1、Designation: B 499 96 (Reapproved 2002)Endorsed by AmericanElectroplaters SocietyEndorsed by NationalAssociation of Metal FinishersStandard Test Method forMeasurement of Coating Thicknesses by the MagneticMethod: Nonmagnetic Coatings on Magnetic Basis Metals1This standard is issued under the fixed d
2、esignation B 499; 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) indicates an editorial change since the last revis
3、ion 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 magnetic instrumentsfor the nondestructive measurement of the thickness of non-magnetic coatings over ferrous or other magnetic base metals.NOTE 1Autocata
4、lytically deposited nickel-phosphorus alloys contain-ing more than 8 % phosphorus are sufficiently nonmagnetic to bemeasured by this test method, as long as the measurement is made priorto any heat treatment.1.2 These instruments measure either the magnetic attrac-tion between a magnet and the basis
5、 metal, as influenced by thepresence of the coating, or the reluctance of a magnetic-fluxpath passing through the coating and the basis metal.1.3 Measurements made in accordance with this test methodwill be in compliance with the requirements of ISO Interna-tional Standard 2178 as printed in 1982.1.
6、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 standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Doc
7、uments2.1 International Standard:ISO 2178 Non-Magnetic Coatings on Magnetic SubstrateMeasurement of Coating ThicknessMagnetic Method23. Significance and Use3.1 The thickness of a coating is often critical to itsperformance. For most nonferrous coatings on steel, themagnetic method is reliable for me
8、asuring coating thicknessnondestructively and is suitable for specification acceptancetesting and SPC/SQC applications. The test method requiresthat the magnetic properties of the substrate used during thecalibration be the same as that of the test specimen.4. Factors Affecting the Measuring Accurac
9、y4.1 The following factors affect the accuracy of a coatingthickness measurement:4.1.1 Coating ThicknessInherent to the test method is ameasurement uncertainty that, for thin coatings, is constant andindependent of the coating thickness. The magnitude of thismeasurement uncertainty is primarily a fu
10、nction of test piecesurface finish (see 4.1.6 on surface roughness). For thicknessesgreater than about 25 m, this uncertainty is proportional to thecoating thickness.4.1.2 Magnetic Properties of the Basis MetalMagneticthickness measurements are affected by variations in themagnetic properties of the
11、 basis metal. (For practical purposes,magnetic variations in low-carbon steel can often be consid-ered to be insignificant. To avoid the influences of severe orlocalized heat treatments and cold working, the instrumentshould be calibrated using a calibration standard having a basismetal with the sam
12、e magnetic properties as that of the testspecimen or, preferably and if available, with a sample of thepart to be tested before application of the coating.)4.1.3 Basis Metal ThicknessFor each instrument, there isa critical thickness of the basis metal above which themeasurements will not be affected
13、 by an increase in thethickness of the basis metal. Since it depends on the instrumentprobe (Note 2) and the nature of the basis metal, its valueshould be determined experimentally if not supplied by themanufacturer.NOTE 2In this method “instrument probe” will also include the term“magnet.”4.1.4 Edg
14、e EffectsThe method is sensitive to 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 calibrated for sucha measurement. The effect may extend to about 20 mm fromthe discontin
15、uity, depending on the instrument.4.1.5 CurvatureThe measurements are affected by thecurvature of the test specimen. The influence of curvaturevaries considerably with the make and type of instrument butalways becomes more pronounced as the radius of curvaturedecreases. Instruments with two-pole pro
16、bes may also producedifferent readings if the poles are aligned in planes parallel or1This test method is under the jurisdiction of ASTM Committee B08 on Metallicand Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 onGeneral Test Methods.Current edition approved April 10, 1
17、996. Published June 1996. Originallypublished as B 499 69. Last previous edition B 499 88. Replaces portions ofA 219.2Available from American National Standards Institute, 11 W. 42nd St., 13thFloor, New York, NY 10036.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohock
18、en, PA 19428-2959, United States.perpendicular to the axis of a cylindrical surface. A similareffect can occur with a single-pole probe if the tip is unevenlyworn.4.1.6 Surface RoughnessMeasurements are influenced bythe surface topography of the basis metal and coating. Surfaceroughness becomes sign
