ASTM B244-2009 Standard Test Method for Measurement of Thickness of Anodic Coatings on Aluminum and of Other Nonconductive Coatings on Nonmagnetic Basis Metals with Eddy-Current In.pdf

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1、Designation: B 244 09Endorsed 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 Instruments1This standard i

2、s 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 () indicates an editorial ch

3、ange since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1. Scope*1.1 This test method covers the use of eddy-current instru-ments for the nondestructive measurement of the thickness ofa nonconductive coating on a nonmagnetic basis m

4、etal. It isintended to supplement manufacturers instructions for theoperation of the instruments and is not intended to replacethem.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

5、by this testmethod.1.3 The values stated in SI units are to be regarded as thestandard. The values given in parentheses are for informationonly.1.4 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 standa

6、rd to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2B 499 Test Method for Measurement of Coating Thick-nesses by the Magnetic Method: Nonmagnetic Coatings onMagnetic Basis Metals3.

7、 Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 accuracy, nthe measure of the magnitude of errorbetween the result of a measurement and the true thickness ofthe item being measured.3.1.2 adjustment, nthe physical act of aligning a instru-ments thickness readings to match those o

8、f a known thicknesssample (removal of bias), in order to improve the accuracy ofthe instrument on a specific surface or within a specific portionof its measurement range. An adjustment will affect theoutcome of subsequent readings.3.1.3 calibration, nthe high-level, controlled and docu-mented proces

9、s of obtaining measurements on traceable cali-bration standards over the full operating range of the instru-ment, then making the necessary instrument adjustments (asrequired) to correct any out-of-tolerance conditions.3.1.3.1 DiscussionCalibration of coating thickness instru-ments is performed by t

10、he equipment manufacturer, an autho-rized agent, or by an authorized, trained calibration laboratoryin a controlled environment using a documented process. Theoutcome of the calibration process is to restore/realign theinstrument to meet/exceed the manufacturers stated accuracy.3.1.4 reference stand

11、ard, na specimen of known thick-ness used to verify the accuracy of a coating thicknessmeasuring instrument.3.1.5 verification of accuracy, nobtaining measurementson a reference standard prior to instrument use for the purposeof determining the ability of the coating thickness instrument toproduce r

12、eliable values, compared to the combined instrumentmanufacturers stated accuracy and the stated accuracy of thereference standard.4. Summary of Test Method4.1 Instruments complying with this test method measurecoating thickness by the use of eddy currents.Aprobe is placeddirectly on the coated surfa

13、ce in a perpendicular position andelectronic circuitry is used to convert a reference signal into acoating thickness measurement.4.2 The instrument probe coil is energized by alternatingcurrent that induces eddy currents in the metal substrate. Theeddy currents in turn create a secondary magnetic fi

14、eld withinthe substrate. The characteristics of this secondary field aredependent upon the distance between the probe and the basismetal. This distance (gap) is measured by the instrument probe1This test method is under the jurisdiction ofASTM Committee B08 on Metallicand Inorganic Coatings and is t

15、he direct responsibility of Subcommittee B08.10 onTest Methods.Current edition approved April 15, 2009. Published May 2009. Originallyapproved in 1949. Last previous edition approved in 2002 as B 244 97 (2002).2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Custo

16、mer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1*A Summary of Changes section appears at the end of this standard.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken,

17、 PA 19428-2959, United States.and shown on the instrument display as the thickness (micronsor mils) of the intervening coating.5. Significance and Use5.1 The thickness of a coating is often critical to itsperformance. This eddy-current method is nondestructive andis suitable for measuring the thickn

18、ess of anodic coatings onaluminum, as well as the thickness of most nonconductivecoatings on nonmagnetic basis metals.5.2 This test method requires that the conductivity of thesubstrate be the same in the reference standard used forcalibration adjustment and in the coated article to be measured.6. A

19、pparatus6.1 Coating Thickness Instrument, based on eddy currentprinciples, commercially available, suitable to measure coatingthickness accurately.6.2 Coating Thickness Standards, with assigned valuestraceable to a National Metrology Institution. They may becoated aluminum plates, or may be foils or

20、 shims of flat,non-metallic sheet (typically polyester).7. Calibration and Standardization7.1 Calibration of coating thickness instruments is per-formed by the equipment manufacturer, an authorized agent, orby an authorized, trained calibration laboratory in a controlledenvironment using a documente

21、d process. A Certificate ofCalibration showing traceability to a National MetrologyInstitution can be issued. There is no standard time interval forre-calibration, nor is one absolutely required, but a calibrationinterval can be established based on experience and the workenvironment. A one-year cal

22、ibration interval is a typicalfrequency suggested by many instrument manufacturers.7.2 Before use, each instruments calibration accuracy shallbe verified in accordance with the instructions of the manufac-turer, employing suitable thickness standards and, if necessary,any deficiencies found shall be

23、 corrected.7.3 During use, calibration accuracy shall be verified atfrequent intervals, at least once a day. Attention shall be givento the factors listed in Section 8 and to the proceduresdescribed in Section 9.7.4 Coating thickness standards of known thickness areavailable either as shims or foils

24、 or as coated specimens.7.4.1 Foils:7.4.1.1 Foils used for accuracy verification and adjustmentof eddy-current instruments are generally made of plastic.They are advantageous for adjustments on curved surfaces, andare more readily available than coated standards.7.4.1.2 To prevent measurement errors

25、, it is necessary toensure that intimate contact is established between foil andsubstrate. Resilient foils should be avoided if possible. Foilsare subject to indentation and should, therefore, be replacedfrequently.7.4.2 Coated StandardsThese calibration standards con-sist of nonconductive coatings

