1、Designation: B920 16Standard Practice forPorosity in Gold and Palladium Alloy Coatings on MetalSubstrates by Vapors of Sodium Hypochlorite Solution1This standard is issued under the fixed designation B920; 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 practice covers equipment and methods forrevealing the porosity of gold a
3、nd palladium coatings, particu-larly electrodeposits and clad metals used on electrical con-tacts.1.2 This test practice is suitable for coatings containing goldor 75 % by mass of palladium on substrates of copper, nickel,and their alloys, which are commonly used in electricalcontacts.1.3 Avariety o
4、f full porosity testing methods is described inthe literature.2,3These porosity Test Methods are B735, B741,B798, B799, and B809. An ASTM Guide to the selection ofporosity tests for electrodeposits and related metallic coatingsis available as Guide B765.1.4 The values stated in SI units are to be re
5、garded asstandard. The values given in parentheses are for informationonly.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 determin
6、e the applica-bility of regulatory limitations prior to use. For specifichazards, see Section 6.2. Referenced Documents2.1 ASTM Standards:4B374 Terminology Relating to ElectroplatingB542 Terminology Relating to Electrical Contacts and TheirUseB735 Test Method for Porosity in Gold Coatings on MetalSu
7、bstrates by Nitric Acid VaporB741 Test Method for Porosity In Gold Coatings On MetalSubstrates By Paper Electrography (Withdrawn 2005)5B765 Guide for Selection of Porosity and Gross Defect Testsfor Electrodeposits and Related Metallic CoatingsB798 Test Method for Porosity in Gold or Palladium Coat-i
8、ngs on Metal Substrates by Gel-Bulk ElectrographyB799 Test Method for Porosity in Gold and PalladiumCoatings by Sulfurous Acid/Sulfur-Dioxide VaporB809 Test Method for Porosity in Metallic Coatings byHumid Sulfur Vapor (“Flowers-of-Sulfur”)3. Terminology3.1 DefinitionsMany terms used in this practic
9、e are de-fined in Terminology B542 and terms relating to metalliccoatings are defined in Terminology B374.3.2 Definitions of Terms Specific to This Standard:3.2.1 corrosion products, nthose reaction products ema-nating from the pores that protrude from, or are otherwiseattached to, the coating surfa
10、ce after a vapor test exposure.3.2.2 metallic coatings, ninclude platings, claddings, orother metallic layers applied to the substrate. The coatings cancomprise a single metallic layer or a combination of metalliclayers.3.2.3 porosity, nthe presence of any discontinuity, crack,or hole in the coating
11、 that exposes a different underlying metal.3.2.4 underplate, na metallic coating layer between thesubstrate and the topmost layer or layers. The thickness of anunderplate is usually greater than 0.8 m (30 in.).4. Summary of Practice4.1 The test practice employs a solution of sodiumhypochlorite, a ma
12、terial readily available as household bleach.The test is recommended primarily as a qualitative means forassessing the plating quality in electrical connectors and isdesirable because it uses readily available reagents and equip-ment and is extremely inexpensive, simple, and fast. In the test,1This
13、practice is under the jurisdiction ofASTM Committee B02 on NonferrousMetals and Alloys and is the direct responsibility of Subcommittee B02.11 onElectrical Contact Test Methods.Current edition approved Oct. 1, 2016. Published October 2016. Originallyapproved in 2001. Last previous edition approved i
14、n 2011 as B920 01 (2011).DOI: 10.1520/B0920-16.2For example see: Nobel, F. J., Ostrow, B. D., and Thompson, D. W., “PorosityTesting of Gold Deposits,” Plating, Vol 52, 1965, p. 1001.3Krumbien, S J., Porosity Testing of Contact Platings, Proceedings, Connectorsand Interconnection Technology Symposium
15、, Oct. 1987, p. 47.4For 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.5The last approved version of this histori
16、cal standard is referenced onwww.astm.org.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1the coated parts to be evaluated are suspended above a solutionof sodium hypochorite in a vessel that is closed but not sealed.Paper towels exte
17、nd from the solution in the bottom part of theway up the sides of vessel providing a wicking action for thesolution. After exposure in this manner for 30 to 45 min, theparts are removed from the vessel, dried with hot air andexamined for the presence of corrosion products that indicateporosity.4.2 F
18、or more quantitative characterization of porosity it isbetter to use one of the previously mentioned porosity teststandards. This practice is oftentimes used as an early predictorof the likelihood of failure in a full mixed flowing gas (MFG)test used as an accelerated environmental test.4.3 Exposure
