerican Chemi-cal Society.38.1.1 The water shall be distilled or deionized conformingto the purity requ
39、irements of Specification D 1193, Type IVreagent water.8.1.2 Unless otherwise specified, phosphate buffered saline(PBS) should be used as the standard test solution. A standardPBS formulation is given in Appendix X2, along with theformulations of two simulated bile solutions for testing im-plantable
40、 medical devices intended for use in the biliarysystem, the formulations of two artificial urine solutions fortesting implantable indwelling materials intended for use in theurinary tract, and the compositions of two other commonlyused physiological solutions.8.1.3 The pH of the electrolyte should b
41、e adjusted based onthe nature of the solution by the addition of NaOH or HCl.When the electrolyte is deaerated, its pH may change signifi-cantly if it is not sufficiently buffered. Several pH controllingmethods are provided in Appendix X2.8.1.4 Nitrogen gas with a minimum purity of 99.99 %should be
42、used for purging the test solution of oxygen.9. Test Specimen9.1 Unless otherwise justified, all samples selected fortesting should be taken from finished, clinical-quality product.Cosmetic rejects or other nonclinical samples may be used ifthe cause for rejection does not affect the corrosion behav
43、ior ofthe device. Sterilization may be omitted if it can be demon-strated that prior sterilization has no effect on the corrosionbehavior of the device.9.1.1 Test specimens used for design parameter studies canbe prepared as detailed in Reference Test Method G5 forworking electrodes, with the requir
44、ement that the metallurgicaland surface conditions of the specimens are the same as theintended implantable medical device.10. Procedure10.1 Prepare the specimen such that the portion exposed tothe test solution is in the same metallurgical and surfacecondition as the implantable form of the medical
45、 device beingstudied.10.1.1 Calculate the total surface area of the specimenexposed to the solution in order to determine the currentdensity (current per surface area) generated by the specimenduring the test.10.2 Prepare enough test solution to immerse the device andauxiliary electrodes and so to a
46、void any appreciable change inthe solution corrosivity during the test through exhaustion ofthe corrosive constituents or by accumulation of corrosionproducts that may affect further corrosion. At a minimum,transfer 500 mL of electrolyte to a clean polarization cell.Measure and record the pH of the
47、solution before and aftereach test.10.3 Place the auxiliary electrodes, salt bridge probe, ther-mometer, and gas purge diffuser in the test chamber and bringthe temperature of the test solution to 37 6 1C.10.4 Purge the solution for a minimum of 30 min withnitrogen gas at a flow rate of 150 cm3/min.
48、10.5 Gently immerse the test specimen in the test solutionand connect it to a potentiostat. Continue the nitrogen purgethroughout the test.10.6 Record Erfor1hor,alternatively, until the restpotential stabilizes to a rate of change less than 3 mV/min.10.7 At the end of the Errecording period, start t
49、hepotentiodynamic scan in the positive or noble (forward)direction, as defined in Practice G3. The scanning programshould be set with the following parameters:10.7.1 Starting or initial potential (Ei)atEr.10.7.2 Ascan rate of either 0.167 mV/s or 1 mV/s should beused. Note that the scan rate may affect the breakdownpotential of the device and the shape of the passive region ofthe polarization curve. Comparisons should not be madebetween test results using different scan rates, even if all otherexperimental parameters are held constant.10.7.3 A current density threshold t
