ASTM E376-17

Designation: E376 − 17Standard Practice forMeasuring Coating Thickness by Magnetic-Field or EddyCurrent (Electromagnetic) Testing Methods1This standard is issued under the fixed designation E376; 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 U.S. Department of Defense.1. Scope*1.1 This practice covers the use of magnetic- and eddycurrent-type thickness instruments (gauges) for nondestructivethickness measurement of a coating on a metal (i.e. electricallyconducting) substrate. The substrate may be ferrous or nonfer-rous. The coating or plating being measured may be electri-cally conducting or insulating as well as ferrous or non-ferrous.1.2 More specific uses of these instruments are covered byPractice D7091 and the following test methods issued byASTM: Test Methods B244, B499, and B530.1.3 Units—The values stated in SI units are to be regardedas standard. The values given in parentheses are mathematicalconversions to inch-pound units that are provided for informa-tion only and are not considered standard.1.4 Measurements made in accordance with this practicewill be in compliance with the requirements of ISO 2178 asprinted in 1982.1.5 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, health, and environmental practices and deter-mine the applicability of regulatory limitations prior to use.1.6 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:2B244 Test Method for Measurement of Thickness of AnodicCoatings on Aluminum and of Other NonconductiveCoatings on Nonmagnetic Basis Metals with Eddy-Current InstrumentsB499 Test Method for Measurement of Coating Thicknessesby the Magnetic Method: Nonmagnetic Coatings onMagnetic Basis MetalsB530 Test Method for Measurement of Coating Thicknessesby the Magnetic Method: Electrodeposited Nickel Coat-ings on Magnetic and Nonmagnetic SubstratesD7091 Practice for Nondestructive Measurement of DryFilm Thickness of Nonmagnetic Coatings Applied toFerrous Metals and Nonmagnetic, Nonconductive Coat-ings Applied to Non-Ferrous MetalsE543 Specification for Agencies Performing NondestructiveTestingE1316 Terminology for Nondestructive Examinations2.2 ASNT Standards:3SNT-TC-1A Recommended Practice for Personnel Qualifi-cation and Certification in Nondestructive TestingANSI/ASNT-CP-189 Standard for Qualification and Certifi-cation of NDT Personnel2.3 AIA Standard:NAS-410 Certification and Qualification of NondestructiveTesting Personnel41This practice is under the jurisdiction of ASTM Committee E07 on Nonde-structive Testing and is the direct responsibility of Subcommittee E07.07 onElectromagnetic Method.Current edition approved Nov. 1, 2017. Published December 2017. Originallyapproved in 1989. Last previous edition approved in 2011 as E376 – 11. DOI:10.1520/E0376-17.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at service@astm.org. For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.3Available fromAmerican Society for Nondestructive Testing (ASNT), P.O. Box28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http://www.asnt.org.4Available from Aerospace Industries Association of America, Inc. (AIA), 1000Wilson Blvd., Suite 1700,Arlington, VA22209-3928, http://www.aia-aerospace.org.(Replacement standard for MIL-STD-410.)*A Summary of Changes section appears at the end of this standardCopyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.12.4 International Standards:5ISO 2178 Nonmagnetic Coatings on Magnetic Substrate—Measurement of Coating Thickness—Magnetic MethodISO 9712 Non-destructive Testing—Qualification and Cer-tification of NDT PersonnelNOTE 1—See Appendix X1.3. Terminology3.1 Definitions—For definitions of terms relating to thispractice, refer to Terminology E1316.4. Significance and Use4.1 General—Most thickness gages are not applicable to allcombinations of coating-substrate thicknesses and materials.The limitations of a particular instrument are generally delin-eated by its manufacturer. The substrate material and coatingcombination to be measured as well as the inherent variationsin the substrate and coating shall be reviewed prior to selectingthe instrument to be used and the measurement accuracyrequired.4.2 Magnetic—Magnetic-type gauges measure either mag-netic attraction between a magnet and a coating or its substrate,or reluctance of a magnetic flux path passing through thecoating and substrate. These gauges are designed to measurethickness of a nonmagnetic coating on a magnetic substrate.Some of them will also measure thickness of nickel coatings ona magnetic or nonmagnetic substrate.64.3 Eddy Current—Eddy