19、ificant when the degree of roughness isgreater than 10 % of the coating thickness, causing increasedscatter in measurements. Therefore, it is necessary, on a roughor scratched surface, to make a greater number of measure-ments at different positions to obtain an average value that isrepresentative o
20、f the mean coating thickness. If the basis metalis rough, it may also be necessary to check the zero of theinstrument at several positions on a portion of the uncoated,rough, basis metal.4.1.7 Direction of Mechanical Working of the Basis MetalMeasurements made by an instrument having a two-pole prob
21、eor an unevenly worn single-pole probe may be influenced bythe direction in which the magnetic basis metal has beensubjected to mechanical working (such as rolling), the readingchanging with the orientation of the probe on the surface.4.1.8 Residual MagnetismResidual magnetism in the ba-sis metal af
22、fects the measurements made by instruments whichemploy a stationary magnetic field. Its influence on measure-ments made by reluctance instruments employing an alternat-ing magnetic field is much smaller.4.1.9 Stray Magnetic FieldsStrong stray magnetic fields,such as are produced by various types of
23、electrical equipment,can seriously interfere with the operation of magnetic thicknessinstruments.4.1.10 Foreign ParticlesMagnetic instruments of alltypes must make physical contact with the test surface and are,therefore, sensitive to foreign material that prevents intimatecontact between probe and
24、coating surface. Both the testsurface and instrument probe should be kept free of foreignmaterial.4.1.11 Conductivity of CoatingSome magnetic instru-ments work at frequencies between 200 and 2000 Hz. At thesefrequencies, eddy currents produced in thick, highly conduc-tive coatings may interfere with
25、 the reading.4.1.12 PressureInstrument readings are sensitive to thepressure with which the probe is applied to the test specimen.Application of the probe should not be allowed to deform thecoating.4.1.13 Probe OrientationInstruments using the principleof magnetic attraction may be sensitive to the
26、orientation of themagnet in relation to the field of gravity of the earth. Thus, theoperation of an instrument in a horizontal or upside-downposition may require a different calibration, or may be impos-sible.5. Calibration of Instruments5.1 Before use, each instrument shall be calibrated inaccordan
27、ce with the instructions of the manufacturer, employ-ing suitable thickness standards.5.2 During use, the calibration shall be checked at frequentintervals, at least once a day. Attention shall be given to thefactors listed in Section 4 and to the procedures described inSection 5.5.3 Calibration sta
28、ndards of known thickness are availableeither as shims or foils or as coated specimens.5.3.1 Calibration Foils:NOTE 3In the following paragraphs, the use of the word “foil willimply a nonmagnetic metallic or nonmetallic foil or shim.5.3.1.1 Because of the difficulty of ensuring adequate con-tact, fo
29、ils are generally not recommended for the calibration ofinstruments based on the principle of magnetic attraction butthey are suitable in some circumstances provided the necessaryprecautions are taken. They can normally be used for thecalibration of other types of instruments.5.3.1.2 Foils are advan
30、tageous for calibration on curvedsurfaces and are more readily available than coated standards.To prevent measurement errors it is necessary to ensure thatintimate contact is established between foil and substrate.Resilient foils should be avoided to prevent indentation errors.Only nonferrous metal
31、foils should be used for thicknesses lessthan 15 m. Calibration foils are subject to wear and indenta-tion and, therefore, should be replaced frequently. Worn foilsshall not be used to calibrate the instrument.5.3.2 Coated StandardsThese calibration standards con-sist of coatings of known, uniform t
32、hickness permanentlybonded to the substrate material.35.4 The basis metal of the calibration standards shall havemagnetic 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 barestan
33、dard and that of the test specimen is recommended.5.5 In some cases the calibration of the instrument should bechecked by rotating the probe in increments of 90 (see 4.1.7and 4.1.8).5.6 The basis-metal thickness for the test and the calibrationshall be the same if the critical thickness, defined in
34、4.1.3, isnot exceeded. It is often possible to back up the basis metal ofthe standard or of the test specimen with a sufficient thicknessof similar material to make the readings independent of thebasis-metal thickness.5.7 If the curvature of the coating to be measured is such asto preclude calibrati