19、 periods may vary, depending upon the degreeof porosity to be revealed. Reaction of the gas with acorrodable base metal at pore sites produces reaction productsthat appear as discrete spots on the gold or palladium surface.Individual spots may be counted with the aid of a loupe orlow-power stereomic
20、roscope.4.4 This porosity test involves corrosion reactions in whichthe products delineate defect sites in coatings. Since thechemistry and properties of these products may not resemblethose found in natural or service environments this test is notrecommended for prediction of the electrical perform
21、ance ofcontacts unless correlation is first established with serviceexperience.5. Significance and Use5.1 Palladium and gold coatings are often specified for thecontacts of separable electrical connectors and other devices.Electrodeposits are the form of gold that is most used oncontacts, although i
22、t is also employed as inlay or clad metaland as weldments on the contact surface. The intrinsic nobilityof gold and palladium alloys enables it to resist the formationof insulating oxide films that could interfere with reliablecontact operation.5.2 In order for these coatings to function as intended
23、,porosity, cracks, and other defects in the coating that exposebase-metal substrates and underplates must be minimal orabsent, except in those cases where it is feasible to use thecontacts in structures that shield the surface from the environ-ment or where corrosion inhibiting surface treatments fo
24、r thedeposit are employed. The level of porosity in the coating thatmay be tolerable depends on the severity of the environment tothe underplate or substrate, design factors for the contactdevice like the force with which it is mated, circuit parameters,and the reliability of contact operation that
25、it is necessary tomaintain. Also, when present, the location of pores on thesurface is important. If the pores are few in number and areoutside of the zone of contact of the mating surfaces, theirpresence can often be tolerated.5.3 Methods for determining pores on a contact surface aremost suitable
26、if they enable 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 proceduresth
27、at detect only highly porous conditions.5.4 The present test practice is capable of detecting virtuallyall porosity or other defects that could participate in corrosionreactions with the substrate or underplate. In addition, it can beused on contacts having complex geometry such as pin-socketcontact
28、s (although with deep recesses it is preferred that thecontact structures be opened to permit reaction of the vaporswith the interior significant surfaces).5.5 The relationship of porosity levels revealed by particulartests to contact behavior must be made by the user of these teststhrough practical
29、 experience or by other forms of testing. Thus,absence of porosity in the coating may be a requirement forsome applications, while a few pores in the contact zone maybe acceptable for others. The acceptable number, sizes andlocations of the pore corrosion products shall be as specified onthe appropr
30、iate drawing or specification.5.6 This test is considered destructive in that it reveals thepresence of porosity by contaminating the surface with corro-sion products and by undercutting the coating at pore sites orat the boundaries of the unplated areas. Any parts exposed tothis test shall not be p
31、laced in service.5.7 The test is simple and inexpensive. The cost associatedwith the test is very low, using standard basic equipment foundin an industrial laboratory. There are minimal waste disposalissues associated with the procedure. The test is very popularbecause of its very quick means of ass
32、essing the likelihood ofplating quality problems, prior to the performance of acceler-ated environmental testing on the 1 to 2 week scale at muchgreater expense.6. Safety Hazards6.1 Carry out this test procedure in a clean, working fumehood. The vapor emitted is toxic, corrosive, and irritating.6.2
33、Because the test is conducted in a reaction vessel usinga loose-fitting cover, it is desirable to insure that the draftsoften found in hoods are not so high as to adversely effect thereproducibility of the test within the reaction vessel.6.3 Observe good laboratory practices when handling thesodium
34、hypochlorite (household bleach) solution. In particular,wear eye protection completely enclosing the eyes, and makeeye wash facilities readily available.7. Apparatus7.1 Test Glassware, a vessel of sufficient size such that thesodium hypochlorite solution at least 1 cm in depth can beplaced in the bo