current-type thickness gauges areelectronic instruments that measure variations in impedance ofan eddy current inducing coil caused by coating thicknessvariations. They can only be used if the electrical conductivityof the coating differs significantly from that of the substrate.4.4 Accuracy—The accuracy of a measurement depends onthe instrument, the foils, its calibration and standardization,and its operating conditions. The accuracy is also affected bythe interferences listed in Section 5, such as part geometry(curvature), magnetic permeability, electrical conductivity, andsurface roughness.NOTE 2—This practice under ideal conditions may allow the coatingthickness to be determined within 610 % of its true thickness or to within62.5 µm (or 60.0001-in.), whichever is the greater. (See exceptions inAppendix X2.)5. Interferences5.1 Thickness of Coating—The precision of a measurementchanges with coating thickness depending on method used andinstrument design. Generally, the precision is a percentage ofthe coating thickness except at the lower end of the rangeswhere it is a fixed thickness.5.2 Magnetic Properties of Basis Metal—Magnetic thick-ness gauges are affected by variations of the magnetic proper-ties of the basis metal. For practical purposes, magneticvariations in low-carbon AISI 1005-1020 steels may be con-sidered to be insignificant. To avoid the influences of severe orlocalized heat treatments and cold working, the instrumentshould be standardized using a reference standard having abase metal with the same magnetic properties as that of the testspecimen or, preferably and if available, with a sample of thepart to be examined before application of the coating.5.3 Thickness of Substrate—For each method there is aneffective depth of penetration of field created by the instrumentprobe. This is the critical depth or thickness beyond which theinstrument will no longer be affected by increase of substratethickness. Since it depends on the instrument probe andsubstrate, it should be determined experimentally.5.4 Structure and Composition of Coating and Substrate—Eddy current instruments are sensitive to variations ofstructure, composition, and other factors affecting electricalconductivity and magnetic permeability of the coating andsubstrate. For example, such instruments are sensitive todifferences between: (1) aluminum alloys, (2) chromium coat-ings deposited at different temperatures, and (3) organiccoatings containing variable amounts of metallic pigments.5.5 Edge Effect—All examination methods are sensitive toabrupt surface changes of test specimens; therefore, measure-ments made too near an edge or inside corner will not be validunless the instrument is specifically standardized for such ameasurement. The effect usually extends 3 to 13 mm (1⁄8 to1⁄2in.) from the discontinuity, depending on method probeconfiguration, and instrument. Edge effect is usually a functionof coil diameter.5.6 Curvature of Examination Surface—Thickness mea-surements are sensitive to curvature of the specimen. Thissensitivity varies considerably between instruments and be-comes more pronounced with increasing curvature.5.7 Smoothness of Surface, Including That of Base Metal—Since a rough surface may make single measurementsinaccurate, a greater number of measurements will provide anaverage value that is more truly representative of the overallcoating thickness. Roughness also may cause certain instru-ments to read high since their probes may rest on peaks.5.8 Direction of Rolling of Base Metal—Instruments withtwo pole pieces may be sensitive to direction of rolling of thebase metal; that is, gauge readings may change depending onalignment of pole pieces with surface of specimen or part underexamination.5.9 Residual Magnetism in Base Metal—Residual magne-tism in base metal may affect readings of magnetic- and eddycurrent-type instruments.5.10 Stray Magnetic Fields—Strong magnetic fields, asfrom arc welding, can seriously interfere with operations ofcertain thickness gauges.5.11 Cleanness of Probe and Test Surface—Measurementsare sensitive to foreign material that prevents intimate contactbetween probe and coating surface.5.12 Pressure of Probe—Instrument readings can be sensi-tive to pressure with which probe is applied to test surface.5Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036, http://www.ansi.org.6Autocatalytically deposited nickel-phosphorus alloys containing more than 8 %phosphorus are sufficiently nonmagnetic to be measured by this method, as long asthe measurement is made prior to any heat