ASTM_B_504_-_90_2011.pdf

Designation:B50490(Reapproved 2011)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 B504;the number immediately 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()indicates an editorial change since the last revision or reapproval.This standard has 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 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 CoatingsMeasurement 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 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 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 coating 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.In 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 over 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 measurements 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 voltage 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 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 error unless the current change is such as to1This test method is under the jurisdiction ofASTM Committee B08 on Metallicand Inorganic Coatingsand is the direct responsibility of Subcommittee B08.10 onTest Methods.Current edition approved Oct.1,2011.Published October 2011.Originallyapproved in 1970.Last previous edition approved in 2007 as B504 90(2007).DOI:10.1520/B0504-90R11.2Available from American National Standards Institute(ANSI),25 W.43rd St.,4th Floor,New York,NY 10036,http:/www.ansi.org.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 displace 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 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 aconstant pressure device.If excessive pressure is applied tosuch a gasket,the test area may be altered undesirably.5.4 AgitationIn most,but not all,co