35、ttom of the vessel without interfering with thesamples. The vessel (see Figs. 1 and 2) shall be made of glassor plastic not having a gastight lid, such as a glass beaker witha watch glass cover or desiccator.7.2 Specimen Holders or Supports, may be made of glass,polytetrafluoroethylene, or other ine
36、rt materials. It is essentialthat the specimens be arranged so as not to impede circulationof the gas. Specimens shall not be closer than 12 mm (0.5 in.)from the wall or paper towels and 25 mm (1 in.) from theB920 162solution surface.Also, the measurement areas of the specimensshall be at least 12 m
37、m (0.5 in.) from each other.7.3 Do not use a porcelain plate or any other structure thatwould cover more than 30 % of the liquid surface cross-sectional area. This is to insure that movement of air and vaporwithin the vessel will not be restricted during the test.7.4 Stereomicroscope, having at leas
38、t 20 magnificationshall be used for pore examination is recommended. Inaddition, a movable source of illumination capable of provid-ing oblique lighting on the specimen surface is required. It isfurther recommended that a graduated reticle be inserted intoone of the eyepieces of the microscope.7.5 H
39、ot Air Dryer.8. Reagents8.1 Sodium Hypochlorite Solution, 5.25 % nominal, (with-out additives).Acquire this material locally to ensure freshness.8.2 Plain Paper Towels, or suitable non-reactive paperproduct which will retain its shape following exposure.9. Procedure9.1 Handle specimens as little as
40、possible, using onlytweezers, microscope-lens tissue, or clean, soft cotton gloves.Prior to the test, inspect the samples under 20 magnificationfor evidence of particulate matter. If present, such particlesshall be removed by blowing them with clean, oil-free air. Ifthis is not successful discard th
41、e sample. Then, clean thesamples with solvents or solutions that do not contain chlori-nated hydrocarbons, CFCs or other known ozone-destroyingcompounds. The procedure outlined in Note 1 has been foundto give satisfactory results for platings with mild to moderatesurface contamination.NOTE 1Suggeste
42、d Cleaning Procedure: (1) Keep individual contactsseparated if there is a possibility of damage to the measurement areasduring the various cleaning steps; (2) Dip in methanol if desired to aid inthe removal of organic residues; (3) Clean samples for 5 min in anultrasonic cleaner, which contains a ho
43、t (65 to 85C) 2 % aqueous solutionof a mildly alkaline (pH 7.5 to 10) detergent (such as Micro or Sparkleen);(4)After ultrasonic cleaning, rinse samples under warm running tap waterfor at least 5 s; (5) Rinse samples ultrasonically for 2 min in freshdeionized water to remove the last detergent resid
44、ues; (6) Immerse in freshmethanol or isopropanol, and ultrasonically “agitate” for at least 30 s inorder to remove the water from the samples; (7) Remove and dry samplesuntil the alcohol has completely evaporated. If an air blast is used as an aidto drying, the air shall be oil free, clean, and dry;
45、 (8) Do not touch surfacemeasurement area of the samples with bare fingers after cleaning; (9)Re-inspect samples (under 20 magnification) for particulate matter on thesurface. If particulates are found, repeat the cleaning steps. Surfacecleanliness is extremely important; contaminants, such as plati
46、ng salts,organic films, and metal filings or flakes may give erroneous indicationsof defects, and are unacceptable.NOTE 2Omit the cleaning steps for samples having corrosion-inhibiting, or lubricant coatings, or both, if it is desired to determine theefficacy of these coatings in the test atmosphere
47、.9.2 The test temperature shall be 23 6 3C, unless other-wise specified, and the relative humidity in the immediatevicinity of the test chamber shall be no greater than 60 %. If therelative humidity is greater than 60 %, results can be unpre-dictable and therefore the test is not acceptable.NOTE 3Pr
48、ocedure to be Performed in Hood or Well-Ventilated Area:(1) Obtain a clean, dry beaker or other vessel (sufficient in size toaccommodate your sample size), a cover glass and a specimen supportstrip, made of an inert material, for example, polyethylene or glass; (2)Attach the contacts to be tested to
49、 the support strip, by any suitable meanssuch as the use of two-sided adhesive tape so that the areas of interest onthe contact extend beyond the edge of the support, and the fumes freelyaccess the surfaces; (3) Place strips of paper towel on the inside surface ofthe vessel, so as to provide a wick for the bleach, and sufficient materialfor reaction with the solution. (This is important, since the bleach alonewill not generate sufficient chlorine to cause a reaction in a shorttime-frame.); (4) Pour in sufficient bleach such that a depth of approxi-mately 1 cm is obta
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