1、Designation: B 504 90 (Reapproved 2007)Endorsed by AmericanElectroplaters SocietyEndorsed by NationalAssociation of Metal FinishersStandard Test Method forMeasurement of Thickness of Metallic Coatings by theCoulometric Method1This standard is issued under the fixed designation B 504; the number imme
2、diately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of 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.This standard h
3、as been approved for use by agencies of the Department of Defense.1. Scope1.1 This test method covers the determination of the thick-ness of metallic coatings by the coulometric method, alsoknown as the anodic solution or electrochemical strippingmethod.1.2 This standard does not purport to address
4、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. Referenced Documents2.1 ISO Standard:ISO 2177 Metallic Coa
5、tingsMeasurement of CoatingThicknessCoulometric Method by Anodic Dissolution23. Summary of Test Method3.1 The thickness of the coating is determined by measuringthe quantity of electricity (coulombs) required to dissolve thecoating anodically from a known and accurately defined area.3.2 As commonly
6、practiced, the method employs a smallmetal cell which is filled with an appropriate electrolyte. Thetest specimen serves as the bottom of the cell and an insulatinggasket between the cell and the specimen defines the test area(about 0.1 cm2). With the test specimen as anode and the cellas cathode, a
7、 constant direct current is passed through the celluntil the coating has dissolved, at which time a sudden changein voltage occurs.3.3 The thickness of the coating may be calculated from thequantity of electricity used (current multiplied by time), thearea, the electrochemical equivalent of the coat
8、ing metal, theanodic-current efficiency, and the density of the coating.Alternatively, the equipment may be calibrated against stan-dards with known coating thicknesses.3.4 Commercial instruments using this principle are avail-able. The method is rapid and versatile, but destructive to thecoating. I
9、n general, its range is considered to be between 0.75and 50 m. Chromium, gold, tin, and other coatings can bemeasured down to 0.075 m.4. Significance and Use4.1 Measurement of the thickness of a coating is essential toassessing its utility and cost.4.2 The coulometric method destroys the coating ove
10、r avery small (about 0.1 cm2) test area. Therefore its use is limitedto applications where a bare spot at the test area is acceptableor the test piece may be destroyed.5. Factors Affecting the Accuracy of the Method5.1 Composition of ElectrolytesElectrolytes used for cou-lometric thickness measureme
11、nts must permit the coatingmetal to dissolve at a constant anodic-current efficiency (pref-erably 100 %); they must have a negligible spontaneouschemical effect on the coating metal and must so differentiateelectrochemically between the coating and the substrate that asuitably sharp and large voltag
12、e change occurs at the end pointof the test.5.1.1 Electrolytes furnished with commercial instrumentsmay be presumed to meet these requirements; others must beevaluated before use by testing standards having knownthicknesses. Appendix X1 lists some electrolytes and coating-substrate combinations that
13、 have been used with some instru-ments.5.2 Current VariationFor coulometric instruments em-ploying the constant-current technique, variation of the currentduring a test will result in errors. For instruments using acurrent-time integrator, variation of the current during a testwill not result in err
14、or unless the current change is such as todisplace the anodic current density beyond the range ofconstant or 100 % anodic-current efficiency.5.3 Area VariationThe accuracy of the thickness measure-ment will not be better than the accuracy with which the testarea is defined or known. Typically, this
15、test area is defined bya flexible, insulating gasket. Area variation is usually mini-mized by using as large an area as practical and by using a1This test method is under the jurisdiction ofASTM Committee B08 on Metallicand Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 o
16、nTest Methods.Current edition approved March 1, 2007. Published March 2007. Originallyapproved in 1970. Last previous edition approved in 2002 as B 504 90 (2002).2Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036, http:/www.ansi.org.1Copyright
17、ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.constant pressure device. If excessive pressure is applied tosuch a gasket, the test area may be altered undesirably.5.4 AgitationIn most, but not all, coulometric thicknessmeasurements, a relativ
