1、Designation: F 746 04Standard Test Method forPitting or Crevice Corrosion of Metallic Surgical ImplantMaterials1This standard is issued under the fixed designation F 746; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of
2、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 covers the determination of resistanceto either pitting or crevice corrosion of metals and alloys fr
3、omwhich surgical implants will be produced. It is a modifiedversion of an established test2and is used as a screening test torank surgical implant alloys in order of their resistance tolocalized corrosion.1.2 This test method applies only to passive metals andalloys. Nonpassive alloys (other than no
4、ble alloys) are suscep-tible to general corrosion and are not normally suitable forimplant use.1.3 This test method is intended for use as a laboratoryscreening test of metals and alloys which undergo pitting orcrevice corrosion, or both.1.4 The values stated in SI units are to be regarded as thesta
5、ndard.1.5 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Refere
6、nced Documents2.1 ASTM Standards:3D 1193 Specification for Reagent WaterF 86 Practice for Surface Preparation and Marking of Me-tallic Surgical ImplantsF 2129 Test Method for Conducting Cyclic Potentiody-namic Polarization Measurements to Determine the Corro-sion Susceptibility of Small Implant Devi
7、cesG 3 Practice for Conventions Applicable to ElectrochemicalMeasurements in Corrosion TestingG 5 Reference Test Method for Making Potentiostatic andPotentiodynamic Anodic Polarization MeasurementsG 15 Terminology Relating to Corrosion and CorrosionTesting3. Summary of Test Method3.1 A cylindrical s
8、pecimen fitted with an inert tapered collaris immersed in a phosphate buffered saline electrolyte at 37Cfor1htoestablish a corrosion potential. Pitting (or crevicecorrosion) is then stimulated by potentiostatically polarizingthe specimen to a potential much more noble than the corrosionpotential. St
9、imulation of pitting (or crevice corrosion) will bemarked by a large and generally increasing polarizing current.3.2 Immediately after the stimulation step, the potential isdecreased as rapidly as possible to one of several preselectedpotentials at, or more noble than, the corrosion potential. If th
10、ealloy is susceptible to pitting (or crevice corrosion) at thepreselected potential, the polarizing current will remain atrelatively high values and will fluctuate or increase with time.A post-test examination of the metal specimen establisheswhether localized corrosion has occurred by pitting of th
11、eexposed surface or by preferential attack at the crevice formedby the tapered collar, or both.3.3 If the pit (or crevice) surface repassivates at the pre-elected potential and localized corrosion is halted, the polariz-ing current will drop to values typical for passive surfaces andthe current will
12、 decrease continuously. The parameter ofinterest, the critical potential for pitting (or crevice corrosion),is defined as the highest (most noble) preselected potential atwhich pit (or crevice) surfaces repassivate after the stimulationstep.4. Significance and Use4.1 This test method is designed sol
13、ely for determiningcomparative laboratory indices of performance. The resultsmay be used for ranking alloys in order of increasing resistanceto pitting and crevice corrosion under the specific conditions ofthis method. It should be noted that the method is intentionallydesigned to reach conditions t
14、hat are sufficiently severe tocause breakdown of at least one alloy (Type 316 L stainlesssteel) currently considered acceptable for surgical implant use,1This test method is under the jurisdiction of ASTM Committee F04 on Medicaland Surgical Materials and Devices and is the direct responsibility of
15、SubcommitteeF04.15 on Material Test Methods.Current edition approved Oct. 1, 2004. Published October 2004. Originallyapproved in 1981. Last previous edition approved in 1999 as F 746 87 (1999).2Syrett, B. C., Corrosion, Vol 33, 1977, p. 221.3For referenced ASTM standards, visit the ASTM website, www
16、.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards 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.and t
17、hat those alloys which suffer pitting or crevice corrosionduring the more severe portions of the test do not necessarilysuffer localized corrosion when placed within the human bodyas a surgical implant.5. Apparatus5.1 The following required equipment is described in Ref-erence Test Method G 5:5.1.1
18、Standard Polarization Cell, of 1000 cm3.5.1.2 Electrode Holders, for auxiliary and working elec-trodes.5.1.3 Potentiostat, calibrated in accordance with ReferenceTest Method G 5.5.1.4 Potential-Measuring Instrument.5.1.5 Current-Measuring Instrument.5.1.6 Anodic Polarization Circuit.5.1.7 Platinum A
19、uxiliary Electrodes.5.1.8 Saturated Calomel Electrode (SCE).5.1.9 Salt Bridge Probe.5.2 A cylindrical working electrode is fabricated from thetest material by machining, grinding, and suggested finalpolishing with 600-grit metallographic paper. It is suggestedthat the part of the cylindrical specime
20、n that is exposed to thetest solution have a length of 20.00 6 1.00 mm (0.787 6 0.039in.) and a diameter of 6.35 6 0.03 mm (0.250 6 0.001 in.) (seeFig. 1).5.3 A crevice is created by fitting the cylindrical specimenwith a tapered collar, machined from commercial puritypolytetrafluoroethylene (PTFE).