treatment.E376 − 1725.13 Probe Position—Some magnetic-type gauges are sen-sitive to position of probe relative to the earth. For example,operation of gauge in a horizontal or upside-down position mayrequire a new standardization or may be impossible.5.14 Temperature—Eddy current instruments may be af-fected by temperature variations.6. Basis of Application6.1 The following items are subject to contractual agree-ment between the parties using or referencing this standard.6.2 Personnel Qualification6.2.1 For direct reading instruments or if specified in thecontractual agreement, personnel performing examinations tothis standard shall be qualified in accordance with a nationallyor internationally recognized NDT personnel qualificationpractice or standard such as ANSI/ASNT-CP-189, SNT-TC-1A, NAS-410, ISO 9712, or a similar document and certifiedby the employer or certifying agency, as applicable. Thepractice or standard used and its applicable revision shall beidentified in the contractual agreement between the usingparties.6.3 Qualification of Nondestructive Testing Agencies—Ifspecified in the contractual agreement, NDT agencies shall bequalified and evaluated as specified in Specification E543. Theapplicable edition of Specification E543 shall be specified inthe contractual agreement.6.4 Procedures and Techniques—The procedures and tech-niques to be utilized shall be as specified in the contractualagreement.6.5 Surface Preparation—The pre-examination surfacepreparation criteria shall be in accordance with 5.11 andrequirements specified in the contractual agreement.6.6 Timing of Examination—The timing of examinationshall be in accordance with the applicable contractual agree-ment.6.7 Extent of Examination—The extent of examination shallbe in accordance with the applicable contractual agreement.6.8 Reporting Criteria/Acceptance Criteria—Reporting cri-teria for the examination results shall be in accordance withSection 9 unless otherwise specified. Since acceptance criteriaare not specified in this standard, they shall be specified in thecontractual agreement.6.9 Reexamination of Repaired/Reworked Items—Reexamination of repaired/reworked items is not addressed inthis standard and if required shall be specified in the contrac-tual agreement.7. Calibration and Standardization7.1 Each instrument should be calibrated in accordance withthe manufacturer’s instructions and standardized before use byemploying suitable thickness standards. Standardization shouldbe checked at frequent intervals during use. Attention shouldbe given to Section 5 and Section 8.7.2 Reference standards of uniform thickness are availablein either of two types, foil or coated substrate, as supplied orrecommended by the manufacturer of the instrument. There areinstances, however, where reference standards are made byother than instrument manufacturers.7.2.1 Standardization Foils (Shims)—Standardization foil isplaced on the surface of uncoated base metal when standard-izing the instrument. Foils are advantageous for standardizingon curved surfaces and are often more readily available than acoated standard. To prevent measurement errors due to poorcontact between foil and substrate, make sure of intimatecontact between them. Foils are subject to indentation andshould, therefore, be replaced when damaged.7.2.1.1 Nonmagnetic foils may be used to standardize mag-netic thickness gauges for measurement of nonmagnetic coat-ings. Nonconductive plastic foils can be used to standardizeeddy current instruments for measurement of nonconductivecoatings.7.2.1.2 Resilient foils should not be used if there is possi-bility that the instrument probe will cause a change in thicknessreading. Use of two or more foils on top of each other shouldbe avoided unless flexibility of thin foils is required for acurved surface.7.2.2 Coated reference standards consist of coatings ofknown thickness permanently bonded to the substrate material.7.3 Thicknesses of reference standards should bracket andbe as close as possible to the coating thickness being measured.7.4 For magnetic instruments, reference standards shouldhave the same magnetic properties as the coated specimen.7.5 For eddy current instruments, the reference standardshould have the same electrical and magnetic properties asthose of coated specimen being measured (see 5.4).7.6 To determine standardization validity, a reading on abare specimen identical in magnetic and electrical properties tothat of the test specimen substrate is recommended.7.7 If the coating process is changed, the standardizationmay no longer