1、Designation: B735 16Standard Test Method forPorosity in Gold Coatings on Metal Substrates by NitricAcid Vapor1This standard is issued under the fixed designation B735; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of las
2、t 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 procedures forusing nitric acid vapor for determining porosity in goldcoatings, gre
3、ater than 0.6 m (25 in.) in thickness, particularlyelectrodeposits and clad metals used on electrical contacts.1.2 This test method is designed to show whether theporosity level is less or greater than some value that, byexperience, is considered by the user to be acceptable for theintended applicat
4、ion.1.3 Avariety of other porosity testing methods are describedin Guide B765 and in the literature.2,3Other porosity testmethods are Test Methods B741, B798, B799, and B809.1.4 The values stated in SI units are to be regarded asstandard. The values given in parentheses are for informationonly.1.5 T
5、his 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 become familiarwith all hazards including those identified in the appropriateSafety Data Sheet (SDS) for this product/material as providedby th
6、e manufacturer, to establish appropriate safety and healthpractices, and determine the applicability of regulatory limi-tations prior to use. Specific precautions are given in Section 8and 9.4.2. Referenced Documents2.1 ASTM Standards:4B374 Terminology Relating to ElectroplatingB542 Terminology Rela
7、ting to Electrical Contacts and TheirUseB741 Test Method for Porosity In Gold Coatings On MetalSubstrates By Paper Electrography (Withdrawn 2005)5B765 Guide for Selection of Porosity and Gross Defect Testsfor Electrodeposits and Related Metallic CoatingsB798 Test Method for Porosity in Gold or Palla
8、dium Coat-ings 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 t
9、his test method aredefined 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 c
10、oating surface after a vapor test exposure.3.2.2 measurement area (or “significant surface”), 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 marke
11、d samples.3.2.2.1 DiscussionFor specification purposes, the signifi-cant surfaces or measurement 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
12、 tarnish films thatinterfere with the function of the part.3.2.3 metallic coatings, ninclude platings, claddings, orother metallic layers applied to the substrate. The coating cancomprise a single metallic layer or a combination of metalliclayers.3.2.4 porosity, nthe presence of any discontinuity, c
13、rack,or hole in the coating that exposes a different underlying metal.1This test method is under the jurisdiction of ASTM Committee B02 onNonferrous Metals and Alloys and is the direct responsibility of SubcommitteeB02.11 on Electrical Contact Test Methods.Current edition approved Oct. 1, 2016. Publ
14、ished October 2016. Originallyapproved in 1984. Last previous edition approved in 2011 as B735 06 (2011).DOI: 10.1520/B0735-16.2For example see: Nobel, F. J., Ostrow, B. D., and Thompson, D. W., “PorosityTesting of Gold Deposits,” Plating, Vol 52, 1965, p. 1001.3Krumbein, S. J., Porosity Testing of
15、Contact Platings, Proceedings, Connectorsand Interconnection Technology Symposium, 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 Do
16、cument Summary page onthe ASTM website.5The last approved version of this historical standard is referenced onwww.astm.org.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.2.5 underplate, na metallic coating layer between thesubstrat
17、e and the topmost layer or layers. The thickness of anunderplate is usually greater than 0.8 m (30 in.).4. Summary of Test Method4.1 This test method employs nitric acid (HNO3) vapor atlow relative humidity. Reaction of the gas mixture with acorrodible base metal at pore sites produces reaction prod
18、uctsthat appear as discrete spots on the gold surface. Individualspots are counted with the aid of a loupe or low-power stereomicroscope.4.2 This test method is suitable for inlays or claddingscontaining 75 % or more of gold or for electroplatings con-taining 95 % or more of gold on substrates of co
19、pper, nickel,and their alloys, that are commonly used in electrical contacts.4.3 The nitric acid vapor test is too severe to be used forgold coatings less than 0.6 m (25 in.) in thickness. It is alsonot suitable for coatings that are less noble than gold orplatinum, such as palladium and its alloys,
20、 or gold-flashedpalladium and its alloys. Gold-flashed is defined as a platedthickness of gold between 3 and 5 in.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 fou
21、nd in natural or service environments, these tests arenot recommended for prediction of the electrical performanceof contacts unless correlation is first established with serviceexperience.5. Significance and Use5.1 Gold coatings are often specified for the contacts ofseparable electrical connectors
22、 and other devices. Electrode-posits are the form of gold that is most used on contacts,although it is also employed as clad metal and as weldments onthe contact surface. The intrinsic nobility of gold enables it toresist the formation of insulating oxide films that couldinterfere with reliable cont
23、act operation.5.2 In order that the nobility of gold be assured, porosity,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 e
24、nvironment orwhere corrosion inhibiting surface treatments for the depositare 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, circ
25、uit parameters, and thereliability of contact operation that it is necessary to maintain.Also, when present, the location of pores on the surface isimportant. If the pores are few in number and are outside of thezone of contact of the mating surfaces, their presence can oftenbe tolerated.5.3 Methods
26、 for determining pores on a contact surface aremost suitable 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 vary.Thi
27、s test method is 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 by judgement.Thus, absence ofporosity in the coating may be a requirement for someapplications, while a f
28、ew pores on the critical surfaces may beacceptable for another. Such acceptance (or pass-fail) criteriashould be part of the product specification for the particularproduct or part requiring the porosity test.5.5 This test method is highly sensitive and is capable ofdetecting virtually all porosity
