ASTM B244-1997(2002) Standard Test Method for Measurement of Thickness of Anodic Coatings on Aluminum and of Other Nonconductive Coatings on Nonmagnetic Basis Metals with Eddy-Curr.pdf

1、Designation: B 244 97 (Reapproved 2002)Endorsed by AmericanElectroplaters SocietyEndorsed by NationalAssociation of Metal FinishersStandard Test Method forMeasurement of Thickness of Anodic Coatings onAluminum and of Other Nonconductive Coatings onNonmagnetic Basis Metals with Eddy-Current Instrumen

2、ts1This standard is issued under the fixed designation B 244; 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 (e) indica

3、tes 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 use of eddy-current instru-ments for the nondestructive measurement of the thickness ofa nonconductive coating on a n

4、onmagnetic basis metal.1.2 This test method is particularly useful for measuring thethickness of an anodic coating on aluminum alloys. Chemicalconversion coatings are too thin to be measured by this testmethod.2. Referenced Documents2.1 ASTM Standards:2B 499 Test Method for Measurement of Coating Th

5、ick-nesses by the Magnetic Method: Nonmagnetic Coatings onMagnetic Basis Metals3. Significance and Use3.1 The thickness of a coating is often critical to itsperformance. This eddy-current method is nondestructive andis suitable for measuring the thickness of anodic coatings onaluminum, as well as th

6、e thickness of most nonconductivecoatings on nonmagnetic basis metals.3.2 This test method requires that the conductivity of thesubstrate be the same in the calibration standard and in thecoated article to be measured.4. Apparatus4.1 The coating thickness shall be determined with anelectromagnetic i

7、nstrument that measures the changes in ap-parent impedance of the coil inducing the eddy currents intothe basis metal.3The design of the instrument shall be such thatvariations in apparent impedance, produced by variations incoil to basis metal spacing, can be calibrated to indicate thethickness of

8、coatings.5. Factors Affecting the Measuring Accuracy5.1 Inherent in the test method is a measuring uncertaintythat, for thin coatings, is constant and independent of thecoating thickness, and, for a single measurement, not less than0.5 m; for thicknesses greater than about 25 m, thisuncertainty is p

9、roportional to the coating thickness.5.2 Electrical Properties of the Basis MetalEddy-currentmeasurements are affected by the electrical conductivity of thebasis metal, which itself is often affected by heat treatments.5.3 Basis-Metal ThicknessFor each measurement, there isa critical thickness of th

10、e basis metal above which themeasurements will not be affected by an increase in thatthickness. Its value should be determined experimentally, if notsupplied by the manufacturer of the measuring instrument,since it depends on both the measuring frequency of the probesystem and the electrical conduct

11、ivity of the basis metal.1This method is under the jurisdiction ofASTM Committee B08 on Metallic andInorganic Coatings and is the direct responsibility of Subcommittee B08.10 onGeneral Test Methods.Current edition approved April 10, 1997. Published March 1998. Originallyapproved in 1949. Last previo

12、us edition approved in 1993 as B 244 79 (1993).2For 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.3The following

13、 instruments have been found satisfactory for this purpose:PermascopeType EC-8-Ty, Twin City Testing Corp., P.O. Box 552,Tonawanda, NY 14150Boonton Metal Film GageType 255-A, Boonton Radio Corp., Boonton, NJ07005 (This instrument is no longer manufactured, but many are still in use.)Dermitron Thickn

14、ess Tester, Unit Process Assemblies, Inc., 53-15 37th Ave.,Woodside NY 11377Filmeter, American Instrument Co., Silver Spring, MD 20910ElcotectorMK-II, Ferro Engineering Div. of Ferro Corp., 4150 East 56th St.,Cleveland, OH 44105; also available from East Lancashire Chemical Co., Ltd.,Fairfield, Manc

15、hester, England.Accuratest Mark II, Coloral Products, Ltd., Steward St., Birmingham 18,England.IsometerType 2080, Institute, Dr. Forster, Reutlingen/Wurtt. Grathwohl-strasse 4, West Germany.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United St

16、ates.5.3.1 General RuleFor a given measuring frequency, thehigher the conductivity of the basis metal, the smaller itscritical thickness. For a given basis metal, the higher themeasuring frequency, the smaller the critical thickness of thebasis metal.5.4 Edge EffectThis method is sensitive to abrupt

17、 changesin the surface contour of the test specimen. Therefore, mea-surements made too near an edge or inside corner will not bevalid unless the instrument is specifically calibrated for such ameasurement.5.5 CurvatureMeasurements are affected by the curvatureof the test specimen. The influence of c

18、urvature varies consid-erably with the make and type of instrument, but alwaysbecomes more pronounced as the radius of curvature de-creases.5.6 Surface Roughness:5.6.1 Measurements are influenced by the surface topogra-phy of the substrate and the coating, and a rough surface willgive individual ins

19、trument readings that will vary from point topoint. In this case, it is necessary to make many readings atdifferent positions to obtain a average value that is represen-tative of the mean coating thickness.5.6.2 If the basis metal is rough it may also be necessary tocheck the zero of the instrument