35、on on a flat surface, the curvature of thecoated standard, or of the substrate on which the calibrationfoil is placed, shall be the same.6. Measuring Procedure6.1 Operate each instrument in accordance with the instruc-tions of the manufacturer giving appropriate attention to thefactors listed in Sec
36、tion 4.6.2 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.6.3 Observe the following precautions:6.3.1 Basis-Metal ThicknessCheck whether the basis-metal thickness exceeds the critic
37、al thickness. If not, either usethe back-up method mentioned in 5.6 or make sure that thecalibration has been made on a standard having the samethickness and magnetic properties as the test specimen.3Coated standards suitable for many applications of the test method may bepurchased from the office o
38、f Standard Reference Materials, National Institute ofStandards and Technology, Gaithersburg, MD 20899.B 49926.3.2 Edge EffectsDo not make readings close to an edge,hole, inside corner, etc., of a specimen unless the validity of thecalibration for such a measurement has been demonstrated.6.3.3 Curvat
39、ureDo not make readings on a curved surfaceof a specimen unless the validity of the calibration for such ameasurement has been demonstrated.6.3.4 Number of ReadingsBecause of normal instrumentvariability and in order to minimize surface roughness effects,a measurement shall be the mean value of seve
40、ral readings.6.3.4.1 For each measurement, make at least 3 readings,removing the probe after each reading, and average thereadings. If any 2 of the readings differ from each other bymore than 5 % of the average reading or 2 m, whichever is thegreater, then the measurement shall be discarded and repe
41、ated.6.3.4.2 The substrate or coating, or both may be too roughto meet this criterion. In such a case it may be possible toobtain a valid measurement by averaging a number of readings.To be valid under this test method, the validity of such aprocedure must be demonstrated (see Appendix X1).6.3.4.3 I
42、nstruments of the attractive force type are sensitiveto vibrations, and readings that are obviously erroneous shouldbe rejected.6.3.5 Direction of Mechanical Working If the direction ofmechanical working has a pronounced effect on the reading,make the measurement on the test specimen with the probe
43、inthe same orientation as that used during calibration. If this isimpossible, make four measurements in various orientations byrotating the probe in increments of 90.6.3.6 Residual MagnetismWhen residual magnetism ispresent in the basis metal, when using two-pole instrumentsemploying a stationary ma
44、gnetic field make measurements intwo orientations differing by 180. With single-pole instru-ments employing a stationary magnetic field, it may benecessary to demagnetize the test specimen to get valid results,and this may also be advisable with two-pole instruments.6.3.7 Surface CleanlinessBefore m
45、aking measurements,clean any foreign matter such as dirt, grease, and corrosionproducts from the surface without removing any coatingmaterial. Avoid any areas having visible defects, such aswelding or soldering flux, acid spots, dross, or oxide whenmaking measurements.6.3.8 Lead CoatingsThe magnet o
46、f an instrument of theattractive force type may stick to lead and lead alloy coatings.Apply a very thin film of oil to improve the reproducibility ofreadings and correct the measurement for the thickness of theoil film. Excess oil shall be wiped off so that the surface isvirtually dry. The correctio
47、n may be determined by measuringthe coating thickness of a nonsticking coating of appropriatethickness with and without the oil film and taking the differ-ence between the two measurements. Do not use this procedurewith other coatings.6.3.9 TechniquesThe readings obtained may depend onthe technique
48、of the operator. For example, the pressure appliedto a probe, or the rate of applying a balancing force to amagnet, will vary from one individual to another. Reduce orminimize such effects either by having the instrument cali-brated by the same operator who will make the measurement orby using const
49、ant-pressure probes. In appropriate cases when aconstant pressure probe is not being used, the use of ameasuring stand is strongly recommended.6.3.10 Positioning of ProbeIn general, place the instru-ment probe perpendicular to the specimen surface at the pointof measurement. For some instruments of the attractive forcetype, this is essential. With some instruments, however, it isdesirable to tilt the probe slightly and select the angle ofinclination giving the minimum reading. If, on a smoothsurface, the readings obtained vary substantially with th