29、or other defects in gold coatingsthat could participate in substrate corrosion reactions. The testis rapid, simple, and inexpensive. In addition, it can be used oncontacts having complex geometry such as pin-socket contacts.However, it is preferred that deeply recessed sockets be openedto expose the
30、ir critical surfaces prior to testing.5.6 This test method is considered destructive in that itreveals the presence of porosity by contaminating the surfacewith corrosion products and by undercutting the coating at poresites or at the boundaries of unplated areas. Any parts exposedto these tests sha
31、ll not be placed in service.5.7 This test method is intended to be used for quantitativedescriptions of porosity (such as number of pores per unit areaor per contact) only on coatings that have a pore densitysufficiently low that the corrosion sites are well separated andcan be readily resolved. As
32、a general guideline this can beachieved for pore densities up to about 100/cm2or per 100contacts. Above this value the tests are useful for the qualita-tive detection and comparisons of porosity.6. Apparatus6.1 Test Chamber, may be any convenient size glass vesselcapable of being sealed with a glass
33、 lid, such as a glassdesiccator of 9 to 12 Lcapacity. The ratio of the air space in thechamber (in cubic centimetres) to the nitric acid surface area(in square centimetres) shall not be greater than 25:1.6.2 Specimen Holders or SupportsSupports or hangersshall be made from glass, polytetrafluoroethy
34、lene or other inertmaterials. It is essential that the holders be so designed, and thespecimens so arranged, that the circulation of the vapor is notimpeded. Specimens shall be at least 75 mm (3 in.) from theliquid surface and at least 25 mm (1 in.) from the vessel walls.Also, the measurement areas
35、of the specimens shall be at least12 mm (0.5 in.) from each other.6.2.1 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 ensure that movement of air and vaporwithin the vessel will not be restricted during the t
36、est.6.3 Stereomicroscope, having a 10 magnification, shall beused for pore counting. In addition a movable source ofillumination capable of giving oblique lighting on the specimensurface is also useful.7. Reagent7.1 Nitric Acid, Reagent Grade Concentrated 70 6 2%HNO3, sp gr 1.415 to 1.420.B735 1628.
37、 Safety Hazards8.1 Carry out this test method in a chemical fume hood,since the gases that are released, mainly when the reactionvessel is opened at the end of each test, are very corrosive.8.2 Use caution, however, to ensure that drafts that are oftenfound in fume hoods do not cause significant coo
38、ling of thechamber walls, that could lead to a rise in the relative humidityand acceleration of the test (see 9.3). It is often convenient toenclose the reaction vessel in a box with a loose-fitting cover.8.3 Observe normal precautions in handling corrosive acids.In particular, wear goggles complete
39、ly enclosing the eyes whenhandling nitric acid, and make eye wash facilities readilyavailable.9. Procedure9.1 Handle specimens as little as possible, and only withtweezers, microscope-lens tissue, or clean, soft cotton gloves.Prior to the test, inspect the samples under 10 magnificationfor evidence
40、of particulate matter. If present, such particlesshall be removed by blowing them off with clean, oil-free air.If this is not successful discard the sample. Then, clean thesamples with solvents or solutions that do not contain chlori-nated hydrocarbons, CFCs, or other known ozone-destroyingcompounds
41、. The procedure outlined in Note 1 has been foundto give satisfactory results for platings with mild to moderatesurface contamination.NOTE 1Suggested cleaning procedure:(1) Keep individual contacts separated if there is a possibility ofdamage to the measurement areas during the various cleaning step
42、s.(2) Clean samples for 5 min in an ultrasonic cleaner which contains ahot (6585C) 2 % aqueous solution of a mildly alkaline (pH 7.510)detergent (such as Micro or Sparkleen).(3) After ultrasonic cleaning, rinse samples under warm running tapwater for at least 5 s.(4) Rinse samples ultrasonically for
43、 2 min in fresh deionized water toremove the last detergent residues.(5) Immerse in fresh methanol or isopropanol, and ultrasonically“agitate” for at least 30 s in order to remove the water from the samples.(6) Remove and dry samples until the alcohol has completely evapo-rated. If an air blast is u
44、sed as an aid to drying, the air shall be oil free,clean, and dry.(7) Do not touch measurement area of the samples with bare fingersafter cleaning.(8) Reinspect samples (under 10 magnification) for particulate matteron the surface. If particulates are found, repeat the cleaning steps. Surfacecleanli
45、ness is extremely important; contaminants, such as plating salts,organic films, and metal flakes may give erroneous indications of defects,and are unacceptable.NOTE 2If large areas of exposed non-noble metal are present,masking these areas may be necessary. However, when masking is donewith platers
46、tape, take care not to inhibit the flow of acid vapor to themeasurement area.9.2 The ambient temperature and the temperature of thespecimens and solution are 23 6 3C at the beginning of thetest and maintained throughout the test period.9.3 The relative humidity in the immediate vicinity of thetest c
47、hamber shall be no greater than 60 %,6although 55 % orbelow is preferred. If the relative humidity is greater than60 %, do not run this test.9.4 Add fresh HNO3to the bottom of the clean and dry testchamber, and immediately close the cover. After 30 6 5 min,load the samples, using suitable fixtures,
48、and replace the cover.The ambient relative humidity shall be no greater than 60 %during both the addition of the HNO3and the insertion of thesamples.6(WarningDo not grease the rim of the desiccatornor its cover. If desired, press a minimum of three strips ofpressure sensitive polytetrafluoroethylene
49、 tape (adhesive sidedown) at equal intervals around the desiccator rim.)9.5 Unless otherwise specified, the exposure time to nitricacid vapor shall be 60 6 5 min. An exposure time of 75 6 5min is also commonly used for gold thicknesses in the 2 to 2.5m (75 to 100 in.) range.Atable of convenient exposure timesis given in the appendix.NOTE 3Variations in exposure time with thickness are often recom-mended because pores in thicker coatings are deeper and their averagesizes are smaller than those in thinner coatings. The nitric acid mediumwould therefore ta