20、at several positions on asample of the uncoated rough substrate. If the roughness of thesubstrate surface is small, relative to the coating thickness, itseffect will probably be negligible.5.7 Foreign ParticlesThe probes of eddy-current instru-ments must make physical contact with the test surface a

21、nd are,therefore, sensitive to foreign material that prevents intimatecontact between the probe and the coating surface.5.8 PressureThe pressure with which the probe is appliedto the test specimen affects the instrument readings, andshould, therefore, be kept constant.5.9 Number of ReadingsThe preci

22、sion of the measure-ments can be improved by increasing the number of readings inaccordance with statistical principles.6. Calibration of Instruments6.1 Before use, each instrument shall be calibrated inaccordance with the instructions of the manufacturer, employ-ing suitable thickness standards. Du

23、ring use, the calibrationshall be checked at frequent intervals, at least once an hour.Attention shall be given to the factors listed in Section 5 and tothe procedures described in Section 7.6.2 Calibration standards of known thickness are availableeither as foils or as coated specimens.6.2.1 Calibr

24、ation Foils:6.2.1.1 The calibration foils used for the calibration ofeddy-current instruments are generally made of plastic. Theyare advantageous for calibration on curved surfaces, and aremore readily available than coated standards.6.2.1.2 To prevent measurement errors, it is necessary toensure th

25、at intimate contact is established between foil andsubstrate. Resilient foils should be avoided if possible. Cali-bration foils are subject to indentation and should, therefore, bereplaced frequently.6.2.2 Coated StandardsThese calibration standards con-sist of nonconductive coatings of known, unifo

26、rm thicknesspermanently bonded to the substrate material.6.3 The basis metal of the calibration standards shall haveelectrical properties similar to those of the basis metal of thecoated test specimen. To confirm their suitability, a comparisonof the readings obtained with the basis metal of the bar

27、estandard and that of the test specimen is recommended.6.4 The basis metal thickness for the test and the calibrationshall be the same if the critical thickness, defined in 4.3, is notexceeded. When possible, back up the basis metal of thestandard or of the test specimen with a sufficient thickness

28、ofsimilar material to make the readings independent of the basismetal thickness. A way to determine if the basis metalthickness exceeds the critical thickness is to make measure-ments before and after backing up the basis metal with copperor aluminum at least 3 mm thick. If there is no differencebet

29、ween the readings, the critical thickness is exceeded.6.5 If the test specimen is soft and thin, it is subject toindentation by the probe. Because of this, and despite the useof special probes or fixtures, measurements on such specimensare sometimes impossible to make.6.6 If the curvature of the tes

30、t specimen to be measured issuch as to preclude calibration on a flat surface, the curvatureof the coated standard or of the substrate on which thecalibration foil is placed shall be the same as that of the testspecimen.7. Procedure7.1 Operate each instrument in accordance with the instruc-tions of

31、the manufacturer. Give appropriate attention to thefactors listed in Section 5.7.2 Check the calibration of the instrument at the test siteeach time the instrument is put into service and at frequentintervals during use to assure proper performance.7.3 Observe the following precautions:7.3.1 Basis M

32、etal ThicknessCheck whether the basismetal thickness exceeds the critical thickness. If not, either usethe back-up method in 6.4, or make sure that the calibration hasbeen made on a standard having the same thickness andelectrical properties as the test specimen.7.3.2 Edge EffectsDo not make reading

33、s close to an edge,hole, inside corner, etc., of a specimen, unless the validity ofthe calibration for such a measurement has been demonstrated.7.3.3 CurvatureDo not make readings on a curved surfaceof a specimen unless the validity of the calibration for such ameasurement has been demonstrated.7.3.

34、4 Number of ReadingsBecause of normal instrumentvariability, it is necessary to make several readings at eachposition. Local variations in coating thickness may also requirethat a number of measurements be made in any given area; thisapplies particularly to a rough surface (see 5.9).7.3.5 Surface Cl

35、eanlinessBefore making measurements,clean any foreign matter such as dirt, grease, and corrosionproducts from the surface without removing any coatingmaterial.B 244 97 (2002)28. Accuracy8.1 The instrument, its calibration, and its operation shall besuch that the coating thickness can be determined w

36、ithin 10 %or 1 m, whichever is greater, of the true thickness.9. Report9.1 The report shall include the following information:9.1.1 Type of instrument used,9.1.2 Size and description of test specimen,9.1.3 Whether special jigs were used,9.1.4 Type of calibration standard and the method used, and9.1.

37、5 Thickness of coating as determined from the measure-ments.NOTE 1Although, theoretically, this test method can be used formeasuring nonconductive coatings on a magnetic basis metal, its use forcoatings below 25 m is not recommended, and the magnetic methodoutlined in Test Method B 499 shall be used

38、.10. Precision and Bias10.1 The precision of this test method is being determined.ASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentionedin this standard. Users of this standard are expressly advised that determination of the v

39、alidity 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 not revised, either reapproved or withdrawn. Your c

40、omments 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, which you may attend. If you feel that your comments ha

41、ve 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,United 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).B 244 97 (2002)3