1、Designation: F746 04 (Reapproved 2009)1Standard Test Method forPitting or Crevice Corrosion of Metallic Surgical ImplantMaterials1This standard is issued under the fixed designation F746; the number immediately following the designation indicates the year of originaladoption or, in the case of revis
2、ion, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon () indicates an editorial change since the last revision or reapproval.1NOTEUnits information was editorially corrected in January 2010.1. Scope1.1 This test method covers the determinatio
3、n of resistanceto either pitting or crevice corrosion of metals and alloys fromwhich 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 meth
4、od applies only to passive metals andalloys. Nonpassive alloys (other than noble 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 for metals and alloys which undergo pitting orcrevice cor
5、rosion, or both.1.4 The values stated in either SI units or inch-pound unitsare to be regarded separately as standard. The values stated ineach system may not be exact equivalents; therefore, eachsystem shall be used independently of the other. Combiningvalues from the two systems may result in non-
6、conformancewith the standard.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
7、 prior to use.2. Referenced Documents2.1 ASTM Standards:3D1193 Specification for Reagent WaterF86 Practice for Surface Preparation and Marking of Me-tallic Surgical ImplantsF2129 Test Method for Conducting Cyclic PotentiodynamicPolarization Measurements to Determine the CorrosionSusceptibility of Sm
8、all Implant DevicesG3 Practice for Conventions Applicable to ElectrochemicalMeasurements in Corrosion TestingG5 Reference Test Method for Making Potentiostatic andPotentiodynamic Anodic Polarization MeasurementsG15 Terminology Relating to Corrosion and CorrosionTesting3. Summary of Test Method3.1 Ac
9、ylindrical specimen 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 corrosionp
10、otential. Stimulation 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 pote
11、ntial. If thealloy 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 p
12、itting of theexposed 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-selected potential and localized corrosion is halted, the polar-izing current will drop to values typical for passive surfacesand the
13、 current will decrease continuously. The parameter ofinterest, the critical potential for pitting (or crevice corrosion),is defined as the highest (most noble) pre-selected potential atwhich pit (or crevice) surfaces repassivate after the stimulationstep.1This test method is under the jurisdiction o
14、fASTM Committee F04 on Medicaland Surgical Materials and Devices and is the direct responsibility of SubcommitteeF04.15 on Material Test Methods.Current edition approved Dec. 1, 2009. Published January 2010. Originallyapproved in 1981. Last previous edition approved in 2004 as F746 04. DOI:10.1520/F
15、0746-04R09E01.2Syrett, B. C., Corrosion, Vol 33, 1977, p. 221.3For referenced ASTM standards, visit the ASTM website, www.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
16、.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.4. Significance and Use4.1 This test method is designed solely for determiningcomparative laboratory indices of performance. The resultsmay be used for ranking alloys in order of incre
17、asing resistanceto pitting and crevice corrosion under the specific conditions ofthis method. It should be noted that the method is intentionallydesigned to reach conditions that are sufficiently severe tocause breakdown of at least one alloy (Type 316 L stainlesssteel) currently considered acceptab
18、le for surgical implant use,and that 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
19、Ref-erence Test Method G5:5.1.1 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 G5.5.1.4 Potential-Measuring Instrument.5.1.5 Current-Measuring Instrument.5.1.6 Anodic Polari
20、zation Circuit.5.1.7 Platinum Auxiliary 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
21、part of the cylindrical specimen 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 puritypoly
22、tetrafluoroethylene (PTFE). 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 mm0.015 in. smaller than the diameter of the specimen to 0.38mm 0.015 in. larger
23、. To be consistent 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 Refer
24、ence Test Method G5, the method of specimenattachment is to drill and tap the specimen to receive a threadedstainless steel connection rod. A4-40 thread is used, typically.However, because many surgical implant alloys are not easilydrilled, external threads may also be machined, ground, or cast,as i
25、llustrated in Fig. 1. A small stainless steel adapter is fittedonto 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 crevice
26、d area), AC;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)
27、, and the 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 F2129)
28、is the following: NaCl 8.0 g/L, KCl 0.2 g/L,NOTE 1Unless shown, dimensional tolerances are given in text.FIG. 1 Dimensions of Specimen and CollarF746 04 (2009)12Na2HPO412H2O 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
29、 the purityrequirements of Specification D1193, Type IV reagent water.6.1.2 After transferring the appropriate amount of electro-lyte to the test cell (7.5), the pH is measured both before andafter the test.7. Preparation of Specimens and Conditioning7.1 Prepare the test specimen surface within1hoft
30、hestartof the experiment by the method described in Reference TestMethod G5.7.2 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
31、 avoidscratching 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 G5.7.4 Ultrasonically degrease the electrode assembly in eitheracetone, toluene, or boiling
32、 benzene (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 pla
33、tinum auxiliary 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 th
34、is is not 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 (
35、below 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 ini
36、tial 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 (saturated calomel electrode (SCE)to stimulate pitting (or crevice corrosion).NOTE 3In the stimulati
37、on step, the change in potential either 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. Howe
38、ver, if a single channelpotentiostat 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 the adjustment is made, the potentiostatshould be switched from the standby mode to the opera
39、te mode to allowstimulation of localized corrosion. After stimulation, the single-channelpotentiostat must remain in the operate mode during the shift to thepreselected potential, and the latter shift should be performed manually asrapidly as possible. Manual shifting of the potential may also be ne
40、cessaryafter the stimulation step when using a two-channel potentiostat if theswitch from +0.8 V (SCE) to the preselected potential would result in apotential transient to values more active than the preselected potential.Such transients could lead to repassivation and to the incorrect assumptiontha
41、t the repassivation occurred at the preselected potential.8.3 The current shall be recorded using a strip chart recorderwith a minimum chart speed of 60 mm/min and a maximumcurrent scale of 0 to 3 mA. The current will be recorded at +0.8V (SCE) for a period that depends upon the reaction (see Fig.3)
42、.8.3.1 If localized corrosion is not stimulated in the initial 20s, the polarizing currents will remain very small or decreaserapidly with time. Proceed to 8.4.FIG. 2 Assembly into G5 Electrode HolderF746 04 (2009)138.3.2 Stimulation of localized corrosion will be markedeither by polarization curren
43、ts that generally increase with timeor by current densities that exceed 500 A/cm2(for thesuggested specimen size this would be equivalent to a currentof approximately 2 mA).8.3.2.1 If the current increases with time, after 20 s proceedto 8.5.8.3.2.2 If at any time a current density of 500 A/cm2isexc
44、eeded, proceed immediately to 8.5. In some instances, 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 a
45、n additional 15 min; thechart speed may be reduced to 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 resi
46、stance to localized corrosion in the testenvironment. 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 wi
47、ll repassivate or if localized corrosion will con-tinue 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.
48、 4(a) for examples). Monitor this current for15 min.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 unsui
49、table for accuratelymonitoring the much smaller repassivation 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 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 sec