21、 The collar should have anouter diameter of 12.70 6 0.05 mm (0.500 6 0.002 in.) and athickness of 3.18 6 0.20 mm (0.125 6 0.008 in.). The insidediameter of the tapered collar should range from 0.38 mm(0.015 in.) smaller than the diameter of the specimen to 0.38mm (0.015 in.) larger. To be consistent
22、 with the dimensionssuggested in 5.2, the inside diameter should taper from 5.97 60.05 mm (0.235 6 0.002 in.) to 6.73 6 0.05 mm (0.265 60.002 in.). See Fig. 1 for drawing of the tapered collar. Therelatively fine tolerances are needed to ensure a reproducible fitand crevice.5.4 In Reference Test Met
23、hod G 5, the method of specimenattachment is to drill and tap the specimen to receive a threadedstainless steel connection rod. A 4-40 thread is used, typically.However, because many surgical implant alloys are not easilydrilled, external threads may also be machined, ground, or cast,as illustrated
24、in Fig. 1. A small stainless steel adapter is fittedon to these threads and the adapter then accepts the connectionrod.5.5 Determine the total exposed surface area of the speci-men before placement of the PTFE collar, AT; determine thearea on the internal surface of the collar (the creviced area), A
25、C;and determine the exposed surface area of the specimen afterplacement of the collar, AS(where: AS= AT AC). Dimensionsshould be measured to the nearest 0.1 mm.5.5.1 ExampleUsing the dimensions suggested previouslyfor the specimen diameter ( d = 6.35 mm), the specimen length( l = 20.00 mm), and the
26、collar thickness ( t = 3.18 mm),AT5pdl 1pd245 431 mm2(1)AC5pdt 5 63 mm2(2)AS5 AT2 AC5 386 mm2(3)6. Reagents6.1 ElectrolyteUnless otherwise specified, phosphatebuffered saline (PBS) should be used as the standard testsolution. A standard PBS formulation (see Table X2.3 of TestMethod F 2129) is the fo
27、llowing: NaCl 8.0 g/L, KCl 0.2 g/L,Na2HPO412H2O 1.15 g/L, KH2PO40.2 g/L, and bring to 1 Lvolumetrically using distilled water.6.1.1 The water shall be distilled conforming to the purityrequirements of Specification D 1193, Type IV reagent water.6.1.2 After transferring the appropriate amount of elec
28、tro-lyte to the test cell (7.5), the pH is to be measured both beforeand after the test.NOTE 1Unless shown, dimensional tolerances are given in text.FIG. 1 Dimensions of Specimen and CollarF7460427. Preparation of Specimens and Conditioning7.1 Prepare the test specimen surface within1hofthestartof t
29、he experiment by the method described in Reference TestMethod G 5.7.2 Using a suitable mechanical jig, force-fit the PTFEcollar onto the cylindrical specimen so that the base of thecollar is up 10 6 2 mm (0.393 6 0.079 in.) from the bottom ofthe specimen (see Fig. 2). Care should be taken to avoidsc
30、ratching the metal surface.NOTE 1Once the collar is removed from the specimen, it should notbe reused.7.3 Mount the specimen on the holder and on the electroderod as described in Reference Test Method G 5.7.4 Ultrasonically degrease the electrode assembly in eitheracetone, toluene, or boiling benzen
31、e (with caution, underhood), rinse in distilled water, and dry.7.5 Transfer 500 mL of electrolyte solution to a cleanpolarization cell. Bring the temperature of the solution to 37 61C by immersing the test cell in a controlled temperaturewater bath or by other suitable means.7.6 Place the platinum a
32、uxiliary electrodes, salt bridgeprobe and other components in the test cell and temporarilyclose the center opening with a stopper. Fill the salt-bridge withthe electrolyte.NOTE 2The levels of the solution in the reference and the polarizationcells should be the same to avoid siphoning. If this is n
33、ot possible, asolution-wet (not greased) stopcock can be used in the salt-bridge toeliminate siphoning.7.7 Transfer the specimen electrode assembly to the test celland adjust the submerged salt bridge probe tip so it is about 2mm (0.08 in.) from the center of the bottom portion of thespecimen (below
34、 the collar).8. Procedure8.1 Continuously record the corrosion potential of the work-ing electrode (specimen) with respect to the saturated calomelelectrode for 1 h, starting immediately after immersing thespecimen. The potential observed upon immersion in theelectrolyte shall be called the initial
35、corrosion potential. Thepotential at the end of the 1 h shall be known as the finalcorrosion potential, E1.8.2 After the 1-h period, the potential should be potentio-statically shifted to +0.8 V (SCE) to stimulate pitting (orcrevice corrosion).NOTE 3In the stimulation step, the change in potential e
