1、Designation: B 798 95 (Reapproved 2009)Standard Test Method forPorosity in Gold or Palladium Coatings on Metal Substratesby Gel-Bulk Electrography1This standard is issued under the fixed designation B 798; the number immediately following the designation indicates the year oforiginal adoption or, in
2、 the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers equipment and techniques fordetermining porosity in noble m
3、etal coatings, particularlyelectrodeposits and clad metals used on electrical contacts.1.2 The test method is designed to show whether theporosity level is less or greater than some value which byexperience is considered by the user to be acceptable for theintended application.1.3 Other porosity tes
4、ting methods are outlined in GuideB 765. Detailed critical reviews of porosity testing are alsoavailable.2Other porosity test methods are B 735, B 741,B 799, and B 809.1.4 The values stated in SI units are to be regarded asstandard. The values in parentheses are for information only.1.5 This standar
5、d 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 become familiarwith all hazards including those identified in the appropriateMaterial Safety Data Sheet (MSDS) for this product/materialas provided by the
6、 manufacturer, to establish appropriatesafety and health practices, and determine the applicability ofregulatory limitations prior to use. For specific hazard state-ments, see Sections 7 and 8.2. Referenced Documents2.1 ASTM Standards:3B 374 Terminology Relating to ElectroplatingB 542 Terminology Re
7、lating to Electrical Contacts andTheir UseB 735 Test Method for Porosity in Gold Coatings on MetalSubstrates by Nitric Acid VaporB 741 Test Method for Porosity In Gold Coatings On MetalSubstrates By Paper Electrography4B 765 Guide for Selection of Porosity and Gross DefectTests for Electrodeposits a
8、nd Related Metallic CoatingsB 799 Test Method for Porosity in Gold and PalladiumCoatings by Sulfurous Acid/Sulfur-Dioxide VaporB 809 Test Method for Porosity in Metallic Coatings byHumid Sulfur Vapor (“Flowers-of-Sulfur”)3. Terminology3.1 DefinitionsMany terms used in this test method aredefined in
9、Terminology B 542 and terms relating to metalliccoatings are defined in Terminology B 374.3.2 Definitions of Terms Specific to This Standard:3.2.1 decorations, nthose reaction products emanatingfrom the pores that provide visual contrast with the gelmedium.3.2.2 measurement area (or “significant sur
10、face”), nthesurface that is examined for the presence of porosity. Thesignificant surfaces or measurement areas of the part to betested shall be indicated on the drawing of the part or byprovision of suitably marked samples.3.2.2.1 DiscussionFor specification purposes, the signifi-cant surfaces or m
11、easurement areas are often defined as thoseportions of the surface that are essential to the serviceability orfunction of the part, such as its contact properties, or which canbe the source of corrosion products or tarnish films thatinterfere with the function of the part.3.2.3 metallic coatings, ni
12、nclude platings, claddings, orother metallic layers applied to the substrate. The coatings cancomprise a single metallic layer or a combination of metalliclayers.1This test method is under the jurisdiction of ASTM Committee B02 onNonferrous Metals and Alloys and is the direct responsibility of Subco
13、mmitteeB02.11 on Electrical Contact Test Methods.Current edition approved Oct. 1, 2009. Published October 2009. Originallyapproved in 1988. Last previous edition approved in 2005 as B 798 95 (2005).2Nobel, F. J., Ostrow, B. D., and Thompson, D. W., “Porosity Testing of GoldDeposity,” Plating, Vol 52
14、, 1965, p. 1001, and Krumbein S. J., “Porosity Testingof Contact Platings,” Proceedings, Connectors and Interconnection TechnologySymposium, October 1987, p. 47.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book o
15、f ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4Withdrawn. The last approved version of this historical standard is referencedon www.astm.org.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, Unit
16、ed States.3.2.4 porosity, nthe presence of any discontinuity, crack,or hole in the coating that exposes a different underlying metal.3.2.5 underplate, na metallic coating layer between thesubstrate and the topmost layer or layers. The thickness of anunderplate is usually greater that 0.8 m (30 in.).