18、ely high anodic-current density isemployed to shorten the test time. It is then necessary to agitatethe electrolyte to maintain a constant anodic-current efficiency.Where agitation is required, insufficient agitation may result inpolarization of the specimen, thereby causing a premature andfalse end
19、point.5.5 Alloying Between Coatings and Metallic SubstratesThe measurement of a coating thickness by the coulometricmethod implicitly assumes that a sharply defined interfaceexists between the coating and the substrate. If an alloy layerexists between the coating and the substrate as, for example, i
20、nthe case of coatings applied by hot dipping, the coulometricend-point may occur at some point within the alloy layer, thusgiving a high value of the thickness of the unalloyed coating.5.6 Purity of CoatingImpurities or additives that code-posit with the coating may change the effective electrochemi
21、calequivalent of the coating and also change the anodic currentefficiency.5.6.1 Alloy CoatingVariations in the composition of alloycoatings will change the effective electrochemical equivalentof the coating.5.7 Cleanliness of Test SurfaceThe surface to be testedmust be clean. Oil, grease, and organi
22、c coatings such as lacquermust be removed with suitable solvents. Oxides, conversioncoatings, and corrosion products are preferably removed bycarefully burnishing the test surface with a clean, soft pencileraser. Tin and nickel surfaces, in particular, should be soburnished prior to testing to remov
23、e passive oxide films.5.8 Density of CoatingThe coulometric method intrinsi-cally measures coating mass per unit area, the equivalent linearthickness being a function of the density of the coating. If thedensity of the coating tested is different from the value of thedensity used for the calibration
24、, the linear thickness obtainedcoulometrically will be different from the actual linear thick-ness of the coating tested.5.8.1 Density of Alloy CoatingsVariation in the composi-tion of alloy coatings will change the density of the coating.5.9 Number and Location of TestsSince the coulometrictest met
25、hod measures, essentially, a local coating thickness, asingle test may not be representative of the coating thicknessover the entire significant surface.6. Calibration of Equipment6.1 The equipment shall be calibrated by means of standardshaving known coating thicknesses. If commercial equipment isu
26、sed, the manufacturers instructions shall be followed insofaras they are compatible with this test method.6.2 Calibration of Direct-Reading InstrumentsDirect-reading instruments shall be calibrated against standards hav-ing known coating thicknesses, and adjusted to produce correctreadings correspon
27、ding to the coating thicknesses of thestandard.6.3 Calibration of Nondirect-Reading Instruments:6.3.1 Nondirect-reading instruments shall be calibratedagainst standards having a known coating thickness by using acalibration constant, C, calculated as follows:C 5 coating thickness of the standards/in
28、strument reading (1)6.3.2 The instrument shall be adjusted so that where stan-dards having known coating thicknesses are tested, the correctthickness is obtained by multiplying the instrument reading bythe calibration constant, C.6.4 Thickness StandardsThe thickness standards shallconsist of the sam
29、e type of coating and substrate as thespecimens to be measured, and they shall have an accuracy of65 % or better.7. Procedure for Making Measurements7.1 If commercial equipment is used, the manufacturersinstructions shall be followed insofar as they are compatiblewith this test method.7.2 The test s
30、urface shall be cleaned of all foreign materialthat might affect the measurement.NOTE 1Certain nickel deposits, frequently dull nickel, may exhibitpassivity. When such coatings are tested coulometrically, the voltageacross the specimen and test cell is markedly higher (approximately 1 V)than normal,
31、 and the coating does not dissolve. Oxygen is evolved at thespecimen and the test may continue indefinitely.NOTE 2Removal of the passivity may be accomplished in some casesby mildly abrading (as with a pencil eraser) the nickel surface prior totesting. Alternatively, the specimen may be made cathodi
32、c in the coulo-metric electrolyte for 10 to 20 s by applying current from an externalsource. Allowing the nickel to be in contact with 10 % volume hydro-chloric acid for approximately 1 min prior to the test may also be usedeffectively.7.3 After completion of the measurement, the test surfaceshall b