36、ither from E1or from one of the preselected potentials to + 0.8 V (SCE) should beessentially instantaneous. Such instantaneous changes are facilitated byuse of a two-channel potentiostat in which the new control voltage can beselected on the channel not in use. However, if a single channelpotentiost
37、at is used, it should be switched temporarily to the standby mode(no impressed current) while the set-potential control is being adjusted toa setting of +0.8 V (SCE); after so adjusting, the potentiostat should beswitched from the standby mode to the operate mode to allow stimulationof localized cor
38、rosion. After stimulation, the single-channel potentiostatmust remain in the operate mode during the shift to the preselectedpotential, and the latter shift should be performed manually as rapidly aspossible. Manual shifting of the potential may also be necessary after thestimulation step when using
39、 a two-channel potentiostat if the switchfrom +0.8 V (SCE) to the preselected potential would result in a potentialtransient to values more active than the preselected potential. Suchtransients could lead to repassivation and to the incorrect assumption thatthe repassivation occurred at the preselec
40、ted potential.8.3 Record the current using a strip chart recorder with aminimum chart speed of 60 mm/min and a maximum currentscale of 0 to 3 mA. The current will be recorded at +0.8 V(SCE) for a period that depends upon the reaction (see Fig. 3).8.3.1 If localized corrosion is not stimulated in the
41、 initial 20s, the polarizing currents will remain very small or decreaserapidly with time. Proceed to 8.4.8.3.2 Stimulation of localized corrosion will be markedeither by polarization currents that generally increase with timeor by current densities that exceed 500 A/cm2(for thesuggested specimen si
42、ze this would be equivalent to a currentof approximately 2 mA).8.3.2.1 If the current generally increases with time, after 20s proceed to 8.5.FIG. 2 Assembly into G5 Electrode HolderF7460438.3.2.2 If at any time a current density of 500 A/cm2isexceeded, proceed immediately to 8.5. In some instances,
43、 uponshifting to +0.8 V (SCE), the current density will almostinstantaneously exceed 500 A/cm2. In such cases, proceeddirectly to 8.5 without pause.8.4 If localized corrosion is not stimulated within the initial20 s, continue at +0.8 V (SCE) for an additional 15 min; thechart speed may be reduced to
44、 a minimum of 5 mm/min afterthe initial 20 s. If localized corrosion is eventually stimulated,proceed to 8.5. If localized corrosion cannot be stimulated evenin 15 min, the test is terminated, and the material is consideredto have a very high resistance to localized corrosion in the testenvironment.
45、 Report the critical potential as +0.8 V (SCE).8.5 If localized corrosion is stimulated at +0.8 V (SCE), thepotential is then returned as rapidly as possible (see Note 3) toE1(which is the first preselected potential) to determine if thespecimen will repassivate or if localized corrosion will con-ti
46、nue to propagate at the preselected potential.8.6 If the pitted or creviced local regions repassivate at thepreselected potential, the polarizing current will drop quicklyto zero or to low values consistent with a passive surfacecondition (see Fig. 4(a) for examples). Monitor this current for15 min.
47、8.6.1 During this 15 min, the chart speed may be reduced toa minimum of 5 mm/min.8.6.2 Adjust the current scale to obtain satisfactory accu-racy. The range used for monitoring the relatively large currentduring stimulation is almost certainly unsuitable for accuratelymonitoring the much smaller repa
48、ssivation currents.8.6.3 If the pitted or local regions do not repassivate at E1,then the critical voltage shall be reported as E1, with thenotation that the specimen never repassivated following theinitial stimulation. The test shall be terminated.8.7 After ensuring repassivation at E1by observing
49、low,decreasing (or constant) polarization currents for 15 min,repeat the stimulation step (8.2 and 8.3) at + 0.8 V (SCE) andthen change the potential as rapidly as possible (see Note 3) tothe second preselected potential which should be the nearest0.05-V increment more noble than E1(on the SCE scale).Repeat 8.6.8.7.1 ExampleIf E1is any value in the range 0.100to 0.051 V (SCE), then the second preselected potentialwould be 0.050 V (SCE).8.8 The test consists of alternating between stimulationat +0.8 V (SCE) and returning to a preselected potential to seeif repassivati
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