17、4. Summary of Test Method4.1 This test method is an electrographic technique, “gel-bulk electrography.” The specimen is made the anode in a cellcontaining a solid or semisolid electrolyte of gelatin, conduct-ing salts, and an indicator. Application of current to this cellresults in the migration of
18、base medal ions through continuouspores. Reaction of cations with an indicator gives rise tocolored reaction products at pore sites which may be countedthrough the clear gel. Individual spots are counted with the aidof a loupe or low power stereomicroscope.4.2 This test method is suitable for coatin
19、gs containing75 % or more of gold on substrates of silver, nickel, copper,and its alloys, which are commonly used in electrical contacts.This test method is also suitable for coatings of 95 % or moreof palladium on nickel, copper and its alloys.4.3 These porosity tests involve corrosion reactions in
20、which the products delineate defect sites in coatings. Since thechemistry and properties of these products do not resemblethose found in natural or service environments, these tests arenot recommended for prediction of the electrical performanceof contacts unless correlation is first established wit
21、h serviceexperience.5. Significance and Use5.1 Noble metal coatings, particularly gold or palladium, areoften specified for the contacts of separable electrical connec-tors and other devices. Electrodeposits are the form of gold orpalladium which is most used on contacts, although gold andpalladium
22、are also employed as clad metal and as weldmentson the contact surface. The intrinsic nobility of gold and to acertain extent palladium enables them to resist the formation ofinsulating films that could interfere with reliable contactoperation.5.2 In order that the nobility of gold be assured, poros
23、ity,cracks, and other defects in the coating that expose base metalsubstrates and underplates must be minimal or absent, exceptin those cases where it is feasible to use the contacts instructures that shield the surface from the environment orwhere corrosion inhibiting surface treatments for the dep
24、ositare employed. The level of porosity in the coating that may betolerable depends on the severity of the environment to theunderplate or substrate, design factors for the contact devicelike the force with which it is mated, circuit parameters, and thereliability of contact operation that it is nec
25、essary to maintain.Also, when present, the location of pores on the surface isimportant. If the pores are few in number or are outside of thezone of contact of the mating surfaces, their presence can oftenbe tolerated.5.3 Methods for determining pores on a contact surface aremost suitable if they en
26、able their precise location and numbersto be determined. Contact surfaces are often curved or irregularin shape, and testing methods should be suitable for them. Inaddition, the severity of porosity-determining tests may varyfrom procedures capable of detecting all porosity to proceduresthat detect
27、only gross defects. The test method in this documentis generally regarded as severe.5.4 The relationship of porosity levels revealed by particulartests to contact behavior must be made by the user of these teststhrough practical experience or judgment. Thus, absence ofporosity in the coating may be
28、a requirement for someapplications, while a few pores in the contact zone may beacceptable for others.5.5 This test method is capable of detecting porosity orother defects in gold or palladium coatings that could partici-pate in substrate corrosion reactions. In addition, it can be usedon contacts h
29、aving complex geometry such as pin-socketcontacts (although difficulty may be experienced in inspectingdeep recesses).6. Limitations6.1 This test is considered destructive in that it reveals thepresence of porosity by contaminating the surface with corro-sion products and by under-cutting the corrod
30、ible metal at poresites and at unplated areas. In addition, the surface is coatedwith a corrosive gel mixture which is difficult to removecompletely. Any parts exposed to the gel test shall not beplaced in service.6.2 The gel-bulk procedure is not as sensitive to small poresand is more complex than
31、porosity tests involving gaseouscorrodants5(see Test Methods B 735 and B 799). It alsoinvolves more chemicals, preparation, and auxiliary equip-ment.6.3 This test is intended to be used for quantitative descrip-tions of porosity (such as number of pores per unit area or percontact) only on measureme
32、nt areas where coatings have poredensities that are sufficiently low so that the corrosion sites arewell separated and can be readily resolved. As a generalguideline this can be achieved for pore densities up to about25/cm2.6.4 For this purpose, the measurement area, or “significantsurface, shall be
33、 defined as those portions of the surface thatare essential to the serviceability or function of the part, suchas its contact properties, or which can be the source ofcorrosion products or tarnish films that interfere with thefunction of the part. When necessary, the significant surfacesshall be ind