33、e examined visually, and if the dissolution of the coatingis not virtually complete, the measurement shall be discardedand repeated.8. Precision and Bias8.1 The equipment, its calibration, and its operation shall besuch that the coating thickness can be determined with anuncertainty of less than 10
34、%.8.2 Instruments suitable for compliance with 8.1 are avail-able commercially. For many coating systems the instrumentsare capable of making measurements with an uncertainty ofless than 4 % (95 % confidence).8.3 Although an uncertainty of less than 10 % may beachieved consistantly for a great numbe
35、r of coating-substratecombinations, the uncertainty may be greater when the coatingthickness is less than 1 m or exceeds 50 m.8.4 The bias of a coulometric measurement is the discrep-ency remaining between the measured thickness and the truethickness if all random errors are eliminated. It is, there
36、fore, nogreater than, and attributable to (1), the calibration error of theinstrument and (2) the quality of the calibration standard usedto calibrate the instrument.B 504 90 (2007)2APPENDIX(Nonmandatory Information)X1. ELECTROLYTESX1.1 Table X1.1 lists electrolytes that have been used forcoulometri
37、c thickness measurements; however, they are notnecessarily suitable for use with all types of coulometricinstruments.X1.2 Use of these electrolytes is not mandatory for com-pliance with this method, and when commercial coulometricinstruments are used, the manufacturers recommendationsshall be follow
38、ed.X1.3 Table X1.2 lists other coating-substrate combinationsthat have been used successfully with commercially availableelectrolytes.ASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentionedin this standard. Users of this standa
39、rd are expressly advised that determination of the validity of any such patent rights, and the riskof infringement of such rights, are entirely their own responsibility.This standard is subject to revision at any time by the responsible technical committee and must be reviewed every five years andif
40、 not revised, either reapproved or withdrawn. Your comments are invited either for revision of this standard or for additional standardsand should be addressed to ASTM International Headquarters. Your comments will receive careful consideration at a meeting of theresponsible technical committee, whi
41、ch you may attend. If you feel that your comments have not received a fair hearing you shouldmake your views known to the ASTM Committee on Standards, at the address shown below.This standard is copyrighted by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959,U
42、nited States. Individual reprints (single or multiple copies) of this standard may be obtained by contacting ASTM at the aboveaddress or at 610-832-9585 (phone), 610-832-9555 (fax), or serviceastm.org (e-mail); or through the ASTM website(www.astm.org).TABLE X1.1 Typical Electrolytes for Electrodepo
43、sited CoatingsACoatingSubstrate (Basis Metal)SteelCopper andAlloys (suchas brass)Nickel Aluminum ZincCadmium 1, 10 1, 10 1 1 . . .Chromium 2, 11 3, 4, 12 2, 13 2, 13 . . .Copper 5, 14 . . . 5, 15 5 16Lead 17 17 17 . . . . . .Nickel 6, 18 6, 19 . . . 6, 18 . . .Silver 7 8 7 . .Tin 3,4,20 3,4,20 3,4 2
44、,21 .Zinc 9 9 9 9 . . .AThe numbers in the table refer to the following aqueous solutions:(1) 100 g KI/L, with traces of I2(2) 100gNa2SO4/L(3) 175 mL HCl (sp gr 1.18)/L(4) 150 g NaOH/L(5) 80 g NaKC4H4O6(sodium potassium tartrate) + 100 g NH4NO3/L(6) 30gNH4NO3+ 30 g NaSCN/L(7) 100 g NaNO3+ 3 ml HNO3(
45、sp gr 1.42)/L(8) 180 g KSCN/L(9) 100 g NaCl or KCl/L(10) 30gKCl+30gNH4Cl/L(11) 100 mL H3PO4(spgr1.75)+10gCrO3/L(12) 100gNa2CO3for coatings up to 5 m (0.2 mil)/L(13)64mLH3PO4(sp gr 1.75)/L(14) 800gNH4NO3+10mLNH4OH (sp gr 0.88)/L(15) 100gK2SO4+20mLH3PO4(sp gr 1.75)/L(16) Pure H2SiF6solution containing
46、 not less than 30 % H2SiF6(Slightly weakeracid may be used, if some MgSiF6is added to the solution.)(17) 200gCH3COONa + 200 g CH3COONH4/L(18) 800gNH4NO3+ 3.8 g CS(NH2)2(thiourea)/L(19) 100 mL HCl (sp gr 1.18)/L(20) 100 g KNO3+ 100 g KCl/L(21)50mLH2SO4(sp gr 1.84) + 5 g KF/LTABLE X1.2 Measurable Coat
47、ing-Substrate CombinationsSubstrateCoatingAlumi-numCopperandCopperAlloysNickel SteelMag-neticStain-lessSteelNonme-tallicBrassAXXXCobalt X XGold X XIndium X X XIron XNickel (Electrodes)AXXXNickel-IronAXXTin-LeadAXXXXTin-NickelATin-ZincAAThe measurement accuracy of these alloy coatings is dependent on thecomposition of the coating.B 504 90 (2007)3
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