ASTM B244-2009(2014) 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: B244 09 (Reapproved 2014) 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 B244; 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 () indicate

3、s 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. Scope1.1 This test method covers the use of eddy-current instru-ments for the nondestructive measurement of the thickness ofa nonconductive coating on

4、a nonmagnetic basis metal. 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

5、 thin to be measured 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 th

6、e 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 ASTM Standards:2B499 Test Method for Measurement of Coating Thicknessesby the Magnetic Method: Nonmagnetic Coatings onMagne

7、tic Basis Metals3. 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 reading

8、s to match those of 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

9、docu-mented process of obtaining measurements on traceable cali-bration standards over the full operating range of theinstrument, then making the necessary instrument adjustments(as required) to correct any out-of-tolerance conditions.3.1.3.1 DiscussionCalibration of coating thickness instru-ments i

10、s performed by the 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

11、 reference standard, na specimen of known thicknessused to verify the accuracy of a coating thickness measuringinstrument.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 instrume

12、nt toproduce reliable 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 th

13、e coated surface in a perpendicular position andelectronic circuitry is used to convert a reference signal into acoating thickness measurement.1This test method is under the jurisdiction ofASTM Committee B08 on Metallicand Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on

14、Test Methods.Current edition approved May 1, 2014. Published May 2014. Originallyapproved in 1949. Last previous edition approved in 2009 as B244 09. DOI:10.1520/B0244-09R14.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For

15、Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States14.2 The instrument probe coil is energized by alternatingcurrent that ind

16、uces eddy currents in the metal substrate. Theeddy currents in turn create a secondary magnetic field 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 probeand shown

17、 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 thickness of anodic coatings onaluminum, as w

18、ell 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. Apparatus6.1 Coating Thickness Instrumen

19、t , 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 shims of flat,non-metallic sheet (ty

20、pically 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 documented process. A Certificate ofCalibratio

21、n 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 calibration interval is a typicalfrequen

22、cy suggested by many instrument manufacturers.7.2 Before use, each instruments calibration accuracy shallbe verified in accordance with the instructions of themanufacturer, employing suitable thickness standards and, ifnecessary, any deficiencies found shall be corrected.7.3 During use, calibration

23、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 or as coated specimens.7.4.1 Foils:7.4

24、.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, it is necessary toensure that intimat

25、e 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 of known, uniform thicknesspermanently

26、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 may beused provided the certified value

27、s 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 the test specimen with a sufficient thi

28、cknessof 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 3 mm (120 mils) thick. If there is no

29、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 sometimes impossible to make.7.7 If the curvatu

30、re 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 Measuring Accuracy8.1 Inherent in the test

31、 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 coating thickness.8.2 Electrical Prope

32、rties 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 above which themeasurements will not be aff

33、ected 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 metal.8.3.1 General RuleFor a given me

34、asuring 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 abruptchanges in the surface contour o

35、f 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.B244 09 (2014)28.5 CurvatureMeasurements are affected by the curvatureof the test specimen. The influence of curvature varies consi

36、d-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 individual instrument readings th

37、at will vary from point topoint. In this case, it is necessary to make many readings atdifferent 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 necessary, thezero of the instrumen

38、t 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 if necessary, the zero of the

39、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 make physical contact with the tes

40、t 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 appliedto the test specimen affects th

41、e 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 with the instruc-tions of the man

42、ufacturer. 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 adjusted in order to improvethei

43、r 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, surfaceroughness), as well as ambient and surface temperatures, mayre

44、quire adjustments to be made to the instrument. Follow themanufacturers 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, or make sure that the calibrationa

45、djustment 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 adjustment for such a measurement has beende

46、monstrated.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 several readings at eachposition. Loc

47、al 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 corrosionproducts from the surface

48、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 the following information:11.1.1 T

49、ype 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 formeasuring nonconductive coatings on a mag