34、icated on the drawings of the parts, or by theprovision of suitably marked samples.6.5 The test applicability to platings of varying thickness isa function of the quality of the plating.6.6 The applicability of this test method to localized plat-ings or claddings with adjacent exposed substrate is l
35、imited bythe efficacy of coatings applied to mask the non-noble areas toprevent gross decoration of the surfaces under test. Users ofthis method are required to develop their own techniques formasking such exposed substrate areas.5For example, Clarke, M., “Porosity and Porosity Tests,” in “Propertie
36、s ofElectrodeposits,” edited by Sard, Leidheiser, and Ogburn, The ElectrochemicalSociety, 1975, p. 122.B 798 95 (2009)27. Apparatus7.1 Test Vessel may be made of glass, acrylic resin, or otherinert uncolored transparent material. It shall have thin-walledflat sides, and be of a size appropriate to t
37、he sample to betested.7.2 Power Supply,0to1Aand0to10Vdc,anelectronically-regulated, constant-current (65 %) apparatus ispreferred.7.3 dc Milliammeter and Separate dc Voltmeter.7.4 Cathode Material in the form of foil or wire made ofplatinum or gold is required. The cathode and specimen(anode) areas
38、shall be approximately the same. Additionally,gold or platinum wire for cathode and anode are needed for thatportion of the hook-up that is in the reagent solution. It may beconvenient to use small alligator clips to secure the lead wiresto the cathode and anode. These clips must be heavily goldplat
39、ed so as to be entirely free of porosity. A variation of thisprocedure, suitable for samples having relatively few pores, isto use a second identical test sample as the cathode. The testcan be run with current first in the forward, then in the reversedirection so that the porosity in both samples ma
40、y be deter-mined. Fig. 1 is a schematic of the test cell setup.NOTE 1A commonly-used alternate cell design incorporates thecathode as part of the cell structure (as shown in Fig. 2). In addition, thesamples may be attached to a common carrier strip or holder, so that onlythe sample surfaces need be
41、in the gel.7.5 Timer capable of indicating seconds. It is convenient touse a timer switch to control the test current.7.6 Stereomicroscope having 103 magnification and anilluminator are required for sample inspection after test. Aneyepiece reticle is recommended for convenience in locatingthe contac
42、t area or other significant measurement areas.8. Reagent8.1 Note that some of the indicating reagents are sensitive toheat and light, particularly the rubeanic acid (dithio-oxamide).The indicator solutions should be stored in the dark instoppered bottles. For rubeanic acid, do not store for more tha
43、na month, and filter prior to use.8.2 Food-Grade GelatinThis type is preferred to USPgrade gelatin, because the latter may not give transparentsolutions. A 10 % solution is prepared by mixing9gofthegelatin in 91 mL of distilled or deionized water, and slowlyheating to 60 to 65C with stirring, until
44、all the gelatindissolves.NOTE 2If the storage bottle is tightly capped, the plain gelatinsolution may be stored for up to 2 days in a refrigerator and kept at 5 to10C, discard it if mold appears on its surface.9. Safety Hazards9.1 Reagents identified in Table 1 have the potential to causeinjury or s
45、kin discoloring if improperly handled. Good labora-tory practice including the use of a fume hood and skin and eyeprotection should be observed, especially during solutionpreparative and the cleaning of the test samples. Properprecautions in the use of electrical power supplies and electri-cal conne
46、ctions should also be scrupulously observed.10. Procedure10.1 This test is suitable for gold coated on silver, nickel, orcopper and its alloys, and palladium coated on nickel, copperand its alloys either as underlayers or substrates, in accordancewith the reagents chosen in Table 1.10.2 Sequence of
47、Operations:10.2.1 Solution Preparation:10.2.1.1 Electrolyte.10.2.1.2 Indicator.10.2.2 Calculate the current to be used.FIG. 1 Schematic of Typical Test-Cell Setup with Anode (Sample)and Cathode Facing Each Other (Preferred Orientation)FIG. 2 Exploded View of Alternate Cell Design IncorporatingCathod
48、e as Part of Cell StructureB 798 95 (2009)310.2.3 Prepare the samples prior to cleaning.10.2.4 Clean the samples.10.2.5 Prepare the gel while the samples are cleaning.Remove from heat when dissolved.10.2.6 Dry the samples.10.2.7 Suspend the samples in the test cells.10.2.8 Prepare the composite gel
49、solution and add to thecells.10.2.9 Solidify the gel thoroughly.10.2.10 Set up the equipment and make electrical attach-ments.10.2.11 Apply the calculated current.10.2.11.1 Examine immediately for gross defects.10.2.11.2 Take readings after the prescribed time period.10.2.12 Record the results.10.3 CleaningHandle specimens as little as possible, andonly with tweezers, microscope-lens tissue, or clean, softcotton gloves. Prior to the test, inspect the samples under 103magnification for evidence of particulate matter. If present,such particles shall be removed by bl