26、of known, uniform thicknesspermanently bonded to the substrate material.7.4.3 The coating thickness of the standards used shallbracket the users highest and lowest coating thickness mea-surement requirement. Standards suitable for many applica-tions of the test method are commercially available and

27、may beused provided the certified values are traceable to a NationalMetrology Institution.7.5 The basis metal thickness for the test and the calibrationadjustment shall be the same if the critical thickness, defined in8.3, is not exceeded. When possible, back up the basis metal ofthe standard or of

28、the test specimen with a sufficient thicknessof similar material to make the readings independent of thebasis metal 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 similarmetal at least

29、 3 mm (120 mils) thick. If there is no differencebetween the readings, the critical thickness is exceeded.7.6 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 sometime

30、s impossible to make.7.7 If the curvature of the test specimen to be measured issuch as to preclude calibration adjustment on a flat surface, thecurvature of the coated standard or of the substrate on whichthe foil is placed shall be the same as that of the test specimen.8. Factors Affecting the Mea

31、suring Accuracy8.1 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 (0.02 mil); for thicknesses greater than about 25 m (1mil), this uncertainty is proportional to the

32、 coating thickness.8.2 Electrical Properties of the Basis MetalEddy-currentmeasurements are affected by the electrical conductivity of thebasis metal, which itself is often affected by heat treatments.8.3 Basis-Metal ThicknessFor each measurement, there isa critical thickness of the basis metal abov

33、e 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 conductivity of the basis

34、 metal.8.3.1 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.8.4 Edge EffectThis test method is sensitive to

35、 abruptchanges in the surface contour of the test specimen. Therefore,measurements made too near an edge or inside corner will notbe valid unless the instrument is specifically adjusted for sucha measurement.8.5 CurvatureMeasurements are affected by the curvatureof the test specimen. The influence o

36、f curvature varies consid-erably with the make and type of instrument, but alwaysbecomes more pronounced as the radius of curvature de-creases.8.6 Surface Roughness:8.6.1 1 Measurements are influenced by the surface topog-raphy of the substrate and the coating, and a rough surface willgive individua

37、l instrument readings that will vary from point topoint. In this case, it is necessary to make many readings atB244092different positions to obtain an average value that is represen-tative of the mean coating thickness. If the basis metal is roughit may also be necessary to check, and adjust if nece

38、ssary, thezero of the instrument at several positions on a sample of theuncoated rough substrate. If the roughness of the substratesurface is small, relative to the coating thickness, its effect willprobably be negligible.8.6.2 If the basis metal is rough it may also be necessary tocheck, and adjust

39、 if necessary, the zero of the instrument atseveral positions on a sample of the uncoated rough substrate.If the roughness of the substrate surface is small, relative to thecoating thickness, its effect will probably be negligible.8.7 Foreign ParticlesThe probes of eddy-current instru-ments must mak

40、e physical contact with the test surface and are,therefore, sensitive to foreign material that prevents intimatecontact between the probe and the coating surface. Both thetest surface and instrument probe should be kept free of foreignmaterial.8.8 PressureThe pressure with which the probe is applied

41、to the test specimen affects the instrument readings, andshould, therefore, be kept constant.8.9 Number of ReadingsThe precision of the measure-ments can be improved by increasing the number of readings inaccordance with statistical principles.9. Procedure9.1 Operate each instrument in accordance wi

42、th the instruc-tions of the manufacturer. Give appropriate attention to thefactors listed in Section 8.9.2 Verify the accuracy of the instrument at the test site eachtime the instrument is put into service and at frequent intervalsduring use to assure proper performance.9.3 Many instruments can be a

43、djusted in order to improvetheir accuracy on a specific surface or within a specific portionof its measurement range. The effects of properties of thesubstrate (composition, shape, roughness, edge effects, electri-cal properties) and coating (composition, mass, surface rough-ness), as well as ambien

44、t and surface temperatures, may requireadjustments to be made to the instrument. Follow the manu-facturers instructions.9.4 Observe the following precautions:9.4.1 Basis Metal ThicknessCheck whether the basismetal thickness exceeds the critical thickness. If not, either usethe back-up method in 7.5,

45、 or make sure that the calibrationadjustment has been made on a reference standard having thesame thickness and electrical properties as the test specimen.9.4.2 Edge EffectsDo not make readings close to an edge,hole, inside corner, etc., of a specimen, unless the validity ofthe calibration adjustmen

46、t for such a measurement has beendemonstrated.9.4.3 CurvatureDo not make readings on a curved surfaceof a specimen unless the validity of the calibration adjustmentfor such a measurement has been demonstrated.9.4.4 Number of ReadingsBecause of normal instrumentvariability, it is necessary to make se

47、veral 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 8.9).9.4.5 Surface CleanlinessBefore making measurements,clean any foreign matter such as dirt, grease, and

48、corrosionproducts from the surface without removing any coatingmaterial.10. Accuracy10.1 The instrument, its calibration, and its operation shallbe such that the coating thickness can be determined within10 % or 1 m, whichever is greater, of the true thickness.11. Report11.1 The report shall include

49、 the following information:11.1.1 Type of instrument used including manufacturer,model number, principle of operation, and date of calibration,11.1.2 Size and description of test specimen,11.1.3 Whether special jigs were used,11.1.4 Type of coating thickness standard and/or referencestandard and the method used for accuracy verification and anycalibration adjustment11.1.5 The number of measurements taken and the value ofeach measurement,11.1.6 Operator identification, and11.1.7 Date.NOTE 1Although, theoretically, this test method can be used formeasuri